FIGHT? Here is another W8JI myth bone!
Yuri Blanarovich
Wanna FIGHT?
Here is another misconception propagated by "guru" W8JI, like "there is no
electrostatic shielding, the grounded piece of tubing (shield) IS the
antenna."
Let the games begin! (I will hold my horses for a while :-)
As posted on Top...@contesting.com reflector:
>>>
Subject:
"The June, 2006 QST has an article about a 160m RX loop.
Comments, advice welcome."
73,
Tim K3HX
Tim, et al,
I just received the rag here. Very interesting this article, in light of
the many discussions on this reflector and antennas reflector,
concluding by our best experts that a shield contributes to NO USEFUL
function whatsoever in such a receiving loop! It just essentially
becomes electrically the loop itself; the shield's outer skin IS the
antenna. No S/N ratio advantage, no anything except adding to a very
critical balancing result. No noise discrimination, no magic. So what
gives with this mythological article?
73, Roy K6XK
W8JI wrote:
Roy and all,
It appears QST isn't as careful as they used to be years
ago.
It is a very well known physical property of a "shield" more
than several skin depths thick that essentially nothing goes
through that shield. It's a Faraday cage, and when the
time-varying electric field goes to zero so does the
magnetic field. This is explained in nearly every handbook
(even ARRL publications) and is the reason coaxial cables
have that "third path" on the outside of the shield for
common mode currents. This is why a bead balun or a coil of
coax works to stop common mode on the outside of the shield
and does not affect the stuff inside the cable.
Obviously any technical explanation that requires fields to
go through the shield isn't accurate.
It is also a very well known fact that radio waves are
electromagnetic, and the interference (unless in the
induction field area of less than a half wave or so) is a
radio wave, and like all radio waves it is neither magnetic
or electric. It has a fixed ratio or electric to magnetic
fields. It is also established that a very short distance
from a small loop the predominant field is electric, not
magnetic. Clearly a shield can't "filter" noise, since noise
is not field sensitive.
So what does the shield in a loop do? The shield is actually
the antenna element. The shield is actually what receives
(and/or transmits) the EM wave. NOT the wire inside the
shield. The wire inside the shield simply couples whatever
device is connected to the antenna (the loop's shield) to
the receiver.
If we have a poor coupling system to the loop, how we
configure the shield can certainly affect the balance of the
loop and the common mode current. Say we use an unbalanced
line or we use an unbalanced amplifier directly attached to
a loop. Now we have created a problem, the feedline feeding
the loop acts just like part of the antenna system. We have
an unbalanced feedline or amplifier tied to a balanced
antenna, and so every conductor going to that point acts
like part of the antenna system. We can couple noise and
signals picked up by the feedline shield and everything in
the house to the input of the receiver.
I go through this problem in detail at:
http://www.w8ji.com/magnetic_receiving_loops.htm
If you read carefully you'll see no has ever said the shield
and how the shield is connected won't change the system when
the system is not designed correctly. When the feedpoint is
done correctly, the presence or absence of a shield has no
effect at all on the system. All the stuff about the shield
"filtering" the fields and blocking the electric field is
nonsense. But some construction methods do result in better
loop balance.
The characteristics of an improperly done feed system are
affected by the construction of the loop, but that isn't
because the shield is necessary or that a solid shield
behaves differently than braided shield. It's because
something was more wrong with the construction in one system
compared to the other, not because of the shield quality.
73 Tom
<<<
Roy Lewallen
> Howdy antenna aficionados!
>
> Wanna FIGHT?
>
> Here is another misconception propagated by "guru" W8JI, like "there is no
> electrostatic shielding, the grounded piece of tubing (shield) IS the
> antenna."
>
> Let the games begin! (I will hold my horses for a while :-)
>
> As posted on Top...@contesting.com reflector:
I don't find anything incorrect with Tom's response. What did you find
in it that wasn't accurate?
Roy Lewallen, W7EL
Tom Ring
> Howdy antenna aficionados!
>
> Wanna FIGHT?
>
>
Yuri
More mixer and more ice would serve you well.
tom
K0TAR
Dave
wet, windy day interesting!
"Tom Ring" <news...@taring.org> wrote in message
news:44668fda$0$1016$39ce...@news.twtelecom.net...
Yuri Blanarovich
news:1147636317.9...@g10g2000cwb.googlegroups.com...
> Tom, this is well written and devoid of any antagonism towards anyone.
> If anybody wants to dispute any point then all relavent data is in
> place in your posting and thus
> forces all who disagree to stay on subject without the need for
> extraneous data when debating their differences. There will ofcourse,
> be some that will be more interested in a fight or profanity in the
> absence of comunicable knoweledge , but you are well positioned to just
> stand by your posting without retaliating in kind.
> Well done
> Art
>
Here is the exchange on the subject from TopBand reflector:
>K3BU< (...)
and W8JI responses:
> Tom is confusing Faraday shield with Electrostatic shield
> and whole reasoning that the grounded shield of small loop
> antenna is THE antenna is all wrong. Wire loops inside the
> electrostatic shield are perfectly OK to receive the RF
> and ARE the antenna.
It's a very well known property that nothing passes through
the walls of a shield more than several skin depths thick.
This is because skin effect keeps the current in the outside
layers and the core of the shield wall is dead. This is the
very thing that allows our coaxial lines to behave like
three conductors, a center conductor, a inner wall, and an
outer conductor. The physical behavior of a shield does not
change with application.
>Electrostatic shield in small loop antennas reduces the
>interference, electrical noise locally generated (prevalent
>electrical fields).
Not so Yuri.
First an electrostatic field by definition is a non-changing
field. Static is stationary or unchanging, and things that
aren't changing can't make RF noise.
(Here he is confused about electrostatic shield, "electrostatic field" and
electrical field and just like with loading coils case, confusing the issue
with behavior of ALL shields, Faraday, Electrostatic, coax, etc. applied to
a wrong case. - Yuri)
The field from an accidental transmitter (noise source) is
just like the field from any intentional signal source like
a transmitter. There is absolutely nothing that says the
field has a high field impedance (electric field dominant).
Even if it was a high impedance at the source, just 1/10th
wave or so from the source the field would change to a low
impedance.
We can't filter noise by virtue of field impedance or a
shield. Even if we could, the noise source in the nearfield
would randomly field dominant depending on distance and
source charateristics.
The only thing the shield can do at radio frequencies is
change the system balance.
73 Tom
K7ITM
It seems to me that when "W8JI" is associated with something, you
assume immediately that it is wrong. If you were to read Ronold W. P.
King's explanation about small loop antennas in "Transmission Lines,
Antennas and Waveguides", would you be any more apt to believe it? How
about Glenn S. Smith's discussion of them in Johnson and Jasik's
"Antenna Engineering Handbook" (second edition)? Each of those begins
with a reasonably detailed description of an "unshielded" loop and
moves on to a "shielded" loop.
In addition, can you expain to us how the current on the wires on the
inside of the shield is NOT balanced by an equal current in the
opposite direction on the inside surface of the shield? Please tell us
in detail just what currents are where on the shielded loop. If you
are going to try to tell us that some explanation is in error, please
provide us with enough detail that we can make up our own minds. So
far, all I've seen here is some vague reference to confusion about
shields.
The descriptions in each of the two references I gave above are far
more detailed than what you have posted here, either of your own or of
W8JI's, and I find them both enlightening--they are slightly different
from each other--but both detailed enough that you can make up your own
mind about what's really going on, and not have to read ranting
generalities or statements with nothing to back them up.
Cheers,
Tom
Yuri Blanarovich
"K7ITM" <k7...@msn.com> wrote in message
news:1147709818.2...@y43g2000cwc.googlegroups.com...
> Yuri,
>
> It seems to me that when "W8JI" is associated with something, you
> assume immediately that it is wrong.
That's what might seem to you, but I point out gross misinformation, when I
come across it. I express my opinion based on what I know or believe. I
could be wrong and I gladly get educated. Mostly, if I see, measure or touch
something, I believe it to be right. Mumbo-jumbo "scientwific explanation",
taking off on tangent to justify the fallacy don't cut it with me.
> If you were to read Ronold W. P.
> King's explanation about small loop antennas in "Transmission Lines,
> Antennas and Waveguides", would you be any more apt to believe it? How
> about Glenn S. Smith's discussion of them in Johnson and Jasik's
> "Antenna Engineering Handbook" (second edition)? Each of those begins
> with a reasonably detailed description of an "unshielded" loop and
> moves on to a "shielded" loop.
>
I don't have the King's book, in Jasik's the treatment of small loops and
shielded loops is dealing with some "medieval" designs. The closest to my
version is his Fig. 5.23a showing balanced shielded loop. But then the
5-23bdoesn't make much sense to me, having small loop on the front of
reflector, when the small loop has the minimum of radiation along the axis
through the loop, and he places the reflector in the minimum - null
direction? The way they show the loops, half of loop solid wire, half coax
line, creates confusion what is antenna, what is shield, or perhaps combines
them. I have not used those designs.
> In addition, can you expain to us how the current on the wires on the
> inside of the shield is NOT balanced by an equal current in the
> opposite direction on the inside surface of the shield? Please tell us
> in detail just what currents are where on the shielded loop. If you
> are going to try to tell us that some explanation is in error, please
> provide us with enough detail that we can make up our own minds. So
> far, all I've seen here is some vague reference to confusion about
> shields.
>
> The descriptions in each of the two references I gave above are far
> more detailed than what you have posted here, either of your own or of
> W8JI's, and I find them both enlightening--they are slightly different
> from each other--but both detailed enough that you can make up your own
> mind about what's really going on, and not have to read ranting
> generalities or statements with nothing to back them up.
>
> Cheers,
> Tom
>
I will not get tangled into currents, phasors, but describe my design of
small shielded loop antenna that I used on 160m and this should perhaps shed
some light on the controversy.
I used 1/2" copper water tubing (non ferrous material passing the magnetic
field) for circular loop about 4 foot diameter. At the top the loop had gap,
at the bottom it was mounted in small metallic box. Loop, box and mast were
all DC connected and grounded. Mast was about 5 ft high, with Ham-m rotor at
the base to rotate the contraption. This formed Electrostatic shield for the
antenna.
From the connection box I threaded three turns of electrical house wire #12
and across the ends connected mica trimmer capacitor C1 (abt 1200 pF?) to
resonate the three wire loop antenna at 1.830 kHz). Not connected to
anything else, nor ground or loop.
Then I threaded one turn of the same #12 wire as a coupling turn. One end
was connected to the coax braid, the other end through another mica trimmer
capacitor C2 to the center conductor of the coax. Floating, not grounded or
connected to other loop or tubing.
I tuned the C1 to resonate the three turns at the desired frequency and C2
to provide 50 ohm match to coax. Circuit wise this mirrors the LC parallel
tuned circuit with link coupling and provide better signal than other
published designs.
I tried version of this without copper tubing shield and with. I had local
AC power line noise (within fractions of wavelength) and shielded loop
attenuated the local noise.
The way I see this works, the three turns were the antenna, it was tunable
across the band. The "link" coupling allowed to keep the symmetry of antenna
and provided some isolation for common mode currents between the antenna and
coupling (well known in LC tuned circuit with link coupling.). The copper
tubing was ELECTROSTATIC SHIELD which let's the EM waves pass through.
If the copper tubing IS the antenna, then how does it work? Short, grounded
in the center bent dipole? Then the radiation pattern should have maximum
perpendicular to the plane of the loop/dipole. But the antenna has NULLS in
that direction, corresponding to the properties of the 3 plus 1 wire loops.
You scientwists can play games with theories how it should behave, but the
reality again shows how it behaves. Anyone can build the antenna as I
described and VERIFY it. Wire loops without electrostatic shield tubing
still work the same way as with the shield. So which IS antenna?
Another description of the subject antenna is at
http://www.tpub.com/content/antennaandmasts/TM-11-5985-352-14/css/TM-11-5985-352-14_31.htm
73 Yuri Blanarovich, K3BU, VE3BMV
Roy Lewallen
Yuri Blanarovich wrote:
> . . .
> I used 1/2" copper water tubing (non ferrous material passing the magnetic
> field) for circular loop about 4 foot diameter. . .
If you believe that, it's no surprise that you're having difficulty
understanding how a shielded loop works.
It's not hard to demonstrate that the (time-varying) magnetic field
doesn't penetrate a non-ferrous shield, if you believe (correctly) that
a time-varying magnetic field will produce a current on a nearby
conductor. Simply put an oscillator or signal source into a copper box
-- you can solder one op out of PC board material. Run some wires all
around the inside which carry the oscillator signal, putting them as
close to the shield wall as you like. Put a battery inside the box to
power the oscillator and seal the box up. Then sniff around the outside
of the box with any kind of magnetic field detector you can devise. If
you have a little potted oscillator of some kind, you should be able to
do this in a couple of hours at most.
Or, just connect your rig to a good dummy load with some double shielded
coax and sniff around the outside of the copper coax shield. If you put
the detector just outside the shield, the current on the inside of the
shield will be much closer to the detector than the current on the
center conductor. So if the shield is transparent to a magnetic field,
your detector should go wild. (Make sure the rig is very well shielded,
though, so no common mode currents make their way from the rig to the
outside of the shield.)
Alternatively, if you'll spend some time with a good electromagnetics
text learning about eddy currents and the like, you'll understand why
you'd be wasting your time with those experiments.
Once you're convinced that the shield blocks the magnetic as well as
electric field, you'll have to revise your theory on how a shielded loop
works. And you'll find that Tom's explanation is correct.
Roy Lewallen, W7EL
Cecil Moore
> "K7ITM" <k7...@msn.com> wrote:
>>It seems to me that when "W8JI" is associated with something, you
>>assume immediately that it is wrong.
>
> That's what might seem to you, but I point out gross misinformation, when I
> come across it.
What gets missed quite often in these discussions is that
everyone agrees on 99 44/100 percent of the technical issues
and we tend not to discuss those issues. We only discuss the
56/100 percent of the issues upon which we disagree. It is
akin to the arguments between Einstein and Bohr. I suspect
that no two people here on r.r.a.a are in 100% agreement
on everything.
--
73, Cecil http://www.qsl.net/w5dxp
w8...@akorn.net
Cecil Moore wrote:
> Yuri Blanarovich wrote:
>
> > "K7ITM" <k7...@msn.com> wrote:
> >>It seems to me that when "W8JI" is associated with something, you
> >>assume immediately that it is wrong.
> >
> > That's what might seem to you, but I point out gross misinformation, when I
> > come across it.
>
> What gets missed quite often in these discussions is that
> everyone agrees on 99 44/100 percent of the technical issues
> and we tend not to discuss those issues. We only discuss the
> 56/100 percent of the issues upon which we disagree.
Make no mistake about it, I disagree with everything Yuri has posted
about the physics behind a "shielded loop".
I certainly don't want to be considered to be 99% in agreement with
anyone who thinks a time-varying magnetic field can pass though a
highly conductive copper wall, or any wall that is several skin depths
thick, just as I don't want to be 99% in agreement with anyone who
thinks a loading coil "replaces" or has the phase shift or "current
drop" of missing electrical degrees.
The basic physical properties have to be understood before I'd be
largely in agreement. If basic building blocks are wrong, our idea of
how the worlds works must also be very distorted.
73 Tom
w8...@akorn.net
Roy Lewallen wrote:
> I haven't gone through this in detail yet, but one misconception is glaring:
> Yuri Blanarovich wrote:
> > I used 1/2" copper water tubing (non ferrous material passing the magnetic
> > field) for circular loop about 4 foot diameter. . .
>
> If you believe that, it's no surprise that you're having difficulty
> understanding how a shielded loop works.
>
> It's not hard to demonstrate that the (time-varying) magnetic field
> doesn't penetrate a non-ferrous shield, if you believe (correctly) that
> a time-varying magnetic field will produce a current on a nearby
> conductor.
The experiment I did years ago used a copper wall in a one or two foot
square thin sheet.
I made a link on one side through a high power 50 ohm load resistor.
This allowed me to run significant known current through the resistor
(and link) from a regular HF transmitter.
Using two probes, one a sensitive low voltage dial light bulb with two
prongs soldered on and the other a multiple turn loop with diode/meter
indicator, I could roughly check current in multiple places on both
sides of the sheet.
It was very easy to see where the magnetic field (and current) goes. It
does not pass through the wall. It's quite amazing to see how even a
direct soldered connection on one side will not produce current (and a
magnetic field) on the other side.
73 Tom
Cecil Moore
> Cecil Moore wrote:
>>What gets missed quite often in these discussions is that
>>everyone agrees on 99 44/100 percent of the technical issues
>>and we tend not to discuss those issues. We only discuss the
>>56/100 percent of the issues upon which we disagree.
>
> Make no mistake about it, I disagree with everything Yuri has posted
> about the physics behind a "shielded loop".
That's part of the 56/100 percent of the issues upon which you
disagree.
> ... I don't want to be 99% in agreement with anyone who
> thinks a loading coil "replaces" or has the phase shift or "current
> drop" of missing electrical degrees.
Just proves that no individual is right 100% of the time. 75m
bugcatcher coils obey the laws of physics and thus suffer a
delay in the real world. Hint: Contrary to the lumped circuit
model, *everything* has a delay in the real world. The only place
coils don't have a delay is in your mind where miracles and
magic are possible.
The loading coil causes a phase shift in accordance with the
laws of physics. The stinger causes a phase shift in accordance
with the laws of physics. The impedance discontinuity between
the coil and stinger causes a phase shift in accordance with
the laws of physics. There are NO missing degrees.
When you comprehend how an electrical 1/4WL stub can be 19 degrees
of 450 ohm line plus 18 degrees of 50 ohm line and be physically
0.1 WL long, then you will have comprehended reality.
K7ITM
magnetic field
doesn't penetrate a non-ferrous shield, if you believe (correctly) that
a time-varying magnetic field will produce a current on a nearby
conductor."
Yes, it's all easy to demonstrate. It's used in practice all the time:
the shielding in a transmitter, the aluminum shield cans around IF and
RF coils, the copper strap around a power transformer (used
specifically to lower the external magnetic field around the
transformer, so it won't couple into low-level audio circuits or affect
colors on a color CRT).
And indeed it all agrees with theory. For this one, you need little
more than Faraday's Law of Magnetic Induction.
It's fine with me if there are people who don't want to be bothered
with theory, but if they profess that something works by means
different from the theory that I understand and which agrees with the
observations I make, they shouldn't expect me to believe them without
putting some very serious effort into explaining why the accepted
theory is wrong. I believe Yuri when he tells me his antenna works.
But I'm not buying into his explanation of HOW it works.
Cheers,
Tom
Roy Lewallen
>
> The experiment I did years ago used a copper wall in a one or two foot
> square thin sheet.
>
> I made a link on one side through a high power 50 ohm load resistor.
> This allowed me to run significant known current through the resistor
> (and link) from a regular HF transmitter.
>
> Using two probes, one a sensitive low voltage dial light bulb with two
> prongs soldered on and the other a multiple turn loop with diode/meter
> indicator, I could roughly check current in multiple places on both
> sides of the sheet.
>
> It was very easy to see where the magnetic field (and current) goes. It
> does not pass through the wall. It's quite amazing to see how even a
> direct soldered connection on one side will not produce current (and a
> magnetic field) on the other side.
I encourage Yuri to add this experiment to the ones I suggested, or just
do this one. Understanding what happens to the currents and fields in
the presence of a shield is essential to understanding "shielded" loop
operation, and his statement about magnetic fields and non-ferrous
shields shows that he lacks this understanding. Any explanation based on
the mistaken idea that a non-ferrous shield is transparent to a magnetic
field is bound to be seriously flawed.
Roy Lewallen, W7EL
Yuri Blanarovich
I understand your and Roy's points. I am not claiming to try to formulate
the infinitesimal theory of wasaaaap and I didn't try that with loading
coils. Ensuing discussions helped me to better understand the mechanaism of
how things work, the theory and how can I better apply them. I thank you for
that.
What I have problem with someone claiming shield is not a shield (Why do
they bother calling it shield or shielded loop?), when I saw the shielding
properties of it in the vicinity of the local interfering signals. It
performs as a shield to the antenna that is wound inside. Tom categorically
denies SHIELD, it IS the ANTENNA he claims. (Like there is no current drop
along the loading coil! - The gospel from the all-knowing guru.)
What I have problem with someone claiming the small loop antenna (three plus
one turn) is not the antenna, but when I remove the shield, the "not
antenna" is still THE ANTENNA.
I am not arguing the mechanics or theory behind how the shield works, it may
be transparency to magnetic field, it may be the voltage generated in the
gap, bla, bla...
Based on my experience with the said antenna, I concluded that wire loops
are THE antenna, shield works as an electrostatic shield.
I know that if I stick oscillator inside of 10' of 1/2" tubing, I will get
hardly or no signal out. I know if I bend that tubing into a circle with gap
and stick wire loop antenna inside, I can get signals out of that "shielded"
antenna and can attenuate close by interfering signals. Shielding doesn't
MAKE my antenna work (it works without shield too), shield enhances its
rejection/shielding properties in near fields.
I know there are small loops and there are small shielded loops and they
work and I have proved it.
Just don't tell me it is called shield because it is antenna, or that
antenna inside the shield doesn't work, or shield doesn't shield from
electrostatic fields, or that my antenna I described doesn't work as I
described.
Tom may pontificate his ideas to his worshippers, but I don't swallow that.
I point out my, and who else cares, disagreement, especially when I see his
"ideas" migrating into ham literature.
Go ahead with your but, but, butts.....
Yuri Blanarovich
Now we have special case of biiiig coils being antenna here.
Let the games begin!
Yuri
"Cecil Moore" <myc...@hotmail.com> wrote to W8JI:
Yuri Blanarovich
"Roy Lewallen" <w7...@eznec.com> wrote in message
news:126irc4...@corp.supernews.com...
>I haven't gone through this in detail yet, but one misconception is
>glaring:
>
> Yuri Blanarovich wrote:
>> . . .
>> I used 1/2" copper water tubing (non ferrous material passing the
>> magnetic field) for circular loop about 4 foot diameter. . .
>
> If you believe that, it's no surprise that you're having difficulty
> understanding how a shielded loop works.
>
> It's not hard to demonstrate that the (time-varying) magnetic field
> doesn't penetrate a non-ferrous shield, if you believe (correctly) that a
> time-varying magnetic field will produce a current on a nearby conductor.
> Simply put an oscillator or signal source into a copper box -- you can
> solder one op out of PC board material. Run some wires all around the
> inside which carry the oscillator signal, putting them as close to the
> shield wall as you like. Put a battery inside the box to power the
> oscillator and seal the box up. Then sniff around the outside of the box
> with any kind of magnetic field detector you can devise. If you have a
> little potted oscillator of some kind, you should be able to do this in a
> couple of hours at most.
That is called Faraday shield and does not function as Electrostatic shield.
> Or, just connect your rig to a good dummy load with some double shielded
> coax and sniff around the outside of the copper coax shield. If you put
> the detector just outside the shield, the current on the inside of the
> shield will be much closer to the detector than the current on the center
> conductor. So if the shield is transparent to a magnetic field, your
> detector should go wild. (Make sure the rig is very well shielded, though,
> so no common mode currents make their way from the rig to the outside of
> the shield.)
>
> Alternatively, if you'll spend some time with a good electromagnetics text
> learning about eddy currents and the like, you'll understand why you'd be
> wasting your time with those experiments.
I learned about shieldings, Faradyas, I use them, in equipment design, in RF
and harmonics suppression, I built shielded room for university. But I also
know the difference between the Farady shield and Electrostatic shield and
seen them work. Maybe lumping all shields is as no good as lumping all coils
ain't no good?
> Once you're convinced that the shield blocks the magnetic as well as
> electric field, you'll have to revise your theory on how a shielded loop
> works. And you'll find that Tom's explanation is correct.
>
> Roy Lewallen, W7EL
Roy,
I have magnetothermia machine which is about 200 W push-pull power generator
at around 27 MHz. It uses single turn, shielded loop, made of coax, about 30
inch in circumference. Loop wire, antenna (center conductor of coax) is fed
from the plates of two tubes, shield is open at the far end and grounded at
the exit from the enclosure. I get those 200 W heating my body tissue with
magnetic field. Maybe it has something to do with shielding being a fraction
of a wavelength distance from the radiator and the properties of the
magnetic and electric components in the antenna reactive near field region?
I know that this loop radiates along its circumference, not just from the
gap in the shield. What's yer theory? Or it don't (ooops, can't) woyk?
You seem to associate and stick to wrongos and I am sorry you find their
explanations correct, for the reality proves them wrong.
73 Yuri Blanarovich, K3BU
K7ITM
Just once, and I'm done with this.
Someone somewhere along the line mistakenly called it a shield. They
didn't understand how it works and what's important. Get over it.
Look at just the "shield" with no wires inside. Isn't that exactly a
single turn loop antenna? Isn't the feedpoint the gap in the loop?
If you put a wire (or several wires) through the inside of that tube
you used to call the shield, they just pick up the signal from the
feedpoint. Consider a single wire through the tube. There is a
voltage across the gap, the feedpoint of the loop. Since there is
essentially no voltage drop along the wire in the center, across the
distance of the gap in the tube, then the voltage across the gap must
appear as transmission line voltage across the coaxial feedlines which
are made up of the wire and the inner surface of the tube. If you've
arranged things symmetrically, then the total gap voltage will divide
equally between the two. Then it's just standard coaxial lines from
there to where you connect your receiver, or where you put a tuned
tank.
Or if you have multiple wires through the tube, the net transmission
line current divides among them. And you can resonate them with a
capacitor, but that doesn't make them have antenna currents on them.
If you have another way to analyze it accurately, fine. I don't care.
My way works for me, and it does not disagree with the _performance_
I've seen you post about. It does disagree with the _theory_ you've
suggested.
As for WHY adding the "shield" helps get rid of local e-field noise
(from sources less than a few wavelengths away, which at VLF might be
kilometers), and why the nulls are more perfect, it's because symmetry
is CRITICAL for that performance, and adding the outside tube allows
you to make a more perfectly symmetrical loop than you can practically
accomplish with just wires and all the tuning stuff you hang off it.
If you are VERY careful to keep things symmetrical, you can also do it
without the tube. But it takes amazingly little imbalance to screw
things up.
Dat's it in a nutshell.
Cheers,
Tom
Roy Lewallen
> . . .
> I point out my, and who else cares, disagreement, especially when I see his
> "ideas" migrating into ham literature.
But Tom's explanation is correct. It's consistent with theory; alternate
explanations aren't. If you're really interested in learning how a
"shielded" loop works and won't accept Tom's explanation because it came
from Tom, you can find a similar explanation in a number of reputable
texts. I'll gladly provide references, if you ask before I leave for
Dayton. Once you gain an understanding of some basic electromagnetic
principles, the correctness of the explanation will be obvious.
Oh, and don't worry about Tom's ideas migrating into the literature.
They were already in the literature well before any of us were born.
Roy Lewallen, W7EL
Roy Lewallen
> . . .
> and harmonics suppression, I built shielded room for university. But I also
> know the difference between the Farady shield and Electrostatic shield and
> seen them work. Maybe lumping all shields is as no good as lumping all coils
> ain't no good?
Sorry, you're not making much sense to me. You said that a non-ferrous
shield is transparent to a (time-varying) magnetic field. The
experiments I proposed illustrate that this is false. This has nothing
to do with what name you attach to a shield.
>
> Roy,
> I have magnetothermia machine which is about 200 W push-pull power generator
> at around 27 MHz. It uses single turn, shielded loop, made of coax, about 30
> inch in circumference. Loop wire, antenna (center conductor of coax) is fed
> from the plates of two tubes, shield is open at the far end and grounded at
> the exit from the enclosure. I get those 200 W heating my body tissue with
> magnetic field.
Hm. How do you know it's from just the magnetic field?
This is really interesting. Just a couple of postings ago, you said that
a non-ferrous shield is transparent to a magnetic field. Now you say
that a magnetic field is heating your body. Do you have some embedded
steel shrapnel or something making your body ferrous, or do you just eat
lots of nails and scrap metal?
Maybe it has something to do with shielding being a fraction
> of a wavelength distance from the radiator and the properties of the
> magnetic and electric components in the antenna reactive near field region?
What has? The heating? That's due to the lossiness of bodily fluids in
the presence of either time-varying magnetic or electric fields.
> I know that this loop radiates along its circumference, not just from the
> gap in the shield. What's yer theory? Or it don't (ooops, can't) woyk?
If you'll read what Tom has posted, or a description in any good text,
you'll find that the whole circumference of a "shielded" loop radiates.
The field comes from current on the outside of the "shield", not from
some field penetrating the shield. That's my theory. It's the same as
Tom's, and that of every respected author I've read.
> You seem to associate and stick to wrongos and I am sorry you find their
> explanations correct, for the reality proves them wrong.
Reality proves Newton wrong -- any fool can see that moving objects come
to rest on their own. There's no conflict between theory and reality --
just between theory and people's interpretations of what they're seeing.
I'll stick with the theory that's been known and confirmed for over a
century. People with alternate theories, like yours, will have to
provide some extraordinary proof to sway my thinking.
It seems you're more interested in proving Tom to be wrong about
something -- anything! -- than taking the effort to really understand
what's actually happening. So nothing else I can post will help you. I
hope the lurkers have gotten something from this, though.
Roy Lewallen, W7EL
nm...@wt.net
>that no two people here on r.r.a.a are in 100% agreement
>on everything.
Surely you jest... :) Fight! Fight! Fight! Kinda reminds me of
Beevis and Butthead after eating too much chocolate..
K7ITM pretty much boiled it down to the raw minerals by
noting that the usual "shielded loops" only advantage is the oft
improved balance. I've already been through all this mess testing
them here...
And I've proven to myself that an open wire loop can be just as
good as a "shielded loop" just as long as balance is taken care
of. It's the balance that matters. If the two types are equally
balanced,
and the same size, they will act the same.
The rest is just fodder for bored old farts on a newsgroup.
Of course, many won't agree with me, and this would include Yuri,
since he believes a shielded loop is quieter than an open loop.
But I don't care.
It's a free country. Or I think it is... Sometimes I wonder these days
with all these goofballs we have in DC running the show.
MK
Mike Coslo
> I suspect
> that no two people here on r.r.a.a are in 100% agreement
> on everything.
I disagree! heh ;^)
- 73 de Mike KB3EIA -
Mike Coslo
> If you'll read what Tom has posted, or a description in any good text,
> you'll find that the whole circumference of a "shielded" loop radiates.
> The field comes from current on the outside of the "shield", not from
> some field penetrating the shield. That's my theory. It's the same as
> Tom's, and that of every respected author I've read.
Game, Set, and Match, Roy. The explanation and the everyday application
of the concept of non-ferrous shielding are both simple and elegant.
Seems like the thread stopper to me! I suspect it will continue
anyhow.... 8^)
Cecil Moore
> The explanation and the everyday
> application of the concept of non-ferrous shielding are both simple and
> elegant.
I'm fairly ignorant when it comes to shielding. Do the
magnetic fields from a magnet penetrate copper? Do the
magnetic fields from 60 Hz devices penetrate the shield
on coax?
Yuri Blanarovich
<nm...@wt.net> wrote in message
news:1147848484.7...@j55g2000cwa.googlegroups.com...
> And I've proven to myself that an open wire loop can be just as
> good as a "shielded loop" just as long as balance is taken care
> of. It's the balance that matters. If the two types are equally
> balanced, and the same size, they will act the same.
Sooo, in shielded loop the shield is the antenna according to W8JI and
worshippers. But you take the shield (W8JI antenna) away, now the wires are
antenna, some say don't need no stinkin' shield and "antenna" to work as an
antenna.
> The rest is just fodder for bored old farts on a newsgroup.
> Of course, many won't agree with me, and this would include Yuri,
> since he believes a shielded loop is quieter than an open loop.
> But I don't care.
Amazing how selective in reading and digestion of postings some people are.
They tend to ignore the reality and description of it, they pick on
selective "proof" of what they were taught and figered out.
I emphasize, that electrostatic shield on the loop antenna is effective on
close proximity radiation, within some fractions of a wavelength from the
source of interference/signal. It does not (significantly) affect band noise
or distant noise/signals. Anyone who can build shielded loop and test it
within local arcing source or test transmitter, can see the attenuation of
the said noise.
So shield works as a electrostatic shield, if you guys like it or not, or
refuse to admit.
It is not that I believe in that, I have experienced it, seen it, measured
it and it works, it is there and anoyne can verify that, contrary to
"theories" of those who "figured" it can't be.
Electrostatic shields work on principle of capacitance plate, being grounded
and side exposed to electrical/electrostatic fields shunting the field to
ground. Capacitor's one "plate" is the interference/signal source
(antenna) - other "plate" is the el. static loop shield, grounded, shunting
electrical fields to ground and preventing from entering the antenna.
(Something like that).
Sooo, antenna works without shield (not just my assertion), but when you
insert it in the shield then shield becomes W8JI antenna. So his shield,
untuned becomes antenna, but my tuned and tunable inside the shield antenna
is not the antenna? Makes as much sense as "there is equal current along the
loading coil doesn't matter what", riiiiight?
> It's a free country. Or I think it is... Sometimes I wonder these days
> with all these goofballs we have in DC running the show.
> MK
>
We were better off with Clintonistas having orgies in WH while Bin Ladin
turbanites were running around, blowing up Americans and using our flight
schools, our planes to demonstrate their "religion of peace" in NYC WTC
inferno?
Let's stick to some reality in antennas.
Yuri, K3BU
Richard Clark
<K3...@optonline.net> wrote:
>Electrostatic shields work on principle of capacitance plate, being grounded
>and side exposed to electrical/electrostatic fields
There's a very simple test of this "shield." It relates to experience
and doesn't need for you to go to the library.
1.) Tack a wire across the gap.
Q. Do you still have signal?
A. No!? None????
Extra Credit Question:
Did the wire make the "shield" better, or worse?
73's
Richard Clark, KB7QHC
p.s.
from your experience, the answer to the initial question above may
vary. If in fact it does, it may bring new material for discussion.
Yuri Blanarovich
"Mike Coslo" <mremovet...@adelphia.net> wrote in message
news:FJCdnXEsfsm...@adelphia.com...
Before you pronounce your verdict, why don't youze guyze build the shielded
loop antenna as I described and test it. Try version without shield, see
what IS antenna, and try the same antenna with shielded loop. Then run
electric drill or another source of arcing or interference in the vicinity
and see if there is shielding effect or not. Then pronounce your verdict and
pontificate on how electrostatic shields suppose to work. Otherwise you look
silly like W8JI cult worshippers.
73 Yuri, K3BU
Yuri Blanarovich
"Richard Clark" <kb7...@comcast.net> wrote in message
news:nggm62t1oj5bqickh...@4ax.com...
> On Wed, 17 May 2006 11:30:55 -0400, "Yuri Blanarovich"
> <K3...@optonline.net> wrote:
>
>>Electrostatic shields work on principle of capacitance plate, being
>>grounded
>>and side exposed to electrical/electrostatic fields
>
> There's a very simple test of this "shield." It relates to experience
> and doesn't need for you to go to the library.
>
> 1.) Tack a wire across the gap.
>
> Q. Do you still have signal?
>
> A. No!? None????
>
That makes it Faraday shield, which stops any signal from entering inside of
the tubing.
I never asserted that Faraday shield or closed metallic enclosure passes any
signals or fields.
We are talking about electrostatic shield, which if removed, antenna works
without change, you put it back, it still works the same way plus it rejects
in its reactive near field region electrical field interference.
If it was to be antenna, then when removed, the rest should stop working as
an antenna, or what is the theory?
> Extra Credit Question:
> Did the wire make the "shield" better, or worse?
>
It turned it to Farady shield and prevented signals from exciting the
antenna inside.
Extra Credit Question for professor:
Q1: If electrostatic shield is added to small loop antenna and it attenuates
the interference or signals from its vicinity, does it perform the function
of a shield or antenna?
Q2: Can the piece of tubing that is grounded by its outside surface, acts as
a capacitor's plate and provide the path to ground for electric field in
vicinity?
> 73's
> Richard Clark, KB7QHC
>
> p.s.
> from your experience, the answer to the initial question above may
> vary. If in fact it does, it may bring new material for discussion.
I just wish that points of discrepancy were addressed, rather than parties
taking off on tangents fitting their convinctions and trying to weasel out
of the wrong statements.
73 Yuri, K3BU
Richard Clark
<K3...@optonline.net> wrote:
>> Extra Credit Question:
>> Did the wire make the "shield" better, or worse?
>It turned it to Farady shield and prevented signals from exciting the
>antenna inside.
It's still the same "1/2 inch copper water tubing (non ferrous
material passing the magnetic field)."
So, does that wire make the "shield" better, or worse?
Super-extra credit question:
If we replaced the non ferrous material (same gap, no link) with (most
have probably anticipated this) a ferrous material, does this allow
near field region electrical field interference to pass un-impeded?
w8...@akorn.net
Yuri Blanarovich wrote:
> That makes it Faraday shield, which stops any signal from entering inside of
> the tubing.
> I never asserted that Faraday shield or closed metallic enclosure passes any
> signals or fields.
> We are talking about electrostatic shield, which if removed, antenna works
> without change, you put it back, it still works the same way plus it rejects
> in its reactive near field region electrical field interference.
> If it was to be antenna, then when removed, the rest should stop working as
> an antenna, or what is the theory?
Yuri,
A shield is a shield.
People made some very good posts explaining how the "shield" works, and
there was nothing wrong with my original explaination.
You stated the shield is an "electrostatic" shield and I pointed out a
static field does NOT cause noise. Static is by definition stationary
or non-varying.
The only reason the shield affects the noise, as I and others have
pointed out, is the shield changes the balance of the system. The
shield IS the actual portion of the antenna that receives the signal,
whether that signal is noise or an intentional desired signal.
The entire shield can be dispensed with without any change in the
system so long as the system remains in balance, and that is quite
possible to do.
As a mater of fact if a non-symmetrical "shield" is added over a
balanced system it will decrease balance and make the system more
susceptable to noise because the feedline will become part of the
actual antenna.
You might look for a copy of "Fundamentals of Electricity and
Magnetism" (McGraw-Hill). This entire book deals with basic field
behavior and entire chapters explain in detail what everyone is saying.
As Roy pointed out, I didn't make this stuff up. It has been in print
since the 1800's and the electric field effects first experimented with
around 600 BC (although it was the 1600's before serious experiments
were done).
There's nothing impossible about what you may have observed but the
reasoning you gave and statements about my explaination being in error
are wrong.
There is absolutely nothing that causes noise to electric field
dominant and the shield absolutely does not "filter" the time-varying
electric field from the time-varying magnetic field. The shield IS the
actual antenna and the stuff inside it, once inside it, is excited only
by the gap. Nothing at frequencies of interest passes through the
shield walls.This is a very well-known behavior and why so many
immediately disagreed with your description.
73 Tom
Richard Harrison
"Electrostatic shields work on principle of capacitance plate, being
Terman on page 1049 of his 1955 edition writes:
"Such a shield ensures that all parts of the loop will always have the
same capacitance to ground irrespective of the loop orientation in
relation to neighboring objects."
Yuri is consistent with Terman, and that is liable to be better than a
bible because it is provable and demands no faith. If wrong, it will be
rewritten with corrections.
Best regards, Richard Harrison, KB5WZI
Mike Coslo
Yuri, you is way too intense! I don't pontificate, and my silliness is
genetic, not involved in any worship of W8JI.
I very much expect that any effects that you see may be due to another
cause than what you attribute it to. I don't know if your antenna is not
completely shielded along it's entire circumference or not. I wonder if
you could put your antenna inside a Faraday cage and see different
results. Perhaps even try the unshielded antenna in the Faraday cage.
Unshielded antenna in cage should equal shielded loop in open. If it
doesn't, I'd look for a problem in the experiment first, not a problem
in the theory.
Mike Coslo
> "Richard Clark" <kb7...@comcast.net> wrote in message
> news:nggm62t1oj5bqickh...@4ax.com...
>
>>On Wed, 17 May 2006 11:30:55 -0400, "Yuri Blanarovich"
>><K3...@optonline.net> wrote:
>>
>>
>>>Electrostatic shields work on principle of capacitance plate, being
>>>grounded
>>>and side exposed to electrical/electrostatic fields
>>
>>There's a very simple test of this "shield." It relates to experience
>>and doesn't need for you to go to the library.
>>
>>1.) Tack a wire across the gap.
>>
>>Q. Do you still have signal?
>>
>>A. No!? None????
>>
>
> Agree!
> That makes it Faraday shield, which stops any signal from entering inside of
> the tubing.
Makes it a short!
> I never asserted that Faraday shield or closed metallic enclosure passes any
> signals or fields.
Aren't both conditions shields?. One just has a short.
> We are talking about electrostatic shield, which if removed, antenna works
> without change, you put it back, it still works the same way plus it rejects
> in its reactive near field region electrical field interference.
> If it was to be antenna, then when removed, the rest should stop working as
> an antenna, or what is the theory?
>
>
>>Extra Credit Question:
>>Did the wire make the "shield" better, or worse?
>>
>
> It turned it to Farady shield and prevented signals from exciting the
> antenna inside.
>
> Extra Credit Question for professor:
> Q1: If electrostatic shield is added to small loop antenna and it attenuates
> the interference or signals from its vicinity, does it perform the function
> of a shield or antenna?
> Q2: Can the piece of tubing that is grounded by its outside surface, acts as
> a capacitor's plate and provide the path to ground for electric field in
> vicinity?
>
>
>>73's
>>Richard Clark, KB7QHC
>>
>>p.s.
>>from your experience, the answer to the initial question above may
>>vary. If in fact it does, it may bring new material for discussion.
>
>
> I just wish that points of discrepancy were addressed, rather than parties
> taking off on tangents fitting their convinctions and trying to weasel out
> of the wrong statements.
Give it a few more posts, Yuri, and it will turn into standing waves in
coils again!! ;^)
Richard Clark
Harrison) wrote:
>Yuri is consistent with Terman, and that is liable to be better than a
>bible because it is provable and demands no faith. If wrong, it will be
>rewritten with corrections.
Hi Richard,
It is one thing to run a Xerox, it is another to apply it to one
particular howler that has Yuri stumped:
Super-extra credit question:
If we replaced the non ferrous material (same gap, no link) with (most
have probably anticipated this) a ferrous material, does this allow
near field region electrical field interference to pass un-impeded?
w8...@akorn.net
Richard Harrison wrote:
> Yuri is consistent with Terman, and that is liable to be better than a
> bible because it is provable and demands no faith. If wrong, it will be
> rewritten with corrections.
If you read what has been said here very carefully you will find Yuri
claims the shield "blocks electric fields" or stops "electrostatic
fields". This is the effect of a Faraday cage or shield.
I did not claim that effect. Terman certainly did not. It is a folklore
or Ham-myth that only appears in amateur circles.
What others (including Terman exactly as you quoted) are trying to
tell Yuri is the shield ONLY affects balance. The shield IS the actual
antenna element that does the radiating. That is written in a half
dozen engineering good engineering references. That is how ANY shield
behaves.
Read here:
http://en.wikipedia.org/wiki/Faraday_cage
or here:
http://members.aol.com/omlcgm/detecknowledgy/faraday.htm
or any of dozens of other places. Unless an off-the-wall hobbyist
publication or rouge opinion, you will see everyone agrees.
What Terman said is absolutely correct. The "shield" (when properly
constructed) balances the capacitance of the antenna to earth. It does
not "stop" the electric field. It certainly does not filter a
time-varying electric field because doing so would by definition of
Maxwell's equations (which everyone who isn't a CFA or EH antenna quack
agrees are true) also stop the time varying magnetic field.
As everyone (including Terman) has tried to explain, the shield only
affects balance. The shield HAS to be the actual antenna element
because by definition of ALL the peer-reviewed textbooks published to
date as well as any description of coaxial cables the inner shield wall
is isolated from the outside by the skin depth of that wall.
This is so very simple to prove, it only takes a moment. It doesn't
even take exotic test gear. These experiments were done in the 18th
century with very crude instruments.
You can take a solid copper sheet for example and place a small loop
antenna near that "wall". If you probe current on the wall near the
loop on the loop side, you will find a current maximum right under that
loop. VERY easy to see. 100% repeatable.
Now if you move the probe to the other side of the wall you will find
current MINIMUM at the sheet center and increasing towards two of the
edges.
This is a TOTALLY open wall with no seal, it isn't even a box.
Shields a few skin depths thick are a virtually perfect barrier to both
magnetic and electric fields. This is true for densely woven coaxial
cable shield or even thin aluminum shields, metallic sheets, or any
good conductive wall.
Saying Terman supports anything to the contrary only proves someone is
misquoting or misunderstanding plain English, since Terman is a very
clear writer. There is no way Terman failed basic physics and his peer
reviewed textbooks are wrong.
Yuri may need to read some basic textbooks, I'd be happy to copy the
applicable pages if there isn't a good library nearby. It is essential
to get the basics down solid.
73 Tom
Richard Harrison
"There is absolutely nothing that causes (sic) noise to electric field
dominant and the shield absolutely does not "filter" the time-varying
A "Faraday shield" is designed to allow magnetic field coupling while
disallowing electric coupling. See page 38 of Terman`s 1955 edition:
"It is possible to shield slectrostatic flux without simultaneously
affecting the magnetic field by surrounding the free space to be
shielded with a conducting cage that is made in such a way as to provide
no low-resistance path for the flow of eddy currents, while at the same
time offering a metallic terminal upon which electrostatic flux lines
can terminate."
I`ve previously described the Faraday picket fences or Faraday screens
used in the medium wave broadcast stations where I worked that were used
to avoid capacitive coupling to the antennas while permitting magnetic
coupling. Capacitive coupling would favor harmonics of the operating
frequency. These are undesirable. The Faraday screen effectively rejects
the capacitive coupling. It shorts the lightning strikes to ground too.
In a Faraday screen one end of pickets or wires is grounded. Their other
ends are open-circuited. So, circulating current can`t flow through the
wires. Thus, no counter-field can be generated to oppose magnetic
coupling but capacitive flux lines land on the wires and are shorted to
ground. It all works very well.
Look at Terman`s shielded loop on page 1048 of his 1955 editiomn.
There`s a gap in the shield opposite the feedpoint. The gap prevents
current circulation in the loop shield thereby making it permeable to
magnetic coupling while shorting the electric field to ground.
Therefore, this loop cover is a Faraday screen.
Why should we care if noise comes from near or far? The near field has 3
components. See "TV and Other Receiving Antennas" by Arnold B. Bailey.
The first near field component is produced by the electric vector and
decays by the cube of the distance. The second is the induction field
and decays as the square of the distance. The third is the radiation
field electric vector which becomes the volts per meter at a great
distance. This decays inversely with distance and its power decays as
the square of the distance. 6 dB every time the distance doubles.
Point is we don`t have to get very far from a noise source to make a big
improvement in noise received, especially if we avoid electric field
coupling which decays especially fast in the near field.
Yuri Blanarovich
"Richard Harrison" <richard...@webtv.net> wrote in message
news:4751-446...@storefull-3255.bay.webtv.net...
Richard,
thanks for the reference and support, coming from one who had his hands
"dirtied" with the antennas.
Terman also says just a sentence before:
"Errors from unbalance can be minimized by using circuit arrangements
that are symmetrical with respect to ground, such as shown in Fig. 26-27b.
It is also helpful to enclose the loop in an electrostatic shield, such as
metal housing broken by an insulated bushing, as show schematically in Fig.
26-27c."
and then sentence quoted above.
Clearly, the shield is functioning as an electrostatic shield, providing
symmetry and is not acting as "W8JI Antenna". Loops are the antenna, shield
is the SHIELD, contrary to W8JI proselytizing.
Small loops are the antennas, with or without the shield. Electrostatic
shield is a shield, provides symmetry for the antenna and helps to reject,
shunt the interference from the sources in the proximity of the antenna by
its virtue of the capacitance to the ground.
Terman didn't say: "yo stupid, you don need no stinkin' loops, jus' use the
shield as antenna" :-)))
73 Yuri Blanarovich, K3BU
Yuri Blanarovich
> Yuri Blanarovich wrote:
>> "Mike Coslo" <mremovet...@adelphia.net> wrote in message
>> news:FJCdnXEsfsm...@adelphia.com...
>>
>>>Roy Lewallen wrote:
>>>
>>>
>>>>If you'll read what Tom has posted, or a description in any good text,
>>>>you'll find that the whole circumference of a "shielded" loop radiates.
>>>>The field comes from current on the outside of the "shield", not from
>>>>some field penetrating the shield. That's my theory. It's the same as
>>>>Tom's, and that of every respected author I've read.
>>>
>>>Game, Set, and Match, Roy. The explanation and the everyday application
>>>of the concept of non-ferrous shielding are both simple and elegant.
>>>
>>>Seems like the thread stopper to me! I suspect it will continue
>>>anyhow.... 8^)
>>>
>>>- 73 de Mike KB3EIA -
>>
>>
>> Before you pronounce your verdict, why don't youze guyze build the
>> shielded loop antenna as I described and test it. Try version without
>> shield, see what IS antenna, and try the same antenna with shielded loop.
>> Then run electric drill or another source of arcing or interference in
>> the vicinity and see if there is shielding effect or not. Then pronounce
>> your verdict and pontificate on how electrostatic shields suppose to
>> work. Otherwise you look silly like W8JI cult worshippers.
>
> Yuri, you is way too intense! I don't pontificate, and my silliness is
> genetic, not involved in any worship of W8JI.
>
Sorry! I didn't mean you specifically, jus' generally those who worship W8JI
gospels.
> I very much expect that any effects that you see may be due to another
> cause than what you attribute it to. I don't know if your antenna is not
> completely shielded along it's entire circumference or not. I wonder if
> you could put your antenna inside a Faraday cage and see different
> results. Perhaps even try the unshielded antenna in the Faraday cage.
Of course it will not work, Faraday cage - shield, shields all RF.
> Unshielded antenna in cage should equal shielded loop in open. If it
> doesn't, I'd look for a problem in the experiment first, not a problem in
> the theory.
It is electrostatic shield, not "shielded, closed" loop shield. Antenna will
still work the same inside the cage, just will not receive any signals if
they are not passed through the cage. I am not overthrowing legitimate
theories, I am describing what I observed and objecting to call the shield
an antenna, when it isn't!!!
> - 73 de Mike KB3EIA -
I have no problem with theories, I have problem with silly claims that
shield is an antenna. I described my experiments, explained behavior and
performance of the shielded loop in the near field interfering
signals/noise.
Build it, if you have problem with local noise, you would see the benefit of
the electrostatic shield on the suppression of it and on symmetry and deep
nulls on other signals. Shield is a shield and not antenna. Rest of
mumbo-jumbo is twist away from the subject and attempt to legitimize
wrongoooo!
73 Yuri Blanarovich, K3BU
Richard Harrison
"I did not claim that effect. Terman certainly did not. (Yuri claims the
shield "blocks electric fields" or stops "electrostatic fields".)"
I`ll requote Terman from page 38 of his 1955 edition which Tom ignored:
"It is possible to shield electrostatic flux without simultaneously
affecting the magnetic field by surrounding the space to be shielded wih
a conducting cage that is made in such a way as to provide no
low-resistance path for the flow of eddy currents while at the same time
offering a metallic terminal upon which electrostatic flux lines can
terminate."
That is a description of the shield on Terman`s direction finding loop.
The loop has a gap in the shield opposite its feedpoint. The gap
prevents current from circulating around the loop shield and thus
prevents creation of an opposing magnetic field by the shield to the
incident field acting on the loop.
The grounded shield nevertheless terminates electric flux shorting it to
ground.
The loop shield is thus a true Faraday screen, not a Faraday car body or
screened room.
w8...@akorn.net
Richard Harrison wrote:
> Tom, W8JI wrote:
> "I did not claim that effect. Terman certainly did not. (Yuri claims the
> shield "blocks electric fields" or stops "electrostatic fields".)"
>
> I`ll requote Terman from page 38 of his 1955 edition which Tom ignored:
> "It is possible to shield electrostatic flux without simultaneously
> affecting the magnetic field by surrounding the space to be shielded wih
> a conducting cage that is made in such a way as to provide no
> low-resistance path for the flow of eddy currents while at the same time
> offering a metallic terminal upon which electrostatic flux lines can
> terminate."
Richard.
I know anything Roy Lewallen agrees with, you disagree with. I know
anything I say (or even what I don't say) sets Yuri off into a foaming
lather. I really wish you guys could put personal hate or dislike aside
and look at facts. This is an imporant issue because the myth about
shields is imbedded in amateur circles despite many clearly written
engineering texts and very simple experiments that prove the concept of
time-varying magnetic fields penetating the shield. It's just a fact
when the time-varying electric field is taken to zero so is the
time-varying magnetic field.
Static by definition is not moving or varying. Don't confuse jargon
describing a different coupling mode with the mechanics of a loop
operating at radio frequencies.
When we receive noise or signals, the fields are time-varying. Just as
with a piece of coaxial cable, the inner wall of a "shielded loop" is
isolated by the skin depth of the conductor from the outside wall. The
electric and magnetic coupling effects are what causes a coaxial cable
with a dense shield more than a few skin depths thick to ALWAYS have
the same current on the inside of the shield as the inner conductor
has, and all radiation or common mode current flow over the outside.
This isn't something I invented. It has been in nearly every textbook
long before I was born.
I'm pleased that Yuri credits me for the work, but unfortunately I had
little to do with it. It really was people from the 1700's and 1800's
that did all the work.
You (and Yuri) appear to be confusing how time-varying fields work.
I suggest you put Terman aside and actually read some textbooks on
fields.
It's helpful to actually make a few measurements. A few minutes spent
with some very simple test equipment would go a long way to "turning on
the light".
> The loop shield is thus a true Faraday screen, not a Faraday car body or
> screened room.
If you say so. And as one, it also must block any time-varying magnetic
field. As K7ITM points out it is the gap in the loop that is actually
the feedpoint, and it is the outside of the loop that is the actual
antenna.
If you do not think a loop behaves this way, you need to get busy doing
some real important work. You need to get all the Handbooks to quit
talking about common mode currents on shield outsides. You need to get
them to quit treating the inside of the shield as a isolated conductor
that is independent of the outside.
As I and others have suggested it only takes a moment to prove the
books are correct. You can prove it with a single sheet of copper and a
minimum of test equipment.
73 Tom
Yuri Blanarovich
<w8...@akorn.net> wrote in message
news:1147910035....@38g2000cwa.googlegroups.com...
>
> Richard Harrison wrote:
>> Tom, W8JI wrote:
>> "I did not claim that effect. Terman certainly did not. (Yuri claims the
>> shield "blocks electric fields" or stops "electrostatic fields".)"
>>
>> I`ll requote Terman from page 38 of his 1955 edition which Tom ignored:
>> "It is possible to shield electrostatic flux without simultaneously
>> affecting the magnetic field by surrounding the space to be shielded wih
>> a conducting cage that is made in such a way as to provide no
>> low-resistance path for the flow of eddy currents while at the same time
>> offering a metallic terminal upon which electrostatic flux lines can
>> terminate."
>
> Richard.
>
> I know anything Roy Lewallen agrees with, you disagree with. I know
> anything I say (or even what I don't say) sets Yuri off into a foaming
> lather. I really wish you guys could put personal hate or dislike aside
> and look at facts. This is an imporant issue because the myth about
> shields is imbedded in amateur circles despite many clearly written
> engineering texts and very simple experiments that prove the concept of
> time-varying magnetic fields penetating the shield. It's just a fact
> when the time-varying electric field is taken to zero so is the
> time-varying magnetic field.
>
That really nails it! His "technical" response!
Perfect picture of a jerk parading as an engineer!
Yep, I hate your guts and I made up phony claims on your web site for all to
see, so I can "hate you"! Brilliant! Keep it up!
Halleluja, now we know that shields are antennas, praise the guru!
Bada BUm
Cecil Moore
> I know anything Roy Lewallen agrees with, you disagree with.
Absolutely false. I'll bet they agree on 99% of technical
topics, e.g. ohm's law, Maxwell's equations, etc.
Richard Clark
wrote:
>It's still the same "1/2 inch copper water tubing (non ferrous
>material passing the magnetic field)."
>
>So, does that wire make the "shield" better, or worse?
Hmm, this one must've been experienced exactly as an existential
question about the infinite cosmos.
>Super-extra credit question:
>If we replaced the non ferrous material (same gap, no link) with (most
>have probably anticipated this) a ferrous material, does this allow
>near field region electrical field interference to pass un-impeded?
This one must never been experienced either. I've always wondered why
perfect academic set-ups like "non ferrous material" (as if it were
lossless) always appear in the context of a populist aw-shucks kind of
posting.
Sorry All,
But when such simple questions become imponderables of the century,
they merit Cecil's 5 forbidden words woven in. Of course, it makes
only the most strained of sense, but there's nothing to compete! ;-)
73's
Richard Clark, KB7QHC
p.s. as viewed through the bottom of a bottle of Dick's Working Man's
Brown Ale (Centralia, Washington)
Richard Harrison
"This is an important issue because the myth about shields is embedded
in amateur circles despite many clearly written engineering texts and
very simple experiments that prove the concept of time-varying magnetic
Some of that poison reached the 2006 ARRL Handbook on page 13.18. Fig
13.26 says:
"Electrostatically-shielded loop for RDF. To prevent shielding of the
loop from magnetic fields, leave the shield unconnected at one end."
Terman`s RDF loop should have better balance than ARRL`s because
Terman`s shield gap is squarely in the center of the loop
and not at one end. However, as long as the shield is broken preventing
induced current from flowing around the shield, Lenz`s law will be
thwarted and magnetic coupling to the coil under the shield will be
obtained. Electric field coupling to the coil beneath the shield will be
disallowed by the shield`s connection to ground wherever it occurs,
though not as elegantly as when care is taken to get the best balance
possible.
I`ve worked with such Faraday screens in my broadcasting career.
nm...@wt.net
>worshippers. But you take the shield (W8JI antenna) away, now the wires are
>antenna, some say don't need no stinkin' shield and "antenna" to work as an
>antenna.
I don't know what a W8JI antenna is, except for those I've heard on
160m... :/
But I do know that I've tested various versions of both shielded and
unshielded
loops, and have never been able to tell a lick of difference as far as
close local
noise pickup. I spent a whole week testing that very thing. It's not
something I just
made up, or picked up from W8JI.
>Amazing how selective in reading and digestion of postings some people are.
>They tend to ignore the reality and description of it, they pick on
>selective "proof" of what they were taught and figered out.
Only my test results were used to come up with my conclusion. So I
guess
I taught myself. I've never built a shielded loop yet that was any
"quieter" to
local noise than any of my good unshielded loops. But my unshielded
loops
are well balanced. Were yours?
> So shield works as a electrostatic shield, if you guys like it or not, or
>refuse to admit.
I refuse to admit it, if I can't prove it. And I haven't been able to
prove it yet.
One thing...How in the heck is a solid shield going to filter one
source of RF,
and ignore another. In reality, it will shield *all* RF, unless I am
missing
something here. So the outer shield *must* be the antenna, unless the
sky is
now green. No RF is going to prevail past the outer skin depth of the
solid
shield. None. Nada...
>Sooo, antenna works without shield (not just my assertion), but when you
>insert it in the shield then shield becomes W8JI antenna.
It does? I'm sure if this is the case, it probably tunes 160m.... :/
So his shield,
>untuned becomes antenna, but my tuned and tunable inside the shield antenna
>is not the antenna? Makes as much sense as "there is equal current along the
>loading coil doesn't matter what", riiiiight?
If you say so....
>Let's stick to some reality in antennas.
Thats all I do. I've made a load of loops. I have a diamond loop 44
inches
per side right next to me. Almost is as tall as the ceiling... Heck, I
even
have tried using shielded loops as the coupling loop to unshielded
loops.
Works pretty well to maintain balance, but mine work just as well with
just
a simple unshielded coupling loop. Probably cuz my loops are very
symmetrical
and balanced naturally. The coax feedline itself is the only real issue
in my case,
and even it's not really very critical. I never saw any indication
that using a
shielded coupling loop made the loop quieter than not using one. Not
once.
Myself, I don't really like small loops for receiving on 160m. They are
good for
cutting the noise when working loud locals, but in my experience they
are
pretty ho-hum when receiving weak dx. For 160m, I would use the biggest
loop
I could manage. Probably outside to have enough room...
My loops are mainly for MW BC receiving, although the one next to me
tunes
500-2300 kc in two stages, by switching cap gangs. I can go LW if I
tack on
more fixed caps. The real value of small loops are not the "quiet", or
the s/n
or whatever. It's the nulls... But nulls have much more value in the BC
band,
than they do on 160m unless maybe you have a noise source in the area
you wanna null out. Thats how a loop reduces noise. Using the nulls...
:/
I do have to agree with Tom. I think the "shielded loop" theory many
hams
adhere to is just another batch of wive tailery.. Along with grounds to
cure
antenna/feedline problems, sticking coax ends in bottles hoping to
thwart
lightning, etc... And I've never once talked to Tom about small loops.
It's all my idea to shun this "shield=quiet" theory, not W8JI's.
MK
Mike Coslo
> w8...@akorn.net wrote:
>
>> I know anything Roy Lewallen agrees with, you disagree with.
>
>
> Absolutely false. I'll bet they agree on 99% of technical
> topics, e.g. ohm's law, Maxwell's equations, etc.
Just like arguing whether Coke or Pepsi tastes best. The closer the
product, the worse the arguing...
Richard Harrison
"I refuse to admit it, if I can`t prove it."
A shield is extra work, weight, and cost but despite that, many are in
use.
As electrons move along a conductor a magnetic field expands from some
depth inside the conductor itself. The magnetic lines of force sweep
outward from the conductor while inducing an emf in the conductor
itself. The self induced emf opposes instantaneous change of current in
the inductance of the conductor. This is the basis of Lenz`s law:
"In all cases of electromagnetic induction, induced electromotive force
and resultant current are in such a direction as to oppose the effect
producing them."
Skin effect prevents penetration of RF very deep into a good conductor.
Skin effect makes RF coil shields impenetrable. Electric hields are
shorted to ground by the conductive shield. Magnetic fields induce
counter fields from the currents they induce on the surface of the
shield.
A Faraday screen breaks the current path on the shield preventing the
counter fields from being magneticly induced. Result is a shield that is
penetrable by the magnetic field but impenetrable by the electric field.
The electric field is still shorted to ground by its conductive path.
Faraday screens are used because they work.
Richard Clark
Harrison) wrote:
>A Faraday screen breaks the current path on the shield preventing the
>counter fields from being magneticly induced. Result is a shield that is
>penetrable by the magnetic field but impenetrable by the electric field.
>The electric field is still shorted to ground by its conductive path.
>Faraday screens are used because they work.
Hi Richard, Yuri,
In regard to my last question that so stumped you two, there is
absolutely nothing inherent in non-ferrous vs. ferrous materials that
changes this one particular aspect of shielding. Both materials are
conductive, and both are selected for their lowest conductivity (hence
their lowest Ohmic loss). The only substantive difference is that a
ferrous material offers the prospect of using a thinner covering for
the same isolation. Above LF, this is hardly useful unless you are
planning on using very thin foils. Art's selection of mylar films
with conductive coatings is one such example that works with a
conductive surface thickness in the 100s of microns.
The "shielded dipole" observes the one principle requirement of
insuring that a break in continuity is maintained. Otherwise, the
shorted turn snuffs the antenna for any design held within it. This
prohibition in continuity is paramount to all designs and is driven by
both magnetic as well as electric considerations, as in the RF field
they are inextricably coupled.
To cut to the chase: there is no way to separate these fields and
select one of them over the other. If you choose to run your arc
welder within a loop's diameter of the antenna while also DXing; then
it is the balance of the antenna design, not the shielding that will
determine your success. Screw up the geometry of that gap, and you
will hear as much hash as if the "shield" never existed.
For the standard single turn "shielded dipole," the arms of the dipole
are the shield. There must be 10 million examples of this particular
model on 1 million repeater installations world-wide. There are also
tri-axial or twin-axial designs of the "shielded dipole" that wholly
divorce themselves of the exterior shield. All such designs, to work
effectively, exhibit a balanced configuration that is identical to the
standard single turn. The balance is the only consideration at issue,
and shielding is a means, but hardly a necessary ends to that
achievement. As such, shielding is simply insurance and a brute force
means to force balance through what in engineering terms is called
"swamping."
Michael Tope
> As electrons move along a conductor a magnetic field expands from some
> depth inside the conductor itself. The magnetic lines of force sweep
> outward from the conductor while inducing an emf in the conductor
> itself. The self induced emf opposes instantaneous change of current in
> the inductance of the conductor. This is the basis of Lenz`s law:
> "In all cases of electromagnetic induction, induced electromotive force
> and resultant current are in such a direction as to oppose the effect
> producing them."
>
> Skin effect prevents penetration of RF very deep into a good conductor.
> Skin effect makes RF coil shields impenetrable. Electric hields are
> shorted to ground by the conductive shield. Magnetic fields induce
> counter fields from the currents they induce on the surface of the
> shield.
>
> A Faraday screen breaks the current path on the shield preventing the
> counter fields from being magneticly induced. Result is a shield that is
> penetrable by the magnetic field but impenetrable by the electric field.
> The electric field is still shorted to ground by its conductive path.
> Faraday screens are used because they work.
>
> Best regards, Richard Harrison, KB5WZI
>
I would agree, Richard, but at HF frequencies the current path
around the shield isn't really broken by the gap. Due to the skin
effect, the RF current flowing on the inside of the loop shield is free
to flow around the edge of the shield conductor and onto the outside
of the shield at the gap. At very low frequencies, where the skin
depth is large, this wouldn't necessarily be true, but at HF as long
as there are a few skin depths between the outside and the inside
surface of the conductor, then the inside surface of the shield and
the outside surface of the shield can be treated as independent
conductors.
73, Mike W4EF........................
Richard Harrison
"I would agree, Richard, but at HF frequencies, the current path around
the shield isn`t real;ly broken by the gap."
To best describe what broken means, a picture helps. There is a picture
on page 13.18 of the 2006 ARRL Handbook. Fig 13.26 has a legend which
says:
"To prevent shielding of the loop from magnetic fields, leave the shield
unconnected at one end."
I think the handbook has it right.
Michael Tope
"Richard Harrison" <richard...@webtv.net> wrote in message
news:21360-446...@storefull-3257.bay.webtv.net...
> "I would agree, Richard, but at HF frequencies, the current path around
> the shield isn`t real;ly broken by the gap."
>
> To best describe what broken means, a picture helps. There is a picture
> on page 13.18 of the 2006 ARRL Handbook. Fig 13.26 has a legend which
> says:
> "To prevent shielding of the loop from magnetic fields, leave the shield
> unconnected at one end."
>
I am a bit behind on ARRL Handbooks, Richard, but from
what you describe, this is the same figure that appears in my
1992 edition (chapter 38, figure 2). In any case, what is
shown in the figure agrees with my understanding of "broken",
although admittedly when I made my previous post, I was
thinking of the case where the shield is broken on the side of
the loop opposite the feedpoint. For the purposes of this
discussion, however, it doesn't matter whether the break is
at the top (opposite the feed) or at the bottom (adjacent
to the feed). In either case, current induced on the inside
of the shield by current flowing on the center conductor loop
has a continuous back to ground via the outside surface of the
shield. IOW, the gap doesn't suppress the eddy current, rather
it forces it to flow on the outside surface of the shield, thereby
causing the loop to radiate.
>
> I think the handbook has it right.
>
Yes, I agree it does. If you connect the shield at both ends, the
loop can't radiate because the eddy current caused by current
flowing on the inner conductor loop will confined to the inside of
the shield. Likewise, eddy currents induced on the outside of the
shield by EM waves passing the antenna will be confined to the
outside of the shield if there is no gap (reciprocity holds - the
antenna won't receive with no gap).
73, Mike W4EF.................................................
w8...@akorn.net
Michael Tope wrote:
>> I am a bit behind on ARRL Handbooks, Richard, but from
> what you describe, this is the same figure that appears in my
> 1992 edition (chapter 38, figure 2). In any case, what is
> shown in the figure agrees with my understanding of "broken",
> although admittedly when I made my previous post, I was
> thinking of the case where the shield is broken on the side of
> the loop opposite the feedpoint. For the purposes of this
> discussion, however, it doesn't matter whether the break is
> at the top (opposite the feed) or at the bottom (adjacent
> to the feed). In either case, current induced on the inside
> of the shield by current flowing on the center conductor loop
> has a continuous back to ground via the outside surface of the
> shield. IOW, the gap doesn't suppress the eddy current, rather
> it forces it to flow on the outside surface of the shield, thereby
> causing the loop to radiate.
Absolutely nothing, neither electic nor magnetic, couplesthrough the
wall of a conductor more than several skin depths thick. This isn't
anything that can be debated, it is simply how it works. It is very
easy to demonstrate, it takes only a few minutes and a minimum of test
equipment.
It is something very basic in physics and underlies how coaxial cables
and things with shields of all types work.
The gap is the feedpoint no matter where the gap is placed. The
radiation and coupling of any time-varying field, magnetic or electric,
occurs on a frequency where the shield is more than a few skin depths
thick comes by the gap.
This is such a very basic thing it is important everyone understand it.
> > I think the handbook has it right.
> >
>
> Yes, I agree it does. If you connect the shield at both ends, the
> loop can't radiate because the eddy current caused by current
> flowing on the inner conductor loop will confined to the inside of
> the shield.
Absolutely. When the gap is closed there is no potential difference
across the gap the outside of the shield is not connected to the inside
of the shield via the potential developed across the gap. The outer
wall is not coupled to the inner wall, the feedpoint is shorted.
When the gap is opened, the outside of the shield IS the antenna. Not
the inside or anything inside the inside.
> Likewise, eddy currents induced on the outside of the
> shield by EM waves passing the antenna will be confined to the
> outside of the shield if there is no gap (reciprocity holds - the
> antenna won't receive with no gap).
Again true. This is a very basic thing we must understand if we are to
understand how shields, walls, or conductors of any kind or form work
with HF currents, voltages, or fields of any type.
There isn't any way to change this effect.
73 Tom
Michael Tope
<w8...@akorn.net> wrote in message
news:1148204558....@j55g2000cwa.googlegroups.com...
>
> Michael Tope wrote:
>>> I am a bit behind on ARRL Handbooks, Richard, but from
>> what you describe, this is the same figure that appears in my
>> 1992 edition (chapter 38, figure 2). In any case, what is
>> shown in the figure agrees with my understanding of "broken",
>> although admittedly when I made my previous post, I was
>> thinking of the case where the shield is broken on the side of
>> the loop opposite the feedpoint. For the purposes of this
>> discussion, however, it doesn't matter whether the break is
>> at the top (opposite the feed) or at the bottom (adjacent
>> to the feed). In either case, current induced on the inside
>> of the shield by current flowing on the center conductor loop
>> has a continuous back to ground via the outside surface of the
>> shield. IOW, the gap doesn't suppress the eddy current, rather
>> it forces it to flow on the outside surface of the shield, thereby
>> causing the loop to radiate.
>
> Absolutely nothing, neither electic nor magnetic, couplesthrough the
> wall of a conductor more than several skin depths thick. This isn't
> anything that can be debated, it is simply how it works. It is very
> easy to demonstrate, it takes only a few minutes and a minimum of test
> equipment.
>
I don't think we disagree on that point, Tom. Perhaps I should
have chosen my words more carefully. I didn't mean to imply
that gap somehow forces the current on the inside of the shield
to pass through shield. When I said that the gap forces the current
to flow on the outside surface of the shield, I meant that in the
sense that the eddy current flows on the inside of the shield until
it reaches the break in the shield at which point the current flow
wraps around the edge of the shield and onto the outside surface
(thereby reversing direction relative to the direction of the eddy
current on the inside of the shield). The skin effect in effect
separates the shield into two distinct conductors, the inner surface
being one conductor and the outer surface of the shield being the
other. The gap is the circuit node where these two independent
conductors are connected. The eddy current flows out of one
conductor (the inner surface of the shield ) and into the other
conductor (the outer surface of the shield).
73, Mike W4EF.........................................................
Cecil Moore
> Absolutely nothing, neither electic nor magnetic, couplesthrough the
> wall of a conductor more than several skin depths thick.
Do 60 Hz magnetic fields penetrate RF coax since
the conductor is not more than several skin depths
thick?
Richard Harrison
"Absolutely nothing, neither electric nor magnetic, couples through the
wall of a conductor several skin depths thick."
That`s wrong for a "Faraday screen".
Terman is right. At the bottom of page 38 of his 1955 edition he writes:
"It is possible to shield electrostatic flux without simultaneously
affecting the magnetic field by surrounding the space to be shielded
with a conducting cage that is made in such a way as to provide no
low-resistance path for the flow of eddy currents, while at the same
time offering a metallic terminal upon which electrostatic flux lines
can terminate."
An example exists in the AM broadcast stations I`ve worked in. Every
tower was coupled to its transmission line through a 1:1 air-core
traansformer. Two identical single-layer solenoids sharing the same
axis. Between the coils was a metal picket fence. One end of the pickets
was firmly grounded to the coupling cabinet. The other end of all
pickets was an open circuit. Electric lines of force were intercepted by
the pickets and directly shorted to ground. However, the fences had no
effect on the magnetic coupling between them because the open circuit at
the ends of the pickets prevented circulating currents which would have
opposed magnetic coupling according to Lenz`s law.
Voila! Magnetic coupling but no electrostatic coupling between coils of
a transformer.
It`s time for W8JI to turn-off his misinformation machine.
Yuri Blanarovich
"this can't be" because "gurus" know otherwise.
Why do you hate Tom? You don't like anything he says on his "myth
overturning" web pages. He describes in a such detail and explains that
"shield is an antenna" - why don't you get it? :-))))
According to Tom, RF gets induced on the outside "wire" of the shield, then
it crolls to the "inside" wire of the shield around the edge of the tubing
and sees another wire and jumps over, and then to coax.
If tubing or shield was the antenna, then it would receive DX and near field
signals the same way. The fact that shield is shielding the near field
signals should make any guru wonder.
There was ZS1 on TopBand reflector reporting that he used shielded loop and
other loop antennas, and shielded loop was the only one that suppressed the
local TV birdies. Tom "explained" to him "how things work" and he apologized
that he did not mean to have this as an example of what I was saying.
There are other examples where shield "doesn't shield" - like link coupling
made of coax with end shield open and center conductor soldered to the
shield. As I mentioned I have magnetothermia machine that produces about
200W from single shielded loop, according to Tom, it should be frying the
coax in the gap, with all that RF power trying to make the corner :-)
There is more nonsense on his web site.
73 Yuri, K3BU
"Richard Harrison" <richard...@webtv.net> wrote in message
news:21360-447...@storefull-3257.bay.webtv.net...
Gene Fuller
Think again about what you wrote. The "Faraday screen" is full of
openings between the wires of the picket fence. There is no evidence
that anything magnetic or electric penetrates the walls of the
conductors beyond a very shallow layer.
Terman certainly did not deny the existence of skin effect that keeps
the fields out of the interior of conductors.
73,
Gene
W4SZ
Michael Tope
"Yuri Blanarovich" <K3...@optonline.net> wrote in message
news:Xh7cg.1170$w05...@fe10.lga...
> There are other examples where shield "doesn't shield" - like link
> coupling made of coax with end shield open and center conductor soldered
> to the shield. As I mentioned I have magnetothermia machine that produces
> about 200W from single shielded loop, according to Tom, it should be
> frying the coax in the gap, with all that RF power trying to make the
> corner :-)
>
Yuri, think about how the "link coupling" magnetic loop you describe
above works. When the loop is energized where does the RF current
leaving the center conductor go? It has to flow onto the outside of the
shield. Where else could it go?
RF current "makes the corner" around to the outside surface of the
shield in coax all the time. If it didn't we wouldn't need choke
balun's.
73, Mike
W4EF.................................................................
w8...@akorn.net
> Richard,
>
> Think again about what you wrote. The "Faraday screen" is full of
> openings between the wires of the picket fence. There is no evidence
> that anything magnetic or electric penetrates the walls of the
> conductors beyond a very shallow layer.
>
> Terman certainly did not deny the existence of skin effect that keeps
> the fields out of the interior of conductors.
Gene,
You might have to find a book that quotes the description of a screen
with parallel wires and large air gaps as compared to a wall or
cylinder several skin depths thick.
:-)
73 Tom
Yuri Blanarovich
"Michael Tope" <W4...@comcast.net> wrote in message
news:tL2dnYM6ZZ55lezZ...@comcast.com...
We need RF chokes and baluns to supress curents induced on the shield from
the unbalance at the antenna feedpoint.
Sooo, according to W8JI "teachings", RF current gets induced onto the
outside surface of tubing, then crolls around the edges and goes inside the
tubing?
Sooo, we should cork the elements, or the current will get confused inside
of dark tubing elements, Eh?
Any formulas to calculate the resonance of such "antenna"??
> 73, Mike
> W4EF.................................................................
--
Yuri Blanarovich, K3BU, VE3BMV
Richard Harrison
"Terman certainly did not deny the existence of skin effect that keeps
true. The point is, shielding from magnetic fields is different from
electric fields. On page 35 of his 1955 edition, Terman writes:
"Magnetic flux in attempting to pass through a shield (copper or
aluminum) induces voltage in the shield which gives rise to eddy
currents. These eddy currents oppose the action of the flux, and in
large measure prevent its penetration through the shield."
On page 38, Terman writes:
"Electrostatic shielding is obtained by enclosing free space to be
shielded by a conducting surface."
On page 45, is problem 2-45 which contains an illustration of a grid of
open-circuit wires which "will provide electrostatic shielding without
magnetic shielding---." This works just like the picket fences used in
broadcast stations to inhibit harmonic transmission.
Terman did not make this stuff up. It was already in wide use at the
time.
Gary Schafer
Yuri,
It is true that current will not flow on the inside of a tube from
current on the outside. The "waveguide beyond cutoff" effect keeps it
from doing so. The currents quickly cancel a short distance inside the
tube.
However, if you put a conductor inside that tube (wire) now it acts
like a coax cable and the energy on the center conductor couples to
the inside wall of the tube. At the end of the tube the current is
free to wrap around to the outside.
73
Gary K4FMX
Michael Tope
>> RF current "makes the corner" around to the outside surface of the
>> shield in coax all the time. If it didn't we wouldn't need choke
>> balun's.
>
> We need RF chokes and baluns to supress curents induced on the shield from
> the unbalance at the antenna feedpoint.
Actually what oftentimes happens with a coax feed is that the RF
current leaving the inside of the feedline shield can flow in two
directions. It can flow down the antenna element half connected to the
shield (desired path), or it can flow down the outside of the shield
(undesired path). The electrons are dumb, all they are looking for is
the path of least resistance. They can't tell that the metal surface
on the outside of the coax isn't supposed to be part of the antenna.
The only way to keep current from flowing down the shield is make
the antenna element-half connected to the shield look like a lower
impedance than the outside of the shield. If you place ferrite beads
around the outside of the shield, this will raise the impedance of the
shield path, thereby diverting the bulk of the RF current into the
element-half and off of the shield's outside surface.
> Sooo, according to W8JI "teachings", RF current gets induced onto the
> outside surface of tubing, then crolls around the edges and goes inside
> the tubing?
As per K4FMX's comments, this can only happen if there is a
center conductor inside the tubing, or if the tubing diameter is greater
than ~1/2 wavelength in diameter, otherwise the inside of the tubing
looks like a circular waveguide beyond cutoff. This is why coax
of a given diameter becomes useless above a certain upper frequency
limit. Once the I.D. of the coax becomes a significant fraction of a
wavelength in diameter, the coax will start to support propagation of
waveguide modes (e.g. non-TEM modes). At HF frequencies, even
large diameter tubing is well beyond waveguide cutoff, so there is no
concern about "corking" open tubing with no center conductor (it
corks itself).
73, Mike W4EF.....................................................
Cecil Moore
> The electrons are dumb, all they are looking for is
> the path of least resistance.
Hmmmm, electrons that know ohm's law sound pretty
smart to me. :-)
Roy Lewallen
> Gene Fuller wrote:
> "Terman certainly did not deny the existence of skin effect that keeps
> fields out of the interior of conductors.'
>
> true. The point is, shielding from magnetic fields is different from
> electric fields. On page 35 of his 1955 edition, Terman writes:
> "Magnetic flux in attempting to pass through a shield (copper or
> aluminum) induces voltage in the shield which gives rise to eddy
> currents. These eddy currents oppose the action of the flux, and in
> large measure prevent its penetration through the shield."
Am I mistaken, but is this not a clear statement that a copper or
aluminum shield will block magnetic flux, along with an explanation of
why it happens?
Roy Lewallen, W7EL
K7ITM
around a mains-frequency power transformer at reducing the exterior
magnetic field is well known and often used. It's all very clear from
Faraday's law of magnetic induction: the net magnetic flux through an
area enclosed by a perfect conductor may not change, so time-varying
magnetic fields are perfectly blocked by perfect conductors. Copper's
a reasonable approximation of a perfect conductor in the case of RF
shields.
Cheers,
Tom
w8...@akorn.net
It would only be a clear statement to those who understand what was
quoted from Terman.
If a person is confused by or somehow DOESN'T understand what Terman is
saying, he or she might take it to mean magnetic fields can travel
unimpeded through a shield.
It sure is difficult to drive a stake through the heart of myths like
the loop shield "shielding the electric field and not the magnetic
field" when clearly written text in dozens of engineering textbooks is
misunderstood.
73 Tom
Richard Clark
Hi Tom,
However, Richard's explanation is the analogue of the effectiveness of
a copper strap (with a non-contacting overlap so as to not be a
shorted turn) between windings of a mains-frequency power transformer,
and grounded to provide electrostatic separation of the two circuits.
Cecil Moore
> If a person is confused by or somehow DOESN'T understand what Terman is
> saying, he or she might take it to mean magnetic fields can travel
> unimpeded through a shield.
I have asked this simple question a number of times and, so
far, no one has answered. It should have a simple answer so
here it is again. Does a 60 Hz magnetic field travel virtually
unimpeded through a coax shield? This question involves 60 Hz
noise being coupled through coax.
Richard Harrison
"Am I mistaken, but is this not a clear statement that a copper or
it happens?"
Yes. And now the rest of the story which I`ve already posted several
times.
At the bottom of page 38 in Terman`s 1955 edition;
"It is possible to shield slectrostatic flux WITHOUT simultaneously
affecting the magnetic field by surrounding the space to be shielded
with a conducting cage that is made in such a way as to provide NO
low-resistance path for the flow of eddy currents, while at the same
time offering a metallic terminal upon which electrostatic flux can
terminate."
I invited readers to look at page 43 in the same book where:
"A grid of wires such as shown in the accompanying figure will provide
electrostatic shielding WITHOUT magnetic shielding---."
I also said that similar grids (metal picket fences) were used in AM
broadcast stations I`d worked in to eliminate capacitive coupling to the
antennas which would otherwise favor harmonics of the broadcast
frequency.
W8JI is yet desiring to drive a stake in the heart of the "myth" that
E&H fields are separable if even for an instant. He is dead wrong.
The magnetic field alone does quite well in transferring all the wave`s
energy through a special transformer which completely bars the electric
field. Of course, current in the transformer`s secondary produces a
voltage and the E-field is immediately restored.
This is not witchcraft. In free-space the electric field and the
magnetic field are repeatedly exchanging all the energy back and forth.
It keeps the wave going.
At a short or open on a transmission line energy is not lost. It is
merely transferred for an instant into the surviving field.
Similarly, all the energy can be transferred through the electric field
with zero magnetic coupling. Imagine two separately shielded coils. Now,
use a coupling capacitor to transfer the energy from one coil to the
other. Voila! E-field transfer with zero magnetic coupling. It`s no
myth. It`s a fact.
Richard Harrison
"Does a 60 Hz magnetic field travel virtually unimpeded through a
Surely many readers have toiled with shielded audio cables similar to
coax and they know the answer is yes, if the shield is broken (this
often happens at the ends of the cable).
Roy Lewallen
> Roy Lewallen, W7EL wrote:
> "Am I mistaken, but is this not a clear statement that a copper or
> aluminum shield will block magnetic flux along with explanation of why
> it happens?"
>
> Yes. And now the rest of the story which I`ve already posted several
> times.
>
> At the bottom of page 38 in Terman`s 1955 edition;
> "It is possible to shield slectrostatic flux WITHOUT simultaneously
> affecting the magnetic field by surrounding the space to be shielded
> with a conducting cage that is made in such a way as to provide NO
> low-resistance path for the flow of eddy currents, while at the same
> time offering a metallic terminal upon which electrostatic flux can
> terminate."
>
> I invited readers to look at page 43 in the same book where:
> "A grid of wires such as shown in the accompanying figure will provide
> electrostatic shielding WITHOUT magnetic shielding---."
>
> I also said that similar grids (metal picket fences) were used in AM
> broadcast stations I`d worked in to eliminate capacitive coupling to the
> antennas which would otherwise favor harmonics of the broadcast
> frequency.
>
> W8JI is yet desiring to drive a stake in the heart of the "myth" that
> E&H fields are separable if even for an instant. He is dead wrong.
Certainly the E or H field can disappear for part of a cycle. This
happens routinely in a transmission line or in free space, as the energy
is swapped back and forth between the two components. Tom has never said
this is not so. And for whole cycles you can locally change the ratio of
E/H, but you cannot separate them. Maxwell's equations show this. And if
you do change the ratio of E/H, the normal free space ratio is restored
within a small distance.
> The magnetic field alone does quite well in transferring all the wave`s
> energy through a special transformer which completely bars the electric
> field. Of course, current in the transformer`s secondary produces a
> voltage and the E-field is immediately restored.
Yes. And does this restored E field not occupy the space between the
winding and the wire grid? If so, how can you tell that the grid has
"stopped" the E field if it exists on both sides of the grid?
> This is not witchcraft. In free-space the electric field and the
> magnetic field are repeatedly exchanging all the energy back and forth.
> It keeps the wave going.
With this I totally agree. That explanation is related to why you can't
simply remove one component or the other.
> At a short or open on a transmission line energy is not lost. It is
> merely transferred for an instant into the surviving field.
>
> Similarly, all the energy can be transferred through the electric field
> with zero magnetic coupling. Imagine two separately shielded coils. Now,
> use a coupling capacitor to transfer the energy from one coil to the
> other. Voila! E-field transfer with zero magnetic coupling. It`s no
> myth. It`s a fact.
It's a myth that there's no magnetic field in the space between a
capacitor's plates.
Roy Lewallen, W7EL
Tom Donaly
The skin depth of copper at 60 hertz is supposed to be 8.53mm. That's
too thick to make a practical shield. No wonder they're having problems.
73,
Tom Donaly, KA6RUH
Richard Harrison
"It`s a myth that there`s no magnetic field in the space between a
capacitor`s plates."
Maxwell`s great speculation was that "displacement current", as between
a capacitor`s plates, produced magnetic flux as does conduction current.
His speculation is now proved.
Roy Lewallen
> Roy, W7EL wrote:
> "It`s a myth that there`s no magnetic field in the space between a
> capacitor`s plates."
>
> Maxwell`s great speculation was that "displacement current", as between
> a capacitor`s plates, produced magnetic flux as does conduction current.
> His speculation is now proved.
Yes. So how does a capacitor between two inductors constitute "E-field
transfer with zero magnetic coupling" as you stated?
Roy Lewallen, W7EL
Tom Donaly
When did you perform this experiment, Richard? (Perfectly shielding two
coils and then coupling them with a capacitor.) And how did you manage
to shield them if, as you seem to think, the magnetic fields are capable
of penetrating the shield?
73,
Tom Donaly, KA6RUH
Cecil Moore
> It's a myth that there's no magnetic field in the space between a
> capacitor's plates.
What quantum particles support that magnetic field?
Cecil Moore
> The skin depth of copper at 60 hertz is supposed to be 8.53mm. That's
> too thick to make a practical shield. No wonder they're having problems.
So 60 Hz magnetic fields penetrate shielded coax?
Michael Tope
"Cecil Moore" <myc...@hotmail.com> wrote in message
news:_KNcg.35204$Lm5....@newssvr12.news.prodigy.com...
Cecil, if ever I had the feeling that I was about to answer a loaded
question, this is it, but here goes anyway - "Yes, I believe a 60 Hz
magnetic field impinging on a piece of shielded coax would penetrate
the shield of that coax significantly if the shield were made of a
non-ferrous conductor."
73, Mike W4EF.............................................
Cecil Moore
> "Cecil Moore" <myc...@hotmail.com> wrote:
>>So 60 Hz magnetic fields penetrate shielded coax?
>
> Cecil, if ever I had the feeling that I was about to answer a loaded
> question, this is it, but here goes anyway - "Yes, I believe a 60 Hz
> magnetic field impinging on a piece of shielded coax would penetrate
> the shield of that coax significantly if the shield were made of a
> non-ferrous conductor."
It's not a loaded question. I just always assumed that coax
would shield the system from 60 Hz noise and I guess I was
wrong.
Yuri Blanarovich
"Richard Harrison" <richard...@webtv.net> wrote
>
> Similarly, all the energy can be transferred through the electric field
> with zero magnetic coupling. Imagine two separately shielded coils. Now,
> use a coupling capacitor to transfer the energy from one coil to the
> other. Voila! E-field transfer with zero magnetic coupling. It`s no
> myth. It`s a fact.
>
> Best regards, Richard Harrison, KB5WZI
>
I wonder how the gurus 'splain the different behavior of vertical vs.
horizontally polarized antennas, where E field determines the way signals
reflect and form the pattern. If E and H fields are so "inseparable" there
should be no difference, right?
They are ignoring behavior of E and H fields within the near field of
antennas and workings of the shield.
Another example of shield's performance was when I had small shielded loop
next to the Beverage. The combination gave better S/N performance and better
signal levels than each of them alone. Again, shield performing shielding
function in the vicinity of near field of both antennas. Loops were tunable
and performing as an antenna, shield was shielding from the near by
interference and providing symmetry. FACTS - verifiable not subject to wild
speculations about current crawling around the edge to inside of tubing, or
antenna inside of electrostatic shield quitting to work as antenna because
of "W8JI shield is antenna teachings".
Michael Tope
Cecil,
Here's a link to an interesting post on TowerTalk by Jim K9YC
discussing the subject of shielding effectiveness of coax at very low
frequencies:
http://lists.contesting.com/_towertalk/2005-07/msg00663.html
73, Mike, W4EF.....................................................
Dr. Honeydew
that is transverse? Perhaps I could get my assistant, Beaker, to run
some tests for us. He sometimes get a little, ah, involved with his
experiments, though, so it may take some time.
Regards,
Bunsen
w8...@akorn.net
Yuri Blanarovich wrote:
> I wonder how the gurus 'splain the different behavior of vertical vs.
> horizontally polarized antennas, where E field determines the way signals
> reflect and form the pattern. If E and H fields are so "inseparable" there
> should be no difference, right?
The E and H fields are at right angles to each other. The E field is
the field used to define polarization. A "vertical" antenna has a
vertical E field. It has a horizontal H field.
The E field is higher impedance, and easily "shorted" by the earth.
When it goes away so does the H field. This is why a vertically
polarized wave (vertical electric field) has good ground wave
propagation and a horizontally polarized wave has no useful ground
wave.
This is the very reason noise that propagates along the earth any
distance is mostly vertically polarized.
> They are ignoring behavior of E and H fields within the near field of
> antennas and workings of the shield.
No, they understand fine. What most people are saying here fits
perfectly with how everything works.
> Another example of shield's performance was when I had small shielded loop
> next to the Beverage. The combination gave better S/N performance and better
> signal levels than each of them alone. Again, shield performing shielding
> function in the vicinity of near field of both antennas. Loops were tunable
> and performing as an antenna, shield was shielding from the near by
> interference and providing symmetry. FACTS - verifiable not subject to wild
> speculations about current crawling around the edge to inside of tubing, or
> antenna inside of electrostatic shield quitting to work as antenna because
> of "W8JI shield is antenna teachings".
Nobody has said a change in antenna design or construction can't cause
a change in performance. The problem is you are trying to tell us is
the "magnetic" portion of time-varying fields can penetrate the wall of
a metal tube. That is NOT true when the wall is a few skin depths
thick. Nothing can go through, neither the time-varying electric nor
magnetic fields. The gap is the feedpoint, the wire inside the tube
simply a coupling mechanism, and the outside of the tube is the actual
antenna. It can't be any other way.
This is something core to how many basic simple things work. This is
the reason a screen room works, why an aluminum can shields IF or RF
coils, how a shield on a radio works, why coaxial cables act like they
do, and so on.
This is why an ungrounded solid metal plate placed between the axis of
two coils diverts the flux. This coupling is why, if we measure the
inductance and Q of a short length of concentric conductors, the center
conductor has more inductance and significantly less Q than the outer
conductor.
It's a large part of why everything works as it does for RF. It's very
important we understand it, otherwise we create all types of bad
science.
73 Tom
Richard Clark
wrote:
Hi All,
Really, this contretemps seems to be over a matter of scale and
application.
Ramo, Whinnery, and Van Duzer make clear distinctions between mutual
couplings and radiative couplings. Most of the discussion in this and
related threads appear to discard these distinctions.
Richard's application of screened air linked couplers and using the
illustration of power transformers is found in "Fields and Waves..."
by these authors:
"Where there is a component of the electric field in phase with
the current, the integral of the electric field cannot be
considered either as a pure "capacitive" or "inductive" voltage
drop since there will be real energy transfer (radiation) from
these terms."
Richard's applications and illustrations do not push this boundary. In
fact, Ramo et. al distinctly offer the case of "electrostatic
shielding" and clearly support the separation of magnetic and electric
flux (fields). And so as to anticipate the conundrum of the "static"
in electrostatic, the authors show no issue. However, they do provide
a rational warning:
"It often happens that electrodes, although grounded
for direct current, may be effectively insulated or floating
at radio frequencies because of impedance in the grounding
leads. In such cases the new electrodes do not accomplish
their shielding purposes but may in fact increase capacitive
coupling."
Insofar as Yuri's complaint, it is an ego trip that wholly ignores the
scales of wavelength, the application of materials, the nature of
balance, and the misapplication of mutual coupling to explain far
field effects. In short, he has been bitten by the "lumped vs.
distributed" distinction once again. The only saving grace of his
argument may be found in that there are two forms of the "shielded
dipole" where one supports Tom's claim, and the other support's
Yuri's.
Unfortunately, as correct as Richard's examples are, they too are
misapplied to the "shielded dipole." The "shielded dipole" may be
small in relation to wavelength, but its response mechanism is NOT
found by using mutual coupling math, but rather through radiation
math.
Roy Lewallen
> . . .
> Richard's applications and illustrations do not push this boundary. In
> fact, Ramo et. al distinctly offer the case of "electrostatic
> shielding" and clearly support the separation of magnetic and electric
Can you direct me to where in the text they do so? All I've found is a
short section (5.28) on "Electrostatic Shielding" where they explain
that introducing a grounded conductor near two others will reduce the
capacitive coupling between them. Obviously this will alter the local
E/H ratio, but in no way does it allow an E or H field to exist
independently, even locally, let alone at any distance.
Roy Lewallen, W7EL
Richard Clark
wrote:
>Richard Clark wrote:
Hi Roy,
Article 5.12 "Circuit Concepts at High Frequencies or Large
Dimensions"
Figure 5.28(a) shows a complete shielding. Of course this is entirely
electric, and arguably magnetic. However, magnetic flux can penetrate
thin shields, electric flux cannot.
This is part and parcel to the world of isolated and shielded
circuits. The electrostatic shields are as effective as they are
complete in their coverage. Their contribution is measured in mutual
capacitance between the two points being isolated. With a drain wire
to ground, and a low enough Z in that wire, then that mutual
capacitance tends towards zero (however, near zero is a matter of
degree as I've offered in past discussion).
Figure 5.28(a) shielding is quite common in medical circuit design,
and mutual capacitance does equal zero; and yet signals and power pass
in and out through magnetic coupling. Isolated relays are a very
compelling example of magnetic transparency in the face of total
electric shielding.
Magnetic shielding operates through reflection or dissipation
(absorption loss due to eddy currents). This loss is a function of
permeability ľ. Unfortunately, permeability declines with increasing
frequency, and with declining field strength. Basically, all metals
exhibit the same characteristic ľ above VLF; hence any appeal to
"magnetic materials" used to build antennas is specious.
This is not to say the magnetic shield is ineffective, merely derated
seriously from what might be gleaned through poor inference by reading
ľ values from tables.
However, it is quite obvious that transformer inter stage shielding
and the faraday shield found in AM transmitters is not seeking to
optimize this attenuation, far from it. Thus the degree in isolation
is found in the ratio of the mutual capacitance between the two points
before and after shielding; and the attenuation in magnetic flux
induction introduced between the two circuits after shielding.
Returning to Ramo, et. al, the introduction of a partial shield.
Figure 5.28(c) is effective insofar as its ability to reduce mutual
capacitance.
w8...@akorn.net
Richard Clark wrote:
> Figure 5.28(a) shows a complete shielding. Of course this is entirely
> electric, and arguably magnetic. However, magnetic flux can penetrate
> thin shields, electric flux cannot.
Only when the shield is thin compared to the skin depth.
When the shield is thick relative to skin depth nothing gets through.
This is very easy to prove. I have been making measurements of a ~ .032
inch thick copper wall and with 0dB reference on a small resonant pick
up loop my analyzer is in the noise (-90dB) on the side directly
opposite that loop.
The same is true for a direct soldered connection to the wall on each
opposite side.
One inch to the side on the same side levels are -10dB. That would be a
two inch long path shorted by the copper the entire way. Go to the
direct opposite side through only .032 thick copper and levels are not
even detectable.
> This is part and parcel to the world of isolated and shielded
> circuits. The electrostatic shields are as effective as they are
> complete in their coverage. Their contribution is measured in mutual
> capacitance between the two points being isolated.
I don't have that reference and so cannot see that shield, but the only
thing the shield can do is reduce field impedance by changing the ratio
of electric to magnetic fields. In order to take either one to zero the
other must also be at zero.
I think the confusion comes from misapplying a grid forming a shunt
capacitance to reduce direct capacitance between two objects (forming a
"T" divider) to the shielded loop antenna or shielded link.
Consider a loop of any size, even a link in a tank coil. That conductor
has a field impedance and radiation characteristics largely set by the
diameter of the coil.
Once we put a wall around that conductor more than a few skin depths
thick NOTHING goes through that wall. The "shield" actually becomes the
coupling coil, the link inside simply develops a voltage across the
shield to drive that external coil. Both electric and magnetic fields
are present on the outer wall of the shield, and while they may be
modified by shield balance they really are not much different than we
had with just the inner conductor alone. We really just change the
balance.
With the grid, we have substantial air gap segmenting the "wall".
Naturally the coupling mechanism is different than we have with a solid
wall. We, in effect, have dozens of long gaps.
Each conductor in that grid is indeed excited by the magnetic and
electric fields, and each conductor has a potential difference across
area and a current flowing. Part of the field, both electric and
magnetic, leaks through. Part is reradiated by the currents and
voltages in the grid.
I think at some point of time MRT or Dave Saloff patented a right angle
grid of two layers with opposing ends in each adjacent conductor in
each layer grounded that I designed. The idea was to more evenly
distribute the fields and prevent "hot spots". This was for a medical
application.
Rest assured this applicator produced both time-varying electric and
magnetic fields, but the balance was so much improved tuning was more
stable. The improved balance and evenly distributed field meant the
feedline did not radiate any significant amount when brought near the
patient, unlike a regular multiple turn loop.
I still have some prototype applicators here, as well as the field
measurements required by the FDA. The applicator actually had to match
with lowest return loss into the buttocks of an average size 30 year
old female.
73 Tom
Richard Clark
>> This is part and parcel to the world of isolated and shielded
>> circuits. The electrostatic shields are as effective as they are
>> complete in their coverage. Their contribution is measured in mutual
>> capacitance between the two points being isolated.
>
>I don't have that reference and so cannot see that shield, but the only
>thing the shield can do is reduce field impedance by changing the ratio
>of electric to magnetic fields. In order to take either one to zero the
>other must also be at zero.
Hi Tom,
There are too many contra-examples too sustain your point. What you
are talking about is radiation, this does not account for common
induction that occurs on the very short scales I've offered.
w8...@akorn.net
> are talking about is radiation, this does not account for common
> induction that occurs on the very short scales I've offered.
Will you give me an example where the electric field is zero and all
coupling is via magnetic flux?
73 Tom
Richard Clark
Tom,
As this has already been discussed not but two postings ago, the
posting your responded to, why are you asking for that content again?
Roy Lewallen
I was going to ask the same question but Tom beat me to it. And I must
have missed the example, too. Would you be so kind as to repost it?
Roy Lewallen, W7EL
Richard Clark
wrote:
>Richard Clark wrote:
>>On Wed, 24 May 2006 12:11:52 -0700, Roy Lewallen <w7...@eznec.com>
>>wrote:
>>
>>>Richard Clark wrote:
>>> > . . .
>>>> Richard's applications and illustrations do not push this boundary. In
>>>> fact, Ramo et. al distinctly offer the case of "electrostatic
>>>> shielding" and clearly support the separation of magnetic and electric
>>>> flux (fields). . .
>>>
>>>Can you direct me to where in the text they do so? All I've found is a
>>>short section (5.28) on "Electrostatic Shielding" where they explain
>>>that introducing a grounded conductor near two others will reduce the
>>>capacitive coupling between them. Obviously this will alter the local
>>>E/H ratio, but in no way does it allow an E or H field to exist
>>>independently, even locally, let alone at any distance.
>>
>>Hi Roy,
>>
>>Article 5.12 "Circuit Concepts at High Frequencies or Large
>>Dimensions"
>>
>>Figure 5.28(a) shows a complete shielding. Of course this is entirely
>>electric, and arguably magnetic. However, magnetic flux can penetrate
>>thin shields, electric flux cannot.
>>
>>This is part and parcel to the world of isolated and shielded
>>circuits. The electrostatic shields are as effective as they are
>>complete in their coverage. Their contribution is measured in mutual
>>capacitance between the two points being isolated. With a drain wire
>>to ground, and a low enough Z in that wire, then that mutual
>>capacitance tends towards zero (however, near zero is a matter of
>>degree as I've offered in past discussion).
>>
>>Figure 5.28(a) shielding is quite common in medical circuit design,
>>and mutual capacitance does equal zero; and yet signals and power pass
>>in and out through magnetic coupling. Isolated relays are a very
>>compelling example of magnetic transparency in the face of total
>>electric shielding.
>>
>>Magnetic shielding operates through reflection or dissipation
>>(absorption loss due to eddy currents). This loss is a function of
>>permeability ต. Unfortunately, permeability declines with increasing
>>frequency, and with declining field strength. Basically, all metals
>>exhibit the same characteristic ต above VLF; hence any appeal to
>>"magnetic materials" used to build antennas is specious.
>>
>>This is not to say the magnetic shield is ineffective, merely derated
>>seriously from what might be gleaned through poor inference by reading
>>ต values from tables.
>>
>>However, it is quite obvious that transformer inter stage shielding
>>and the faraday shield found in AM transmitters is not seeking to
>>optimize this attenuation, far from it. Thus the degree in isolation
>>is found in the ratio of the mutual capacitance between the two points
>>before and after shielding; and the attenuation in magnetic flux
>>induction introduced between the two circuits after shielding.
>>
>>Returning to Ramo, et. al, the introduction of a partial shield.
>>Figure 5.28(c) is effective insofar as its ability to reduce mutual
>>capacitance.
>>
Cecil Moore
> I was going to ask the same question but Tom beat me to it. And I must
> have missed the example, too. Would you be so kind as to repost it?
Not sure of the context but ideally at a voltage node in an
unterminated transmission line, the E-field is very close to
zero while almost all of the EM energy exists in the H-field.
According to "Optics", by Hecht, the same thing can
happen in free space with light waves.
--
73, Cecil, W5DXP
Roy Lewallen
magnetic field in the absence of the other. Such a condition is, in
fact, impossible.
Richard Clark wrote:
>>>>> . . .
>>>>> Richard's applications and illustrations do not push this boundary. In
>>>>> fact, Ramo et. al distinctly offer the case of "electrostatic
>>>>> shielding" and clearly support the separation of magnetic and electric
>>>>> flux (fields). . .
>>>> Can you direct me to where in the text they do so? All I've found is a
>>>> short section (5.28) on "Electrostatic Shielding" where they explain
>>>> that introducing a grounded conductor near two others will reduce the
>>>> capacitive coupling between them. Obviously this will alter the local
>>>> E/H ratio, but in no way does it allow an E or H field to exist
>>>> independently, even locally, let alone at any distance.
>>> Hi Roy,
>>>
>>> Article 5.12 "Circuit Concepts at High Frequencies or Large
>>> Dimensions"
>>>
>>> Figure 5.28(a) shows a complete shielding. Of course this is entirely
>>> electric, and arguably magnetic. However, magnetic flux can penetrate
>>> thin shields, electric flux cannot.
We've been talking about *time-varying* fields, and must have forgotten
to explicitly state that qualification. The figure in question deals
with static fields. Time-varying electric flux can indeed penetrate thin
shields of finite conductivity, although the E/H ratio within the shield
is very small. If a shield could block time-varying electric fields, the
time-varying magnetic field which remained would create an electric
field. A time-varying magnetic fields creates a time-varying electric
field and vice-versa; this is dictated by Maxwell's equations. The
answer to question 4.06d in Ramo, et al, "Can a time-varying field of
any form exist in space without a corresponding electric field? Can a
time-varying electric field exist without the corresponding magnetic
field?" is no.
A gapless shield made of a perfect conductor of any thickness will
completely block both electric and magnetic fields.
>>>
>>> This is part and parcel to the world of isolated and shielded
>>> circuits. The electrostatic shields are as effective as they are
>>> complete in their coverage. Their contribution is measured in mutual
>>> capacitance between the two points being isolated. With a drain wire
>>> to ground, and a low enough Z in that wire, then that mutual
>>> capacitance tends towards zero (however, near zero is a matter of
>>> degree as I've offered in past discussion).
>>>
>>> Figure 5.28(a) shielding is quite common in medical circuit design,
>>> and mutual capacitance does equal zero; and yet signals and power pass
>>> in and out through magnetic coupling. Isolated relays are a very
>>> compelling example of magnetic transparency in the face of total
>>> electric shielding.
The mutual capacitance at DC equals zero. Time-varying electric fields
penetrate the shield if it's thin in terms of skin depth.
>>> Magnetic shielding operates through reflection or dissipation
>>> (absorption loss due to eddy currents). This loss is a function of
>>> permeability ต. Unfortunately, permeability declines with increasing
>>> frequency, and with declining field strength. Basically, all metals
>>> exhibit the same characteristic ต above VLF; hence any appeal to
>>> "magnetic materials" used to build antennas is specious.
This is not true. Metals do indeed exhibit varying permeabilities at RF
and above. This can be illustrated by a number of means, a common one
being the efficacy of a powdered iron core.
Electric field shielding also operates through reflection and
dissipation. Permeability affects both, because of its effect on
material wave impedance and skin depth.
>>> This is not to say the magnetic shield is ineffective, merely derated
>>> seriously from what might be gleaned through poor inference by reading
>>> ต values from tables.
Permeability does indeed change with frequency for a variety of reasons.
Consequently, some intelligence (and often measurement or guesswork) has
to be used to determine what it will be at the frequency in question.
>>> However, it is quite obvious that transformer inter stage shielding
>>> and the faraday shield found in AM transmitters is not seeking to
>>> optimize this attenuation, far from it. Thus the degree in isolation
>>> is found in the ratio of the mutual capacitance between the two points
>>> before and after shielding; and the attenuation in magnetic flux
>>> induction introduced between the two circuits after shielding.
>>>
>>> Returning to Ramo, et. al, the introduction of a partial shield.
>>> Figure 5.28(c) is effective insofar as its ability to reduce mutual
>>> capacitance.
Indeed it is. This is not, however, an example of a (time-varying)
magnetic or electric field existing in isolation.
I readily agree that a static electric or magnetic field can exist in
isolation from the other, as I'm sure all other participants to this
discussion do. But not time-varying ones. You can greatly change the E/H
ratio, but you can't make it zero or infinite. And whatever you do will
have only a local effect -- the ratio will rapidly approach the
intrinsic Z of the medium as you move away from the anomaly which
modified the ratio. Rapidly, that is, in terms of wavelength -- it can
be quite a physical distance at very low frequencies.
Roy Lewallen, W7EL
Richard Clark
wrote:
>Time-varying electric flux can indeed penetrate thin
>shields of finite conductivity, although the E/H ratio within the shield
>is very small.
>A gapless shield made of a perfect conductor of any thickness will
>completely block both electric and magnetic fields.
Hi Roy,
Given the vast gulf that separates these two observations above, and
the oblique reply in general that does not flow from your previous
question that I responded to.... It seems you are answering a topic I
have not entered into, or restating what I've already offered.
Roy Lewallen
Sorry, once again I miss your point. I maintain that time-varying
electric and magnetic fields cannot exist independently, while you claim
that they can. Tom and I asked for an example of a case where they do,
and your response did not contain such an example.
Roy Lewallen, W7EL