colinear representation in NEC
Owen Duffy
a) | b) |
| |
| |
| |
| |
| |
| |
| |
| |
-------| |
-------| |||
| |||
| |||
| |||
| |||
| |||
| ---
S S
--------------- -----------------
Fig a) above is an attempt to portray a colinear vertical over infinite
ground with a source at "S". The configuration is easy enough to model in
NEC with sensible results.
The common explanation for operation of a) is that the U shaped section
is a quarter wave s/c stub, that it is responsible for delivering direct
in-phase drive to the upper section, and that it plays no part itself in
radiation ie, that the common mode current on the pair of conductors is
zero at all points. Notwithstanding the conventional wisdom, it seems
unlikely that there is no common mode current on that section, and NEC
models suggest that there is, and that it accounts for some small
asymmetric distortion of the pattern.
Fig b) above is an attempt to represent a coaxial arrangement of tubes
where the lower end of the tubes are connected together, and that is fed
at S against an infinite ground.
My questions are:
1. To what extent is b) equivalent to a)?
2. How is b) modelled in NEC?
Thanks
Owen
Roy Lewallen
I can't answer that question right off, except that at first glance they
look quite similar in operation. I'd build both models with EZNEC, then
take a look at the reported currents in the View Antenna display. You
can get the same information from tabular NEC results, but most people
find the graphical display quicker and easier to interpret.
You can see the significance of the seemingly small common mode current
on the a) model stub by replacing it with a transmission line model stub
which of course has zero common mode current. The results are quite
different than for the wire model stub.
> 2. How is b) modelled in NEC?
A coaxial line can be modeled as a combination of a transmission line
(for the inside of the coax) and a wire (for the outside of the coax).
Download the EZNEC demo program and look in the manual index under
Coaxial Cable, Modeling. It'll direct you to one of the furnished
example files which illustrates how. Then you can do the same thing with
NEC if you're so inclined.
Roy Lewallen, W7EL
Richard Clark
>My questions are:
>
>1. To what extent is b) equivalent to a)?
Hi Owen,
To no extent as far as I can tell without modeling. I don't think the
phases are going to equivalent.
>2. How is b) modelled in NEC?
This takes some presuming of your intent, and my presumption, given
the symmetry of the two smaller elements (why two otherwise?), is you
are attempting to portray a skeletal sleeve with them. Two is
insufficient by my standards, six are barely worth chasing the numbers
and I typically use 16. One such example, complete with a link to the
design can be found at:
http://home.comcast.net/~kb7qhc/antennas/verticals/Cage/cage.htm
It deviates only by a small degree, but could prove a useful boost in
adding the longer element after opening the top end of the thick
radiator.
73's
Richard Clark, KB7QHC
Owen Duffy
news:h7mdnax9a96mICHU...@posted.easystreetonline:
Hello Roy,
Thanks for the response.
>
...
>> My questions are:
>>
>> 1. To what extent is b) equivalent to a)?
>
> I can't answer that question right off, except that at first glance
> they look quite similar in operation. I'd build both models with
> EZNEC, then take a look at the reported currents in the View Antenna
> display. You can get the same information from tabular NEC results,
> but most people find the graphical display quicker and easier to
> interpret.
Ok, here is the model I constructed of b) (the coaxial tubes
construction). For simplicity, the upper and lower outer tubes are the
same diameter, the same wire in this model.
CM
CE
GW 10 1 0 -2 2 0 -2 2.1 0.005
GW 1 47 0 0 0 0 0 15 0.005
GE 1
GN 1
EK
EX 6 1 1 1 0
TL 10 1 1 16 50 5 1e+99 1e+99 0.0001
FR 0 0 0 0 15 0
EN
I have a 3/4 wave vertical over perfect ground, and I have inserted a
quarter wave s/c transmission line into the vertical at 1/3 height. I
have shunted the TL with 10k ohm to represent some loss in the stub.
The currents report shows the currents in the top half wave to be
approximately 180° out of phase with the bottom quarter wave.
The question is whether such a construction yields three quarter waves in
phase, or whether the NEC model is correct that they are not in phase.
>
> You can see the significance of the seemingly small common mode
> current on the a) model stub by replacing it with a transmission line
> model stub which of course has zero common mode current. The results
> are quite different than for the wire model stub.
>
My initial feeling is that the wire model of a) is correct. I have not
yet done as you suggest in the previous par.
>> 2. How is b) modelled in NEC?
>
> A coaxial line can be modeled as a combination of a transmission line
> (for the inside of the coax) and a wire (for the outside of the coax).
> Download the EZNEC demo program and look in the manual index under
> Coaxial Cable, Modeling. It'll direct you to one of the furnished
> example files which illustrates how. Then you can do the same thing
> with NEC if you're so inclined.
Is my model above what you suggest?
Appreciate your comments Roy, thanks.
Owen
Owen Duffy
Richard Clark <kb7...@comcast.net> wrote in
news:n2hqr4lau204b4h76...@4ax.com:
> On Sat, 14 Mar 2009 21:33:15 GMT, Owen Duffy <no...@no.where> wrote:
>
>>My questions are:
>>
>>1. To what extent is b) equivalent to a)?
>
> Hi Owen,
>
> To no extent as far as I can tell without modeling. I don't think the
> phases are going to equivalent.
Ok. Your view is contrary to common explanation... but of course that
doesn't make it wrong.
NEC models of the wire construction at a) show in phase operation, but a
small distortion of the pattern due to common mode current on the stub...
so they support the common explanation in the phase aspect, but not in
respect of the stub causing phase change with no other effects.
The explanation of b) as a) where the stub is relocated coaxially sounds
appealing, but that explanation might be wrong.
>
>>2. How is b) modelled in NEC?
>
> This takes some presuming of your intent, and my presumption, given
> the symmetry of the two smaller elements (why two otherwise?), is you
> are attempting to portray a skeletal sleeve with them. Two is
You seem to have mininterpreted my ASCII art, and that would be easy to
do. I am describing at b), two coaxial tubes, the lowest tube is 1/4
wave, the longer tube is 3/4 wave. The lower tube ends are connected, and
fed between ground and the bottom of the tube assembly.
See the model that I have posted in response to Roy.
Thanks.
Owen
K7ITM
Hi Owen,
I suppose that R.W.P. King disagrees with the "common explanation."
He makes it quite clear that there is interaction of the antenna field
with the stub perpendicular to the axis of the antenna wire, and that
the coaxial stub does not interact in the same way and the antenna
performance is therefore different. (Antennas chapter of Transmission
Lines, Antennas and Wave Guides, King, Mimno and Wing.) This is why I
like using a feedline to guarantee the phasing. It can be done by
driving collinear dipoles with equal lengths of transmission line, or
by using an arrangement like the "coaxial collinear," where the
radiating elements are outer conductors of coaxial transmission lines
used to insure that the multiple feedpoints are at least fed in-phase
voltages (and you have to consider that the currents are not exactly
in phase).
Cheers,
Tom
Owen Duffy
K7ITM <k7...@msn.com> wrote in
news:aeea71bd-ad84-41db...@r36g2000prf.googlegroups.com:
...
> I suppose that R.W.P. King disagrees with the "common explanation."
> He makes it quite clear that there is interaction of the antenna field
> with the stub perpendicular to the axis of the antenna wire, and that
> the coaxial stub does not interact in the same way and the antenna
> performance is therefore different. (Antennas chapter of Transmission
> Lines, Antennas and Wave Guides, King, Mimno and Wing.) This is why I
> like using a feedline to guarantee the phasing. It can be done by
> driving collinear dipoles with equal lengths of transmission line, or
> by using an arrangement like the "coaxial collinear," where the
> radiating elements are outer conductors of coaxial transmission lines
> used to insure that the multiple feedpoints are at least fed in-phase
> voltages (and you have to consider that the currents are not exactly
> in phase).
That it interesting that Prof King declares that there is more than just
a transmission line action with the external style of stub.
An NEC model of a) works well, showing in phase operation and a nice
pattern. I have played around with two stubs of shorter length on
opposite sides of the vertical and stacked on top of each other, and they
worked fine (ie in phase current distribution with zero near the stubs)
at about 0.15+ wavelenths each... which doesn't fit with a propagation
delay around the conductor path explanation. Interesting!
I am trying to support the common explanation of the coaxial colinear in
my diagram b) using NEC, but I haven't yet been sucessful.
Owen
Roy Lewallen
>
> Ok, here is the model I constructed of b) (the coaxial tubes
> construction). For simplicity, the upper and lower outer tubes are the
> same diameter, the same wire in this model.
>
> CM
> CE
> GW 10 1 0 -2 2 0 -2 2.1 0.005
> GW 1 47 0 0 0 0 0 15 0.005
> GE 1
> GN 1
> EK
> EX 6 1 1 1 0
> TL 10 1 1 16 50 5 1e+99 1e+99 0.0001
> FR 0 0 0 0 15 0
> EN
>
> Is my model above what you suggest?
No. But I did take the time to see what would be necessary to actually
model it. And what I ended up with is identical to a) except that the
wire stub is replaced by the shorted transmission line model, and the
lower wire has become the outside of the coaxial structure so is
increased in diameter. So those are the two differences between a) and
b). As Tom mentioned and I alluded to, there's some interaction between
the wire stub and the antenna which doesn't exist between the ideal
transmission line and the antenna, so performance is different.
You might as well leave your source open circuited as to connect it to
the shorted end of the transmission line stub. The current into one
transmission line conductor always equals the current out of the other,
so if the two are shorted, no more current can go into or out of the
shorted end. Therefore, any external connection to it looks like an open
circuit since no current will flow through the external connection.
What's a type 6 source (EX 6)? The NEC-2 and NEC-4 documentation I have
defines only types 1 - 5.
Roy Lewallen, W7EL
Roy Lewallen
> Hi Owen,
>
> I suppose that R.W.P. King disagrees with the "common explanation."
> He makes it quite clear that there is interaction of the antenna field
> with the stub perpendicular to the axis of the antenna wire, and that
> the coaxial stub does not interact in the same way and the antenna
> performance is therefore different. (Antennas chapter of Transmission
> Lines, Antennas and Wave Guides, King, Mimno and Wing.) This is why I
> like using a feedline to guarantee the phasing. It can be done by
> driving collinear dipoles with equal lengths of transmission line, or
> by using an arrangement like the "coaxial collinear," where the
> radiating elements are outer conductors of coaxial transmission lines
> used to insure that the multiple feedpoints are at least fed in-phase
> voltages (and you have to consider that the currents are not exactly
> in phase).
>
> Cheers,
> Tom
In most phased arrays, the objective is to get the fields from the
elements to be in some particular ratio. Driving them with currents in
that same ratio doesn't always accomplish the desired field ratio,
though, when elements have different current distributions as they often
do. (See http://eznec.com/Amateur/Articles/Current_Dist.pdf.) The
difference between field ratio and feedpoint current ratio is
particularly great when base feeding half wave elements. As it turns
out, you'll often get better field ratios by feeding with voltages
having the desired magnitude ratio and phase difference than feeding
with properly ratioed currents, when dealing with end fed half wave
elements. The coaxial collinear requires a pretty delicate balance of
outer and inner velocity factors as well as the effects of mutual
coupling, particularly when there are more than a couple of elements. So
I suspect that the current distribution can either help or hinder
depending on how the factors are traded off. I wouldn't be surprised,
though, if ratioing the voltages rather than currents is actually helpful.
As an illustration, open the EZNEC example file Cardioid.EZ. Change the
number of segments to 10 per wire for better accuracy. (It can still be
run with the demo program.) Click FF Plot and note the nice cardioid
pattern. Then change the Z coordinates of End 2 of the two wires to 0.47
m to make them nearly anti-resonant, and click FF Plot again. The
pattern deterioration is due to the elements having different current
distributions. Finally, change the source types from I to V. This will
force the voltages, rather than currents, at the antenna bases to be in
the desired ratio. Run FF Plot again. You still won't have the nice
cardioid back, but it's quite an improvement over the pattern with
"correctly" ratioed base currents. The bottom line is that the element
currents are more closely related to the base voltages than the base
currents, when the elements are near anti-resonance (parallel, or half
wave, resonance).
Roy Lewallen, W7EL
Owen Duffy
> Owen Duffy wrote:
>>
>> Ok, here is the model I constructed of b) (the coaxial tubes
>> construction). For simplicity, the upper and lower outer tubes are
>> the same diameter, the same wire in this model.
>>
>> CM
>> CE
>> GW 10 1 0 -2 2 0 -2 2.1 0.005
>> GW 1 47 0 0 0 0 0 15 0.005
>> GE 1
>> GN 1
>> EK
>> EX 6 1 1 1 0
>> TL 10 1 1 16 50 5 1e+99 1e+99
>> 0.0001 FR 0 0 0 0 15 0
>> EN
>>
>> . . .
>
>> Is my model above what you suggest?
>
> No. But I did take the time to see what would be necessary to actually
> model it. And what I ended up with is identical to a) except that the
> wire stub is replaced by the shorted transmission line model, and the
> lower wire has become the outside of the coaxial structure so is
> increased in diameter. So those are the two differences between a) and
> b). As Tom mentioned and I alluded to, there's some interaction
I think that is what I had done, but I used the same diameter top to
bottom.
Here is a revised deck with different diameters:
CM
CE
GW 10 1 0 -2 2 0 -2 2.1 0.005
GW 1 15 0 0 0 0 0 5 0.015
GW 2 30 0 0 5 0 0 15 0.005
GE 1
GN 1
EK
EX 0 1 1 1 0
TL 10 1 2 1 50 5 1e+99 1e+99 0.0001
FR 0 0 0 0 15 0
EN
In the above, the lower conductor is three times the diameter of the
upper conductor. The TL is wired into the lowest segment of the upper
conductor. Again, I have shunted the TL with 10k R to represent loss in a
real TL.
This model does not show in phase currents in upper and lower parts of
the vertical.
> between the wire stub and the antenna which doesn't exist between the
> ideal transmission line and the antenna, so performance is different.
>
> You might as well leave your source open circuited as to connect it to
> the shorted end of the transmission line stub. The current into one
I don't think I did that.
> transmission line conductor always equals the current out of the
> other, so if the two are shorted, no more current can go into or out
> of the shorted end. Therefore, any external connection to it looks
> like an open circuit since no current will flow through the external
> connection.
>
> What's a type 6 source (EX 6)? The NEC-2 and NEC-4 documentation I
> have defines only types 1 - 5.
I have been playing with this in EZNEC and 4NEC2. The deck I offered was
from 4NEC2 as my EZNEC files are binaries and couldn't go inline. The EX
6 is an extension for a current source. It is immaterial in this case,
and the 6 can be changed to a 0.
Thanks.
Owen
K7ITM
> > Hi Owen,
>
> > I suppose that R.W.P. King disagrees with the "common explanation."
> > He makes it quite clear that there is interaction of the antenna field
> > with the stub perpendicular to the axis of the antenna wire, and that
> > the coaxial stub does not interact in the same way and the antenna
> > performance is therefore different. (Antennas chapter of Transmission
> > Lines, Antennas and Wave Guides, King, Mimno and Wing.) This is why I
> > like using a feedline to guarantee the phasing. It can be done by
> > driving collinear dipoles with equal lengths of transmission line, or
> > by using an arrangement like the "coaxial collinear," where the
> > radiating elements are outer conductors of coaxial transmission lines
> > used to insure that the multiple feedpoints are at least fed in-phase
> > voltages (and you have to consider that the currents are not exactly
> > in phase).
>
> > Cheers,
> > Tom
>
> In most phased arrays, the objective is to get the fields from the
> elements to be in some particular ratio. Driving them with currents in
> that same ratio doesn't always accomplish the desired field ratio,
> though, when elements have different current distributions as they often
Thanks for the clarifications, Roy. Indeed, with my last slightly
cryptic comment about considering that currents might not be in phase,
I was wanting to communicate that you always want to check the
currents on the elements to make sure they do what you want. That's
true no matter how you feed the antenna, though as you say the feed
you use may aid in insuring that the currents stay the way you want.
I'm a bit surprised about your comment about the coaxial (fed)
collinear requiring a "pretty delicate balance" between coax
propagation velocity and (presumably) radiating element geometry.
What I've found in my simulations is that I could change the coax vf,
keeping the elements a transmission-line half wave long so that the
feedpoints were all the same in-phase voltage, and the net gain of the
antenna for a given physical length was only slightly affected. I'd
typically see a couple of the elements in a ten element array with
considerably lower current magnitude, but the currents were nearly in-
phase on all the elements, and the pattern was always the desired
"flat pancake". On the other hand, I wasn't trying for any up or down
slope to the pattern, and I can see that things might change in that
case. With the propagation velocities I was using, between 0.66 and
about 0.9, and the element diameters I was using, I suppose the
elements were always shorter than resonance, and the self and mutual
impedances were not changing in any dramatic fashion.
Or, perhaps my model was all screwed up! ;-)
Cheers,
Tom
Roy Lewallen
>
> I think that is what I had done, but I used the same diameter top to
> bottom.
Sorry, my mistake when looking at the source. Your model is just as I
described. I apologize for the error.
>
> Here is a revised deck with different diameters:
>
> CM
> CE
> GW 10 1 0 -2 2 0 -2 2.1 0.005
> GW 1 15 0 0 0 0 0 5 0.015
> GW 2 30 0 0 5 0 0 15 0.005
> GE 1
> GN 1
> EK
> EX 0 1 1 1 0
> TL 10 1 2 1 50 5 1e+99 1e+99 0.0001
> FR 0 0 0 0 15 0
> EN
>
> In the above, the lower conductor is three times the diameter of the
> upper conductor. The TL is wired into the lowest segment of the upper
> conductor. Again, I have shunted the TL with 10k R to represent loss in a
> real TL.
>
> This model does not show in phase currents in upper and lower parts of
> the vertical.
I've been running your model without the loss, and I'm seeing currents
in the upper and lower wires which are nearly 180 degrees out of phase.
>> between the wire stub and the antenna which doesn't exist between the
>> ideal transmission line and the antenna, so performance is different.
For sure -- maximum gain is about 46 degrees above the horizon.
>> You might as well leave your source open circuited as to connect it to
>> the shorted end of the transmission line stub. The current into one
>
> I don't think I did that.
You're right, you didn't. My mistake.
> . . .
In playing with the model, I noticed something surprising -- length and
Z0 of the transmission line have very little effect on the pattern, even
over wide ranges (5 to 5000 ohm Z0, lengths from essentially zero to one
wavelength). In fact, try removing the transmission line altogether,
leaving the wires connected directly together and look at the pattern.
Then try changing one wire end slightly to break the connection between
them -- again, very little change in the pattern. The fact is that the
junction of the two wires is at a point of very little current, so you
can connect or disconnect them with almost no change. Likewise, you can
insert just about anything (of zero physical size), including an ideal
transmission line of any length, without any real effect. So the
transmission line stub doesn't really do anything significant at all.
What I don't understand yet is exactly why the wire stub does what it
does. It sure doesn't work like the simplified explanations imply.
Roy Lewallen, W7EL
Owen Duffy
Roy Lewallen <w7...@eznec.com> wrote in
news:eIednRTIvf4bPiPU...@posted.easystreetonline:
...
Thanks, all noted.
> What I don't understand
> yet is exactly why the wire stub does what it does. It sure doesn't
> work like the simplified explanations imply.
Returning to my diagram a), below is an expansion of the detail at the
junction of the stub and vertical sections.
|
|
|
|
|
|
|
| A
B |
---------------------|
--------------------|
|
C |
| D
|
|
|
|
|
|
|
|
It strikes me that if we omit the stub all together, and leave a gap in
its place, we have two unconnected resonant elements, the top half wave,
and the bottom quarter wave with a driving source. The two elements are
field coupled to some extent, and currents will setup in each section out
of phase. NEC models support this, and I think they are correct in doing
so.
Returning now to a) with the stub connected and very close to resonance,
and with reference to the diagram above, for A, B, C and D very close to
the corners, I(A)=I(B) and I(C)=I(D).
If the desired outcome of using the stub is that the upper and lower
sections are in phase, then I(A)~=I(D). That implies common mode current
in the stub, so to cause I(A)~=I(D), the stub must have common mode
current (equal to (I(A)+I(D))/2 per conductor).
If that is true, then reduction of the physical stub to a pure
differential mode TL element is discarding part of what makes it "work".
That implies that replacement of the stub with a two terminal equivalent
impedance, eg by insertion of a load in an NEC segment, or insertion of
one port of a TL network in an NEC segment is an inadequate model.
Am I on the wrong track here?
Owen
K7ITM
> Hi Roy,
>
> Roy Lewallen <w...@eznec.com> wrote innews:eIednRTIvf4bPiPU...@posted.easystreetonline:
For what it's worth, I think you're on exactly the right track, Owen.
Some things to ponder: does it make any significant difference if the
stub is, say, 2mm wires spaced 20mm apart or 1mm wires spaced 10mm
apart (that is, the same impedance line, but different physical size),
and does it make any significant difference if the wires are kept in a
plane that includes the antenna elements, or if they are twisted near
their attachment point so they lie in a plane perpendicular to the
antenna wire, or if they are twisted throughout their length? What if
they are coiled in a spiral out from the antenna wire, so their
shorted end lies much closer than a quarter wave from the axis of the
antenna? I don't have any answers to these questions; they just seem
like an interesting and reasonable extension of your original
question.
Cheers,
Tom
K7ITM
> Hi Roy,
>
I'm sorry...perhaps I don't understand your notation. Don't you
expect that the current at A will be (rather roughly) out of phase
with the current at D? If I think about a collinear with three half-
wave elements end to end, and drive the center of the center element,
if it's going to act like I want, I'll have high current near the
middle of each element, and those three will be in-phase. Because of
the mutual impedances among the elements, things get a bit funny at
the ends. I suppose there is a fairly large voltage across the gap
between adjacent elements, and therefore there will be moderately high
current near those ends to account for the capacitive current in the
air between them. That's what I'm seeing in the EZNEC model I just
hacked, and it's as I'd expect. The currents near the ends of the
central element are considerably higher than the currents near the
open ends of the outer elements.
(Now to spend a few minutes playing with changing the length of the
stubs through resonance...)
Cheers,
Tom
Owen Duffy
K7ITM <k7...@msn.com> wrote in
news:cb900fbc-ab06-44c8...@o2g2000prl.googlegroups.com:
...
> I'm sorry...perhaps I don't understand your notation. Don't you
I am taking a convention that the sense of currents in segments is from
bottom to top. That means that I defined all segments in order from bottom
to top.
My notation ~= is to mean approximately equal.
Does that clarify things?
Cheers
Owen
K7ITM
> Hi Tom,
>
Yes--and then if they were exactly equal, would that not imply only
transmission line current on the stub? Obviously, they are exactly
equal if you simply connect the ends of the elements together...but
that isn't what gets us to in-phase currents at the centers of each
element (in the case of the symmetrical 3 element design; or the base
current in the bottom quarter wave in phase with the center current in
the top half wave...), and (nearly) equal currents at those current
maxima. To the extent that the currents A and D in your diagram
differ, there is common-mode or "antenna" current on the stub.
Cheers,
Tom
Owen Duffy
news:9b11c6f7-d079-4c8b...@d19g2000prh.googlegroups.com:
...
> Yes--and then if they were exactly equal, would that not imply only
> transmission line current on the stub? Obviously, they are exactly
Thinking some more about it, my current thinking is that my analysis was
flawed. I was using the standing wave currents, when I should be using
the travelling wave components.
I suspect that when NEC models the conductor arrangement at my fig a), it
correctly accounts for propagation delay and the phase relationships
compute correctly.
If we replace the stub with a TL element, I suspect that NEC reduces the
TL to a two port network and loads a segment of the vertical with an
equivalent steady state impedance of the s/c stub network. If that is
done, the reduction to a lumped load means that there is zero delay to
travelling waves, and the computed currents (amplitude and phase) in the
vertical will be incorrect. This means that you cannot replace a resonant
stub with a high value of resistance, it doesn't work.
If that is the case, it suggests that NEC cannot model such phasing
schemes using TL elements.
Owen
Cecil Moore
> Thinking some more about it, my current thinking is that my analysis was
> flawed. I was using the standing wave currents, when I should be using
> the travelling wave components.
That's exactly the flaw committed by w8ji and w7el when
they tried to measure the delay through a 75m loading
coil using standing wave current which doesn't appreciably
change phase through a loading coil or through the entire
90 degree length of a monopole. Using standing wave
current, w8ji measured a 3 nS delay through a 10 inch
long coil, a VF of 0.27.
http://www.w8ji.com/inductor_current_time_delay.htm
W7EL reported: "I found that the difference in current
between input and output of the inductor was 3.1% in
magnitude and with *no measurable phase shift*, despite
the short antenna... The result from the second test was
a current difference of 5.4%, again with *no measurable
phase shift*."
Of course, phase shift is not measurable when one is
using standing wave current with its almost unchanging
phase. EZNEC supports that assertion. Bench measurements
support that assertion.
When traveling waves are used to measure the delay through
a 75m loading coil, the correct delay through w8ji's 10
inch coil turns out to be about 26 nS (~37 degrees) at 4 MHz
with a more believable VF of 0.033.
http://www.w5dxp.com/current2.htm
--
73, Cecil http://www.w5dxp.com
"Government 'help' to business is just as disastrous as
government persecution..." Ayn Rand
Tom Donaly
Why would NEC reduce a TL two-port to a lumped load? Two-port
parameters can handle transmission line problems quite well without
the simplifying assumption that all components are of zero length.
73,
Tom Donaly, KA6RUH
Tom Donaly
Cecil, if I ever have a dead horse on my hands, I won't let you
near it because you'll beat it even deader.
73,
Tom Donaly, KA6RUH
K7ITM
Of course, if the TL model doesn't "know about" the antenna field
(which I believe is in fact the case), there will be no common-mode
current on it because of that field. It's pretty clear to me that the
common-mode current is very important to correctly simulating the
situations you are interested in. In fact, figure (B) of your
original posting puts the stub in a position where it does not see the
antenna field, and I would expect it to behave much differently from
the perpendicular stub of figure (A).
One of the things I did in my simulation playing last night was to
delete the stubs, leaving just the three 1/2 wave elements end-to-end
with a bit of gap between them. (0.01m gap between 0.5m elements, 1mm
diameter, 11 segments each.) I'm sure you know what that pattern and
current distribution look like. Then I added sources at the centers
of the outer elements. I set all the sources to 1 amp, in-phase. The
pattern was somewhat sharper (though just marginally more gain) than
the stub-coupled case. What I didn't try, but will as I have a
chance, is to put sources at the centers of the outer elements and set
them to the values (magnitudes and phases) I see in the stub-coupled
collinear, and see how much the current distribution near the ends
looks like the stub coupled case. I suppose it will be pretty close,
and the antenna pattern will look very similar to the stub coupled
pattern.
Thanks for bringing this subject up. I'm learning something from it.
Cheers,
Tom
Owen Duffy
news:QoQvl.13889$8_3....@flpi147.ffdc.sbc.com:
...
> Why would NEC reduce a TL two-port to a lumped load? Two-port
> parameters can handle transmission line problems quite well without
> the simplifying assumption that all components are of zero length.
Hi Tom,
I expect that NEC does model the propagation delay from end to end on a
transmission line. My comment was that NEC reduces a s/c TL stub to a
lumped load for the stub input end which is inserted in the vertical.
The problem here perhaps is our viewing the phasing section as a s/c stub
of two wire line, when perhaps is it better described as a single wire TL
of a half wave length.
With that thought in mind, I have constructed a model where the phasing
section is configured in a double triangular shape, but with the same
conductor length, and NEC suggests in-phase currents. In fact, it has
slightly better pattern symmetry than a).
CM
CE
GW 1 15 0 0 0 0 0 5 0.0005
GW 5 30 0 0 5.1 0 0 15 0.0005
GW 10 7 0 0 5 1.47 0 5. 0.0005
GW 11 10 1.47 0 5. 0 1.47 5. 0.0005
GW 12 15 0 1.47 5. 0 -1.47 5.1 0.0005
GW 13 7 0 -1.47 5.1 -1.47 0 5.1 0.0005
GW 14 15 -1.47 0 5.1 0 0 5.1 0.0005
GE 1
GN 1
EK
EX 0 1 1 0 1.0 0
FR 0 0 0 0 15 0
EN
Owen
K7ITM
> "Tom Donaly" <dtdon...@sbcglobal.net> wrote innews:QoQvl.13889$8_3....@flpi147.ffdc.sbc.com:
>
> ...
>
> > Why would NEC reduce a TL two-port to a lumped load? Two-port
> > parameters can handle transmission line problems quite well without
> > the simplifying assumption that all components are of zero length.
>
> Hi Tom,
>
> I expect that NEC does model the propagation delay from end to end on a
> transmission line. My comment was that NEC reduces a s/c TL stub to a
> lumped load for the stub input end which is inserted in the vertical.
>
> The problem here perhaps is our viewing the phasing section as a s/c stub
> of two wire line, when perhaps is it better described as a single wire TL
> of a half wave length.
>
I'm not sure why you want to reduce it to something less complex than
it is. Transmission lines like this support both even and odd mode
propagation, I guess what we'd normally call "transmission line
currents" and "antenna currents." It seems perfectly OK to me to let
both exist on the line at the same time. It also seems to me there is
value in doing that, because I believe there's insight to be gained
from understanding how each of those currents contributes to the net
performance of the antenna. It's important that the stub be in the
field of the antenna so that antenna current is excited on it, and
it's also important that the stub be shorted a quarter wave away from
where it attaches to the collinear elements, so that the differential
transmission line currents do the right thing.
On the other hand, you may well discover some insights looking at it
in a different way, so I hope my comments won't discourage you from
doing that!
Cheers,
Tom
Owen Duffy
news:f3665982-bd52-4957...@e5g2000vbe.googlegroups.com:
> On Mar 17, 12:41 pm, Owen Duffy <n...@no.where> wrote:
>> "Tom Donaly" <dtdon...@sbcglobal.net> wrote
>> innews:QoQvl.13889$8_3.3071@f
> lpi147.ffdc.sbc.com:
>>
...
> On the other hand, you may well discover some insights looking at it
> in a different way, so I hope my comments won't discourage you from
> doing that!
My real objective is to model b) in NEC.
Trying to understand a) and to deconstruct it is part of an approach to
finding a solution to b).
The 'stub' in a) cannot simply be replaced by a s/c TL element, so that
suggests that a s/c TL element is not a solution for b) either.
The last configuration with the triangular / diamond configuration of the
phasing line seems to work in an NEC model, and the deconstruction
suggests that having a half wave of conductor is fundamental, and that it
need not be in the form of a two wire TL.
I have also tried removing the 'stub' from a) and using a half wave TL to
drive segments each side of the gap from each other. If the segments are
close to, but not the last, this does produce a current distribution that
is not 180° out of phase, but it does not produce the almost perfect in-
phase outcome of modelling the wire structure. Nevertheless, playing with
the length of that TL, being very close to half wave in length is
essential to overriding the natural tendency to out of phase currents.
This hasn't solved the problem of modelling a coaxial configuration,
expecially where the coaxial section was coax cable, apart from excluding
some approaches as invalid.
Owen
Roy Lewallen
It's easy to reason yourself into traps by dividing currents into
"standing wave" and "traveling wave" components. They're different
things and don't add or superpose. Results of attempts to make this
differentiation can be seen in a vast number of postings on this forum
in the past.
Rather, I recommend considering a current to be a single value or, at
most, made of differential and common mode components which *can* be
added to obtain the total current.
In a steady state single frequency analysis, which is what NEC performs,
there is no such thing as delay. All time relationships can be expressed
as phase difference, which can't be tied to a unique delay -- you can't
even tell if the phase difference was due to time delay or magical
prescience-caused time lead. In a steady state analysis there is no way
to distinguish a half wave lossless transmission line from a 1-1/2 wave
line; they act exactly the same in all ways. So does a magical -1/2
wavelength line whose output appears a half cycle *before* the input
appears. Only in a time-domain analysis will you be able to tell the
difference. So yes, NEC models the transmission line as a two port
network. It does force the correct voltage and current amplitude and
phase relationships between the input and output. And it's
indistinguishable in the steady state analysis from an ideal
transmission line which effects the phase difference by means of delay.
The NEC transmission line model is equivalent to a real (but lossless)
transmission line on which the current is purely differential, e.g., a
coax line with a large number of ferrite cores on the outside. The model
is accurate within the constraints of a steady state analysis. If you're
interested in looking at the effects of delay in a transient system,
you'll need to use an analysis tool other than NEC. But if you let your
transient analysis run until steady state is reached, the results will
be the same as NEC.
Roy Lewallen, W7EL
Cecil Moore
> Cecil, if I ever have a dead horse on my hands, I won't let you
> near it because you'll beat it even deader.
The horse is alive and well - the nonsense that I quoted
is still on W8JI's web page.
Cecil Moore
> If you're
> interested in looking at the effects of delay in a transient system,
> you'll need to use an analysis tool other than NEC. But if you let your
> transient analysis run until steady state is reached, the results will
> be the same as NEC.
But in NEC, if you load a transmission line with its
characteristic impedance, reflections are eliminated
and the delay along the wire is proportional to the
phase shift *even during steady-state*.
Jim Kelley
I agree that electromagnetic traveling waves are the kinds of waves that
propagate on and cause radiation to emanate from an antenna. But your
claims about 'standing waves not changing phase along the antenna'
provoke the following questions:
1.) what relation (if any) do you believe the wavelength of the standing
wave has to the wavelength of the radio frequency waves traveling on an
antenna? And,
2.) what relation (if any) does the phase of a sinusoidal wave have to
its amplitude?
73, ac6xg
Cecil Moore
> I agree that electromagnetic traveling waves are the kinds of waves
that propagate on and cause radiation to emanate from an antenna. But
Jim, I thought you have EZNEC. Here are the currents at all of
the segments along a 20m dipole with 21 segments from end to end.
Please note that in a dipole that is 180 degrees long, the phase
of the (mostly standing-wave) current varies by less than 3 degrees.
How can the current in a 180 degree antenna vary by less than 3 degrees?
Quoting my web page: "Standing wave current cannot be used to directly
measure either a valid amplitude change or a valid phase shift through
a loading coil. All of the reported conclusions based on loading coil
measurements using standing-wave current on standing-wave antennas are
conceptually flawed." Owen had an epiphany of a sort when he realized
that fact of physics.
20m dipole 3/18/2009 5:28:50 PM
--------------- CURRENT DATA ---------------
Frequency = 14.2 MHz
Wire No. 1:
Segment Conn Magnitude (A.) Phase (Deg.)
1 Open .0836 -2.75
2 .23595 -2.57
3 .37707 -2.38
4 .50791 -2.17
5 .62692 -1.95
6 .73226 -1.71
7 .82218 -1.44
8 .89511 -1.13
9 .94979 -0.78
10 .98539 -0.37
11 1 0.00
12 .98539 -0.37
13 .94979 -0.78
14 .89511 -1.13
15 .82218 -1.44
16 .73226 -1.71
17 .62691 -1.95
18 .50791 -2.17
19 .37707 -2.38
20 .23595 -2.57
21 Open .0836 -2.75
--
73, Cecil http://www.w5dxp.com
"Government 'help' to business is just as disastrous as
government persecution..." Ayn Rand
P.S. I posted this reply but it didn't show up on my server.
I apologize if it is a duplicate.
Jim Kelley
> Jim Kelley wrote:
> > I agree that electromagnetic traveling waves are the kinds of waves
> that propagate on and cause radiation to emanate from an antenna. But
> your claims about 'standing waves not changing phase along the antenna'
> ...
>
> Jim, I thought you have EZNEC.
> Here are the currents at all of
> the segments along a 20m dipole with 21 segments from end to end.
> Please note that in a dipole that is 180 degrees long, the phase
> of the (mostly standing-wave) current varies by less than 3 degrees.
> How can the current in a 180 degree antenna vary by less than 3 degrees?
It seems to me that computers are completely stupid about certain
things. Could it be a case of garbage in, garbage out?
> Quoting my web page: "Standing wave current cannot be used to directly
> measure either a valid amplitude change or a valid phase shift through
> a loading coil. All of the reported conclusions based on loading coil
> measurements using standing-wave current on standing-wave antennas are
> conceptually flawed."
And what more authoritative reference could someone cite than their own
web page? :-)
I've never actually known what it was that made you believe Roy had
measured standing wave current - whatever that means. Or, how his
measurements compare with your own measurements of the phenomenon.
> Owen had an epiphany of a sort when he realized
> that fact of physics.
It may not even be as elusive a fact as one is given to believe around here.
73, ac6xg
Cecil Moore
> I've never actually known what it was that made you believe Roy had
> measured standing wave current - whatever that means.
Good Grief! Could it be that a monopole is a "STANDING WAVE ANTENNA"?
Cecil Moore
> Could it be that a monopole is a "STANDING WAVE ANTENNA"?
Here's an EZNEC simulation of a 1/4WL monopole. It is
a 1/4WL stub with the wire resistivity adjusted to
simulate monopole radiation. The standing wave current
distribution (lack of phase) and feedpoint resistance
are similar to a monopole.
http://www.w5dxp.com/stub_dip.EZ
Add a short at the top and a load of 600 ohms in the
shorted segment and observe the traveling wave.
http://www.w5dxp.com/stubsht.EZ
Turn on the current phase display and observe the
traveling wave phase shift.
Jim Kelley
> Could it be that a monopole is a "STANDING WAVE ANTENNA"?
The supposition is true, so the intended implication must be that only
standing wave current can be measured on monopole antennas. And Roy
therefore would have to have measured standing wave current (whatever
that is).
I must decline to agree. :-)
73, ac6xg
Richard Clark
wrote:
>I must decline to agree. :-)
Couldn't you incline to disagree?
73's
Richard Clark, KB7QHC
Cecil Moore
About 90% of the total current on an open-ended 1/4WL
monopole is standing wave current with close to unchanging
phase. That's why a 1/4WL monopole is called a "standing
wave antenna".
That is the current that Roy and Tom used so the component
traveling wave, accounting for about 10% of the total current
where the phase shift actually occurs, was mostly ignored and
swamped by the huge component standing wave.
This is such a simple concept - I don't see the problem
in understanding that a wave with the following equation
doesn't change phase with position (x). The phase is the
same over 90 degrees of length no matter what fixed x and
fixed t are chosen. EZNEC supports that fact of physics.
Here's the standing wave equation from "Optics", by Hecht:
E(x,t) = 2E01*sin(kx)*cos(wt) quoting "Optics", by Hecht:
"[Standing wave phase] "doesn't rotate at all, and the resultant
wave it represents doesn't progress through space - its a standing
wave."
Another interesting thing about the standing wave equation
is that the sign of (wt) can be reversed, i.e. standing waves
don't move in either direction - they just stand there. EM
waves cannot stand still so "EM standing wave" is an oxymoron.
Quoting one of my college textbooks, "Electrical
Communication", by Albert:
"Such a plot of voltage is usually referred to as a
*voltage standing wave* or as a *stationary wave*.
Neither of these terms is particularly descriptive
of the phenomenon. A plot of effective values of
voltage, appearing as in Fig. 6(e), *is not a wave*
in the usual sense. However, the term "standing wave"
is in widespread use."
From "College Physics", by Bueche and Hecht:
"These ... patterns are called *standing waves*, as
compared to the propagating waves considered above.
They might better not be called waves at all, since
they do not transport energy and momentum."
One can use EZNEC's VERT1.EZ to view the essentially
unchanging phase on a standing wave monopole. Just look
at the difference in phase between the feedpoint and a
point 45 degrees up the antenna. In 45 degrees of antenna,
the current phase changes by 3.65 degrees. That is the
current Roy used to measure phase shift through a coil
in order to support w8ji's 3 nS delay "measurements".
Jim Lux
> "Tom Donaly" <dtdo...@sbcglobal.net> wrote in
> news:QoQvl.13889$8_3....@flpi147.ffdc.sbc.com:
>
> ...
>> Why would NEC reduce a TL two-port to a lumped load? Two-port
>> parameters can handle transmission line problems quite well without
>> the simplifying assumption that all components are of zero length.
>
> Hi Tom,
>
> I expect that NEC does model the propagation delay from end to end on a
> transmission line. My comment was that NEC reduces a s/c TL stub to a
> lumped load for the stub input end which is inserted in the vertical.
>
>
just another two port that gets dumped into a giant matrix which is
solved as a system of linear equations. Think of TL as a special case of NT.
Cecil Moore
> No it doesn't do prop delay.
The prop delay is easily calculated by loading the TL
with Rload=Z0 and observing the resulting traveling wave
phase shift while taking VF into account. In the same
manner, the prop delay through a loading coil can be
calculated.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com
Tom Donaly
> Jim Lux wrote:
>> No it doesn't do prop delay.
>
> The prop delay is easily calculated by loading the TL
> with Rload=Z0 and observing the resulting traveling wave
> phase shift while taking VF into account. In the same
> manner, the prop delay through a loading coil can be
> calculated.
What's the Z0 of a loading coil, Cecil?
73,
Tom Donaly, KA6RUH
Tom Donaly
What kind of two port does NEC use, Jim? What is "just another two port?"
73,
Tom Donaly, KA6RUH
Jim Kelley
> Jim Kelley wrote:
>> Cecil Moore wrote:
>>
>>> Could it be that a monopole is a "STANDING WAVE ANTENNA"?
>>
>> The supposition is true, so the intended implication must be that only
>> standing wave current can be measured on monopole antennas. And Roy
>> therefore would have to have measured standing wave current (whatever
>> that is).
>>
>> I must decline to agree. :-)
>
> About 90% of the total current on an open-ended 1/4WL
> monopole is standing wave current with close to unchanging
> phase. That's why a 1/4WL monopole is called a "standing
> wave antenna".
Presumably they're not called that because of 'standing current'. A
standing wave is just the stationary pattern that results from the
interference of waves. It doesn't really have a 'life' of it's own.
> This is such a simple concept - I don't see the problem
> in understanding that a wave with the following equation
> doesn't change phase with position (x).
Cecil - as the equation is written, the phase term IS position. The
phase of the sine function changes with x.
> The phase is the
> same over 90 degrees of length no matter what fixed x and
> fixed t are chosen. EZNEC supports that fact of physics.
> Here's the standing wave equation from "Optics", by Hecht:
The phase of a time varying function changes with time except in the
special case of a 'standing wave' function, where it changes with position.
> E(x,t) = 2E01*sin(kx)*cos(wt) quoting "Optics", by Hecht:
>
> "[Standing wave phase] "doesn't rotate at all, and the resultant
> wave it represents doesn't progress through space - its a standing
> wave."
Right. He could (and should) have gone on to say that standing waves
don't really *DO* anything at all.
> Another interesting thing about the standing wave equation
> is that the sign of (wt) can be reversed, i.e. standing waves
> don't move in either direction - they just stand there. EM
> waves cannot stand still so "EM standing wave" is an oxymoron.
> Quoting one of my college textbooks, "Electrical
> Communication", by Albert:
>
> "Such a plot of voltage is usually referred to as a
> *voltage standing wave* or as a *stationary wave*.
> Neither of these terms is particularly descriptive
> of the phenomenon. A plot of effective values of
> voltage, appearing as in Fig. 6(e), *is not a wave*
> in the usual sense. However, the term "standing wave"
> is in widespread use."
>
> From "College Physics", by Bueche and Hecht:
>
> "These ... patterns are called *standing waves*, as
> compared to the propagating waves considered above.
> They might better not be called waves at all, since
> they do not transport energy and momentum."
Right. All of which deepens the mystery of why you would continue to
insist on claiming that Roy measured standing wave current.
> One can use EZNEC's VERT1.EZ to view the essentially
> unchanging phase on a standing wave monopole. Just look
> at the difference in phase between the feedpoint and a
> point 45 degrees up the antenna. In 45 degrees of antenna,
> the current phase changes by 3.65 degrees. That is the
> current Roy used to measure phase shift through a coil
> in order to support w8ji's 3 nS delay "measurements".
I still need you to explain what standing wave _current_ is, and, just
as importantly how it phase shifts by *TRAVELING* through a coil.
73, ac6xg
Roy Lewallen
> Jim Lux wrote:
>>>. . .
>> No it doesn't do prop delay. It does a steady state model. The TL is
>> just another two port that gets dumped into a giant matrix which is
>> solved as a system of linear equations. Think of TL as a special case
>> of NT.
>
> What kind of two port does NEC use, Jim? What is "just another two port?"
> 73,
> Tom Donaly, KA6RUH
NEC network objects are two port networks described by a set of Y
parameters -- see the NEC-2 documentation on the NT "card". When a
transmission line is specified via a TL "card", the Y parameters
appropriate for the specified line length and Z0 are calculated for a
standard Y parameter network which is then used in the model. In the
code, this is done in the NETWK subroutine between line labels 16 and 17.
EZNEC v. 5.0 allows the user to specify a skin-effect proportional
transmission line loss. It accomplishes this internally by appropriately
modifying the network Y parameters.
Roy Lewallen, W7EL
Cecil Moore
> What's the Z0 of a loading coil, Cecil?
Z0 and VF depend upon the geometry of the coil
*and the frequency*. A 75m Texas Bugcatcher coil
has a Z0 of ~3800 ohms and a VF of ~0.02. The
coil that w8ji used for his 3 nS "measurements"
has a Z0 of ~5300 ohms and a VF of ~0.033. I've
generated an EXCEL file that does the calculations:
http://www.w5dxp.com/CoilZ0VF.xls
I've also got a web page that explains why the
current phase in a standing-wave antenna cannot
be used to measure delay.
http://www.w5dxp.com/current2.htm
I have done the suggested bench experiments myself
and the results are nowhere near w8ji's results.
When traveling wave current is used instead of
standing wave current, the delay is obvious on a
dual-trace O'scope.
This is nothing new. It is based on the information
in the IEEE paper which someone presented years ago:
http://www.ttr.com/TELSIKS2001-MASTER-1.pdf
Owen Duffy
news:rN2dnf-Ppf1If1rU...@posted.easystreetonline:
...
>
> NEC network objects are two port networks described by a set of Y
> parameters -- see the NEC-2 documentation on the NT "card". When a
> transmission line is specified via a TL "card", the Y parameters
> appropriate for the specified line length and Z0 are calculated for a
> standard Y parameter network which is then used in the model. In the
> code, this is done in the NETWK subroutine between line labels 16 and
> 17.
>
> EZNEC v. 5.0 allows the user to specify a skin-effect proportional
> transmission line loss. It accomplishes this internally by
> appropriately modifying the network Y parameters.
I suspected as much from model behaviour.
My question is then, can the 'stub' in figure a) be replaced by a TL
element for a valid model of a)?
Owen
Cecil Moore
> [A standing wave] doesn't really have a 'life' of it's own.
My point exactly, Jim. We may be closer than you think.
> Cecil - as the equation is written, the phase term IS position. The
> phase of the sine function changes with x.
My point exactly, Jim. We may be closer than you think.
Tom and Roy did NOT use position to determine the phase.
That is the entire point of my posting.
> The phase of a time varying function changes with time except in the
> special case of a 'standing wave' function, where it changes with position.
Again, my point exactly - something that (apparently)
neither w8ji or w7el wants to admit.
> Right. He could (and should) have gone on to say that standing waves
> don't really *DO* anything at all.
My point exactly! Now try to tell it to w8ji and w7el
who used primarily standing wave current to prove their
points.
> I still need you to explain what standing wave _current_ is, and, just
> as importantly how it phase shifts by *TRAVELING* through a coil.
My point exactly, Jim. We may be closer than you think.
I am the one who is saying that it doesn't phase shift
while traveling through a wire or a coil. W8JI and W7EL
apparently think that it does phase shift through a coil
and can therefore be used to measure the delay through
a coil. I am the one who disagrees with that concept.
You and I are on the same side, Jim. This reminds me of
the time someone else realized the gurus were wrong and
simply stopped posting in order to save guru face and
to avoid proving them wrong.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com
Jim Kelley
That's true more than you know, which is why I can't figure out why you
keep claiming that someone has measured standing wave current phase
shifts (whatever they are).
73, ac6xg
Roy Lewallen
> Roy Lewallen <w7...@eznec.com> wrote in
> news:rN2dnf-Ppf1If1rU...@posted.easystreetonline:
> ...
>> NEC network objects are two port networks described by a set of Y
>> parameters -- see the NEC-2 documentation on the NT "card". When a
>> transmission line is specified via a TL "card", the Y parameters
>> appropriate for the specified line length and Z0 are calculated for a
>> standard Y parameter network which is then used in the model. In the
>> code, this is done in the NETWK subroutine between line labels 16 and
>
> I suspected as much from model behaviour.
>
> My question is then, can the 'stub' in figure a) be replaced by a TL
> element for a valid model of a)?
>
> Owen
No, it can't. The NEC two-port network object can't have any common mode
current -- the current out of one terminal of a port is always exactly
equal to the current into the other terminal of that port regardless of
external connections, which means that common mode current is zero by
definition. The wire stub, on the other hand, couples to external fields
which can cause common mode current on the wires.
The only time you can substitute a transmission line (network) object
for a wire transmission line is when the transmission line is carrying
no common mode current. An example would be a transmission line
connected to the center of a symmetrical dipole and positioned
symmetrically with respect to the dipole so it gets equal coupling from
both legs.
A coaxial line can be modeled as a combination of a transmission line
(network) object and a wire, the former carrying the differential mode
current and the latter the common mode current, as described in the
EZNEC manual. This is possible because the two components are physically
separated on a coax line. However, I don't know of any way to do the
equivalent thing with a parallel-wire line because the two components
aren't physically separated as they are on coax.
Roy Lewallen, W7EL
Cecil Moore
> That's true more than you know, which is why I can't figure out why you
> keep claiming that someone has measured standing wave current phase
> shifts (whatever they are).
I previously published the currents in a 20m dipole
with 21 segments. Here they are again. Can you comprehend
why there is only a maximum phase shift of 4.54 degrees
in 180 degrees of antenna? That is NOT the characteristic
of traveling wave current. That is the characteristic of
primarily standing wave current which has constant phase.
Roy reported that the phase shift across a loading coil wasn't
measurable which is a true statement because the standing wave
current indeed doesn't change phase across a coil or through a
wire. But he then used that same evidence to support w8ji's
ridiculous 3 nS delay through a 75m mobile loading coil when
there is no relationship between standing wave current phase
and the delay through a loading coil. Traveling wave current
must be used to measure the delay through a loading coil,
something I have been saying for years.
EZNEC+ ver. 4.0
20m dipole 3/23/2009 8:43:06 PM
--------------- CURRENT DATA ---------------
Frequency = 14.2 MHz
Wire No. 1:
Segment Conn Magnitude (A.) Phase (Deg.)
1 Open .09631 -4.54
2 .2561 -4.25
3 .39868 -3.93
4 .5289 -3.60
5 .64603 -3.25
6 .74868 -2.86
7 .83539 -2.43
8 .90483 -1.94
9 .95592 -1.38
10 .98795 -0.68
11 feedpoint 1 0.00
12 .98795 -0.68
13 .95592 -1.38
14 .90483 -1.94
15 .83539 -2.43
16 .74868 -2.86
17 .64602 -3.25
18 .5289 -3.60
19 .39868 -3.93
20 .2561 -4.25
21 Open .09631 -4.54
Owen Duffy
> Owen Duffy wrote:
...
>> My question is then, can the 'stub' in figure a) be replaced by a TL
>> element for a valid model of a)?
>>
>> Owen
>
> No, it can't. The NEC two-port network object can't have any common
> mode
...
Thanks Roy.
Is NEC capable of modelling the configuration shown at
http://www.vk1od.net/lost/King-22.3b.png (which is the same type of
problem as my figure b)?
My attempts to model b) by modelling a plain conductor and inserting a
load in the segment where the open end of the coax stub would otherwise
be, does not result in an in-phase current distribution.
King discusses the coaxial stub and suggests that the conductors need to
be significantly large in diameter, and the stub length would be less
than a quarter wave for in-phase radiator currents.
Owen
Roy Lewallen
>
> Thanks Roy.
>
> Is NEC capable of modelling the configuration shown at
> http://www.vk1od.net/lost/King-22.3b.png (which is the same type of
> problem as my figure b)?
>
> My attempts to model b) by modelling a plain conductor and inserting a
> load in the segment where the open end of the coax stub would otherwise
> be, does not result in an in-phase current distribution.
>
> King discusses the coaxial stub and suggests that the conductors need to
> be significantly large in diameter, and the stub length would be less
> than a quarter wave for in-phase radiator currents.
>
> Owen
Your description of the model is correct. Technically, the wire
representing the outside of the coax (which in the model is located
where the coax line is) should be the diameter of the shield, as we've
discussed before. The stepped wire diameter error of NEC-2 might,
however, result in less accurate results by doing this than by leaving
the diameter the same as the other wires. Experiments with your earlier
b) model showed that the transmission line object characteristics have
almost no effect on the wire currents when it's inserted at a point of
very low current, and that it doesn't result in in-phase current
distribution. That will be true here also at frequencies where the
current is very low near the open end of the stub.
Roy Lewallen, W7EL
Tom Donaly
> Tom Donaly wrote:
>> What's the Z0 of a loading coil, Cecil?
>
> Z0 and VF depend upon the geometry of the coil
> *and the frequency*. A 75m Texas Bugcatcher coil
> has a Z0 of ~3800 ohms and a VF of ~0.02. The
> coil that w8ji used for his 3 nS "measurements"
> has a Z0 of ~5300 ohms and a VF of ~0.033. I've
> generated an EXCEL file that does the calculations:
>
> http://www.w5dxp.com/CoilZ0VF.xls
>
> I've also got a web page that explains why the
> current phase in a standing-wave antenna cannot
> be used to measure delay.
>
> http://www.w5dxp.com/current2.htm
>
> I have done the suggested bench experiments myself
> and the results are nowhere near w8ji's results.
> When traveling wave current is used instead of
> standing wave current, the delay is obvious on a
> dual-trace O'scope.
>
> This is nothing new. It is based on the information
> in the IEEE paper which someone presented years ago:
>
> http://www.ttr.com/TELSIKS2001-MASTER-1.pdf
Still using the Tesla coil fella's ideas, are you? A frequency
dependent Z0 is a good trick. What happens when you double the
length of the coil? Does the Z0 stay the same? What if the coil is
infinite? Can you make a quarter wave shorted stub with it? If you
make it a half wavelength long - keeping in mind the velocity factor -
will the impedance looking into the coil equal the impedance of
the load? How do you attach a load to it?
73,
Tom Donaly, KA6RUH
Owen Duffy
> Owen Duffy wrote:
...
>> Is NEC capable of modelling the configuration shown at
>> http://www.vk1od.net/lost/King-22.3b.png (which is the same type of
>> problem as my figure b)?
A point made by King is that if the three half waves are in phase,
radiation resistance will be quite high (one third current required for
same distant field strength), around 316 ohms against 105 ohms for three
half waves not-in-phase. Presumably these figures are for free space.
This effect is certainly observable in models using my Fig a) (though
half the respective resistances due to the vertical over perfect ground).
The feedpoint impedance looks like it might provide a hint as to whether
currents are actually in-phase.
Exploring that thought, an example (to some extent) of King's Fig 22.3b
is the W5GI Mystery Antenna (see
http://www.w5gi.com/images/w5gimsteryantennaschematic.gif ) which claims
to be three half waves in phase at 14.2MHz. It is very similar to the
diagram above in King though I note that the phasing sections are 105° in
electrical length.
The W5GI is fed with a half wave (at 14.2MHz) of 300 ohm line, then 34'
of RG8X. W5GI reports impedance looking into the RG8X as 42+/-j18. That
suggests the load on the RG8X is 31+j2 or 70-j18. The feedpoint impedance
should be about the same value due to the half wave of 300 ohm low loss
line. Neither impedance is within a bull's roar of 316+j0, and are so low
as to question whether the three half waves are indeed in-phase. (The
highest impedance that would yeild 42+/-j18 on a short length of RG8X
would be around 80+j0, closer to the not-in-phase configuration than the
in-phase configuration).
W5GI's reported feed impedance seem inconsistent with three half waves in
phase, and questions whether the phasing arrangement works as suggested.
Thoughts?
Owen
Cecil Moore
> Still using the Tesla coil fella's ideas, are you?
The title of the article is "RF Coils, ..." The block
diagram of a Tesla coil with a top hat is identical
to a 160m mobile antenna with top hat.
> A frequency dependent Z0 is a good trick.
It's no trick - just based on empirical measurements
as explained in the IEEE paper. Measurements proved
that the Z0 of a coil varies with wavelength so
wavelength is included in the empirical formula.
I observed that phenomenon during my own experiments.
> What happens when you double the length of the coil?
Same thing as doubling the length of a stub. At a fixed
frequency, the delay through the coil is (roughly) doubled.
> Does the Z0 stay the same? What if the coil is infinite?
Length of the coil does not appear in the empirical
formula for Z0 of a coil. Coil diameter, TPI, and
wavelength are the variables. Wire diameter would
obviously have some effect but is not included in
the empirical formula.
> Can you make a quarter wave shorted stub with it?
Yes, but you need a ground plane close by. Mininec ground will do.
Here's a 75m Texas Bugcatcher coil loaded with its Z0 impedance
modeled over Mininec ground.
http://www.w5dxp.com/coil505u.EZ
The current phase shift through the coil is clearly visible
by displaying "Load Dat". The delay through the coil (EZNEC)
is roughly proportional to the phase shift, i.e. about 38
degrees. The coil is 0.5 feet long with a calculated VF
of 0.02 so the calculated phase shift (without EZNEC) is
about 36 degrees. That's pretty close agreement.
> If you make it a half wavelength long - keeping in mind the velocity
factor -
> will the impedance looking into the coil equal the impedance of the load?
> How do you attach a load to it?
Here's the Texas Bugcatcher coil modeled at the first (1/4WL)
self-resonant frequency of 7.96 MHz:
http://www.w5dxp.com/coil505s.EZ
I have not experimented with 1/2WL self-resonance. The above
file seems to be 1/2WL self-resonant at about 19.2 MHz but
the 75m Texas Bugcatcher coil, at 19.2 MHz, does not meet
the guidelines for validity given in the IEEE article.
Reference:
http://www.w5dxp.com/current2.htm
http://www.w5dxp.com/current.htm
http://www.w5dxp.com/CoilZ0VF.xls
Cecil Moore
> The feedpoint impedance looks like it might provide a hint as to whether
> currents are actually in-phase.
At a 1/4WL monopole's resonant frequency, the forward antenna
current and reflected antenna current are in phase. The two
component voltages are 180 degrees out of phase. The feedpoint
resistance is [|Vfor|-|Vref|]/[|Ifor|+|Iref|] where these are
antenna voltages and currents on a standing-wave antenna.
If the feedpoint impedance is purely resistive it appears
that the two component waves must be in phase or 180 degrees
out of phase.
Jim Kelley
> Roy reported that the phase shift across a loading coil wasn't
> measurable which is a true statement because the standing wave
> current indeed doesn't change phase across a coil or through a
> wire.
He wasn't measuring "standing wave current", whatever that is. You
should probably examine his test setup more carefully.
> But he then used that same evidence to support w8ji's
> ridiculous 3 nS delay through a 75m mobile loading coil when
> there is no relationship between standing wave current phase
> and the delay through a loading coil. Traveling wave current
> must be used to measure the delay through a loading coil,
> something I have been saying for years.
And after all those years you still haven't provided any measurements
that support what you've been saying. And as far as I know, neither has
anyone else. But I'm happy to stand corrected.
73, ac6xg
Cecil Moore
> He wasn't measuring "standing wave current", whatever that is.
Sorry Jim, of course he was, since standing wave current
is the primary current that exists on standing wave
antennas like the antenna Roy used to measure his currents.
You keep saying "whatever that is" when it is well
defined in most any antenna book. That you don't
understand standing wave current on standing wave
antennas is just a statement of ignorance - no
offense intended - apparently Roy is just as ignorant.
Perhaps you should study and understand the
difference between a standing wave antenna like
a dipole and a traveling wave antenna like a
terminated Rhombic. Balanis has a good discussion
of such. Here's a quote: "Standing wave antennas,
such as the dipole, can be analyzed as traveling
wave antennas with waves propagating in opposite
directions (forward and backward) and represented
by traveling wave currents..."
An inverted-V dipole can be converted from a
standing wave antenna to a traveling wave antenna
by terminating the ends with a load connected to
mininec ground. Here is an inv_V and a terminated
inv_V modeled in EZNEC. Please look at the "Currents"
display until you understand the meaning of the
phase angles.
http://www.w5dxp.com/inv_v.EZ (standing wave antenna)
Phase angle of the current varies by 2.72 degrees
along each 90 degrees of antenna. This is the current
that Roy used.
http://www.w5dxp.com/inv_vT.EZ (traveling wave antenna)
Phase angle of the current varies by 90 degrees
along each 90 degrees of antenna. This is the current
that Roy should have used.
Jim Kelley
> Jim Kelley wrote:
>> He wasn't measuring "standing wave current", whatever that is.
>
> Sorry Jim, of course he was, since standing wave current
> is the primary current that exists on standing wave
> antennas like the antenna Roy used to measure his currents.
The only current flowing on an antenna is the current traveling from one
end to the other.
> You keep saying "whatever that is" when it is well
> defined in most any antenna book.
I have the ARRL Antenna Book. Where might I find 'Standing Wave
Current' defined, or at least a description of how to measure it?
Perhaps it's in a section about 'Standing Wave Power'?
> That you don't
> understand standing wave current on standing wave
> antennas is just a statement of ignorance - no
> offense intended - apparently Roy is just as ignorant.
Sounds authoratative. I wonder if anyone is buying it?
73, ac6xg
Cecil Moore
> The only current flowing on an antenna is the current traveling from one
> end to the other.
Since standing waves cannot exist without the underlying
component traveling waves, to avoid conceptual blunders,
one needs to deal directly with the component traveling
waves. Your statement is based on a purely mathematical
shortcut which exists only in the human brain, not in
reality, and obscures the actual speed-of-light physics
necessary for an EM wave to even exist.
The current can be artificially parsed the way you
are doing it but that parsing leads to the very
misconception under which you are laboring. The same
thing happened with w8ji's and w7el's "measurements"
involving delays through loading coils. The actual
component physics, as explained in any reasonably
technical antenna book is:
Total current = forward current + reflected current
Itot = Ifor + Iref (phasor addition)
Reference: "Antenna Theory", Balanis, 2nd edition
Balanis, page 488:
"The sinusoidal current distribution of long open-ended
linear antennas is a standing wave constructed by two
waves of equal amplitude and 180 degrees phase difference
at the open end traveling in opposite directions along
its length. ... The current and voltage distributions
on open-ended wire antennas are similar to the standing
wave patterns on open-ended transmission lines."
Balanis, page 489:
"Standing wave antennas, such as the dipole, can be
analyzed as traveling wave antennas with waves
propagating in opposite direstions (forward and
backwards) and and represented by traveling wave
currents, If and Ib in Figure 10.1a."
In a standing wave antenna, e.g. a 1/2WL dipole, there
exists a forward wave that gives up about 10% of its
energy content to radiation. The remaining 90% of the
wave encounters the open end of the antenna and is
reflected. So, just as in the case of an open-circuit
stub, we have a forward current component flowing in
one direction and a reflected current component flowing
in the other direction. Many of the mistakes and mis-
conceptions about antennas are based on your false
assertion above.
> I have the ARRL Antenna Book.
:-) The ARRL Antenna Book doesn't even have "traveling
wave antennas" in its index. It does state: "Unterminated
long-wire antennas are often referred to as 'standing
wave antennas'". Please reference a reasonably technical
antenna book like "Antennas", by Kraus. "A sinusoidal
current distribution (on a standing wave antenna) may be
regarded as the standing wave produced by two uniform
(unattenuated) traveling waves of equal amplitude moving
in opposite directions along the antenna."
> I wonder if anyone is buying it?
It doesn't matter if anyone is buying it. What matters
is technical validity. Your first statement above is
technical invalid. Given the free space description of
standing waves of light given by Hecht in "Optics", your
assertion above would lead one to believe that the photons
comprising the standing wave of light must be at rest even
though that's an impossibility (except in the human mind).
Here's what a couple of references say about standing waves.
"Electrical Communication", by Albert:
"Such a plot of voltage is usually referred to as a
*voltage standing wave* or as a *stationary wave*.
Neither of these terms is particularly descriptive
of the phenomenon. A plot of effective values of
voltage, appearing as in Fig. 6(e), *is not a wave*
in the usual sense. However, the term "standing wave"
is in widespread use."
"College Physics", by Bueche and Hecht:
"These ... patterns are called *standing waves*, as
compared to the propagating waves considered above.
*They might better not be called waves at all*, since
they do not transport energy and momentum."
Cecil Moore
> The only current flowing on an antenna is the current traveling from one
> end to the other.
Let's assume you are correct. Here are a few questions:
1. Given a 90 degree monopole fed against an infinite
ground plane, what would be the phase at the top of the
antenna compared to the phase at the feedpoint for any
instant in time?
2. Why would the feedpoint impedance of a 1/4WL monopole
be more than a magnitude less than the feedpoint impedance
of an infinite monopole?
3. Where does the above current go when it hits the open-
circuit at the top of the monopole?
4. Why is the total energy in the E-field at the top of the
monopole so high?
Roy Lewallen
> Roy Lewallen <w7...@eznec.com> wrote in
> news:0qOdnaGj7oOsGVXU...@posted.easystreetonline:
>
>> Owen Duffy wrote:
> ...
>>> Is NEC capable of modelling the configuration shown at
>>> http://www.vk1od.net/lost/King-22.3b.png (which is the same type of
>>> problem as my figure b)?
>
> A point made by King is that if the three half waves are in phase,
> radiation resistance will be quite high (one third current required for
> same distant field strength), around 316 ohms against 105 ohms for three
> half waves not-in-phase. Presumably these figures are for free space.
I looked up the section in King, Mimno, and Wing and was pretty
disappointed. It's one of my favorite references, and I usually find the
explanations clear. But the description of that antenna is pretty vague,
with considerable hand waving ("[Operation of coaxial stubs] is much
less satisfactory than that with the open-wire stubs. . ." without
explaining why). And in the explanation of the open-wire stubs, the
authors seem to state that the wires must carry purely differential
currents. And their models (Fig. 22-4) do show purely differential
coupling from the antenna to the stubs.
I speculate that they really didn't understand how these antennas
worked, had discovered that the coaxial sleeve versions didn't work or
at least didn't work as well -- and didn't show the proper impedance --,
but didn't fully understand why. King, in particular, was and is one of
the giants of antenna theory, and leaves us a lifetime of brilliant
insight and rigorous mathematical analysis. But at least at the time
that book was published, they lacked the modeling tools we have today.
> This effect is certainly observable in models using my Fig a) (though
> half the respective resistances due to the vertical over perfect ground).
>
> The feedpoint impedance looks like it might provide a hint as to whether
> currents are actually in-phase.
It surely does. Given the currents on and locations of the end wires,
the modification to the center wire can be calculated from mutual
coupling considerations. And I think this is a clue that led King,
Mimno, and Wing to conclude that something was amiss with the coaxial
version.
> Exploring that thought, an example (to some extent) of King's Fig 22.3b
> is the W5GI Mystery Antenna (see
> http://www.w5gi.com/images/w5gimsteryantennaschematic.gif ) which claims
> to be three half waves in phase at 14.2MHz. It is very similar to the
> diagram above in King though I note that the phasing sections are 105° in
> electrical length.
>
> The W5GI is fed with a half wave (at 14.2MHz) of 300 ohm line, then 34'
> of RG8X. W5GI reports impedance looking into the RG8X as 42+/-j18. That
> suggests the load on the RG8X is 31+j2 or 70-j18. The feedpoint impedance
> should be about the same value due to the half wave of 300 ohm low loss
> line. Neither impedance is within a bull's roar of 316+j0, and are so low
> as to question whether the three half waves are indeed in-phase. (The
> highest impedance that would yeild 42+/-j18 on a short length of RG8X
> would be around 80+j0, closer to the not-in-phase configuration than the
> in-phase configuration).
>
> W5GI's reported feed impedance seem inconsistent with three half waves in
> phase, and questions whether the phasing arrangement works as suggested.
>
> Thoughts?
I doubt that it does.
Roy Lewallen, W7EL
Owen Duffy
Roy Lewallen <w7...@eznec.com> wrote in
news:VsSdnUxUxq60P1fU...@posted.easystreetonline:
> Owen Duffy wrote:
>> Roy Lewallen <w7...@eznec.com> wrote in
>> news:0qOdnaGj7oOsGVXU...@posted.easystreetonline:
>>
>>> Owen Duffy wrote:
>> ...
>>>> Is NEC capable of modelling the configuration shown at
>>>> http://www.vk1od.net/lost/King-22.3b.png (which is the same type
>>>> of problem as my figure b)?
>>
>> A point made by King is that if the three half waves are in phase,
>> radiation resistance will be quite high (one third current required
>> for same distant field strength), around 316 ohms against 105 ohms
>> for three half waves not-in-phase. Presumably these figures are for
>> free space. . . .
>
> I looked up the section in King, Mimno, and Wing and was pretty
> disappointed. It's one of my favorite references, and I usually find
> the explanations clear. But the description of that antenna is pretty
> vague, with considerable hand waving ("[Operation of coaxial stubs] is
> much less satisfactory than that with the open-wire stubs. . ."
> without explaining why). And in the explanation of the open-wire
This lesser mortal was encouraged that he noted the difference, but there
really was no explanation. My feeling is that to note the difference but
to be unable to explain it, other than nebuluous conditions like the
coaxial tubes must be large diameter ratio, is incomplete... a problem
yet to be solved.
I have come to the conclusion that the coaxial tubes are not simply a
relocation of a TL as popularly explained. Over the years, I have
accumulated a few projects that were works of art, but didn't work
properly... and they all used coaxial phasing sections.
> stubs, the authors seem to state that the wires must carry purely
> differential currents. And their models (Fig. 22-4) do show purely
> differential coupling from the antenna to the stubs.
>
> I speculate that they really didn't understand how these antennas
> worked, had discovered that the coaxial sleeve versions didn't work or
> at least didn't work as well -- and didn't show the proper impedance
> --, but didn't fully understand why. King, in particular, was and is
> one of the giants of antenna theory, and leaves us a lifetime of
> brilliant insight and rigorous mathematical analysis. But at least at
> the time that book was published, they lacked the modeling tools we
> have today.
Understood... but, I think after our discussion on this, NEC is not up to
the task, it may take a more advanced EM field modelling tool.
My suspicion is that NEC's shortfall is that a TL element does not
properly represent the coaxial stub and its interaction with the other
elements near resonance, though well away from resonance, it is possible
that it may be quite ok. King raises the issues of diameter ratios, and
the difference with whether the stub is inboard or outboard of the o/c
end... but it is not resolved quantitatively.
Now W5GI does introduce his antenna with the statement "A multi-band wire
antenna that performs exceptionally well even though it confounds antenna
modeling software".
I know that is almost always a harbinger of bunk, the proverbial "Danger
Will Robinson...", but in fairness, it does appear that one modelling
package, NEC, cannot adequately model the coaxial arrangement near
resonance, though in his antenna, the coax section would be resonant
around 12MHz and King suggests it ought to be much shorter (resonant well
above 14MHz).
That is not to say there aren't other BS warnings in the W5GI explanation
of operation, or claims of performance.
Thanks for your comments, I find this an interesting subject.
Owen
Roy Lewallen
> . . .
> Understood... but, I think after our discussion on this, NEC is not up to
> the task, it may take a more advanced EM field modelling tool.
I don't agree with this.
> My suspicion is that NEC's shortfall is that a TL element does not
> properly represent the coaxial stub and its interaction with the other
> elements near resonance, though well away from resonance, it is possible
> that it may be quite ok. King raises the issues of diameter ratios, and
> the difference with whether the stub is inboard or outboard of the o/c
> end... but it is not resolved quantitatively.
I believe that NEC can do a fine job of modeling any of the variations
we've been discussing. But like all modeling systems, it has to be used
properly -- the transmission line object isn't an adequate model for
either a coaxial structure or an open wire stub, if either is carrying
any common mode current. And in these antennas it is, so you can't
insist on using nothing more than a transmission line object and then
bemoaning that the result isn't correct. The wire stub variation can be
correctly modeled as wires. The coaxial structure can be correctly
modeled as a combination of a wire and transmission line object. In
either case I have high confidence that carefully and accurately
measured results will agree closely with NEC predictions.
> Now W5GI does introduce his antenna with the statement "A multi-band wire
> antenna that performs exceptionally well even though it confounds antenna
> modeling software".
>
> I know that is almost always a harbinger of bunk, the proverbial "Danger
> Will Robinson...", but in fairness, it does appear that one modelling
> package, NEC, cannot adequately model the coaxial arrangement near
> resonance, though in his antenna, the coax section would be resonant
> around 12MHz and King suggests it ought to be much shorter (resonant well
> above 14MHz).
It doesn't appear this way to me at all. What has led you to the
conclusion that it isn't possible to accurately model it with NEC?
Again, it's certainly impossible if you use only a transmission line
object to represent a structure which has common mode current. There are
many ways to build a model which doesn't accurately represent the
antenna being modeled. But just because it's possible to make a bad
model doesn't mean it's impossible to make a good one.
What is the evidence that results from a properly designed NEC model
disagree with careful measurements of pattern, current, or impedance of
an actual antenna of these types? You've noted that the W5GI antenna
impedance isn't consistent with a correctly phased collinear. I'd be
surprised if the impedance isn't close to what a correct NEC model
predicts -- or that the phases of the currents aren't also what NEC
predicts.
> That is not to say there aren't other BS warnings in the W5GI explanation
> of operation, or claims of performance.
>
> Thanks for your comments, I find this an interesting subject.
Me too, and thanks for bringing it up. I'd never taken a really close
look at this class of antenna before, and the results have been interesting.
Roy Lewallen, W7EL
Owen Duffy
> Owen Duffy wrote:
...
> I believe that NEC can do a fine job of modeling any of the variations
> we've been discussing. But like all modeling systems, it has to be
> used properly -- the transmission line object isn't an adequate model
> for either a coaxial structure or an open wire stub, if either is
> carrying any common mode current. And in these antennas it is, so you
> can't insist on using nothing more than a transmission line object and
> then bemoaning that the result isn't correct. The wire stub variation
> can be correctly modeled as wires. The coaxial structure can be
> correctly modeled as a combination of a wire and transmission line
> object. In either case I have high confidence that carefully and
> accurately measured results will agree closely with NEC predictions.
Taking the W5GI as an example, here is a deck that models the coaxial
stub section as a conductor of 5mm dia, whilst the wires for the other
sections are 2mm diameter. I have calculated the impedance looking into
16.5' of RG8X (W5GI's specified stub) as 14.5-j179 at 14.2MHz, and
inserted that load in both of the segments where the o/c end of the stub
is located. I have not used a TL element, rather I have separately
calculated the input Z of the stub using the technique used at
http://www.vk1od.net/calc/tl/tllc.php , that should be more accurate than
using a lossless TL element.
The model assumes an effective balun, ie that there is no common mode
feedline current since I have not provided such a path.
CM W5GI Mystery Antenna
CM Extended thin wire kernel used
CM
CE
GW 1 31 -5.033 0.000 10.563 5.033 0.000 10.563 0.001000
GW 2 15 -10.067 0.000 10.563 -5.033 0.000 10.563 0.002500
GW 3 15 -15.100 0.000 10.563 -10.067 0.000 10.563 0.001000
GW 4 15 5.033 0.000 10.563 10.067 0.000 10.563 0.002500
GW 5 15 10.067 0.000 10.563 15.100 0.000 10.563 0.001000
GE
EK
FR 0,1,0,0,14.200
EX 0 1 16 0 1 0
LD 5 0 0 0 5.7E7
LD 4 1 1 1 14.505 -191.739 0
LD 4 1 31 31 14.505 -191.739 0
GN 2 0 0 0 13 0.005
XQ
EN
This model indicates out of phase operation of the antenna, a multi lobed
pattern and feedpoint Z of 115-j179. (Although there is a half wave of
300 ohm line in between, this feedpoint Z would cause VSWR=8 on the 50
ohms line.
I think that I have dealt with the common mode path properly.
Try as I might changing stub lengths etc, I cannot get this configuration
to deliver in-phase operation of the radiator.
I suspect the model is not valid.
Owen
Jon K Hellan LA4RT
cage of wires around the center conductor? Would the orders of
magnitude difference between shield/center distance and wire lengths
cause problems?
73
Jon LA4RT, Trondheim, Norway
Owen Duffy
I have spent a lot of time exploring different modelling options over
recent weeks.
One view that one might take re my fig a) is that at connection of the
stub with the main vertical, the stub offers low impedance to common mode
current and high impedance to differential current. It leads to thinking
of it as a kind of mode trap that guides the system into in-phase
operation.
I have played around with ways of trying to represent that without using
the wire segments of the stub.
One method was to place a transformer with only one centre tapped
winding. The top and bottom of the winding connect to the upper half wave
and the lower quarter wave respectively, and the centre tap connects to a
horizontal quarter wave. My thinking was that this structure provides low
impedance to common mode current on the horizontal section, but a high
impedance to differential input to the top and bottom of the transformer
winding.
The model achieves reasonably good in-phase operation, but works best
with about 0.35 wave horizontal. I have used an NT card to insert the
transformer windings in the two segments. Here is the deck.
CM
CE
GW 1 15 0 0 0 0 0 5 0.005
GW 2 15 0 0 5 0 0 15 0.005
GW 3 15 0 0 5 7.2 0 5 0.005
GE 1
NT 1 15 2 1 0 0.01 0 -0.01 0 0.01
GN 1
EK
EX 0 1 1 1 0
TL 1 15 2 1 100 0
FR 0 0 0 0 15 0
EN
I then tried changing the horizontal section to two opposed radial wires,
and found that worked well with each radial being about 0.2 wave long.
CM
CE
GW 1 15 0 0 0 0 0 5 0.005
GW 2 15 0 0 5 0 0 15 0.005
GW 3 15 0 0 5 4 0 5 0.005
GW 4 15 0 0 5 -4 0 5 0.005
GE 1
NT 1 15 2 1 0 0.01 0 -0.01 0 0.01
GN 1
EK
EX 6 1 1 1 0
FR 0 0 0 0 15 0
EN
One can achieve similar outcome by wiring an appropriately phased zero
length TL between the segments each side of the horizontal wire.
If these models indicate that the common mode path on the horizontal wire
is important, one loses control of the length of that in the case of the
coaxial configuration because there isn't an o/c end indpendent of the
vertical conductor.
The coaxial construction gives the opportunity to create a high impedance
to differential current between the adjacent segments, but lacks the
ability to create a low impedance common mode path independently of the
vertical structure.
Thoughts?
Owen
Jim Kelley
> Jim Kelley wrote:
>> The only current flowing on an antenna is the current traveling from
>> one end to the other.
> Let's assume you are correct. Here are a few questions:
>
> 1. Given a 90 degree monopole fed against an infinite
> ground plane, what would be the phase at the top of the
> antenna compared to the phase at the feedpoint for any
> instant in time?
>
> 2. Why would the feedpoint impedance of a 1/4WL monopole
> be more than a magnitude less than the feedpoint impedance
> of an infinite monopole?
>
> 3. Where does the above current go when it hits the open-
> circuit at the top of the monopole?
>
> 4. Why is the total energy in the E-field at the top of the
> monopole so high?
In what way are any of the questions relevant to, or deterministic of
the assumption?
73, ac6xg
Richard Clark
wrote:
Ah Jim!
You have the essence of Cecil's (r) Sub-optimal Conjugated Hypothesis
Information Transform before you, the SCHIT (c) model.
He has taken the ordinary postulate of current flow, conjugated it
into a new hypothesis through his sub-optimization. By removing
random bytes, it becomes more intelligible (I will take a stab at it
here):
1. a 90 degree monopole fed against an infinite ground plane
2. the feedpoint impedance of a 1/4WL monopole
3. current go[es]
4. the total energy
now makes perfect sense and whitens your teeth at the same time.
Returning to the process - through sub-optimization by adding
bafflegab, the future deconstruction (posts that would follow the one
above and for which I have already deconvoluted) would find Cecil
eventually unwinding the original conjugation, proving he was right by
proving you right - except you were wrong in what you "thought" (the
information transform) because he thought you were wrong.
73's
Richard Clark, KB7QHC
Cecil Moore
> In what way are any of the questions relevant to, or deterministic of
> the assumption?
Answering a question with a question is a well known
diversion. Please answer my questions and you will
automatically answer yours.
Here's some more: How can a current that changes
phase by 3 degrees in 90 degrees of wire be used
to measure the EM wave delay through the wire?
How can that current be used to measure the delay
through a coil positioned in the middle of that wire?
How fast does EM wave energy travel through a wire?
Cecil Moore
> Returning to the process - through sub-optimization by adding
As far as bafflegab goes, Richard, no one can hold a candle
to you. Your posting is a perfect example.
Richard Clark
wrote:
>Richard Clark wrote:
>> Returning to the process - through sub-optimization by adding
>> bafflegab, ...
>
>As far as bafflegab goes, Richard, no one can hold a candle
>to you. Your posting is a perfect example.
You must be flattered (an example of information transformation) at
this imitation of you then (your comment here so unabashedly basking
in the intended conjugate of these congratulations).
[Gad this so easy, I should have gotten an AIG bonus for derivative
design!]
:-0
Cecil Moore
> You must be flattered (an example of information transformation) at
> this imitation of you then (your comment here so unabashedly basking
> in the intended conjugate of these congratulations).
Just send me some of what you are smoking and I will die happy. :-)
Richard Clark
wrote:
>Richard Clark wrote:
>> You must be flattered (an example of information transformation) at
>> this imitation of you then (your comment here so unabashedly basking
>> in the intended conjugate of these congratulations).
>
>Just send me some of what you are smoking and I will die happy. :-)
A double conjugation which reveals the source of this side thread.
Deconstructing the bafflegab by random byte dispersal gives us Cecil's
information transform:
>what I am smoking isn't good enough.
[gad, this is so easy I could be double-dipping AIG bonuses and
getting favored IRS status too!]
Owen Duffy
news:Xns9BDA99F99B...@61.9.191.5:
...
> I suspect the model is not valid.
I should have explained that the reason for that suspicion that the model
does not predict behaviour similar to W5GI's claims, most importantly three
half waves in phase on 20m.
Owen
Owen Duffy
61.9.191.5:
> NT 1 15 2 1 0 0.01 0 -0.01 0 0.01
>
Ouch, the signs of the Y values should be the opposite, so
NT 1 15 2 1 0 -0.01 0 0.01 0 -0.01
Leakage reactance is usually +ve, so Y11 should be -ve, etc.
It has a similar effect, but correct signs is better.
Apologies.
Owen
Richard Clark
wrote:
>I'm very naive in these matters. Could a coaxial stub be modeled as a
>cage of wires around the center conductor? Would the orders of
>magnitude difference between shield/center distance and wire lengths
>cause problems?
Hi Jon,
I offered that model long ago in this thread - as it was ignored, I
was condemned to use it myself.
I had given some thought ahead of plunging ahead into the model (it
was originally a thick radiator for which the model was perfectly
suitable). The concept of coaxial tube shielding proceeds along the
premise of the shield supporting separate conduction paths, isolated
by skin effect of the tube conductor. That is, the currents of the
shield on the inside surface are separate and distinct from those on
the outside surface. I knew full well that NEC would not maintain
that distinction for any wire in a cage simply because it lacks the
ability to report separate currents along the same wire as would be
found in this inside/outside tube surface.
The model I published and provided the link to here in this thread was
not strictly faithful to the concept of the cage model for a coaxial
tube, however. I enhanced it into roughly 1000 wires emulating a cage
10.5M long, 2M in diameter, with hoops every 33cM along its length,
and closed at both ends. Think of it as a roll of mesh with a 1 foot
grid capped at both ends with radial wires. Within it is a length of
wire that is roughly 10M long and isolated from the cage at both ends.
With the wire loss set to perfect, the central wire was driven and it
was as though no shielding cage existed. Within tenths of a dB, the
radiation characteristic across HF was roughly the same as from a
simple wire dipole. Conceptually, it would appear that the Faraday
shield does not exist in the world of NEC.
When I introduced the copper setting for wire loss, this assemblage
exhibited the following "loss"
MHz
1 24.2
2 16.6
3 14.2
4 12.8
5 12.1
6 12.1
7 13.4
8 18.8
9 19.0
10 6.6
11 3.0
12 2.2
13 2.0
14 1.8
15 1.6
16 1.7
17 1.5
18 1.4
Following this, I connected the ends of the coaxial interior wire to
the caps at the tube ends (a complete short circuit). Losses in the
left column (where significant); lobe peak in the right column (where
significant):
MHz
1 56.1
2 39.4
3 29.3
4 21.9
5 15.8
6 10.5
7 6.0
8 2.4
9 0.2 1.8 dBi
10 2.7 dBi
11 2.8 dBi
12 2.7 dBi
13 2.5 dBi
14 2.3 dBi
15 0.2 2.1 dBi
16 0.5
17 0.7
18 1.1
So, to your question:
>Could a coaxial stub be modeled as a
>cage of wires around the center conductor?
No, not if my experience bears any relevance.
Owen Duffy
news:v99vs4h2ffv9n3a26...@4ax.com:
> On Thu, 26 Mar 2009 09:35:06 +0100, Jon K Hellan LA4RT <d...@null.com>
> wrote:
>
>>I'm very naive in these matters. Could a coaxial stub be modeled as a
>>cage of wires around the center conductor? Would the orders of
...
> So, to your question:
>>Could a coaxial stub be modeled as a
>>cage of wires around the center conductor?
> No, not if my experience bears any relevance.
Hi Jon, Richard,
I considered the same, and I did model some simpler structures to explore
some possible effects.
Although it would be possible to create a cylindrical structure of GW
elements, my concern was that it would not have the near complete
isolation of inner and outer surfaces of the outer conductor, that it
might need be be very large in diameter in terms of wavelength, and that
it moves further away from practical commercial coaxial lines.
I have been quiet here, but have been modelling and writing notes up on
the results. I have asked for comment on a draft model, and subject to
that, I will post the URL for further comments, hopefully in a day or
two.
The effort was really about understanding whether the stub in my fig a)
could simply be replaced by a pure differential mode transmission line,
and whether that could then be coaxially collinear with the main
radiator. I think the answer to the first question is NO, and that drives
the answer to the second question.
Owen
Jim Kelley
> Jim Kelley wrote:
>> In what way are any of the questions relevant to, or deterministic of
>> the assumption?
>
> Answering a question with a question is a well known
> diversion. Please answer my questions and you will
> automatically answer yours.
One could claim that the questions exemplify your point about diversion.
:-)
> Here's some more: How can a current that changes
> phase by 3 degrees in 90 degrees of wire be used
> to measure the EM wave delay through the wire?
> How can that current be used to measure the delay
> through a coil positioned in the middle of that wire?
Assuming the antenna is 90 degrees in length, the relevant currents can
be measured, the maximum is known and the minimum is zero, then:
According to the plots that I've seen, the standing wave pattern will
show a discontinuous change in amplitude at positions where there is an
abrupt change in phase of the traveling waves. Since it's fair to
assume propagation velocity is the same in both directions, waves would
be phase delayed by the same amount in both directions at a
discontinuity, and the combined sum of the two delays would account for
the total delay and for the resulting change in amplitude. Since a
standing wave can be considered an amplitude vs phase plot (where both
phase and amplitude vary with position) and the amplitude is known on
both sides of the discontinuity, the amplitude on each side of the
discontinuity relates functionally to a corresponding phase on the
abscissa of the standing wave curve. The total change in phase is equal
to the difference in phase on the two sides of the discontinuity. The
phase delay for each traveling wave is then half the total phase change.
Whether all of the assumptions are true for the cited case, I don't
know. The assumptions that you've made are not always clearly or
completely communicated, but would obviously weight heavily in the
results. This is also true for EZNEC results.
Why not take some actual phase shift measurements for yourself?
73, ac6xg
Cecil Moore
> Why not take some actual phase shift measurements for yourself?
I have already done that at my previous QTH and
reported it two years ago. Remember these graphs
from software that you recommended?
http://www.w5dxp.com/travstnd.gif
My dual-trace scope measurements agreed within
the accuracy to which I could measure.
Point is that the delay through a transmission
line, a wire, or a coil is the same no matter
what type of current (standing wave or traveling
wave) is flowing. EM waves are EM waves. If the
current is primarily standing wave current with
essentially unchanging phase, the phase shift
in the standing wave current is unrelated to the
delay through the T-line, wire, or coil. Yet
standing wave current phase is what was used to
"prove" a 3 nS delay through a 100T, 2" dia, 10TPI
coil on 75m. If traveling wave current had been
used, as I did on my 75m Texas Bugcatcher coil,
the delay would have been shown to be ~26 nS.
In a 1/4WL monopole or 1/2WL dipole, the total
current is about 90% standing wave current.
Did you take a look at the current phase in these
two inverted-Vs?
http://www.w5dxp.com/inv_vt.EZ
Jim Kelley
It's like having a conversation with a recorded message.
ac6xg
Roy Lewallen
>
> It's like having a conversation with a recorded message.
>
> ac6xg
Exactly why I plonked him a few years ago. The relative silence is
refreshing, and I haven't missed a thing.
Roy Lewallen, W7EL
Cecil Moore
> Exactly why I plonked him a few years ago.
Roy, I remember it well. You plonked me when I
reminded you that an antenna is a distributed
network and NOT a lumped circuit. Anyone can
verify that simply by googling the newsgroup.
Cecil Moore
> It's like having a conversation with a recorded message.
On my end, it's like trying to communicate with
an I/O interface that is all output and no input.
I have no other choice but to repeat the questions
that you, so far, have refused to answer.
So here is it again: How can one use the total
current in a 1/4WL monopole, which changes phase
by 3 degrees in 90 degrees of antenna, to measure
the delay through a wire or a loading coil?
It's a really simple question, Jim. Either one
can or one cannot.
Owen Duffy
...
> I have been quiet here, but have been modelling and writing notes up
> on the results. I have asked for comment on a draft model, and subject
> to that, I will post the URL for further comments, hopefully in a day
> or two.
http://www.vk1od.net/antenna/ccps/index.htm
Owen
Jim Kelley
Cecil Moore wrote:
> Jim Kelley wrote:
> > It's like having a conversation with a recorded message.
> I have no other choice but to repeat the questions
> that you, so far, have refused to answer.
Beeep. Check Google newsgroups, yesterday at 7:11 PM.
ac6xg
Richard Clark
Hi Jim,
You have experienced the famous time conjugated
answer-preceding-the-question paradox of Cecil's information
transformation. What Cecil is saying (we are now employing the random
byte discard from the data babblefield):
>>you have misunderstood your answer to the question I am asking now.
Cecil Moore
> Beeep. Check Google newsgroups, yesterday at 7:11 PM.
Beeep. Check this very newsgroup. I already responded
to that posting. You didn't answer the question. We
are not talking about discontinuities. We are talking
about a straight 1/4WL piece of wire. So allow me to
keep asking until I get a reasonable response:
EZNEC says there is ~3 degrees of phase change in the
current in 90 degrees of monopole. How can that current
be used to measure the delay through 'n' degrees of
monopole?
For instance - in 30 degrees of monopole, the current
shifts phase by one degree. What *exactly* does that
indicate? Wouldn't the delay be more accurately
measured by comparing the ARCCOSine of the amplitudes?
Cecil Moore
> You have experienced the famous time conjugated
> answer-preceding-the-question paradox of Cecil's information
> transformation.
OK, Richard, I admit that you caught me asking rhetorical
questions - Congratulations!
Richard Clark
Hi Owen,
This is a lot to digest at this time, but at least it is all in one
place and done well.
You offer several many solutions (to what are arguably straw men
hypothesis such as the W5GI mystery antenna) and there are certainly
gaps that beg filling: Fig. 2 is rather anemic as is the original
supporting commentary.
I offered a comment long ago that my best guess was that phasing would
seem to separate your two examples. That seems to have been both
vindicated and rejected through the numerous examples - I've lost
track of the focus.
I will have to revisit the comments in this thread and tie them to the
cogent points of your page.
Owen Duffy
Richard,
If it doesn't come through on an initial read, I have tried to deconstruct
the fig a) / Fig 1 configuration, and then synthesise it with another
structure with and without the common mode current path, and observe the
effect on current distribution... then compare that with the TL
representation.
If the development isn't clear, I have some more work to do!
...
> I will have to revisit the comments in this thread and tie them to the
> cogent points of your page.
Constructive comments are always welcome, and appreciated.
Thanks
Owen
Jim Kelley
> Jim Kelley wrote:
> current in 90 degrees of monopole. How can that current
> be used to measure the delay through 'n' degrees of
> monopole?
I have absolutely no idea. Sounds like you've made an error somewhere.
ac6xg
Richard Clark
>> I will have to revisit the comments in this thread and tie them to the
>> cogent points of your page.
>
>Constructive comments are always welcome, and appreciated.
Hi Owen,
I can well appreciate the issue of common mode driven by coupling to
the field. The work-arounds I would have expected Roy to have offered
would have been a combination of the TL faculty of NEC for the
differential mode, and an appendix-like wire to support the common
mode contribution. The lack of this discussion where it often appears
in other threads leaves me to wonder if other issues are being
discussed here; hence my problem with topic focus.
As for the modeling of a coaxial transmission line by wires, I have
fairly convinced myself that that approach is thoroughly dead (having
seen no contrary response to my comment about the concept of a Faraday
Shield being unknown to NEC).
By these two, it would seem that modeling coaxial components in NEC is
intractable and claims applied to their use will only be
proven/disproven in the lab or the field.
Proceeding from this last conclusion, I cannot see any purpose to the
comparison of the two colinear representations. You certainly bring
many issues to bear, but except for vague references that are 60 years
old, I don't see any solution to your original questions (which is
where I thought the focus resided).
Richard Clark
wrote:
>OK, Richard, I admit that you caught me asking rhetorical
>questions - Congratulations!
Congratulations? In noting the absolute uniform homogeneity for the
technical equivalent of:
"Are we there yet?"
Cheap kudos with the equivalent buying power of shares in Lehman
Brothers.
"Are we solvent yet?"
Cecil Moore
> Cecil Moore wrote:
>> EZNEC says there is ~3 degrees of phase change in the
>> current in 90 degrees of monopole. How can that current
>> be used to measure the delay through 'n' degrees of
>> monopole?
>
> I have absolutely no idea. Sounds like you've made an error somewhere.
Nope, there's no error. Roy once verified that the total
current in a standing wave antenna, like a dipole, changes
phase very little over the 180 degree length of the 1/2WL
dipole. Yet, he used that same total current with its unchanging
phase to try to measure the delay through a loading coil.
In "Antennas", Kraus' plot of the total current on a dipole,
(Figure 14-2 on page 464 in the 3rd edition) also shows that
same 3 degree phase change in the total current over the 180
degree length of a 1/2WL dipole thus agreeing with EZNEC.
If one cannot detect a phase shift in 25 degrees of 1/4WL
monopole or 1/2WL dipole using the total current, how can
one expect to detect a phase shift in 25 degrees of loading
coil using that same constant phase current?
Cecil Moore
> According to the plots that I've seen, the standing wave pattern will
> show a discontinuous change in amplitude at positions where there is an
> abrupt change in phase of the traveling waves.
There's no discontinuity because the 180 degree phase shift
occurs at an amplitude zero crossing, i.e. when the phase
shift occurs, the amplitude is zero. But please note the
phase shift doesn't occur at all on a 1/4WL (or shorter)
monopole.
> Since a
> standing wave can be considered an amplitude vs phase plot (where both
> phase and amplitude vary with position) ...
For the standing wave function, I(x,t)=Io*cos(x)*cos(wt),
the phase at any point x, for a particular (t), doesn't
change phase. Set t=0 and vary x to see what happens.
Only the amplitude changes with x. That's why standing
wave current phase cannot be used to measure the delay
through a loading coil.
Jim Lux
> On Tue, 31 Mar 2009 22:27:57 GMT, Owen Duffy <no...@no.where> wrote:
>
> By these two, it would seem that modeling coaxial components in NEC is
> intractable and claims applied to their use will only be
> proven/disproven in the lab or the field.
>
Depends on what you want to do with modeling coaxial components. A wire
to represent the (radiating)outside, and an appropriate NT or TL to
represent the (non radiating) inside works fairly well.
If you actually want to model the cable itself (including the fields
inside), I suspect it won't work so well. MoM codes in general often
don't deal with modeling the fields inside closed boxes very well.
I suspect that the cases where it doesn't are basically in the category
of things that MoM codes don't do well with in general, and you need to
go to a different kind of model (FDTD? etc.)
Jim Kelley
> Jim Kelley wrote:
>
>> (where both phase and amplitude vary with position) ...
>
> For the standing wave function, I(x,t)=Io*cos(x)*cos(wt),
> the phase at any point x, for a particular (t), doesn't
> change phase. Set t=0 and vary x to see what happens.
> Only the amplitude changes with x. That's why standing
> wave current phase cannot be used to measure the delay
> through a loading coil.
The term x is the phase of the cosine function, Cecil. The phase of the
standing wave function varies by 90 degrees along the length of a 1/4
wave resonant standing wave antenna.
ac6xg
Cecil Moore
> The term x is the phase of the cosine function, Cecil. The phase of the
> standing wave function varies by 90 degrees along the length of a 1/4
> wave resonant standing wave antenna.
I'm sorry, Jim, that is just not true. The standing-wave
function has a *constant phase* along the length of a 1/4WL
monopole for any fixed (t). Cos(x) is the *envelope amplitude*
function (not phase function) for any fixed (t).
What Gene Fuller said previously is true regarding the
cos(kz)*cos(wt) term in a standing wave:
Gene Fuller, W4SZ wrote:
> In a standing wave antenna problem, such as the one you describe,
> there is no remaining phase information. Any specific phase
> characteristics of the traveling waves died out when the startup
> transients died out.
>
> Phase is gone. Kaput. Vanished. Cannot be recovered. Never to be
> seen again.
>
> The only "phase" remaining is the cos (kz) term, which is really
> an amplitude description, not a phase.
One can ask EZNEC to display the phase of the total current.
When one does that, one will see that the phase is ~constant
for a 1/4WL thin wire monopole over mininec ground. The change
in amplitude is what allows us to calculate the actual delay
through the wire using an ARCCOS function.
Jim Kelley
> Jim Kelley wrote:
>> The term x is the phase of the cosine function, Cecil. The phase of
>> the standing wave function varies by 90 degrees along the length of a
>> 1/4 wave resonant standing wave antenna.
>
> I'm sorry, Jim, that is just not true. The standing-wave
> function has a *constant phase* along the length of a 1/4WL
> monopole for any fixed (t). Cos(x) is the *envelope amplitude*
> function (not phase function) for any fixed (t).
Refer to a table of sines and observe the two things which vary
throughout the period of any sinusoidal wave. One of them is amplitude.
What would you prefer I call the other one?
ac6xg
Cecil Moore
> Refer to a table of sines and observe the two things which vary
> throughout the period of any sinusoidal wave. One of them is amplitude.
> What would you prefer I call the other one?
Did you not understand what Gene Fuller said?
Io*cos(kx) is the amplitude term. If kx=0 then
the amplitude is Io. If kx=pi/4, the amplitude
is 0.707*Io. If kx=pi/2, the amplitude is zero.
cos(wt) does not vary with (x), only with time.
At any snapshot in time, e.g. t=0, the phase
does not vary at all.
Jim Kelley
> Jim Kelley wrote:
>> Refer to a table of sines and observe the two things which vary
>> throughout the period of any sinusoidal wave. One of them is
>> amplitude. What would you prefer I call the other one?
>
> Did you not understand what Gene Fuller said?
Somebody once made a claim about answering a question with a question.
He said that it was a means of diversion. Clearly that is the case.
It's no coincidence that the phase of the standing wave varies by 90
degrees along the length of a 90 degree standing wave antenna.
ac6xg