COAX RF CHOKE
Scott
> I just purchased a Mosley TA53 with the 40 meter
> add on traps. The instructions call for a RF coax
> choke made of 10 turns at 10 inch diameter to be
> placed at 3 to 4 inches from the feed point.
>
> Is this necessary? Is there a substitute I can use
> instead of 26 feet of coax? Can the coax coil be placed
> at the bottom of the tower before the entry point to the
> shack?
>
> Any help would be appreciated.
>
> Jerry W8DLD
>
> Sent via Deja.com http://www.deja.com/
> Before you buy.
I don't think its a good idea to put the Choke at the entry to the
house,
you want to make sure you Choke off current at the feedpoint, to prevent
feedline interaction with pattern, among other things. I suggested
either buying
a Unadilla W2DU 1:1 balun or making one up yourself with some
ferrite beads - Its detailed in the 1999 ARRL Antenna book on page
26-19.
I certainly prefer it to that bulky air-coil.
Scott.
w8...@my-deja.com
Tom Hobby
All logperiodics have the same type balun near the feedpoint.
Tom
<w8...@my-deja.com> wrote in message news:7uoi52$2bv$1...@nnrp1.deja.com...
Brian Kelly
wrote:
Follow the manufacturer's instructuons to the letter!! Common mode
currents on the coax can destroy the pattern of any antenna. The choke
balun *has* to be loctated as close to the feedpoint as possible -
inches. Mosely is absoluterly correct, the coiled coax is a far more
effective choke than any string of ferrite beads or any of the
baluns-in-a-plastic-pipe thingeys. 'Way less expensive, too.
>
Brian Kelly w3rv
Reg Edwards
then coiling the surplus into a large-diameter hank at a considerable
height above ground, is just as likely to increase radiation from that
feedline as it is to suppress it.
A hank of coax with its turn-to-turn self capacitance is a high Q
tuned circuit. Peculiarly enough, Its Q is limited by its radiation
loss resistance.
At its resonant frequency it has a much higher impedance than
necessary. At higher frequencies it has insufficient impedance. This
is easily checked indoors by coiling a spare length of coax and
measuring the end-to-end impedance using a small hand-held 'antenna
analyser' on the Z range. Look for very low impedance series
resonances above the basic resonant frequency.
To be effective a choke must be physically small to minimise its self
capacitance and to raise its lowest resonant frequency When a choke
has to handle large powers a small ferrite core or cores are needed.
--
Reg, G4FGQ
Brian Kelly
<G4fgq...@btinternet.com> wrote:
>In my opinion, greatly increasing the length of a coaxial feedline and
>then coiling the surplus into a large-diameter hank at a considerable
>height above ground, is just as likely to increase radiation from that
>feedline as it is to suppress it.
>
It's only 26ft of RG-8/213 at 7Mhz, not very much loss in this case.
How would the RF "get back onto the feedline" below the choke?
Capacitve coupling thru the choke?
>
>A hank of coax with its turn-to-turn self capacitance is a high Q
>tuned circuit.
>
Wouldn't the ideal choke be a tank circuit resonant at the operating
freq? A series "trap" if you will?
>
>Peculiarly enough, Its Q is limited by its radiation
>loss resistance.
>
>At its resonant frequency it has a much higher impedance than
>necessary. At higher frequencies it has insufficient impedance.
>
In this particular case the antenna is a 7mhz monoband affair and the
choke is solenoid wound rather than just a hank. The recommended 10
turns on a 10 inch diameter form doesn't strike me as very high Q
coil, the coil length should be more like 2x it's diameter. The 7Mhz
antenna is on the same boom as a tribander, is this where concerns
about higher freq resonances come into play?
>
What are the implications of a much higher impedance than necessary?
The losses associated with the additional length of coax?
>
>This is easily checked indoors by coiling a spare length of coax and
>measuring the end-to-end impedance using a small hand-held 'antenna
>analyser' on the Z range. Look for very low impedance series
>resonances above the basic resonant frequency.
>
I understand the purpose of the measurement of Z which I think should
be in the hundreds of ohms range at the lowest operating freq for a
multiband choke but I don't understand the implications of low
impedances above the operating band for a monoband antenna/choke.
>
>To be effective a choke must be physically small to minimise its self
>capacitance and to raise its lowest resonant frequency.
>
Kindly expand if you are so inclined. I obviously don't understand the
relationships between the multiple resonances one should find in a
wound coax choke with a grid-dip meter and the effectiveness of the
choke if the resonances do not fall in the ham bands.
>
>When a choke
>has to handle large powers a small ferrite core or cores are needed.
>
. . . maybe both . . ?
>--
>Reg, G4FGQ
>
Tnx,
>
Brian Kelly w3rv
K1BQT
then coiling the surplus into a large-diameter hank at a considerable
height above ground, is just as likely to increase radiation from that
feedline as it is to suppress it.<
Roy Lewallen might disagree with you on that point, as do I. Try this, Reg.
Wind a feedline balun as Roy prescribes for a 10 or 15 meter dipole and hang
the antenna about 10 feet off the ground. Then, clamp a sensitive RF ammeter
onto the cable and walk it up the line toward the antenna. Next, uncoil the
balun and repeat this proceedure. You'll have your answer.
Rick K1BQT
dm
it at the rig , and smiling??
Brian Kelly
>Has nobody ELSE thought of feeding the thing with twinlead , matching
>it at the rig , and smiling??
>
>
Fuhgeddit. It's a manufactured rotatable antenna designed specifically
for feeding with 50 ohm coax. You'd have to use a tuner plus sort out
all kinds of feedline installation hassles which come with balanced
feedlines. Rigging a balanced line around a rotating mast is not my
idea of a smile generator. A few turns of coax at the antenna is a
whole bunch simpler, more reliable and less expensive.
>
Brian Kelly w3rv
dm
antennas isn't quite as hard as it sounds, and big deal if have
a"tuner"....73 Anyhoo
Reg Edwards
satisfactory. Very convenient to construct, no soldered joints etc. It
is necessary only to prune the length of coax and number of turns and
hank diameter for the correct resonant frequency. The impedance at
resonance between the ends will be up in the >100 K-ohm region.
Insofar as longitudinal current is cocerned the trap can be considered
an open circuit. For fewer turns and less weight a fixed capacitor
could be connected between the two coaxial ends but that would have
the effect of narrowing the useful bandwidth.
Above the fundamental resonant frequency the uneven distribution of
stray capacitance among the turns, combined with the length of coax
used and the tight mutual coupling between the various sections,
result in a number of random unrelated resonances. In between each
'parallel' high impedance resonance there will be a low impedance
'series' resonance.
Make a hank of 13 turns of RG-58 over a diameter of 9 inches. Loosely
tape it at a couple of places. It will resonate crudely on 160 metres.
Use of a hand-held 'antenna analyser' will show a higgledy-piggledy
impedance-vs-frequency response. Some high impedance points will be
far beyond the range of the analyser. Some of the low impedance points
may be below 10 ohms. Over wide frequency ranges the impedance may not
rise above 200 or 300 ohms.
A better, more predictable behaviour will be obtained with a longish
neatly wound solenoid but to obtain resonance at a particular
frequency an even longer length of coax will be needed.
The coax feedline below the trap will behave as a very low-loss,
sensitive, vertical or sloping, receiving antenna ready to pick up and
re-radiate anything it can collect from the VERY near field of the
main antenna. Or lead the RF right back into the shack. Depending on
coax resonant length, probably both ! There's no way of getting rid
of it completely. When the RF power level on the feeder is already
down to a few percent of that on the main antenna there's just no
point in doing anything about it until there are real grounds for
complaint. If it works don't fix it !
To obtain an HF CHOKE fill up a large ferrite ring with coax or twin
line. Its impedance can be made greater than 1000 ohms from 80 to 10
metres. Or use TWO rings to make sure of 160m. Or slide over the coax
a dozen or so, 1.5" long, snug fitting ferrite sleeves. It will have
no pronounced resonances. For a real good job space the sleeves along
the whole feedline. Don't give the RF a chance to sneak back on the
feeder below the choke.
By the way, 10 turns of coax outer conductor wound on a 10-inch
diameter former, spaced apart by twice the thickness of the PVC jacket
has a VERY high Q at 7 MHz. May be greater than 1000. It is
self-resonant somewhere around 7 MHz and the impedance between one end
and the other at 7 MHz is probably greater than 1 Megohm. But it's not
a choke - it's a trap at the frequency of use.
But Brian, don't get me wrong. I am not being in any way critical of
those amonst us who strive for perfection in each of the many
components which come together to form a radio station - and of course
our hobby. We all have different ways of obtaining kicks.
--
Regards, Reg.
=================================
Brian Kelly wrote..
> On Sun, 24 Oct 1999 10:24:29 +0100, "Reg Edwards"
>
>
> >In my opinion, greatly increasing the length of a coaxial feedline
and
> >then coiling the surplus into a large-diameter hank at a
considerable
> >height above ground, is just as likely to increase radiation from
that
> >feedline as it is to suppress it.
> >
Reg Edwards
that the current falls to a low value near the end of an antenna
terminated with a rather high impedance. The line being the antenna
in this case. However, there will now be a high voltage at that point
which your ammeter will not indicate.
When the longitudinal RF on a long feeder gets there because it is
coupled to the near field of the main antenna it will still be there
when a choke is inserted. All the choke does is shift the longitudinal
current and voltage nodes up or down the feeder. The electro-magnetic
coupling between the antenna's near field and feeder remains. The
feedline may now be acting as a sort of reflector instead of a
director because of the different phasing. But it is still radiating
!
If by chance a choke is inserted at a point of maximum voltage (and
minimum current) it has a small effect if any at all. So what happens
depends critically on feeder length which you have not mentioned.
Consider what happens when the choke is inserted in the coax at a
point which is an exact number of half waves from where the braid
finds a good ground connection. Will the effects be different from
when the distance to ground is an odd number of quarterwaves ? And
how will you know ?
Yours, Reg.
=================================
K1BQT <k1...@aol.com> wrote in message
news:19991024093843...@ng-fq1.aol.com...
> >In my opinion, greatly increasing the length of a coaxial feedline
and
> then coiling the surplus into a large-diameter hank at a
considerable
> height above ground, is just as likely to increase radiation from
that
> feedline as it is to suppress it.<
>
K1BQT
Hmm. Well, I always thought current might find its way onto the untreated
feedline in two ways. First, it may flow directly onto the outer (if it's not
choked). The amount of current drawn depends upon the impedance the coax outer
presents to the feedpoint. If the impedence is high, not much current is
stolen from the dipole's half-element. If it's low (referenced to 25 ohms),
quite a lot may be stolen and radiated.
Second, current may get on by induction. If the feedline is prependicular to
the element and/or the length of the line is non-resonant, little or no current
will be induced. If the line doubles back on one leg and also happens to
present a parasitic resonance, your argument may begin to hold a trickle of
water. That's about it. A trickle.
Worst case, if you have a bothersome resonant line length--you can always break
it up with a few sleeves midway or add another coax coil to detune it.
Currents induced on the feed by proximity are usually far less troublsome than
currents induced by load-sharing between two wires connected in parallel.
More succintly stated, the odds a feedline will radiate with a common-mode
choke installed at the feedpoint is akin to the odds of draining a wine bottle
with the cork stuck in your throat. Pretty slim, either way, Reg.
73
Rick K1BQT
Reg Edwards
> More succintly stated, the odds a feedline will radiate with a
common-mode
> choke installed at the feedpoint is akin to the odds of draining a
wine bottle
> with the cork stuck in your throat. Pretty slim, either way, Reg.
Rick, I knew we were on common ground. If it works - why fix it !
I'm all in favour of feeding a one-band dipole all the way with 75-ohm
coax. With only 1 percent of the Tx power being radiated by the feeder
what is there to fix ?
And who needs a great hank of coax causing the dipole centre to sag to
2/3rds its natural height ?
Yours, Reg.
Thomas C. Sefranek
Reg Edwards wrote:
> I'm all in favour of feeding a one-band dipole all the way with 75-ohm
> coax. With only 1 percent of the Tx power being radiated by the feeder
> what is there to fix ?
Hi Reg,
How do you figure the 1%?
Almost NO "real world" antenna is 75 ohms across the band.
> And who needs a great hank of coax causing the dipole centre to sag to
> 2/3rds its natural height ?
Yea, thats why I use "a great hunk of " powdered iron core,
it drags the antenna to the ground. :) And what with that RG-214,
it weighs a ton!
> Yours, Reg.
--
Thomas C. Sefranek WA1RHP
ARRL Instructor, Technical Specialist, VE Contact.
http://www.harvardrepeater.org
http://hamradio.cmcorp.com/inventory/Inventory.html
Reg Edwards
> How do you figure the 1%?
> Almost NO "real world" antenna is 75 ohms across the band.
Who said it was ?
And the 75-ohm coax is not what you think it is either. It could be
50 ohms and no-one would notice the difference.
Your SWR, power and S-meters are even more uncertain.
But we all need to obtain occasional relief in the land of
make-belief.
What are hobbies for anyway ?
---
Yours Reg, G4FGQ.
Brian Kelly
<G4fgq...@btinternet.com> wrote:
>Brian, if all you need is a single band trap then a hank of coax is
>satisfactory. Very convenient to construct, no soldered joints etc. It
>is necessary only to prune the length of coax and number of turns and
>hank diameter for the correct resonant frequency. The impedance at
>resonance between the ends will be up in the >100 K-ohm region.
>Insofar as longitudinal current is cocerned the trap can be considered
>an open circuit. For fewer turns and less weight a fixed capacitor
>could be connected between the two coaxial ends but that would have
>the effect of narrowing the useful bandwidth.
>
Good. That answers my basic question, single-band "traps" will serve
as effective "choke baluns". The original poster was only concerned
about a 7Mhz choke. I did not realize how high the impedance could
get. This is good.
>
>Above the fundamental resonant frequency the uneven distribution of
>stray capacitance among the turns, combined with the length of coax
>used and the tight mutual coupling between the various sections,
>result in a number of random unrelated resonances. In between each
>'parallel' high impedance resonance there will be a low impedance
>'series' resonance.
>
>Make a hank of 13 turns of RG-58 over a diameter of 9 inches. Loosely
>tape it at a couple of places. It will resonate crudely on 160 metres.
>Use of a hand-held 'antenna analyser' will show a higgledy-piggledy
>impedance-vs-frequency response. Some high impedance points will be
>far beyond the range of the analyser. Some of the low impedance points
>may be below 10 ohms. Over wide frequency ranges the impedance may not
>rise above 200 or 300 ohms.
>
I wouldn't wrap a scrambled hank. I have an inexpensive MFJ-259B
handheld "antenna analyzer" which also serves as a grid-dipper and
operates to 170Mhz. It won'r measure impedances over 650 +/- jX ohms
of impedance but that should be good enough for ham purposes.
>
>A better, more predictable behaviour will be obtained with a longish
>neatly wound solenoid but to obtain resonance at a particular
>frequency an even longer length of coax will be needed.
>
There ya go, I can relate to a "real coil" because the behavior can be
at least somewhat predictable.
>
>The coax feedline below the trap will behave as a very low-loss,
>sensitive, vertical or sloping, receiving antenna ready to pick up and
>re-radiate anything it can collect from the VERY near field of the
>main antenna. Or lead the RF right back into the shack. Depending on
>coax resonant length, probably both ! There's no way of getting rid
>of it completely. When the RF power level on the feeder is already
>down to a few percent of that on the main antenna there's just no
>point in doing anything about it until there are real grounds for
>complaint. If it works don't fix it !
>
Agreed. Interesting. I certainly have observed that effect but never
considered it in this light. The primary purpose of a choke balun of
any flavor the way I understand it is to significantly reduce the
common mode currents on the feedline which would distort and/or
degrade antenna radiation patterns right at the antenna. The much
smaller induced currents anywhere and everywhere in the system are
just something we live with and which have little or no practical
impact.
>
>To obtain an HF CHOKE fill up a large ferrite ring with coax or twin
>line.
>
Wouldn't that booger a balanced line??
>
> Its impedance can be made greater than 1000 ohms from 80 to 10
>metres. Or use TWO rings to make sure of 160m. Or slide over the coax
>a dozen or so, 1.5" long, snug fitting ferrite sleeves. It will have
>no pronounced resonances. For a real good job space the sleeves along
>the whole feedline. Don't give the RF a chance to sneak back on the
>feeder below the choke.
>
. . . sounds like a bit of overkill . . ? Actually I'll do that if I
can't get a solenoid-wound coax choke to work well between my xcvr and
my ant. tuner on all HF bands.
>
>By the way, 10 turns of coax outer conductor wound on a 10-inch
>diameter former, spaced apart by twice the thickness of the PVC jacket
>has a VERY high Q at 7 MHz. May be greater than 1000. It is
>self-resonant somewhere around 7 MHz and the impedance between one end
>and the other at 7 MHz is probably greater than 1 Megohm. But it's not
>a choke - it's a trap at the frequency of use.
>
Ya ran SOLENOID for that, didn't you? It certainly confirms the design
of the choke recommended by the original poster's 7Mhz antenna
manufacturer. I'll be building some 20-10m quads in the not too
distant future and apply the same approach to single-band chokes for
those.
>
>But Brian, don't get me wrong. I am not being in any way critical of
>those amonst us who strive for perfection in each of the many
>components which come together to form a radio station - and of course
>our hobby. We all have different ways of obtaining kicks.
>
I get mine from 160/80 dxing, not from winding chokes. If I can
squeeze a bit more performance out of the antenna or reduce RFI to
some extent or keep the hot stuff out of my xvcr with a choke I'll
build one. But I seriously doubt the lack of any chokes at all on my
feedlines in the past have hurt my dx totals or contest scores very
much.
>--
>Regards, Reg.
>=================================
>
Thanks for one more increment of education, Reg.
>
Brian Kelly w3rv
Thomas C. Sefranek
Reg Edwards wrote:
> Thomas wrote -
> > How do you figure the 1%?
> > Almost NO "real world" antenna is 75 ohms across the band.
> =================================
> Who said it was ?
Ahhh, I ASSUMED you were attributing the 1% to the mis-match.
> And the 75-ohm coax is not what you think it is either. It could be
> 50 ohms and no-one would notice the difference.
Agreed.
> Your SWR, power and S-meters are even more uncertain.
I think my Bird is somewhat more certain than my MFJ analyzer.
But my HP power meter IS certain.
My S-Meter in the TS-440S is incomprehensible.
(Have you ever done a power to S- Units calibration? TOTALLY
non-linear!)
[ It's curve reminds me of the flight of a butterfly.)
> But we all need to obtain occasional relief in the land of
> make-belief.
Internet?
> What are hobbies for anyway ?
Expression of a desire to learn, build, communicate, teach, and then...
have fun.
>
> ---
> Yours Reg, G4FGQ.
JDer8745
Someone sed:
"Has nobody ELSE thought of feeding the thing with twinlead, matching it at
the rig, and smiling??"
====================
Yes. It's a good idea. That's what I would have thought of.
But most hams today are "hung up" on the idea of coaxial TL. Sigh!
73 de Jack, K9CUN
Roy Lewallen
ago I posted on this newsgroup some measurements I made of coax coils
which shows that they form a fairly low-Q resonant circuit with
sufficiently high impedance over about three bands to be a very
effective choke. Some of the best dimensions appear in the _ARRL
Handbook_. The measurements were made with an HP vector impedance meter.
Roy Lewallen, W7EL
Dick Carroll
C'mon Jack. Maybe some of them have read Walt Maxwell's
"Reflections" and found that he knew what he was writing
about.
And open type feedline isn't always practical. My coax
goes underground for >100 feet. Past the trees instead of
through them.
Dick W0EX
pot
Twinlead radiates and no "new" radios are made for twinline.....
Of course you properly don't know that because your old piece of crap is made for
twinlead.
Happy RFI/TVI........
"Hotrod"
W6RCecilA
> Go back to the old day where you came from.
> Twinlead radiates and no "new" radios are made for twinline.....
Actually, the same phenomenon that causes twinlead to radiate
also causes coax to radiate.
--
73, Cecil, W6RCA http://www.bigfoot.com/~w6rca
Dick Elliott
information that "new" radios are not made for twin line? I have had several
"new" transceivers and some old........twin line does a fine
job............sure some..... "radiates".......will take place. You need to
get with some "OLD TIMERS" and get trained the proper way TO BE AN AMATEUR
RADIO OPERATOR and along the way you might try to learn some
manners.........or do you simply love "PLUG AND PLAY" while you are ON THE
POT.........or INHALING THE POT? signed....young Dick Elliott......N9IPA
pot <p...@gate.net> wrote in message news:3815E7F6...@gate.net...
> Go back to the old day where you came from.
> Twinlead radiates and no "new" radios are made for twinline.....
Fort Davis, TX
instructions.
They are cheap to make, it doesn't take a rocket scientist to do it; JUST
DON'T USE FOAM INSULATED COAX, as power will 'burn' right through it on the
curves of the choke. Been there, done that.
Tom K0TH
Dennis C. O'Connor
Denny
strum...@gmail.com
>
> > I just purchased a Mosley TA53 with the 40 meter
> > add on traps. The instructions call for a RF coax
> > choke made of 10 turns at 10 inch diameter to be
> > placed at 3 to 4 inches from the feed point.
> >
> > Is this necessary? Is there a substitute I can use
> > instead of 26 feet of coax? Can the coax coil be placed
> > at the bottom of the tower before the entry point to the
> > shack?
> >
> > Any help would be appreciated.
> >
> > Jerry W8DLD
> >
> house,
> you want to make sure you Choke off current at the feedpoint, to prevent
>
> feedline interaction with pattern, among other things. I suggested
> either buying
> a Unadilla W2DU 1:1 balun or making one up yourself with some
> ferrite beads - Its detailed in the 1999 ARRL Antenna book on page
> 26-19.
> I certainly prefer it to that bulky air-coil.
>
>
> Scott.
I am trying to get this combination of antennas to work. I wonder if the original poster of this thread is still around? I would love to talk to you as to whether you had success in getting the 40m add on to work! -M