Putting together a new Cushcraft A4S (10,15, 20m) beam.

Cushcraft recommends using an 8-turn, 6-inch diameter RG8U coax cable coil
as an RF choke for feeding the driven element. HyGain suggested the same
thing for the TH2MK3 antenna I used many years ago -- only using 12 turns of
RG8 (with the 6-inch outside diameter), rather than 8 turns. I have a few
questions on the best way to do this.

1) Is it fair to assume that the more turns the better?

2) Should the coil be as close to the antenna feedpoint as possible, or is a
foot or two of straight coax between the antenna input and the coil OK?

3) Would placing the coax coil around the mast (instead of taping one side
of the coil to the mast as suggested) be a bad thing to do?

4) Is there a good website or book that describes the technical tradeoffs of
a coax RF choke? ( I am sort of wondering if the choke can be skipped. Some
time ago, I used a W2AU balun on a multiband dipole. All it did was create
TVI when I went on 10 meters. Took the balun out, and the resonant points
didn't change. I figured that if there were antenna currents on the coax,
they would bring down the resonant frequency without the balun. Since that
didn't happen, I figured the balun wasn't helping - - but balun core
saturation at 30 MHz (with the KW on) was what was racking up the TVs.)

5) One last question - not having to do with the coax RF choke:
Cushcraft "operating tips" say to not use vinyl tape on SO239 or PL-239
connectors. Why? What problem would the tape cause??

Thanks for any info - AK

AK wrote:
Putting together a new Cushcraft A4S (10,15, 20m) beam.

Cushcraft recommends using an 8-turn, 6-inch diameter RG8U coax cable coil
as an RF choke for feeding the driven element. HyGain suggested the same
thing for the TH2MK3 antenna I used many years ago -- only using 12 turns of
RG8 (with the 6-inch outside diameter), rather than 8 turns. I have a few
questions on the best way to do this.

1) Is it fair to assume that the more turns the better?

No. The coil makes a broadly resonant circuit, which greatly increases
its common mode impedance. Increasing the number of turns will lower it.

2) Should the coil be as close to the antenna feedpoint as possible, or is a
foot or two of straight coax between the antenna input and the coil OK?

No problem.

3) Would placing the coax coil around the mast (instead of taping one side
of the coil to the mast as suggested) be a bad thing to do?

Probably. It's likely to lower the resonant frequency, reducing the
common mode impedance.

4) Is there a good website or book that describes the technical tradeoffs of
a coax RF choke? ( I am sort of wondering if the choke can be skipped. Some
time ago, I used a W2AU balun on a multiband dipole. All it did was create
TVI when I went on 10 meters. Took the balun out, and the resonant points
didn't change. I figured that if there were antenna currents on the coax,
they would bring down the resonant frequency without the balun. Since that
didn't happen, I figured the balun wasn't helping - - but balun core
saturation at 30 MHz (with the KW on) was what was racking up the TVs.)

For general information on baluns and common mode chokes (called
"current baluns" in the article), see
http://eznec.com/Amateur/Articles/Baluns.pdf. For dimensions of air core
chokes like yours, see the ARRL Handbook.

That's an interesting story about the W2DU balun. There must have been a
huge amount of common mode current, and insufficient balun impedance.

5) One last question - not having to do with the coax RF choke:
Cushcraft "operating tips" say to not use vinyl tape on SO239 or PL-239
connectors. Why? What problem would the tape cause??

Can't help you there.

Roy Lewallen, W7EL

Thanks for the reply, Roy.

"Roy Lewallen" wrote in message
...
AK wrote:
Putting together a new Cushcraft A4S (10,15, 20m) beam.

Cushcraft recommends using an 8-turn, 6-inch diameter RG8U coax cable
coil as an RF choke for feeding the driven element. HyGain suggested the
same thing for the TH2MK3 antenna I used many years ago -- only using 12
turns of RG8 (with the 6-inch outside diameter), rather than 8 turns. I
have a few questions on the best way to do this.

1) Is it fair to assume that the more turns the better?

No. The coil makes a broadly resonant circuit, which greatly increases its
common mode impedance. Increasing the number of turns will lower it.

So it's not just working on inductance. I'm still wondering why Cushcaft
recommends 8 turns and HyGain 12.


2) Should the coil be as close to the antenna feedpoint as possible, or
is a foot or two of straight coax between the antenna input and the coil
OK?

No problem.

3) Would placing the coax coil around the mast (instead of taping one
side of the coil to the mast as suggested) be a bad thing to do?

Probably. It's likely to lower the resonant frequency, reducing the common
mode impedance.

OK

4) Is there a good website or book that describes the technical tradeoffs
of a coax RF choke? ( I am sort of wondering if the choke can be skipped.
Some time ago, I used a W2AU balun on a multiband dipole. All it did was
create TVI when I went on 10 meters. Took the balun out, and the resonant
points didn't change. I figured that if there were antenna currents on
the coax, they would bring down the resonant frequency without the balun.
Since that didn't happen, I figured the balun wasn't helping - - but
balun core saturation at 30 MHz (with the KW on) was what was racking up
the TVs.)

For general information on baluns and common mode chokes (called "current
baluns" in the article), see http://eznec.com/Amateur/Articles/Baluns.pdf.
For dimensions of air core chokes like yours, see the ARRL Handbook.

Very informative; still reading.

That's an interesting story about the W2DU balun. There must have been a
huge amount of common mode current, and insufficient balun impedance.

Actually, it was a W2AU ( http://www.unadilla.com/w2awire.htm ) I used in
the early 70's.

5) One last question - not having to do with the coax RF choke:
Cushcraft "operating tips" say to not use vinyl tape on SO239 or PL-239
connectors. Why? What problem would the tape cause??

Can't help you there.

Reg thinks it's the tape's lack of water sealing. I have been using some
putty-like sealer on my 239s lately, but it is a mess to clean off when
disconnecting and re-connecting the coax - as sometimes required.

Roy Lewallen, W7EL

73, AK

"AK" wrote in message
news:WbGYf.666481$084.607593@attbi_s22...
Thanks for the reply, Roy.

5) One last question - not having to do with the coax RF choke:
Cushcraft "operating tips" say to not use vinyl tape on SO239 or PL-239
connectors. Why? What problem would the tape cause??

Can't help you there.

Reg

Please make that "Dave Platt" --

thinks it's the tape's lack of water sealing. I have been using some
putty-like sealer on my 239s lately, but it is a mess to clean off when
disconnecting and re-connecting the coax - as sometimes required.

Roy Lewallen, W7EL

73, AK

On Wed, 05 Apr 2006 02:46:14 GMT, "AK" wrote:

Reg thinks it's the tape's lack of water sealing. I have been using some
putty-like sealer on my 239s lately, but it is a mess to clean off when
disconnecting and re-connecting the coax - as sometimes required.

Hi OM,

Lay down a layer of tape first.
Seal it with the sealer (much cheaper from flower shops).
Lay down another layer of tape.

73's
Richard Clark, KB7QHC

"AK" wrote in message
news:_7CYf.908089$xm3.833232@attbi_s21...
Putting together a new Cushcraft A4S (10,15, 20m) beam.

Cushcraft recommends using an 8-turn, 6-inch diameter RG8U coax
cable coil
as an RF choke for feeding the driven element. HyGain suggested the
same
thing for the TH2MK3 antenna I used many years ago -- only using 12
turns of
RG8 (with the 6-inch outside diameter), rather than 8 turns. I have
a few
questions on the best way to do this.

1) Is it fair to assume that the more turns the better?

2) Should the coil be as close to the antenna feedpoint as possible,
or is a
foot or two of straight coax between the antenna input and the coil
OK?

3) Would placing the coax coil around the mast (instead of taping
one side
of the coil to the mast as suggested) be a bad thing to do?

4) Is there a good website or book that describes the technical
tradeoffs of
a coax RF choke? ( I am sort of wondering if the choke can be
skipped. Some
time ago, I used a W2AU balun on a multiband dipole. All it did was
create
TVI when I went on 10 meters. Took the balun out, and the resonant
points
didn't change. I figured that if there were antenna currents on the
coax,
they would bring down the resonant frequency without the balun.
Since that
didn't happen, I figured the balun wasn't helping - - but balun core
saturation at 30 MHz (with the KW on) was what was racking up the
TVs.)

5) One last question - not having to do with the coax RF choke:
Cushcraft "operating tips" say to not use vinyl tape on SO239 or
PL-239
connectors. Why? What problem would the tape cause??

Thanks for any info - AK

=========================================

A choke made of 8 turns, 6 inches diameter, has a resonant frequency
around 21 MHz and has an impedance greater than 1000 ohms in the 20
meter and 10 meter bands.

Chokes are best wound neatly in the form of a single layer solenoid.
Jumble wound choke coils can have funny effects.

For design of chokes made with coax cable download program SELFRESS
from website below and run immediately.
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

"Reg Edwards" wrote in message
...
=========================================

A choke made of 8 turns, 6 inches diameter, has a resonant frequency
around 21 MHz and has an impedance greater than 1000 ohms in the 20
meter and 10 meter bands.

Thanks Reg. I'm wondering now what 12 turns would work out to??

Chokes are best wound neatly in the form of a single layer solenoid.
Jumble wound choke coils can have funny effects.

Mine is a single layer with lots of electrical tape keeping the turns close
and pretty much all the same.

For design of chokes made with coax cable download program SELFRESS
from website below and run immediately.
----
.................................................. .........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. .........

Thanks again, and 73 - AK

"Reg Edwards" wrote in message
...

"AK" wrote in message
news:_7CYf.908089$xm3.833232@attbi_s21...
Putting together a new Cushcraft A4S (10,15, 20m) beam.

Cushcraft recommends using an 8-turn, 6-inch diameter RG8U coax
cable coil
as an RF choke for feeding the driven element. HyGain suggested the
same
thing for the TH2MK3 antenna I used many years ago -- only using 12
turns of
RG8 (with the 6-inch outside diameter), rather than 8 turns. I have
a few
questions on the best way to do this.

1) Is it fair to assume that the more turns the better?

2) Should the coil be as close to the antenna feedpoint as possible,
or is a
foot or two of straight coax between the antenna input and the coil
OK?

3) Would placing the coax coil around the mast (instead of taping
one side
of the coil to the mast as suggested) be a bad thing to do?

4) Is there a good website or book that describes the technical
tradeoffs of
a coax RF choke? ( I am sort of wondering if the choke can be
skipped. Some
time ago, I used a W2AU balun on a multiband dipole. All it did was
create
TVI when I went on 10 meters. Took the balun out, and the resonant
points
didn't change. I figured that if there were antenna currents on the
coax,
they would bring down the resonant frequency without the balun.
Since that
didn't happen, I figured the balun wasn't helping - - but balun core
saturation at 30 MHz (with the KW on) was what was racking up the
TVs.)

5) One last question - not having to do with the coax RF choke:
Cushcraft "operating tips" say to not use vinyl tape on SO239 or
PL-239
connectors. Why? What problem would the tape cause??

Thanks for any info - AK

=========================================

A choke made of 8 turns, 6 inches diameter, has a resonant frequency
around 21 MHz and has an impedance greater than 1000 ohms in the 20
meter and 10 meter bands.

Chokes are best wound neatly in the form of a single layer solenoid.
Jumble wound choke coils can have funny effects.

For design of chokes made with coax cable download program SELFRESS
from website below and run immediately.
----
.................................................. .........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. .........



I like to wind them on a piece of PVC pipe tie wrapping or lacing the turns
to the pipe.