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