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