"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
.................................................. ..........