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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 |
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