What about placing a choke at the coax antenna junction on a Cushcraft R8
vertical or a Gap Titan? Would this help or is the matching network on the
Cushcraft already doing the job.
Art WA8VSJ
----- Original Message -----
From: "Jim Brown K9YC" <k9yc@audiosystemsgroup.com>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Friday, August 06, 2010 1:02 PM
Subject: Re: [TenTec] [Fwd: Line Isolator Balun (sorta) question.]
> On Fri, 06 Aug 2010 11:48:22 -0500, Dr. Gerald N. Johnson wrote:
>
>>The purpose of these "baluns" is to attenuate any RF current on the
>>outside of the shield. That helps cut feed line radiation when the
> coax
>>is hooked to an antenna. Sometimes that helps cut TVI and received
>>interference from local sources. Sometimes that hurts working
> stations
>>that need the vertically polarized component from feedline radiation.
>
> First, let's call these things by their real name so that we
> understand what they do. They are COMMON MODE CHOKES, NOT BALUNS.
>
> As Jerry notes, the function of a common mode choke is to kill current
> on the feedline -- that is, to prevent the feedline from acting as
> part of the antenna. That's good for at least four reasons. First, it
> preserves the directional properties of the antenna. Second, it
> prevents the feedline from receiving noise and coupling it to the
> antenna. Third, it prevents the feedline from radiating RF into your
> living room (and your neighbor's living room). Fourth, it prevents the
> feedline from coupling RF into your shack.
>
> Also as Jerry notes, there is no benefit from using a choke on jumpers
> in the shack. The ideal place for a common mode choke is at the
> feedpoint of the antenna (that is, up in the air where the coax
> connects to the antenna). Chokes are also useful as "egg insulators"
> to break up feedlines into non-resonant lengths (like an egg insulator
> in a guy wire) so that they can not act as parasitic elements to other
> antennas (like a vertical nearby).
>
> Strings of beads are next to useless at HF frequencies, both because
> their impedance is far too low to be effective, and because they are
> inductive, not resistive at HF. A common mode choke must be resistive
> so that it cannot resonate with the line. If it resonates, it is
> useless. A string of beads IS quite useful at VHF and UHF.
>
> There is a lot of detail and a tutorial discussion about this in
>
> http://audiosystemsgroup.com/RFI-Ham.pdf
>
> 73, Jim Brown K9YC
>
>
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