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Re: Topband: Placement of RX feedline chokes

To: topband@contesting.com
Subject: Re: Topband: Placement of RX feedline chokes
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Wed, 3 Feb 2016 14:59:34 -0800
List-post: <topband@contesting.com">mailto:topband@contesting.com>
On Wed,2/3/2016 10:10 AM, Herbert Schoenbohm wrote:
It has come to my attention by a seasoned topbander that I have this all wrong and the decoupling coil should be close as possible to the Beverage transformer. He reasoned that the long RG6 cable (some 500 feet long) couple random pickup right back into the Beverage transformer and could also reduce the side lobe rejection from the Beverage itself. So where is the best placement for for a feed-line choke?

He's right -- if you're using only one feedline choke, it should be as close as practical to the feedpoint (that is, out by the transformer, or up in the air for a TX antenna). There is also benefit to one or more additional chokes on a feedline. Two benefits. First, it minimizes interaction between that feedline and other antennas, fulfilling the same function as egg insulators breaking up guy wires.

Second, it minimizes feedline current, which, due to imperfections in the coax shield, can convert noise picked up in the shield to a differential signal on the coax. The metric for this conversion is called the transfer impedance of the cable, and is defined as the differential voltage divided by the shield current. EMC guru Henry Ott notes that the lower limit for transfer impedance is the resistance of the shield at the frequency of interest. The uniformity and the "coverage" of the shield also contributes.

Coax made for CATV is optimized for its performance at VHF-UHF. Shields are typically thin Al braid plus one or more layers of Al foil. These are pretty good shields at VHF/UHF, but their relatively high resistance can seriously degrade shielding at MF and HF.

73, Jim K9YC

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