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Re: [RFI] shielding question

To: RFI List <rfi@contesting.com>
Subject: Re: [RFI] shielding question
From: Jim Smith <jimsmith@shaw.ca>
Date: Mon, 22 Dec 2003 14:59:31 -0800
List-post: <mailto:rfi@contesting.com>
Ooohhh Yuckkk - I didn't want to hear this. A while ago I bought a 500 ft spool of Belden 9501 single pair with Beldfoil aluminum polyester shield with drain wire and used it to run every conceivable audio line (except mic) in the shack to a jack field. As all the lines are mono I just tied the ends of the two conductors together to give me single conductor plus shield. Sounds like I may have wasted a little money and a lot of time.

Could you explain the mechanism (or point me to a source) whereby the sheld is rendered useless by the presence of a drain wire at HF?

73 de Jim Smith VE7FO

Jim Brown wrote:

2. If the shield on your cable is a foil with a drain wire, it is worse
than no shield at all for frequencies below about 20 MHz. That is
because any current flowing on the shield in this part of the spectrum
flows almost entirely on the drain wire, which usually has the same
twist ratio (lay, in cable-speak) as the twisted pair, and lies closer
to one conductor than the other. In this condition, the current flowing
on the shield will be converted to a differential mode voltage on the
signal pair! Above roughly 20 MHz, the current distributes more evenly
over the shield, and this becomes less of an issue.

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