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Re: [TowerTalk] Coax

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Coax
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Fri, 22 Jul 2011 21:04:38 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 7/22/2011 5:04 PM, Jim W7RY wrote:

> Jim, this is also from your white paper.  Cables such as RG-223, RG-141 and 
> RG-400. All double shielded and the size of RG-58.
>
> Q: I have a high power, multi-transmitter station. Can I use RG58 or RG8X 
> between my transceiver and amplifier?
>
> A: Smaller coax cables like RG58 and RG8X provide less shielding than larger 
> coax, thanks to the higher resistance of their shields. To minimize 
> inter-station interference, use coax with a beefy copper shield for all 
> cables.
It's a matter of the shield resistance, as well as the density and the 
uniformity of the shield. First, as I noted early on,  RG-numbers have 
been next to meaningless for nearly 50 years. We have to study the spec 
sheets for the cables we are considering. Look at the resistance of the 
shield and compare it to larger cable with a beefy shield.

While I didn't discuss it, there's an standard engineering spec for coax 
called the transfer impedance. It's the ratio of voltage induced on the 
center conductor by current flowing on the shield. The lower limit on 
the transfer impedance is the resistance of the shield.  It's the 
standard specification for the quality of the shield, and the leakage 
from the coax (and to the coax).  The lower the transfer impedance, the 
better the shielding. Obviously, the transfer impedance can vary with 
frequency.

73, Jim Brown K9YC

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