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Re: [TowerTalk] Life expectancy of RG-213

To: "" <>
Subject: Re: [TowerTalk] Life expectancy of RG-213
From: "Jim Brown" <>
Date: Fri, 27 Aug 2010 15:23:20 -0700
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On Fri, 27 Aug 2010 13:43:32 -0700, Rick Karlquist wrote:

>One thing no one has mentioned so far is that when the shield
>oxidizes, the wires may no longer make good electrical contact 
>each other.  I have heard that the coax loses it's shielding
>effectiveness due to this, but I have seen no scientific tests.

Books have been written on the this. The classic is by Tsalovich 
(I own it, but can't lay my hands on it right now to verify the 
spelling of his name), and it goes into great detail about shield 
construction. There's another by Vance that Ott references. 

Magnetic field rejection in coax is provided by mutual coupling 
between the center conductor and the shield. That depends upon 
uniform distribution of the shield current throughout the shield, 
AND upon a low value of shield resistance. In effect, the 
resistance is in series with the mutual inductance. Ott has a 
great discussion of this in both editions of his book on EMC. If 
there is zero resistance and ideal uniformity, the coupling 
coefficient is unity. 

Rejection is the result of the two induced voltage cancelling the 
coupled voltage. The resistance can be viewed as a plateau of 
voltage that cannot be cancelled -- it's in series with the mutual 
inductance. This is the limiting factor at low frequencies, when 
the reistance is greater than mutual inductive reactance. For 
typical good coax with a very robust copper shield, the shield 
starts working at about 1 kHz, and is increasingly ineffective 
below that.

73, Jim Brown K9YC  


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