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## Re: [TowerTalk] Life expectancy of RG-213

 To: "towertalk@contesting.com" Re: [TowerTalk] Life expectancy of RG-213 "Jim Brown" Fri, 27 Aug 2010 15:23:20 -0700 mailto:towertalk@contesting.com>
 ```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 with >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 _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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