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
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