[TowerTalk] Lightning Protection Question

[Jim Brown]

On Sun, 12 Sep 2010 11:13:04 -0500, Gary Schafer wrote:

>The reason that the coax protectors (polyphasers) should be at the single
>point ground panel (shack entrance) is because any appreciable run of coax
>can have current induced onto the CENTER conductor of the cable when there
>is high current induced or carried on the outer shield of the cable.
>Inducing current onto the center conductor of coax cable may sound strange
>but that is what happens. For non believers a simple google of the term
>"transfer impedance" will explain how it happens.

YES!  In ideal coax, there should be NO induced voltage, because the mutual 
coupling would be ideal. That requires a shield with zero resistance, and a 
shield that is perfectly uniform. But in real coax, there is resistance in 
the shield, and the shield has SOME degree of non-uniformity. Ott observes 
that at low frequencies, the transfer impedance is approximately equal to the 
shield resistance. 

73, Jim K9YC