[TowerTalk] signals on inside of a pipe.. RE: TT SHUTDOWN (was: 4 awg copper wire and Amp locks)

[Jim Brown]

At 12:18 PM 7/12/2006, Gary Schafer wrote:

>If the shield of a coax cable carries a lightning strike about half of the
>energy gets transferred to the center conductor of the cable and propagates
>down the cable just like a radio signal would. This includes the propagation
>delay caused by the velocity factor of the coax.

No. Coax acts as a nearly ideal common mode choke for signal injected end to end. 
The only component of that common mode signal that shows up between center and 
shield is the part below about 1 kHz, where the common mode choke behavior kicks 
in (see Ott for a proof of this). 

Now, if there is an antenna connected to the coax, the lightning will induce some 
voltage between the center conductor and the shield, just like any other EM field 
would, and THAT signal will propagate down the coax just like any other signal 
induced on the antenna.

And, of course, when the center conductor of the coax vaporizes or the dielectric 
melts, all bets are off. :) 

I also agree that for lightning protection, cable running in conduit should be 
bonded at both ends so that the conduit doesn't choke the discharge. 

73,

Jim Brown K9YC