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

[Gary Schafer]

> -----Original Message-----
> From: towertalk-bounces at contesting.com [mailto:towertalk-
> bounces at contesting.com] On Behalf Of Jim Brown
> Sent: Wednesday, July 12, 2006 3:10 PM
> To: Tower Talk List
> Subject: Re: [TowerTalk] signals on inside of a pipe.. RE: TT SHUTDOWN
> (was:4 awg copper wire and Amp locks)
> 
> 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

Jim,

What common mode are you talking about? Coax carrying current usually
happens on a tower when one end of the coax is at the top and the other end
is at the bottom with lightning striking at the top.

The energy between the center conductor and the shield at the bottom is not
common mode energy as the signal on the center conductor is delayed by the
velocity factor of the coax and arrives at different times. That gives a
differential voltage at the bottom.   See Polyphaser.

73
Gary  K4FMX