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

[Jim Brown]

On Wed, 12 Jul 2006 21:51:07 -0500, Gary Schafer wrote:

>What common mode are you talking about? 

Common mode voltage is that which is equal on the two conductors 
and induced between one end of the cable and the other. In other 
words, lightning is lighting up the coax end to end.

>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. 

Yes, and much of that voltage is common mode -- that is, induced 
from one end of the coax to the other. Voltage that propagates 
down the coax is DIFFERENTIAL MODE voltage -- voltage induced on 
the ANTENNA that is connected to the coax. And as I said in an 
earlier post, that differential signal WILL behave like any other 
signal on coax until the coax vaporizes. Remember that the 
differential mode voltage is defined as the voltage between the 
center conductor and the shield. 

Another point. There are many forms of antennas and ways to hang 
them. Many of us have WIRE antennas that are connected by coax to 
our stations, and may not have them associated with a tower. I've 
never had a dipole with the coax coming down a tower. 

But that coax on the tower is going to be in parallel with the 
tower, and if it is bonded to the tower at both ends, nearly all 
of the current is going to flow on the tower, both by virtue of 
its cross section (less inductance and resistance), and because 
the coax acts as a common mode choke. 

Jim Brown K9YC