You two guys don't seem to be communicating because you are both correct
(neglecting a few misworded comments).
Currents traveling down a tower will be partly shared by the coax. How
much is questionable.
It is also true that currents flowing common mode down a coaxial cable
will produce a differential signal at the coax end due to the shield
transfer impedance. If you look at the Polyphaser book you can see some
experimental data which illustrates this. They injected 1050 amps common
mode into a 51 ft piece of 1/2 inch hardline. There is a picture of the
resulting currents from both the center and shield. They didn't show
the differential, so you have to manually subtract these two signals.
When you do, you can see that there is a differential current peak of
about 600 amps. This current peak reverses direction about 100 us later.
How much of this actually happens on your tower is a function of how
much current actually flows in the coax. If grounded properly at top
and bottom, most of the current flow is in the tower and not in the coax
because the tower is much lower impedance. In the Polyphaser book they
made a calculation for the current in the coax for a 150 ft tower. I
don't think it is correct. I disagree with their value for tower
inductance and they treated the coax as having a free-space inductance.
It won't be the same when taped against a tower leg. Using their
method, two coax cables in parallel would produce a lower impedance than
the tower. They got a big number.
In polyphaser's experiment, they essentially had both the center and
shield shorted at each end of the coax. This doesn't happen on a coax on
a tower. One end is usually connected to an antenna. This provides an
oportunity to couple in more differential signal, because the center and
shield are not referenced to the same potential. In addition to that,
the antenna that it is connected to will ring like crazy, producing very
large currents pulses at the antenna resonant frequency. The exact
amplitude produced by this is hard to determine, but it is not
insignificant.
Jerry, K4SAV
Gary Schafer wrote:
>
>
>>-----Original Message-----
>>From: towertalk-bounces@contesting.com [mailto:towertalk-
>>bounces@contesting.com] On Behalf Of Jim Brown
>>Sent: Thursday, July 13, 2006 11:24 AM
>>To: Tower Talk List
>>Subject: Re: [TowerTalk] signals on inside of a pipe.. RE:
>>TTSHUTDOWN(was:4 awg copper wire and Amp locks)
>>
>>On Thu, 13 Jul 2006 10:40:38 -0500, Gary Schafer wrote:
>>
>>
>>
>>>As explained above there is a large voltage difference between the top of
>>>the tower and the bottom of the tower during a strike.
>>>
>>>
>>Gary,
>>
>>It is clear that you haven't understood a word that I have written, nor do
>>you care to learn anything that isn't already in your head. Thus, I give
>>up.
>>
>>73,
>>
>>Jim
>>
>>
>
>I suppose I could say the same Jim.
>
>73
>Gary K4FMX
>
>
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