> -----Original Message-----
> From: towertalk-bounces@contesting.com [mailto:towertalk-
> bounces@contesting.com] On Behalf Of Jim Brown
> Sent: Thursday, July 13, 2006 1:54 AM
> To: Tower Talk List
> Subject: Re: [TowerTalk] signals on inside of a pipe.. RE: TT
> SHUTDOWN(was:4 awg copper wire and Amp locks)
>
> 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.
It is far from common mode. The voltage difference between the top of a
tower and the bottom of it can be many thousand volts due to its inductance.
A coax cable hung on the tower, even if grounded at several points as it
comes down the tower, will share a portion of the strike current that the
tower carries. That puts a large differential voltage between coax ends.
As explained in my earlier post because of the large low frequency current
on the shield, energy will be induced into the center of the coax and onto
the center conductor. Because of the delay in propagation inside of the coax
compared to that outside the coax there will be a differential voltage
developed between center and shield at the bottom end. This can all occur
even with the center conductor shorted to the shield at the top of the coax
even with no antenna attached. Or with shorted type feed system on antenna.
>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.
As explained above there is a large voltage difference between the top of
the tower and the bottom of the tower during a strike. All lines in parallel
with the tower will share the total current. All have inductance including
the tower. To think that there will be no or little current carried by
these lines is a mistake.
73
Gary K4FMX
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