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[TowerTalk] Disconnecting coax

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Subject: [TowerTalk] Disconnecting coax
From: (Bill Nelson)
Date: Wed, 3 Jun 1998 21:45:24 -0700 (PDT)
>I too have wondered why one would ground one's incoming coax cables, thus
>providing a much better lightning target than a "floating" situation.
>Based on the reasoning, would lightning rather strike a well-grounded
>vertical metal pole, or a dry fiberglass pole?  I opt for floating.

The people who put the disconnected end of the coax in a mayonaise
jar (or any other type of jar) are fooling themselves.  Do they really
belive that - after a stroke that has traveled thousands of feet - that
a few more feet is going to matter?  In other words, the strike would
jump from the terminator in the jar to the best ground path. 

There is a major problem with a floating coax shield/antenna. Any
strike is then going to travel down both the center conductor and
the shield - so much of the full strike will travel the full length
of the coax.  If the coax is grounded to the mast/tower near the
antenna, as well as at the base, then about half of the current that
is being carried by the coax will be shunted to ground there. The
tower itself will also ground much of the current - if the antenna
is of the DC ground design.

>I guess that in the case of a very extensive ground field, like ones for
>commercial tv towers?) the massive charge of a direct hit can be
>dissapated with no harm, but I wonder if any normal ham will actually
>spend the money and time to go to that extent.  Thus, for the "regular"
>ham, isn't it better advice to float antenna coax incoming terminations? 

It isn't that expensive.  A very good protection system can be installed
for a couple of thousand dollars - including a good entrance window. With
the cost of a top-of-the-line transceiver being several times that - plus
all the other gear Hams usually have in their shacks - it is foolish to
cut corners in a lightning prone area.

>This is such a field of unknowns.  de K4VUD

Not really. The phenomenon is pretty well understood - as far as antennas
and transmission lines are concerned.  See the web pages by Polyphasor and
I.C.E. for more detail.

Bill Nelson

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