[TowerTalk] Ice Bridges

[EUGENE SMAR]

Greg:

     Read the Polyphaser book, <The Grounds for Lightning and EMP
Protection.>  In it their engineers recommend bringing ALL coax cables down
to ground level (both physically and electrically).  Where a commercial
installation uses an ice bridge to support the coax lines, there exists the
possibility that the coax shield will be raised to lethal voltages above
ground at the point it departs from the tower leg.  Bringing the coax to the
bottom of the tower and connecting it at this point brings the coax shield's
potential very close to that of the buried ground conductors, i.e., much
less than higher up the tower.

     What I take from reading Polyphaser's book is that all coax cable
should be bonded to and run away from the tower at or very close to ground
level where possible.  Most ham installations (except, possibly, for
repeaters at commercial sites) will be able to follow this design guideline.

73 de
Gene Smar  AD3F
-----Original Message-----
From: Greg Fox <fox@obsolete.org>
To: towertalk@contesting.com <towertalk@contesting.com>
Date: Thursday, July 20, 2000 12:08 PM
Subject: [TowerTalk] Ice Bridges


>
>
>Hi there,
>
>While working around large broadcast and celluar towers I've noticed
>one signifigant feature on these towers which the amateur equivelents
>don't have -- ice bridges.
>
>An ice bridge consists of a grating held up by two to three inch steel
>piping. The purpose is to protect the hardline/waveguide runs from the
>falling ice which is expected to break off the tower. More or less it
>creates a "bridge" from the tower to the shelter/cabinets which the
>lines run underneath of, and are protected.
>
>Is this important for an amateur radio installation?
>
>In most cases the towers are much smaller, the transmission line isn't
>in the 1 5/8" - 2" range, and there are not SHP microwave dishes atop.
>However, I still think it might be a issue with all the alluminum up
>in the air, due to the nature of HF beams.
>
>Any opionons/stories?
>
>
>73s de Greg/KE9R
>
>
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