----- Original Message -----
From: "Richard L. King" <k5na@ecpi.com>
To: <towertalk@contesting.com>
Sent: Sunday, January 06, 2008 5:09 AM
Subject: [TowerTalk] Grounding an Insulated Tower
> Hi everyone. HNY.
>
> I am near completion of a 155 foot 25G tower with an insulator 30
> feet above ground to use on 160M. I am interested in techniques for
> grounding for lightning protection and to bleed off noise from rain
> static, etc.
>
> The tower will be fed directly with the coax to the upper insulated
> part and the coax shield will be connected to several raised radials.
> The coax and the radials will NOT be connected to earth ground at the
> tower.
>
> I am worried that a direct lightning hit might cause me sever
> problems if I cannot bleed it off somehow. I have thought about
> lightning spark gaps and maybe a large inductor between the insulated
> section and the lower part of the tower, but not sure how that will
> work and what should be the values of those components.
>
> I do plan to ground the lower, unattached, section of the tower with
> ground rods.
>
> Thanks.
>
> 73, Richard - K5NA
>
Hi Richard,
I recommend that you bond the coax shield to the bottom 30 feet of
the tower (use some sort of coax grounding kit at the top and bottom
of the 30 foot length of tower) and then use a common mode choke
(i.e. current balun) at the feedpoint to keep the elevated radials
de-coupled from the grounded tower sections. In addition a 1.5:1 Unun
shunted between the upper tower sections (i.e. the vertical radiator) and
the raised radials would provide the impedance match between the 35 ohm
feedpoint impedance of the 1/4 wave vertical and your 50 ohm coax as
well as providing a DC path between the upper tower sections, the raised
radials, and the grounded lower sections. Lastly as K1TTT suggests, I
would add a spark gap between the upper tower section and ground to
clamp any voltages induced across the Unun or the common-mode choke
during a lightning hit. In fact I would add two spark gaps, one between
the raised radials and the grounded lower-tower section and one
between the upper tower section and the lower tower section.
Sounds like very nice antenna in the making. Good luck!
73, Mike W4EF...............................
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