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[TowerTalk] Shunt Feed Capacitor for Tower

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Subject: [TowerTalk] Shunt Feed Capacitor for Tower
From: (Pete Smith)
Date: Thu, 07 Feb 2002 08:52:34 -0500
At 07:14 AM 2/7/02 -0500, Tom Rauch wrote:
>> Remember that the F12 antennas have elements which are insulated from
>> the boom.  I wouldn't be surprised if that made for a less-effective
>> top-loading effect.
>......and worse yet they can arc across the insulators. The 
>elements need to be electrically grounded to the boom at the shunt-
>feed frequency in many cases to avoid arcing.

In an earlier message, Tom warned against this problem and suggested
modeling the voltage across a small load placed at the top of the tower as
a check on the voltages to be expected.  I've tried to do this, but must
not be doing something right, because for a 1-ohm resistive load I'm only
seeing a fraction of a volt across it.  Can anyone clarify how this should
be done properly in EZNEC?

I would be a little leery of a direct short between element centers and the
grounded boom, because of the potential effect on serendipitous resonances
and the resulting interaction between antennas.  In another earlier message
(which I can't find now), Tom suggested using a small inductance instead,
sized to pass 160m current but effectively block higher frequencies.

Because my antennas were already up, I elected to try low power and leave
the elements ungrounded.  At 200w, so far so good.  I am wondering, though
-- if and when I go to high power, should I expect to have to do this for
both elements in the highest antenna, or all elements in all the
insulated-element antennas on the tower?

Re capacitors, I'm using an omega match with a couple of 3000v, 500 pf
variables that I got from Ted Shannon at Henry Radio ("Ted Henry"
<>).  The price was right, and I think he has more.

73, Pete N4ZR

Sometimes a tower is
just a tower

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