[Towertalk] Shunt-Fed Tower EZNEC Simulation

[Guy Olinger, K2AV]

The tower resonance is the tower resonance, period. Nothing changes
that. That is a frequency at which RF current flows most easily in the
tower/antenna combo.

Everything else is forcing energy into the system at a non-resonant
frequency, transforming impedance, and tuning out the reactance. This
will require devices (in addition to a tap and drop wire) for sure. Or
another way of looking at it is that you will require a TUNER at the
end of a tap and drop wire.

Certain tap heights, and drop wire spacing on a given tower will allow
the TUNER to be as simple as a series capacitor, but whatever you do
will be forcing energy into a non-resonant system and tuning for match
to the feed coax.

Don't look for SWR at the end of the tap wire, that doesn't mean dink
in a non-resonant system. Look for Z=X+jY on your intended operating
frequencies that will allow you a straight-forward, and easily
switchable tuning device. There are a number of excellent ways to
accomplish this. Gamma and Omega match seem to be the most-touted
means.

Note also that once matched, almost anything you do on the tower will
require a rematch.

73, Guy

----- Original Message -----
From: "John Bodey" <jsbmjb@theriver.com>
To: <towertalk@contesting.com>
Sent: Friday, July 05, 2002 11:58 PM
Subject: [Towertalk] Shunt-Fed Tower EZNEC Simulation


> What don't I understand???
>
>
>
> EZNEC tells me that my 78' Rohn 45 with a small Warc-band beam is
> resonant at 3.0 MHz.  Moving the shunt feed connection higher or
lower,
> closer or farther from the tower only changes the match around 3.0
MHz!
> Everywhere else the VSWR is infinity.
>
>
>
> I don't understand how I get this tower to load on 160 or 75 meters?
>
>
>
> The 3.0 MHz resonance was verified with an antenna analyzer
connected to
> a 12 AWG wire dropped from the top with a 4-foot separation.
>
>
>
> Thanks and 73,
>
> W7AG, John
>
>
>
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