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Re: [TowerTalk] RE: TowerTalk Digest, Vol 22, Issue 80

To: "Dudley Chapman" <>, <>
Subject: Re: [TowerTalk] RE: TowerTalk Digest, Vol 22, Issue 80
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Fri, 22 Oct 2004 15:33:49 -0400
List-post: <>
>   One caveat, though.  The scenario above, of bringing 60
ohms down to 32
> ohms applies to a vertical element that is physically
close to 1/4 wave.  In
> that case, the approx. 32 radiation resistance of the
vertical element
> radiates half the power and the rest goes into heating up
the 30+ ohms of
> ground resistance.

We have to be very careful assuming distributed losses are
directly conveyed to the feedpoint and that they are always
conveyed without modification by standing waves in the
ground system! There are a lot of things going on in that
"soup" we call ground.

Consider this. I can use a very short single radial loaded
with a high-Q inductor, and the base impedance will look
like the ground system has very low loss resistance. The
efficiency will be terrible if the antenna is near earth,
and yet I can actually have a feed resistance using a full
1/4 wl element that is below 32 ohms!

I've measured FS increases with additional radials long
after base impedance quits changing. I've had some radial
configurations that showed very low base resistance with
very modest efficiency and relatively high base resistance
with almost ideal efficiency.

The bottom line in complex resonant systems is when we want
to know something like efficiency or FS changes when we are
changing the system, we need to directly measure what we
want to know. We can't watch an indirectly related parameter
and expect it to be accurate.

While I've seen this with big antennas, a more common
example of this is my pickup truck. I can use antenna and
antenna mounting configurations that produce very low base
impedances that would make it appear to be low ground
resistance, yet the efficiency is worse than configurations
that have higher base impedance.

I think N6RK posted something on topband confirming this
with data he took with his land-based antennas.

73 Tom


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