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Re: [TowerTalk] WTB: GAP VOYAGER and a ????

To: "Daniel Hileman" <>,
Subject: Re: [TowerTalk] WTB: GAP VOYAGER and a ????
From: "Dan Zimmerman N3OX" <>
Date: Thu, 8 May 2008 19:41:24 -0400
List-post: <">>
>Just a thought, I've always heard on verticals that generally the
>more radials the better (up to about 120 of them) but the literature says
>this antenna doesnt really need them (other than the 3, I think, that comes
>with it)

That should raise a big red flag ;-)

>I thought about putting out 60,
>120' radials out, BUT will this mess up the performance or tuning of the

If you can put out 60 120' radials, don't waste your time with a
no-radials antenna... but more on that in a second.  To directly
answer your question, yes, as it turns out, it's very likely a big
radial field underneath a GAP antenna (but not attached to it) will
help its performance on the lower bands.  It will probably make the
tuning narrower.  Very short "vertical dipole" antennas like the GAP
designs end up losing energy to the ground because of coupling from
the densely stored electromagnetic energy around the antenna into the
earth (near-field ground loss).  The fact that no part of the antenna
touches the ground is kind of a red herring that no-radials
manufacturers like to trick people with.

The bottom of the antenna is at quite high RF voltage, and as the
bottom of the antenna oscillates back and forth from positively to
negatively charged at RF frequencies, the electric field around it
shoves  charges in the earth  around too... in other words, you get
*ground currents* even if the antenna isn't touching the earth.

If the electric field is shoving around electrons on a dense radial
system installed below the antenna, that's not nearly as bad.  I can't
tell you how many dB this is worth and neither can the manufacturers
(I bet they don't want you to know :-) )   I don't think models
capture this well, if at all.  Maybe NEC4 does better.  All I know is
that EZNEC seems awfully optimistic about a capacitance hat /small
counterpoise a few feet off the earth.  I can nearly guarantee you
that measurements would show much, much worse performance than my
models of short, low vertical dipoles seem to.  The high RF voltage
also causes serious currents to flow to lossy earth on your feedline
and supporting mast.

If you can put down radials in such quantity, you can do SO much
better than a short, no-radials antenna... really.  Putting up
someting as tall as possible and fed against those radials is really
the way to go.

Now, since you're saying Voyager first, more later, I'd do the radials
and you can recycle the aluminum from the GAP.  If you're paying full
price though... I dunno... I think there could be better choices.  If
you do put those radials on the ground underneath the Gap to screen
the earth from the GAP's electric field... well, then you're done with
that chore ;-)


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