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[TowerTalk] vertical antenna tradeoffs

To: "Towertalk" <>
Subject: [TowerTalk] vertical antenna tradeoffs
From: "Jim Lux" <>
Date: Fri, 1 Oct 2004 18:14:26 -0700
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Here's an interesting question...

Assuming you have a certain length radiator as a vertical, and you have a
choice between feeding it as a monopole or a dipole.  In both cases, the
radiator is short (compared to a 1/4 wavelength):

Putting a loading coil somewhere on the radiator, and feeding it as a
monopole.  You'll have to deal with the loss in the ground system.

Putting loading coils in the middle at the feed point like a "shorty 40", or
at 1/4 and 3/4 points (equivalent to center loading)

In the dipole case, the ground losses don't affect the antenna itself.

In both cases, the ground does affect the far field (in the usual vertical
antenna scheme).

If we assume NO ground losses in the monopole case, for 40 meters, it's
almost a wash between, say, a 12 foot center loaded monopole and a 12 foot
shorty forty scheme.  The Rrad is somewhat higher for the dipole case (2.4
ohms vs 1.4) but the loss resistance is also higher (two coils, vs one..).

This somehow seems incorrect....

I used the antenna handbook equations for calculating the radiation
resistances of the loaded antennas and the L required, etc. (15 uH in the
monopole case, 34 in the dipole case)

This also doesn't take into account any losses in the matching networks to
get the feedpoint impedance to 50 ohms, but I assume that would be similar.

There is also a big pattern difference between a dipole with the feed at
zero feet (i.e. the monopole) and a dipole at lambda/4 off the ground, at
least over conductive ground.


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