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Re: [TowerTalk] elevated short vertical dipole or quarterwavemonopole?

To: "Jim Lux" <>,"Towertalk" <>
Subject: Re: [TowerTalk] elevated short vertical dipole or quarterwavemonopole?
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Thu, 2 Dec 2004 20:23:51 -0500
List-post: <>
> I assume you meant "without" making the feedline part of
the antenna, and,
> yes, that is a problem. But, for the sake of discussion,
assume you run the
> feedline up the inside of the lower half of the dipole, so
the "exposed"
> part of the feedline is basically the same for both..
something coming out
> at the bottom of the antenna near the ground.

The feedline shield exits the antenna at a point with the
full potential of the lower tip of the antenna, and that
isn't good. You might as well tape the feedline to the
outside of the antenna as the inside, since the bottom of
the antenna is NOT an electrical 1/4 wl with respect to the
shield length along that area and thus there is almost no
decoupling at all.

Decoupling the shield is a real major PITA. Ferrites will
overheat and fracture, air-wound coiled coax is pretty bulky
and restricted BW.

You need many many kilo's of impedance on the shield, or the
shield to the rig is a major part of the radiating system
ala EH antenna and Isotron antenna.

> Interesting.  Did you ever contemplate where the
difference was coming from?
> Ground losses? A pattern difference because the phase
center is at a
> different height?

No idea. That was beyond the scope of what I could measure,
and no one cared. Loss is loss.

The two major vertical "dipole" problems I found were
decoupling the feedline (big PITA to do correctly) do to the
fact the electric field was so high (which means big
dielectric losses in anything lossy around the antenna base)
and loss of signal strength compared to a moderate ground
system with the same overall peak height.

I never tested a hat loaded 1/4wl monopole that would
increase voltage and decrease current at the antenna base
while increasing radiation resistance. The feedpoint was a
major headache because somehow you have to get the feedline
past that very intense electric field at the antenna base
when the antenna bottom is floated from ground.

The common solution is to simply ignore the problem and
pretend like the feedline isn't radiating.

73 Tom


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