----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: "Jim Lux" <jimlux@earthlink.net>; "Towertalk" <towertalk@contesting.com>
Sent: Thursday, December 02, 2004 3:50 PM
Subject: Re: [TowerTalk] elevated short vertical dipole or quarterwave
monopole?
<snip> in the loading
> > coil, however, you wouldn't have the loss in the ground
> radial array.
>
> You're neglecting or ignoring the fact that the earth itself
> has significant loss because of the high field intensity.
Precisely the effect I'm trying to get a handle on. In the elevated dipole
case, the field distribution at the ground surface is quite different from
the monopole. The high currents are farther from the soil, and I think that
it's H field losses (that is, eddy currents in the soil), not E field
(dielectric losses) that's going to be the dominant mechanism. If the
losses are the same, then it's close to being a wash, performance wise,
because they're basically the same physical size made of the same materials,
etc.etc.etc.
>
> You also have to get the feedline away from the dipole with
> making the feedline part of the antenna, and that's no easy
> task!
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.
>
> You also could end-load the dipole and have about the same
> radiation resistance as a quarter wave antenna.
>
> Too many variables!
Indeed.
>
> I made quite a few actual FS measurements when MFJ and
> HyGain were doing no-radial verticals. Antennas were in the
> clear, nothing else around. Feedlines were properly
> decoupled.
>
> As a general rule (although I'm sure there are exceptions) I
> found a 1/4 wl vertical with a modest ground of 15-30 1/8th
> wave radials was a few dB better than the best vertical
> dipole of the SAME tip height I could build, and I didn't
> have the horrid feedline isolation issues with the Marconi
> system.
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?
>
Tnx,
Jim, W6RMK
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