Jim -
I agree completely with your assessment of the expected performance of 4 low
elevated radials.
Such an arrangement takes care of current return (so will 1 radial), but
does a lousy job regarding the other fuction of a ground plane, namely
screening the near-field from lossy earth. ("lousy" equals about 2 to 4 dB
worse than it could be)
The main reason for this response is the reference to "radials" that
continues to be made regarding the WWVH antenna referred to. That antenna
is essentially a coaxial sleeve type half wave vertical dipole. The sleeve
in that case just happens to be a conical wire cage. It has a very nice
feature, in that it can be fed underneath by coax. The reference to the
lower part of the antenna as "radials" probably confuses many folks as to
what they are looking at. The radial field at ground level is, of course, a
screen, not a current return, and would be just as effective if it were laid
out as perpendicular wires in a mesh, as long as the spacing was adequately
small.
Vy 73, Ken
----- Original Message -----
From: "Jim Reid" <jimr.reid@verizon.net>
To: <rthorne@tcac.net>; "Josh T" <kb0gus@hotmail.com>
Cc: <towertalk@contesting.com>
Sent: Saturday, July 21, 2001 4:53 PM
Subject: Re: [TowerTalk] homebrew 20m ground mouted vertical... good idea??
>
>
> Some one suggested:
>
>
> > If you can elevate the vertical about 10' or so and use
> > 3 or 4 elevated radials...
>
> You will be dissapointed!! The E-field lines at the ends
> of the resonant elevated radials will couple strongly to
> earth at such a low height! The result will be that both
> xmit'd and rcvd signals will be down from 3 to 6 dB
> from those of a horizontal dipole up 1/2 wave or so
> above ground. The strong voltage points at the radial
> ends couple RF current into the soil where half or more
> of your rcvd and output power will be lost to heat!!
>
> If you put such an array up at least 1/2 wave, or about
> 33 feet or so to the feedpoint of the vertical element and
> the four "radials" you will not loose so much power, but
> you also will not have the "gain" off the sides of the dipole;
> but at least reasonalbe omni spreading of all of your output
> power. Also, the radials need to be horizontal up there
> at that height to avoid coupling to earth.
>
> Out here on Kauai at WWVH, they found they had to put
> in a very extensive radial field beneath their elevated
> vertical 15 mHz antennas, which also use the down
> sloping radials. Have a look at this photo of the installation
> at WWVH; you can see the radial lines, about 120 of
> them around the antenna base. Look under the fence
> rails about the enclosure:
>
> http://www.boulder.nist.gov/timefreq/stations/wwv.html
>
> or a close up:
>
>
http://www.boulder.nist.gov/timefreq/images/radiostations/wwvh-large/wwvh5.j
> pg
>
> This is a 15 mHz, 1/2 wave vertical array, with several down
> sloping radials. The ends of the radials terminate at those long
> white insulators at the lower ends. These towers are spaced
> 1/4 wave apart and driven 90 degrees out of phase: the
> result, a cardioid pattern favoring out at the far Western
> Pacific. Note that the feed point is up something over 1/4 wave in
> height and the ends of the "radials" are up nearly an eighth
> wave; 120 ground screen radials were still needed to
> recover the field strength out in the Western Pacific.
>
> 73, Jim KH7M
>
>
>
>
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