>From: "clive penna" <clive@gm3poi.prestel.co.uk>
To: <topband@contesting.com>
>Date: Sat, 14 Mar 1998 10:59:04 -0000
>
>Eric and everybody interested in the discussion,
>
>I would like to see the results of an experiment involving the
>two types of antenna carried out in a manner similar to those
>carried out in Forensic experiments where to two items are
>compared in a LIKE with LIKE manner. In this context two
>verticals at the same site but not interacting, constructed in
>the same way, with the same uncluttered foreground, feed with
>same feed system. Always keeping in mind ' Like with Like, I
>feel sure that a meaningful outcome could be obtained where Mr
>average can decide whether to go with elevated radials or ground
>mounted.
This experiment has been done a number of times by various
researchers for various organizations ranging from the Broadcast
Industry and Universities to the Military (where I was involved
in a few tests). The results of non-flawed studies always have
returned the same results. The correctly sized, sufficiently
dense screen is superior to four resonant radials in close
proximity to earth.
The quantification of the differece has varied. The largest
number that I have seen referred to anywhere is 7 dB. The
Largest I have personally measured is 6 dB (5.8 actually). The
smallest difference I have observed is 3 dB. That was over
really good ground at a place called Wilcox Playa.
Wilcox Playa is a "dry" salt lake bed in the desert about 70
miles east of here (Tucson AZ). The center of the playa is
optically flat and (large) vegetation free for a radius of 5
miles or so. I put the word dry in quotes because it is seldom
actually dry. Usually it varies from moist but not muddy to 1/4
inch of standing water over the entire central area. Our tests
were done during moist but not muddy conditions. I don't recall
for sure now, but I think the conductivity for that area at the
time was taken to be near 20 mS/meter.
I can only assume that the soil under the 7 dB study was quite
poor. I would think that it would not be unreasonable for
someone with 3 to 5 mS/meter soil to expect 4 to 5 dB
improvement. Someone over dry sandy soil or rocky mountainous
terrain might get 6 or 7 dB. Again, it mostly depends on the
effective bulk conductivity (at RF) of the material under the
radiator.
>Perhaps the biggest problem in doing such an experiment is doing
>it at topband it may be easier at 40m with an adjustment in
>elevated radial height. By the way I use elevated radials in a
>80m 4 Square system and i'm happy with the results but I would
>sure like another 6db if thats what a 'quality' amateur band
>experiment came up with !!
>
>Clive GM3POI
>
>----------
Our tests were actually done at various frequencies between 9 and
18 MHz. But the results agreed well with work done for the
broadcast industry.
If you have the space, and 4 to 5 dB is worth the expense and
effort to you, by all means lay in a screen. You can save some
wire by configuring the screen as radials only at the corners of
the array over 90 degree arcs. Outside the array between the
arcs, the wires can be spaced 0.015 (or 0.03 if you don't need
the last 0.5 dB) wavelengths apart and run parallel to each
other. Within the central area of the array, you can run bonded
intersection perpendicular mesh made from wires at the same
spacing as above. You won't loose measurable effeciency if the
entire screen is several inches below grade. But be careful how
you do your bonding if the system is to be buried.
0.03 wavelength on topband is about 15 feet. Of course for 80M
it is half that.
If you do this, please make some ground lervel far field FS
measurements before and after and report your findings to the
group here. They don't have to be in terms of absolute
volts/meter. Merely dB relative to the same measuring
configuration for each ground system. You need only be a few
wavelengths out to get meaningful results.
>
>Snip... (quoted insert of my ground screen post) for brevity
>
73, Eric N7CL
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