I got the idea of the mesh at work from a Tech Class engineer but dont
remember him referencing HRM. This was in the mid 80's and I was complaining
about the poor performance of a shunt fed 100' tower with 10/15/20 4el yagis
for top loading and 60 radials of 60-130' or so.
Since chicken wire is only twisted, thin, and very cheap, I decided its
utility would be very short lived and went with the 2X4" rabbit wire which
is welded, hot dip galvanized and plastic coated. Quite expensive (no China
brands) but I found 5 4x50' reels in the next town in the local "Want
Advertiser" that had been used less than a year and was a fraction the cost.
When I moved here it came with me and its been rolled up in the woods since
1989, still looking fine. I went with elevated radials instead for the 2
element 1/4 waves.
Carl
KM1H
----- Original Message -----
From: "Jim Brown" <jim@audiosystemsgroup.com>
To: "'TopBand'" <topband@contesting.com>
Sent: Monday, September 24, 2012 7:31 PM
Subject: Topband: Ground Screens - Another Small Space Option
Those of us who have worked in broadcasting know that most AM broadcasters
use a heavy copper mesh (typically 40' - 64' square) surrounding the base
of a tower, with radials connected at the perimeter. I recall KM1H saying
that he's used a mesh for years, and I've recently come across NC0B's
discussions on the topic. He first wrote about it in Ham Radio in May
1977, and did a presentation at Dayton in 2009.
He describes an installation of a 60 ft Tee vertical with 200 ft
horizontal and 600 sq ft of mesh laid out in strips at 90 degrees to each
other, centered at the feedpoint, then with the addition of 20 or 40
radials 100 ft long.
The gist of his work is that 400 - 600 sq ft of mesh is a good design goal
for antennas of that height, that 1/2-inch galvanized hardware cloth
works.Measurements of the 60 ft Tee vertical with the ground screen alone
was 4 dB below an ideal ground, adding 20 or 40 radials 100 ft long
increased field strength by 0.4 and 0.9dB respectively.
When thinking about this, bear in mind that the radius illuminated by the
fields from a vertical are directly related to the height of the vertical,
so a taller antenna would require longer strips of mesh to achieve
comparably high values of efficiency. But in his Ham Radio article
describes some quite small ground screens that had to fit in very limited
spaces, and says "that they worked well." On the other hand, his 2009
slides consider 100 sq ft a minimum.
Both the paper and the presentation are well worth study. The Ham Radio
article is on his own website, the Dayton slides are on K3LR's website.
73, Jim K9YC
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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