Topband: Ground Screens - Another Small Space Option

[ZR]

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 at audiosystemsgroup.com>
To: "'TopBand'" <topband at 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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