Topband: Ground Screens - Another Small Space Option

[Jim Brown]

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