On Tue, 16 Jun 2009 15:33:15 -0400, Bill Tippett wrote:
> I believe this might be another broadcast myth (like
>supposedly needing 120 1/4 wl radials) and which is NOT mandated by
>the FCC. About 10 years ago I decided to make a ground screen for my
>180' Topband vertical using ~16 50' X 4' rolls of welded galvanized
>fence wire (with rectangular openings about 2" x 4"...not the smaller
>gauge woven hexagonal chicken wire). I couldn't tell any significant
>difference in performance with this laid over my existing radial
>field (about 110 radials of 100-150') although I didn't actually make
>before and after measurements.
I generally agree about the "broadcast myth" part -- there was a lot of
"go for broke" mentality in it. Remember -- in the days where this was
developed and practiced, AM broadcasting was all there was, and it was
a very big deal! HOWEVER -- one place ground screens were employed was
multi-tower directionals, where very small differences between towers
make a big difference in the depth of nulls, which is the whole point
of those patterns. This IS a very good reason for using them (and for
making the radial system as good as practical). This is the direct
equivalent of a ham 4-square vertical array, and the arrays at K9DX.
The only important difference is that AM BC arrays were optimized for
the nulls, and ham arrays are usually optimized for gain. If you want
the nulls to be wonderful, you need to pay lots of attention to the
symmetry of everything, including the radial system.
I DO fully agree with with Rudy's analysis of the relative importance
of copper close to the tower, and as far as ham radio is concerned, I
see the ground screen as simply another way of achieving that.
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