Topband: Inverted "L"
Thu, 26 Apr 2001 14:46:21 -0300
Another means of reducing the unwanted radiation from the top section and still
getting the top loading you require for a reasonable impedance at the base is
to reduce the flattop portion to 1/8 wave by folding back 50 percent of its
length. (Leave the electrical end floating and connect the far end together.)
This would require decent spreaders. The RF voltage at the end would require
some reasonable spacing such as 8-12 inches, especially near industrial areas.
The T is a much better approach since much of the straight up radiation is
reduced. However, some top banders don't have equal distance supports in both
directions from where there tower is located making the folded back 1/8 wave
flattop a possible solution in getting more vertical and thus lower angle
If the vertical portion is only 50 feet you can add some inductance to raise
the feed point impedance or match it with a suitable network. Some limited
space installations use folded back radial in the same manner. I think they
would be better of with a wire mesh screen under the vertical portion and at
least 6 full length 1/4 wave radials even if there is some zig zagging to do on
the ends to make them fit into a small lot.
Herb Schoenbohm (ex-KV4FZ)
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