On the subject of unnatural shunt feeds, I recently helped
my neighbor, N0OQW with a shunt feed on his 70' tower. The
shunt wire is offset 1 meter and attached at the 69' level.
A ten foot mast on top contains a 3 el yagi tribander, then
a 6m dipole, and on top a 10 el 2m yagi. ALso, at the 70'
level are 80m and 40m inv vee dipoles which are electically
shorted to the tower when not in use. All of these provide
excellent top loading, especially the 80m dipole.
The guys at the 70' level are insulated at the top, the
guys at the 35' level are connected right to the tower.
When I put my Autek RF1 to the shunt wire, I detected a
min Z value of 440 ohms at 1.92 Mhz. Min SWR occurred
around 1.85 Mhz which indicates some reactance present.(The
tower is adjacent to metal buildings, including barn with
metal roof.)
We inserted a 9:1 transformer and adding about 300 pf of
capacitance in series resonated the system around 1.85 Mhz.
Saturday night during the contest, we were about 10 miles
apart. We broke at one point and went to SSB. His signals
on my meter were 25 over S9 with his gain down as far as it
would go(about 2 watts out)!!!.
Modeling his tower and all appendages with Nec2 shows
similar results. In particular, removing the 80m dipole
decreses Zr considerably. Also, the lower guys show
considerable current. The Nec2 plot shows almost a perfect
vertical pattern radiation. Takeoff angle 25 degrees,
about 1+ dbi gain.
Listening to N0OQW during the contest Saturday nite, he
had quite a pileup going at one point. If anyone remembers
working him and can recall his signal strength, please
reply to me offline. We would like some reports to see if
this is really working as well as it seems.
Thanks, Pat W0OPW
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