I have wondered the same thing many times. I used to shunt feed my old
tower. It had (my tower went down in a storm about 3 weeks ago) a tribander
and a bunch of other hardware at the top. The shunt feed had an impedance
of about 400 +j160 or so. Which meant I needed a 9:1 transformer and a
200pf cap to feed it. It worked pretty well with the auto tuner and I could
tune anywhere on the 160 meter band using the tuner (although it would have
trouble at about1890 from time-to-time.
With a side mounted yagi, I don't think the pattern gets distorted, but it
does change the characteristic impedance of the tower when resonated on 160.
The more stuff you hang on the tower, the more capacitance it sees at the
feedpoint.
In a shortened vertical, the inductance to load it is doubled if loaded at
the middle versus the base. Hmmmm I wonder if the reverse is true if you
hang a bunch of stuff, like a yagi, in the middle. I might try and model it
and see what happens. My gut tells me that I will see more capacitance due
to the center mounted yagi but that the "hat" effect would be 1/2 of what is
expected if mounted at the top.
Ford-N0FP
ford@cmgate.com
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