Topband: shunt feeding anomaly?

[Pete Smith]

After hearing CN8WW rolling in on 160 during CQWW, I've got the bug.  The
best thing for me to do to get on 160 is to shunt feed my tower, which has
a side-mounted C-3 at 69 feet, one top-mounted at 98 feet, and an EF-240S
at 104 feet.

Reading ON4UN's book, I was expecting to have to tap to the tower somewhere
between the middle and top tribanders.  However, an EZNEC model has me
wondering.  I find, as I "move" the tap point down the tower from the top,
keeping a constant spacing between the shunt wire and the tower center, the
real part of the feedpoint impedance (at the bottom of the shunt wire)
starts quite high (~300 ohms), and doesn't reach 50 ohms until around 22
feet above ground.  At that height the source data show 52 +j191.  My
assumption is that series C is then used to bring it to (more or less) 50 +j0.

I've been pretty careful with the model, increasing the number of segments
till the results converge, and also keeping segments carefully aligned
between the tower and the closely-spaced gamma wire.  I guess what I'm
looking for here on Topband is a real-world sanity check and a fresh eye on
what I've been doing.

Thanks in advance!

73, Pete N4ZR

Contesting is ... Extreme Radio

The World Contest Station Database 
is back up and running at
http://www.qsl.net/n4zr 


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