Topband: Shunt feed - how high is high enough?

[Pete Smith]

Fascinating, Earl, and thanks for doing all that work!

My NEC-2 model (EZNEC 1.0) probably incorporated all the inaccuracies that
you anticipated, because I used a 12" "wire" 97 feet tall for the tower, a
#14 shunt wire and 30" spacing, plus a 2" diameter mast and 2.5" diameter
yagi booms to simulate at least part of the top loading.  I tried to make
sure the segmentation of the parallel "wires" matched, but I'm sure all
those different diameters, plus the close spacing of the shunt wire to
another wire of vastly different diameter, probably loaded the model with
inaccuracy.  My 50-ohm point appeared at 25 feet above ground, and the
60-foot point gave a resistive component of almost 450 ohms.    I've never
shunt fed a tower, so I didn't know whether to believe the model or not.
I'm sure you have sufficient real-world experience so that you feel pretty
comfortable about your tap points making more sense.

I think I'll fabricate the shunt arm and put it on the tower as soon as we
have a break in the weather, measure the resulting feedback, adjust as
necessary, and report back.  I also think I'll start at your 50-ohm tap
point, not mine!

Would you be willing to share your Rohn 25 model?  I'd love to play with
the two models -- with and without the loading -- and see how the
comparison works out. 

Thanks again for your help.

73, Pete N4ZR
Happy Holidays!



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