Topband: Shunt feed - how high is high enough?

[Pete Smith]

>From answers to a query on Towertalk, as well as some modeling with NEC2,
it appears that I can obtain 50+j250 or so at the bottom of a shunt feed
wire spaced 30' from my tower with the tap to the tower at about 30 feet,
or a considerably higher resistive component (and lower inductive) if I tap
at 60 feet (right below the first tribander in a 2-high stack).  

Convenience of a smaller value series capacitor aside, is there any reason
from a loss or field strength standpoint to prefer the higher tap?  If so,
I had in mind using an UNUN after the cap to convert the feedpoint down
close to 50 ohms.

The total tower is 97 feet, plus 8 feet of mast.  The tribanders are at 69
and 97 feet, with a Force 12 shorty forty at 105 feet.  I will initially
just be using the tower's lightning ground, laying in radials on or just
under the ground as time and weather permit.

Thanks for any advice.

73, Pete N4ZR
Happy Holidays!



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