[TowerTalk] Shunt-feeding tower with stacked tribanders

[Pete Smith]

I'm firming up plans for my stack to go up this spring, having actually
found a used C-3 that I can afford.  At the same time, I'm looking to shunt
feed the tower for 160m, and am not finding much info on shunt feeding
towers loaded this way.

My tower is 97 feet of Rohn 25, with a Force 12 C-3 at 98 feet and a Force
12 EF-240S at about 105 feet.  The second C-3 will go on a side mount at
approximately 69 feet.  Later, a third one may go on at 35 feet, but first
things first.

The first order of business is to figure out how high on the tower the tap
will need to be.  ON4UN's book gives good data for the electrical length of
a tower loaded with various yagis on top, which leads me to conclude that
my tower's electrical length without the lower C-3 is probably about 120
degrees, which equates to tap heights of 19-27 meters depending on spacing
of the gamma wire from the tower. 

How much will the second C-3 affect the electrical length, given that it is
not placed near the top?  Can I more or less ignore it?

Once I get the gamma wire mounted properly, it appears that I should be
able to use my antenna analyzer to measure the R+X and figure out how large
a capacitance will be needed to cancel the reactance.  I've been told that
I can use an open stub of RG-8 cable to provide the capacitance, but don't
see any references to that technique in ON4UN.

Will it work?  For the moment, power level on 160 will be only ~100 watts.

Thanks for any advice, anecdotes, etc. - and Happy New Year, everyone!



73, Pete Smith N4ZR
n4zr@contesting.com 

Sometimes a tower is just a tower

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