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[TowerTalk] Shunt Feed Help

Subject: [TowerTalk] Shunt Feed Help
From: Kim Elmore <>
Date: Sat, 17 Nov 2007 21:24:02 -0600
List-post: <>
I'm sure shunt feeding towers has been beaten to death numerous 
times, but I have a specific question.

I'm shunt feeding my 48 ft tower at 1.8 MHz, with various antennas on 
an 8 ft mast above the feed point. I measured the impedance with a 
noise bridge and found it to be Xr = 65 ohms X = +445 ohms (inductive).

The resistive part of the feed point impedance will probably get 
closer to 50 ohms as I add radials (there are none right now -- only 
three ground rods and the connection to the service ground and shack 
ground so all grounds are strapped together).

I initially I tried feeding it through a 500 pF vacuum variable, and 
I also tried varying the spacing of the shunt from the tower, all 
while watching the SWR. I couldn't find any combination that worked. 
Tomorrow, I'll look at how the impedance changes as I change the 
capacitance. I also have a 1000 pF vacuum variable that I've not 
tried yet. I've seen examples that use a 500 pF variable in series 
with the shunt and 1000 pF variable between the shunt and ground, but 
I haven't tried this, yet.

Am I on the right track? What other tack should I take?else should I 
try? I have a good sized B&W coil should work for an L network. 
Should I simply try that?

Kim Elmore, N5OP


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