[TowerTalk] Shunt Feed Help

[W5LT]

Kim:
I question the accuracy of the impedance measurement. If it was an accurate
value at the base of the tower, you should have found a good match at around
200pfd of series capacitance. 
Z = R+jX = 65 + (+445-Xc) = 65 + (445-445) = 65 Ohms resistive. 
Then: VSWR = 65/50 = 1.3 

Try measuring again with a different type of instrument. You may be getting
some BC interference to the measuring instrument (quite common at 160m). I
use the AIM4170 from Array Solutions for all my antenna measurements and
find it quite accurate, even with BC interference.
73s

Bob, W5LT

-----Original Message-----
From: Kim Elmore [mailto:cw_de_n5op at sbcglobal.net] 
Sent: Saturday, November 17, 2007 9:24 PM
To: towertalk at contesting.com
Subject: [TowerTalk] Shunt Feed Help

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