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@sbcglobal.net]
Sent: Saturday, November 17, 2007 9:24 PM
To: towertalk@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
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