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Re: [TowerTalk] Shunt Feed Help

To: cw_de_n5op@sbcglobal.net, towertalk@contesting.com
Subject: Re: [TowerTalk] Shunt Feed Help
From: TexasRF@aol.com
Date: Sun, 18 Nov 2007 11:25:41 EST
List-post: <mailto:towertalk@contesting.com>
 
Kim, assuming the 65 +j445 is a good number, adding -445 ohms (198 pF) in  
series with the feed should result in 65 +j0. This in turn would yield a vswr 
of 
 1.3.
 
So, a good first step would be adding the capacitance and confirming the  
expected reduction in vswr as a result. From that point there are a number of  
matching schemes to reduce the impedance from 65 to 50 ohms. A simple network  
for example would be a shunt C across the feed line and a series L between the  
feedline and shunt feed wire.
 
In actual practice, the series L can be created by increasing the value of  
the series C enough to end up with 65 +j25 or 30. This is in fact exactly what  
the Omega match does and works just fine in the case you described.
 
You can expect a fairly narrow vswr curve due to the Q of the shunt feed  and 
to a less extent by the Q of the omega capacitor.
 
If you are willing to add a motor to the series C you can resonate the  
system remotely from your shack, allowing operation across the whole band.
 
73,
Gerald K5GW
 
In a message dated 11/17/2007 9:24:51 P.M. Central Standard Time,  
cw_de_n5op@sbcglobal.net writes:

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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