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[TowerTalk] Shunt Fed Tower

To: "Tower and HF antenna construction topics." <towertalk@contesting.com>
Subject: [TowerTalk] Shunt Fed Tower
From: Kelley <kshaddrick@jetup.net>
Date: Sun, 07 Nov 2010 16:55:37 -0600
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
I'm having some tuning issues with my shunt fed tower.

  The current configuration is a 48' Rohn HDBX tower with a Cushcraft 
A3S beam (with the A743 40m add-on kit), a Cushcraft A502S 6m beam, and 
a 2m/440 vertical on top. I also have a 80m inverted V with the apex at 
the top of the tower.

The shunt feed is connected to the top of the tower (opposite the 80m 
inverted V) and runs down the tower about 18" from the tower to the 
bottom of the tower. There are currently 4 64' radials that cover an arc 
from roughly the northwest to the east (the house prevents anymore 
radials to the west and another vertical prevents radials farther to the 
south). I plan on adding additional radials within the existing arc 
(hopefully before the snow comes!).

At 2 MHz, I get a SWR reading of 1.2 (R of 49 and X of 10). However, as 
I go lower in frequency, the SWR goes up. At 1.9 MHz it's 6.1 with an R 
of 13 and an X of 26. At 1.8 MHz it's 25+ with an R of 0 and an X of 49. 
All measured with a MFJ-259B.

I do have a variable capacitor in the circuit at the bottom of the 
antenna. I do not know the range of the capacitor (see my earlier email 
concerning the Dentron AT-1K tuner). About 50% mesh of this capacitor 
gives the above readings. Anything other setting and the SWR goes to 25+.

Reading a number of articles on the internet, it appears that almost 
every installation is in some way unique. Some are very simple like 
mine, some involve multiple wires and complex matching circuits.

Does anyone have any thoughts on how I can get this setup to match 
better at a lower frequency? Will adding more radials do it? Would 
moving the wire farther from the tower? I guess this antenna is kind of 
a mystery to me and I'm not sure what direction to take.

I appreciate any direction that anyone can point me in.

Thanks,

Kelley - W0RK

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