[TowerTalk] Shunt Fed Tower

[Kelley]

Thanks for all the replies both on the list and privately.

Today, being a beautiful day here in Minnesota, 60 and sunny, I took the 
afternoon off and ran 8 more radials. It didn't really impact the SWR on 
my shunt feed much at all.

I also added another variable capacitor as many of you suggested. I was 
able to get the SWR down to 2:1 @ 1.8 MHz (R of 28, X of 28).

It's a bear to tune though, whenever I get near the capacitors, my 
physical presence changes the readings by quite a bit, resulting in a 
lower SWR reading until I back away.

Kelley - W0RK

On 11/7/2010 4:55 PM, Kelley wrote:
> 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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