[TowerTalk] 160m Shunt Fed Tower

[Stan Stockton]

Kelley,

Use an Omega Match.  You will  need two variable capacitors - one in  
series between center conductor of feedline and the shunt feed wire  
and another between the shunt feed wire and ground.

Stan, K5GO

Sent from Stan's IPhone



On Oct 8, 2011, at 3:47 PM, Kelley <kshaddrick at jetup.net> wrote:

>
> I have a 48 foot free standing tower. At the top is a Cushcraft A3S
> 3-element tri-bander with the 40m add-on kit. About 5 feet above  
> that is
> a 3-element 6m beam. About 2 feet above that is a 2m/440 vertical.  
> Also
> at the top of the tower about 1 foot out is the apex of a 80m  
> inverted V.
>
> On the opposite side of the tower from the 80m inverted V is my shunt
> wire. It is attached at the top of the tower and runs the entire  
> height
> of the tower. It is about 18 inches away from the tower. It is a  
> single
> 14 gauge stranded wire.
>
> There are 46 65' radials at the base of the tower spread out over 180
> degrees from north thru east to south. The area to the west of the  
> tower
> is blocked by the house so I can't put radials there.
>
> I have been unable to get any kind of reasonable match on this setup.
> I've tried a variety of variable capacitors and coils in various
> configurations with no luck.
>
> Connecting my MFJ-259B to the shunt wire and ground and adjusting  
> for 50
> ohms of resistance, I get a frequency of 9.145 MHz, R=50, X=46, and  
> SWR
> = 2.2.
>
> Setting the frequency to 1.8 MHz, I get a R=0 (it seems to stay at  
> zero
> no matter what frequency I set it at until I get close to 9.145 MHz),
> X=637, and SWR = >25.
>
> To be honest, I don't really know what these numbers are telling me.  
> The
> folks who taught me to build antennas only looked at SWR. Low SWR good
> to go. None of this R and X stuff. I suppose with dipoles one might  
> get
> away with this, but this antenna seems to be a different game.
>
> I'm looking for some ideas on what direction to head. I'm open for any
> suggestions. Is the wire too close to the tower? Too far away? Do I  
> need
> more wires? Is this tower configuration just not suitable for a shunt
> fed antenna? What additional data might you need to determine what's
> going on here?
>
> Thanks for any input you might have,
>
> -- Kelley - WØRK
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