Kelly:
You might have to move the tap to the tower up or down to get an
appreciable dip at resonance on 160M. Another degree of freedom available for
shunt-feeding is moving the shunt wire even farther away from the tower face.
On my tower I use two #14 wires separated from each other by 18 inches and held
off the tower face two feet by T-shaped insulators made from 3/4 inch PVC
tubing.
If that fails, you might have to try an Omega match at the bottom of the
shunt tubing. This match is essentially a variable cap in series between the
coax and the tubing, and another cap in shunt to ground at the junction of the
series cap and the tubing.
Finally, you might have to ground the reflector of the XM240 to the boom
to provide more capacitive loading for the tower. But caution here: I can't
say what will happen to the performance of your Yagi when/if you do this. My
Skyhawk antenna is simlar in design to a F12 C3 and the designer of that
antenna ok'ed the parasitic-to-boom connections. Caveat Amateur.
GL es 73 de
Gene Smar AD3F
-------------- Original message ----------------------
From: "Kelly Jones" <kjones@virtualcohesion.com>
> TTs,
>
> A couple of us spent the day today trying to get a shorty tower loaded
> for 160m. Specifically, the tower is 42 feet tall with 4 feet of mast
> out the top. On top of the tower is an XM240 2 element 40m beam. The
> reflector is not grounded to the boom.
>
> Being that I sit on top of decomposing granite (IE: the Colorado
> Rockies), there isn't such as thing as Earth ground. We were able to
> string out twelve 64 foot radials. These are either just laying on the
> ground (in the wooded areas) or just under the surface in the "yard".
>
> We originally started with 1 shunt made of 1/2" copper tubing 6" from
> the face, but ultimately went with two shunts of 1/2" copper tubing
> spaced 1 foot from the tower face. It seemed no matter what combination
> we tried (just inductance, just capacitance, and both), we could not get
> any resonance for 160m. We were able to find a chunk of 80 that gave me
> about 100 khz with reasonable SWR.
>
> I see that guys have made "shorty" towers work on 160 but just could not
> find the magic combination today. So I defer to the experts for
> suggestions.
>
> 73
> Kelly - N0VD
>
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