Mike wrote:
> I have a 48' Delhi Self supporting tower, TH6DXX @ the tower top and 6m
beam
> on top of the 16' mast that extends past the trust bearing. See yesterdays
> posts for FULL description and link to URL with a .jpg photo
> Electrically this is all maybe 80'-90' tall (?)
> I attached a piece of 3/4" PVC at about the 34'-35' level.
> I attached a #14 AWG insulated stranded wire to the tower at this level
and
> fed it throught the PVC and brought
> it down vertically to ground level at aprox 2'-2.25' out from the side of
> the tower.
> I brought my coax from the shack and inserted a 800pF air variable in the
> center line between the coax and the gamma wire.
...snip...
> The cap is barely meshed (lowest capacitance) for resonance @ 3.525MHz.
SWR
> is below 3.1:1 @ 3800MHz
> Moving the cap 1/2" (roughly) moved resonance from 3.450 to 3.525 (quite
> touchy)
>
This tower is quite tall for 80 meters and appears to look inductive. The
shunt will also look inductive. The capacitive reactance required for
resonance requires a very small cap. I'm wondering how much capacitance he
has here. I'm not sure about the 1/2" measure on a capacitor but perhaps
50pF? 3.6MHz & 50 pF is 884 ohms Xc. 100 pF is 442 ohms, etc.
It would be nice for him to get a tool for measuring feedpoint and try two
things. 1) measure what he has; 2) move the tap point further up the tower
if R<50 ohms, down if >50 ohms; and 3) move the shunt closer to the tower.
On 160, I had great success with a similar sized tower running 450 ohm
feedpoint and having the shunt 1 meter away from the tower and tapped at
68'. For 80 meters, he may be close to where it should be but for an
elevated feedpoint impedance. It is very difficult to advise without
knowing some real data other than SWR. Knowing Z and SWR you can calculate
R +/-j to make more sense of the data.
Just some thoughts. I hope this helps.
Ford-N0FP
ford@cmgate.com
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