Greetings,
I'm looking for some guidance/tips/etc. on my 160m shunt-fed tower
experiment. Tower is 84' of Rohn 45, guyed in two places, guy wires
broken up with insulators. First set of insulators is 5' from torque
arms. For antennas, I have a XM240 at 95', a KT34A at 85', and another
KT34A at 45' (fixed east). I have a couple of inverted vee's hanging
off of sidearms. 80m is at 80', 40m is at 37'.
I've read the info in ON4UN's Low Band DX'ing book, as well as ARRL
Antenna Handbook. The gamma wire is #12 THHN stranded. The gamma arm
is 5' of 1" diameter aluminum tubing. I have a 150~370 pf air variable
capacitor (big honkin' breadslicer) in a wx-proof housing for the series
capacitor. There is a 5' long pvc water pipe mounted at 20', and the
gamma wire slopes from this point to the series cap mounted at base of
tower. The gamma wire spacing is constant from this insulator point to
the gamma tap arm.
Iteration Tap Point Spacing Resonant Freq.
--------- --------- ------- --------------
1 36' 42" 2.6 MHz
2 36' 42" 3.0 MHz *
2 42' 42" 2.8 MHz
3 50' 42" 3.2 MHz **
4 25' 42" unstable ***
5 36' 24" 5.0 MHz
6 34' 60" 2.6 MHz ****
* => Gamma wire spacing constant to bottom of tower, did not slope
from
20' as rest of
interations did.
** => Not a viable solution, as gamma wire passes through horizontal
plane of lower tribander.
*** => Could not find "clean" vswr dip, as freq. of dip varied
randomly.
**** => Tap arm moved out to 60", sloped to pvc pipe insulator at 42"
spacing, then sloped from
there to series cap. Ran out of time to lengthen pvc pipe
insulator
to 60" spacing, so
not sure if a solution exists with increased gamma wire spacing
or
not.
I seem to be stuck at 2.6 MHz. I was suprised that both moving tap
point higher up the tower and decreasing gamma wire spacing both
resulted in raising resonant frequency. The highest gamma arm tap point
that I feel would work is about 40', as I don't want to disrupt the
lower tribander. Even here, not sure that 5' spacing won't cause lower
tribander to complain. I feel that there is a workable solution....
from the charts I have seen, I should have enough loading (from the
yagi's) to make the tower electrically close to 90 degrees in length.
If you've followed along so far, I would appreciate any thoughts that
you might have as to how to proceed from here, especially so if you've
gotten your similiarly size/equipped tower to shunt-feed on 160. I'm
aware of the omega-matching technique, where another capacitor is added
from the feedpoint to the tower (and ground). Any feedback relating to
shunt-feeding towers on 160 are welcomed.
73 es hope to cu on topband soon....
Charlie N9CO
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