Forgive me, I know this subject has cropped up several times here. I am
hoping someone among you tower-talkians can give me some guidance on what my
next move should be.
I have a crank up tower which is about 65ft fully extended, on top of which
I have 18ft of HD stub mast. At 65ft I have dipoles for 160m and 80m, at
67ft a F12 C31XR, at 76ft a Cushcraft XM240 and at 85ft a 5 ele 6m yagi.
I am trying to shunt feed the tower for 160m. As I want to retain the 80m
and 160m dipoles at this stage, I have fitted each of them with decoupling
traps resonant at 1830 kHz. I have dropped a shunt wire from 65ft at a
distance of 30 inches from the tower.
Results so far:
The lowest SWR I can get at 1830 kHz is 2:1, which I get with 225 pf of
series capacitance.
The frequency on which I can get my minimum SWR of 1.3:1 is 2220 kHz with a
series capacitance of 100 pf.
Bringing the shunt wire closer to the tower brings down the frequency at
which I can get minimum SWR, however the minimum SWR increases as I go away
from 2220 kHz. I have tried a shunt capacitor and this produces roughly the
same effect.
At this point I am unsure whether to reduce shunt height, alter shunt
spacing or attach some light weight alloy tubing to the top of the stub mast
to increase overall height. Perhaps I will need to do all three?
Every time I need to make an adjustment I have to crank the tower down, luff
it over and partially dismantle my C31XR and XM240 so I can reach the parts
I need to adjust. As you will understand, I'm rather keen to minimise the
number of times I have to do this.
Words of wisdom gratefully received.
73
Bob 5B4AGN, P3F
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