NW7US, Tomas wrote:
Hi,
I am using an AL-80A and have need as an Army MARS and National Guard
Net member to utilize frequencies far out of normal Amateur Radio
allocations. I am finding it hard to get the Amp to tune on certain
frequencies "out of band" - I can get about 100 watts on those problem
frequencies.
I suspect that the filter netwoks in this Amp prevent it from being
used too far out of frequency. Is that true?
Check the SWR on your exciter. If the SWR is over 2 or 3 to 1, then possibly
the exciter output is folding back, providing less drive. And this would
indicate that the input pi nets are probably attenuating the drive that does get
through. So in that case, the answer to your question is 'yes'. I'm not sure
if it's possible to broaden these input networks, since it would significantly
reduce the input network Q, causing other problems.
You could retune the 80 or 40 meter input network to reduce the SWR on 5 MHz.
For example if you are an SSB only operator (God forbid) then you could center
the 80 meter network at 3.9 MHz. or higher. It would still work on the 75 meter
phone band, but the SWR aboove the band would be lowered.
There is also an incorrect Q in the plate circuit. For example, if you are
using the 80 meter position to operate on 5 MHz, the Q would be lower than
normal. This would reduce harmonic suppression, but should actually increase
efficiency slightly. If the plate tuning and loading controls are not at
extremes, then the problem is not related to the plate tank.
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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