It does sound like a parasitic of some kind. But why suddenly? Another possibility:
something in the system is arcing. An arc might have a similar 'jumping' characteristic.
What I would do at this point is hook the amplifier up to a good dummy load with a known
good hunk of coax and see if you can recreate this without any of the other variables like
antenna switches, etc.
On 4/12/2013 8:02 AM, N1BUG wrote:
I just said:
What makes me think oscillation is:
Suppose I have the amp tuned for maximum output at 28 MHz. Indicated
antenna SWR is low, output is 1500 watts.
Now assume that for the following procedures I maintain RF drive at
all times.
Now I vary the amp loading slightly. As I decrease loading
capacitance just a bit, I see a slow, gradual drop in output of
maybe 50 watts, then all of a sudden a drop of at least 500 watts,
and at that same instant, indicated SWR jumps up wildly. This is
absolutely repeatable on several bands with different antennas.
Once the sudden "jump" has occurred, returning the load cap to the
previous setting does not return things to normal. Going well past
the proper spot, toward higher capacitance, does at some point
produce a sudden decrease in SWR and increase in output power.
FURTHER to this....
If I now leave the load cap at the max output spot and vary the tune cap... I see a
sudden decrease in output and increase in indicated SWR when going to slightly less OR
slightly more capacitance. Unlike the load cap, returning this one to the previous max
output spot does cause things to return to normal (I don't need to continue past the
previous sweet spot).
With the load cap, the issue only occurs if I go toward less capacitance. With the tune
cap, it occurs going either way.
73,
Paul
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Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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