Neil Carr G0JHC wrote:
I set my input matching up for 1:0 SWR with the MFJ, at this time the
heaters are not on. Soon as I switch them on SWR shoots up to 2:1, then over
a 6 minute period gradually to 4:1.
It really doesn't mean anything because the impedance changes when the amplifier
is operating. In that case a large portion of the drive power is fed through to
the output, there may be grid current, etc. I suspect that these effects would
swamp out the effect caused by the expansion of the cathode.
You can use an MFJ analyzer without firing the amp up by connecting a resistor
whose r matches the expected input impedance under operating conditions from
cathode to ground, and measure the SWR with the heater on.
I may (of course) be wrong, but I bet that if you set it up this way, you won't
see the variation that you see now as it warms up because the resistor
(simulating the actual input impedance) will swamp the capacity change.
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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