Impedence mismatch into FET preamp

Hi all.

I have a FET preamp, tuned by means of two fixed capacitors and one variable
inductor that serves as an impedance transformer.

Am I correct in thinking that if there is an impedance mismatch (say the
input circuit was tuned into a 50 ohm load, then was finally presented with
say a 105 ohm load), then the preamp's noise figure would increase and the
FET would start to become artificially noisy?

Then if the FET was pushed into overload by an adjacent strong signal, the
noise would then start to reduce.


I have this odd situation on a repeater, and I can only put it down to
mis-tuned cavities. I've never come across it before though, where the
repeater actually IMPROVES in sensitivity when the transmitter keys up
(?!?!), so I would like to know what's going on before I fix it.


Thanks all,

Sam
M1FJB


.

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