[CQ-Contest] Stubs and Amplifier Failures

[Michael Tope]

We have recently suffered a rash of amplifier failures at the
club station with which I am affiliated (3 major failures over
the last two years). Specifically the problem has been fried
bandswitch components in our two ETO Alpha 78's.

The station is configured with a set of coaxial stub filters on
the various antennas for purposes of minimizing inter-station
interference. The stubs are extremely effective, but I am
beginning to wonder if they are a contributing factor in the
amplifier failures.

Normally, when you select the wrong antenna, it may still
present a finite VSWR to the amplifier due to the losses
in the coax cable and the high, but finite VSWR of the
antenna when operated off its intended operating
frequency. On the other hand, when you place a stub
filter at the output of the amplifier, it will look like a dead
short at the frequencies its intended to null. Since its
almost inevitable that someone will transmit into the wrong
antenna over the course of a contest, I am wondering if
having highly reflective stubs inline is responsible for the
seemingly high failure rate we are experiencing?

Has anyone else observed or experienced this sort of
problem with stub filters? If so, what did you do about it?

Thanks,

Mike, W4EF....................................................