On Thu, 7 Oct 2010 12:45:02 EDT, Dick Grady AC7EL
wrote:

Our ARES group serves an area in Nevada consisting of 2 separate
valleys with a mountain range between them. On the mountain we have a
2-meter FM repeater which serves both valleys. Most of our members
live in one valley and only one or two in the other valley (due to low
population there).

On last night's ARES net, it was Andy's turn to be net control. Andy
lives in the second valley. He normally hits the repeater solidly,
but that night he was scratchy. But even stranger, whenever Andy
transmitted, the repeater's output power, as indicated on my S meter,
was significantly lower than when anyone else was on. We questioned
Andy whether he was on simplex (simplex transmission is possible
between the valleys, or more correctly parts of the valleys), but Andy
assured us that he was duplex. And everyone else heard Andy the same.
I was mobile at 45 mph; if he was simplex I would have gotten terrible
picket-fencing on his signal. I did not get picket-fencing, which
shows that I was receiving it line-of-sight from the repeater. Andy
explained that he was operating on battery and was on low power out to
save battery. Finally, he went to full power output, and his signal
through the repeater was noise-free AND the repeater's output was at
full power.

Based on my understanding, technical training (Masters degree in
Electrical Engineering) and 12 years ham experience, the above should
not have happened. An FM repeater's output signal is (or should be)
either full power or off. The repeater receiver's squelch acts in a
digital fashion in controlling the transmitter: on or off. A weak
input signal will sound scratchy. But the repeater's RF output power
is either full on or it's off. Or should be.

Can anyone explain the above phenomonon?

Dick AC7EL
It is possible that the repeater only seemed weaker to you because its
transmitter was being overmodulated by the noise from Andy's weak
signal. (If an FM transmitter is overmodulated, the power is spread
over a wider bandwidth than your receiver can receiver). That's a
real stretch, but I can't think of anything else other than pure
coincidence.
Pat