I'm so surprised that so many repeaters are without any permanent S-meter!
Yes, you can haul an expensive & delicate service monitor out to a site, but...
It can be very useful to see what's going on in the sprectrum slice that the
repeater receiver is listening. With FM, it's often difficult (or impossible)
to tell if signals are noisy due to a lack of signal strength OR due a rise in
background noise level, which some call "antenna noise". (Then non-human stuff
comes from the sky, it's really galactic noise!)
I remember trouble-shooting a repeater which had complaints of "noisy weak
signals". After I connected the Motorola service box (meter & switches, not a
service monitor) to the rx, I noted a vastly increased no-signal level at the
input to the first limiter. It turned out to be a bad substation insulator,
right next door to the repeater. After that was fixed by the electric company,
normal operation.
An S-meter would tell the story quickly.
As there is no AGC on a typical FM receiver (that's the usual source of S-meter
voltage in AM receivers), we can look at the input to the first limiter/output
of last IF amp. In a pinch, the output of the 1st limiter will do but only at
very low signal/no signal situations--its output is "compressed" by normal
limiter action.
I log the "S-meter" readings at the site for future reference.
It is more difficult to connect an S-meter on FM receivers that have
IF/limiters/demodulator on a chip. In the olde daze, the last IF output had a
test point. There are some S-meter circuits on the web. However, some of them
are metering before all the rx selectivity is in so the readings would have to
be interpreted somewhat for off-channel contributions.
--John
