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.

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.

Several possibilities here, I think:

- As you suggest, the receiver's S-meter indication may have been
misleading. Some FM receiver S-meters operate entirely by looking
at the behavior of the limiter stage(s), and should respond only
to the actual signal strength (i.e. amount of IF amplification needed
in order to push the signal into limiting).

Others, though, might display a "composite" signal, based on both
the limiter behavior (incoming RF signal strength) and the squelch
analysis. Most radios these days use a noise-based squelch... a
weak or absent signal results in a lot of high-frequency
(ultrasonic) noise coming out of the limiter/discrimimator, and a
high-pass filter and detector produces a voltage which rises when
the signal is noisy or absent.

It's entirely possible to display an "S-meter" indication, which
reflects both the limiter voltage, and the amount of noise in the
audio (as detected by the squelch).

If Andy's signal was noisy into the repeater, and if the
receiver-transmitter audio link doesn't include a low-pass filter,
then the transmitter would faithfully remodulate and retransmit the
noise embedded in Andy's signal, and it would be picked up by the
receivers listening to the repeater's audio output. The excess
noise would cause noise-sensitive "S-meter" readings to be lower
than you would see on a clean carrier of the same actual strength.

Many repeaters are designed to run the received audio through the
appropriate de-emphasis filter and (voice-band) low-pass filter,
and the transmitters provide pre-emphasis and limiting/clipping
(and usually stick a low-pass anti-splatter filter just before the
modulator). However, there are repeaters which are designed with a
"straight pass-through" architecture... the demodulated audio from
the receiver is pushed right into the transmitter's modulator with
little or no processing. If the ARES repeater is of this design,
it could easily "pass through" the noise inherent in Andy's
under-strength signal.

- In a similar scenario (as the previous poster suggested) the noisy
signal might actually be causing the transmitter to "splatter"
power far outside of its normal RF bandwidth. If this power was
distributed outside the IF passband of the receiver, it wouldn't be
"seen" by the limiter and would result in less limiting and a lower
S-meter reading. The tighter the receiver's IF passband skirts,
the more that this effect would be visible.

- In yet another scenario like this, the excess noise going into the
transmitter, and the resulting broadband splatter, could result in
the transmitter generating significant power at frequencies outside
the passband of the transmitter's duplexer cavities. At these
frequencies, the duplexer would be presenting a difficult impedance
load to the transmitter... high, low, or highly reactive... and the
transmitter would "see" a high SWR. Enough out-of-band splatter
energy might trigger the transmitter's high-SWR protection circuit,
which would automatically reduce the transmitter power in order to
protect the finals from the high voltages or currents that a high
SWR would cause.

Since all of these scenarios could involve a repeater transmitter
which can be emitting significant power outside of its usual transmit
pass-band, it might be worth your investigating further to make sure
that your system is operating properly and isn't splattering into
adjacent frequencies or emitting significant spurs elsewhere in the
band.

The way I'd do this (if it were me) would be to monitor the
transmitter's output with an RF spectrum analyzer, while having
somebody transmit a clean carrier-plus-PL and gradually reduce the
test signal's RF output level. See what the repeater's RF signal
looks like (both close-in and at some distance from the nominal
carrier frequency) as the RF signal level drops down towards the point
at which the receiver closes its squelch and the transmitter drops out.

Another possibilty (entirely different): Andy wasn't hitting your
repeater at all, but was hitting another one on the same (or nearby)
frequency. Maybe his signal was too weak to open your repeater, maybe
he'd forgotten to set the appropriate PL. When you heard him, it was
through a different repeater located at a substantial distance, and
hence the signal was both noisy and RF-weak. When he boosted his
power, he actually did hit your repeater and the signal cleaned up.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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