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

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

On 10/7/2010 12:45 PM, Dick Grady AC7EL wrote:

Can anyone explain the above phenomonon?

Dick AC7EL


I can not explain that one, though the next person's suggestion makes
sense at first glance (I've not looked at it 2nd glance) I do agree the
signal strength (As indicated on an S-Meter) of the repeater should NOT
be dependent on the strength of the received signal.

I have a slightly different story (And it's resolution)

People told me that when I transmitted the repeater moved off frequency
a few hertz. (I forget how much) Plus I could not access DTMF controlled
features.

Finally I hooked a 'scope to the modulator line in the radio and saw a
very interesting trace.. The top half was nearly square while the bottom
half was sinusoidal (As it should be) Turns out the radio had a couple
of clipping diodes in the audio,, One was open.

Adjusted mic gain till the trace was all sinusoidal. then adjusted
modulation gain till the traqce was the original P-P size, hit the
4-digit command to force a repeater ID on the DTMF pad. and Got the
CWID back from the repeater.. several times, Worked fine after that.

--
Nothing adds Excitement like something that is none of your business.

On 10/7/2010 12:45 PM, Dick Grady AC7EL wrote:

Can anyone explain the above phenomonon?

Dick AC7EL
One thought.. What is the "Hang time" on that repeater... (From the time
received signal stops to the time xmtr cuts off.

It could be the signal was not holding it and it was fluttering the
transmitter (Rapidly switching on and off) and your S-meter was averaging.



--
Nothing adds Excitement like something that is none of your business.

On Fri, 8 Oct 2010 10:10:09 EDT, John Davis
wrote:

On 10/7/2010 12:45 PM, Dick Grady AC7EL wrote:

Can anyone explain the above phenomonon?

Dick AC7EL
One thought.. What is the "Hang time" on that repeater... (From the time
received signal stops to the time xmtr cuts off.

It could be the signal was not holding it and it was fluttering the
transmitter (Rapidly switching on and off) and your S-meter was averaging.

The hang time is zero, literally. A while ago, I asked the tech
committee why there was no hang time, and they said that to save money
they did not add a controller board.

So your explanation makes sense.

Dick AC7EL

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
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

On Fri, 8 Oct 2010 12:30:07 EDT, I wrote:

On Fri, 8 Oct 2010 10:10:09 EDT, John Davis
wrote:

On 10/7/2010 12:45 PM, Dick Grady AC7EL wrote:

Can anyone explain the above phenomonon?

Dick AC7EL
One thought.. What is the "Hang time" on that repeater... (From the time
received signal stops to the time xmtr cuts off.

It could be the signal was not holding it and it was fluttering the
transmitter (Rapidly switching on and off) and your S-meter was averaging.

The hang time is zero, literally. A while ago, I asked the tech
committee why there was no hang time, and they said that to save money
they did not add a controller board.

So your explanation makes sense.

Dick AC7EL

Further thoughts on the mechanism:

Our repeater has zero hang time, because we don't have a controller
board. So if he was just barely opening the squelch, and the squelch
was fluttering rapidly, then the repeater output was being switched on
and off rapidly, and my S meter was averaging the on/off signal. We
could understand Andy because of the redundancy in human speech.

I can think of two mechanisms for the rapid flutter of the squelch.
One is noise in the receiver front-end.

The other is desensing of the receiver from the transmitter. Every
time the transmitter keys, the receiver desenses slightly (those
filter cans cannot suppress all of the transmitted signal at the
receiver input), which makes the received signal seem weaker, which
closes the squelch, which turns off the transmitter, which stops the
desensing, which makes the receiver sense Andy's signal large enough
to open the squelch, ... and this cycle repeats and repeats. On a
stronger input signal, the desensing will not cause the squelch to
close, and the transmitter would not be rapidly switched on and off.

My guess is that desensing is the reason for the observed phenomenon.

Dick AC7EL

In article ,
Dick Grady AC7EL wrote:

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.

Dick-

Your comment on the S meter reading suggests a second repeater on the
same frequency.

Otherwise, I've learned one thing after many years in Ham Radio: Most
people will not give a critical report, regardless of how bad a signal
sounds!

Fred
K4DII

On Oct 7, 11:45 am, Dick Grady AC7EL wrote:
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.
I was mobile at 45 mph; Snip
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.

Given you where going 45 mph mobile, I'd not be suprised if your S-
meter observations where not skewed or perhaps the locations where you
could safely observe the S-meter where places where the repeater's
coverage is a bit lower than normal. Without actually seeing a direct
indication of the repeater's output power (like a power meter in the
coax between the transmitter and the duplexer), I'd consider the S-
meter evidence as hearsay and not get too worried about it.

-= Bob =-

On 10/8/2010 12:30 PM, Dick Grady AC7EL wrote:

The hang time is zero, literally. A while ago, I asked the tech
committee why there was no hang time, and they said that to save money
they did not add a controller board.

So your explanation makes sense.

Dick AC7EL


Wow all you need to add a bit of hang time is a 50 cent capicator.

Personally.. .I'd have given that suggestion a probbility ration down in
the single digits or lower But hey... Glad I brought it up.


--
Nothing adds Excitement like something that is none of your business.