There has been considerable discussion here in the past on the merits of mono vs stereo Part 15 (or equivalent in whatever country you reside in) FM transmission.
I had some ancedotal evidence that suggested that mono FM signals had greater range than stereo. But others suggested that that was false, that the amount of power used to separate the stereo channels was very small, and that most radios automatically switch into mono mode for weak signals.
Well, theory is one thing. And practice is another. I recently acquired a stereo Decade MS-100, which can also easily be switched into mono mode. It, of course, was tuned for Canada's slightly more liberal FM unlicensed rules than in the U.S. And using the exact same location and various other parameters including music, reception equipment etc., I gave it a try.
A little background. I now reside in an apartment. The transmitter is located in the bedroom in front of a window at the back of the apartment building, overlooking my desired coverage area. The front of the apartment overlooks the water (I don't believe the aquatic life owns radios).
Here are the results. On the side of the apartment closest to the transmitter, I could hear the stereo signal clearly - what I would call listenable - approximately a long city block, or about 300 meters. After that, it became very choppy and I certainly wouldn't want to listen to it for any length of time. It was there for another block or so, but not great. In the front of the apartment, where I park (basically, on the other side from the transmitter), there were dead spots intermixed with a reasonably strong signal.
I could not hear the mono signal any further, but the sound certainly got cleaned up! The listenable range increased to almost 2 city blocks, or around 500 meters or so - after that, it completely died out. If I could hear it, I could listen to it. And the dead spots disappeared on the other side of the apartment building.
All this goes to prove that theory isn't everything. My conclusions - there is no difference in range between a mono and stereo signal, but there certainly is a difference in the quality and listenability (sic?) of the two signals. Mono is far more preferable if you want the farthest, cleanest signal.
Another conclusion. Everyone needs to experiment and try things out to get the best results for your particular installation.
My experience with FM mono yielding a more listable signal in the signal area is the same as Artisan found.
The notion that stereo radios switch to mono when reception declines is based on ideal radios in ideal fixed positions where maybe that might happen sometimes.
Also, Artisan, assuming your transmitter is vertically polarized, it makes a difference if the auto radio is horizontally polarized, which might often be the case.
But the receiver is cleaner on mono. I listen to stations over 80 miles away. In the stereo mode, the signal is nasty. Change to mono and it clears up. (IF you can switch it on/off) Most modern receivers don't have that function anymore..
We have a few low powered FM stations around that tramsmit in mono. It's easy to hear them through the hash and trash..
When you're working with the little power that we do, getting the best signal possible over the range that we have available is most important. When in stereo, besides audio, the pilot signal and the two channels have to be transmitted in the same bandwidth and this takes more transmitter power. Yes the extra power needed is small, but when you have little power to start with, that little power needed for the stereo signal is more significant....like a car with A/C and only 80 horsepower or a car with 200 horsepower. The A/C will use up a lot of power on the small engine but not be noticable with the 200 horsepower engine...just drawing a parallel here.
Yes, you can squeeze out a little more listenable range with part 15 power in mono and people will usually only listen if the signal is good.
Mark
Arstisan Radio wrote: On the side of the apartment closest to the transmitter, I could hear the stereo signal clearly - what I would call listenable - approximately a long city block, or about 300 meters.
I could not hear the mono signal any further, but the sound certainly got cleaned up! The listenable range increased to almost 2 city blocks, or around 500 meters or so - after that, it completely died out.
My conclusions - there is no difference in range between a mono and stereo signal, but there certainly is a difference in the quality and listenability (sic?) of the two signals.
Sorry, but it is a bit puzzling to read these conclusions.
The received signal-to-noise ratio of monaural analog FM compared to stereo analog FM favors monaural analog FM by about 23 dB, mostly due to the wider r-f bandwidth needed for the analog FM stereo signal, and the demodulated, spectral noise distribution in that bandwidth.
This noise penalty reduces the coverage range for a given signal-to-noise ratio in the audio output waveforms of analog FM stereo receivers compared to analog monaural FM transmissions.
Other things equal, analog FM stereo receivers progressively falling back to mono mode for weaker received signals do not permit a fair comparison of the useful fields from analog FM stations for those two transmission modes.
I (think I) understood what Artisan said about his FM mono-stereo observation, and was not confused by it. Perhaps I should be confused? But I don't know what to be confused about.
To make it more.....less.....unconfusing, I find what Rich said to be understandable and it seems right, based on what I know, so, let's combine both results.
Artisan and Rich make sense to me. But, Rich seemed to raise a confusion aspect, which I cannot fuse. I mean, defuse, uh.... infuse?
I refuse!
Yes, Artisan's range comparison of stereo to mono seems to be understandable.
He could detect both the stereo signal and the mono signal for about 2 city blocks.
The stereo signal became unuseable beyond one block even though the signal was present at 2 blocks away.
The mono signal remained useable for the entire distance of 2 blocks away.
Both, mono and stereo signals, were lost beyond 2 city blocks.
Maybe not a labratory style experiment but shows an advantage of mono over stereo.
So, if your desire is range over stereo image go mono.
Our car rdio is very very sensitive.
We are in West Hartford, Connecticut.
There is a station in Orleans, Massachusetts
(on Cape Cod) about 140 niles away. This
is on 107.5 MHz. Fairly aften, driving around town,
I can hear it fade out of the noise.
Sometimes I can actually listen to it.
But - anyhow - I guess I'm trying to say
that we have a great car radio.
When I run FM transmitters here at DOGRADIO,
I try to keep them as close to 15.239 as I can.
(Carl and MRAM mentioned a Part 15 rule that
I didn't know about on another thread. It basically
said - if you employ good engineerng practice,
you don't have to be exactly on the nose with
transmitter measurements. The FCC ruling assumes
that you don't have expensive measuring gear. But
implies - I think - that you have to REALLY TRY to get as
close as you can to what is correct.)
So, with 15.239 in practice, I get in the car
and (I don't drive) my wife and I go up to
the top of a hill about 800 to 1000 feet
from here. Any DOGRADIO FM signal
cannot get past that point - no matter
how good the car radio is. If you go
straight - or turn right or left - the
transmission path is just plain blocked.
So I can't say how far any remanants
of the signal are left beyond that point.
(I don't think the FM signal is powerful
enough to be listenable on a typical clock radio
in a house across the street from my location!
Again the receiver is the thing.)
Anyway - - I have been limited by the scope
of these tests.
Now after all of that, here is my main point.
Because of the hill the signal path goes away.
So because of this - the transmission seems
to travel about the same distance in either
stereo or mono. Just at the top of the hill,
it is just a trace of something in the noise.
Beyond that - I haven't been able to observe any remanants
or even a "fade up" because the signal path
is blocked at that point.
I can tell you this:
Following the same path - along the way, the
stereo transmission is choppy even when you are
only about 100 feet away. There are progressively
deeper and more numerous fades as the distance
from the transmitter increases.
If you go back into the house and flip the switch
from stereo to mono, and drive away again: the
fades and dropouts are much much less. Until
about 600 or 700 feet away, when everything
starts to fall apart and drop into the noise.
So that's my experience.
I find all of this to be very
interesting.
Bruce, DOGRADIO