Curious if FM mono is common?
Also, I have a Ramsey fm100b. Is there a mod to have it not broadcast stereo?
My Ramsey FM25B can be jumpered for stereo or mono, and I would guess that your manual instructs how to do the same with your 100b.
Some consumer FM transmitters are stereo only, including the C.Crane.
The Wholehouse 2.0 can be switched to mono.
Modifying a transmitter to shut down the stereo circuitry requires specialized electronic knowledge.
As a member in good standing I will share the secret knowledge of how to disable the stereo capabilities. Shhhhhh....quietly find the 38 KHZ crystal and lift one leg of it.
It's marginal, but airing a mono signal gives you a tiny bit more range as well. At Part 15 levels, it's probably another backyard at best.
Stereo FM signals are more susceptible to noise and multipath distortion than are mono FM signals.[4]
In addition, for a given RF level at the receiver, the signal-to-noise ratio for the stereo signal will be worse than for the mono receiver. For this reason many stereo FM receivers include a stereo/mono switch to allow listening in mono when reception conditions are less than ideal, and most car radios are arranged to reduce the separation as the signal-to-noise ratio worsens, eventually going to mono while still indicating a stereo signal is being received.
WDCX advised... "find the 38 KHZ crystal and lift one leg of it."
Never having paid much attention to how the stereo generating circuitry is set up in these micro-transmitters I looked at the only two schematics I have for the transmitters I own.
The circuit for the C.Crane appears to have two crystals and indeed one of them is 38 kHz as said by WDCX. The other one is marked "75 kHz".
The Ramsey 25B transmitter is different. There is only one crystal marked "7.6 MHz" and the jumper (J1) when shorted grounds IC chip U2 which is not the stereo generator, but enables MONO operation. Chip U2 is the DIP settings brain which sets operating frequency.
You can set your FM100B to mono mode by going into Setup (holding the up and down buttons simulataneously) - I can't remember what you do at that point (if anything) but you should be able to find documentation for it.
There is a significant enough difference between mono and stereo Part 15 signals in the quality of the sound that I don't believe that it's worthwhile to broadcast in stereo (unless, of course, you're only interested in a few feet of range for your own personal use). Much more noise as you go further away from your antenna, particularly the fringe areas.
The Panaxis ACC-100 was FM mono, FCC certified but is no longer made. The much more expensive Decade MS-100 (no S on the end for the mono model) is a good choice. The cheaper transmitters don't tend to give you a choice - they're almost always stereo, but then, they're intended for personal use music transmitting at much shorter ranges.
I don't think I'd fool around with lifting the leg of a crystal - it would void certification at the very least. Better off to get a quality mono transmitter from the get go.
I have modified a few transmitters
for mono. The North Country Radio
MPX-96 FM transmitter kit that I
have runs mono. My friend who
built it for me rigged it up that way.
Although my station isn't on right now,
I did have some listeners in the very
nearby neighborhood, back a while ago.
There were some people listening on a
stereo system in their living room. It was
not necessary for then to fool around trying
to improve a noisy stereo signal, because
I was transmitting mono, and it sounded
good. At that time, my son was doing
most of the shows, and that family
was listening. It was really just between
us and them. And their house was close.
It was probably 200 feet away with a good
FM tuner on their end.
This was about 10 years ago. My son had
the idea for a radio staton and he knew I
could build one. (Ï didn't know how to make
a good AM set-up at that time, however.)
The transmitter was a Ramsey FM-10. Ha!
So I made it do mono, and put a fan on it
and used a frequency counter. What a kick!
It was a really goofy set-up, but it worked.
I also attenuated the RF out to comply with
15.239.
It was actually really fun.
Bruce, Mon. Stn., CT
I mentioned the old Ramsey FM-10
transmitter in my above post.
For those who don't know, it was not
a synthesized transmitter. It had
a free running oscillator and was hard
to get on channel. It also drifted. Before
more modern transmitters came out, it
was widely used by Part 15 people. A lot
of people came up with ideas to try to
make it better. One guy even put a
temperature controlled heater and feedback
system in the oscillator stage to keep it
on channel. Actaully many people had
fun experimenting with the FM-10.
And I think I may have mentioned it
above, but anyway - my FM-10 was
rigged for mono.
Bruce, Mon. Stn., CT
I was not an FM pirate at first. I built a tube transmitter based on a project in an electronics magazine in the 1960s, and it sounded beautiful, was tuned to whatever... there were no digital radios, I just picked a spot somewhere on the dial and there it was. I spent many hours trying to beef up the power of the thing, but it stayed weak no matter what I tried.
Then a buddy, who owned an electronic shop and now owns an AM station, built me a 1 Watt FM transmitter with three transistors, also free tuning and set to a non-specific spot on the dial. each stage had a peaking coil and it fed a channel 7 antenna located in the attic. It covered over a mile.
Every Friday night a preacher and his wife tuned in a mile away and I played wild comedy records and said ridiculous things, we got on the phone and laughed back and forth.
It was down around 88 and in those days (1970s) there was nothing else down there.
Hey Carl, I'm pretty sure you've
told that story before. I love it
every time I hear it.
Because it reminds my when the FM
band was a mysterious place. I couldn't
get stereo in 7th grade (fall 1966), but the Hi FI blew
me away. (A Magnavox vacuum tube record
player/FM Hi FI radio. The record player was
stereo, but the FM receiver wasn't.)
The very few college stations of the day signed
on in the afternoon. I thought it was like the
AM band and you could only get those stations
at night. Only 5 or 6 FM stations were listed in
the newspaper. (Yes the newspaper every Sunday.)
I asked my father why there were only a couple stations
and he said it was because they just didn't go very far,
and then he said he was pretty sure that was the reason.
Bruce, Mon. Stn., CT
CARL: 7.6 X 5= 38
What to Carl Blare is a maze of confusing numbers is to John WDCX the clarification by way of math. Knowledge mathers. Ah, matters.
“When you can measure what you are speaking about, and express it in numbers, you know something about it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind; it may be the beginning of knowledge, but you have scarely, in your thoughts advanced to the stage of science.”