I'm sure that there are many people here more knowledgeable than me. I'm just reporting my experience - I can get a listenable signal much farther transmitting in mono rather than stereo. I'm not sure why.
As for the stereo indicator - I think that is pretty much a moot point, as even in Canada a stereo signal would not remain stereo very long before the receiver switched ovear to mono (and that's with our increased field strength limits) - I figure less than 100 meters from the antenna line of sight for a sensitive car receiver. In the U.S., that would be less than 25 meters on that same car radio, and much less for an ordinary portable stereo.
The comment (I think by MRAM) that the addition of a 19kHz constant tone at a certain level, not usually noticed by listeners, but yes, it lights the stereo light in the receiver. You may already have mentioned that this is the "pilot" tone. Some years ago I volunteered for a poor mono FM station (12.5kW) and we did the 19kHz pilot tone once in awhile, since we couldn't afford the double pair of equalized phone lines to go stereo.
This is just silly, but I think I'll try a pilot tone on my mono streaming station.
The actual stereo signal is up higher at about 56kHz (I forget the number) and it is modulated with the "L-minus-R" signal, which I will leave for someone else to explain.
But it's these high frequencies that get lost when receiving a weak signal, causing noise or distortion because the received bandwidth narrows. Plain mono without the upper "sub-carriers" is more robust at a distance.
Also, the full audio spectrum is rolled off at the top to make room for stereo. The fidelity is much better in mono.
If you inject a 19 kHz pilot tone on a mono transmitter, assuming that the audio circuitry will pass it, that will cause the stereo decoder in the receiver to switch on. The stereo light will go on, but it will also reduce the signal to noise ratio of the detected signal. Why is that? Mainly because most of the noise that is added in the stereo mode comes from the difference channel between 23 and 53 kHz (30 kHz worth, to be exact). Further adding to the S/N deficit is the fact that the post-detection FM noise spectrum is parabolic, which means that there is significantly more noise per Hz at higher frequencies. Stereo transmission incurs a penalty of approximately 23 dB S/N degradation as compared to mono.
So, the bottom line is: if you want to maximize your range by transmitting in mono, do NOT turn on the stereo decoder of the receiver by transmitting a 19 kHz pilot.
Have little plastic static-cling stickers made up of a stereo indicator light and perhaps the text "Stereo" and then promote your station:
"Want to hear us in stereo? Order our stereo sticker!"
The listeners can paste the sticker over their own stereo light and happily listen "in stereo"
That sounds like the "color TV" static sticker from years ago.
A sheet of clear plastic; the bottom 3 inches green, the middle a faint red and the top 3 inches sky blue.
Somebody made money on it.
Introducing color radio in 3D.
I know I have one as well. Part 15 rules state that no operation is permitted below 88.1. very confusing.