I've thought for a very long time that the idea of turning off the stereo pilot to increase an FM station's range was complete hogwash. Virtually all stereo radios made for the last few decades have a stereo blend circuit that mixes the signal to mono when it gets weak. You get stereo when you're within a decent range and you get mono rather than a noisy stereo signal when you get out to the fringes. Everybody wins. Someone, please convince me of why I'm wrong.
Michael J can probably answer this if he checks in. He's posted some info before about the mono vs. stereo debate, and as I recall he was the one who advised KYFO-FM 95.5 a couple decades ago to turn off their stereo pilot to increase the station's range. To this day KYFO still broadcasts in mono. KNRS-FM has been broadcasting in mono from the time that 105.7 (now 105.9) dropped their music format in favor of a simulcast of KNRS 570. KUER FM's stereo pilot has also been turned off for at least three or four years. With all three stations being mostly talk, I can't think of any advantage that broadcasting in stereo would provide other than to add some depth to the bumper music. There's got to be some compelling reason for three FM stations in a major radio market like Salt Lake to not be broadcasting in stereo. Also, let's keep in mind that there are thousands, if not millions, of older FM receivers/tuners still in daily use that don't have automatic mono/stereo switching capabilities. The radios in my 2003 Chevy S10 and 1993 GMC Sierra don't, and they sound like crap when I'm on the fringes of an FM stereo signal. Can you say "mobile flutter"?
David, I'd be willing to bet serious money that your radios have it and you just don't even realize it. GM radios back in the mid 80's had it. (I drove an '86 S10 Blazer.) There's not going to be any visual indication that it's happening, the sound will just blend to mono. The stereo indicator will stay lit just the same. If the mobile flutter is severe enough that the radio still has problems, there's nothing anyone can do about that. Even broadcasting in mono isn't going to solve that problem. I know a lot of engineers swear by this method, but I'm still not buying what they're selling.
Here's an interesting note. Blend circuits were used in receiver advertising. In 1961.
All I know is that when I'm driving my '93 GMC Sierra in an area with fringe stereo reception, the stereo indicator starts flickering on and off and I hear a lot of static/white noise as the stereo signal cuts in and out. Maybe it's more appropriate to call that multi path FM reception. A similar phenomenon occurs on the '93 GMC's radio at night on the AM band. With several AM sky wave signals on the same frequency, the stereo indicator will flicker on and off. I don't know enough about electronics to explain it in more technical terms, so hopefully someone more versed in radio engineering will offer an explanation. Or maybe it's just a radio "urban legend" that broadcasting in mono on FM increases the range of the signal--kind of like the myth that broadcasting an RDS signal on FM reduces the station's range.
It may also explain why KNRS and KUER broadcast in mono. Of the three FM stations in the Salt Lake area which broadcast in mono, only KYFO-FM has an ERP above 25 KW, and KUER broadcasts with an ERP of just over 20 KW (20.5 to be exact).
I've been reading this nonsense for decades. Putting a receiver in mono mode (which is the extreme version of what a blend circuit does) ignores everything from the 19kHz pilot carrier up, exactly the same thing as broadcasting in mono. It was designed that way deliberately so that mono only radios (which also ignore all of that) would be compatible with the new stereo signals.