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Inconvenient Truths about Part 15 FM

Ermi Roos said:
Just as a wild guess, I would not expect the maximum range of a mono transmitter to be increased by much greater than about 1/4 of the maximum stereo range.
Click to expand...
The amount of r-f voltage needed at the antenna input connector of an analog FM broadcast receiver/tuner to produce the same SNR in stereo and monaural modes depends on its design. Lower-end FM receivers typically don't specify this.

But as an example of one currently-available product that does, a Teac T-680RS AM/FM Stereo Tuner specifies an input sensitivity of 15 dBf in mono mode, and 20 dBf in stereo mode for 50 dB SNR.

The field intensity arriving at a matched dipole antenna to produce those dBf values in a 75-ohm receiver environment are about 3.16 µV/m and 5.6 µV/m, respectively. Although the specs are silent about this, let's assume that no stereo blend is used in the tuner to achieve 50 dB SNR in stereo mode.

Referring to the FCC's F50,50 charts of FM broadcast field intensity for a given ERP and HAAT at a receive antenna elevation 30 feet above ground level, here are the distances to those field intensities for an LPFM station with 100 W ERP at a transmit antenna elevation of 100 feet HAAT:

3.16 µV/m = 50.7 miles
5.6 µV/m = 40.6 miles

These distances ignore the likelihood that such low fields could be received without co- and/or adjacent-channel interference.

Field intensity is reduced in approximately a linear manner for VHF receive antennas lower than 30 feet above ground . For example at 10 ft AGL the field would be 1/3 of the value at 30 ft AGL. The distances to the above field intensities for that receive antenna height then would be 31.8 miles and 25.4 miles, respectively.

In both of these scenarios this is a range reduction of about 20% for stereo reception, compared to monaural. So Ermi Roos' estimate of 25% is fairly close (for this case). Cheaper receivers/Walkmen/boomboxes may not do this well.
 
Very nice analysis, Rich. I assume that there will not be any objections to your technical work this time because it would not interfere with anybody's "rice bowl."

["Rice bowl" means "source of income." I learned this expression from the 1966 Steve McQueen movie, Sand Pebbles, about a U.S. Navy river patrol boat in China employing native labor in its crew.]
 
I learned this expression from the 1966 Steve McQueen movie, Sand Pebbles, about a U.S. Navy river patrol boat in China employing native labor in its crew

I thought I was the only one who watched that movie. Richard Crenna played a weasel of a skipper who came through in the end.
 
The operation of the ship was inefficient because each of the auxiliary Chinese crew members had a specific duty that only he was permitted to perform. Sort of like union rules! For anybody else to perform a particular coolie's duty was interfering with his "rice bowl." This impressed upon me (I was just starting out in the working world when the movie came out) the importance of being careful about doing something that affects somebody's livelyhood, because it can produce a strong reaction.

Otherwise, the movie sucked, and is not worth seeing (in my humbe opinion).
 
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