Philip J. Smith said:
It is indeed somewhat pessimistic for reception of WLFM as a radio station. WLFM's licensed facilities are 3kw/385m - a bit less than half the power of WXRT. (6.7kw/399m) (most other major Chicago FM stations have similar facilities) WXRT is predicted to deliver a 61dBu signal at 50km (32mi.); WLFM is predicted to deliver 57dbu. 4dB weaker, but that's not a huge difference.
The difference is explained in part by the fact WLFM is a TV station, not a radio station. The map reflects the coverage of the WLFM video transmitter. At a "normal" analog TV station, the audio transmitter is running considerably less power. (the old full-power rules limited the aural power to no more than 22% of visual, and most stations ran closer to 10%) The map shows the 62dBu interference-protected contour of the WLFM video signal; assuming the audio power was 10% of visual, the map would also reflect the 52dBu contour of the audio signal. Class B "real" FM stations like WXRT are protected to 54dBu; the difference between 52dBu and 54dBu is essentially negligible.
That said, there appears to be a loophole in the low-power TV rules applicable to stations like WLFM; these stations are apparently not held to the 22%-of-visual rule that applied to full-power analog stations. Analog power can apparently be equal to visual. At that point, the map does in fact become rather pessimistic -- it assumes WLFM is running about 1/10 of the audio power it's actually running.
But again, the map does reflect the area in which WLFM is protected from interference. WLFM may be almost as strong as WXRT on a car radio in Joliet, but it's still legal for someone to build another low-power TV station on the same frequency in Joliet, as long as it doesn't cause any interference inside the blue line.
