FM Booster Rules - Coverage Variances Observed?

[RF101]

In general, FM boosters' contours are not supposed to "spill" outside the primary stations' contours.

I have seen cases where waivers have been granted because the "booster spillover zone" is unpopulated such as farmland, wilderness, or open waters. I have even noted cases where the original engineereing paperwork claimed that a booster's signal was fully contained inside the main station's contour. However, updated terrain/recalculated patterns clealy showed several hundred residents coming under a booster's official contour despite being outside the primary station's official coverage.

Here's the question: Does anyine know of an FM booster that has an explicit waiver to extend beyond the main station's contour? Underlying reasons might be weird geography (like an intervening ridgeline blocking the primary signal) or excessive interference from a co-channel/adjacent-channel station.

I would welcome examples.

 

[nd2023]

Boosters are able to exist within the 60 dBu because the contours are only determined by terrain within 10 miles of the transmitter. When there’s a terrain shadow further away, FM curves might predict a 60 dBu signal but the actual signal will be weaker. A booster would fill in the signal.
There are some stations which have their main intentionally terrain shadowed from the target market, with a booster to fill in

 

[RF101]

Things look pretty strict to me. The "plain speak" interpretation from the FCC says:

"The service contour of an FM booster facility must remain within the predicted service contour of the main station. For all booster stations relaying a noncommercial educational FM primary station, and for all boosters relaying a commercial FM primary station (except Class B and B1 commercial stations), the 60 dBu (1 mV/m) F(50,50) service contour of the booster facility must be wholly encompassed by the 60 dBu F(50,50) service contour of the primary station. ...."

"Wholly encompassed" seems pretty strict. Besides the 20% power limit of the main station (once HAAT is factored in...) , the only other big "gotcha rule" is that the booster's (50,10) interference contour must be down 20 db versus the primary station's (50,50) contour over the community of license. This prevents a self-destruction scenario. (I am not sure how all those San Francisco area boosters handle that rule.)
What would be a great reform is if stations were allowed to extend booster (and fill-in translator) coverage out to their "planning radius", for example Class A 28.3 km, Class C3 39.1 km, of course provided that the main station is full-powered over some of its contour.

I guess I will have to submit a change-in-policy request.

 

[Kelly A]

Things look pretty strict to me. The "plain speak" interpretation from the FCC says:

"The service contour of an FM booster facility must remain within the predicted service contour of the main station. For all booster stations relaying a noncommercial educational FM primary station, and for all boosters relaying a commercial FM primary station (except Class B and B1 commercial stations), the 60 dBu (1 mV/m) F(50,50) service contour of the booster facility must be wholly encompassed by the 60 dBu F(50,50) service contour of the primary station. ...."

That's a correct read-back of the rule. Boosters are solely intended to provide a signal inside the 60dBu contour due to terrain. The downside of boosters; is there will always be a field strength overlap somewhere that causes severe multipath reception. In theory, you design the booster pattern to put the overlap in a unpopulated/less-populated area, but theoretical and actual results can vary greatly.

"Wholly encompassed" seems pretty strict.

That's because you're not supposed to exceed the station contour to comply with adjacent/co-channel distance separation.

I guess I will have to submit a change-in-policy request.

Looking for an exception to the; 'stay inside your primary coverage'-rule? HAHA, yeah you do that.

 

[Schroedingers Cat]

The fact that 54 dBu F(50,50) is the contour allowed for Class B boosters, and 57 dBu F(50,50) is the contour allowed for Class B1 boosters is usually ignored or forgotten in these discussions. A booster with a few watts ERP and a narrow directional beam down a highway stretch or valley near the edge of the 54 dBu of a Class B station could be beneficial.

Also, remember than in hilly rather than mountainous terrain, very low ERP in terrain holes is preferable to anything near 20%. People who advocate anywhere near 20% usually are in a severe mountain shadowing area, or they haven't thought out the interference fringe situation. The best you can do is simulate a giant interferometer. Radio waves are coherent, and you cannot change The Laws of Physics. Things can be done to improve the performance, but never can you change the Laws of Physics.

I think opening spillover considerations except in a case by case basis is a slippery slope can of worms.