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LPFM stations have a specific range allowed under FCC rule making. The maximum signal level for LP-100 stations is 1 mv/m at one mile (60 dbu contour). L-10 stations would be allowed 10 db less signal at the same distance from the antenna. The L-10’s would be allowed to cover approximately one-third the distance of the L-100’s. Height above average terrain does calculate into the maximum range formula. The formula used to be on the FCC web site. As far as I know, this part of the LPFM rules has not and will not change.
Additionally, LPFM antennas are only allowed a maximum elevation of 100 feet. Higher elevations would cause the actual effective radiated power (ERP) to be decreased logarithmically. So, at 650 feet elevation, a normal ERP of 100 watts would be substantially reduced (2 watts). By statute the minimum allowed ERP for a LPFM station is one watt, regardless of the elevation. At one watt there is a point that the increase in elevation will cause the station to not be able to cover the one mile 60 dbu contour (the law of diminishing marginal return). Theoretically, radio signals attenuate logarithmically in all dimensions simultaneously.
Only if the LPFM station is within so many miles of the Canadian or Mexico borders, are low power FM stations allowed to use directional antennas. This is done by application to the Commission for a waiver of the specific rule. Otherwise, LPFM stations are not allowed, by law, to use directional antennas. LPFM antennas can be vertically, horizontally or circular polarized and are not considered directionalized.
I have engineered and built a couple LPFM stations over the years. I am always amazed how well they work. Hopefully, this simplified explanation of LPFM rules will answer some of the questions.