Total posts : 45366
Answer to your question on legality of the elevated antenna is it is legal as long as the field strength 3 meters in any direction from the antenna is less than or equal to 250 uV/m. Transmitter power output can be related to field strength but to keep this post reasonably short I will save this until asked.
There are big unknowns when trying to relate field strength to range in a part 15 FM station such as the receiver sensitivity and the amount of signal the receiver antenna delivers to the receiver.
These calculations are to illustrate what factors determine the range. It is possible to calculate the field strength at a given distance from a transmitter with a field of 250 uV/m at 3 meters as FS(uv/m) = (250 uV/m) x (3meters/distance(meters)). For example, the field strength at 100 meters is FS = 250 x (3/100) = 7.5 uV/m. Conversely, the distance (which is the range) can be calculated if the required field strength for a receiver/antenna to operate is known. Distance(meters) = (750)/(required field strength). If the receiver requires 15 uV/m then the range is = 750/15 = 50 meters.
The problem is that the required field strength at the receiver number is usually not known. An average or approximate number, say 7.5 uV/m can be used but the answer will be no better than this assumed number. Also, the path between the transmitter and receiver can alter the field strength in ways which are not easily predictable. If the transmitter antenna is elevated so there is nothing but “free space” between the transmitter and receiver antennas then the equations above can be used to estimate the range.
The point being is that the path unknowns between the transmitter antenna and the receiver and the unknowns regarding the receiver sensitivity make it very difficult to establish a firm number for the range.
Reports from hobbyists regarding their range experiences usually come from installations where the actual radiated field strength is not known so these are not reliable if compliance is your goal. The closest estimates for a compliant system would be from those who are using unmodified certified transmitters based on the assumption that the field strength from these is less than the legal maximum. This is not very scientific but is probably the best we can do with limited test equipment.