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“…Rich, is it possible to reasonably estimate the error range in this type of situation? Though Robert’s measurements may not be precise it may be useful if the error range is known. …”
This method would be subject to a considerable number of unknowns, unfortunately. The VHF wavelengths used for FM broadcasting are much more subject to obstruction losses and reflections than those used in AM broadcasting. Such effects at the receive antenna on the signal using a line-of-sight path to the transmit antenna cannot easily be predicted.
Below are some Powerpoint clips I wrote before I retired, showing some of these conditions. The clips were from a microwave talk I gave, but the same principles apply at VHF.
Here is a page from a microwave system planning handbook, showing how the received signal varies with Fresnel clearance:
Another factor is the azimuth and elevation plane radiation from the transmit antenna used by the FM station, which can vary as much as 20 dB depending on the radial path to the receive point. This is a function of the transmit antenna configuration, and re-radiation by the tower supporting that antenna. Almost no FM broadcast station radiates a truly omnidirectional signal.
The FCC uses a statistical approach when predicting the fields of an FM broadcast station, giving a result for 50% of locations, 50% of the time.
Short paths are much less vulnerable to obstuctions and reflections, which probably is why the FCC uses 3 meters as the reference distance in §15.239.