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Here is a bit more detail on the subject of Part 15 FM systems.
A rated input sensitivity of 1.7 µV for an FM receiver is the r-f voltage needed across its antenna input terminal impedance to produce a certain signal-to-noise ratio in its audio output waveform. So that sensitivity rating is not the field intensity in µV/m that is needed at its receive antenna.
For example at 107.9 MHz, a typical “whip” receive antenna about 30″ long needs an arriving field intensity of about 5 µV/m to produce 1.7 µV across the typical 75 ohm impedance at the antenna input terminals of the receiver.
Below is a graphic showing the free space fields for an FCC-legal Part 15 FM system (the red line in the graph) and several others. The power needed to radiate the maximum legal field is extremely small: 11.43 nanowatts (0.000 000 011 43… watts). Systems operating with transmitter powers of even 1/10 mW (and above) can produce fields far above the FCC limit for Part 15 FM unless their antennas radiate almost none of that power.
Reflections of these signals from the earth and from buildings in the area of the transmit and receive antennas can produce net fields at some locations that can be as much as twice as great as for a free space path. But in other locations they might be only ~1/10 as great as the free space path. This effect often is easy to notice when listening to a weak FM broadcast station on a car radio, where the audio has short intervals of high noise/distortion as the car changes its location by as little as five feet (sometimes called “picket fencing”).
There are two considerations for this, for Part 15 FM systems:
1. If the field intensity that the FCC measures is in a location where the fields are higher than the legal free-space value, they might conclude that the transmit system was non-compliant even though it was not.
2. If an unlicensed operator judges the amount of power being radiated by the performance of an FM receiver at a location where the fields are lower than the legal free space value, it might lead to excess radiated power — and draw attention to that operator.