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A “Part 15” (not really) FM system radiating 25 mW from a 1/2-wave dipole can produce a field strength of ~420 uV/m over a reflection-free, line-of-sight path to a point 1/2 mile away, if that power is radiated in that direction. This is plenty of field for almost any FM receiver, and more field than a legal Part 15 FM system should produce only 9.84 feet (3 meters) from its antenna.
OTOH, a truly legal Part 15 FM system would produce about 9 uV/m for those conditions, at a distance of 1/2 mile. But that still is a useful signal for a decent FM receiver.