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Rich’s calculations used the same gain for all the NOUO Field Strength measurements he cited, a power gain of 1.6.
The Part 15 FM (88-108MHz) allowed field strength is tiny, the power required to get there is also tiny. If someone uses a transmitter that is calibrated for micro power use, if the lowest power setting was 1mW (0.001W), and that was accurately the power at the antenna port on the transmitter, the feed line and antenna would need to have a gain of 0.00001875 (-47.3dB of loss) for the radiated field strength to be part 15 complaint. In that case, care would be needed to make sure the feed line and attenuators/pads did not radiate enough to violate the part 15 rules.
Even if a very sensitive radio is used on a channel with no co or adjacent channel interference, getting much more than 750/800 ft of reception range is a clue a transmitter is exceeding the part 15 field limit.