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[quote:b064c02119=”RadioheadC”]So, what accounts for the difference between the 10 or 11.5 nanowatts that the FCC and you respectively estimate and the 1 to 10 milliwatts that manufacturers seem to use?[/quote:b064c02119]
The FCC Public Notice you linked to refers to the “Maximum Effective Radiated Power” permitted — which is the peak directional power actually radiated by the antenna — not the power input or output of the Part 15 FM transmitter itself.
The transmitter output power can be whatever value it needs to be to produce no more than 250 uV/m at a 3-meter distance in any direction from the antenna. So if the antenna has a peak gain of 1X, the power applied to its input connector could not exceed the nominal 10 nanowatt value.
I think the FCC Rules for “type acceptance” require commercial manufacturers of Part 15 FM transmitters to supply the antenna, and that it should either be permanently attached to the transmitter, or use a unique connector so that it can’t be replaced with an antenna that wasn’t tested and certified as meeting the Rules with that transmitter.
In any case, antennas for the FM band usually are very efficient, because even though they are short physically, they are long in terms of wavelength (unlike for Part 15 AM antennas). A 1/2-wave FM dipole, for example, is only 4 or 5 feet long, and has a peak gain of 1.64X compared to an isotropic radiator. A “whip” type antenna has somewhat less gain than that, but still far more gain than would permit using anything approaching 1-10 mW with it, and still meet Part 15 Rules.
Manufacturers quoting Part 15 FM transmitter power of 1 to 10 mW may say (if pressed) that these powers are for use only “where authorized.” Or maybe some of them either haven’t done the math, or are referencing inapplicable/outdated FCC documents. Good question to ask the ones who do this.