Total posts : 45366
18.75 nanowatts is tiny. It is so small accurately measuring it is difficult. It is easier to design a lossy antenna system so the TPO is greater than the allowed ERP. As an example, the C Crane FM2 does not need a resonant antenna because they can get to the max allowed field strength without one. Adding wire to the C Crane’s antenna to make it resonant increases the efficiency of the transmitter thus exciding the allowed field strength.
Anyone can slap a part 15 compliant sticker on a product and sell it on ebay or amazon. If end user of the product gets a visit by the FCC, the violation falls on the end user. The FCC can issue a NOUO, and if the end user pisses off the FCC, a NAL. If the FCC wanted, they could go after the seller of the device, but the end user still could be facing a several thousand dollar fine if the FCC wants to be aggressive.
So it is important to get a transmitter that is actually part 15 certified and use it in a way that will not increase the field strength. Without a field strength meter, that is the only way the end user has some hope of not getting a NOUO.
A transmitter that can do 87.5, 87.7, or 87.9 that has a part 15 sticker is probably not actually part 15 certified.
A C Crane can’t even do 88.1; when they went to certify their transmitter and tested it on 88.1 they may have found emissions below 88MHz that exceeded 100uv/m @3m, so rather than have side band energy fall outside of the FM band they decided to limit how far down their device could transmit. So the part 15 certification of the C Crane transmitter is presumably a valid one.