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WARNING: The FCC also states it is illegal in the USA to posess,transport,sell,market,own or operate any transmitter that is not a kit that does not comply with and bear an FCC certification ID tag. They are certified with the supplied antenna and are illegal to modify. A kit need not be certified but is still expected to meet field strength limits,which is the users responsibility.
FM transmitters are not specified to a power input or output level,they are restricted to a field strength limit which can be exceeded at a power level of a few microwatts,again it is the users responsibility to insure compliance.
The best way to insure FM compliance is to use the FCC estimate of range expected from a legal transmitter which works out to about 300 feet maximum. If you are getting more than that,you are probably in violation. If you want more range,try the AM band.
On another note,I heard about a company in California who has portable AM and FM stations mounted in vans they park at malls and stores with banners on the van and advertising on the air,giving specials and discounts for listeners. They even had a live remote in the mall or parking lot. They were making big money at it too. YMMV