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The best approach to staying within the law would probably be to buy an FCC certified FM transmitter, operate it in its fully certified state and try to maximize range by getting the transmitter and antenna high and in the clear.
A short list of inexpensive FCC Certified (as in FCC certification clearly provided by the manufacturer – not “compliant” or “compatible” – but really, actually certified with an FCC number) is:
C Crane Digital FM Transmitter at $69.95
UX300 USB FM Transmitter at $99.95
ACC100 Part 15 FM Transmitter at $199.95 in mono or $399.95 in Stereo
Info Radio, priced at $499.95
The other way to go would be to buy one of the dozens of other transmitters on the market (the PCS unit is certainly a good choice in the “not certified” category) and roll the dice. With a good faith effort to stay within the rules you would likely do pretty well, worst case scenario might be an NOUO by mail if there was complaint made by someone.
In reading over the FCC Field Citations you can see that most folks who get pinched are in the thousands in uV/m so you would probably not be in that category – just be ready to turn it off if asked 🙂