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Originally Posted by Chris is me
As long as you stay under 3.2 miles, your low power transmitter is completely legal.
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Bill, I strongly recommend you do your own research with the FCC who holds the authority for these types of devices. Be wary of any advice you receive on a forum like this (including mine) as any of it can be misleading or wrong, and if you follow it and end up breaking the law by accident they will not accept that as an excuse. A prime example here is Chris' assertion that "as long as you stay under 3.2 miles it's legal" - this is flat-out wrong.
Here is the FCC website describing low-power FM stations, which is probably where Chris got his 3.2 mile limit from. Note that you would likely not qualify for a LPFM license (and yes, you still need a license):
http://www.fcc.gov/lpfm/
Choice quotes from that document:
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This page contains information about the Low Power FM (LPFM) radio service, which was created by the Commission in January 2000. These stations are authorized for noncommercial educational broadcasting only (no commercial operation) and operate with an effective radiated power (ERP) of 100 watts (0.1 kilowatts) or less, with maximum facilities of 100 watts ERP at 30 meters (100 feet) antenna height above average terrain (HAAT). The approximate service range of a 100 watt LPFM station is 5.6 kilometers (3.5 miles radius).
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A construction permit is required before a LPFM station can be constructed or operated.
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LPFM stations are available to noncommercial educational entities and public safety and transportation organizations, but are not available to individuals or for commercial operations.
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Ramsey has some good real-world information on the use of low-power FM kits or homebuilt devices here:
http://www.ramseyelectronics.com/res...t.asp?page=fcc
One of the important limitation is probably this one:
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The most specific FCC regulation of 88-108 MHz FM Broadcast band unlicensed operation is that the "field strength" of the signal must not exceed 250 microvolts/meter at a distance of 3 meters from the transmitter (FCC rule 15.239). If you have any concern about this emission limit, have your device checked by a technician with accurate measuring equipment. Remember that the "field strength" of a signal is determined as much by the antenna as by the RF output of the transmitter itself.
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They also have a table which shows the field strength at various distances if you adhere to this rule. If you modify a kit in any way, though, you'd need to somehow verify compliance with this rule.
And, most importantly, you cannot interfere with any licensed broadcaster in the band. If you cause interference for any of your neighbors who are trying to listen to a licensed station, you are violating federal law. The FCC has been known to crack down on offenders too, so don't just assume that no one will notice and you'll get away with it.