What is the maximum distance you can legally broadcast? I see transmitters for sale online that can go anywehere from 1 mile up to 10 miles. They obviously can't be FCC legal. But I would have no problem getting one if I wanted one. They are readily available.
Well that's a bunch of Mumbo jumbo to me. The FCC should have written that in English. How far can I transmit a signal before I have to worry about the feds knocking on my door?
Saying the FCC should have "written that in English" is a lot like saying that you shouldn't have to learn how to drive a car; you should be able to just "turn the key and it goes". Spectrum is finite and it requires proper engineering for stations to share it appropriately without causing interference to each other.
That said, it's actually easier to decipher than you might think. The rule is 250 uV/m (microvolts per meter) measured at 3 meters' distance from the radiating antenna. It helps here to remember that for, say, a Class A FM (6000 watts ERP at 30m HAAT, or the equivalent) the rule is that the service contour is 1 V/m (volt per meter) measured at 28.3 kilometers (or 28,300 meters). One volt equals one million microvolts, so the legal limit for unlicensed broadcasting in the FM band is 0.00025 volts/meter measured at 3 meters. In other words, a tiny fraction of the power of your average radio station. So small that you're not really "broadcasting"...which is rather the point of legal, unlicensed transmissions in the FM band (governed by Part 15 of the Code of Federal Regulations). They're not supposed to be reaching a wide audience.
To really put it in layman's terms: I've had discussions with people in the know that have told me that if your Part 15 FM broadcast is audible at more than 250ft it's unofficially considered "illegal". After all, for legal Part 15 "broadcast" purposes, there is no reason why you would ever need a range greater than 250ft. In most cases, you'd never need more than about 25-30ft, actually. If you want more than 250ft, you need a license to operate...with all the benefits and responsibilities that entails. Dem's da rules.
It may be illegal every non-comm ever licensed has had to protect channel 6. IF they are running 5KW they pretty much have to be using a certified rig. I have never seen a 5KW pirate rig on he market. If they were interfering with the aircraft band they would have ben shut down by now.
If any pirate station is running a 5000 watt transmitter I'll eat my hat. It's VERY expensive (usually over $10,000) to purchase a transmitter over 1000 watts, and even that's pretty pricey. It's not easy to build a 5kW transmitter, either...quite hard to do it right without killing yourself or creating something that spews out spurious emissions all over the place and sounds like garbage, to boot. Theoretically, I could see a pirate getting a three-bay, vertically-polarized, full-wavelength-spaced antenna array. That will get you about 3.0 gain (I'm speaking quite roughly here) and that could mean a fairly hefty signal. But a 3-bay/F-W means at least 25-30ft of vertical real estate. That's kinda hard to obtain at all, much less hide.
It's also thoroughly unnecessary. On a reasonably "clear" frequency, even a 20 watt FM transmission will go 10-20 miles if you've got a little height and not too much terrain in the way. Our LPFM has 100 watts ERP and it'll go 15 miles in some directions...and it'd go further if it weren't for terrain and two (distant) co-channel stations. Saying you have 5000 watts
sounds a lot more impressive...and from a marketing perspective, who listening is ever gonna know the truth?