Total posts : 45366
One of the things that Rich has shown us with some of the antenna analysis he’s done is that predicting the range of an FM transmitter solely based on power cannot easily be done. It’s not simply just linear.
Range (or field strength) for a given power level depends on a huge number of factors, the most important being transmitter antenna height (and the type of antenna), receiver antenna height (and the type of antenna), obstructions between the two antennas, and of course, distance.
And then you have the disparity in receivers. Most people won’t use car radios to listen, and there is a large difference in the sensitivity and selectivity of modern portable and handheld receivers.
So, if you have a lousy portable radio and a transmitter with a lousy antenna (i.e., a rubber duck) at ground level getting a range of, say 600 feet, that range could be many times more that with a transmitter mounted at 10 meters, a good antenna and a much better radio. Milliwatts with a 1/4 wave vertical, elevated, can get you several miles of range under good conditions (I know, I’ve experimented).
Johny C in New Zealand stated in one of the ALPB meetings that line of sight, his 1 watt in New Zealand could probably get 20 km to a good car radio. The FCC isn’t going to look at that 600 feet – they’re going to look at the maximum range you could get, as that will dictate potential interference.
And that’s also the danger with specifying the rules as output power. Field strength maximums put hard, measureable (admittedly with expensive equipment) range limits on your signal. With output power, not so much.