Last fall I wrote a post on unlicensed broadcasting on the AM band in the US. The impetus was reading an article on a rare FCC action against a station in Portland, Oregon, which caused me to track down the few other Commission actions against AM pirates in 2010.
The so-called Part 15 rules that govern permissible unlicensed broadcasting on the AM dial are somewhat more lax than the FM dial. This makes it a popular band for hobbyists and folks who want to serve small localized areas without breaking commission rules. However, it can also be easy to overstep these rules in the pursuit of gaining a bit more broadcast area.
As a service to would-be legal micropower broadcasters, the Low-Power Radio Blog recently compiled the technical reasons behind FCC actions against unlicensed AM stations going back to 2003.
One of the interesting elements of Part-15 rules for AM is that they specify the total amount of power a transmitter can use, along with the total length of the transmission cable to the antenna combined with the length of the antenna itself. Many clever amateur radio engineers have figured out ways to eek the most out of these constraints. And, from looking at this list of enforcement actions, it would appear that abiding these limits is well advised.
Six of the twenty-one cited actions were for stations that had transmission cables or antennas that were too long. The limit for both combined is three meters (9.8 feet), and in five cases the Commission notes the lengths they measured were anywhere from 5.4 to 28.2 meters too long.
For the other fifteen cases the FCC field agents reported a field strength–a measure of the power of the signal received at a particular distance from the transmitter. In these cases agents measured field strengths of anywhere from 43 to 9000 times the Part 15 limit. What this indicates to me is that these broadcasters were making no attempt to stay under that limit, and therefore making no obvious attempt to operate unlicensed stations in compliance with Part 15 rules.
This also indicates that there are few, if any, broadcasters who are trying to adhere to Part 15 rules but slightly overstepping the limit and getting a visit or nasty letter from the FCC. It all adds up to mean that if you want to dip your toes into the unlicensed broadcasting pool with low risk, going with Part 15 compliant low-power AM is probably the best way to go.