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Unlicensed AM broadcasting a little safer than FM


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.

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2 Responses to Unlicensed AM broadcasting a little safer than FM

  1. March 25, 2011 at 3:32 pm #

    Part 15 low power AM broadcasting has been around as early as the 50’s thanks to phono oscillators, a device which allowed the operator to broadcast their phonographs to nearby AM radios. In more than 6 decades it’s only been in the last 10 or so years that Part 15 AM has come under any intense scrutiny thanks in part to the alarmists who think one-tenth of a watt will somehow put the big radio conglomerates out of business.

    The family of web sites focus on low power AM and micropower FM broadcasting under Part 15 regulations not only for the hobbyist and electronics enthusiast, but for specialty business applications and campus-limited broadcasting for K-12, college and university radio. Utilizing properly engineered systems, Part 15 AM and FM have their benefits but for the average radio enthusiast Part 15 low power AM is a much safer alternative. With the proper resources most anybody can build their own station while remaining legally compliant with the FCC Part 15 regulations.

  2. John Poet April 7, 2011 at 6:35 am #

    I used to do low-power AM as a young teenager in the 70s, before becoming a shortwave radio pirate later on. It was a “quick and dirty” operation using the old Radio Shack “AM wireless mic kit”. Of course I hooked the thing up to the SWL antenna, so I was way over the allowable antenna length. Even so, the range seemed to be limited to about 100 yards down the street. I don’t know that anyone ever heard it other than those doing it. I didn’t know how to do a direct connection for audio, so the sound was fairly dirty.

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