Micro-Broadcasting:
Getting The Most Out Of Part 15 Radio
I was an electronics enthusiast all the way back to my pre-teen years. It all started with my introduction to low power radio "broadcasting" - a Radio Shack AM wireless microphone, called a "P-Box" kit, which I built back in the mid-1960's. I decided to attend a vocational-technical high school to pursue an education in electronics. My curiosity continued as I started designing and building transmitters with a handbook from the Amateur Radio Relay League serving as a guide. Radio intrigued me and I wanted to learn more about the world of radio broadcasting.
In my teen years I got involved with what was then a 330-watt community FM station located at my hometown high school. The station's founder and technical director became my mentor and, under his guidance, I served as student chief engineer during most of my tenure prior to graduating and moving into the working world. This valuable experience opened doors for me later in life to not only become chief engineer for a 1000 watt directional AM daytimer, but to work as a contract engineer for several other AM and FM stations. For nearly three decades I've been employed as an electronics technologist for a municipal school district. With the renovation of one of the district's high schools, I was surprised to learn that, not only did they plan on building a television media center, but they planned a two-studio radio station as well. As is typical with such things, little thought was given on how to put it all together. Since many people in the school district were aware of my broadcast background it was up to me to figure out how to build the station, given limited funds and coming in after much of the renovation had already taken place. I did know that I wanted to give our students an opportunity similar to what I was fortunate enough to have had.The campus is a stone's throw away from both the New York City and Long Island radio markets, meaning that the radio spectrum is crowded to the point where it would be impossible to apply for even the smallest licensed radio facility, a low power FM (LPFM) station. Limited by both spectrum space and available budget, I knew I would have to be creative.
I realized I would need to make the station a campus-limited operation. Luckily for me, this was the perfect opportunity to explore Part 15 radio.
What is Part 15?
The Code of Federal Regulations is a compilation of all final regulations issued by United States federal agencies. Title 47 of this compilation is specific to telecommunications. Contained within Title 47 is Part 15, the section addressing the regulation and use of license-free, low power radio devices that emit signals across the radio spectrum. Subpart C addresses the use of an intentional radiator, otherwise known as an antenna. I focused on the regulations addressing operation within the AM and FM broadcast bands.
There are several rules governing the use the Part 15 radio devices on the AM broadcast band. Part 15.209 explains that legal operation is defined as field strength dependent on the operating frequency. The formula of 24000 divided by the operating frequency in kilohertz equals the allowable signal strength in microvolts per meter, which is measured at a distance of 30 meters from the antenna. Other than the specified field strength there are no physical limitations imposed regarding the antenna or ground system.
Since a typical hobbyist doesn't have access to calibrated measurement equipment, there is an alternative regulation. Part 15.219 allows operation by defining the input power to the transmitting device's final amplifier as not exceeding 100 milliwatts in combination with a total length of transmission line, antenna and ground lead not exceeding 3 meters in length.
Pertinent to operating on the grounds of an educational institution, Part 15.221 outlines field strength measurement requirements for three different operating modes: carrier-current (where the transmitted signal is injected into the AC power lines of the campus), radiating coax (special "leaky" coaxial cable that allows a controlled amount of the transmitted signal to radiate along its entire length) and antenna-based or "free radiating" transmission.
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