Radio Amateurs attempt Low Power FM

Back in October of 2013, members of the K5UTD Amateur Radio Club met up at Raising Canes after the monthly meeting. We discussed RadioUTD, and how the internet wasn’t providing enough exposure. Apparently the FCC allows you to setup very low power AM stations in which the AC mains act as your antenna. For UT Dallas, and a station like RadioUTD, this was unsatisfactory. We discussed another option, Low Power FM.

Back in the early 2000’s, the FCC created the Low Power classification of Part 73 FM broadcast stations. LPFM stations are community-driven entities that transmit at no greater than 100 watts ERP at 100 feet above ground level. The goal of LPFM is to bring more community-based programming to served areas, as opposed to commercial broadcast interests.

The dinner at Canes finished up, and we all went home. Sometime later in the night, I got a text from another K5UTD club member, Landon. It said something along the lines of, “LPFM application process open. Act soon!”

[K5UTD is the Amateur Radio Club. Our focus is on communications, RF engineering, and public service. RadioUTD is the internet-only radio station located in the Student Union. These two organizations are independent, but are collaborating for a common goal of establishing an LPFM station. RadioUTD wants to be on the air, and K5UTD enjoys a challenge.]

Applying for a License

It’s not like driving over to the DMV, waiting in line, getting an awful picture taken, and regretting said photo for the next 10 years. The application was 26 pages long. The window to file the application was from October 15, 2013 to November 14, 2013 (at 5:00PM). Per the college student standard, ours was submitted at 4:45PM on the 14th of November.

In the application, there were a lot of technical details, details about the organization applying for the license, and an odd favoring of Native American applicants. Everything is based on a point system. Established Community Presence is worth one point. This is defined as being established for two years. Local Program Origination will earn you another point. For at least 8 hours of every day, the programming must be locally produced. Having a main studio that is staffed for at least 20 hours a week will earn a point. A bonus point is awarded for having local programming and a main studio! The final point that we were eligible for certifies that we don’t have an interest in any other broadcast station. This totals to 5 points. Without being affiliated with tribes, tribal lands, or tribal organizations, we cannot claim a sixth point. Unfortunately, we’re not affiliated with anything tribal.

We collaborated with RadioUTD to compose the application, and were ultimately confident with what we had submitted.

The Waiting Game

As with every government-based application process, progress isn’t exactly quick. There were many applications submitted, and I imagine the FCC wasn’t prepared to handle all of them. Several months after the applications were submitted, the FCC released “MX Groups.” In the event that there are multiple applicants competing for the same frequency in the same geographic area, a mutually-exclusive group is formed.

The first factor in resolving MX groups is the point system. Applicants with the most points will be chosen over those with less. We’re in a favorable situation, as we qualify for the maximum number of points attainable in the DFW area. Because DFW is in the top 10 radio markets, there are other applicants that also qualify for 5 points. In this case, we’re given an option to voluntarily enter a time-sharing agreement. For example, if Station A and Station B want to enter a time-sharing agreement, Station A can opt to take 5:00AM to 1:00PM. Station B can take 1:00PM to midnight. The final method of resolving an MX group is involuntary time-sharing. This is where the FCC will tell you when you can be on the air as to not conflict with the other stations. This is the method of last resort.

As of the writing of this post, our application is in an MX group with others from the DFW area. The FCC has not yet resolved the MX groups in Texas.

The Signal Chain of a Radio Station

Let’s take a minute to discuss how the audio moves through a radio station. There are several sources of audio – a computer, microphones, a telephone line, and maybe CD players, record players, or even tape decks. All of these sources are brought to a mixer. The primary function of a mixer (in a radio application) is to control the levels of each source. There’s typically a START/STOP button (think unmute/mute), a fader to control the level, and some routing options. The audio that’s intended to go over the air is played on the Program Bus. Contrary to the Program Bus, the Audition Bus is where you can play music to only be heard in the studio. This is useful for cuing music, screening callers, and level checking.

Processing is the next major step in the signal chain. Most commercial radio stations will heavily compress their audio. Compression is a way to level out the “loudness” of a track.This is an oversimplification of compression, but it effectively turns up the quiet parts and turns down the loud parts. Broadcasters will compress their signal so they can make every part of the song sound even. With everything even, it can be turned up to the maximum allowable level. The FCC defines this maximum as 100% modulation, where 100% = 75KHz FM deviation. Thus far, the signal (music, talking, etc) is generated, mixed, and compressed.

FM Stereo was introduced in the early 60’s. For stereo audio, you need two channels, Left and Right. In FM broadcast, the transmission itself is a single channel, or mono. To multiplex (simultaneously transmit) both the left and right channels, the audio is fed from the compressor to a stereo generator. The stereo generator uses math to split the signal into L + R and L – R. Your receiver will use the reverse process to separate that out into the left and right channels. The L + R technique is intended to be backwards compatible with mono receivers (if it were just L and R, the mono receivers would only get one channel instead of the sum of both). In our case, the stereo generator and compressor/limiter are an all-in-one solution (Orban Optimod 8100A).

The next step in the signal chain doesn’t change anything, but rather moves the signal from one location to another. It’s called the Studio to Transmitter Link, or STL. In the case of our station, the transmitter will be located in the Engineering and Computer Science building, and the studio is located across the street in the Student Union. In commercial radio, the distance between studio and transmitter is typically measured in miles, not yards. STLs were traditionally 950MHz RF links (strictly wireless), but modern radio stations make use of phone lines, traditional 950MHz STLs, and even the internet. Depending on how the station is built, the compression/limiting and stereo generation can happen at the transmitter site or at the studio. In our case, the STL is going to be an internet link, and the compression/limiting and stereo generation will happen at the transmitter site.

The final step is the transmitter. This is the unit that actually puts the station on the air. We give it audio and power, and it gives us RF. This will be amplified from about 15 watts to 80. This RF is then sent to an antenna. The antenna will induce a charge on other antennas in the receiving range, allowing you to receive the transmission with your car or home stereo.

Here’s the TL;DR: Music/Speech > Mixer > Compression/Limiting > Stereo Generation > Transmitter > Antenna.

In the meantime…

Internally at K5UTD, we discussed our likelihood of getting the license. Based on this chance, and perceived financial risk, we set an arbitrary limit on broadcast equipment expenses. Is this enough to setup a full station? No. Is it enough to maintain interest with the involved parties? Yes, in fact having the dollar limit motivates us to work a little harder at finding good equipment at bargain prices. Remember, we’re Amateur Radio operators. Bargain hunting is our specialty.

Finding a transmitter was our first goal. We did some research, and two main contenders seemed to meet what we were looking for – the Harris MX-15 and the Continental 802A. Both are 80’s era, solid-state transmitters, going for far less than the all-digital solutions seen today. The signal chain in both of these units is completely analog. Oddly enough, we found an MX-15 on eBay, and were able to locally meetup with the seller. She appreciated what we were trying to do, and gave us a “student discount.”

Our next acquisition was mostly by chance. By sheer coincidence, I met the Chief of Engineering at an 8 station cluster. He also appreciated what we were doing, and offered to “fill up a pickup truck worth of gear.” A few months later, we returned with a truck, and were given Marti Remote Units, an Optimod 8100A, and some other kibbles and bits that will help in the long run. The 8100 was state-of-the-art back in the 80’s, and is commonly used as a backup today. It’s not overly aggressive, but will easily keep our station compliant and near 100% modulation all of the time. Remember, we’re not attempting to compete with KISS stations, or The EDGE, or whoever else. We’re not trying to squeeze as many dB as humanly possible out of 75KHz of deviation. Our goal sound will be lightly compressed, lightly equalized, and easy to listen to.

The most recent acquisition came from a local production company. They’ve migrated entirely to digital mixers, and with digital gear they don’t need outboard analog processing. They kindly donated some equalizers, compressors, and CD players. The CD players will be perfect for auto failover, as they can be controlled via RS-232 or TTL logic. The equalizers will be perfect for ever-so-slightly adjusting our curve to sound good in mobile applications.

We sincerely appreciate both the donors and what they’ve done to help us out with this process. We’ve spent a little over $600, and have almost built an entire transmitter site. None of this would have been possible without your help!

Final Thoughts

We’re still waiting for our construction permit, but the process thus far has definitely been educational and a lot of fun. It’s not every day that an opportunity to setup an FM radio station comes along, and I’m very glad that we’ve had support from everybody at K5UTD, RadioUTD, the various engineers I’ve harassed about FCC rules and regs, and from the equipment donors.