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fm_broadcast_primer

A Primer for Broadcasting on the FM band

Preface

Ahh, good old analog radio broadcasting.
The radio is a beacon, outlasting the era of its birth. Persisting as a communication form even in this modern day of hyper connectivity. Maybe it's nostalgia, more likely it's simplicity and ubiquity, that has helped “the radio” stay relevant for a century.

At any case, the modern spectrum of radio markets, are more saturated and controlled by corporate and christian interests than ever before. With the consolidation of stations under companies like clear channel watch and new powerful religious stations sprouting up all the time.
A small group of platforms, devoted to local communities free speech and expression, is a dwindling minority on the dial. The same thing that has happened to our democracy is happening to the airwaves.

In 2000 response to much public outcry, the FCC created the LPFM licenses, for stations under 100 Watts. Unfortunately corporate lobbying in congress halted the project for a decade.
In 2013 2,800 groups applied for LPFM licenses, and their are currently 2,714 stations. The FCC however has no plan to open any more application windows for LPFM licenses.
Even with a new application window, the cost associated with licensing would be prohibitive for many smaller communities and stations.

In light of all this, broadcasting without a license, seems the only way for many communities to air their voices. Unlicensed broadcasts, that do not significantly interfere with any other stations, allow for increased diversity of the airwaves.
Broadcasting equipment is very inexpensive now. You can cover a small town or neighborhood with a $60 transmitter like this.(Note: this TX can be easily killed by close lightning)

Taking Back the Airwaves

Starting an FM radio station in your community can be very fulfilling project. There are many facets to running a station. This wiki has lots of info to guide you in the process. Here we will go over getting a signal up on the FM band and what your options are to get there.

Broadcast Equipment

FM broadcasting takes some special equipment.

At the core of this equipment, are the transmitter(TX) and antenna. These are connected with a coax cable.
A transmitter often has multiple built in amplifier stages, to give you a higher power output. You may also connect additional amplifier stages, to some systems, to increase transmission power.

A filter should also be used, to prevent harmonic interference on aircraft and law enforcement bands. A filter is already be built into some transmitters, and can be added easily if not.
A SWR meter can be connected, between the antenna and the rest of the system, to measure transmitter vs reflected power. This device can help you troubleshoot issues and tune your antenna and transmitter systems. A good SWR reading is around 1.2, 1.4 is very good and less than one is not so good. Excessive reflected power can damage your transmitter.
Antenna analyzers and spectrum analyzers can be used to dial in your signal even more, if you have access to them. (Here is a cheap option for a spectrum analyzer)
Small coax jumpers and adapters will be needed to hook up separate pieces of equipment.

This is the order in which to connect the components, starting from the Antenna

  1. Antenna
  2. Coax cable (to run to a building/enclosure)
  3. SWR meter (optional)
  4. Filter (if not built into the TX)
  5. Amplifier (if not built into the TX)
  6. Transmitter
  7. Audio Equipment

Audio Equipment

At the minimum, you need some sort of audio source to feed into the transmitter. This can come straight from a computer or be produced from a whole studio.
A mixing board can be used to combine many sources of audio to produce shows.
One or more compressor limiter stage, can help greatly boost your stations loudness and presence, without over modulating. Over modulation interferes with adjacent stations, so it needs to be avoided.
Here is an inexpensive compressor that takes up very little space.

Finding Equipment

You can find broadcast equipment in person at flea markets and occasionally thrift stores or pawn shops.
Finding them online is easier and allows for a much greater choice in your equipment.
Here are some good sources for broadcast equipment online.
eBay, Amazon, broadcast warehouse

Here is more info on Setting Up a Studio

fm_broadcast_primer.txt · Last modified: 2016/07/29 00:57 by skyfly200