Elements of the FM Broadcast Air Chain
1) The Sources
2) The mixing console
3) The Processor
4) The Studio Transmitter Link (STL)
5) The FM Exciter
6) The FM Transmitter
7) The Antenna
Let’s look at the elements in detail…
The Sources – This simply refers to the sources available for broadcast…These includes microphones, CD players, Computers, etc.
The Mixing Console – All of the sources are connected to this device, and the board operator uses it to direct the appropriate source onto the program feed. Typically, these consoles have several outputs.
Program (PGM) – This output is typically used to send to the transmitter so that the listening public can enjoy your programming
Audition (AUD) – This output is typically only used in-house, and is only heard by folks in the studio. Typical uses include feeding audio to the on-air telephone system, feeding audio recording equipment, etc.
Utility (UTL) – This is quite simply an extra “audition” output. Had to call it something. J. It’s used in the same manner as the Audition output.
Whether or not something is sent to these outputs is up to the operator. Typically there’s buttons near each “Fader” to feed a particular source to any or all outputs.
Monica Gannon of WDOK operating a Broadcast Mixing Console
The Processor – The program output of the console is connected to the Audio Processor. This is a device used to control audio levels to ensure legal operation of the radio station. They are also used to provide a distinct on-air sound for the radio station.
Fm Audio Processor
The Studio Transmitter Link (STL) – The STL is simply that. The system used to connect the studio to the broadcast transmitter so that the listening public can hear what you’re doing in the studio. This can be simply a long cable run between the studio and transmitter, or a sophisticated “microwave” link provided by the devices pictured below.
In this picture, the top device is the sending unit (transmit), located at the studio. The other is the receiver, located at the transmitter site. The output processor is connected to the sending (transmit) unit. The output of the receiver is then connected to the transmitter.
“Microwave” radio STL Units
The FM Exciter – This is the first step to being on the air… The FM exciter takes the signals from the STL or Processor output, and modulates a radio wave with it. This radio wave is then a tiny version of your big signal. It is typically only a few watts. With just this signal alone, you can broadcast about a mile or so with it, so you’re going to need some extra power. That’s where the transmitter comes in….
The FM Exciter
The FM Transmitter – This is essentially a big ‘ol amplifier for FM radio signals. The amount of amplification used depends on what the Federal Communications Commission licenses a station to use. The signal from the FM exciter can be amplified to a level starting at 100 Watts (the current minimum for FM) to as much as 100,000 Watts (the maximum power level for FM). Again, the Federal Communications Commission determines the exact level for each FM station in the country.
The tolerance of the output power is as follows:
The output may vary no more than: 105% over the licensed output power, and no less than 90% below the licensed output power. Because of this critical range, operators are usually required to watch the transmitter “readings” to ensure that the parameters are met.
The FM Antenna – The FM Transmitter is attached to the FM antenna, which is usually placed at the highest point possible. The FM antenna is not the entire radio tower, but is attached to the radio tower, typically attatched on a thin pole at the very top of the tower, if possible. The picture below is that of a “two bay” FM Antenna…This means that two individual antennas are used to operate as one unit.
A Two Bay FM Broadcast Antenna
Hope this sheds some light on how broadcasting works!