FM Broadcasting

Posted in Technical Information

FM Radio  88 MegaHertz to 108 Mhz

 

FM broadcasting has gained popularity in the past 20 years, especially in urban areas. The modulation techniques used in FM broadcasting make it virtually immune to the “noise” and interference that make listening to shor twave or medium wave broadcasts difficult in the cities.

It needs to be stressed, however, that the coverage area or reach of an FM broadcast is limited to approximately 80 kilometers. The coverage area of an FM transmitter is basically line of sight. FM does not follow the curvature of the earth the way medium wave and short wave does and will be blocked by mountains located in the path from transmitter to receiver.

Because of the reduced coverage area, lower power transmitters can be used. Pictured to the right (Figure 1) is a 500 watt FM transmitter typical of the type of transmitters used in many locations in Africa.

When a proposal is made to install an FM transmitter or a series of transmitters in a network, there are a number of steps to go through. First, a broadcast license needs to be secured from the government. A detailed survey of the potential transmitter sites needs to be carried out. Information regarding availability of power, buildings and a tower needs to be collected.

Once the license is granted and a location selected, a frequency for broadcasting will be issued. Advances in technology now make it possible to determine the coverage area of a potential transmitter site based on GPS readings taken at the location.

The coverage area of an FM transmitter will depend on the terrain and the height of the antenna in relation to the target area. Shown at right (Figure 2) is a typical coverge plot for a 1000 watt Crown transmitter connected to a four element dipole array mounted on a 100 foot tower.

The dipole array pictured at the right (Figure 3) consists of the four T-shaped objects on the tower. The antennae here are mounted directly on the leg of a guyed tower.

When self-supported towers are used, the physical size of the structure requires that the antennae be mounted away from the tower in order to minimize the effect of the tower on the broadcast signal.

This requires skilled workers to install the antennae on the tower. The dipoles are installed on the support structure and then hoisted into position.

Once the antennae are installed, the feeder cable which carries the signal from the transmitter to the antennas is installed. (Figure 4)

And, finally, the equipment rack containing the transmitter is installed inside a building and connected to the antennas.

A series of FM transmitters/translators can be used to form a network to cover a larger area. This allows one broadcast source to feed a network that gives countrywide coverage. Each translator receives the signal from the adjacent station and rebroadcasts the programme on a different frequency. In this case, an additional receive antenna must be installed to receive the signal from the previous site so that it can be re-broadcast on a different frequency.

Figure 1 - FM Transmitter

 

Figure 2 -Coverage

 

Figure 3 - Dipole Antenna

 

Figure 4