FREQUENCY MODULATION (FM) BASICS AND TUTORIALS

Frequency-modulation (FM) broadcasting refers to the transmission of voice and music received by the general public in the 88- to 108-MHz frequency band. FM is used to provide higher-fidelity reception than is available with standard broadcast AM.

In 1961 stereophonic broadcasting was introduced with the addition of a double sideband suppressed carrier for transmission of a left-minus-right difference signal. The left-plus-right sum channel is sent with use of normal FM.

Some FM broadcast systems also include a subsidiary communications authorization (SCA) subcarrier for private commercial uses. FM broadcast is typically limited to line-of-sight ranges. As a result, FM coverage is localized to a range of 75 mi (120 km) depending on the antenna height and ERP.

Frequency Allocations
The 100 carrier frequencies for FM broadcast range from 88.1 to 107.9 MHz and are equally spaced every 200 kHz. The channels from 88.1 to 91.9 MHz are reserved for educational and noncommercial broadcasting and those from 92.1 to 107.9 MHz for commercial broadcasting. Each channel has a 200 kHz bandwidth.

The maximum frequency swing under normal conditions is ±75 kHz. Stations operating with an SCA may under certain conditions exceed this level, but in no event may exceed a frequency swing of ±82.5 kHz. The carrier frequency is required to be maintained within ±2000 Hz. The frequencies used for FM broadcasting generally limit the coverage to the line-of-sight or a slightly greater distance.

The actual coverage area is determined by the ERP of the station and the height of the transmitting antenna above the average terrain in the area. Either increasing the power or raising the antenna will increase the coverage area.

Station Classifications
In FM broadcast, stations are classified according to their maximum allowable ERP and the transmitting antenna height above average terrain in their service area. Class A stations provide primary service to a radius of about28 km with 6000 W of ERP at a maximum height of 100 m.

The most powerful class, Class C, operates with maximums of 100,000 W of ERP and heights up to 600 m with a primary coverage radius of over 92 km. The powers and heights above average terrain (HAAT) for all of the classes are shown in Table 69.5.

All classes may operate at antenna heights above those specified but must reduce the ERP accordingly. Stations may not exceed the maximum power specified, even if antenna height is reduced. The classification of the station determines the allowable distance to other co-channel and adjacent channel stations.


Field Strength and Propagation
The field strength produced by an FM broadcast station depends on the ERP, antenna heights, local terrain, tropospheric scattering conditions, and other factors. A factor in the determination of new licenses for FM broadcast is the separation between allocated co-channel and adjacent channel stations, the class of station, and the antenna heights.

Although FM broadcast propagation is generally thought of as line-of-sight, larger ERPs along with the effects of diffraction, refraction, and tropospheric scatter allow coverage slightly greater than line-of sight.

Transmitters
FM broadcast transmitters typically range in power output from 10 W to 50 kW.  The highest-powered solid-state transmitters are currently 10 kW, but manufacturers are developing new devices that will make higher-power solid-state transmitters both cost-efficient and reliable.

Antenna Systems
FM broadcast antenna systems are required to have a horizontally polarized component. Most antenna systems, however, are circularly polarized, having both horizontal and vertical components. The antenna system, which usually consists of several individual radiating bays fed as a phased array, has a radiation characteristic that concentrates the transmitted energy in the horizontal plane toward the population to be served, minimizing the radiation out into space and down toward the ground.

Thus, the ERP towards the horizon is increased with gains up to 10 dB. This means that a 5-kW transmitter coupled to an antenna system with a 10-dB gain would have an ERP of 50 kW. Directional antennas may be employed to avoid interference with other stations or to meet spacing requirements.

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