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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