Radio broadcasting

(communications) Radio transmission intended for general reception.

The transmission, via radio-frequency electromagnetic waves, of audible program material for direct reception by the general public. Electromagnetic waves can be made to travel or propagate from a transmitting antenna to a receiving antenna. By modifying the amplitude, frequency, or relative phase of the wave in response to some message signal (a process known as modulation), it is possible to convey information from the transmitter to the receiver. In radio broadcasting, this information usually takes the form of voice or music. See also Electrical communications; Electromagnetic wave transmission; Modulation; Radio.

Radio broadcasting occurs in seven frequency bands. Longwave broadcasting is permitted by international agreement in a portion of the low-frequency band from 150 to 290 kHz in Europe. The most widely used broadcast band is in the medium-frequency (mf) range between 525 and 1700 kHz. It is commonly known as AM after amplitude modulation, the technique employed. Shortwave broadcasting is permitted worldwide in eight frequency bands between 5950 and 26,100 kHz. The very high frequency (VHF) band of 88 to 108 MHz is used for what is commonly called FM broadcasting, after frequency modulation that is used for transmissions. During the 1990s, a digital audio broadcasting (DAB) service in the 1452–1492- and 174–240-MHz frequency bands was put in place in Europe, Canada, and other countries. This band is unavailable in the United States for this service, so an alternate DAB system is being devised for use there. Radio broadcasting from satellites to listeners has been authorized in the 2310–2360-MHz frequency band. See also Amplitude modulation; Amplitude-modulation radio; Frequency modulation; Frequency-modulation radio.

AM medium-frequency band

Broadcast stations in the medium-frequency band use amplitude modulation of a carrier wave to transmit information. The amplitude of the wave is modified in response to the changing amplitude of an audible voice or music signal. The AM receiver detects these amplitude changes and converts them back into audible signals, which can then be amplified and reproduced on acoustical transducers or speakers. See also Radio receiver; Radio transmitter.

The audible frequency range is generally considered to extend from 20 to 20,000 Hz. As a practical matter, AM broadcasting transmissions are limited to a range of 50 to 10,000 Hz. Because of transmission components and directional antennas, the fidelity of many stations is more severely restricted, resulting in voice transmission that is still acceptable but music transmission that is of relatively low fidelity.

An AM station may use a single tower for an antenna, resulting in an omnidirectional radiation pattern; or two or more transmitting towers to augment the radiation in certain directions while suppressing it in others, in order to comply with station allocation criteria. Since the allocation restrictions may be different during the daytime and nighttime hours, many stations employ two different directional antennas. The radiation from the antenna is expressed in millivolts per meter at 1 km (0.62 mi) from the antenna. See also Antenna (electromagnetism).

AM broadcast signals propagate from the transmitter by three mechanisms: ground-wave, space-wave, and skywave. Ground waves travel along the ground surface (the boundary between the Earth and the atmosphere). Because they are surface waves, they penetrate into the ground, resulting in the energy being diminished because of losses in the ground. The current flowing in the antenna also produces space waves, which travel through the atmosphere from transmitter to receiver. Space-wave propagation is usually limited by intervening terrain obstacles or the curvature of the Earth. Sky-wave propagation occurs when space waves directed toward the ionosphere are reflected toward the Earth. This phenomenon can result in substantial signal strengths at distances of several hundred miles from the antenna. AM sky-wave propagation occurs primarily during nighttime hours by reflections from the E and F layers of the ionosphere at about 60 and 130 mi (100 and 220 km) altitude above the Earth's surface, respectively. See also Ionosphere; Radio-wave propagation.

The daytime ground-wave signal level protected by allocation criteria is usually 0.5 mV/m, although much higher signal strengths may be necessary to overcome noise from atmospheric and artificial sources, especially in highly urbanized areas. During nighttime hours, the service area for most AM stations (other than class A stations) is usually limited by interference from other cochannel stations. For low-power stations, this may be only 10 mi (16 km) or less from the transmitter. For the highest class of stations (class A), nighttime service is protected by the allocation criteria to the 0.5-mV/m, 50%-time sky-wave contour. Because such service is subject to the time variations and fading of sky-wave propagation, the 0.5-mV/m, 50%-time contour is considered to be a secondary service area.

FM VHF band

FM broadcasting has become the dominant broadcast service in the United States primarily because of its better fidelity and its superior reception, which is less subject to noise and interference than that of AM. Information is conveyed by frequency modulation or deviation of a carrier wave. In the United States, the carrier frequency may be deviated ±75 kHz around the assigned carrier frequency. The carrier frequencies, or channels, are spaced at 200-kHz intervals in the United States; a few other countries use slightly different channel spacings. Nearly all FM stations transmit in stereo.

The service area of an FM station depends on the propagation of space waves from the transmitter to the receiver. Space waves propagate through the atmosphere and are diffracted around, and reflected off, mountains, buildings, and other objects. Propagation within areas that have an unobstructed line-of-sight from transmitter to receiver is most reliable and predictable.

Shortwave broadcasting

For reaching audiences in foreign countries or other distant places, shortwave broadcasting is most often used. Nearly 600 million shortwave radio receivers are in use worldwide. Shortwave broadcasting is permitted worldwide in eight frequency bands from 5950 to 26,100 kHz. The assigned transmitting frequencies are spaced at 5-kHz intervals, resulting in a limited usable audio bandwidth. Voice transmissions are most effective, while music transmissions have limited fidelity.

Shortwave signals propagate via sky waves that are reflected one or more times from the E and F layers of the ionosphere. Multiple reflections are possible because a signal can also bounce off the Earth's surface after reflecting off the ionosphere in a “Ping-Pong” effect.

Digital audio broadcasting

To enhance audio quality, a digital audio broadcasting service has been put in place in Europe and elsewhere, operating primarily in the 1452–1492-MHz band. As in VHF FM broadcasting, DAB uses transmitters located at elevated locations (mountaintops, building roofs) that provide the best line-of-sight paths to the intended service area. Digital broadcasting differs from VHF FM broadcasting in that the audio signal (voice or music) is first converted to a stream of binary digits (data bits) that represent the audio signal. These data bits are then used to modulate the radio-frequency carrier signal using one of several techniques. After transmission via the radio waves, the radio-frequency carrier is demodulated at the receiver to recover the stream of data bits, and the bits are then converted back to the audio signal. DAB offers improved reception quality and fidelity because error-correcting codes in the digital signal can be used to eliminate many flaws that may occur during transmission. See also Information theory; Modulation.

Satellite broadcasting

Satellite broadcasting also uses digital modulation techniques. However, in this case the transmitters are located on satellites high above the Earth. From this position, satellite broadcasting can achieve essentially universal coverage of an entire nation or most of a continent from one or two transmitters. Both geostationary and low-Earth-orbit satellites are used for these systems. Specialized receiving antennas placed in locations visible to the sky are usually needed, along with a special radio receiver specifically designed for satellite service. To extend coverage into areas that are not visible to the sky, some satellite broadcasting networks employ a network of ground-based transmitters (repeaters) to supplement the coverage of the satellite signal. Satellite broadcasting is capable of providing 100 or more channels of audio programming, usually on a subscription or fee basis. See also Direct broadcasting satellite systems.