Radio spectrum allocations

(communications) The specification of the frequencies of the radio spectrum which are available for use by the various radio services.

The specification of the frequency bands of the radio spectrum which are made available for use by the various radio services. The radio spectrum is the part of the natural spectrum of electromagnetic radiation lying between the frequency limits of 10⁴ and 3 × 10¹¹ hertz. Since the characteristics of the radiation do not change abruptly, these limits are not sharply defined and may change, depending upon the demands for changes in service and upon changes in technology.

For purposes of identification the radio spectrum is divided into bands differing from adjacent bands by frequency ratios of 10. These bands are identified by the metric wavelength of the shortest waves in each band (kilometric band, and so forth) or by adjective or numerical designators (for example, very-low-frequency band or band 4). The wavelength &lgr: in meters is related to the frequency f in hertz by the relationship c = f&lgr:, where c, the velocity of propagation of radio waves in space, is about 3 × 10⁸ m/s.

The major classes of radio services are provided with international allocations within the various frequency bands. Within the United States there is a national allocation which conforms in general to the international allocation. The Radio Regulations divide the Earth into three regions and contain a detailed table of allocations, with some differences between regions, together with exceptions to the listed allocations and conditions for frequency use. Regional and national allocations can be made within the worldwide framework, but any departures by signatories of the Regulations must be accomplished on the basis of noninterference with services operating in accordance with the agreed allocations.

The characteristics of radio waves, due to natural phenomena such as atmospherics, vary greatly with frequency. Attempts are made to take advantage of propagation characteristics when making allocations, and normally the frequencies allocated have characteristics satisfying the operational requirements of the service with regard to distance, bandwidth, and other technique factors. Allocations are sometimes made for a new service which has already begun to use a particular band. Since many services have varied and continually growing requirements, allocations to them will be found in several frequency bands. In making specific frequency assignments to individual radio stations, the separation between assignable frequencies must be such as to avoid harmful interference in view of the bandwidth requirements, the frequency stability of the transmitters, the frequency selectivity of the receivers, the transmitter power and its distance separation from receivers operating on adjacent channels, and other factors. Thus the separation between adjacent assigned frequencies varies widely between services and between different locations in the frequency spectrum.

In most of the frequency bands it has been found practical to make block allocations of frequencies to given services in each region. Individual frequency assignments can then be made to radio stations operating in that service without the need for coordination with frequency assignments to stations in other services. Interference problems are more easily resolved, as they occur between compatible users. Some of these allocations are exclusive to a particular service, and some are shared between specified services to varying degrees. Similarly, in some services the stations have individual frequency assignments which are protected from interference from other stations, whereas the stations in other services may share time or geographic coverage on frequencies common to several stations. These arrangements are based upon the nature and the needs of the particular services being provided.