Microwave radio relay is a technology for transmitting digital and analog signals, such as long-distance telephone calls and the relay of television programs to transmitters, between two locations on a line of sight radio path. In microwave radio relay, radio waves are transmitted between the two locations with directional antennas, forming a fixed radio connection between the two points. Long daisy-chained series of such links form transcontinental telephone and/or television communication systems. The term digital signal is used to refer to more than one concept. ... It has been suggested that this article or section be merged into Analog electronics. ... In telecommunication, signalling (or signaling) has the following meanings: The use of signals for controlling communications. ... A telephone call is a connection over a telephone network between the calling party and the called party. ... When viewing a scene, as in optics, photography, or even hunting, the line of sight is the straight line between the observer and the target. ... Radio frequency, or RF, refers to that portion of the electromagnetic spectrum in which electromagnetic waves can be generated by alternating current fed to an antenna. ... Log-periodic dipole array A directional antenna is an antenna which transmits or receives maximum power in a particular direction. ...

How microwave radio relay links are formed

Because a line of sight radio link is made, the radio frequencies used are occupied only in a narrow path between stations (with the exception of a certain radius of each station). Antennas used must have a high directive effect; these antennas are installed in elevated locations such as large radio towers in order to be able to transmit across long distances. Typical types of antenna used in radio relay link installations are parabolic reflectors, shell antennas and horn radiators, which have a diameter of up to 4 meters. The high directive effect of the antennas permits an economical use of the available frequency spectrum, despite long transmission distances. Image File history File links Download high-resolution version (392x1280, 339 KB) Please credit the photographer, David Jordan, when using this image. ... A parabolic reflector (also known as a parabolic dish or a parabolic mirror) is a reflective device formed in the shape of a paraboloid of revolution. ...

Planning considerations

Because of the high frequencies used, a quasi-optical line of sight between the stations is generally required. Additionally, in order to form the line of sight connection between the two stations, the first Fresnel zone must be free from obstacles so that propagation of the radio waves can occur across a nearly uninterrupted path. Obstacles in the signal field cause unwanted signal absorption, and are as a result only acceptable in exceptional cases. In optics and radio communications, a Fresnel zone (pronounced as FRA-nel Zone), named for physicist Augustin-Jean Fresnel, is one of a (theoretically infinite) number of a concentric ellipsoids of revolution which define volumes in the radiation pattern of a (usually) circular aperture. ... Radio propagation is a term used to explain how radio waves behave when they are transmitted, or are propagated from one point on the Earth to another. ...

Obstacles, the curvature of the Earth, the geography of the area and reception issues arising from the use of nearby land (such as in manufacturing and forestry) are important issues to consider when planning radio links. In the planning process, it is essential that "path profiles" are produced, which provide information about the terrain and Fresnel zones affecting the transmission path. The presence of a water surface, such as a lake or river, in the mid-path region also must be taken into consideration as it can result in a near-perfect reflection (even modulated by wave or tide motions), creating "multi-path" distortion as the two received signals ("wanted" and "unwanted") swing in and out of phase. Manufacturing, a branch of industry which accounts for about one-quarter of the worlds economic activity, is the application of tools and a processing medium to the transformation of raw materials into finished goods for sale. ... A decidous beech forest in Slovenia. ...

The effects of atmosphere stratification cause the radio path to bend downward in a typical situation so a major distance is possible as the earth equivalent curvature increase from 6370 km to about 8500 km (a 4/3 equivalent radius effect). Rare events of temperature, humidity and pressure profile versus height, may produce large deviations and distortion of the propagation and affect transmission quality. High intensity rain and snow must also be considered as an impairment factor, especially at frequencies above 10 GHz. All previous factors make it necessary to compute suitable power margins, in order to maintain the link operative for high percentage of time, like the standard 99.99% or 99.999% used in 'carrier class' services of most telecommunication operators.

Over-horizon microwave radio relay

In over-horizon, or tropospheric scatter, microwave radio relay, unlike a standard microwave radio relay link, the sending and receiving antennas do not use a line of sight transmission path. Instead, the stray signal transmission, known as "tropo-scatter" or simply "scatter," from the sent signal is picked up by the receiving station. The signal clarity obtained by this method is dependent on the weather and other factors, and as a result a high level of technical difficulty is involved in the creation of a reliable over horizon radio relay link. Over horizon radio relay links are therefore only used in cases where standard radio relay links are unsuitable (for example, in providing a microwave link to an island). Tropospheric scatter (or troposcatter) is the scattering of distant TV and FM radio stations by the troposphere so that they travel farther than the line of sight. ...

Usage of microwave radio relay systems

While the technology initially lost its forefront position to newer technologies such as fiber-optic cable and optical radio relay link (both of which offer larger data capacities at lower cost per bit), microwave radio relay systems are being used increasingly in portable radio applications. The technology is particularly suited to this application because of lower operating costs, a more efficient infrastructure, and provision of direct hardware access to the portable radio operator. Fiber Optic strands An optical fiber in American English or fibre in British English is a transparent thin fiber for transmitting light. ... Hardware is the general term that is used to describe physical artifacts of a technology. ...

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External links

• Photograph of typical self-supporting microwave radio relay tower in Western Ohio.

• AT&T's Microwave Radio-Relay Skyway introduced in 1951

• Bell System 1951 magazine ad for Microwave Radio-Relay systems.

• RCA vintage magazine ad for Microwave-Radio Relay equipment used for Western Union Telegraph Co.