- Microwave radio relay
Microwave radio relay is a technology for transmitting digital and analog signals, such as long-distance
telephone calls and the relay of televisionprograms to transmitters, between two locations on a line of sightradio path. In microwaveradio relay, radio wavesare 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.
How microwave radio relay links are formed
Because a line of sight radio link is made, the radio frequencies used occupy only 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. Highly directive antennas permit an economical use of the available frequency spectrum, despite long transmission distances.
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 zonemust be free from obstacles so the radio waves can propagate across a nearly uninterrupted path. Obstacles in the signal field cause unwanted attenuation, and are as a result only acceptable in exceptional cases. Obstacles, the curvature of the Earth, the geographyof the area and reception issues arising from the use of nearby land (such as in manufacturingand 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 terrainand 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 multipathdistortion as the two received signals ("wanted" and "unwanted") swing in and out of phase. Multipath fades are usually deep only in a small spot and a narrow frequency band, so space and frequency diversity schemes were usually applied in the third quarter of the 20th century.
The effects of atmospheric stratification cause the radio path to bend downward in a typical situation so a major distance is possible as the earth equivalent curvature increases 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, collectively known as
path loss, make it necessary to compute suitable power margins, in order to maintain the link operative for a 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. Signal clarity obtained by this method depends 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 where standard radio relay links are unsuitable (for example, in providing a microwave link to an island).
Usage of microwave radio relay systems
During the 1950s the
AT&T Communicationssystem of TD radio grew to carry the majority of US Long Distancetelephone traffic, as well as intercontinental television networksignals. Similar systems were soon built in many countries, until the 1980s when the technology lost its share of fixed operation to newer technologies such as fiber-optic cableand optical radio relay links (of which offer larger data capacities at lower cost per bit). Communication satellites, which are also microwave radio relays, better retained their market share, especially for television.
At the turn of the century, 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 hardwareaccess to the portable radio operator.
British Telecom microwave network
* [http://www.att.com/attlabs/reputation/timeline/51microwave.html AT&T's Microwave Radio-Relay Skyway introduced in 1951]
* [http://www.long-lines.net/documents/radio_relay_ad_51.jpgBell System 1951 magazine ad for Microwave Radio-Relay systems.]
* [http://coldwar-c4i.net/WU/WU-RCA-ad.html RCA vintage magazine ad for Microwave-Radio Relay equipment used for Western Union Telegraph Co.]
* [http://digital-microwave-radio.at-communication.com/en Digital Microwave Radio]
* [http://www.drgibson.com/towers/ AT&T Long Lines Microwave Towers Remembered]
* [http://www.porticus.org/bell/longlines.html AT&T Long Lines]
Microwave Radio Transmission Design Guide, Trevor Manning, Artech House, 1999
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