page_title: Microwave transmission
section_title: Microwave radio relay
hierarchical_section_title: Microwave transmission / Microwave radio relay
caption_attribution_description: English: Microwave horn antennas on the roof of a telephone switching center near Madison St. and 17th Ave, Capitol Hill, Seattle, Washington state, USA. This is part of the TD-2 AT&amp;T Long Lines microwave relay system, created in 1955 to relay telephone calls, television shows, and other data from point to point in the US. The system uses C-band frequencies in two bands at 4 and 6 GHz. This type of antenna is called a Hogg or horn-reflector antenna, invented by Albert Beck and Harald Friis in 1941 and developed at Bell Labs in the 1950s. They consist of a vertical flaring metal horn fed with a 2.8 in. circular waveguide, with a reflector mounted in the mouth at a 45° angle, so the beam of microwaves is emitted horizontally (the aperture of these antennas, facing right, is covered by a plastic sheet to keep out rain). The reflector is a segment of a parabolic reflector, so the antenna is equivalent to a parabolic antenna fed off-axis. The advantage of this type of antenna over an ordinary parabolic (dish) antenna is that it has very small sidelobes; that is it picks up very little radiation from directions outside its main beam axis. This allows the same microwave frequencies to be used by several nearby antennas without interfering with each other. These AT&amp;T model KS-15676 L9 antennas have a gain of about 43 dBi and produce a beamwidth (HPBW) of about 1° at 6 GHz. Backlobe sensitivity is -60 to -80 dBi. (From KS-15676 Horn Reflector Antenna Description, Bell System standards, AT&amp;T Also: Bell Labs Record Nov 1955) Microwave links like this use 2 antennas (one transmitting and one receiving) to send a beam of microwaves of a few watts power to a similar station about 30 miles away. Each antenna can carry tens of thousands of telephone calls.
context_page_description: Microwave transmission is the transmission of information by microwave radio waves. Although an experimental 40-mile microwave telecommunication link across the English Channel was demonstrated in 1931, the development of radar in World War II provided the technology for practical exploitation of microwave communication. In the 1950s, large transcontinental microwave relay networks, consisting of chains of repeater stations linked by line-of-sight beams of microwaves were built in Europe and America to relay long distance telephone traffic and television programs between cities. Communication satellites which transferred data between ground stations by microwaves took over much long distance traffic in the 1960s. In recent years, there has been an explosive increase in use of the microwave spectrum by new telecommunication technologies such as wireless networks, and direct-broadcast satellites which broadcast television and radio directly into consumers' homes.
context_section_description: Microwave radio relay is a technology widely used in the 1950s and 1960s for transmitting signals, such as long-distance telephone calls and television programs between two terrestrial points on a narrow beam of microwaves.  In microwave radio relay, microwaves are transmitted on a line of sight path between relay stations using directional antennas, forming a fixed radio connection between the two points. The requirement of a line of sight limits the separation between stations to the visual horizon, about 30 to 50 miles (48 to 80 km).  Before the widespread use of communications satellites, chains of microwave relay stations were used to transmit telecommunication signals over transcontinental distances.
  Beginning in the 1950s, networks of microwave relay links, such as the AT&T Long Lines system in the U.S., carried long distance telephone calls and television programs between cities. The first system, dubbed TD-2 and built by AT&T, connected New York and Boston in 1947 with a series of eight radio relay stations. These included long daisy-chained series of such links that traversed mountain ranges and spanned continents. Much of the transcontinental traffic is now carried by cheaper optical fibers and communication satellites, but microwave relay remains important for shorter distances.
caption: C-band horn-reflector antennas on the roof of a telephone switching center in Seattle, Washington, part of the U.S. AT&T Long Lines microwave relay network
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