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109-502: TD-2 was a microwave relay system developed by Bell Labs and used by AT&T to build a cross-country network of repeaters for telephone and television transmission. The same system was also used to build the Canadian Trans-Canada Skyway system by Bell Canada , and later, many other companies in many countries to build similar networks for both civilian and military communications. The system began with

218-477: A Los Angeles-San Francisco link on 1 September. The two coasts were linked in 1951. Equipment improvements in 1953 increased capacity to 600 calls per channel. Looking to further improve throughput, Bell Labs introduced the TH system, which operated in a higher band, around 6 GHz. It also added polarization to the signals allowing two channels per band. This allowed it to carry 1,200 calls per channel, but required

327-419: A beam of radio waves in the microwave frequency range to transmit video , audio , or data between two locations, which can be from just a few feet or meters to several miles or kilometers apart. Microwave links are commonly used by television broadcasters to transmit programmes across a country, for instance, or from an outside broadcast back to a studio. Mobile units can be camera mounted, allowing cameras

436-535: A connection between New York City and Murray Hill, the location of Bell Laboratories in 1946. The TDX system was set up between New York and Boston in 1947. The TDX was upgraded to the TD2 system, which used [the Morton tube, 416B and later 416C, manufactured by Western Electric] in the transmitters, and then later to TD3 that used solid-state electronics . Remarkable were the microwave relay links to West Berlin during

545-419: A diameter of up to 4 m (13 ft). Highly directive antennas permit an economical use of the available frequency spectrum, despite long transmission distances. Because of the high frequencies used, a line-of-sight path between the stations is required. Additionally, in order to avoid attenuation of the beam, an area around the beam called the first Fresnel zone must be free from obstacles. Obstacles in

654-413: A distance of 56 km (35 miles), was followed in 1935 by a 300 MHz telecommunication link, the first commercial microwave relay system. The development of radar during World War II provided much of the microwave technology which made practical microwave communication links possible, particularly the klystron oscillator and techniques of designing parabolic antennas. Though not commonly known,

763-427: A feed horn for microwave antennas such as satellite dishes and radio telescopes . An advantage of horn antennas is that since they have no resonant elements, they can operate over a wide range of frequencies , a wide bandwidth . The usable bandwidth of horn antennas is typically of the order of 10:1, and can be up to 20:1 (for example allowing it to operate from 1 GHz to 20 GHz). The input impedance

872-488: A few kilometers, not enough for long-distance communication. The electronic technologies needed in the millimeter wave band are also in an earlier state of development than those of the microwave band. More recently, microwaves have been used for wireless power transmission . Microwave radio relay is a technology widely used in the 1950s and 1960s for transmitting information, such as long-distance telephone calls and television programs between two terrestrial points on

981-509: A fibre. In 1976, AT&T installed its first experimental fibre system, a 2,000 feet (610 m) run under the streets of Atlanta , and many similar projects emerged around the world. In 1976, Masaru Horiguchi of NTT introduced a new optical fibre that was optically clear at 1.3 micrometers. That same year, J. Jim Hsieh of the Lincoln Laboratory introduced a solid-state laser operating at this frequency. In 1979, AT&T built

1090-420: A horn as spherical wavefronts, with their origin at the apex of the horn, a point called the phase center . The pattern of electric and magnetic fields at the aperture plane at the mouth of the horn, which determines the radiation pattern , is a scaled-up reproduction of the fields in the waveguide. Because the wavefronts are spherical, the phase increases smoothly from the edges of the aperture plane to

1199-530: A line of sight limits the separation between stations to the visual horizon, about 30 to 50 miles (48 to 80 km). For longer distances, the receiving station could function as a relay, retransmitting the received information to another station along its journey. Chains of microwave relay stations were used to transmit telecommunication signals over transcontinental distances. Microwave relay stations were often located on tall buildings and mountaintops, with their antennas on towers to get maximum range. Beginning in

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1308-587: A lower cost per bit. During the Cold War, the US intelligence agencies, such as the National Security Agency (NSA), were reportedly able to intercept Soviet microwave traffic using satellites such as Rhyolite/Aquacade . Much of the beam of a microwave link passes the receiving antenna and radiates toward the horizon, into space. By positioning a geosynchronous satellite in the path of the beam,

1417-409: A microwave relay would cost less to install for the same network, although there were some questions about the ongoing operational costs. Given concerns about the company's ability to raise capital, the microwave system was seen as a more attractive choice. Continued experiments through this period demonstrated that interference from rain was significant above 10 GHz, while operation below 1 GHz

1526-487: A narrow beam of microwaves. In microwave radio relay, a microwave transmitter and directional antenna transmits a narrow beam of microwaves carrying many channels of information on a line of sight path to another relay station where it is received by a directional antenna and receiver, forming a fixed radio connection between the two points. The link was often bidirectional, using a transmitter and receiver at each end to transmit data in both directions. The requirement of

1635-499: A network using this technology in Lake Placid, New York , to carry the television signals of the 1980 Winter Olympics . By the early 1980s, long-distance fibres were rapidly replacing all other technologies. AT&T continued using its microwave network for telephone service through this period, but Sprint's 1980s all-fibre, all-digital network forced the company to switch to digital as well, using new fibre rather than updating

1744-573: A new special project group was set up as the war was clearly winding down and a return to civilian work was approaching. This led to a microwave relay group being set up in the Research Department under the direction of Gordon Thayer. On 13 March 1944, AT&T announced they would be installing 7,000 miles (11,000 km) of coaxial cable to carry telephone and television signals, and then extended that in 1950 to 12,000 miles (19,000 km). However, engineering studies demonstrated that

1853-489: A pyramidal horn, the dimensions that give an optimum horn are: For a conical horn, the dimensions that give an optimum horn are: where An optimum horn does not yield maximum gain for a given aperture size . That is achieved with a very long horn (an aperture limited horn). The optimum horn yields maximum gain for a given horn length . Tables showing dimensions for optimum horns for various frequencies are given in microwave handbooks. Horns have very little loss, so

1962-430: A second line using TH. By the late 1960s, almost all of the population of North America was linked using TD-2 and TH. Television signals moved to satellite distribution in the 1970s and 80s, and the network was mostly used for telephone from that time. During the late 1980s and especially 1990s, the installation of fibre optic lines replaced the microwave networks. Some of the towers are in use today for other purposes, but

2071-401: A short length of rectangular or cylindrical metal tube (the waveguide), closed at one end, flaring into an open-ended conical or pyramidal shaped horn on the other end. The radio waves are usually introduced into the waveguide by a coaxial cable attached to the side, with the central conductor projecting into the waveguide to form a quarter-wave monopole antenna. The waves then radiate out

2180-417: A standard parabolic antenna is that the horn shields the antenna from radiation coming from angles outside the main beam axis, so its radiation pattern has very small sidelobes . Also, the aperture isn't partially obstructed by the feed and its supports, as with ordinary front-fed parabolic dishes, allowing it to achieve aperture efficiencies of 70% as opposed to 55–60% for front-fed dishes. The disadvantage

2289-448: A suitable antenna design was produced. TD-2 stations after 1955 used the new horn design. At the same time, this allowed the existing TD-2 stations to be upgraded to also use polarized signals, and new multiplexer designs emerged, which in combination allowed up to 600 calls per channel. This over doubled the capacity of the original links. Thus, the design effort that considered whether TH could take over existing TD-2 sites instead delayed

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2398-547: A tapered transmission line , or an optical medium with a smoothly varying refractive index. In addition, the wide aperture of the horn projects the waves in a narrow beam. The horn shape that gives minimum reflected power is an exponential taper. Exponential horns are used in special applications that require minimum signal loss, such as satellite antennas and radio telescopes . However conical and pyramidal horns are most widely used, because they have straight sides and are easier to design and fabricate. The waves travel down

2507-449: A time, these networks were also used to send television signals for cross-country broadcast, and later, computer data. Communication satellites took over the television broadcast market during the 1970s and 80s, and the introduction of long-distance fibre optic systems in the 1980s and especially 90s led to the rapid rundown of the relay networks, most of which are abandoned. In recent years, there has been an explosive increase in use of

2616-645: Is an antenna that consists of a flaring metal waveguide shaped like a horn to direct radio waves in a beam. Horns are widely used as antennas at UHF and microwave frequencies, above 300 MHz. They are used as feed antennas (called feed horns ) for larger antenna structures such as parabolic antennas , as standard calibration antennas to measure the gain of other antennas, and as directive antennas for such devices as radar guns , automatic door openers , and microwave radiometers . Their advantages are moderate directivity , broad bandwidth , low losses, and simple construction and adjustment. One of

2725-406: Is often used. The aperture efficiency increases with the length of the horn, and for aperture-limited horns is approximately unity. A type of antenna that combines a horn with a parabolic reflector is known as a Hogg-horn, or horn-reflector antenna, invented by Alfred C. Beck and Harald T. Friis in 1941 and further developed by David C. Hogg at Bell Labs in 1961. It is also referred to as

2834-408: Is regulated by International Telecommunication Union ( ITU-R ) and local regulations ( ETSI , FCC ). In the last decade the dedicated spectrum for each microwave band has become extremely crowded, motivating the use of techniques to increase transmission capacity such as frequency reuse, polarization-division multiplexing , XPIC , MIMO . The history of radio relay communication began in 1898 with

2943-408: Is slowly varying over this wide frequency range, allowing low voltage standing wave ratio (VSWR) over the bandwidth. The gain of horn antennas ranges up to 25 dBi , with 10–20 dBi being typical. A horn antenna is used to transmit radio waves from a waveguide (a metal pipe used to carry radio waves) out into space, or collect radio waves into a waveguide for reception. It typically consists of

3052-408: Is some flare angle that gives minimum reflection and maximum gain. The internal reflections in straight-sided horns come from the two locations along the wave path where the impedance changes abruptly; the mouth or aperture of the horn, and the throat where the sides begin to flare out. The amount of reflection at these two sites varies with the flare angle of the horn (the angle the sides make with

3161-454: Is that it is far larger and heavier for a given aperture area than a parabolic dish, and must be mounted on a cumbersome turntable to be fully steerable. This design was used for a few radio telescopes and communication satellite ground antennas during the 1960s. Its largest use, however, was as fixed antennas for microwave relay links in the AT&;T Long Lines microwave network. Since

3270-441: Is that signals travel somewhat slower in fibre than through the air, about 200,000 km/s instead of 299,700 km/s. Much more important is that the fibre networks generally follow existing infrastructure like railways and tunnels rather than the relatively straight point-to-point connections of the microwave system. The packets are not routed between the two stations, they are simply forwarded, further improving performance. In

3379-559: The Cold War , which had to be built and operated due to the large distance between West Germany and Berlin at the edge of the technical feasibility. In addition to the telephone network, also microwave relay links for the distribution of TV and radio broadcasts. This included connections from the studios to the broadcasting systems distributed across the country, as well as between the radio stations, for example for program exchange. Military microwave relay systems continued to be used into

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3488-575: The English Channel to provide a link back to headquarters in the UK. Bell did carry on some continued work with telephony during the war, experimenting with systems working at 3, 4.6 and 9.5 GHz over a 40 miles (64 km) line between New York and Neshanic, New Jersey . A shorter link was also tested at 0.7 and 24 GHz. In April 1944, the company announced their plans to use this technology to build an intercity telephony system. In December,

3597-426: The English Channel using 10-foot (3 m) dishes. Telephony, telegraph, and facsimile data was transmitted over the bidirectional 1.7 GHz beams 40 miles (64 km) between Dover , UK, and Calais , France. The radiated power, produced by a miniature Barkhausen–Kurz tube located at the dish's focus, was one-half watt. A 1933 military microwave link between airports at St. Inglevert, France, and Lympne, UK,

3706-580: The International Telecommunication Union was called to allocate the spectrum, which was ratified by the FCC in the summer of 1948. This set aside three bands for common carrier use, 3.7 to 4.2, 5.925 to 6.425 and 10.7 to 11.7 GHz. So while TDX was still at the stage of only being a breadboard model, the decision was made to move ahead with a production system at the newer and slightly lower frequencies. In October 1946,

3815-499: The directivity of a horn is roughly equal to its gain . The gain G of a pyramidal horn antenna (the ratio of the radiated power intensity along its beam axis to the intensity of an isotropic antenna with the same input power) is: For conical horns, the gain is: where The aperture efficiency ranges from 0.4 to 0.8 in practical horn antennas. For optimum pyramidal horns, e A = 0.511., while for optimum conical horns e A = 0.522. So an approximate figure of 0.5

3924-411: The intermediate frequency pre-amplifier were successfully addressed, raising its useful life from as little as 100 hours to 10,000. Another important improvement was a rapid switching system that allowed any channel to be switched to a stand-by channel without dropping the signal. One channel was normally left open for this purpose, with the other five being actively used. Another significant issue with

4033-429: The "sugar scoop" due to its characteristic shape. It consists of a horn antenna with a reflector mounted in the mouth of the horn at a 45 degree angle so the radiated beam is at right angles to the horn axis. The reflector is a segment of a parabolic reflector, and the focus of the reflector is at the apex of the horn, so the device is equivalent to a parabolic antenna fed off-axis. The advantage of this design over

4142-400: The 1950s a unit of the US telephone carrier, AT&T Long Lines , built a transcontinental system of microwave relay links across the US which grew to carry the majority of US long distance telephone traffic, as well as television network signals. The main motivation in 1946 to use microwave radio instead of cable was that a large capacity could be installed quickly and at less cost. It

4251-563: 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 TDX and built by AT&T, connected New York and Boston in 1947 with a series of eight radio relay stations. Through the 1950s, they deployed a network of a slightly improved version across the U.S., known as TD2 . These included long daisy-chained links that traversed mountain ranges and spanned continents. The launch of communication satellites in

4360-489: The 1960s, when many of these systems were supplanted with tropospheric scatter or communication satellite systems. When the NATO military arm was formed, much of this existing equipment was transferred to communications groups. The typical communications systems used by NATO during that time period consisted of the technologies which had been developed for use by the telephone carrier entities in host countries. One example from

4469-411: The 1970s provided a cheaper alternative. Much of the transcontinental traffic is now carried by satellites and optical fibers , but microwave relay remains important for shorter distances. Because in microwave transmission the waves travel in narrow beams confined to a line-of-sight path from one antenna to the other, they do not interfere with other microwave equipment, so nearby microwave links can use

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4578-501: The 1970s this design has been superseded by shrouded parabolic dish antennas , which can achieve equally good sidelobe performance with a lighter more compact construction. Probably the most photographed and well-known example is the 15-meter-long (50-foot) Holmdel Horn Antenna at Bell Labs in Holmdel, New Jersey, with which Arno Penzias and Robert Wilson discovered cosmic microwave background radiation in 1965, for which they won

4687-406: The 1990s. Frequency bands below 10 GHz, and above all, the information to be transmitted, were a stream containing a fixed capacity block. The target was to supply the requested availability for the whole block ( Plesiochronous digital hierarchy , PDH, or synchronous digital hierarchy , SDH). Fading and/or multipath affecting the link for short time period during the day had to be counteracted by

4796-594: The British Army used the Wireless Set Number 10 in this role during World War II. The need for radio relay did not really begin until the 1940s exploitation of microwaves , which traveled by line of sight and so were limited to a propagation distance of about 40 miles (64 km) by the visual horizon. After the war, telephone companies used this technology to build large microwave radio relay networks to carry long-distance telephone calls. During

4905-542: The GHz range would offer far more bandwidth and allow hundreds of calls on a single link. Bell went so far as to show illustrations of what such a system might look like, the illustration using long horn antennas . The opening of World War II ended these experiments. The development of the cavity magnetron and improvements in the power of klystrons along with the associated waveguides , crystal detectors , and microwave switches as part of radar development provided all of

5014-523: The New York to Chicago route was selected as the basis for a nationwide network. A planning team outlined two plans, one would be completed in June 1949 and the other in June 1950, different mostly in that the former, known as TD1, would use the existing TDX equipment while the later, TD-2, would use improved equipment with six channels instead of four and new receivers that would allow greater distances between

5123-493: The TD-2 system was that only half of the available bandwidth could be used, as microwave frequency filters of the era were not particularly narrow so the channels had to be spaced out significantly. This also limited the angles at which the antennas could be pointed; any two signals closer than 60 degrees would begin to interfere. In 1951, the development of slot filters using ferrite cores solved this issue and would allow almost double

5232-540: The USA is the RCA CW-20A 1–2 GHz microwave relay system which utilized flexible UHF cable rather than the rigid waveguide required by higher frequency systems, making it ideal for tactical applications. The typical microwave relay installation or portable van had two radio systems (plus backup) connecting two line of sight sites. These radios would often carry 24 telephone channels frequency-division multiplexed on

5341-743: The Wireless Set No. 10, which used microwave relays to multiplex eight telephone channels over long distances. A link across the English Channel allowed General Bernard Montgomery to remain in continual contact with his group headquarters in London. In the post-war era, the development of microwave technology was rapid, which led to the construction of several transcontinental microwave relay systems in North America and Europe. In addition to carrying thousands of telephone calls at

5450-470: The antennas either, the towers are perfectly sited for use with new equipment. Microwave relay Although an experimental 40-mile (64 km) 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. During the war, the British Army introduced

5559-400: 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, along

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5668-411: The axis). In narrow horns with small flare angles most of the reflection occurs at the mouth of the horn. The gain of the antenna is low because the small mouth approximates an open-ended waveguide, with a large impedance step. As the angle is increased, the reflection at the mouth decreases rapidly and the antenna's gain increases. In contrast, in wide horns with flare angles approaching 90° most of

5777-433: The bandwidth. Combined with wider bands and new encoding, TH could carry 1,200 calls per channel, and have double the number of channels. In theory, because they operated on different bands, TH systems could be added to existing TD-2 sites to increase the station's capacity. Unfortunately, the TD-2 antennas could not be used with polarized signals, and TH planned to use horn antennas which preserved polarization. That led to

5886-477: The behaviour of lower-frequency radio waves to follow the curvature of the Earth to provide over-the-horizon performance. Around the same time, the first experiments with MHz frequency radios were showing the ability to use ionospheric scatter to provide long-distance radio propagation at these higher frequencies. A new link between New York and London started in 1928, and was quickly followed by other users around

5995-501: The case of the New York-Chicago link, third-party measurements showed an average overall drop in latency of 2.5 milliseconds around 2011. This corresponded to the opening of the first new microwave link. By 2013, 15 such links were in operation between the two cities, and similar networks have been started between London and Frankfurt and other locations. Although these do not use the original equipment, and generally don't use

6104-406: The center, because of the difference in length of the center point and the edge points from the apex point. The difference in phase between the center point and the edges is called the phase error . This phase error, which increases with the flare angle, reduces the gain and increases the beamwidth, giving horns wider beamwidths than similar-sized plane-wave antennas such as parabolic dishes. At

6213-514: The channels using frequency modulation . The network used seven repeaters along the link. The system was completed in November 1947 and experimental television transmissions began on the 13th. The signals were transmitted from Boston to New York and then on to Washington, D.C., on an existing coax link. The link remained free for use until May 1948, at which point it was offered as a commercial service. The TDX link remained in place until 1958. As

6322-408: The conductive walls causes an abrupt impedance change at the aperture, from the wave impedance in the waveguide to the impedance of free space , (about 377 Ω ). When radio waves travelling through the waveguide hit the opening, this impedance-step reflects a significant fraction of the wave energy back down the guide toward the source, so that not all of the power is radiated. This is similar to

6431-469: The consideration of whether TD-2 could also move to a horn design, and whether a single horn could work at both frequencies. To do this, the waveguide would have to be circular as far as the point where the TH signal would be tapped off, and large enough to carry the 3.7 GHz TD-2 as opposed to the shorter 6 GHz TH signals. Extensive research and testing was required to answer the question, but eventually,

6540-465: The decision to delay service until the fall of 1950, allowing for multiplexer systems to be installed that would allow 480 calls per channel. At the same time, plans were made for a second line between Los Angeles and San Francisco. The equipment on the Chicago route was installed by the spring of 1950. These early systems were built in tall concrete towers that allowed the radio equipment to be mounted in

6649-865: The detrimental factors mentioned in this section, 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. The longest known microwave radio relay crosses the Red Sea with a 360 km (220 mi) hop between Jebel Erba (2,170 m (7,120 ft) a.s.l., 20°44′46.17″N 36°50′24.65″E  /  20.7461583°N 36.8401806°E  / 20.7461583; 36.8401806 , Sudan) and Jebel Dakka (2,572 m (8,438 ft) a.s.l., 21°5′36.89″N 40°17′29.80″E  /  21.0935806°N 40.2916111°E  / 21.0935806; 40.2916111 , Saudi Arabia). The link

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6758-419: The distribution of television signals as these generally started at a single transmitter site, the network's main studios, and were broadcast to many receivers, at the local television stations. This could be easily accomplished by a single satellite and relatively inexpensive receivers at the local stations. As television moved off the microwave systems, the freed channels were turned over to use for telephone, or

6867-542: The diversity architecture. During 1990s microwave radio links begun widely to be used for urban links in cellular network . Requirements regarding link distance changed to shorter hops (less than 10 km (6.2 mi), typically 3 to 5 km (1.9 to 3.1 mi)), and frequency increased to bands between 11 and 43 GHz and more recently, up to 86 GHz (E-band). Furthermore, link planning deals more with intense rainfall and less with multipath, so diversity schemes became less used. Another big change that occurred during

6976-524: The early 1970s emerging market for dedicated data lines. The replacement of its use for telephone was also taking place during the 1970s. At Corning Glass , a team led by Robert Maurer developed a new method of making optical fibre that had much higher quality and lower loss than previous designs. At almost the same time, Bell Labs developed the first room-temperature semiconductor laser . This could be switched on and off at very high speed, allowing it to create pulse-code modulation (PCM) signals within

7085-432: The end of 1947 and all the other pieces by early 1948. Western Electric would gear up production lines so deliveries could start in late 1948 and be completed in six months. Meanwhile, AT&T Long Lines would survey and purchase the repeater sites and build the associated buildings and towers. Management was initially concerned with television signals, but as time went on, telephone signals grew in importance. This led to

7194-531: The equipment needed to move radiotelephony into the microwave region. In the UK, these technologies were used to produce the world's first microwave relay telephone system: Wireless Set No. 10 (WS.10), which multiplexed eight telephone calls into a single microwave link that could be used to the limit of the line of sight. This was used during the Second World War 's Normandy landings : in the field to communicate with forward units, and on either side of

7303-469: The experimental TDX , completed in November 1947, carrying television and telephone between Boston and New York City. TD-2 was a minor improvement on TDX, moving to the 3.7 to 4.2 GHz band set aside in 1947 for common carrier use. The system had six channels, and using frequency-division multiplexing , each could carry up to 480 telephone calls or a television signal. The first TD-2 link between New York and Chicago opened on 1 September 1950, followed by

7412-442: The first horn antennas was constructed in 1897 by Bengali-Indian radio researcher Jagadish Chandra Bose in his pioneering experiments with microwaves. The modern horn antenna was invented independently in 1938 by Wilmer Barrow and G. C. Southworth The development of radar in World War II stimulated horn research to design feed horns for radar antennas. The corrugated horn invented by Kay in 1962 has become widely used as

7521-509: The flare angle, the radiation of the beam lobe is down about 20 dB from its maximum value. As the size of a horn (expressed in wavelengths) is increased, the phase error increases, giving the horn a wider radiation pattern. Keeping the beamwidth narrow requires a longer horn (smaller flare angle) to keep the phase error constant. The increasing phase error limits the aperture size of practical horns to about 15 wavelengths; larger apertures would require impractically long horns. This limits

7630-414: The freedom to move around without trailing cables. These are often seen on the touchlines of sports fields on Steadicam systems. Terrestrial microwave relay links are limited in distance to the visual horizon, a few tens of miles or kilometers depending on tower height. Tropospheric scatter ("troposcatter" or "scatter") was a technology developed in the 1950s to allow microwave communication links beyond

7739-460: The gain of practical horns to about 1000 (30 dBi) and the corresponding minimum beamwidth to about 5–10°. Below are the main types of horn antennas. Horns can have different flare angles as well as different expansion curves (elliptic, hyperbolic, etc.) in the E-field and H-field directions, making possible a wide variety of different beam profiles. For a given frequency and horn length, there

7848-453: The horizon, to a range of several hundred kilometers. The transmitter radiates a beam of microwaves into the sky, at a shallow angle above the horizon toward the receiver. As the beam passes through the troposphere a small fraction of the microwave energy is scattered back toward the ground by water vapor and dust in the air. A sensitive receiver beyond the horizon picks up this reflected signal. Signal clarity obtained by this method depends on

7957-415: The horn end in a narrow beam. In some equipment the radio waves are conducted between the transmitter or receiver and the antenna by a waveguide; in this case the horn is attached to the end of the waveguide. In outdoor horns, such as the feed horns of satellite dishes, the open mouth of the horn is often covered by a plastic sheet transparent to radio waves, to exclude moisture. A horn antenna serves

8066-420: The last decade was an evolution toward packet radio transmission. Therefore, new countermeasures, such as adaptive modulation , have been adopted. The emitted power is regulated for cellular and microwave systems. These microwave transmissions use emitted power typically from 0.03 to 0.30 W, radiated by a parabolic antenna on a narrow beam diverging by a few degrees (1 to 3-4). The microwave channel arrangement

8175-408: The long-distance traffic in the U.S. was being carried by TD-2. It also carried 95% of the country's inter-city television signals. Two events in 1970 led to the ending of AT&T's microwave expansion and its eventual demise. The first geostationary communications satellites were launched in the 1960s, but widespread commercial service did not start until the 1970s. Satellites quickly took over

8284-479: The majority of the sites are abandoned. Radio telephone systems had been experimented with as early as 1915, the year after AT&T bought Lee de Forest 's patents on the audion vacuum tube . Experiments were carried out between Arlington, Virginia , Hawaii and Paris. After being interrupted by World War I , such experiments began again and led to the creation of a permanent link between New York City and London in 1927. This system operated at 60 kHz, using

8393-466: The microwave band has a bandwidth 30 times that of all the rest of the radio spectrum below it. A disadvantage is that microwaves are limited to line of sight propagation; they cannot pass around hills or mountains as lower frequency radio waves can. Microwave radio transmission is commonly used in point-to-point communication systems on the surface of the Earth, in satellite communications , and in deep space radio communications . Other parts of

8502-409: The microwave beam can be received. At the turn of the century, microwave radio relay systems were 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. A microwave link is a communications system that uses

8611-438: The microwave carrier (i.e. Lenkurt 33C FDM). Any channel could be designated to carry up to 18 teletype communications instead. Similar systems from Germany and other member nations were also in use. Long-distance microwave relay networks were built in many countries until the 1980s, when the technology lost its share of fixed operation to newer technologies such as fiber-optic cable and communication satellites , which offer

8720-458: The microwave radio band are used for radars , radio navigation systems, sensor systems, and radio astronomy . The next higher frequency band of the radio spectrum , between 30 GHz and 300 GHz, are called " millimeter waves " because their wavelengths range from 10 mm to 1 mm. Radio waves in the millimeter wave band are strongly attenuated by the gases of the atmosphere , which limits their practical transmission distance to

8829-570: The microwave spectrum by new telecommunication technologies such as wireless networks , and direct-broadcast satellites which broadcast television and radio directly into consumers' homes. Larger line-of-sight links are once again popular for handing connections between mobile telephone towers, although these are generally not organized into long relay chains. Microwaves are widely used for point-to-point communications because their small wavelength allows conveniently-sized antennas to direct them in narrow beams, which can be pointed directly at

8938-433: The microwave system. By the late 1990s, most of the microwave network had been turned off. In 1999, AT&T sold off the towers to any buyers. Most towers went unpurchased and now stand derelict. A small number of former TD-2 towers have been brought back to use under third-party ownership. The original New York to Chicago link is one of these. There are two reasons for their re-use, both related to end-to-end time. The first

9047-406: The nation. Over the next years, AT&T and Bell Labs continually worked on the system to improve it. Among the most important improvements were those on the lifetime of the tubes. The primary concern was the main transmitter, the 416A, which was raised from about 2000 hours when it entered service to about 6 to 8000 hours by 1952, and 20,000 hours by 1967. Likewise, problems with the 417A used in

9156-449: The number of channels and allow the antennas to be pointed to within 9 degrees, meaning a single tower could service two closely spaced endpoints. In 1955, Bell Labs had begun work on a new relay system known as TH, which operated in the 6 GHz band. A significant feature of TH was that it used polarization to separate the signals, allowing the channels to operate very close to each other in frequency and thereby make much better use of

9265-496: The outcome of the FCC's efforts, Bell decided to install an experimental system as a prototype of what they believed would be the commercial system. This was built as the TDX line between New York and Boston. The FCC granted them an allocation between 3.9 and 4.4 GHz in May 1945. The system had four channels of 10 MHz each spaced over the allocation, and the signals were encoded into

9374-416: The path also must be taken into consideration since it can reflect the beam, and the direct and reflected beam can interfere with each other at the receiving antenna, causing multipath fading. Multipath fades are usually deep only in a small spot and a narrow frequency band, so space and/or frequency diversity schemes can be applied to mitigate these effects. The effects of atmospheric stratification cause

9483-653: The publication by Johann Mattausch in the Austrian journal, Zeitschrift für Elektrotechnik. But his proposal was primitive and not suitable for practical use. The first experiments with radio repeater stations to relay radio signals were done in 1899 by Emile Guarini-Foresio. However the low frequency and medium frequency radio waves used during the first 40 years of radio proved to be able to travel long distances by ground wave and skywave propagation. In 1931, an Anglo-French consortium headed by Andre C. Clavier demonstrated an experimental microwave relay link across

9592-556: The radio path to bend downward in a typical situation so a major distance is possible as the earth equivalent curvature increases from 6,370 km (3,960 mi) to about 8,500 km (5,300 mi) (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 making rain fade must also be considered as an impairment factor, especially at frequencies above 10 GHz. All of

9701-408: The receiving antenna. This use of tightly-focused direct beams allows microwave transmitters in the same area to use the same frequencies, without interfering with each other as lower frequency radio waves would. This frequency reuse conserves scarce radio spectrum bandwidth. Another advantage is that the high frequency of microwaves gives the microwave band a very large information-carrying capacity;

9810-460: The reflection at an open-ended transmission line or a boundary between optical mediums with a low and high index of refraction , like at a glass surface. The reflected waves cause standing waves in the waveguide, increasing the SWR , wasting energy and possibly overheating the transmitter. In addition, the small aperture of the waveguide (less than one wavelength) causes significant diffraction of

9919-454: The reflection is at the throat. The horn's gain is again low because the throat approximates an open-ended waveguide. As the angle is decreased, the amount of reflection at this site drops, and the horn's gain again increases. This discussion shows that there is some flare angle between 0° and 90° which gives maximum gain and minimum reflection. This is called the optimum horn . Most practical horn antennas are designed as optimum horns. In

10028-407: The same frequencies. The antennas must therefore be highly directional (high gain ), and are installed in elevated locations such as large radio towers in order to be able to avoid the obstructions closer to the ground and transmit across long distances. Typical types of antenna used in radio relay link installations are parabolic antennas , dielectric lens, and horn-reflector antennas , which have

10137-413: The same function for electromagnetic waves that an acoustical horn does for sound waves in a musical instrument such as a trumpet . It provides a gradual transition structure to match the impedance of a tube to the impedance of free space, enabling the waves from the tube to radiate efficiently into space. If a simple open-ended waveguide is used as an antenna, without the horn, the sudden end of

10246-403: The signal field cause unwanted attenuation . High mountain peaks or ridges are often ideal positions for the antennas. In addition to the use of conventional repeaters with back-to-back radios transmitting on different frequencies, obstructions in microwave paths can also be dealt with by using Passive repeater or on-frequency repeaters. Obstacles, the curvature of the Earth, the geography of

10355-460: The stations. AT&T filed an application with the FCC in January 1947 to build the link. Management demanded that they use the more advanced TD-2 system but meet the original 1949 date, as television stations were clamouring for new links. Engineering accepted the goal and said it could be met if everything went right. Their initial plan was to develop the radio, antenna and power plant designs by

10464-405: The television spectrum was being bought up, AT&T faced increasing pressure to give up its existing VHF allocations for new television channels. This would only be possible if the FCC opened new frequencies for them to use for telephony. As early as 1946 the FCC was already concerned about potential crowding in the GHz range and began to consider its formal allocation as well. In 1947, a meeting of

10573-411: The tower to keep it as close to the antennas as possible and thus avoid losses in the transmission lines. Tests began in June, initially with little success and problems with noise continued to plague the system into July. Things were finally improving by August, at which time an experiment sent a signal from New York to Chicago, back to New York and then again to Chicago. The total length of transmission

10682-478: The use of Schottky barrier diodes and tunnel diodes , allowing the number of telephone channels to be increased once again to 1,200. To reach these levels, there needed to be improvements to the physical plant and antennas as well. Taking advantage of just these changes resulted in the TD-2A, which could carry 900 telephone channels, which could be rapidly deployed while waiting for TD3 to arrive. By 1968, 40% of all

10791-496: The use of horn antennas to retain polarization. After considerable research, Bell developed an antenna that worked for both TD-2 and TH, but these improvements also helped TD-2 and increased its capacity again to 900 calls, delaying a widespread rollout of TH which was added only to the busiest links. Bell Canada began building a similar TD-2 system, the Skyway, which went into service 1958. The Canadian railway companies then built

10900-464: The waves issuing from it, resulting in a wide radiation pattern without much directivity. To improve these poor characteristics, the ends of the waveguide are flared out to form a horn. The taper of the horn changes the impedance gradually along the horn's length. This acts like an impedance matching transformer , allowing most of the wave energy to radiate out the end of the horn into space, with minimal reflection. The taper functions similarly to

11009-1009: 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. Troposcatter links are therefore only used in special circumstances where satellites and other long-distance communication channels cannot be relied on, such as in military communications. ELF 3 Hz/100 Mm 30 Hz/10 Mm SLF 30 Hz/10 Mm 300 Hz/1 Mm ULF 300 Hz/1 Mm 3 kHz/100 km VLF 3 kHz/100 km 30 kHz/10 km LF 30 kHz/10 km 300 kHz/1 km MF 300 kHz/1 km 3 MHz/100 m HF 3 MHz/100 m 30 MHz/10 m VHF 30 MHz/10 m 300 MHz/1 m UHF 300 MHz/1 m 3 GHz/100 mm SHF 3 GHz/100 mm 30 GHz/10 mm EHF 30 GHz/10 mm 300 GHz/1 mm THF 300 GHz/1 mm 3 THz/0.1 mm Horn antenna A horn antenna or microwave horn

11118-458: The widespread use of TH as the capacity of the existing TD-2 systems improved. TH rollout did not begin until 1961, and by the mid-1960s, the majority of the network still used TD-2. In April 1962, it was decided to re-engineer the TD-2 system as TD3. This was a solid state system with the only remaining tube being the microwave transmitter, which moved from a klystron to a lower noise travelling-wave tube . The receiver had far less noise, through

11227-423: The world. The main problem with this system is that the scattering meant the ultimate range of the signals could not be predicted, which made it difficult to ensure that any two stations could use the same frequencies and be safe from interference. Research continued on moving to ever-higher frequencies in an effort to avoid interference as well as expand bandwidth . A single-line link between Boston and Cape Cod

11336-445: Was built in 1979 by Telettra to transmit 300 telephone channels and one TV signal, in the 2 GHz frequency band. (Hop distance is the distance between two microwave stations). Previous considerations represent typical problems characterizing terrestrial radio links using microwaves for the so-called backbone networks: hop lengths of a few tens of kilometers (typically 10 to 60 km (6.2 to 37.3 mi)) were largely used until

11445-410: Was difficult as the required antenna sizes were too large to be practical. One problem for the project was that AT&T was not the only one with big post-war plans for radio spectrum; during the war television production was cancelled and those companies were expecting a huge post-war buying spree. During early testing, UHF signals would sometimes be detected at very long ranges that theory suggested

11554-413: Was expected at that time that the annual operating costs for microwave radio would be greater than for cable. There were two main reasons that a large capacity had to be introduced suddenly: Pent-up demand for long-distance telephone service, because of the hiatus during the war years, and the new medium of television, which needed more bandwidth than radio. The prototype was called TDX and was tested with

11663-525: Was impossible. This led to the discovery of tropospheric scatter , which would become another important long-range telephony system in the future. It also led to the "television freeze" of 1948, as the FCC attempted to understand the problem and come up with solutions. As this would almost invariably mean a reallocation of frequencies, AT&T was also frozen in their relay efforts while they waited to learn which frequencies they might get to use. While they waited

11772-407: Was set up in 1934 at 60 MHz, moving to what was then relatively unused spectrum. A more advanced system was set up across the entrance of Chesapeake Bay in 1941, operating at 150 MHz. This system had enough bandwidth to allow 12 telephone calls to be sent on the single connection using the same multiplexing system used on long-distance calling wires. It was already clear that moving to

11881-534: Was the same as New York to San Francisco, and the degradation of the signal was "barely perceptible" even on an oscilloscope. The New York-Chicago line was opened for service on 1 September 1950, and the Los Angeles-San Francisco link on the 15th. The two sections were linked in time for it to broadcast Harry S. Truman 's opening address at the Treaty of San Francisco on 4 September 1951 across

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