Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft, spacecraft, guided missiles, motor vehicles, weather formations, terrain. A radar system consists of a transmitter producing electromagnetic waves in the radio or microwaves domain, a transmitting antenna, a receiving antenna and a receiver and processor to determine properties of the object. Radio waves from the transmitter reflect off the object and return to the receiver, giving information about the object's location and speed. Radar was developed secretly for military use by several nations in the period before and during World War II. A key development was the cavity magnetron in the UK, which allowed the creation of small systems with sub-meter resolution; the term RADAR was coined in 1940 by the United States Navy as an acronym for RAdio Detection And Ranging The term radar has since entered English and other languages as a common noun, losing all capitalization.
The modern uses of radar are diverse, including air and terrestrial traffic control, radar astronomy, air-defense systems, antimissile systems, marine radars to locate landmarks and other ships, aircraft anticollision systems, ocean surveillance systems, outer space surveillance and rendezvous systems, meteorological precipitation monitoring and flight control systems, guided missile target locating systems, ground-penetrating radar for geological observations, range-controlled radar for public health surveillance. High tech radar systems are associated with digital signal processing, machine learning and are capable of extracting useful information from high noise levels. Radar is a key technology that the self-driving systems are designed to use, along with sonar and other sensors. Other systems similar to radar make use of other parts of the electromagnetic spectrum. One example is "lidar". With the emergence of driverless vehicles, Radar is expected to assist the automated platform to monitor its environment, thus preventing unwanted incidents.
As early as 1886, German physicist Heinrich Hertz showed that radio waves could be reflected from solid objects. In 1895, Alexander Popov, a physics instructor at the Imperial Russian Navy school in Kronstadt, developed an apparatus using a coherer tube for detecting distant lightning strikes; the next year, he added a spark-gap transmitter. In 1897, while testing this equipment for communicating between two ships in the Baltic Sea, he took note of an interference beat caused by the passage of a third vessel. In his report, Popov wrote that this phenomenon might be used for detecting objects, but he did nothing more with this observation; the German inventor Christian Hülsmeyer was the first to use radio waves to detect "the presence of distant metallic objects". In 1904, he demonstrated the feasibility of detecting a ship in dense fog, but not its distance from the transmitter, he obtained a patent for his detection device in April 1904 and a patent for a related amendment for estimating the distance to the ship.
He got a British patent on September 23, 1904 for a full radar system, that he called a telemobiloscope. It operated on a 50 cm wavelength and the pulsed radar signal was created via a spark-gap, his system used the classic antenna setup of horn antenna with parabolic reflector and was presented to German military officials in practical tests in Cologne and Rotterdam harbour but was rejected. In 1915, Robert Watson-Watt used radio technology to provide advance warning to airmen and during the 1920s went on to lead the U. K. research establishment to make many advances using radio techniques, including the probing of the ionosphere and the detection of lightning at long distances. Through his lightning experiments, Watson-Watt became an expert on the use of radio direction finding before turning his inquiry to shortwave transmission. Requiring a suitable receiver for such studies, he told the "new boy" Arnold Frederic Wilkins to conduct an extensive review of available shortwave units. Wilkins would select a General Post Office model after noting its manual's description of a "fading" effect when aircraft flew overhead.
Across the Atlantic in 1922, after placing a transmitter and receiver on opposite sides of the Potomac River, U. S. Navy researchers A. Hoyt Taylor and Leo C. Young discovered that ships passing through the beam path caused the received signal to fade in and out. Taylor submitted a report, suggesting that this phenomenon might be used to detect the presence of ships in low visibility, but the Navy did not continue the work. Eight years Lawrence A. Hyland at the Naval Research Laboratory observed similar fading effects from passing aircraft. Before the Second World War, researchers in the United Kingdom, Germany, Japan, the Netherlands, the Soviet Union, the United States, independently and in great secrecy, developed technologies that led to the modern version of radar. Australia, New Zealand, South Africa followed prewar Great Britain's radar development, Hungary generated its radar technology during the war. In France in 1934, following systematic studies on the split-anode magnetron, the research branch of the Compagnie Générale de Télégraphie Sans Fil headed by Maurice Ponte with Henri Gutton, Sylvain Berline and M. Hugon, began developing an obstacle-locatin
Pakistan Air Force
The Pakistan Air Force is the aerial warfare uniform service branch of the Pakistan Armed Forces, tasked with the aerial defence of Pakistan, with a secondary role of providing air support to the Pakistan Army and the Pakistan Navy. The PAF has a tertiary role of providing strategic air transport and logistics capability to Pakistan; as of 2017, per IISS, the PAF has 70,000 personnel. It operates 146+ aircraft, its primary mandate and mission is "to provide, in synergy with other inter-services, the most efficient and cost effective aerial Defence of Pakistan." Since its establishment in 1947, the PAF has been involved in various combat operations, providing aerial support to Inter–Services's operations and relief efforts. Under the Article 243, the Constitution of Pakistan appoints the President of Pakistan as the civilian Commander-in-Chief; the Chief of Air Staff, by statute a four-star air officer Air Chief Marshal, is appointed by the President with the consultation and confirmation needed from the Prime Minister of Pakistan.
The Pakistan Air Force is commanded by Air Chief Marshal Mujahid Anwar Khan. On 10 April 1959, on the occasion of the Islamic Eid ul-Fitr festival holiday in Pakistan, an Indian Air Force English Electric Canberra B58 of No. 106 Squadron entered Pakistani airspace on a photo reconnaissance mission. Two PAF F-86F Sabres of No. 15 Squadron on Air Defence Alert were scrambled from Sargodha Air Base to intercept the IAF aircraft. Butt attempted to bring down the Canberra by firing his Sabre's machine guns, but the Canberra was flying at an altitude of more than 50,000 feet – beyond the operational ceiling of the F-86F; when Yunis took over from his leader, the Canberra lost height while executing a turn over Rawalpindi. Yunis fired a burst that struck the Canberra at an altitude of 47,500 feet and brought it down over Rawat, near Rawalpindi, marking the first aerial victory of the PAF. Both crew members of the IAF Canberra and were captured by Pakistani authorities and were subsequently released after remaining in detention for some time.
The PAF fleet at the time consisted of 12 F-104 Starfighters, some 120 F-86 Sabres and around 20 B-57 Canberra bombers. The PAF claims to have had complete air superiority over the battle area from the second day of operations. While, Air Chief Marshal Arjan Singh of the Indian Air Force claimed, despite been qualitative inferior, IAF achieved air superiority in three days in the 1965 War. Many publications have credited the PAF's successes to U. S. equipment, claiming it to be superior to the aircraft operated by the IAF and giving the PAF a "qualitative advantage". However some Pakistanis refute this argument; as per them, the IAF's MiG-21, Hawker Hunter and Folland Gnat aircraft had better performance than the PAF's F-86 fighters. According to Air Cdre Sajad Haider, the F-86 Sabre was inferior in both power and speed to the IAF's Hawker Hunter. According to Air Commodore Sajjad Haider who flew with No. 19 squadron, the F-104 Starfighter did not deserve its reputation as "the pride of the PAF" because it "was unsuited to the tactical environment of the region.
It was a high-level interceptor designed to neutralize Soviet strategic bombers in altitudes above 40,000 feet." The IAF is believed to have feared the Starfighter although, it was not as effective as the IAF's Folland Gnat. According to Indian sources, the F-86F performed reasonably well against the IAF Hawker Hunters but not as well against the Folland Gnat, nicknamed Sabre Slayer by the IAF. According to Indian sources most aircraft losses of IAF were on ground while PAF lost most in aerial combat. Though the IAF flew a larger offensive air campaign by devoting 40% of its air effort to offensive air support alone, according to Indian sources the majority of its losses came from aircraft destroyed on the ground through PAF air strikes; the PAF had achieved far more in terms of enemy aircraft destroyed on the ground but without doubt, the IAF had achieved much more in the close support role. The two countries have made contradictory claims of combat losses during the war and few neutral sources have verified the claims of either country.
The PAF claimed it shot down 104 IAF planes and lost 19 of its own, while the IAF claimed it shot down 73 PAF planes and lost 59. According to the independent sources, the PAF lost some 20 aircraft while the Indians lost 60–75. Despite the intense fighting, the conflict was a stalemate. By late 1971, the intensification of the independence movement in erstwhile East Pakistan lead to the Bangladesh Liberation War between India and Pakistan. On 22 November 1971, 10 days before the start of a full-scale war, four PAF F-86 Sabre jets attacked Indian and Mukti Bahini positions at Garibpur, near the international border. Two of the four PAF Sabres were shot down and one damaged by the IAF's Folland Gnats. On 3 December, India formally declared war against Pakistan following massive preemptive strikes by the PAF against Indian Air Force installations in Srinagar, Sirsa and Jodhpur. However, the IAF did not suffer because the leadership had anticipated such a move and precautions were taken; the Indian Air Force was quick to respond to Pakistani air strikes, following which the PAF carried out defensive sorties.
Hostilities ended at 14:30 GMT on 17 December, after the fall of Dacca on 15 December. The PAF destroyed 45 IAF aircraft while Pakistan lost 75 aircraft. In 1979, the PAF's Chief of Air Staff, Air Chief Marshal Anwar Shamim, was told by President, Chief o
Royal Thai Air Force
The Royal Thai Air Force or RTAF is the air force of the Kingdom of Thailand. Since its establishment in 1913 as one of the earliest air forces of Asia, the Royal Thai Air Force has engaged in numerous major and minor conflicts. During the Vietnam War era, the RTAF was supplied with USAF-aid equipment. In February 1911 Belgian pilot Charles Van Den Born was responsible for the first aircraft demonstration in Siam at Bangkok's Sapathum Horse Racing Course. King Rama VI was sufficiently impressed that on 28 February 1912 he sent three Army officers to France to learn to fly. After receiving their wings and qualification, the officers returned to Siam in November 1913, bringing with them eight aircraft: four Breguets and four Nieuport IVs). In March 1914, Thai aviation moved from Sapathum to Don Muang north of Bangkok; the Ministry of Defence placed the Siamese Flying Corps under the Army Engineer Inspector General Department. Prince Purachatra Jayakara, Commander of the Army Engineers, his brother Prince Chakrabongse Bhuvanath, were instrumental in the development of the Royal Siamese Aeronautical Service as it was renamed in 1919.
In 1937, it became an independent service known as the Royal Siamese Air Force. Two years when the kingdom's name was changed to Thailand, it became the Royal Thai Air Force; the Air Force during the years before the Second World War was seen as a moderately-well equipped force with modern aircraft. During the French-Thai War, the Thai Air Force achieved several air-to-air-victories in dogfights against the Vichy Armée de l'Air. During World War II, the Thai Air Force supported the Royal Thai Army in its occupation of the Shan States of Burma as somewhat reluctant allies of the Japanese and took part in the defense of Bangkok against allied air raids in the latter part of the war, achieving some successes against state-of-the-art aircraft like the P-51 Mustang and the B-29 Superfortress. During these times, the RTAF was supplied by the Japanese with Imperial Japanese Army Air Force aircraft such as the Ki-43 "Oscar," and the Ki-27 "Nate." Other RTAF personnel took an active part the anti-Japanese resistance movement.
The Thai Air Force sent three C-47s to support the United Nations in Korean War. The Wings Unit, operating the C-47 joined the anti-communist forces in the Vietnam War. Along the border, the Thai Air Force launched many operations against communist forces, including the Ban Nam Ta Airfield Raid in Laos, clashes between Thai and communist Vietnamese troops along the Thai-Cambodian border; when the Cold War ended, the Thai Air Force participated in Operation Border Post 9631 along the Thai-Burmese border in 1999, launched the evacuation of foreigners during the 2003 Phnom Penh riots in Cambodia. For fiscal year 2018 the air force's budget is 39,931 million baht; the Royal Thai Air Force is commanded by the Commander of the Royal Thai Air Force. The Royal Thai Air Force Headquarters is located in Don Muang Airbase, Thailand; the RTAF command structure consists of five groups: headquarters, logistics support, special services, combat forces. The headquarters group in Bangkok performs the usual general staff functions, including planning and directing operations of the combat elements.
Combat Group. The support group provides engineering, ordnance, transportation and medical services support; the education group supervises all air force training programmes. The special service group is responsible for the welfare of air force personnel and coordinates the activities of Thai civil aviation with those of the air force; the Royal Thai Air Force maintains a number of modern bases which were constructed between 1954 and 1968, have permanent buildings and ground support equipment. All but one were built and used by United States forces until their withdrawal from Thailand in 1976 when Thai air force assumed use of the installations at Takhli and Nakhon Ratchasima. In the late 1980s, these bases and Don Muang Air Base outside Bangkok, which the air force shares with civil aviation, remain the primary operational installations. Maintenance of base facilities abandoned by the United States exceeded Thai needs. Nonetheless, all runways were still available for emergency use. By 2004 the Royal Thai Air Force had its main base at Don Muang airport, adjacent to Don Mueang International Airport.
The RTAF had large air fields and facilities at Nakon Ratchasima Ubon Ratchathani, Takhli. The following squadrons are active with the Royal Thai Air Force; this 100 man unit, part of the Royal Thai Air Force's Special Combat Operations Squadron, was formed in the late 1970s and are based near Don Muang Airport and provide anti-hijacking capabilities. They have three assault platoons, each divided into two sections; the Royal Thai Air Force Combat Group is divided into 11 wings plus a training school, plus a few direct-reporting units. Directorate of Air Operations Control, RTAF RTAF Security Force Command Flying Training Schoolcomposed of 1st, 2nd and 3rd Flying Training Squadrons. Based at RTAFB Kamphang Saen in Nakhon Pathom ProvinceWing 1Interceptor and fighter wing based at RTAFB Korat in Nakhon Ratchasima Province. Wing 2Helicopter wing providing utility/transport and search and rescue. Based at RTAFB Lopburi in Lopburi ProvinceWing 4Light attack / Interceptor wing based at RTAFB Takhli in Nakhon Sawan Province.
Wing 5 Transport and special mission wing based at RTAFB Prachuap Khiri Khan in Ao Manao, Prachuap Khiri Khan Province. Wing 6Multi-role non-combat wing providing transport, mapping and surveying. Based at RTAFB Don Muang/B
Turkish Air Force
The Turkish Air Force is the aerial warfare service branch of the Turkish Armed Forces. The Turkish Air Force can trace its origins back to June 1911 when it was founded by the Ottoman Empire, the air force as it is known today did not come into existence until 1923 with the creation of the Republic of Turkey; the Turkish Armed Forces initiated a $160 billion modernization program. $45 billion is earmarked to go to the overhaul of the Turkish Air Force. As part of this program, Ankara aims to commission helicopters. According to Flight International and the International Institute for Strategic Studies, the Turkish Air Force has an active strength of 60,000 military personnel and operates 668 manned aircraft; the history of Ottoman military aviation dates back to between June 1909 and July 1911. The Ottoman flight squadrons participated in the Balkan Wars and World War I; the fleet size reached its apex in December 1916, when the Ottoman aviation had 90 active combat aircraft. Some early help for the Ottoman Air Force came from the Imperial German Fliegertruppe, with future Central Powers 13-victory flying ace Hans-Joachim Buddecke flying with the Turks early in World War I as just one example.
The General Inspectorate of Air Forces trying to reconstruct itself on July 29, 1918 had no personnel, but only remained as a title on paper. After the end of World War I and the occupation of the Ottoman Empire by the Allies in 1919, some Turkish aviators tried to build new units in Istanbul, İzmir, Elazığ and Diyarbakır with planes left over from World War I and tried to bring together flight personnel. During the Turkish War of Independence, Turkish pilots joined the Konya Air Station. With the formation of the Grand National Assembly by Mustafa Kemal and his colleagues on April 23, 1920, in Ankara, the reorganization of the army, the Branch of Air Forces was established under the Office of War of the GNA. A few damaged aircraft belonging to the GNA were repaired, afterwards used in combat. On 1 February 1921, the Branch of Air Forces was renamed as the General Directorate of Air Forces at Eskişehir and on 5 July 1922 reorganized as the Inspectorate of Air Forces at Konya. After the proclamation of independence and sovereignty with the Treaty of Lausanne and the establishment of the Republic of Turkey on 29 October 1923, approaches were made to form a modern Air Force.
Consisting of 3 normal and 1 naval aviation units, an air school, the number of units was increased to 10 normal and 3 naval aviation units. Starting in 1924, personnel were sent abroad for flight education. In 1925 the Air School was reestablished in Eskişehir and its first students graduated in that same year; the Inspectorate of Air Forces was reorganized as Underdecretariat of the Ministry of Defense in 1928 and new schools were found for non-pilot personnel. Some personnel were sent to France for training. On July 1, 1932, air regiments were considered to be a separate combat arm and started training its own personnel; the Turkish aviators began to wear blue uniforms from 1933. Sabiha Gökçen became the first female fighter pilot in military history in 1937. Another key event in 1937 was the establishment of the Air War College. By 1940, Turkish air brigades had more than 500 combat aircraft in its inventory, becoming the largest air force in the Balkans and the Middle East; the growing inventory of air brigades required another structural change, made in 1940.
The Air Undersecretariat under the Ministry of National Defense for logistical affairs and the General Staff for educational affairs were united to form the Air Force Command in 1944. Thus, the Air Force became a separate branch of the Turkish Armed Forces; the first Commander of the Turkish Air Force was General Zeki Doğan. Turkey did not enter World War II on the side of the Allies until February 1945. However, the Turkish Armed Forces went on full alert and were prepared for war following the military alliance between neighbouring Bulgaria and the Axis Powers, formalized in March 1941, the occupation of neighbouring Greece by the Axis Powers in April 1941. Within a year, Turkey's borders were surrounded by German forces in the northwest and west, Italian forces in the southwest; the Turkish Air Force made daily reconnaissance flights over Bulgaria, the Greek Islands in the Aegean Sea, the Dodecanese Islands which belonged to Italy, to monitor the positions of the Axis forces. The large cities in western Turkey were darkened at nights, anti-aircraft guns and searchlights were deployed for defence against possible enemy planes.
All available money in the Turkish Government Treasury was used to purchase new weapons from any available provider in the world. The Turkish Air Force received large numbers of new aircraft in this period, including Supermarine Spitfire Mk. I/V/IX/XIX, Curtiss Falcon CW-22R/B, Fairey Battle-I, Avro Anson-I, Hawker Hurricane I/II, Morane-Saulnier M. S.406, Curtiss P-40 Tomahawk, Curtiss P-40 Kittyhawk, Westland Lysander-I, Consolidated B-24D Liberator B-24, Bristol Blenheim IV/V, Bristol Beaufort, Bristol Beaufighter Mk. I/X, Focke-Wulf Fw 190-A3, Martin 187 Baltimore, De Havilland DH.98 Mosquito Mk. III/IV, Do
General Dynamics F-16 Fighting Falcon
The General Dynamics F-16 Fighting Falcon is a single-engine supersonic multirole fighter aircraft developed by General Dynamics for the United States Air Force. Designed as an air superiority day fighter, it evolved into a successful all-weather multirole aircraft. Over 4,600 aircraft have been built since production was approved in 1976. Although no longer being purchased by the U. S. Air Force, improved versions are being built for export customers. In 1993, General Dynamics sold its aircraft manufacturing business to the Lockheed Corporation, which in turn became part of Lockheed Martin after a 1995 merger with Martin Marietta; the Fighting Falcon's key features include a frameless bubble canopy for better visibility, side-mounted control stick to ease control while maneuvering, an ejection seat reclined 30 degrees from vertical to reduce the effect of g-forces on the pilot, the first use of a relaxed static stability/fly-by-wire flight control system which helps to make it a nimble aircraft.
The F-16 has an internal M61 Vulcan cannon and 11 locations for mounting weapons and other mission equipment. The F-16's official name is "Fighting Falcon", but "Viper" is used by its pilots and crews, due to a perceived resemblance to a viper snake as well as the Colonial Viper starfighter on Battlestar Galactica which aired around when the F-16 entered service. In addition to active duty in the U. S. Air Force, Air Force Reserve Command, Air National Guard units, the aircraft is used by the USAF aerial demonstration team, the U. S. Air Force Thunderbirds, as an adversary/aggressor aircraft by the United States Navy; the F-16 has been procured to serve in the air forces of 25 other nations. As of 2015, it is the world's most numerous fixed-wing aircraft in military service. Experiences in the Vietnam War revealed the need for air superiority fighters and better air-to-air training for fighter pilots. Based on his experiences in the Korean War and as a fighter tactics instructor in the early 1960s, Colonel John Boyd with mathematician Thomas Christie developed the energy–maneuverability theory to model a fighter aircraft's performance in combat.
Boyd's work called for a small, lightweight aircraft that could maneuver with the minimum possible energy loss, which incorporated an increased thrust-to-weight ratio. In the late 1960s, Boyd gathered a group of like-minded innovators who became known as the Fighter Mafia, in 1969, they secured Department of Defense funding for General Dynamics and Northrop to study design concepts based on the theory. Air Force F-X proponents remained hostile to the concept because they perceived it as a threat to the F-15 program. However, the Air Force's leadership understood that its budget would not allow it to purchase enough F-15 aircraft to satisfy all of its missions; the Advanced Day Fighter concept, renamed F-XX, gained civilian political support under the reform-minded Deputy Secretary of Defense David Packard, who favored the idea of competitive prototyping. As a result, in May 1971, the Air Force Prototype Study Group was established, with Boyd a key member, two of its six proposals would be funded, one being the Lightweight Fighter.
The Request for Proposals issued on 6 January 1972 called for a 20,000-pound class air-to-air day fighter with a good turn rate and range, optimized for combat at speeds of Mach 0.6–1.6 and altitudes of 30,000–40,000 feet. This was the region; the anticipated average flyaway cost of a production version was $3 million. This production plan, was only notional, as the USAF had no firm plans to procure the winner. Five companies responded, in 1972, the Air Staff selected General Dynamics' Model 401 and Northrop's P-600 for the follow-on prototype development and testing phase. GD and Northrop were awarded contracts worth $37.9 million and $39.8 million to produce the YF-16 and YF-17 with first flights of both prototypes planned for early 1974. To overcome resistance in the Air Force hierarchy, the Fighter Mafia and other LWF proponents advocated the idea of complementary fighters in a high-cost/low-cost force mix; the "high/low mix" would allow the USAF to be able to afford sufficient fighters for its overall fighter force structure requirements.
The mix gained broad acceptance by the time of the prototypes' flyoff, defining the relationship of the LWF and the F-15. The YF-16 was developed by a team of General Dynamics engineers led by Robert H. Widmer; the first YF-16 was rolled out on 13 December 1973. Its 90-minute maiden flight was made at the Air Force Flight Test Center at Edwards AFB, California, on 2 February 1974, its actual first flight occurred accidentally during a high-speed taxi test on 20 January 1974. While gathering speed, a roll-control oscillation caused a fin of the port-side wingtip-mounted missile and the starboard stabilator to scrape the ground, the aircraft began to veer off the runway; the test pilot, Phil Oestricher, decided to lift off to avoid a potential crash, safely landing six minutes later. The slight damage was repaired and the official first flight occurred on time; the YF-16's first supersonic flight was accomplished on 5 February 1974, the second YF-16 prototype first flew on 9 May 1974. This was followed by the first flights of Northrop's YF-17 prototypes on 9 June and 21 August 1974, respectively.
During the flyoff, the YF-16s completed 330 sorties for a total of 417 flight hours. Increased interest turned the LWF into a serious acquisition program. North Atlantic Treaty Organization allies Belgium, the Netherlands, Norway were seeking to replace their
A duplexer is an electronic device that allows bi-directional communication over a single path. In radar and radio communications systems, it isolates the receiver from the transmitter while permitting them to share a common antenna. Most radio repeater systems include a duplexer. Duplexers can be based on polarization, or timing. In radar, a transmit/receive switch alternately connects the transmitter and receiver to a shared antenna. In the simplest arrangement, the switch consists of a gas-discharge tube across the input terminals of the receiver; when the transmitter is active, the resulting high voltage causes the tube to conduct, shorting together the receiver terminals to protect it, while its complementary, the anti-transmit/receive switch, is a similar discharge tube which decouples the transmitter from the antenna while not operating, to prevent it from wasting received energy. In radio communications, the transmitted and received signals can occupy different frequency bands, so may be separated by frequency-selective filters.
These are a higher-performance version of a diplexer with a narrow split between the two frequencies in question. With a duplexer the high- and low-frequency signals are traveling in opposite directions at the shared port of the duplexer. Modern duplexers use nearby frequency bands, so the frequency separation between the two ports is much less. For example, the transition between the uplink and downlink bands in the GSM frequency bands may be about one percent. Significant attenuation is needed to prevent the transmitter's output from overloading the receiver's input, so such duplexers employ multi-pole filters. Duplexers are made for use on the 30-50 MHz, 136-174 MHz, 380-520 MHz, plus the 790–862 MHz, 896-960 MHz and 1215-1300 MHz bands. There are two predominant types of duplexer in use - "notch duplexers", which exhibit sharp notches at the "unwanted" frequencies and only pass through a narrow band of wanted frequencies and "bandpass duplexers", which have wide-pass frequency ranges and high out-of-band attenuation.
On shared-antenna sites, the bandpass duplexer variety is preferred because this eliminates interference between transmitters and receivers by removing out-of-band transmit emissions and improving the selectivity of receivers. Most professionally engineered sites ban the use of notch duplexers and insist on bandpass duplexers for this reason. Note 1: A duplexer must be designed for operation in the frequency band used by the receiver and transmitter, must be capable of handling the output power of the transmitter. Note 2: A duplexer must provide adequate rejection of transmitter noise occurring at the receive frequency, must be designed to operate at, or less than, the frequency separation between the transmitter and receiver. Note 3: A duplexer must provide sufficient isolation to prevent receiver desensitization. Source: from Federal Standard 1037C The first duplexers were invented for use on the electric telegraph and were known as duplex rather than duplexer, they were an early form of the hybrid coil.
The telegraph companies were keen to have such a device since the ability to have simultaneous traffic in both directions had the potential to save the cost of thousands of miles of telegraph wire. The first of these devices was designed in 1853 by Julius Wilhelm Gintl of the Austrian State Telegraph. Gintl's design was not successful. Further attempts were made by Carl Frischen of Hanover with an artificial line to balance the real line and Siemens & Halske, who bought and modified Frischen's design; the first successful duplex was designed by Joseph Barker Stearns of Boston in 1872. This was further developed into the quadruplex telegraph by Thomas Edison; the device is estimated to have saved Western Union $500,000 per year in construction of new telegraph lines