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Authors: Bickers Richard Townshend
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that a chain of radio detection stations should be established between the Tyne and Southampton, to the number of about 20. These were to constitute âChain Homeâ, referred to as âCHâ.
The expected ranges at which these would detect aircraft were:
83 miles at 13,000ft (132km at 3,939m).
50 miles at 5,000ft (80km at 1,515m).
35 miles at 2,000ft (56km at 606m).
25 miles at 1,000ft (40km at 303m).
The original scheme envisaged a transmitting station every 20 miles (32km), alternate ones to have a receiver also. Each mast was to be not less than 200ft (60.6m) high, on land not less than 50ft (15m) above sea level and not more than two miles (3.2km) from the coast.
Range measurement by a transmitter-receiver station would fix an aircraft as lying in a certain circle. Measuring the time interval between the pulse transmitted by a neighbouring station would put the aircraft in a certain ellipse. Thus a transmitter-receiver with its two flanking transmitters could get a fix. Height finding was not to be introduced at first,although the height of an aircraft at 7,000ft (2,121m) altitude and 15 miles (24km) range had been measured to within one degreeâs error in elevation (l,200ft/363m) by comparing signals received by two vertical aerials. In February 1936 the main experimental work was moved to Bawdsey, south of Orfordness, where a CH station was built. There, on March 13, a Hawker Hart was seen at a range of 75 miles (120km) at a height of 15,000ft (4,545m).
Trials in 1937 showed that most aircraft appearing in the observed area were reported with good accuracy up to 80 miles (128km); bearing was less reliable than range; height was good above 8,000ft (2,425m), unreliable below 5,000ft (1,515m). Estimates of the size of formations were not reliable. By the time the annual Home Defence Exercise was held in the summer of 1939, C-in-C Fighter Command Air Chief Marshal Sir Hugh Dowding reported: âThe system worked extremely well, and although doubtless capable of improvements as a result of experience, may now be said to have settled down to an acceptable standard.â At Easter 1939, with the outbreak of war expected at any moment, the radar chain had begun continuous watch-keeping. The Germans were curious about the tall towers that had sprung up on the English coast. In May General Wolfgang Martini, Chief of Luftwaffe Signals, flew up the east coast in the airship
Graf Zeppelin
, which radar picked up and, by the size and slow speed of the response on the cathode ray tube, identified. In August the airship, without Martini aboard, made a second sortie that radar did not detect, but which was seen by people in Scotland and intercepted outside the three-mile (5km) limit by a section of Auxiliary Air Force fighters. Both espionage ventures were abortive, as the Germansâ receivers picked up only a miscellany of confused noises that betrayed nothing about the existence of radar. Generalleutnant Adolf Galland has since revealed that right up to the Battle of Britain the Luftwaffe thought that the main purpose of the towers was the detection of shipping.
Chain Home operated on a wavelength of 10 to 13.5 metres, on frequencies of 22 to 30MHz, with 200kW peak power output. Its range was 120 miles (192km) and it could read height. The aerials were stationary, the transmitter was on a 350ft (106m) steel tower and the receiver on a 240ft (73m) wooden one. The coverage was described as âfloodlightâ, i.e., it was diffused in a wide arc or complete circle, which both inhibited the obtaining of accuracy in azimuth and allowed low-flying aircraft to go undetected. A shorter wavelength was needed to pickup at low altitude, and a rotating aerial to enable a narrow âsearchlightâ beam to sweep from side to side or be pointed in any required direction.
Such equipment was devised and formed the Chain Home Low, or CHL. Its wavelength was 1.5m, frequency 200MHz, and power output 150kW. Range
The expected ranges at which these would detect aircraft were:
83 miles at 13,000ft (132km at 3,939m).
50 miles at 5,000ft (80km at 1,515m).
35 miles at 2,000ft (56km at 606m).
25 miles at 1,000ft (40km at 303m).
The original scheme envisaged a transmitting station every 20 miles (32km), alternate ones to have a receiver also. Each mast was to be not less than 200ft (60.6m) high, on land not less than 50ft (15m) above sea level and not more than two miles (3.2km) from the coast.
Range measurement by a transmitter-receiver station would fix an aircraft as lying in a certain circle. Measuring the time interval between the pulse transmitted by a neighbouring station would put the aircraft in a certain ellipse. Thus a transmitter-receiver with its two flanking transmitters could get a fix. Height finding was not to be introduced at first,although the height of an aircraft at 7,000ft (2,121m) altitude and 15 miles (24km) range had been measured to within one degreeâs error in elevation (l,200ft/363m) by comparing signals received by two vertical aerials. In February 1936 the main experimental work was moved to Bawdsey, south of Orfordness, where a CH station was built. There, on March 13, a Hawker Hart was seen at a range of 75 miles (120km) at a height of 15,000ft (4,545m).
Trials in 1937 showed that most aircraft appearing in the observed area were reported with good accuracy up to 80 miles (128km); bearing was less reliable than range; height was good above 8,000ft (2,425m), unreliable below 5,000ft (1,515m). Estimates of the size of formations were not reliable. By the time the annual Home Defence Exercise was held in the summer of 1939, C-in-C Fighter Command Air Chief Marshal Sir Hugh Dowding reported: âThe system worked extremely well, and although doubtless capable of improvements as a result of experience, may now be said to have settled down to an acceptable standard.â At Easter 1939, with the outbreak of war expected at any moment, the radar chain had begun continuous watch-keeping. The Germans were curious about the tall towers that had sprung up on the English coast. In May General Wolfgang Martini, Chief of Luftwaffe Signals, flew up the east coast in the airship
Graf Zeppelin
, which radar picked up and, by the size and slow speed of the response on the cathode ray tube, identified. In August the airship, without Martini aboard, made a second sortie that radar did not detect, but which was seen by people in Scotland and intercepted outside the three-mile (5km) limit by a section of Auxiliary Air Force fighters. Both espionage ventures were abortive, as the Germansâ receivers picked up only a miscellany of confused noises that betrayed nothing about the existence of radar. Generalleutnant Adolf Galland has since revealed that right up to the Battle of Britain the Luftwaffe thought that the main purpose of the towers was the detection of shipping.
Chain Home operated on a wavelength of 10 to 13.5 metres, on frequencies of 22 to 30MHz, with 200kW peak power output. Its range was 120 miles (192km) and it could read height. The aerials were stationary, the transmitter was on a 350ft (106m) steel tower and the receiver on a 240ft (73m) wooden one. The coverage was described as âfloodlightâ, i.e., it was diffused in a wide arc or complete circle, which both inhibited the obtaining of accuracy in azimuth and allowed low-flying aircraft to go undetected. A shorter wavelength was needed to pickup at low altitude, and a rotating aerial to enable a narrow âsearchlightâ beam to sweep from side to side or be pointed in any required direction.
Such equipment was devised and formed the Chain Home Low, or CHL. Its wavelength was 1.5m, frequency 200MHz, and power output 150kW. Range
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