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Chain Home

Marconi tower at sunset.

Chain Home transmitter

Chain Home was the codename for the ring of coastal radar stations built by the British before and during World War II. The system comprised two types of radar. The Chain Home stations, or AMES Type 1 (Air Ministry Experimental Station), provided long-range detection. The Chain Home Low stations, or AMES Type 2, were shorter-ranged but could detect aircraft flying at lower levels.

Development
Operations
Chain Home Sites
Use by the Germans
See also
References
Notes
Further reading
External links

Development

The Chain Home system was fairly primitive, since in order to be battle-ready it had been rushed into production by Sir Robert Watson-Watt's Air Ministry research station near Bawdsey. Watson-Watt, a pragmatic engineer, believed that "third-best" would do if "second-best" would not be available in time and "best" never available at all. Chain Home certainly suffered from glitches and errors in reporting.

It was in many ways technically inferior to German radar developments, but the better German technology proved to be a disadvantage. The Chain Home stations were relatively simple to construct and comprehensive coverage was available by the start of the Battle of Britain. By contrast, the Germans had only commissioned around 8 of their Freya stations by this time. In spite of the simplicity of the Chain Home technology, the system had a surprising level of functionality. It could determine distance and direction of incoming aircraft formatting, giving rise to its initial name of RDF (Range and Direction Finding), later rechristened 'radar' by the Americans. Most stations were also able to measure the elevation of the formation, which knowing the range gave the height. Local geography prevented some stations from measuring elevation. Although not originally a design goal, the operators became very adept at estimating the size of detected formations from the shape of the displayed returns.

Chain Home looked nothing like the radar sets found at modern commercial airports. There was no rotating antenna sending out a "searchlight" beam of radio energy, but instead an array of fixed antennas. The transmitting array was formed of wires strung between high (110 m) metal towers, and this array sent out a "floodlight" of radio energy covering a swath over the ground of about 100 degrees.^[1] The receiving array was on wood towers (about 73 m high) and consisted of two antennas at right angles to each other.^[2] The receiving antennas were directional in their sensitivity, so the signal strength received by each depended on the angle between it and the target. An operator would manually adjust a comparator device to find what angle to the target best matched the relative strengths of the two received signals. The angle of elevation to the target was estimated by similar comparisons to the signal strengths from a second pair of receiving antennas. This second pair was located closer to the ground, which produced a different sensitivity in elevation.^[3] The time delay of the echo determined the range to the target.

The Chain Home stations were designed to operate at 20-50 MHz although typical operations were at 20-30 MHz, or about a 12 metre wavelength.^[4] The availability of multiple operating frequencies gave some protection from jamming. The detection range was typically 120 miles (192 km), but could be better.^[5]

The Chain Home Low stations operated at 200 MHz, or about a 1.5 metre wavelengh. Technically, they were not closely related to Chain Home, and they employed a rotating antenna.^[6]

Compare with the German Freya radar.

From May to August 1939, the German Zeppelin LZ130 made flights along Britain's North Sea coast to investigate the 100 m high radio towers the British had erected from Portsmouth to Scapa Flow. LZ130 performed a series of radiometric tests and took photographs. German sources report the 12 m Chain Home signals were detected and suspected to be radar; however, the chief investigator was not able to prove his suspicions, so Germany went to war uncertain of British radar defenses.^[7] Other sources are said to report different results.^[8]

Operations

The Chain Home stations were arranged around the British coast, initially in the South and East but later the entire coastline, including the Shetland Islands. They were first tested in the Battle of Britain in 1940 when they were able to provide adequate early warning of incoming Luftwaffe raids.

During the battle, Chain Home stations, most notably the one at Ventnor, Isle of Wight, were attacked a number of times between 12 and 18 August, 1940. On one occasion a section of the radar chain in Kent, including the Dover CH, was put out of action by a lucky hit on the power grid. However, though the wooden huts housing the radar equipment were damaged, the towers survived owing to their open steel girder construction. Because the towers were untoppled and the signals soon restored, the Luftwaffe concluded the stations were too difficult to damage by bombing and so left them alone for the rest of the war. Had the Luftwaffe realised just how essential the radar stations were to British air defences, it is likely that they would have gone all out to destroy them.

The Chain Home system was dismantled after the war, but some of the tall steel radar towers remain, converted to new uses. One such -high (110 m) transmitter tower (picture above) can now be found at the BAE Systems facility at Great Baddow in Essex (2008). It originally stood at Canewdon, and is said to be the only Chain Home tower still in its original, unmodified form.

Use by the Germans

The Germans deployed^[26] a passive radar system, the Kleine Heidelberg Parasit, which allowed them to track British aeroplanes using the radio signals from the Chain Home radars.^[27] The "floodlight" nature of the Chain Home transmissions would provide a pair of signals which could be used to locate aircraft. The primary signal was the direct flight of the radio signal from the Chain Home transmitter to the German receiver. The second, weaker signal was that reflected from the aircraft. The time delay between these two signals established how much longer was the reflected path compared to the direct path, and from geometry this longer path described an ellipse on which the aircraft must lie. The focal points of this ellipse were the transmitting and receiving antennas, and the Germans knew the location of both. A direction finding antenna searching for the echo could be used to establish where on the ellipse the aircraft was.^[28] This system gave the Germans a radar with a range of up to 400 km and an accuracy in range of 1 to 2 km and in bearing of about 1 degree.^[29] The Heidelberg Parasit was not affected by Window.^[30]

References

Neale, B. T., The GEC Journal of Research, Vol.3 No.2 1985, pages 73-83, a copy of which could be found on The Radar Pages, 2007-06

Pritchard, David, The Radar War: Germany's Pioneering Achievement, 1904-45, Patrick Stephens Limited, Wellingborough, England, 1989, ISBN 1-85260-246-5.

Pritchard's book draws mainly on original German sources but also includes information from interviews with British researchers of the era, including R. V. Jones. Unfortunately, it includes neither references nor a bibliography, although the text often indicates the original source.