Chain Home Extra Low
Introduction
In the early stages of the Second World War a German pilot could approach to within 80 miles of the British coast at 10,000 before CH detected him and by descending to 5,000 feet he could avoid CH detection to within 50 miles of the coast. With the introduction of CHL an aircraft flying at 500 feet up to 25 miles away could be tracked with very good accuracy. By 1941 the Germans had discovered that they could evade detection by CH and CHL by flying below 100 feet and skilled German pilots used this deficiency to devastating effect during raids on coastal targets.
By that time it was known that ultra-high radio frequencies could be formed into very narrow beams that could skim the earth's surface without being distorted by it. Research by Randall and Boot at Birmingham University had, by early 1940, led to the development of the Boot Strap Cavity Magnetron, a device that could produce microwave frequency waves with a power level some ten times greater than previous microwave devices. By early 1942 an experimental radar using this magnetron was successfully tracking aircraft flying at 50-200 feet to about 30 miles and on occasions out to 45 miles. On the basis of these experiments the Air Ministry decided to procure a number of these sets to form a further Home Chain to be known as Chain Home Extra Low. The original equipments were Admiralty Type 271's and Army coastal defence CD Mk IV to Mk VI, eleven of which were transferred to R.A.F. service by December 1942, a further three were brought into operation by May 1943. The Army CD Mk VI became Types 52 to 56 in R.A.F.service.
Meanwhile, parallel work had been taking place at the Telecommunications Research Establishment (TRE). Their work had led to the development of a centimetric height finder (CMH) for use at CHL sites. The early CMH equipments were not too successful and they were used, with the nodding turned off, for low angle detection. By 1943 CMH was known as Type 13 in the R.A.F. The Admiralty continued development of their Type 271 radar and the improved model, known as the Type 277 entered R.A.F service as the Type 14.
By April 1943 the R.A.F. had four Type 13's operating as low level watching stations, along with five mobile Type 14's. During the summer of 1943 six more Type 14's were installed on top of 200 foot towers down the east coast. Later on Type 14's were installed on cantilevers on the steel CH towers. It is a testament to the huge efforts of the scientists, engineers and technicians that the entire Home Chain, comprising of CH, CHL and CHEL equipment, was complete by the end of 1943.
