Accidental discovery CHL traces its origins to early experiments with
aircraft interception radar systems in 1936. These were developed as a short-range radar that would be used to close the gap between
Chain Home's (CH) approximate accuracy and the visual range of a
night fighter pilot at about . Developed by a team at Bawdsey Manor led by
"Taffy" Bowen, the new radar had to operate at much shorter
wavelengths in order to limit the antenna sizes to something that could be practically fit on an aeroplane. After considerable experimentation, the team settled on a set working at 1.5 meter wavelength, about 193 MHz in the
VHF band. In early experiments with the new set, the team found that detection of other aircraft was problematic due to their target's relatively small size, but especially due to reflections off the ground. The latter caused a very strong signal that appeared to be at a range equal to the aircraft's current altitude, and everything beyond that was invisible in the resulting clutter. This meant that a typical night bombing run by German aircraft at altitude would only become visible at that range, far less than the desired minimum of . These same experiments demonstrated an unexpected side-effect. As the aircraft flew around over Bawdsey, which is located on the coast of the
English Channel, the team found strong constant returns that they later realised were the cranes at the
Harwich docks, miles away. Other smaller returns were quickly identified as boats in the Channel. These were being detected at ranges far beyond the maximum range against aircraft, in spite of the antennas not being designed for this role. The potential of this discovery was not lost, and
Robert Watson-Watt asked the team to demonstrate the concept in a real-world setting. A series of
military exercises in the Channel in September 1937 provided a perfect test. On 3 September the team's test aircraft,
Avro Anson K6260, detected several
Royal Navy ships in the Channel, and the next day repeated this performance in spite of almost completely overcast skies.
Albert Percival Rowe of the
Tizard Committee later commented that "This, had they known, was the writing on the wall for the German Submarine Service."
Coastal Defence (CD) The
British Army was actually the first to consider radar, when
Alan Butement and
P. E. Pollard submitted a paper in 1931 suggesting using pulses of radio signal to measure the distance to ships. The Army was uninterested until they heard about Watt's work at Bawdsey, when they suddenly became very interested. In October 1936 a liaison team led by Edward Paris and
Albert Beaumont Wood was set up at Bawdsey, officially known as the Military Applications Section, but universally referred to as the "Army Cell". Ironically, the only two technicians with the required experience available were Butement and Pollard. The two quickly began development of two projects, the Mobile Radar Unit (MRU) which was a mobile version of Chain Home, and
Gun Laying radar, a much smaller unit designed to provide range measurements against aircraft as an aid to aiming their
anti-aircraft artillery. Both operated at the longer wavelengths typical of the RAF radars of the era, which led to them being relatively large. The teams had made considerable progress on both projects by the summer of 1937, with
Gun Laying radar, Mk. I (GL) about to enter initial production, and the MRUs later taken over in 1938 by the RAF as the AMES Type 9.
CD becomes CHL Type T3026 transmitter During early tests against Chain Home in 1938, RAF pilots had noticed they could escape detection by flying at low altitudes. This was due to the minimum angle of the CH being about 1.5 degrees above the horizon, which meant aircraft were below the radar's sight until they approached within a few miles. They could escape detection entirely by flying between two CH stations at altitudes around . At first this was not considered to be a serious limitation, as bombers typically flew at altitudes of 15,000 feet or greater, and at that altitude they could be detected over France. But as the magnitude of the problem became clear, Watt became concerned. In July 1939 he placed an order for twenty-four CD sets under the name AMES Type 2 (Type 1 being Chain Home), intending to place one at each Chain Home station to allow coverage at lower altitudes, as low as . in which units were placed on trucks for movement matching the enemy's, extending the RAF's options in engaging the enemy. CHL Station, 2012
AMES Type 11 Although GCI began to supplant CHL during 1941 and especially 1942, CHL continued to provide an important early warning role. In February 1942 the Germans mounted the
Channel Dash, moving two of their battleships to harbours in Germany by sailing them right up the English Channel. This major embarrassment was due largely to supremely effective jamming on the part of the Germans, who managed to render the CH and CHL radars covering the coast entirely ineffective without the operators even noticing. To address this, the RAF began development of the
AMES Type 11, a truck-mounted CHL system operating at 500–600 MHz. This frequency was chosen to match that of German anti-aircraft radars, in the hopes that the signals would be more difficult to notice, and that jamming would have negative effects on the German's own radars. Type 11's were deliberately used only in times of jamming in order to avoid giving the Germans
signals intelligence about them, and in the end were little used. ==List of Chain Home Low sites==