Chain Home Low

Chain Home Low^[1]

Chain Home Low station at Hopton-on-Sea
Country of origin	United Kingdom
Introduced	July 1939 (1939-07)
No. built	100+
Frequency	200 MHz
PRF	400 Hz
Beamwidth	1.5° (horizontal)
Pulsewidth	3 μs
RPM	Up to 3.3
Power	150 kW
Other Names	AMES Type 2, CD Mark I

Chain Home Low (CHL) was the name of a British early warning radar system operated by the RAF during World War II.^[2] The name refers to CHL's ability to detect aircraft flying at altitudes below the capabilities of the original Chain Home (CH) radars, where most CHL radars were co-located. CHL could reliably detect aircraft flying as low as 500 feet (150 m). The official name was AMES Type 2, referring to the Air Ministry Experimental Station at Bawdsey Manor where it was developed, but this name was almost never used in practice.

The system had originally been developed by the British Army's research group, also based at Bawdsey, as a system for detecting enemy shipping in the English Channel. It was built using the electronics being developed for the aircraft interception radar systems, which worked on the 1.5 m band. This high frequency (200 MHz), for the era, allowed it to use smaller antennas that could be swung back and forth to look for returns, in contrast to the enormous fixed antennas of the 6.7 m wavelength (45 MHz) Chain Home system.

When the war began, the Luftwaffe began mine-laying missions where the bomber aircraft would fly almost all of their mission at low altitude. Chain Home could only see targets above 1.5 degrees over the horizon, so these aircraft only became visible at short range. Robert Watson Watt seized several dozen of the Coastal Defense (CD) systems that were in final construction and installed them at CH stations and key locations along the seashore to fill this critical gap in the coverage.

CHL remained an important part of the Chain network for the rest of the war, and was retained in the post-war era until it was replaced during the ROTOR upgrades by the AMES Type 80. The electronics, notably the high-power transmitter, was also re-used in a number of other systems, including the AMES Type 7.