The Chain Home Low radar system
Operational
experience showed that the Chain Home (CH) system had significant gaps in its
low level cover below about 2o and the 1938 R.A.F. exercises demonstrated
that low-flying aircraft could escape detection completely.
In 1936 the War Office had established a small group at Bawdsey under Dr. E. T. Paris and Dr. A. B. Wood. This group had been working on gun-laying (GL) radar for antiaircraft guns and coastal defence (CD) radar for the direction of coastal artillery. The CD equipment worked on the higher frequency of 180-210 MHz and the aerial comprised of a broadside 32 dipole array that produced a narrow beam in both azimuth and elevation. By July of 1939 the CD set could detect an aircraft flying at 500 feet up to 25 miles away with very good accuracy and in August 1939, on Watson-Watt's recommendation, the Air Ministry ordered 24 CD sets from Pye Radio with the intention of placing one at each CH site. These stations became known as Chain Home Low (CHL) stations and the equipment as Radar Type 2.
The early CHL radars used the type A display, similar to that used in CH. The type A display comprised of a cathode ray tube (CRT) that displayed slant range, usually across the horizontal (X) axis, and a deflection proportional to the strength of the received signal on the vertical (Y) axis. The target bearing was read off of a vernier scale representing the direction the aerial was pointing (or goniometer in the case of CH). A major improvement in interception techniques came in June 1940 when the PPI display was introduced. A fuller description of the impact of the PPI on interception can be found in the GCI section.
Later in the war the fear of jamming in the now heavily used 200 MHz band led to developments in the 500-600 MHz band. In January 1942 six 50 cm Type 11 CHL/GCI equipments were ordered and all had been delivered by the end of the year. The performance of this radar was somewhat disappointing though, due mainly to a large gap in the vertical cover that could not be removed by tilting the aerial.
