June 1938. The Air Ministry adopted some of these sets to augment the CH network in case of enemy damage. GL Mk. I sets were used overseas by the British Army in Malta and Egypt in 1939–40. Seventeen sets were sent to France with the British Expeditionary Force; while most were destroyed at the Dunkirk evacuation in late May 1940, a few were captured intact, giving the Germans an opportunity to examine British RDF kit. An improved version, GL Mk. II, was used throughout the war; some 1,700 sets were put into service, including over 200 supplied to the Soviet Union. Operational research found that anti-aircraft guns using GL averaged 4,100 rounds fired per hit, compared with about 20,000 rounds for predicted fire using a conventional director. Coastal Defence In early 1938, Alan Butement began the development of a Coastal Defence (CD) system that involved some of the most advanced features in the evolving technology. The 200 MHz transmitter and receiver already being developed for the AI and ASV sets of the Air Defence were used, but, since the CD would not be airborne, more power and a much larger antenna were possible. Transmitter power was increased to 150 kW. A dipole array high and wide, was developed, giving much narrower beams and higher gain. This "broadside" array was rotated 1.5 revolutions per minute, sweeping a field covering 360 degrees. Lobe switching was incorporated in the transmitting array, giving high directional accuracy. To analyze system capabilities, Butement formulated the first mathematical relationship that later became the well-known "radar range equation". Although initially intended for detecting and directing fire at surface vessels, early tests showed that the CD set had much better capabilities for detecting aircraft at low altitudes than the existing Chain Home. Consequently, CD was also adopted by the RAF to augment the CH stations; in this role, it was designated Chain Home Low (CHL). Centimetric gun-laying When the cavity magnetron became practicable, the ADEE co-operated with TRE in utilising it in an experimental 20 cm GL set. This was first tested and found to be too fragile for army field use. The ADEE became the ADRDE in early 1941, and started the development of the GL3B. All of the equipment, including the power generator, was contained in a protected trailer, topped with two 6-foot dish transmitting and receiving antennas on a rotating base, as the transmit-receive (T-R) switch