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Chain Home

Description of the radar set, tactical-technical characteristics

Figure 1: The Chain Home tower in Great Baddow, Chelmsford

Figure 1: The Chain Home tower in Great Baddow, Chelmsford

Chain Home

Chain Home was the code name for the coastal chain of radar stations built by the British before and during World War II. Their technical name was AMES (Air Ministry Experimental Station). The system included three types of radars: metric Chain Home stations, which provided long-range radar alerts (Type 1), metric Chain Home Low (Type 2), and centimetric Chain Home Extra Low, which had a shorter range but could detect aircraft at low altitudes.

The British government feared a German air attack and set up a research group on remote sensing of aircraft in 1933. Robert Watson-Watts proved the feasibility of radio detection in 1935 and funds were released to build a network in 1937. These are not the first radars as it is often erroneously said since other countries developed similar and often more powerful devices at the same period (see history of radar), but the Chain Home was the first large-scale network integrated into a country’s air defense to be set up.

The Chain Home was a bistatic radar. For each radar, the transmitter used a network of electrical wires stretched between towers measuring 110 m high which gave a 100° wide beam. Two wooden towers measuring 73 m, supporting receiving antennas oriented at 90 degrees to each other, completed the system.

The way of knowing the position of the targets was similar to the use of a direction finder: manually orient the receiving antennas in azimuth and elevation. In azimuth, the operator then compared the relative power of the signals coming from the two antennas until he found the direction where the signal was maximum. Similarly, he compared the signal from one antenna at the top and another at the base of the towers in elevation. The distance was derived from the delay between the transmission and the return time of the echoes from the targets.

The stations of the Chain Home operated at a variable frequency at the limit between high frequency and very high frequency. Most often the range of 20 to 30 MHz was used, but the possibility to vary over a wide range of frequencies allowed in part to counter interference. The usual range was 190 km but it depended on the altitude of the target: 40 km for a target flying at an altitude of 500 meters and 134 km for one at 4,000 meters. The vacuum tube transmitters were built by Metropolitan-Vickers from the plans of the radio transmitter. The receiver was built by A. C. Cossor Ltd. according to the plans given by the Telecommunications Research Establishment.

The Chain Home Low, established in 1939, used a thinner beam at a wavelength of 1.5 meters (200 MHz) for longer range detection at low altitudes (as low as 150 meters). The towers of this radar were smaller and therefore transportable on a truck to a site designated by the RAF for air defense. It was a monostatic radar with the transmitting and receiving antenna at the top of the tower. It was made up of 5 groups, of 4 dipoles each one, placed in front of a metal reflector.

Description of the radar set, tactical-technical characteristics
 Chain Home Chaine Home Low Chain Home Extra Low
frequency:  20 to 50 MHz 200 MHz 3 GHz
pulse repetition frequency (PRF) 12,5 and 25 Hz; 400 Hz 500 Hz
pulsewidth (τ): 20 µs 3 µs 6,6 and 1,9 µs
peak power: 350 kW
later: 750 kW 
150 kW 500 kW
instrumented range: up to 300 km
range resolution: up to 190 km 40 km at 150 m altitude  30 km at 15 m altitude, 145 km at 1 800 m 
beamwidth: Type 13: 1,5° vert. 7,5° horiz.
Type 14 1° horiz. x 3° vert.
Rotation: 1, 1,5, 2 or 3,33 min⁻¹. Type 13: 6 cycles per minute.

In 1943, the Chain Home Extra Low (types 13 and 14) centimeter (10 cm or S band) will be developed for detection at very low altitudes (as low as 15 meters). Type 13 was a monostatic site radar powered by a magnetron performing vertical scans from -1° to +20°. The antenna was initially formed by dipoles mounted on a grid of 6 meters high by 90 cm wide. Subsequently a section of a parabola of similar size, and fed by a slotted waveguide near the focal point, replaced the initial antenna. Problems with the vertical scanning mechanism caused most of these radars to operate with a fixed pointing to the horizon.

Type 14 was similar to Type 13 but the antenna was horizontal to give an azimuthally scanned radar. To have good vertical coverage, two antennas were superimposed: the first pointing towards the horizon and the second-placed above it had an angle of elevation.

In all, 59 units of type 1, 24 of type 2, and about as many of types 13 and 14 were built.

Source: www.radarpages.co.uk : Chain Home, Chain Home Low et Chain Home Extra Low