2. World War Two – The Chain Home Low Radar

In the 1930s the government were growing more worried about the situation developing in Europe and decided that some form of warning system was needed as part of the defence of this country.

Fortunately, as a result of some pioneering work by Hülsmeyer (Germany), and Marconi (Italy) the idea of using the reflection of radio waves to determine the location of distant objects was already being investigated. Appleton had used radio echoes to determine the height of the ionosphere in 1924 and Hülsmeyer had actually proposed a system for ships to use radio waves as a collision avoidance system. Many other scientists were carrying out similar experiments around the world so radar, as such, had many independent births.

 In the mid 1930s H E Wimperis of the Air Ministry asked Robert Watson-Watt, superintendent of a radio department at the National Physical Laboratory, if some form of ‘death ray’ was possible using a radio beam to disable remote targets. This was rejected as being unrealistic but Watson-Watt prepared a report, drafted with the assistance of Arnold Wilkins, in February 1935 entitled ‘The Radio Detection of Aircraft by Radio Methods’. This report was accepted by Sir Henry Tizard’s scientific committee and an experiment was carried out near Daventry on February 26th 1935. The BBC’s short wave transmitter at Daventry was used to detect a Heyford bomber at ranges up to 8 miles and heights of up to 6,000 feet. Even though the experimenters knew what to look for, and the aircraft is rumoured to have been trailing a length of wire ideally cut to act as a radio reflector on the frequency being used, the experiment was considered to be a great success and the system underwent rapid development to become the UK’s first radar early-warning system.

By 1939 a chain of Air Ministry Experimental Station (AMES) Type 1 Radar Stations along the south and east coasts of Britain had been established to detect aggressors in the air or at sea. The new installations were typified by a set of four high wooden towers supporting fixed aerials acting as a transmitter station and four nearby smaller towers acting as a receiver site. These formed the ‘Chain Home’ (CH) system. The CH transmitters acted like radio floodlights and the receiving aerials were made up of pairs of dipoles at right angles to each other. A device controlled by the operators, called a goniometer, tuned the signals from the dipoles and displayed the result on a cathode ray tube trace. This gave the operator the direction and distance to the target but the accuracy was subject to many outside influences. By switching to alternate pairs of dipoles the height could also be calculated. Only aircraft at fairly high altitudes could be tracked by this method, however, and low-flying aircraft or surface shipping could not be detected.

 

 

 

The origins of this thumbnail photograph of the first Chain Home station at Bawdsey Manor is not known. Anyone knowing its ownership is requested to contact the author.

 

To overcome the problem of detecting the lower targets, a second chain, more or less alternating with the first and integrated into the same reporting system, was created and known as the ‘Chain Home, Low’ (CHL). The radars were based on the Coastal Defence radars used by the Army and one of the sites chosen was on top of Truleigh Hill and work began in 1939 to install an AMES Type 2 radar there. This Chain Home Low station consisted of two separate aerials, one for the transmitter and one for the receiver, mounted on 20 foot high gantries, with the equipment housed in a hut underneath each gantry. Since CHL operated on a wavelength of 1.5 metres the aerials were short enough that the arrays could be rotated, which was done manually by an operator using cranks and a chain-driven system from inside the hut. The operators had to synchronise both aerials but the result was a beam about 25° wide, rather than the floodlight effect of CH. The aerials were not continuously rotated but, instead, were aimed at the target and moved from side to side to get the best reading. The received echo was displayed on a single trace on a cathode-ray tube. This display gave slant range to target only. Information about possible targets, detected at fairly long range by CH, could be passed to the CHL for tracking at lower levels as well as the CHL feeding information back through the CH stations to the filter rooms.

 It was not until well after the Battle of Britain that power turned, single arrays (which combined transmitting and receiving aerials) were introduced. These motorised aerials rotated constantly and gave a continuous reading. This new system also allowed the target to be more accurately detected and allowed multiple targets to be tracked on a single display. When the new cathode ray tube equipment known as Plan Position Indicator (PPI) was introduced the result was a stunning advance. The PPI displayed the beam as a rotating line, much as modern radars do, and targets could now be clearly seen in two dimensions. Height was still not given but the operators became very skilful at estimating height from the reflections given by known targets and the signal strength. The operator’s console now consisted of two cathode-ray tubes side by side. the left-hand display was the old single trace with range to target shown as a ‘blip’ on the line. The right-hand display was the new PPI. In normal use the left-hand display would be set to about 100 miles full scale and the PPI would be set at about half this range. Between the two, excellent coverage was achieved.

The picture is from an unidentified source and copyright status is not known. It was not taken at Truleigh Hill but does shows a typical late pattern CHL array.

The main search direction was out towards the English Channel where the risk was seen to be greatest and the equipment installed was capable of spotting low-flying aircraft at ranges up to about 50 miles. Information from the radar was initially interpreted in the buildings at ground level below and details were passed to the nearby CH station at Poling.

The first picture above is from an unidentified source and copyright status is not known.

The CHL radar could detect aircraft flying at 500 feet at ranges of up to 30 miles and higher aircraft could be tracked at up to 100 miles. This meant that CHL radars could be used for medium as well as low-level coverage and thus more than filled the gaps left by the CH systems. The importance of these radar systems during the Battle of Britain is hard to over-estimate. Without them the Royal Air Force would almost certainly have lost the battle. The early warning provided by CHL radars enabled the RAF to intercept incoming aircraft which would otherwise not have been detected until they were within visual range. In this case, fighters would not have had sufficient time to reach the bombers before they had attacked their target. The only alternative would have been to mount standing patrols of aircraft in the path of likely attacks but the severe shortage pilots and aircraft in 1940 would have made such tactics infeasible.

At Poling the information from Truleigh was combined with that from their own CH radar and the interpreted results were passed on to the main ‘Filter Room’ at Stanmore. This extended reporting system slowed down information being passed to Fighter Command but was designed to overcome the inaccuracies in the range and height measurements being interpreted as multiple targets. In fact, due to the dedication and very high skill levels of the operators, it proved to be an extremely successful and efficient system throughout the war and was critical to the success of the whole system.

Towards the end of the war, as hostilities were confined to the Continental arena, Truleigh Hill and its like were no longer seen to be of any great value and they were run down. Many of the CH, CHL and other sites were simply gutted and abandoned, others were vandalised. Some were put under ‘care and maintenance’ but a lack of manpower, or even a real interest in them, led to most of these also becoming run down.

This period was short-lived however, as a new threat was perceived to exist to the East of Europe. The Soviets first successful atomic bomb test and the subsequent ‘Cold War’ led to massive development programs in the UK defence system and Truleigh Hill was part of the system.