Chain Home and Chain Low
The pioneering work that Watson-Watt managed at the Bawdsey Research Station resulted in the design and installation of a chain of radar stations along the East and South coast of England in time for the outbreak of war in 1939. This system, known as Chain Home and Chain Home Low, provided the vital advance information that helped the Royal Air Force to win the Battle of Britain. But the development of the system was not easy and we owe the miraculously rapid start of radar not only to Watson-Watt’s imagination and drive but also to his skill in cutting red tape.
The first demonstration of the military uses of radar was nearly disastrous. It took place in September 1936; an air exercise was planned in which a formation of bombers would approach the coast and a chain of five radar stations would report their position to Fighter Command so that they could be intercepted. In the event the only station ready in time was the one at Bawdsey Manor, which had only just been completed and hadn't been properly tested. It worked on a wavelength of 23 m, a receiving array consisting of a vertical stack of three sets of crossed dipoles and a six element transmitting array mounted on 75 m wooden towers.
On the first day of the exercise the scene in the old stables of Bawdsey Manor was impressive. A line of three radar operators sat silently in front of their cathode-ray tubes; silently because there was nothing to report. Watson-Watt hovered about anxiously giving a running commentary on this non-event. Behind them in semi-darkness stood a row of VIP's looking over their shoulders. They included Lord Swinton, Harry Wimperis, Sir Henry Tizard and the Commander-in-Chief of Fighter Command, Air-Vice-Marshal Sir Hugh Dowding. Far out at sea an "invading" force of flying boats from Felixstowe turned and headed for Bawdsey. In the stables the three operators watched their cathode-ray tubes with increasing embarrassment since no echoes could be seen. By this time Watson-Watt had switched from promotion to mitigation. Finally even he stopped talking and in the ensuing deep silence, the faint hum of many distant engines was heard and, at the same moment, some weak radar echoes were seen! There was only just enough time to send a few hastily measured positions to Fighter Command before the aircraft roared overhead. As a first major demonstration of the use of radar for air defence it could not have been worse; a good sound locator would have done better.'
As was soon discovered, the trouble was that the transmitter was not putting out enough power, and when this was put right the performance of the station during the next few days of the Exercise was much better. Nevertheless it was a close shave; the construction of the chain of radar stations might well have been delayed if Watson-Watt hadn't personally persuaded the Chief of the Air Staff to decide, on the basis of the exercise, that 'the RDF system was already proved'. For that we should be profoundly thankful; if the radar chain had not been working in 1940, only four years later, we might well have lost the Battle of Britain.
In this first exercise, the technique of reporting aircraft positions to Fighter Command was not a success. The original plan was to send them via telephone lines to a device at Fighter Command which showed the position of the aircraft as a bright spot on a map drawn on the face of a cathode-ray tube; but the whole thing, which might have been designed by Heath Robinson, proved so unreliable that it was soon discarded and the results were telephoned. As already stated, Watson-Watt's aim was to develop a new system of air defence of which the technique of reporting the results to Fighter Command was an integral part. In close collaboration with the RAF the group at Bawdsey developed the so-called 'filter room' in which the raw data from several radar stations were correlated and assessed before being passed on to the people whose job was to alert and control the fighters. Coupled with some experiments on controlled interceptions at Biggin Hill, prompted by Tizard, this work led to the remarkably effective operations rooms used by Fighter Command in the Battle of Britain
A second air exercise was staged in 1937 using three of the original five radar stations and was sufficiently successful for the future of the coastal chain of radar stations (Home Chain-CH) to be firmly assured; in August 1937 an order was placed for twenty more stations. Construction went ahead at a great pace and by the outbreak of war there were about 19 stations on the east coast and six on the south, giving radar coverage from the north of Scotland to Portsmouth. By the end of the war there were some 50 CH stations giving coverage more or less all round Great Britain.
A typical CH station on the east coast was equipped with four 110 m steel towers which supported the main transmitting array, a vertical stack of eight half-wave dipoles. The transmitter used water-cooled, continuously evacuated demountable tetrodes to radiate about 350 kW at one of four spot frequencies in the range 20-30 MHz; the pulse width was 20 us and the pulse recurrence frequency was 25 or 12.5 Hz. The receiver was equipped with either three or four wooden towers (73 m high) for different spot frequencies; each tower supported a pair of crossed dipoles (with remotely switched reflectors) at a height of 65 m; there were also auxiliary dipoles at 30 m and 15 m which were used for height finding and filling gaps in the vertical polar diagram of the main system. At the receiver, direction-finding was accomplished by comparing the signals from the crossed dipoles in a goniometer and the 1800 ambiguity in the direction of the target (the 'sense') was resolved by using the switched reflectors. Height-finding was accomplished by measuring the ratio of the signals from the dipoles at 65 m and 30 m with a goniometer and feeding this ratio, together with range, into an electro-mechanical computer which calculated the target's height. The receiver itself had low-noise pentodes (EF8s) in the input stage, a triode-hexode mixer and an IF frequency of 2 MHz with optional bandwidths of 500, 200 and 50 kHz. The receiver output was displayed on a cathode-ray tube whose timebase could be calibrated with marker pips. The typical maximum range on a medium-sized aircraft depended on its height: for an aircraft at 3000 m it was about 130 km and for an aircraft at 6000 m was about 200 km.
The 'invention' of radar
It is often pointed out that Watson-Watt was not the only man to invent radar. For example, Sean Sword's book 'Technical history of the beginnings of radar' notes that the suggestion that radio waves should be used to detect distant metallic objects was made several times before 1935, notably by Christian Hulsineyer in Germany in 1904. Young and Page carried out experiments with pulsed radar in the USA at about the same time as Watson-Watt's team at Orfordness. Indeed, although Watson-Watt may not have been the first man in the world to invent radar he was the first man to apply it successfully to an urgent and important problem and at the right time. To quote one of the most difficult bodies to convince of the value of an invention, the Royal Commission on Awards to Inventors:
'At four critical periods of the war, radar made a major contribution to success:
1 in the Battle of Britain;
2 in defence against night bombing on which the enemy concentrated after his defeat in the Battle of Britain;
3 in dealing with the submarine menace in the critical years of 1941-42;
4 in enabling our bombers to reach their objectives in Germany and to drop their bombs with a high degree of accuracy.’
Robert Watson-Watt was knighted in 1942. Immediately after the war he acted as Scientific Adviser to various Ministries and leading delegations to international meetings on radio aids to marine navigation and civil aviation. Later (1947) he set up a private firm of consultants in Westminster: 'Sir Robert Watson-Watt and Partners'. In the early 1950's he moved to Canada and later to the USA then returned to the UK. He and his wife lived in London during the winters and had a summer home in Pitlochry Perthshire. His wife Dame Kathryn died in 1971 and Sir Robert died two years later in Inverness on the 5th of December 1973.
The Proposed Memorial to the father of radar, Sir Robert Watson-Watt
AN INTERNATIONAL fund-raising appeal has been launched to commemorate the father of radar, Sir Robert Watson-Watt, with a £50,000 memorial in his hometown of Brechin. The Watson-Watt Society of Brechin, established a few months ago, is now a registered charity, having been recognised by the Office of the Scottish Charity Regulator, and Dr Robert Martin, the Chairman of the Society, has kicked-started the fund raising campaign with a personal donation. Dr Martin said that when he retired in January 2006 he received a generous presentation from patients and he wished to put back into the community with a donation to start up the Watson-Watt fund. Statues have been erected to some of our distinguished airmen of the last war, such as Air Marshall Sir Hugh Dowding from Moffat in Dumfriesshire and Air Marshall Sir Arthur Harris, but not to one of our most distinguished electrical engineers, Sir Robert Watson-Watt who made their achievements possible. That is, of course, partly because the British culture habitually under-estimates the value of science and engineering but also because Watson-Watt's achievements were not as visible or comprehensible nor as dramatic as those of the airmen.
So I would urge all who may read this to make a donation, small or substantial, to the Watson-Watt Society of Brechin’s fund. Contact the Society by telephoning 01356 624356 or e-mail: bmitch@mmitch58.wanadoo.co.uk
