The origins of Radar - 'Sound Mirrors'

Following the air raids on London during the First World War, it became obvious to the Air Ministry that any future conflict would undoubtedly include the use of enemy bombers flying over British cities. In November 1932, a speech was given in the House of Commons by Stanley Baldwin, in which he concluded that in any future war, expenditure on deterrents against attacking aircraft was a waste of time as "...the bomber will always get through".

Though a widely held opinion at the time, thankfully it was not the belief of a few, determined men who felt sure that some technology, perhaps unknown at that time, could be brought to bear on the problem. During the First War, several forms of sound detector had been used to locate enemy gun positions. These small dish-shaped detectors were now considered as possible detectors for aircraft and were to be scaled up accordingly. Experiments were carried out in great secrecy along the cliff-tops near Dover with a variety of concrete sound reflector mirrors that had been invented by Dr. William Tucker. Through the late 1920s and into the early 1930s a series of massive, long-range aircraft detector systems, were constructed along the coast from Romney Marshes, near Hythe towards Dungeness.

The 'sound mirror' above is a 200-foot long wall

Built of reinforced concrete, these enormous 'ears' would, it was hoped, be large enough to pick up the noise from an approaching, but still distant aircraft, and focus the sound by means of a probe placed at the centre of the dish to a central listening position. This probe was nothing more elaborate than a collecting horn for the amplified sound collected by the huge dish. From this, it was expected, a bearing could be determined.

This is one of the smaller 'sound mirrors', but is still over 40 feet in diameter

The massive dish itself was nothing more than a huge amplifier, with the operator listening at the base of a periscope tube, down which two sound tubes fed directly into a stethoscope-like apparatus. The listener would then wait for the approaching aircraft, take a bearing and pass the information on to headquarters nearby. When two or more bearings were taken from several sound detectors along the coast, the direction of the aircraft could be calculated.

There was however, an unforeseen problem with the system. Not only did it pick up the sound of an approaching aircraft, there was nothing to stop the dish from picking up and amplifying the sound of everything else on Romney Marshes. This included sheep grazing, passing cars and seagulls flying overhead.

In summer, the 200-foot wall of one of the mirrors was often used as a giant wind-break by picnickers; during one demonstration set-up for the Air Ministry, their officials were almost deafened by the sounds coming from a horse-drawn milk float.

Earliest Experiments - 1935

The initial demonstration was to take place on Tuesday, 26 February 1935, just 13 days after the meeting at Farnborough, which indicates the urgency with which The Air Ministry was treating the matter. The RAF loaned a Handley Page Heyford bomber (K6902) for the experiment to act as a 'target' to be tracked.

The Heyford was a slow and lumbering aircraft which, having first flown in 1930, was already some years out of date by 1935. As an aircraft design it was remarkable for few reasons, perhaps the most obvious of which was it was only capable of a maximum speed of 142 mph; it was also the last heavy bomber of the biplane design to serve with the RAF. For the purposes of the test however, with a wing span of 75 feet, it did provide quite a large object to 'aim at' in the sky.

The test was to be carried out in a field outside the little town of Weedon, near Daventry. The Heyford was instructed to fly on a path between Weedon and the BBC transmitter at Daventry. The detection equipment used consisted of a rather large receiver which was fitted with one of the early oscilloscopes, all of which had come from the laboratory at National Physical Laboratory, and tuned to the 49-meter wavelength that the BBC transmitter worked on.

The equipment was loaded into the back of a small van and driven on the evening of Monday, 25 February 1935, to the field, where Arnold Wilkins and an assistant prepared, in the cold and the dark, for the test the next day.

The pilot of the Heyford, Flight Lieutenant R. S. Blucke, took off from Farnborough the next morning, and climbed to 6,000 feet and started to fly the course on his flight plan.

In the van Arnold Wilkins and his assistant tuned their radio receiver to the frequency of the BBC transmitter at Daventry. As the Heyford bomber flew overhead, the steady signal of the transmitter which was being received and displayed on the oscilloscope, began to move up and down, indicating that a measurable amount of radio energy was being reflected from the aircraft above.

The Air Ministry men in the van watched as the signal indicated the aircraft in their vicinity, they were able to track it for nearly five minutes (which corresponded to a range of approximately eight miles). The experiment was a complete success and had proved conclusively that detection of aircraft with radio means was possible.

From that first test, an initial sum of £10,000 was granted to continue the work, staff were drawn from the people at the Radio Research Station and these included Watson-Watt and Wilkins of course. Radio Detection Finding (RDF) as it was known, was immediately shrouded in the highest level of secrecy - Radar, had been discovered.

From 'Chain Home' to H2S

Following the success of the initial February 1935 demonstration, the team of early RDF developers had now moved to the remote site of Orfordness, on the Suffolk coastline, 90 miles Northeast of London. Here they began the erection of the first experimental radar system. In mid-June 1935, they had succeeded in detecting radar echoes from a flying boat at a range of 17 miles, and by September the range for the detection of aircraft had increased to 40 miles. By the end of 1935, they had increased the range so that they could detect aircraft as far away as 80 miles.

In March 1936, the Orfordness group were moved to Bawdsey Manor a little further down on the Suffolk coast. By this time plans were being put into action to construct enormous radar chain of detection aerials all around the eastern coastline of England and Scotland. The first of these were built between June 1936, and June 1937, and became part of the system that would eventually become known as the Air Ministry Experimental Stations, Type One, or 'Chain Home' (CH).

Chain Home consisted of a series of enormous towers, 300 feet high which started to appear all along the coastline of Eastern Britain. It was the Chain Home system which was used to such dramatic effect during the Battle of Britain in 1940, to guide RAF Fighter pilots towards incoming German bombers.

Later in the war, a radar system was required to aid Bomber Command to find and identify targets in Germany. This was the system which became known as H2S. H2S produced a map-like image of the ground below, allowing quick identification of an area and even specific targets.

Alan Blumlein was head of the EMI team which was responsible for developing electronic circuitry for the H2S radar programme. It was while working on H2S that Blumlein and several of his colleagues were killed during a demonstration flight in a Halifax bomber.

Despite the loss of Alan Blumlein and other key members of the H2S development team, the project was completed. H2S went on to become one of the most important radar developments of the Second World War, allowing accurate bombing of enemy targets with a precision never before achievable.

It has often been said that the atomic bomb ended World War Two, but radar won it.

If that is the case, then H2S was the most important of those radar systems which won the Second World War, and it was Alan Blumlein's input to the development of H2S that was critical.