How microwaves changed the course of the Battle of the Atlantic

Sir Robert Watson Watt is credited as the inventor of radar. In Britain radar was known as RDF (radio direction finding). The way that radar works is that pulses of microwave radiation of controlled frequency and polarisation are emitted from a transmitter. Some of these microwaves reach an object (an aircraft or submarine for example) directly or after reflecting off the Earth or sea surface. For radar to work, the object must be metallic or have a partially conducting surface that absorbs and re-radiates the radio waves.

The time for pulses to travel directly from transmitter to receiver and indirectly after pulses from the transmitter are re-radiated from, for example, a hostile aircraft (the ‘echo’) are measured. In practice the distance between transmitter and receiver is small and can be neglected so that the location of the aircraft can be determined as half the distance from transmitter to aircraft and back again. Results are displayed on an oscilloscope that identify the echo, the re-radiated signal. Radar played a critical role in the Second World War, especially in the Battle of Britain in 1940, and the value of the contribution made by the air tracking radar system developed by Watt in the 1930s is immeasurable.

The system consisted of high-frequency direction-finding radio detectors (HF-DF). In 1938 the Royal Air Force erected a series of tall pylons in a chain along the east and south coasts of England. Chain Home, as the project would be code named, could detect objects in the sky 100 miles away and relay the information to a home defence network. This gave traffic controllers the direction and size of incoming aircraft before they reached the English coasts, allowing Flight Command to prepare itself. For example, on 14 August 1940 there was a flank attack by the Luftwaffe’s No. 3 fleet (from Norway) against Newcastle,  the aircraft were intercepted and 15 German bombers were destroyed with no RAF losses. This interception would not have been possible without radar.

The Allied merchant fleet lost a massive 7.3 million tonnes over the course of 1942, about 6.3 million tonnes of which had been sunk by German U-boats. By early 1943, British imports were one-third less than those in 1939. Many merchant seamen died. This loss of shipping threatened everything in the British war effort. The population faced mass starvation while heavy losses suffered by oil tankers meant only two or three months of fuel remained in Britain’s storage tanks.

Some, but not all, ships possessed HF-DF detectors, and HF-DF detectors could not detect submarines. While U-boat radio emissions could be detected 20 miles away from a convoy, a plane dispatched in that direction did not have a system for detecting small objects such as a conning tower on a submarine. The Allies needed a miniature radar system that could detect such small objects. The breakthrough was made by John Randall and Henry Boot in the spring of 1940 in a wooden building at Birmingham University. They invented the cavity magnetron, which generated microwave pulses with a wavelength of 10 cm, was the size of a saucer and from late spring 1943 could be fitted in RAF aircraft. The device did not crack or melt even though it emitted high energy microwaves.

In September 1940 at the height of the Battle of Britain the Tizard Mission arrived in the United States from England, bringing a prototype cavity magnetron. James Baxter wrote in Scientists against Time (1946) that the cavity magnetron was, “the most valuable cargo brought to these shores.” This device was far ahead of any comparable miniature radar device under development in the United States. In March 1943 squadrons of fully equipped aircraft began to join the Allied forces in the Battle of the Atlantic. Now the Allies could search out and destroy German U-Boats from the air with unparalled accuracy. US industry had manufactured one million cavity magnetrons before the end of the Second World War.

After the war, the United States increased its research and development into the cavity magnetron and microwaves in general. Among the leaders in the field was the physicist Percy Spencer. There is a story that while working at the defence contractor Raytheon, Spencer walked past a bank of equipment that was generating microwaves, and the chocolate bar in his pocket softened, giving him the idea that microwaves could be used to heat food. That led very quickly to the development in the late 1940s of the first type of microwave oven, the early models of which contained cavity magnetrons.

Featured Image: Convoy WS-12 en route to Cape Town, 1941‘ used with permission from Wikimedia Commons.