There were working radar systems before Chain Home, but they were large, power hungry and provided only the most basic information; the direction in which a target might be found. Robert Watson-Watt and his team were the first to develop an effective range finding radar by timing the delay between the outgoing pulse and the return from the target.
Chain Home had to operate at an effective range: far enough to allow fighter aircraft in the air to be notified and ideally to allow planes on the ground to be sent up. The theoretical range of a radar system can be derived from a number of operational parameters to give the Range Equation, a fundamental idea in radar and one which appears on the blackboard several times in “Castle in the Sky”:
Leaving the constants aside, the various symbols are
- R is the range – how far away the target is
- PS is the transmitter power – the more you can send out the more likely you are to get some back
- G is the antenna gain – how efficiently the aerial can convert electrical signals into radio waves and vice versa.
- λ is the wavelength – long wavelengths work better than short ones
- σ is the radar cross section of the target – the bigger it is, the more signal is sent back
- PE is the received power at the antenna – the more sensitive the equipment attached to the antenna is, the smaller the signals which can be processed and the further away the target which can be detected. When the received signal is the smallest possible, the range is the maximum possible.
When Chain Home was being developed, the aerials used had to be large for several reasons:
- to provide enough aerial gain (efficiency)
- to give strongly directional reception, needed to establish the direction of the target
- to allow the use of longer wavelengths. Aerial size is generally proportional to wavelength, and Chain Home used signals of around 12m – ludicrously high by modern standards and high even in comparison with later WW2 systems.
These large antennae had many disadvantages. They were expensive to build, they were obvious to the enemy and they were easy to attack – though, curiously it seems to have taken some time for the Germans to decide that it was worth doing so. They also had to be set up to send signals a long distance and as a result were unable to detect lower, closer aircraft.
Making use of a shorter wavelength
To fill this gap, a second system was added, called Chain Home Low, which used a much shorter wavelength (around 1.5m) and could therefore have much smaller antennae. Some of these were mounted on lorries to make portable radar systems and some used steadily rotating antennae – as we are now familiar with on ships and at airports.
Having smaller antennae was only half the battle, though. The transmitting – and to a lesser extent the receiving – stations also had to be reduced in size, and that meant significant improvements in radio frequency engineering technology. Some of this improvement was underway before the war; Watson Watt and his team were able to make use of transmitter valves which had been developed for the Marconi electronic TV system, but even so radar transmitters were large and power hungry.
The breakthrough needed for smaller radars came at the University of Birmingham in 1940, where John Randall and Harry Boot invented the cavity magnetron, a very small microwave generator which works by passing electrons pass the openings to a series of cavities. This process isn't unlike producing a note by blowing across the mouth of an empty bottle.
The magnetron revolutionized wartime radar allowing not only much more powerful and accurate ground-based systems but also the development of airborne radar, starting with the H2S system which was fitted to Stirling and Halifax bombers in 1943 and still in use on RAF Handley Page Victors fifty years later.
Although magnetrons were powerful and relatively efficient, their output was not particularly stable and they have largely been replaced in radar systems by now. However, they are still very widely in use as the source of microwaves inside microwave ovens, and it has been estimated that over one billion have now been produced for that use.
This article is part of the Radar: What happened next? collection, which looks at different aspects of radar technology since the introduction of Chain Home, the first functioning radar defence system developed during the Second World War. This collection was inspired by the OU/BBC drama Castles in the Sky, featuring Eddie Izzard as Robert Watson-Watt, the father of radar.