Three SHF valves, 1939-1945.
© Science Museum / Science & Society Picture Library
SHF (super high frequency) radio waves have a frequency of 3-30 GHz and are used for radar. Radar (radio detection and ranging) revolutionised British offensive and defensive capability in World War II, playing a vital role in locating incoming German bomber formations during the Battle of Britain in 1940. It works by bouncing powerful, ultra-short radio waves (microwaves) off distant objects. By measuring the total time taken for the beam to travel to the target and back, it is posible to calculate how far away it is. The bearing of the target is obviously the direction from which the reflected beam is received.