A proximity fuze (also called a Variable Time (VT) fuze; shells using these fuzes can be seen referred to as "VT shells") was a fuze designed to detonate an artillery shell automatically when the distance to target became smaller than a preset value or when the target passed through a given plane.
Proximity fuzes operated on different sensing principles, including:
Radio Frequency Sensing
Radio frequency sensing was the main sensing principle for VT shells.
During the Second World War, the principle worked as follows:
The shell contained a radio transmitter which used the shell body as an antenna and emitted a continuous wave of roughly 180 - 220 MHz. As the shell approached a reflecting object, an interference pattern was created. This pattern changed as the distance to the object diminished and every half wavelength in distance, the transmitter was in or out of resonance. This caused a small oscillation of the radiated power and consequently the oscillator supply current of about 200 - 800 Hz, the Doppler frequency. This signal was sent through a band pass filter, amplified, and triggered the detonation when it exceeded a given amplitude.
Previously, detonation of artillery shells required direct contact with the target, or else a timer set at launch, or (in the case of anti-aircraft shells) an altimeter. The immediate effect of the proximity fuze was that shells only had to pass close by a target in order to detonate, rather than make direct contact (hence the name).
The proximity fuze was invented in the UK in 1940, but developed mainly by the United States in collaboration with the British. The US Office of Scientific Research and Development (OSRD) credited the VT shell for three significant outcomes:
From George G. Blackburn's Where The Hell Are The Guns?:
Optical sensing was also developed in the Second World War, using a toroidal lens that concentrated all light out of a plane perpendicular to the missile's main axis onto a photo cell. When the cell current changed a certain amount in a certain time interval, the shell exploded. These fuzes were mainly used for anti-aircraft work. More modern fuzes (such as in air-to-air missiles) used lasers to trigger the detonation.
Acoustic sensing used a microphone in a missile and was still in development by the Germans when the Second World War ended.
Magnetic sensing was used in naval mines and torpedoes, triggered by magnetic detection of the masses of iron used in ship hulls.