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The 721A is a type of radar T/R switch. Pulsed radars using large dish-shaped aerials pointed in the direction of search need to use the same aerial port alternately for transmission of high-power RF pulses (megawatts) and for listening for the faint echoes (microwatts) between pulses. It is necessary to prevent 'spillage' of transmission power from entering and damaging the highly sensitive receiver connected to the same aerial.

One means of doing this is to mount one or more 'T/R cells' (switches) in the waveguide branch leading from the aerial port to the receiver input. The thin copper flanges of the cell are clamped to the upper and lower walls of the waveguide at some point calculated to coincide with a node of the standing wave pattern. The T/R cell contains a rarified gas (often water vapour) which ionises when RF power passes down the guide. Wanted radar echoes are too weak to ionise the cell so the waveguide 'looks' transparent to ordinary received signals. Excess transmitter spillage, however, ionises the cell and the ionised gas absorbs most of the RF power, thus 'blocking' the waveguide and protecting the receiver.

Unfortunately, full ionisation of the cell can take an appreciable fraction of a microsecond so that some part of the leading edge of the transmitter pulse may get through to the receiver before the cell gives full protection. This can be overcome by 'priming' the cell by applying a moderately high voltage to the priming cap seen on one end of the cell. This causes local ionisation inside one of the (hollow) gap electrodes and enables ionisation to spread almost instantaneously to the whole cell if there is any whiff of RF spillage.

In some applications a modest priming voltage may be applied continuously. In cases where a more substantial priming is necessary the priming voltage may be applied in the form of a pulse overlapping the RF transmitter pulse.

The wide glass tube envelope is 27 mm in diameter and the overall length is 75 mm.

Reference: Observation & private communication.