The AT20 HF radio transmitter was built by Standard Telephones and Cables Pty Ltd of Sydney for use by the Australian military forces in World War II. The Airways Museum's unit was built in 1945.

This transmitter was a single channel, non-tunable unit capable of operation in the 2-20 Mc/s band (2-8 Mc/s was the civil aviation band).

After the War, surplus AT20 transmitters were purchased in large numbers by DCA for general air-ground and point-to-point use until equipment designed to DCA specifications arrived in the 1950s. The AT20s were then used for many years as 'back up' equipment.

The equipment initially suffered from power supply (mercury vapour tube) problems but eventually proved reasonably reliable provided the tray plugs and tuning mechanism were properly maintained.

The AT20 was designed to be portable for military use and comprised two cabinets, each nominally able to be manually lifted, which could be stacked either vertically or side by side. Each cabinet was divided into a number of trays, from top to bottom as follows: