Ferranti R3121A Mk III Radio Receiver

Ferranti Ltd model R3121A Indicator Friend or Foe Mark III valve radio receiver, c.1940.

Ferranti Ltd model R3121A Identification Friend or Foe Mark III valve radio transponder, c.1941.

The Ferranti Ltd model R3121A was part of an important Second World War electronics system that helped radar operators tell friendly aircraft apart from enemy aircraft. Manufactured by the Manchester-based Ferranti company, the R3121A was installed in aircraft as part of the IFF Mark III system. It functioned as a radio transponder, detecting a radio signal from a radar station and sending back a signal of its own.

In the late 1930s Britian established a network of military radar systems, with radar stations built around the country and connected to control rooms. The radars could spot enemy aircraft from long distances, allowing the control rooms to direct fighter planes to shoot them down. However, enemy aircraft and friendly aircraft both looked the same on a radar screen. Identification Friend or Foe (IFF) was developed to help tell them apart. An IFF system works by fitting an aircraft with a radio set that detects radio waves from a radar station and responds by sending back a signal of its own that tells the radar controllers that the aircraft is friendly.

As radar became widely used during the Second World War, early IFF systems were unable to keep up with the many different wavelengths of radar in use. IFF Mark III was developed in response, based on an idea by Stockport-born Freddie Williams, later one of the developers of the Manchester “Baby” computer. Instead of being tuned to a particular radar IFF Mark III responded to a separate radio system installed alongside at the radar station, offering a number of advantages over the earlier IFF system. It was widely installed in Allied aircraft during the Second World War.