Radio Compass Indicator I-81-A of SCR-269-G and AN/ARN-7
The I-81-A Radio Compass Indicator (aka Pilot's Bearing Indicator) is a component of the Radio Compass SCR- 269-G system (which evolved into the AN/ARN-7), used during WWII. According to the Airborne Radio Equipment Handbook of the Air Service Command dated 1943, this system appeared in versions of the US Army Air Force bombers B-17, B-24, B-25, B-26, B-29, and transports C-46, C-47, and C-54, as well as in any models of these aircraft used by the US Navy. The system was designed primarily to be used as a navigational instrument (see photos of system),and this is the indicator which appeared on the pilots instrument panel to provide directional indication for the signal being received from the transmitting installation, e.g., an airfield or other navigational beacon.
The equipment is basically a 15 tube superheterodyne radio receiver with the addition of certain essential circuits necessary for radio compass operation. The equipment is designed for remote control operation and may be installed with provisions for dual control or for signal control. For dual-control installations, two remote controls are provided and, although only one remote control functions at one time, control may be readily switched from one to the other. When installations are made using only one remote control, no switching of control is necessary and the one radio control box has control at all times. When used in conjunction with a suitable nondirectional (vertical) antenna, one or two head sets, a 28- volt direct- current supply, a 1 1 5- volt, 400- cycle power supply, and necessary interconnecting wiring, radio compass SCR-269-G is a complete operable unit capable of providing: (a) automatic bearing indication of the direction of arrival of radio frequency energy and simultaneous aural reception of modulated or unmodulated radio frequency energy; (b) aural reception of modulated or unmodulated radio frequency energy using a nondirectional antenna; (c) aural reception of modulated or unmodulated radio frequency energy using a loop antenna (d) aural-null directional indications of the arrival of modulated or unmodulated radio frequency energy using a loop antenna.
The indicator is in reasonable condition given its age. It is mounted on a single aluminum panel. Measures 3.25 inches diameter by ~ 4 inches deep.