SCR-269-G Radio Compass

American SCR 269-G Radio Compass.

The radio compass is basically a radio receiver fitted with a loop aerial. The loop aerial itself is very directional and if rotated through 360 degrees a signal will disappear quite sharply when the loop is square on to the station that is being received and at a maximum when the loop is in line with the bearing of that station. The null and peak will obviously occur at two points in the rotation and providing precautions are taken to ensure that the correct point is chosen, the direction of the station can be determined using the null, that being the most precise point to detect. If the bearing of two or more stations can be measured, the stations identified and if their map locations are known ones position can be determined.

The photograph shows my own loop, the casing was already broken when I found it so it was cut away to show the actual loop aerial inside. The motor/gearbox and the position transducer is in the lower part of the housing where the part number is shown.

Radiolocation using this method appeared well before WWII and was used in aircraft in the 30's. The British R1082, a battery powered radio, the technology of which almost dates from the 20's, could be fitted with a DF loop and the American Bendix Company developed a specialised piece of equipment, the MN26 for commercial aircraft use in the 30's. The familiar R1155 also had provision for DF capabilities but all these systems used manual control to rotate the loop in order to find the appropriate null. The SCR 269 was different in that as well as being a dedicated Radio Compass it could automatically rotate the loop to the correct position without manual intervention with the loop position being displayed on remote reading meters.
My own equipment consists of a BC- 433-G Receiver, a BC- 434-A Control Unit, an LP-21-A Loop, Indicator Unit type I-81-A and Indicator Unit type I-82-A. The equipments operated from a 115v 400Hz supply together with a 24v DC supply for the relays. It was possible to fit two Control Units, together with a Relay Box (BK-22-A), allowing the equipment to be operated by either the Pilot or the Navigator, but my own equipment uses only a single Control Unit.

In order to demonstrate the equipment to the general public I mounted all the equipment on a display panel (see picture), all that is except for the receiver and powers supplies that weighs a ton.

The equipment is powered from a 24v DC supply with a square wave static inverter operating from the same supply in order to produce the 115V 400 Hz. RF noise from the inverter was a problem at first but the equipment now functions reasonably well from square waves. The receiver tuning is by Bowden Cables from the Control unit, frequency range changing by a motor in the receiver, again operated from the Control Unit.

The equipment is simple to use and to demonstrate. A switch on the Control Unit has three positions labelled OFF, COMP, ANT and LOOP. In the LOOP position the loop can be turned manually using a key switch (labelled LOOP) to drive the loop motor at a fast or slow rate. In the ANT position the receiver is switched to a long wire antenna, essential for the operation of the auto locate system but also used to tune in the appropriate station. By switching to the COMPASS position the system automatically turns the loop to the correct bearing which is displayed on both Indicators.