This paper relates to loop radio transmitters for direction-finding or navigation purposes, and particularly to a method of directional transmission which has been developed by the Air Ministry at the Instrument Design Establishment, Biggin Hill, Kent, and later at the Royal Aircraft Establishment, Farnborough, Hampshire. The chief object of the method is to eliminate direction-finding equipment and to reduce operational difficulties in the aircraft. The method is one in which a characteristic of the radiation is “timed” by means of a chronograph, the time interval between a Morse signal and the characteristic indicating the bearing. A loop aerial is employed and the electromagnetic energy radiated from the loop is a maximum in one direction and a minimum in another. The loop rotates in azimuth at a speed of 1 revolution per minute and is made to send out a continuous signal. This signal is interrupted when the line of minimum radiation is in the true North direction, and a special Morse signal is transmitted at that moment to enable an observer to start a chronograph. The observer notes the time interval between “North signal” and the instant at which he perceives minimum radiation, thus obtaining his bearing from the beacon.The paper describes the various stages in the development of the beacon and refers to certain difficulties encountered and how they were met.An appendix gives typical results obtained from tests on the ground and in aircraft during experimental trials.From the results obtained, it is considered that bearings can be determined with an accuracy at least equal to that of any other wireless system of direction-finding. The work of the authors has been more particularly in connection with the use of the rotating-loop system for the navigation of aircraft, and for ranges up to about 200 miles the results show that bearings can be determined with such accuracy as is required for the purposes of aerial navigation.The investigations as to its use on ships is a matter which the Radio Research Board have undertaken.