T~IE radio direction finder is a device for determining in a simple manner the direction of a radio transmitting station with reference to the point at which the direction finder is located. The direction finder has a number of very practical applications, of which one of the most important is its use as an aid to navigation.
Sound and visual signaling devices have been employed for many years as aids to navigation. Lighthouses and lightships with their characteristic light flashes and sound signals are established and maintained along the coasts and at harbor entrances in order that shipping may be carried on with maximum safety. During fog or thick weather, however, the sound and visual signaling devices do not give reliable service.
The radio direction finder is not affected by fog, and has the further advantage that it will operate over much greater distances than sound and visual signaling devices.
A common type. of direction finder, as developed by the Bureau of Standards for installation on shipboard, and as at present used, consists of a coil of ten turns of copper wire wotmd on a frame four feet square which is mounted so that it may be rotated abo.ut a vertical axis. Associated with the coil is suitable radio receiving apparatus and also a device for reading the radio bearing.
As the coil is revolved, the intensity of this signal which is being received from the station whose location is to be determined, diminishes until a minimum is reached, which occurs when the plane of the coil comes to a position at right angles to the line of direction to the radio transmitting station. At this point of mininmm signal, the radio bearing is read on a suitable scale, which mav be either a fixed scale, or the card of a magnetic compass. A radio transmitting station intended primarily for direction finding work is often called a "radio beacon."
This paper deals briefly with the principles of the operation of the direction finder, but is primarily concerned with practical develop,ment which has made possible a device sufficiently simple