The Radio Compass or Goniometer

Author: J.B. Hoag

In order to determine the line of direction of a given transmitter, the loop is slowly rotated about a vertical axis. When the output of sound, or the reading of an output meter is a maximum, the plane of the loop is in line with the transmitter. When the output is a minimum, the plane of the loop is at right angles to the line of propagation. In practice it is found possible to judge the minimum more accurately than the maximum.

The reason for this will be clear from a study of Figs. 34 C and D.

Fig. 34 C. Intensity of output as the loop is rotated

Fig. 34 D. Sharper direction finding can be obtained using the minimum rather than the maximum output of a loop

A horizontal pointer is fixed to the vertical shaft of the loop and moves over a circular disc marked off in degrees. The loop is rotated until the radio signal can no longer be heard, or is as faint as it can be made. At that position, the plane of the loop is at right angles to the line joining the receiver and the transmitter. The use of the minimum has the disadvantage that it may be obscured by background noises from other stations, static, etc.

This simple compass, however, shows only the path of the radio wave. It does not tell whether the transmitter lies ahead of, or behind, the receiver.

Cross bearings may be taken on two transmitters. The receiver is tuned to one station, its loop is rotated to a minimum, and the angle noted. This procedure is repeated for the second transmitter. The two directions are then laid out on a map. Their intersection is at the location of the receiver. See Fig. 34 E.

Fig. 34 E. The method of cross bearings

A different method, often used by commercial airlines in the United States, when a plane is flying along a known path, say along a radio beam, is to fix the loop with its plane the same as the heading of the airplane and to tune the receiver to a station located somewhere off the known path. See Fig. 34 F.

Fig. 34 F. Position fixing

Then, just as the airplane passes directly opposite the transmitter, the signal drops to a minimum. From his maps, the pilot may then say that he is on such-and-such a path and is so-and-so many miles due south, say, from the particular town where the transmitter is placed.