Homing Devices

Author: J.B. Hoag

A homing device is mounted on a moving body and continuously points the way home to a fixed transmitter. In this apparatus, a loop and a vertical antenna are used to produce a cardioidal response curve like the one at the left in Fig. 34 H. A switch is incorporated in the circuits which automatically reverses the polarity of the loop at an audio-frequency rate. The response curve then shifts back and forth between the two patterns, solid and dotted, of Fig. 34 M.

Fig. 34 M. Principle of a homing device

The loop voltages, which are thus reversing in direction at an audio frequency, are amplified, rectified, and passed into a meter whose zero is at the center of its scale. If the two voltages e₁ and e₂ are equal in amount, as is the case when moving directly toward the oncoming radio wave, the meter reads zero. If, however, the ship or airplane is moving to the right of the true course, the two voltages e₁ and e₂ (see right side of Fig. 34 M) are not equal, and the pointer on the meter moves to the right. If off-course to the left, the pointer moves to the left of zero.

In a slightly different form, a double loop gives two currents which, at an audio rate, are alternately connected to flow through two magnets, one to pull a pointer to the right, the other to pull it to the left.

The extent of the left or right movements of the pointer is not a satisfactory measure of how far from the left or right the body is moving from the on-course path. An automatic homing device has been devised to overcome the difficulty. The pointer is fitted with contacts which turn on a battery to run a motor in one direction or the other. The motor is fastened to the shaft of the double loop and automatically rotates it into a position pointing to the transmitter whenever it gets out of line. The pilot does not need to turn the loop manually; he merely notes the angle of the loop as registered on its dial. If he changes his heading, the loop rotates so as to point toward the transmitter, and the dial reading measures the amount by which he is off-course.

Two complete automatic direction finders, tuned to two separate transmitters, are used in one device to control two electron beams in a vacuum tube. Two lines of light appear on the fluorescent screen at the end of the tube, giving the bearing of the stations. A map of the flight area is projected at the same time on the screen so that the pilot sees his location as the intersection of the two lines on the map. The apparatus is controlled in position with a gyro compass so that north, for example, is always at the top of the screen, regardless of the direction of flight.