"A and N" Radio Beacon

Author: J.B. Hoag

Instead of using a loop as a receiving antenna, one may use it as the transmitting radiator. A transmitting loop does not send out the same strength signal in all directions. The radio wave is stronger in the plane of the loop and falls off to zero at right angles, yielding a radiation pattern of the same shape as the receiving response curve.

A second loop, placed at right angles to the first one, will give a similar field-strength pattern, but rotated by a quarter of a turn. In use, energy from a common transmitter is connected first to one loop, then the other, back again to the first, and so on. While connected to the " A " loop, Fig. 34 N, it transmits a dot (of one second duration) and a dash (three seconds), the International Morse code for the letter A; and while on the other loop, a dash and a dot, the letter N.

Fig. 34 N. Principle of the A and N beacon

A plane, flying in the A-A direction, will pick up a series of A's; while one flying in the N-N direction will pick up the N signal. Along the A-N line of flight, however, the A's and the N's will both be heard in equal strength and, because they are properly interlaced, will give a continuous buzzing sound. The pilot follows a path which gives him a steady note in his earphones. He knows when he is off the beacon for then he will hear an A on one side or an N on the other. The farther off the beam, the more prominently does the letter sound above the steady background note.

At present, the beacon transmitters are located approximately every 125 miles apart along the main air lanes. Each transmitter has its own frequency, in the range from 200 to 400 kHz, corresponding to a wave-length range of from 750 to 1,500 meters.

In order to overcome the troublesome night error, it has been found greatly helpful to eliminate those parts of the loop which radiate energy in an upwards direction. This has been accomplished by the use of four towers arranged in pairs like those in Fig. 34 O, each supporting a vertical antenna wire, with each pair replacing a single loop.

Fig. 34 O. One-half of an A and N transmitting antenna system

The power from the transmitter is carried to the towers in buried coaxial transmission cables, a system which prevents the radiation of energy vertically. Opposite towers of a given pair are so " phased " that the horizontal field strength pattern is like that from a loop.

By changing the phase of the currents in the various towers, it is possible to so orient the field patterns as to give equi-strength or on-course signals in any of the preferred directions from the antennas.