Communication E V a

Direction Finding.

Wireless apparatus has been developed in recent years for determining the bearing of a distant transmit ting station with an accuracy of two degrees of arc at distances up to zoo miles, and with nearly equal accuracy at much greater distances, provided that the electrical conditions of the atmos phere are fairly stable. By means of the information thus ob tained a mariner or aviator can navigate his vessel during foggy weather, since fog has no bad effect on wireless signals. This branch of our subject has therefore become of very great im portance. Several methods are available and in active use. In one method the direction finder is situated at a land station, the bearing of the ship or aircraft from that station is measured on receipt of a wireless message from the vessel, and is trans mitted to it. In another method the direction finder is on the ship and the ship's operator measures the bearing of any charted wireless shore station that happens to be transmitting or which can be requested to transmit for the purpose of the measurement. This method has also been used in aeroplanes but is being dis carded. Still another method is seen in the so-called rotating "beacon," which is really a wireless transmitting station fixed on land and provided with a directional antenna that can be rotated in azimuth so as to sweep its signals round the compass as it rotates. The beacon automatically emits a characteristic Morse signal continuously and also a special signal when its directional antenna is in a standard position. An observer at a distance, equipped with ordinary receiving apparatus, hears the signal wax and wane as the directional antenna rotates. Usually the beacon rotates through six degrees per second, and emits the special signal when an observer on the north-south line would receive minimum signals ; therefore an observer off that line need only count the number of seconds that elapse between the special signal and the time when he himself perceives that the continuous signals are least audible, in order to determine his angle from the north-south line by simply multiplying by six. The easier way of performing this operation is to use a stop watch with a seconds hand which makes, like the beacon, a com plete revolution in one minute. The observer starts the watch on hearing the special signal and stops it at the instant of minimum signal ; the angle turned through by the seconds hand is his bearing from the beacon.

Two types of apparatus have been much used in practice in carrying out the above-described methods of finding the bearing of a ship or an aeroplane. In one form the antenna is a flat coil of several turns of wire, fixed on a vertical axis so that the plane of the coil can be pointed in any desired azimuth. Such a coil emits radiation most strongly in its own plane when used as a transmitter, i.e., when strong high frequency currents are passed through it ; it absorbs radiation most strongly in its own plane when used as a receiver. On the other hand substantially nothing is radiated or absorbed in the horizontal direction perpendicular to its plane. Such a coil, used as the antenna of a receiving station, can locate the direction of any distant transmitting station either by turning it until signals are strongest, when its plane must point to the station, or until signals are weakest, when it must be broad side-on to the station. The minimum is sharper than the maximum

and is therefore usually used in practice in order to gain accuracy. But measuring on the minimum implies that the message cannot be read ; so if reading is desired the maximum setting must be sought, or, preferably, the method designed by J. Robinson may be employed. This consists in fixing to a vertical axle two flat coils with their planes vertical and perpendicular to each other. One coil is always connected to the receiving apparatus and is pointed to absorb the maximum signal from the distant station under observation. The other coil is therefore roughly broadside to that station—roughly, because the setting of the main coil to the maximum is intrinsically an inaccurate process. To im prove this setting, the auxiliary coil is switched into series with the main coil, first with its ends connected one way and then the opposite way. If the setting is perfect, the auxiliary coil neither augments nor diminishes the signal strength ; but if it is imperfect one position of the switch augments, the other di minishes the signal, and the setting is altered by trial to abolish this difference.

In the type of apparatus just described the antennae are coils of wire which are small enough to be easily manipulated. In the other type, which was invented by Bellini and Tosi in 1907 and developed by H. J. Round of recent years, the antennae are also coils but they are not movable and may therefore be large struc tures. Two coils are necessary and they are fixed in perpendicu lar vertical planes ; as a rule each coil has only one turn and is supported on a mast or masts. Each coil is connected to one of a pair of small fixed coils inside the station building, these coils also being in perpendicular vertical planes. Within these fixed coils a smaller coil can rotate upon a central vertical axis, and this coil is connected to the detecting apparatus. The principle under lying the invention is that waves coming from any definite azimuth excite an antenna in proportion to the cosine of the angle their path makes with the plane of that antenna ; the oscillatory cur rent thus produced in an antenna passes through the correspond ing small connected coil ; the oscillatory field within the crossed coils has its resultant parallel to the direction of the waves; and the rotatable search coil is swung about to determine the direc tion of this resultant field. The combination of the two crossed coils and the search coil within them is known as a goniometer. It has been described in the preceding lines as applied to the reception of waves; but if powerful oscillatory currents be sent from any source into the search coil these induce currents in the crossed coils and the connected antennae which produce external radiation whose resultant direction is parallel to that of the search coil. Thus a radio goniometer may be used for directional transmitting as well as for directional receiving; in fact the Bellini-Tosi invention has effectively the same properties as the single moving coil antenna already described, both for receiving and transmitting, the principal difference in receiving being that more amplification is required with the moving coil antenna be cause it must be small enough to be rotated quickly by hand.