THE LOOP AERIAL

Radio Record, Volume I, Issue 10, 23 September 1927, Page 5

THE LOOP AERIAL

REMARKABLE PECULIARITIES _

It has been ‘pointed ont that radio waves are an invisible sort of light waves. They travel outward in all directions from the transmitting station’ just as the light from a powerful beacon reaches to all quarters of the hotizon. Yor broadcasting purposes this is a great advantage, siuce it enables the radio waves to cover all the territory around the transmitting station practically impartially. (There are some regions around a transmitting station which may be partially shielded from the radio waves by great groups of. steel buildines. hills containing metallic ore de-

posits and the like, but otherwise the waves travel fairly indifferently in all directions and are capable of rendering equally good service at all points which are at the same distance from the trans-| mitting station). It is this feature which gives radio broadcasting its peculiar adaptability for covering an area in contradistinction to wire commutnica‘tion, which, by its nature, is readily adaptable to the connection of two fixed points on land. While it is possible to trausmit radio messages, par‘ticularly on the higher frequencies or shorter wave-lengths, in particular directions, so that they can be received rendily only within certain regions or sectors, Can Receive From Any Point

In view of the way in which radio waves trayel in all directions, it is possible for an ordinary receiving station using a wire antenna to receive messages coming from any point of the compass. Every broadcast listener has noted and appreciated the fact that the concerts from cities in all directions can be teceived equally well on occasion. He has, however, also noticed that messages coming, say, from a land or ship spark station in one direction can easily interfere with concerts on a nearby frequency or wave-length com-

ing from a different direction. If one had available a form of radio receiver which could receive messages from a desired direction, instead of being open to reception of messages from all directions, it might be possible to eliminate undesired signals, and not, as usual, by tuning only, but by the use of this directional receiver, The Simplest Loop Aerial. Such a receiver, which would broadly receive messages from a definite direction and fail to respond at all to messages from some other direction, would be a convenient kind of ‘"‘radio telescope" and would have interferencereducing capabilities. The simplest form of such a radio telescope is the loop or coil aerial. It consists of a number of turns of wire wound. rcener-

ally, in square form for convenience and mounted on a frame which cen be readily rotated. The side of the square is from about one foot to as imuch as five or six feet, but .the smaller sizes are by far the more convenient. The terminals of the coil are connected in place of the "antenna" and ‘‘ground’’ connection, but an ordinary receiving set will generally not be satisfactory for use with coil aerials, because it is not sufficiently sensitive. As a matter of fact, the signal strength which can be delivered by a coil is only a small fraction of what can be obtained by a suitably proportioned antenna system of the straight-wire type, and the difference must be made up by the use of additional amplification in the form of several more radiotrons, since otherwise strong signals will not be obtained. | Directional Effects,

Let it be supposed that the signal comes from the north. When the loop points north the signal is loud; and as the loop is rotated: to the east the signal dies down until, when the loop poitts due east, the signal in a well-designed and properly used set of this type has almost entirely disappeared, As the loop is again turned to the south the signal comes back with full strength, to disappear once more when the loop is turned to the west. Three facts are at otice evi-

} dent : . Points for Operation. 1. The loop receives the strongest signals when it points in the direction of’ the incoming waves (which, at times, and especially in city locations, may not be the direction of the station which is being received, since the incoming waves may be swung out of a straight line of travel by obstacles or reflection). 2. The loop receives little or not at all when it poimts at tight angles to the direction of the incoming waves. 8. It is possible to tell only the line of direction of the incoming waves from the loop direction, but not the actual direction. Thus one can tell in the case just described that the signals are coming from the north or the south, but one cannot tell whether the true direction is north or south. Generally this indefiniteness will not cause any practical difficulty.

Tor Loudest Reception. Accordingly, the simplest way to get a station loudly, as a matter of, practical operation after having picked it up, is to swing the loop until the desired signal disappears, and then to swing it exactly at right angles to the, disappearing position. This will be found a little more definite and positive than fhe more obvious method of pointing the coil by trial in the direction giving the strongest signal, In order to get rid of an undesired station the loop is merely

swung until the undesired signal dis: appears. If the undesired signal comes from a very different direction this method will work; but if the desired and undesired signals come from nearly the same direction, . this method of reducing interference will not be effective. Of course, normal tuning is also used to get rid of the undesired station. The coil aerial receiver has the advantage that no antenna wires need be strung, either outdoor or indoor. This simplifies the installation of the set and enables it to be used in almost auy desired location. In steelframe buildings it is advisable to keep such sets near the windows and not in back of steel columns or long ‘sections of wall. Stronger signals are generally obtained in the mote open locations. Coil aerials also enable pottable sets to be readily employed and transported, with minimum delay atid inconvenience in setting them up and using them.

Loops Get Distant Statious. Another interesting point in connection with loop receivers is that it has been found possible by actual trial to listen to stations many hundreds of miles away, operating on practically the same frequency or wavelength as a powerful local broadcasting station by utilising the difference of direction only. We thus realise not only "tuning selectivity" of. the ordinary kind, but also ‘directional selectivity." ;

Baffling Kkitects. =.=. A few rather curious and interesting effects will be found in the use- of such receivers, In the interior rooms of steel-fraine buildings it will sometimes’ be found that all signals seem to come from the same direction, The reception is fairly good, but the directions of all signals are the same. This is because the reception ig chiefly from magnetic fields of currents induced by the travelling waves cither in the steel or in the electric wiring system

of the building. A nearby wire acrial of considerable length will increase the signal strengtl on a coil receiver when both are tuned to ihe same frequency. If the long aerial lias a regenerative receiver connected to it, with the tickler or intensity control brought well up the scale, it will very greatly increase the strength of the signals on the coil receiver, because regetierative reception actually strengthens the local field of the incoming radio waves.

Coil Receiver Overcomes Fading. Sometimes at night, and particularly in certain country locations, signals from stations roughly a hundred miles away fade markedly and vary rapidly in intensity. When using an ordinary long wire aerial receiver nothing can be done about the fading effect. With the coil receiver, however, it will sometimes be found that when such a signal fades rapidly, swinging the coil approximately 90 degrees to a new position at right angles to the original position will bring the signal ‘back again. This is sometimes a useful way of following the fading effects of an incoming signal, since it has the advantage that it enables holding ‘the signal at a critical moment-for example, when some important statement is being broadcast or when the station signature is being given. In view of its portability, directional selectivity and general interest and convenience of installation, the coil aerial receiver or radio telescope 15 widely used in modern sets and has a real sphere of usefulness.