THE LOOP AERIAL

Radio Record, Volume I, Issue 15, 28 October 1927, Page 15

THE LOOP AERIAL

THEORY OF OPERATION CORRECT DESIGNING. The loop aerial is a very interesting device, It is quite different in its method of operation from the outdoor aerial. The outdoor aerial is in effect nothing more nor less than a condenser. It is. a very large condenser, to be sure, so far as its physical dimensions are concerned, but electrically it is a relatively small condenser. The loop, on the other hand, is an inductance. , This fundamental difference between the two is the reason why it is necessary to use different methods of tuning in the two cases. ‘There is a very close parallel between the ordinary direct current generator or dynamo and the loop aerial exposed to, passing radio waves. In the dynamo a mumber of coils corresponding to the’ loop aerial are rotated in a powerful magnetic field. The purpose of rotating them is in order that they may move with respect to the field and thus have a voltage generated in them. | LOOP ACTS AS COIL, In the radio case the loop, acting as a , coil, stands still, but the fiel@ moves ‘ swiftly past the coil, thus accomplishing the same result. The speed at which the field moves cannot, of course, be varied and is always the speed of light; that is, 186,000 miles per second Let us see now what form of loop would have the greatest voltage generated in it by a passing radio wave..Let us think of this radio wave as very much like great smooth waves on the ocean, which move forward with a verv definite velocity. The turns of wire on our loop aerial are necessarily in series with each other; that is to say, they form a continuous winding. If the maximum voltage is to be generated in any one turn of the loop then the voltage generated in the two sides of this turn shonld be in opposite directions, so that they may add and not oppose each other. If the voltage generated in both sides of the loop were in the upward direction in any one instance then these two voltages would cancet each other, but if the voltage on one side of the turn was up and on the other side of turn it was down then they would add, and if the loop were cotnected to a receiver a cnrrent would flow around the turns of the A QUESTION OF SIZE. Now, in order to have the voltage generated on one side of the loop in the opposite direction to that generated on the other side of the loop the loop would have to: be one-half a wave length long; that, is to say, it would have to be lone enough in the horizontal direction so that one side was in. the crest of the wave when the other side. was in the trough of the wave. Since the distance between the crest of the wave is the wave leneth itself, then the distances from the crest to the trough is one-half the wave length. The higher the sides of the loop are, that is, the longer the vertical wires are, the greater will be the voltage senerated, and of course the voltage generated in each turn is added to the voltage generated in all the other turns. HOW LOOP FUNCTIONS. But a loop one-half a wave length long is quite ont of the question. It wonld be as long as a steamship and almost as difficult to handle. The loops which we are using every day are of quite reasonable’ dimensions. They are only a few thausandths of a wave length long. How do they function? In order to answer this question let us ask ourselves how we would build a coil of wire in order that absolutely no voltage should be generated in it by the passing wave. The only way in which this could be accomplished would be to so build the coil that the same voltage would be generated in both sides of it and that the voltage generated in the two sides would be opposed to each other This wonld give a complete cancellation and no voltage at all at the terminals of the | loop or coil. . It is obvious that the only way in which this could be done would be by so arranging the loop that it had no length at all. ‘That is to say, arranging it so that the two sides were exactly in the same position in space. This would mean that the horizontal wires across the top and bottom of the loop would cease to exist and the loop would become nothing but a wire laced up and down between pegs on the plain surface | , of a board. TO OBTAIN VOLTAGE. If there is any distance at all be tween the two sides of the loop then there will be some difference, not in the amount of voltage generated in the two sides, but in the time at which this voltage is generated, and there will consequently be some voltage at the terminals of the loop. since complete cancellation of voltages cannot occur. if the loop is rotated so that its horizontal wires are at right angles to the direction in which the signal is coming then the loop has no length so far as" these signals are concerned. The passing wave strikes both sides of each turn in the loop at exactly the same instant and the voltages generated are therefore equal and opposed, and there is no terminal voltage. This is of course the fact which gives the loop antenna its" very wseful directional property. | In applying the loop aerial to an actnal radio receiver it is necessary that provision be made to tune it to reson ance with the desired signal. This is accomplished by means of a variable air condenser, and since this condenser hhas a very definite maximum capacity the amonnt of inductance which the loop can have is also limited. BEST TYPE OF LOOP, The specification for the best loop aerial, therefore, is that it shall have just as many. turns as possible, each

turn being just as long as possible and just as high as possible and still have flo more than. the ‘required maximum inductance. The higher the loop is the greater will be the voltage generated in each side of each turn, and the longer it is the greater will be the difference in time at which these voltages are generated in the two sides of the loop, and consequently the greater will be the voltage at the terminals, but it must not have at inductance value greater than that required for tuning. In order to get the maximum num ber of turns for a given inductance which is what our loop requires, the turns should be wound iust as far apart as possible. It is fomnd that this spac ing is best accomplished by winding the loop on a frame which has the form of a vertical cylinder,