Radio age (May 1922-Dec 1923)

RADIO AGE— "THE MAGAZINE OF THE HOUR' Proper Antenna for Tuning By F. CONRAD Assistant Chief Engineer, Westinghouse Electric & Manufacturing Company THE ability to hear a desired station alone, or "selectivity" as it is called, depends in part on the receiving apparatus and in part on the antenna system to which it is connected. Many believe that the better the antenna, the better the signals. This is true, but it does not necessarily mean that the best antenna is the largest. The function of the antenna is to transfer to the receiving apparatus the electric forces which are set up by the waves being transmitted through space. This receiving apparatus must discriminate between the electric forces due to the radio wave it is desired to receive, and the forces due to the undesired waves, among which are the waves from "Dame Nature" herself, or "static" as they are called. The selective receiver is one that offers a high resistance to the flow of current which would be set up by the electric forces from undesired waves, and offers a low resistance path for the flow of current due to the electric forces from the waves it is desired to receive. In other words, it permits you to hear the stations you wish to hear, and to tune out those you do not wish to hear. The receptive ability of an anten sistance in the receiving set to this desired wave, would have to be decreased as the antenna height is decreased. Experiments have shown that when the antenna height is increased and a receiver, such as a crystaldetector set or a tube set not using regeneration, is used, the signal at first increases but soon reaches a maximum strength, which can not be exceeded by further increase of antenna height. This height is such -""-■»wji«^L^i^"ianiriirr~]ii— 1^_ i H|^W|Bi^^|||HM|B^BBB^ JJBHS^mS^ i^BKmS^t ■/'•>* '^ ■ m^ *li ^' ^^ 1 ^^^ '^ ^ '^^^"'^ >SUJ^^^H Wm mt^ :,_^^m:. ' : . '"^ . WKKKK^ WA '^H^^^HP*' ' ■ ^^r^ia^ '• .':/^ ■'• . "x" ■:' . ■■,■ r-^ -1 .■-■^ This fan, snapped in his garden in Sheffield, England, as aerial and counterpoise. He easily picks up London (U&U Foto). that the electric forces set up by the incoming wave is sufficient to drive through the receiving apparatus the full current strength which is equivalent to the received signals. To express it in another way, this maximum current is that which would itself set up the same strength of na is, in general, determined by the radio wave around the receiving anheight of its horizontal portion tenna as is induced by the transmitabove the ground, or, stated dif ting antenna sending out the signals ferently, the strength of the electric it is desired to hear, forces induced in an antenna by the A vacuum-tube receiving set, in radio waves is proportional to the which the principle of regeneration height of this antenna. Therefore, is employed, tends to reduce the to tune out or discriminate between resistance to the flow of current from different waves, the selectivity or a wave corresponding to that for resisting power of the receiver to in which it is tuned. Therefore, if a terfering waves would have to be regenerative receiver is used, it will increased as the antenna height is in be found possible to maintain the creased, while to receive an equal maximum strength of signal, even signal from a desired wave, the re with a reduced antenna height. However, as the same resistance will be maintained by this receiver against undesired waves the reduction of height will therefore give a greater selectivity. Of course, in general practice it usually will not be possible to obtain quite the same strength of signal with the low as with the high antenna, as there is a certain amount of absorption or loss near the ground which tends to reduce the possible signal strength. Should the location be such that the antenna is perfectly clear and free from surrounding objects, the low one will be found to be practically equal to the high one, when a regenerative receiver is used. But should the antenna be located where it is considerably shielded, as where it is surrounded by high buildings, it is possible that the signal strength will be greatly influenced by height. In this latter condition, it will probably be necessary to make up for the poor selectivity of the high antenna by using a somewhat elaborate receiving apparatus. Under the conditions surrounding the average residence district, it usually is possible, with care in the location of the antenna, to maintain good signals, even though the height is considerably less than with the scheme generally employed of attaching the horizontal wire to some point near or on the roof of a two-story house. The actual selectivity required divides itself into two classes or conditions of service ; one in which it is desired to discriminate between two relatively nearby stations of approximately equal signal strength but separated by some interval of wave length, the other where it is desired to discriminate against a nearby station and receive from a (Continued on page 23.) uses two tin pails and Manchester.