Radio age research, manufacturing, communications, broadcasting, television (1941)

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Antennas Point Ahead LONG AND INTENSIVE RESEARCH BRINGS NEW AERIAL DEVICES TO ASSIST IN GROWTH OF SOUND BROADCASTING AND TELEVISION By Dr. George H. Brown RCA Laboratories Princeton, N. J. THE painstaking development of radio antennas has contributed immeasurably to the growth of the radio industry—ofttimes preceding engineering advances in sound broadcasting, television, facsimily and "FM" broadcasting. Investigations and study of the properties of antennas date back to a period almost 30 years before Marconi's success in radio-teleg- raphy. Maxwell, Hertz, and Poynt- ing are among the great scientists whose discoveries and theories have been the basis for the modern an- tenna systems that have sprouted since the advent of radio communi- cation. About 1900, it was Max Abraham who probed the action of a half-wave antenna in free space and, as far as I know, was the first to evaluate the radiation resistance of a half-wave antenna. As radio developed, the long waves were first used for broadcast purposes and the shorter waves were used for point-to-point com- munication. Just as this division in wave length came about because of the difference in application, so was there a division in the develop- ment of antenna structures. In the first broadcasting stations, the antenna usually consisted of a giant T, supported by two wooden or steel towers. Since the action of this type of antenna was not well known, the horizontal portion of the T was regarded as highly im- portant. It accordingly was built (if cages of wires or a number of parallel wires held in position by spreaders. The vertical portion was iiiiikcd upon as a carrier of energy til the horizontal portion. But, as a matter of fact, the vertical part of the average T antenna radiates 98 per cent of the total radiated power. It soon was found in a number of cases that the antenna induced strong currents in the steel sup- Iiorting towers. The re-radiation from these currents was enough to greatly distort the radiation pat- tern of the system. Measured hori- zontal patterns were found to be oval and sometimes the shape of a figure 8. Reduction of the distor- tion was achieved by developing insulators to isolate the tower from the earth. The next obvious step was to use a single insulated tower for the antenna proper. Used Model Antennas The theoretical work of Stuart IJallantine, published in 1924, brought another important ad- vance. It showed that where the antenna consisted of a straight wire, the maximum field strength along the horizontal was achieved when the wire was six-tenths of a wave length. In 1931, with the con- struction of the first self-support- ing tower antennas, the height set forth by Ballantine was taken as optimum. But oiierating experience with this type of structure soon showed a grave difference between theory and practice. Because of the difficulty in per- forming critical experiments on tnwors several hundred feet in height, work was carried out in our laboratories on the use of model tower antennas. Experiment brought out the fact that, since the towers were not a uniform cross-section throughout their length, the current distribution dif- ^< ■'irUN STILE" ANTE.VNA USED WITH THE FM TRANSMITTER OF STATION WCAU, IN PHILADELPHIA. fered from the sinusoidal distribu- tion assumed by Ballantine. The model antenna technique not only explained the action of existing an- tennas, but also pointed the way to improvement of these antennas and assisted in justifying the construc- tion of "constant cross-section" an- tennas. When the first "constant cross- section" antennas were constructed and tested, it was found that the results predicted by Ballantine were realized. However, by 193-4, there were a number of deared-channel stations in operation. Almost in- variably, the night service distance was not limited by signal intensity. The limit of service was determined by the so-called selective fading zone, that region where the direct ground wave and the sky wave re- flected from the Kennelly-Heaviside layer were about equal in magni- tude. Here, these two waves first reinforced and then tended to can- cel, with consequent fluctuation and distortion. Studies of these effects, together with consideration of the properties of antennas by RCA en- gineers, led to the almost universal adoption of the "constant cross- section" antenna for cleared-chan- nel stations, with the total antenna height equal to 53 per cent of a [14 RADIO AGE]