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

Record Details:

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wave length, the so-called 190 de- gree antenna. Theoretical investigations iiuli- cated that the efficiency of broad- cast antennas could be increased materially by the use of more ex- tensive ground systems. Experi- ments showed that by using larger ground systems there was an ap- preciable saving in antenna heights without heavy loss in efficiency. The extent of our investigations may be seen in the fact that before their completion sixteen separate and complete ground systems were laid. For tests on each system, the antenna height was varied from 10 to 100 feet, in 10-foot steps. For each one of these 160 combinations, complete measurements were made of antenna resistance and reac- tance, field intensity one mile away, current distribution in the buried wires, and total earth current dis- tribution. The trend to use constant cross- section antennas for high-power cleared-channel broadcasting sta- tions grew, and it became evident that the guyed-type of antenna would be more economical than others. But from the field came ob- jections that the guy wires would distort the radiation pattern. To obtain data on this point, our in- vestigations included theoretical studies, experiments with models and actual full-scale tests to con- firm the model experiments. In a number of cases, it was not possible to erect a 190 degree an- tenna because of the proximity to airports or airways. Top-hat an- tennas were then resorted to. here a large set of outriggers was placed on the top of the tower to act as a capacity, thus altering the current distribution so that the radiation pattern of a 190 degree antenna could be obtained with an antenna which was actually shorter than the optimum. In general, it is not physically possible to construct a hat of suffi- cient diameter to reach the best point. To get around this obstacle, the hat was mounted on insulators and connected to the top of the tower by means of an inductance coil. The addition of this coil not only allowed the use of a smaller hat, but gave great flexibility in exactly achieving the desired re- sults, for the proper adjustment was achieved simply by moving a tap on the coil. In practice, the addition of a top hat in one instance more than doubled the effective night service area. Further Studies Made Further studies by RCA engi- neers showed that more efficient operation, with lower voltages, could be obtained by using a sec- tionalized antenna. In this antenna, the insulators were used to break the tower at a point approximately one-third of the distance from the top. The coil was again used to con- nect the two parts of the tower. In certain special cases, a combina- tion of sectionalizing and top-hat seemed desirable. Model measure- ments have been especially helpful in studying top-loading problems. As the number of broadcasting stations increased, directional an- tennas began to appear. Arrays used in the broadcast band had two purposes: First, so that the energy may be concentrated in a given area, thus increasing field strength to equal that attained by pumping more power into a single non-direc- tive antenna; secondly, and per- haps more important, to protect the service area of another station in the same or adjacent channel. In the latter case, the action is to "chop a hole" in the radiation pat- tern so that the signal traveling toward the other station is cut to very small values. Such action makes possible the operation of several regional stations on the same channel. According to my information, the first commercial broadcasting station to use directive antenna for that purpose was VVFLA, at Clear- water, Florida. The array there consisted of two quarter-wave an- tennas separated a quarter of a wave length, and fed so that the currents in the antennas were equal and in time quadrature. The an- tennas were so set up that the sig- nal in the direction of the primary service area of WTM.I, at Mil- waukee, Wisconsin, was reduced to very small values. Successful re- sults at WFLA inspired many sta- tions to install similar equipment. Our antenna development group concentrated on the development of directive antennas. These experi- ments made it possible for us to design the first "asymmetric" direc- tional antenna system, now in use at Station WTAR. at Norfolk, Vir- ginia. The model technique was further used to study problems arising when individual towers in an array are of unequal height, and infor- mation obtained through our tests proved valuable in the installation of this system at Station KDYL, Salt Lake City. More, the use of small model an- tennas, operated at ultra-high fre- quencies, helped us develop methods and apparatus which could be put to work immediately when it be- came desirable to produce antennas for that type of broadcasting. Close INFORMATION 0BT.\INED BY RCA ENGINEERS IN THE STUDY OF MODEL ANTENNAS MADE POSSIBLE THE DIRECTIONAL TYPE OF ANTENNA NOW IN USE AT STATION KDYL AT SALT LAKE CITY. [RADIO AGE 15]