NAB reports (Mar-Dec 1933)

The National Association of Broadcasters NATIONAL PRESS BUILDING * * * * * WASHINGTON, D. C. PHILIP G. LOUCKS, Managing Director NAB REPORTS * * * * Copyright, 1933, The National Association of Broadcasters Vol. 1 No. 22 JULY 26, 1933 ANTENNAS Increasing agitation for power increases for regional and local stations, and for revision of the Federal Radio Commission quota system, has brought attention to the subject of antenna systems used by broadcasting stations. While there is much information available, a concise compilation of data pertinent to broadcasting uses has not been available pre¬ vious to the preparation of a report of a subcommittee (consisting of J. C. McNary, NAB chief engineer, and T. A. M. Craven, Wash¬ ington consulting engineer) to the Committee Preparing for the North American Conference. Inasmuch as this report is com¬ prehensive in its outline of the effects of antenna configuration, power, and operating frequency, its publication is considered timely. The report follows: ASSUMPTIONS For the purpose of calculations and in order more easily to dis¬ close trends, the following are used as a basis: (a) Conductivity over the earth’s terrain is assumed to be 1013 units as an average. This assumption closely approximates the average to be found in the United States and Europe. (b) Antenna loss resistance is taken as 10 ohms. This represents a fair average for practical installations of antennas at broadcasting stations. (c) The height of the Heaviside layer is assumed to have an average of 100 miles with a reflection coefficient of unity. This is considered a close approximation of average conditions encoun¬ tered in this country during the winter months. (d) The propagation curves used are those giving distances for 1 KW radiated calculated by the Dellinger Committee for various frequencies. These curves are generally accepted as representing approximate conditions in practice. (e) Calculations are made for true vertical radiators from .1 to .6 wave length in height. These have representative voltage patterns which have been corroborated in practice and therefore can be used as a basis of comparison for any other type of antenna. (f) Most of the calculations were confined to heights of antenna less than 1,000 feet, since this maximum height of 1,000 feet is considered the limit of practical mechanical design for antenna structures within reasonably economical costs. (g) The radiation resistance and voltage distribution formulas of Ballantine are used, since these are accepted as showing a close approximation to actual facts. (h) Fading is considered objectionable when the ratio between the sky and ground waves exceeds 2. This permits a variation of field intensity at a receiver of 3 to 1. (i) The attenuation for sky wave radiations is assumed to follow the inverse distance law. This permits a reasonable degree of accuracv in the calculations. GENERAL EFFECT OF ANTENNA DESIGN Antenna performance is an important factor in allocation en¬ gineering, both from the standpoint of distribution of energy in the vertical plane as well as field intensity in the horizontal plane. The distribution of radiated energy in the vertical plane is: an important factor in fading, as well as in; useful efficiency. Figure 1 shows the relation between antenna height and frequency, antenna efficiency, radiation efficiency, and the field intensity at 1 mile for various vertical radiators having a 10 ohm loss resistance with 1 KW input power. An illustration of how this figure may be utilized is as follows: A 500-foot antenna at 1000 kc. is a half-wave radiator, having a radiation efficiency of approximately 99 per cent, and an antenna efficiency of 80 per cent with a field intensity of 235 millivolts at 1 mile. Lowering this height to 250 feet would make the antenna a quarter-wave radiator having a radiation efficiency of 78 per cent, an antenna efficiency of 42 per cent, and a field intensity of 170 millivolts at 1 mile. Decreasing the height to 150 feet would mean a radiation efficiency of 50 per cent, and an antenna efficiency of 2iy2 per cent, with a field intensity of 135 millivolts at 1 mile. In order to secure the same field intensity at 1 mile from a 150-foot antenna as could be obtained with 1 KW input power on a 500-foot antenna, it is necessary to increase the 1 KW power of the 150foot antenna to 3 KW. VOLTAGE PATTERN IN THE VERTICAL PLANE The actual height of a true vertical radiator also controls the voltage distribution in the vertical plane. In Figure 2 is given various voltage patterns in the vertical plane for different antennas of designated percentages of the wave length in height. As a general approximation, antennas having a height of .2 wave length or less have a semicircular voltage distribution in the vertical plane. FADING Fading is caused bv the interaction between the radiations trans¬ mitted to the Heaviside layer, where they are reflected back to the earth and then intermingle with the radiations in the horizontal plane. Fading may also occur by the intermingling of radiations in two vertical angles in the event that they are reflected from different heights of the Heaviside layer. Figure 3 shows the distances at which objectionable fading com¬ mences for various types of antennas and frequencies. In this the fading limit indicated is that of the fading wall nearest the trans¬ mitter and is confined to the intermingling of the ground and sky waves when the conductivity of the terrain is 1013 units. Fading is independent of power input and depends solely upon the design of antenna, upon conditions in the Heaviside layer and upon ground conductivity. Thus, if any one of these factors is changed, the fading distance will likewise be affected. SERVICE RANGES The distance to which a specified field intensity can be transmitted over the ground depends primarily upon (1) Power input. (2) Design of antenna. Page 95