NAB reports (Mar-Dec 1933)

(3) Conductivity of terrain. (4) Loss resistance in antenna. The distance to which a specified field intensity can be transmitted without objectionable fading is dependent not only on the fore¬ going factors but also upon conditions in the Heaviside layer. Figures 4 (a), (b), etc., indicate the distances at which 10,000, 2,000, 1 000 and S00 microvolts, respectively, will be transmitted with 1 KW input with various heights of antennas having 10 ohms loss resistance. These also show approximately the expected service radii for each frequency from ISO to 1700 kc. Figures 4 (a), etc., are intended solely to indicate general trends. It should be noted that for a field intensity of 500 microvolts, which naturally is transmitted over the longer , distances, the “carry¬ ing capacity” of low frequencies over the earth outbalances the loss in initial field intensity caused by inefficient antennas; however, as the distance becomes less and the field intensity thus greater, this advantage of the low frequencies becomes less pronounced. These figures illustrate very clearly the advantage of low fre¬ quencies for long distance service, as well as the necessity for having good antenna design to obtain these advantages for low frequency transmission at shorter distances. THE EFFECT OF POWER The effect of increasing power is shown in Figures S (a), (b) and (c), which show the distances to which 500 microvolts will be transmitted with 25 KVV input in various antennas and frequen¬ cies. In these figures there is also clearly illustrated the effect of antenna design, the relative advantages of low frequencies over high frequencies with respect to attenuation, and the effect of fading. It should be noted that even with greater heights of antennas on low frequencies, the value of increasing power, while highly advan¬ tageous in the day, may be nullified to a large extent at night by fading, except for the increased signal strength up to the fading wall. An interesting calculation illustrating the effect of Dower, antenna design and fading on the probable rural service radius of a broad¬ casting station under specified conditions and on various frequen¬ cies is indicated in Figure 6. An illustration of the relation of conductivity, frequency, power, and antenna design to secure equal suburban coverage from broad¬ casting stations is indicated in Figure 7. It will be noted that the carrying capacity of the lower frequencies is of paramount impor¬ tance in making these lower frequencies of greater value than the higher frequencies. The optimum frequency is about 550 kc. In Figure 8 is illustrated the effect of antenna efficiency at the shorter distances where relatively high field intensities are encoun¬ tered. It is here that the higher frequencies, properly used, are of some value as compared to the lower frequencies. Brief Summary of Trends Field Intensity 500 microvolts Antenna Height 100 ft. 200 ft. 500 ft. # 1000 ft. « Antenna Input Power 1 KW 25 KW 1 KW 25 KW 1 KW 25 KW 1 KW 25 KW Frequency 150 kc 60 200 Radii in Miles Day 75 230 125 325 170 370 550 kc 70 150 85 180 110 210 120 245 1000 kc 46 87 53 103 62 120 65 127 1700 kc 28 55 32 65 36 68 36 68 150 kc Day 140 Day Night 140 Day 140 140 140 550 kc Day 85 Day 90 102 102 Day 190 1000 kc Day 55 Day 55 Day 110 Day Day 1700 kc Day 35 Day 40 Day Day Day Day Field 2000 microvolts for 1 KW Intensity Antenna 2500 microvolts for 25 KW Height Antenna 100 ft. 200 ft. 500 ft. # 1000 ft. # Input Power 1 KW 25 KW 1 KW 25 KW 1 KW 25 KW 1 KW 25 KW Radii in Miles Day Frequency 150 kc 550 kc 1000 kc 1700 kc 17.5 25.0 22.0 15.0 60.0 70.0 46.0 28.0 24.0 37.0 27.0 18.0 75 85 53 32 40 48 34 20 125 110 62 36 58 63 37 20 170 120 65 36 150 kc Day Day Day Night Day Day Day Day 140 550 kc Day Day Day Day Day 102 Day Day 1000 kc Day Day Day Day Day Day Day Day 1700 kc Day Day Day Day Day Day Day Day * Optimum height less than 500 and 1000 ft. respectively — 150 kc. 1000 ft. 1000 kc. 500 to 600 ft. 550 kc. 1000 ft. 1700 kc. 300 to 350 ft. (a) At 150 kc., increasing the power from 1 KW to 25 KW with a 100-foot antenna is equivalent to an increase of 3.3 times in radius for the 500-microvolt signal. Increasing the height 10 times increases the radius of a 500-microvolt signal approximately 2.85 times. (b) At 1700 kc., with 100-foot height, increasing the power from 1 KW to 25 KW increases the radius of the 500-microvolt signal 1.96 times. Increasing the height 3.5 times is equivalent to in¬ creasing this radius 1.28 times. (c) At 150 kc., with 100-foot height, increasing power from 1 KW to 25 KW will increase the 2-millivolt radius by approxi¬ mately 3.5 times, while multiplying the height by 10 will increase the radius of the 2-millivolt line approximately 3 times. (d) At 1700 kc., with 100-foot height, increasing power from 1 KW to 25 KW, with a 100-foot antenna, is equivalent to increas¬ ing the 2 -millivolt radius by approximately 1.85 times. Increasing the height 3.3 times is equivalent to increasing the radius of the 2-millivolt line approximately 1.35 times. (e) From the foregoing it can be seen that input power is slightly more important than antenna height in so far as day radius i$ concerned. However, it can be seen that the antenna factor is of primary importance. At night the antenna factor, because of its control of fading, is of greater importance than power. (f) It is only at the relatively shorter distances with the higher field intensities where the difference in the effect of conductivity is not so apparent that high frequencies with proper antennas are of relatively greater value as compared to the lower frequencies on small antennas. (g) Noise level should be an important influence in the choice of frequencies for specific services. A reference to Figure 7 of the report of the Committee on Radio Propagation Data, submitted March 28, 1933, illustrates the noise intensity to be expected on various frequencies. Applying this data to the foregoing table, we find that on 1700 kc., 25 KW will transmit a signal intensity of 500 microvolts to a distance of approximately 68 miles. At this distance the signal to noise ratio will be 50 in the daytime and 12.5 at night. 1 KW input on an antenna 100 feet high at 150 kc. will deliver 500 microvolts at 60 miles, at which distance there will be a signal to noise ratio of 16.5 in the day and 1.22 in the night. However, increasing this power from 1 KW to 25 KW, the signal to noise ratio will be increased to over 50 in the day and to 6.1 at night. Thus it is seen that for daytime the relatively greater value of low frequencies is not seriously impaired in the northern climates. (h) However, at night 25 KW input on a 300-foot antenna at 1700 kc. will transmit a signal intensity of 500 microvolts, without fading, to a distance of approximately 68 miles, where there will be a signal to noise ratio of approximately 12.5. To approximately equal this signal to noise ratio on 150 kc. at this distance, it will be necessary to transmit with 25 KW input power on a 1,000-foot antenna. Thus the greater value of low frequencies is impaired by the noise level factor at night, particularly in tropical regions, and as a corollary the higher frequencies assume greater relative value. Page 96