Broadcasting Telecasting (Jan-Mar 1955)

ANTENNA 1 FIGURE ! antenna of a certain height, over a smooth terrain. Such an estimate is useful only as a starting point, however, and must be modified if the station is located in hilly or mountainous territory. Where it can be had, insist on measured, rather than estimated, coverage information. Radio waves of the frequencies of the standard broadcast band (550-1600 kc) tend to follow the curve of the earth and to flow over or around obstructions, with the topography of the landscape having only minor effect on their reception. But in the tv frequencies (the lowest tv frequency is more than 30 times the highest am frequency, the waves travel in virtually straight lines, like light waves, and are blocked when an obstacle comes between the transmitting and receiving points. Propagation of a tv signal (engineering talk for the way it spreads out from the transmitter) is better on vhf channels 2-6 than on vhf channels 7-13 and these in turn are superior to the uhf channels, 14-83. As the channel number increases the adverse shadow effect of hills and buildings, and even trees, becomes increasingly more noticeable. Recognizing these differences, the FCC has defined grades of service in terms of signal intensities, which have lower values for channels 2-6 than for channels 7-13, with those in turn lower than channels 14-83. However, in so doing, advantages in coverage accrued to the lower channels. Therefore, the Commission has attempted to equalize these differences by permitting the use of more power and taller antennas (which clear the hills and decrease the. shadow) by stations on the higher channels. The FCC rules and their estimated effect on smooth terrain are indicated in Table I. It will be noticed that the height of vhf antenna towers is limited to 1,000 feet in Zone I (the thickly populated northeast and northcentral parts of the country), whereas in Zones II (everyplace not in Zone I and III) and III (the coastal area of the Gulf of Mexico), they may rise to twice that height. The powers shown in Table I are effective radiated powers which are the transmitter power multiplied by the gain of the antenna. In effect the signals radiating from the antenna are focused into a circular plate, going out horizontally in all directions from the antenna but not shooting up into the sky, where they would be wasted. Different antenna designs produce apparent gains of from 3 to 50 times the transmitter power. Vhf stations with only 20-25 kw transmitter powers can attain ERP's of 316 kw; uhf stations with transmitter powers of 25 kw can achieve ERP's as high as 1000 kw. The effect of power on tv station coverage is shown in Table II, which uses uhf channels as illustrations, although a similar effect is also achieved on vhf channels. By increasing the ERP from 1 kw to 1000 kw, the radius of Grade A coverage is extended from 9 miles to 43 miles. Grade B coverage would similarly be extended from 16.5 miles for 1 kw power to 58 miles for 1000 kw. The antenna heights indicated in Table I are heights above the average elevation of the terrain within two to ten miles of the transmitter antenna and do not indicate height above ground. In mountainous country, for example, an antenna could be located on a mountaintop several thousand feet above the valley below it without exceeding the maximum height allowed by the FCC. In level country, where tall towers must be built to obtain maximum coverage, TABLE I Tv Station Coverage Areas with Maximum Power and Antenna Height the antenna height above the ground am above the average terrain are more nearl the same. Increasing the height of the antenna is ai even more effective way of extending the station's coverage area than increasing it; power. This is illustrated in Table III, basec on 100 kw ERP on chs. 2-6. The effect oi antenna height on other channels is similar As the table shows, increasing the antenna height from 100 feet to 2,000 feet extends the radius of the Grade A contour (border line around the Grade A coverage area) from 13 miles to 50 miles. Grade B coverage would be extended from a radius of 36 miles at 100 feet to 86 miles at 2,000 feet. The chart on the opposite page shows the I Grade A and Grade B coverage areas of 1 low band vhf (ch. 2-6) stations, high band I vhf (ch. 7-13) stations and uhf (ch. 14-83) stations, all operating with maximum power I allowed by the FCC, but at different antenna 1 heights. Antenna height is of greater value than power in overcoming the adverse effect of 1 shadow areas, as is shown in Figure 1. If antenna 1 is used, the entire area behind each hill is in shadow (with little or no re-i ception), whereas by using antenna 2 the shadow area is cut in half. An increase in « power might be of some assistance in the < case of antenna 1, but the improvement i would be insignificant as compared to that obtained by increasing the antenna height. There is one more term of tv station coverage measurement the timebuyer should know: decibel (db), which is the engineer's way of expressing ratios of power and voltage (signal intensity). For example, the TABLE II Effect of Power on Coverage of Uhf Tv Stations Effective power 1.0 kw 5.0 10.0 25.0 50.0 100.0 200.0 316.0 500.0 1000.0 TABLE Radius to Grade A contour at 1000 feet { antenna height 9.0 miles 14.0 16.5 20.5 24.0 28.0 32.5 35.0 38.5 43.0 (Zone 1) Effect of Antenna Height on Maximum permissible Estimated radius of contours Coverage of Low Band Vhf Tv Stations Channels Power Antenna height Grade A Grade B Antenna height Radius of Grade A contour 2 6 100 kw 1000 feet 36.5 miles 69.5 miles 100 feet 13 miles 7-13 316 kw' 1000 feet 45.5 miles 63.5 miles 200 17 14-83 1000 kw 2000 feet (Zones II and III) 56.5 miles 74.0 miles 300 500 750 21 27 32 2 6 100 kw 2000 feet 50.0 miles 86.0 miles 1000 36.5 7-13 316 kw 2000 feet 60.5 miles 80.0 miles 1500 44.0 14-83 1000 kw 2000 feet 56.5 miles 74.0 miles 2000 50.0 Page 42 • January 17, 1955 Broadcasting • Telecasting