ข้อมูลทรัพยากร

เนื้อหาย่อ : About the Cover The photograph on the front cover is of an antenna used to send and receive microwave telephone and television-radio signals between Florida City, Florida, and Guanabo, Cuba, 185 miles away. Normally, such signals are confined to ''line-of-sight"' transmission between relay stations located only 30 miles apart. By this new method, which is called ''over-the-horizon'' transmission, the signals are aimed at the troposphere, a layer of electrified air particles lying about five miles and more above the earth's surface. The greatest portion of these signals passes through the troposphere, never to return. A minute portion, however, is reflected back and is scattered over the surface of the earth. Only about one ten-trillionth of the transmitted signal reaches the receiving antenna. Nevertheless, so sensitive is the receiver that reliable com- munication is thus maintained. The photograph on the back cover is that of an ordinary "'line-of-sight"' relay station. The installation at Florida City, consisting of a "line-of-sight " 'relay station and two "'over- the-horizon'' antennas, is pictured on the inside cover. All photographs are courtesy of the American Telephone and Telegraph Company, Long Lines Department.