12/31/2023 0 Comments Ravio sign a link between worldsHowever the newer FM and TV transmitters used the VHF band, in which radio waves travel by line-of-sight, so they are limited by the visual horizon. The ground-hugging waves allowed the signals to travel beyond the horizon, out to hundreds of kilometers. The earlier AM broadcasting used LF and MF bands, where radio waves propagate as ground waves which follow the contour of the Earth. The rise of FM radio and television broadcasting in the 1940s and 50s created a need for even taller masts. By the 1940s the AM broadcast industry had abandoned the Blaw-Knox design for the narrow, uniform cross section lattice mast used today, which had a better radiation pattern. During the 1930s it was found that the diamond shape of the Blaw-Knox tower had an unfavorable current distribution which increased the power emitted at high angles, causing multipath fading in the listening area. The first, a 665 foot (203 m) half-wave mast was installed at radio station WABC's 50 kW Wayne, New Jersey transmitter in 1931. ![]() The pointed lower end of the antenna ended in a large ceramic insulator in the form of a ball-and-socket joint on a concrete base, relieving bending moments on the structure. This had a diamond ( rhombohedral) shape which made it rigid, so only one set of guy lines was needed, at its wide waist. One of the first types used was the diamond cantilever or Blaw-Knox tower. Masts of the Rugby VLF transmitter near Rugby, Englandīy 1930 the expense of the T-antenna led broadcasters to adopt the mast radiator antenna, in which the metal structure of the mast itself functions as the antenna. In a second paper the same year he showed that the amount of power radiated horizontally in ground waves reached a maximum at a mast height of 5⁄ 8 wavelength. ![]() He found that the radiation resistance increased to a maximum at a length of 1⁄ 2 wavelength, so a mast around that length had an input resistance that was much higher than the ground resistance, reducing the fraction of transmitter power that was lost in the ground system without using a capacitive top-load. ![]() In the first he derived the radiation resistance of a vertical conductor over a ground plane. In 1924 Stuart Ballantine published two historic papers which led to the development of the single mast antenna. The antenna used for broadcasting through the 1920s was the T-antenna, which consisted of two masts with a wire topload strung between them, requiring twice the construction costs and land area of a single mast. The allocation of the medium wave frequencies for broadcasting raised the possibility of using single vertical masts without top loading. Multiwire broadcast T-antenna of early AM station WBZ, Springfield, Massachusetts, 1925.ĪM radio broadcasting began around 1920.
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