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New Antenna to Multiply Field
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Amateur radio enthusiasts are very familiar with burying ground radials below the surface (or sometimes just laid on top) in order to increase antenna efficiency by affecting impedance and, more importantly, the radiation pattern. Long distance (DX) operators generally prefer low launch angles over high angle "cloud warmers." Pittsburgh's KDKA, the country's first commercial broadcast radio station, built what would have been the mother of all ground radial arrays for its time - 360 (one every degree), 700-foot copper wires (8 AWG), for a total of a quarter million feet! It was laid using a farm tractor drawing a non-motorized trench cutter that looks like it came straight from the John Deere factory. It also implemented a new type of passive vertical suppressor element array. New Antenna to Multiply Field
The tall streak of light is the 718-foot antenna mast of KDKA in the glare of searchlights. It weighs 60 tons and rests on a socket in a large porcelain insulator.
Laying 50 miles of copper wire which is an unseen factor in the efficiency of the new KDKA antenna; it is buried about a foot deep, to form a ground. In the background, masts of KDKA's multiple system. (Westinghouse photo) KDKA, which began systematic broadcasting on election night, 1920, last month completed the new antenna equipment, and the anniversary was celebrated (a couple of days early) on October 30. It is a double triumph for Dr. Frank Conrad, who sent out those first election returns; for the big antenna carries out an idea which he had in 1929, but which was unsuccessful at that time. The great self-supporting steel mast, 718 feet high, is a 3/4-wave antenna ; it is broken at the 336-foot level, and different voltages are fed to the upper and lower sections, so that current flows in the same direction in both, instead of reversing. Around the mast, 504 feet away, are 8 wooden masts each supporting - and this is a novel feature - a "suppressor" antenna. These buck the "skywave" from the main tower, and thus prevent fading and other undesired effects in the nearby service area. It is expected to increase efficiency tenfold, as regards "local" reception. Another feature of the antenna system is a "ground" of 360 No. 8 copper wires, radiating from the foot of the mast to a distance of 700 feet. These, laid in shallow trenches dug by a tractor, are 48 miles in length overall and give a perfect counterpoise.
Looking from the control room at the transmitter of KDKA, now located at Saxonburg, Pennsylvania, the 17th anniversary of whose original faint broadcasts was last month celebrated. A tenfold increase in service area is to be obtained with the new antenna.
Posted November 26, 2020 |
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