Radio Broadcasting at 500 kilowatts

Sixty years ago this month, the PROCEEDINGS OF THE INSTITUTE OF RADIO ENGINEERS (IRE) included a paper on station WLW in Cincinnati, OH, USA, which recently had begun broadcasting at the unprecedented output power of 500 kW. The WLW superpower transmitter was interesting technologically but it was also the center of a public policy debate which pitted an interest group favoring a few clear-channel, high-power stations against a rival group favoring a much greater number of lower power stations.

Joseph A. Chambers, Chief Engineer of the Crosley Radio Corporation, which owned WLW, was the lead author of the October 1934 paper, although five other engineers were named as having cooperated in the design of the station and preparation of the paper. Station WLW had been transmitting at 50 kW for some time when it was granted permission by the U.S. Federal Radio Commission in June 1932 to use the existing transmitter as an exciter for the new power amplifiers which employed 12 type UV862 vacuum tubes, each rated at 100 kW. The new amplifier consisted of three units of four tubes each and the transmitter could continue to operate at reduced power with one or two units out of service. Two identical modulator units using eight UV862 tubes in push-pull configurations also were incorporated in the design. The final stage of each modulator was connected to its load using a 180-kVA transformer weighing about 19 tons and with a height of 11 ft. The power tubes were water-cooled, with each 10-kW tube using 20 gallons per minute. The transmitter was linked to the antenna by a concentric aluminum tube transmission line which was 780 ft long and had an outer diameter of 10 in. The vertical tower antenna had a height of 831 ft and rested on a porcelain insulator with a concrete foundation. After the installation and testing were completed, WLW began operating at 500 kW in May 1934 as the highest power broadcasting station in the United States.

Chambers and his coauthors reported that the station’s performance had shown that the increased power was the “most effective static eliminator yet devised.” The station also produced interference which was especially bothersome to listeners to station CFRB in Toronto, Canada, which operated at a frequency 10 kHz away from the frequency of WLW. In May 1935, it was reported that the WLW engineers had installed a “suppressor antenna” designed to reduce radiation toward Toronto. The modification was in response to complaints from the Canadian government and involved the addition of two vertical elements with a height of 326 ft each. The new towers were installed while the station was operating and it was found necessary to hoist the steel elements with rope and to make sure the metal was well grounded before it was touched by construction workers.

In November 1936, the U.S. Federal Communications Commission (FCC) convened hearings to consider the pros and cons of clear-channel, superpower broadcasting. Powell Crosley, Jr., owner of WLW, was the chief witness in favor of high power. He argued that stations such as WLW could afford better programming and provided better service to rural audiences. He stated that mail received from listeners had increased about 400% after the increase to 500 kW. At the time there were 13 pending applications from stations wanting to increase power to match WLW, but opponents predicted that permitting this would be ruinous to the lower powered local stations.

By 1936, a more efficient power amplifier invented by William H. Doherty was available for superpower broadcast transmitters. The Doherty amplifier was used in a transmitter made by the Western Electric Company for WHAS in Louisville, KY. and which was intended to operate at 500 kW if authorized by the FCC. However, the FCC decided to restrict U.S. broadcast stations to a maximum power of 50 kW, and WLW was required to cut back to that level. Papers on the Doherty and its application to radio broadcasting appeared in the PROCEEDINGS of September 1936 and September 1939. A 500-kW transmitter using the Doherty design was installed in Mexico. where it was not subject to the FCC power restriction.

James E. Brittain

School of History, Technology, and Society
Georgia Institute of Technology

Submitted by Dick Reiman, Historian


Copyright 1994 IEEE. Reprinted with permission from the IEEE publication, “Scanning the Past” which covers a reprint of an article appearing in the Proceedings of the IEEE Vol. 82, No.4, October 1994.