Florida radio stations revolutionized AM with first-ever directional antenna array

April marks the anniversary of the world’s first use of a directional antenna array to keep a station’s signal from interfering with distant stations on the same frequency. This innovation, engineered and installed at Florida stations WFLA/WSUN in Tampa Bay, enabled AM stations to continue to utilize “sky wave propagation,” or more simply “signal skip,” to beam their signals to northern markets. Here’s the story, which begins on page 82 of Towers in the Sand.

WFLA WSUN directional antenna array (1)

As the crow flies, it’s over a thousand miles from Clearwater, Florida, to Milwaukee, Wisconsin. Yet in the world of AM broadcasting, where signals bounce off of the ionosphere and are reflected back to earth hundreds of miles away in a phenomenon known as “sky wave propagation,” or simply, “signal skip,” a thousand miles may not be enough to prevent interference from stations sharing the same frequency.
Early radio stations established by city governments (such as WJAX Jacksonville and WCOA Pensacola) as well as real estate developers like Carl Fisher (WIOD Miami) used signal skip to send their “fun in the sun” messages to frigid audiences up north. This engineering and physics reality was the raison d’etre for WFLA and WSUN, two stations owned by the St. Petersburg and Clearwater chambers of commerce. Because the stations shared the frequency of 620 kHz on the lower part of the AM band, their signal enjoyed even greater skip. With only 2,500 watts of daytime power and 1,000 watts nighttime, WFLA and WSUN could tout the area’s tropical climate, sandy beaches, and relaxed lifestyle to thousands of winter-worn northerners.
The signal skip that was cherished by the chambers of commerce was cursed by one particularly powerful station in Milwaukee. WTMJ-AM, owned by the influential Milwaukee Journal, also broadcast on 620 kHz, one of the few in the nation on that frequency. Interference from WFLA/WSUN prohibited its growth in its own market, and as the older of the stations (WTMJ signed on May 1922), it had priority with the Federal Radio Commission.
WTMJ petitioned the FRC to abolish WFLA/WSUN, change its frequency, or reduce its power. A frequency change or power cut would have severely impacted the mission of the Florida stations, so much so that the chambers contemplated giving up the broadcast license. Fortunately, its station manager, W. Walter Tison, was both an engineer and a fighter. For help, he turned to his engineering consultant, Commander T. A. M. Craven, to see if they could “direct” his stations’ signals to reduce interference in Milwaukee yet still reach other northern audiences.
In the 1930s engineers had done some experiments with directing the signals of short wave, point-to-point transmissions, but most had dismissed its value for AM broadcasting. Commander Craven enlisted the help of Ray Wilmotte, a longtime Tampa Bay broadcast engineer, British expatriate, and direction-finder expert who was working on expanding the concept to directional antennas. Wilmotte was so sure that he could solve WFLA/WSUN’s Milwaukee problem “that he agreed to be paid only if he was successful,” according to radio historian Barry Mishkind.
It was quite a project, and it created what would become a landmark gateway to Clearwater on the Courtney Campbell Causeway. After detailed calculations, Wilmotte laid out two, two-hundred-foot self-supporting towers, one on either side of the causeway, “a quarter wavelength apart, in line with the bearing toward Milwaukee, 346-degrees,” Mishkind explained. Calling the process “quite simple,” Mishkind described how “the transmission lines [were] cut to quarter-wavelength, and routed in a curved way to the towers.” The result, Wilmotte predicted, would be a “null” (low signal) toward Milwaukee.
It worked! So well, in fact, that an FRC inspector in the path of the null “who was to measure the frequency of the station, instead had to ask if WFLA/WSUN had authority to be off the air,” Mishkind reported. “He could not hear the station at all!” The first directional broadcasting antenna array in the world was deployed in April 1932, and served the Florida chambers well for years, beckoning northerners (other than those in Milwaukee) to “come on down.” In 1950 the original towers and directional antenna array were relocated to the Gandy Causeway where they still straddled the roadway.
Ronald D. Rackley, who later became consulting engineer for WFLA/WSUN and at this writing is a partner at the consulting firm of duTreil, Lundin & Rackley in Sarasota, had a longtime fascination with the pioneering directional antenna array. After years of research, Rackley, along with Wilson Welch, Clear Channel radio’s Gulf Coast director of engineering, went searching for the original towers’ site on the Courtney Campbell, eventually discovering the location from an old aerial photo. The south tower site is now home to a sewage plant, but the original base for the north tower “was hidden in a clump of trees in a field next to a gas station,” wrote Mishkind. “Clearly visible was the date, ‘Mar 8 ’32,’ when the tower bases were poured.”
(Photo courtesy of jeff560.tripod.com)

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