Birth of Commercial FM Broadcasting

Sunday , 22, January 2012 Leave a comment

March 1, 1941 can be considered the day of the appearance of FM radio broadcasting in its usual form, that is, as a commercial industry, when the W47NV radio station with a transmitting center in Nashville, Tennessee, USA, went on the air for the first time. Here is how the American weekly “Broadcasting” wrote about this on March 10, 1941 in the article “New FM Station of WSM, in Nashville, Starts Operation With 70 Hours Weekly”:

“BECOMING the first FM station to start operating on a regular schedule under full-commercial authorization by the FCC, W47NV, Nashville, FM adjunct of WSM, on March 1 started a weekly schedule totaling 70 hours of FM broadcasting. The station, operating with 20 kw. power on 44.7 mc., broadcasts from 1 to 11 p.m. weekdays and 11 a.m. to 9 p.m. Sundays.

The station incorporates several unique operating factors. Its antenna, completely insulated from WSM signals, utilizes the 878-foot transmitter tower of WSM, and technical operation of both WSM and W47NV centers in the WSM transmitter house. The FM transmitter, designed by WSM Chief Engineer Jack DeWitt, is said to utilize a new method of generating Armstrong wide-swing FM signals and was custom-built under direction of Mr. DeWitt by members of the WSM engineering staff.

The four-element FM turnstile array on the WSM tower, connected by coaxial cable to the transmitter house, is located just below the flagpole topping the tower. The coaxial line from the transmitter terminates at the base of the tower in a matching section which feeds the open wire line on the tower, an arrangement claimed to operate not only as an effective filter separating the AM signals of WSM and the FM signals of W47NV but also as a lightning ground for the entire structure. The FM signal is genetated in a single relay rack unit in the control room of the transmitter house, coupled successively to a three-stage 1,000-watt amplifier and a 20,000-watt amplifier. The main rectifier and other power supply equipment are located in the basement of the transmitter house.

Although a complete separate staff has not been set up, Program Director Tom Stewart and Announcers Bill Terry Jr. and Herbert Oglesby are handling programs of the station. Coverage of the station is being tested, but preliminary reports from listeners indicate that coverage comes up to calculations, it was stated. First regular listener reporting lived in Beaver Dam, Ky., about 90 airline miles from the transmitter”.

Looking over the new station’s custom-built 20 kw. FM transmitter, constructed by the WSM engineering staff, is Jack DeWitt, WSM chief engineer, and Warren McNeil, Tennessee bureau chief of Associated Press.
Relay rack with 20kW FM transmitter. In the left section there is a “build-up” amplifier made on powerful GL-833A triodes, and in the right section there is a water-cooled power amplifier based on a GL-889A.
The same relay rack of the transmitter, but from the back (only the section with the power amplifier is visible).
Power triode GL-833A (General Electric).
Power triode GL-889A (General Electric). On such lamps, the output stage of the power amplifier was made.
High voltage switchboard purchased from General Electric. Provides 7.8 kV anode voltage for powerful GL-889A triodes in the output stage of the power amplifier.

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