Armstrong prototype FM transmitter 1935

The first prototype FM (frequency modulation) transmitter, invented and built by Edwin Armstrong at the Empire State Building. Licensed as experimental station W2XDG, beginning June 2, 1934 Armstrong used it for a series of extensive secret tests of his new frequency modulation broadcasting system which he had been developing since 1928. The tests were successful, and in summer 1935 he announced his new system. The device transmitted on 41 MHz with an output power of 50 W, later increased to 2 kilowatts. It consisted of a low frequency crystal oscillator which was phase modulated by a balanced modulator, then was multiplied by a number of frequency doubler stages to the output frequency, which was applied to an ordinary transmitter RF amplifier. It had a frequency deviation of about 75 kHz. The 2 kW FM transmitter produced much better reception at a distance of 75 miles than a 50 kW AM transmitter broadcasting the same material. Information from Kruse, Robert S., "The Armstrong System of Modulation" in Radio magazine, No. 204, January 1936, p. 62-67. It took a number of years for FM broadcasting to catch on with listeners, but by 1970 most music broadcasting migrated to FM due to its superior audio quality.