Walt Hayden’s Rotary Spark Transmitter

CHRS Member Walt Hayden shows us the operating rotary spark transmitter he built with help from Denny Monticelli. It is a brilliant piece of equipment built with great attention to detail. Beautiful workmanship from Walt. Thank you! Here is Walt’s explanation.

Rotary Spark Gap 500 Watt 700 KHz Radio Transmitter c1915

This amateur radio transmitter is made with period components. It is representative of transmitters used by amateurs in the years before and after World War I. During this era many amateur radio operators used fixed spark gap transmitters. However, amateur radio operators that could afford the components had rotary spark gap transmitters because they produced much higher output power than fixed gap transmitters. Amateur transmitters in this era communicated using morse code.

Although better than a fixed gap transmitter, this rotary gap transmitter is not very efficient because it produces 10 watts of radio frequency output using a power input of about 500 watts. Although spark gap transmitters are tuned to one frequency, they produced a broadband radio frequency output. This made communications difficult because transmissions interfered with one another. A better transmitter was needed.

The spark transmitter era begins to fade in the early 1920s when vacuum tubes for radio transmitters became available. By the mid-1920s many amateur radio operators were using vacuum tube radio transmitters. In1929 the Federal Radio Commission (now the FCC) prohibited amateur radio operators from using spark transmitters of all types.