New 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.
A fancier version of one I built many years back. This used a helix inductor from an old Marconi 1/4 Kw lifeboat spark set as the primary coil and a variably coupled secondary. The spark was synchronous using a 200Hz AC supply from a modified computer UPS so you got a 400 Hz spark frequency. Several of the parts were contemporary including the high voltage transformer and key. I think that it might have been a bit more efficient though as it could light a 15 watt bulb across the “jigger” when set up properly. It used a bank of old TX type mica HV capacitors for the spark circuit tuning. The spark gap was the open studded wheel type and looked VERY impressive when operating!
I might be able to find a photo somewhere as it now lives in the local “Museum of transport and technology”. If anyone is interested I will look it up.
It went with a “loose coupler” receiver which had various detectors. A pyrite and steel “Crystal”, an electrolytic detector and a couple of versions of the magnetic detector by Marconi and Fessenden. The museum has that too! BTW the Fessenden “Maggie” was better than the better known Marconi one, smaller, slightly more sensitive and needing a lot less winding up as it just rotated a small bar magnet rather than using iron wire.
Oddly enough I was actually using spark technology until just before I retired 20 years ago. I was one of NZ’s best known acousticians electronics engineer and I built several several versions of triggered spark systems. These were used in model studies of auditoria when a known energy spark was used with an ultrasonic microphone to evaluate a design. We did the acoustic design of the Orange County hall in the US and the Barbican in London among many others.
Please amend my posting on my rotary gap TX. The other magnetic detector is credited to De Forrest not Fessenden. This is from a reference I looked up, but De Forrest had a rather “dodgy” reputation so he may have stolen the idea perhaps???
I thought the magnetic detector was credited to E. Rutherford. He was a from NZ. Marconi turned it into a practical instrument. I don’t remember reading that De Forrest, another dodgy character, doing anything with the magnetic detector.