On Christmas Eve, Christmas Day, New Year's Eve and New Year's Day, Brian Justin (WA1ZMS/4) will transmit voice and music on 486 kHz using a homebrew transmitter under experimental license WG2XFQ.
The transmissions will honor Reginald Fessenden's Christmas Eve 1906 AM voice transmission. Justin, an RF engineer, provided details, “While his original transmissions used a set of carbon microphones in the antenna lead to modulate the signal, WG2XFQ will be utilizing true Heising modulation in honor of Raymond Heising, who developed this early form of amplitude modulation during World War I.”
Justin constructed his 5 watt master oscillator power amplifier using 1920's vintage components, with the addition of a modern 500 watt FET linear amplifier to meet WG2XFQ's ERP limit of 20 watts.
“Raymond Heising developed the first form of AM voice modulation during World War I. The goal was to find a way to take an existing aircraft CW transmitter, used to send air reconnaissance information to the ground with few as possible parts or tubes required.”
“Since the CW rigs of the day were a simple keyed power oscillator, Heising figured that, if one could control the plate current of the CW tube at an audio rate, amplitude modulation (AM) would result. The same crystal detectors used to receive spark transmissions could demodulate the AM without any modification.”
“His design was very simple. If you add a large iron choke to the output of the B+ supply, it will become a constant-current supply. You then only need to feed that B+ to both the CW power oscillator and to an identical power tube that is grid modulated. The two tubes compete for the constant current from the B+ supply, and if the audio tube is driven hard, less current flows in the CW tube and vise-versa. Thus, his design is sometimes referred to as constant current modulation from a technical perspective.”
“A limitation is the need for identical and perfect matching of the two tubes. One is running at RF, the other at AF. The typical result is only about 50 percent modulation.”
EDITOR’S NOTE: Several researchers have concluded that the fabled Fessenden Christmas Eve "broadcast" may never have occurred. While there is no doubt that Mr. Fessenden did perform a public demonstration of the transmission of speech and music on the afternoon of Dec. 21, 1906, there is absolutely no contemporary documentation to back up the claim Fessenden made more than a quarter of a century later that he had done the "world's first broadcast" on Christmas Eve that year.