As he has over the past several years, Brian Justin, WA1ZMS/4 — an active participant in the ARRL’s WD2XSH 600 meter experimental project — will transmit voice and music on 486 kHz as WG2XFQ on Christmas Eve and Christmas Day and again on New Year’s Eve and New Year’s Day. Transmissions will begin at 0001 UTC and end at 2359 UTC.

Justin, who may be better known for his microwave exploits on ham radio, will use an AM audio loop modulating his vintage-style, homebrew transmitter to honor Reginald Fessenden’s Christmas Eve 1906 AM voice transmission.

“While his original transmissions used a set of carbon microphones in the antenna lead to modulate the signal,” Justin explained, “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 W master oscillator power amplifier (MOPA) transmitter using 1920s vintage components. He said a modern 500 W FET linear amplifier allows him to meet his WG2XFQ ERP limit of 20 W.

An RF engineer, Justin collects pre-1920 wireless gear and has a World War I Heising-modulated aircraft transmitter he’s planning to restore. 

A Word on Heising Modulation

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. — Brian Justin, WA1ZMS