just like our ancestors did in the days before valves with their powerful transmitters. Were they spark-gated? To generate a high-frequency alternating current and apply it to an antenna, they used huge alternators in the stations, like the one in the photo in a magazine article. Voltages of thousands of volts were generated and applied to the antenna. The 100 kW alternator at the New York Radio station produced a signal in the 300 (1MHz) band. At the time, according to the article, there were about 600 of these alternators in operation in radio stations throughout the country, in cities like Chicago, San Francisco, etc. I'll tell you how the modulation was done some time.