The Audion

Vacuum tubes revolutionized radio, changing it more than any other single invention. When first introduced, however, even the scientists and engineers working with them did not fully understand how they worked. One of the first tubes to appear in QST was the Audion, by DeForest. Although it had been introduced back in 1905, it was expensive and amateurs did not begin to use it until seven years later, when 22-year-old Edwin Armstrong demonstrated its practical use as a regenerative detector. … Continue reading

Spark Radio

Before tubes became available and affordable and made electronic oscillators practical, the spark gap circuit was the most widely used method for generating radio frequency (RF) signals. Its basic design and operation are simple. A capacitor is connected in series with an inductor and a pair of electrodes separated by a small distance—a spark gap. The capacitor, commonly called a condenser at the time, is charged by a high voltage supply. When this voltage reaches a critical level, a spark … Continue reading