Ham Radio and the Discovery of the Ionosphere (Keynote)

Ham radio's involvement in the discovery of the ionosphere during the early 20th century constitutes a core part of the radio amateur community's collective memory. I will review this episode in a broader historical context. Why radio waves could propagate over long distances along the earth's curvature had been debated since the invention of wireless telegraphy in the late 1890s. By the 1910s, physicists' consensus was that radio waves bounced back from an electrically conductive surface in the upper sky known as the "Kennelly-Heaviside layer." Meanwhile, electrical engineers' empirical studies led to the so-called "Austin-Cohen formula" that predicted a decrease of propagating range with wavelength, implying that transoceanic or transcontinental wireless communication could only be achieved at wavelengths longer than 200 m. Despite these scientific convictions, the American Radio Relay League (ARRL) and its sister organizations in the UK and France in the 1920s embarked large-scale collective experiments for transatlantic wireless signal transmission at wavelengths shorter than 200 m. Their success challenged the Austin-Cohen formula. In addition, ARRL members collaborated with US naval researchers to experiment with medium-range radio-wave propagation. Their studies resulted in the identification of the skip zone—that radio signals disappeared at certain distances from a transmitter but emerged again at a further range. These findings from radio amateurs’ activities paved a crucial ground for the British and American scientists Edward Appleton, Miles Barnett, Gregory Breit, and Merle Tuve to perform radio experiments that provided direct evidence for the ionosphere—a more complex geophysical entity than the Kennelly-Heaviside layer. In this talk, I will examine the radio amateurs' collective experiments in the discovery of the ionosphere. I will also discuss the implications of this form of collaboration to ham radio's later collective technical activities and engagements with "citizen science."