Jonathan Zenneck (1871–1959) und die Anfänge der Ionosphärenforschung in Deutschland

Jonathan Zenneck (1871-1959) is considered a pioneer of radio technology. He began his career as an assistant to Ferdinand Braun (1850-1918), who received the Nobel Prize in Physics in 1909 together with Guglielmo Marconi (1874-1937) for the development of wireless telegraphy. A long unsolved problem concerned the propagation of radio signals over long distances. Zenneck and Arnold Sommerfeld (1868-1951) developed a theory according to which the waves emitted by an antenna could not only propagate as space waves like light, but also as surface waves and thus follow the curvature of the earth. In the 1920s, however, experiments with short waves showed that their propagation over long distances was due to reflection from an electrically conductive layer in the higher atmosphere, the so-called ionosphere. With an experimental station in the Bavarian Alps at Herzogstand, Zenneck made echo soundings at the ionosphere his priority research project. This made it possible to determine the height of the reflecting ionospheric layers, which depended on the sun’s UV radiation and were decisive for the propagation of short waves over long distances. The height from which the signals were reflected varied according to frequency and time of day. Thus echo sounding of the ionosphere became a tool for suggesting frequencies for long-range shortwave communication. Such measurements achieved strategic importance during the Second World War. At the same time, ionospheric research became a new geophysical discipline that deserved interest also for scientific reasons. This Janus-faced profile is reflected in Zenneck’s role as advisor to the Reich Aviation Ministry and in his reputation as a pioneer of ionospheric physics in Germany.