Rudolf Kühnhold

Rudolf Kühnhold (1903–1992) was an experimental physicist who is often given credit for initiating research that led to the Funkmessgerät (radio measuring device – radar) in Germany.

Early life A native of Schwallungen, Saxe-Meiningen, Kühnhold received his higher education in physics at the University of Göttingen. After graduating with a Ph.D. in physics in 1928, he took a position at the Nachrichtenmittel-Versuchsanstalt (NVA – Navy Transmissions Laboratory) of the Kriegsmarine (Germany Navy) in Kiel. There he worked in acoustical research, specifically in Unterwasser-Schall (sonar) with the objective of improving the accuracy of detection of vessels using near-surface, underwater signals. Although his efforts led to a patent, and in 1931, he was promoted to Scientific Director of the NVA, he became convinced that the desired accuracy would only be attained by using electromagnetic, rather than acoustical, techniques.

Radar research Background The first demonstration of radio signals for detecting ships was made in 1904 by Christian Hülsmeyer, with an apparatus called the Telemobilskop (Telemobiloscope). This device, however, could neither distinguish between multiple targets nor directly measure the distance to a target, and was thus not accepted as of practical value. Other similar sets had come forth in the intervening decades, but none were successful.

Kühnhold’s approach Kühnhold's analytical studies indicated that a very narrow beam could solve the multiple-target problem. In 1933, he obtain transmitting and receiving sets operating at 13.5 cm (2.22 GHz), both units using Barkhausen–Kurz tubes. A reflecting target was set up at 2-km distance. The transmitter produced only 0.1 watt – too small for the 4-km total path – and the experiment failed.

For further experiments, Kühnhold turned to Paul-Gunther Erbsloh and Hans-Karl von Willisen, amateur radio operators who had started a project in a narrow-beam, VHF system for secure communications. For the effort, In January 1934, Erbsloh and von Willisen, with backing from Kühnhold, formed a new company – Gesellschaft für Electroakustische und Mechanische Apparate. From the start, this firm was always called simply GEMA.

A split-anode magnetron, producing 70 W at 50 cm (600 MHz) was purchased from the Philips Research Laboratory in the Netherlands. Hans E. Hollmann and Jakob Theodor J. Schultes, both affiliated with the prestigious Heinrich Hertz Institute in Berlin, were added as consultants for developing a regenerative receiver and Yagi antennas, respectively.

The transmitting and receiving antennas were set up some distance apart. In June 1934, large vessels passing through the Kiel Harbor were detected by Doppler interference at a distance of about 2 km. The apparatus had poor reliability of detection due to frequency instability of the magnetron, a characteristic of all early split-anode devices.

Partnership with GEMA Kühnhold worked closely with GEMA and led their attempts to improve the continuous-wave system, but also retained his position at the NVA. In October 1934, strong reflections were observed from an aircraft that happened to fly through the beam; this opened consideration of targets other than ships and brought funding from NVA. ELAC had major financial problems, resulting in selling its Nautik Division and ending Kühnhold's professional career in the 1960s.