Hollmann's doctoral research included the development of an ultra-short-wave transmitter and receiver for centimetre and decimetre waves.
There he continued studies in microwaves and cathode ray tubes and also worked on the ionosphere research and radio astronomy.
In January 1934, Hans-Karl von Willisen and Paul-Günther Erbslöh started a company called Gesellschaft für elektroakustische und mechanische Apparate (GEMA).
Telefunken set up a radar business in 1933 based on Hollmann's work and developed a much shorter-range gun-laying system called Würzburg.
He invented and patented a prototype of the cavity magnetron in 1935,[1] but the German military considered the frequency drift of Hollman's device to be undesirable, and based their radar systems on the klystron instead.
During the war he supervised many research institutes in occupied countries and saved many scientists from being deported to Germany.