History of Microwave Passive Components with Particular Attention to Directional Couplers

After Hertz verified Maxwell's electromagnetic wave theory using a spark-gap microwave source, activity in the microwave range was negligible until about 1930, and quite limited until 1939. In that year, an explosion of activity commenced, motivated by military needs for radar and the invention of the high-power pulsed magnetron. Microwave passive component development during World War II (1939- 1945) is discussed briefly, and then, because of space limitations, this paper concentrates on the important sub-field of directional couplers. The history of microwave filters is covered by the coauthors in a companion paper, while certain other passive components are touched on in the other historical review papers in this issue.