75 YEARS OF MICROWAVE TUBES .1.

The succesful development of powerful microwave tubes reaching up to t he millimeter wave range was enabled above all by two steps: utilizing the finite transit time of electrons for the generation of density mo dulated electron beams and by a planar junction of the high frequency electrodes with the coaxial-line or cavity resonators respectively. Fu rthermore the application of concentrated linear electron beams made p ossible a separation of the highfrequency electrodes from the beam gui ding electrodes. These principles and also the method of extracting en ergy from the beam by induction were applied in the past 75 years to d ifferent designs of microwave tubes. They are beginning with the retar ding field tube (1920) and culminate in the gyrotron whose development is not at all concluded. After a short description of the mechanism a nd design of these tubes the most important electrical properties are discussed.