The author got his first degree in the Department of Electrical Engineering, University of Hong Kong (HKU) in 1977. He is interested in various areas of electronics including microwave electronics. When he was a student in the Department of Electrical Engineering, HKU, there was a laboratory session on the study of a reflex klystron. (Note: A reflex klystron is a vacuum tube used to generate microwave.) The author together with Chun Wah Lee (another male HKU student) and Po Ching Leung (a female HKU student) were arranged to attend a laboratory session on the study of a reflex klystron. This happened before we had the chance to attend a course on vacuum tubes. I admitted that I was ignorant of reflex klystron theory. We anyhow finished the laboratory session. The author admitted that he actually did not understand what was going on. We submitted our report and that was the end. Sometime later, the author attended a HKU course on vacuum tubes and learned that there were two important vacuum tubes used as microwave generators. The first one was the reflex klystron as mentioned above. The second one was the magnetron. Both devices were important in the history of microwave electronics because they were used in radars.
It is 2024 now. The reflex klystron is now more or less an obsolete electron device. However, the magnetron is still mass produced, for example, for microwave generation in microwave ovens. At this point, the author would like to give a brief history of magnetrons. Albert Wallace Hull (1880-1966) was an US engineer working for General Electric [1]. He invented a vacuum tube device which had a cathode emitting electrons and an anode to collect electrons; a magnetic field was used to control electron motion [2]-[3]. Hull named the vacuum tube device “magnetron” in 1921 [2]. A magnetron is just a vacuum tube diode with an additional magnetic field perpendicular to the electric field. In the beginning, magnetrons did not appear to be particularly useful. Within a couple of years of its invention, scientists and engineers around the world realized that magnetrons could work as efficient microwave generators. Arthur Lee Samuel (1901-1990) was an US scientist or engineer working for Bell Laboratories, USA. Samuel has been considered as the inventor of cavity magnetron; he applied for patent in 1934 and got it in 1936 [4]. A cavity resonator is a small open cavity in a metal block. The metal block served as the anode. Basically a cavity magnetron is a magnetron with microwave resonance cavities incorporated in the anode. At that time, the United Kingdom (UK) was expecting war to break out soon in Europe. UK was developing RADAR for her defense. A powerful microwave generator was important. It turned out that UK scientists and engineers managed to develop cavity magnetrons which could generate sufficient amount of microwave power for RADAR. Henry Albert Howard Boot (1917-1983) and Sir John Turton Randall (1905-1984) were famous for this. On 21 February 1940 (Wednesday), Boot and Randall managed to demonstrate a powerful cavity magnetron operating at a wavelength of 9.8 cm [5]. (Note: The frequency corresponding to a wavelength of 9.8 cm is about 3 GHz.)