FIGURE 2-1 Electromagnetic spectrum and frequencies used in microwave processing (Sutton, 1993).

Major advances in microwave generation and generators occurred in the early 1940s with the invention, rapid development, and deployment of the cavity magnetron on the heels of the earlier (1938) invention of the klystron. What started as "flea powered" curiosities are now capable of generating hundreds of megawatts of power.

The genesis of microwave generator development was the introduction of DeForest's audion tube, the first electron tube amplifier, in 1907. In addition to being developed to be capable of generating megawatts of continuous-wave power (albeit limited in operating frequencies to the region below 1,000 MHz), the audion tube led to the lineage of microwave tubes. Figure 2-2 traces, in time, that beginning and the inventions and developments that followed. The dependence of one tube on the understanding and development of others is illustrated.

A historical discussion of microwave generators would be incomplete without highlighting two major developments. The first involved space charge and the transit time of electron motion within a vacuum, which represented a fundamental limitation to the operating frequency and output power of conventional gridded tubes. When the time of transit became an appreciable part of a microwave frequency cycle, performance degraded, forcing the designer to smaller and smaller sizes to achieve higher frequency. The invention of the kylstron obviated this limitation by utilizing space-charge effects and transit-time effects in a device whose dimensions encompass many wavelengths. A klystron today is capable of peak output power of 100 megawatts at 10 GHz.