Microwave techniques are extensively used nowadays in long-distance communication, radio astronomy, medical field, radar systems, space navigation and in missile electronic systems. Frequency range of microwave is 1 to 300 GHz. Wavelength lie in the micron range. The concept of microwave tube emerged in 1939. A theory related to velocity modulation for microwave tubes was proposed by W.C. Hahn and G.F. Metcalf. R.H.Varian and S.F.Varian described a two-cavity klystron amplifier and oscillator by using velocity modulation. Many microwave solid state devices fabricated later, for example, tunnel diodes, transferred electron devices, Gunn diodes, avalanche transit-time devices etc. A microwave system has the following components: a microwave oscillator, waveguides, transmitting antenna, receiving antenna and a microwave amplifier.
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