Microwaves are electromagnetic waves with frequency greater than 1 GHz. Capacitors used in microwave system are of a few picofarads. All components used in microwave system are smaller in size. Unique category of coaxial cables is required in microwave system. Connectors used here possess low loss factor and excellent impedance matching and control. Triodes, tetrodes, and pentodes are conventional vacuum tubes which can be used for low microwave frequencies. Vacuum tube serves the purpose of controlling and amplifying small electrical signals. The basic components of a vacuum tube are heater filament, anode (plate) and a grid. Some common terms used here are interelectrode capacitances, lead inductance, transit time, gain-bandwidth product limitation and velocity modulation. Broadly microwave tubes are divided into two: Linear beam tubes and crossed-field tubes. Klystrons ( two-cavity oscillators, reflex oscillators, multicavity oscillators and extended interaction oscillators), hybrids (thystrons), gystrons, helix, backward wave oscillator, coupled cavity, ring-bar ring-loop and clover leaf centipole long-slot are belonging to linear beam tubes category. Examples for crossed-field tube are magnetrons, crossed field amplifier and carcinotrons. Klystrons are velocity modulated tubes that are used in radar and communication equipments as oscillators and amplifiers. Magnetron is a self contained microwave oscillator.
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