Specialized vacuum tubes are required for RF operation at frequencies above 300 MHz. These bands are known as ultrahigh frequency (UHF) band, which ranges from 300 MHz to 3 GHz, and the microwave band, which ranges from 3 GHz up. The magnetron and the Klystron are examples of tubes that are used to generate and amplify signals at these frequencies.
A magnetron contains a cathode and a surrounding anode. The anode is divided into sections, or cavities, by radial barriers. The output is taken from an opening in the anode, and passes into a waveguide that serves as a transmission line for the RF output energy.
The cathode is connected to the negative terminal of a high-voltage source, and the anode is connected to the positive terminal. Therefore, electrons flow radially outward. A magnetic field is applied lengthwise through the cavities. As a result, the electron paths are bent into spirals. The electric field produced by the high voltage, interacting with the longitudinal magnetic field and the effects of the cavities, causes the electrons to bunch up into clouds. The swirling movement of the electron clouds causes a fluctuating current in the anode. The frequency depends on the shapes and sizes of the cavities. Small cavities result in the highest oscillation frequencies; larger cavities produce oscillation at relatively lower frequencies.
A magnetron can generate more than 1 kW of RF power at a frequency of 1 GHz. As the frequency increases, the realizable power output decreases. At 10 GHz, a typical magnetron generates about 20 W of RF power output.
A Klystron has an electron gun, one or more cavities, and a device that modulates the electron beam. There are several different types. The most common are the multicavity Klystron and the reflex Klystron.
In a multicavity Klystron, the electron beam is velocity-modulated in the first cavity. This causes the density of electrons (the number of particles per unit volume) in the beam to change as the beam moves through subsequent cavities. The electrons tend to bunch up in some regions and spread out in other regions. The intermediate cavities increase the magnitude of the electron beam modulation, resulting in amplification. Output is taken from the last cavity. Peak power levels in some multicavity Klystrons can exceed 1 MW (106 W), although the average power is much less.
A reflex Klystron has a single cavity. A retarding field causes the electron beam to periodically reverse direction. This produces a phase reversal that allows large amounts of energy to be drawn from the electrons. A typical reflex Klystron can produce signals on the order of a few watts at frequencies of 300 MHz and above.