This page covers Microwave tubes which include klystron,magnetron and Traveling Wave tube(TWT).
Before the invention of transistors, vacuum tubes are used for electronic circuit design. These tubes are used for amplification,oscillation,switching and other applications. In many applications such as for VSAT Earth stations,TV stations and military applications high power is needed which is generated with the use of microwave tubes.
The microwave tube, klystron is basically a vaccum tube designed with cavity resonators to produce velocity modulation of electron beam for amplification purpose.
As shown in the figure, cathode in a vacuum tube is heated by a filament, this cathode at high temperature, emits electrons,which are attracted by collector. This causes current to be established between cathode and collector.
Klystrons are available in wide range of sizes. Small size units produce mwatts of power while large size units produce thousand watts of power. They are used at UHF as well as 100 GHz frequency band of operation.
Special variation to the basic klystron tube is called as reflex klystron. It is used as microwave oscillator. The difference between normal klystron and reflex klystron is that reflex klystron uses single cavity.
Reflex klystron devices are targetted for low power and are small in sizes. Power ranges from 100mwatt to several watts. Output of reflex klystron can easily be Freq-Modulated with the addition of AC modulating signal in the series with repeller voltage. Gunn diodes have replaced this type of klystrons.
Combination of simple diode vacuum tube with cavity resonators and extremely powerful permanent magnet is called magnetron. This microwave tube assembly is depicted in the figure. The magnetron consists of circular anode made of copper and connected with high voltage +ve DC. Even no. of resonant cavities having diameter equal to one half wavelength of the operating frequency is placed in this circular anode. In the center there is a interaction chamber which acts as cathode and emits electrons when heated up.
Here direction of electrons is not as in normal diode, but changed by application of strong magnetic field with the use of C shaped magnet. Megnetron help generate power in millions of watts. In pulsed mode, it can even generate mega watts of power.
There are two main types of magnetrons viz. pulsed magnetron and CW magnetron. Pulsed magnetrons are used in radar and Continuous Wave(CW) magnetrons are used in microwave ovens.
Traveling Wave Tube-TWT
This microwave tube is used as microwave RF power amplifier.The main benefits of Traveling Wave Tube(TWT) is wide bandwidth of operation and can generate hundreds and even thousands of watts. TWTs can also be used as both continuous and pulsed mode of operation. Length of this tube is about 1 ft to few feet, many wavelengths to the operating frequency.
Let's see how it works. There is a coil surrounding the tube. Electron beam passes through the center of the helix due to magnets and they are tightly coupled. The microwave signal need to be amplified is applied at the cathode end of helix. The amplified output is taken from the collector end of the helix.