Microwave & Microwave generation | How do they work?

Microwave Communication

Microwaves mean high frequencies than 300 MHz and composed of short wavelength less than 1m waves. So those electromagnetic waves which have high frequencies but having short wavelength are called microwaves. The communication which is done through microwaves is called microwave communication. In other words, the communication in which the microwaves are used as the transmission medium is called microwave communication.

Role of EM Waves and Antennas in Microwave Communication

In microwave communication, the modulated signals are radiated through an antenna in the atmosphere while the receiving antenna receives these information signals. Microwaves depend mostly on the resonant cavities and waveguides through which microwaves are generated and transmitted (The metallic cavity or empty cavity present in the conductive material which is tuned on a specific frequency is called cavity resonators). Microwaves transmission is done through electromagnetic waves which are also called light of sight of communication. A line of sight communication is generally that communication in which the information signals are transmitted in the atmosphere without any physical medium and the receiving antenna receives the signals. Point to be noted that in a line of sight communication both the antennas must be face to face on the transmitting and receiving towers.
Microwaves (0.3—300Ghz) are categorized into three groups according to their bands.

Ultra-high-frequency band (UHF)

This frequency band is (0.3 GHz to 3 GHz) ranges.

Super-high-frequency band (SHF)

This frequency band is (3 GHz to 30 GHz) ranges.

Extra high-frequency band (EHF)

This frequency band is (30 GHz to 300 GHz) ranges.

Due to some limitations of the conventional tubes i.e. inductance and capacitance balance b/w the electrodes, the speed of electrons from one electrode to other electrodes etc. Microwaves generation is not possible from them. There is need to use some special tubes through microwaves can be generated in few watts up to hundred watts. For that purpose, there are some special tubes called microwave generators which are frequently using as microwave generators in microwave communication. I.e. Magnetron, klystron, traveling wave tube. Which are used as microwave oscillators and microwave amplifiers as well? Here we are going to explain in detail the microwave tube magnetron.

What is Magnetron and How it Works?

A cylindrical diode which is used as microwave oscillator in microwave communication is called magnetron. In other words, the magnetron is used in microwave communication as a microwave generator. Scientists Randall and Boot from Britain has invented this magnetron tube.

Magnetron Construction

Construction wise magnetron is basically cylindrical in shape. A magnetron is consists of eight permanent copper anode cavities around a heated tungsten cathode. The number of cavities must be even in the magnetron. Cavities size is generally kept according to the oscillation frequency. All the cavities are made at 300 or 450. Co-axial cable is connected to one of the cavity to get the desired output microwaves. While generating high microwaves waveguides are used to get the output signal.

Cavity magnetron working principle is the same of traveling wave tube. According to that, the high microwave frequencies are generated by the interaction of radial electric field and axial magnetic field. So forgetting microwave frequencies we must provide an electric field around the magnetron tube. As shown in the figures.

  The electric field produces movement in a straight line b/w cathode and anode while the magnetic field produces circular movement. That’s why the electrons emitted from the heated cathode don’t go in a straight line due to the interaction b/w electric field and magnetic field.

                         When the magnetic field value is zero then the emitted electron goes directly to the anode around. There is only electric field effect on it that time as shown in the figure with X-line. But when a magnetic field intensity increases a bit then the electron path is not that much straight like before but it becomes a little curved as shown in the figure with Y-line.
This technique is very progressive to generate high-frequency microwaves because when the emitted electron nears to the anode electrode their velocity increases due to the effect of magnetic field. But when the magnetic field is increased further the emitted electron couldn’t reach to anode but just touches the anode electrode and come back to cathode due to the effect of a magnetic field as shown in the figure with Z-line. This is called cut off the field. Similarly, if we keep increasing the magnetic field the emitted electrons radiate and come back to cathode again which in turn heat up the cathode more up to dangerous temperature value.

Hence we concluded that if we provide and maintain a certain value of magnetic field around the magnetron tube we can get the desired frequency of microwaves. Magnetron produces microwave frequency from 900 MHz to 2.5 GHz with a power of 3oo watt to 10k watt, while it works about 70% efficiency.

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