PRESENTATION ON

MAGNETRON

DEVENDRA PRATAP SINGHLecturer(ECE Department)

Introduction . Physical construction of a magnetron. Basic magnetron operation:

Phase I : Production and acceleration of an electron beam. Phase II : Velocity-modulation of the electron beam Phase III : Forming of a Space-Charge Wheel. Phase IV : Dispense energy to the ac field.

ApplicationsAdvantages.Disadvantages

CONTENTS

INTRODUCTION

Microwaves are very high frequency waves or waves having very small(micro) wavelength.

A magnetron is a high-powered vacuum tube that generates non consistent microwaves with built-in resonators.

The electromagnetic energy created from a magnetron can travel at the speed of light and is the same type of energy used in radio and television broadcasting.

MICROWAVE FREQUENCIES

SPECTRUM • 300Mhz-3Ghz =UHF• 3Ghz-30Ghz =SHF• 30Ghz-300Ghz =EHF• 300-375*10^3

=INFRARED• 375*10^3-790*10^3 =VISIBLE• 790*10^3-225*10^5 =U.V Rays• 225*10^5-450*10^8 =X Rays• 450*10^8-270*10^9 =GAMMA• 270*10^9-INFINITE =COSMIC

(300Mhz-300Ghz)-microwave

Microwave frequency band 1-2 GHz L 2-4 GHz S 4-8 GHz C 8-12 GHz X 12-18 GHz KU 18-27 GHz K 27-40 GHz KA 40-300 GHz MILLIMETER >300 GHz

SUBMILIMETER

HISTORY

1921 Albert Wallace Hull invented Magnetron.

1940 John Randall and Henry Boot developed a high power Magnetron for Radar application during world war II.

1946 Dr. Percy debaron Spencer invented microwave oven.

1960 Scientists detected background noise. These are cosmic microwaves background radiation.

CONSTRUCTION

The cathode is at the center of the tube.

The anode of a magnetron is fabricated into a cylindrical solid copper block.

8 to 20 cylindrical holes around the cathode circumference are resonant cavities.

The open space between the plate and the cathode is called the interaction space.

In this space the electric and magnetic fields interact to exert force upon the electrons.

Physical construction of a magnetron

• The magnetic field is provided by a strong permanent magnet mounted around the magnetron so that the magnetic field is parallel with the axis of the cathode.

• The output lead is usually a probe or loop extending into one of the tuned cavities and coupled into a waveguide or coaxial line.

Contd.....

MAGNETRON OPERATION: Phase I : Production and acceleration of an electron beam

When no magnetic field exists, heating the cathode results in a uniform and direct movement of the field from the cathode to the plate (the blue path ).

If the strength of the magnetic field is increased, the path of the electron will have a sharper bend.

when the critical field value is reached, and the electrons just fail to reach the plate in their circular motion(red path).

This critical field has been setup in our device.

Phase II : Velocity-modulation of the electron beam

The dc field extends radially from adjacent anode segments to the cathode.

The ac fields, extends between the adjacent segments in the cavities.

In order to sustain oscillations in a resonant circuit ,it is necessary to continuously input energy in the correct phase to the RF signal.

This energy will be taken from the DC source and supplied to AC field.

Contd…

This ac field work in addition to the to the permanently available dc field

The ac field of each individual cavity increases or decreases the dc field.

The electrons which fly toward the anode segments loaded at the moment more positively are accelerated in addition. These get a higher tangential speed.

The electrons which fly toward the segments loaded at the moment more negatively are slow down. These get consequently a smaller tangential speed.

Phase III : Forming of a Space-Charge Wheel

The cumulative action of many electrons returning to the cathode.

While others are moving toward the anode forms a pattern resembling the moving spokes of a wheel known as a Space-Charge Wheel.

When one of the spokes just is near an anode segment which is loaded a little more negatively.

The electrons are slowed down and pass their energy on to the ac field.

Phase IV : Dispense energy to the ac field.

The electron spends energy to each cavity as it passes and eventually reaches the anode.

The electron has helped sustain oscillations because it has taken energy from the dc field and given it to the ac field.

Applications

RADAR HEATING LIGHTING

APPLICATIONS 1.RADAR

In radar devices the waveguide is connected to an antenna. The magnetron is operated with very short pulses of applied voltage, resulting in a short pulse of high power microwave energy being radiated. As in all radar systems, the radiation reflected off a target is analyzed to produce a radar map on a screen.

It detects mobile objects or targets by sending electromagnetic wave and analyzing the received echoes.

In microwave ovens the waveguide leads to a radio frequency-transparent port into the cooking chamber. It is important that there is food in the oven when it is operated so that these waves are absorbed, rather than reflecting into the waveguide where the intensity of standing waves can cause arcing. The arcing, if allowed to occur for long periods, will destroy the magnetron.

Contd..

2.HEATING :MICROWAVE OVENS

Contd..

3.LIGHTING

In microwave-excited lighting systems, such as Sulphur Lamps, a magnetron provides the microwave field that is passed through a waveguide to the lighting cavity containing the light-emitting substance (e.g. Sulfur, metal halides etc.)

Health hazards

The lens of the eye has no cooling blood flow, it is particularly prone to overheating when exposed to microwave radiation.

This heating can in turn lead to a higher incidence of cataracts in later life.

ADVANTAGES

The magnetron is a fairly efficient device. In a microwave oven, for instance, an 1100 watt input will generally create about 700 watts of microwave energy, an efficiency of around 65%.

The combination of the small-cavity magnetron, small antennas, and high resolution allowed small, high quality radars to be installed in aircraft.

DISADVANTAGES

They are costly and hence limited in use.

Although cavity magnetron are used because they generate a wide range of frequencies , the frequency is not precisely controllable.

The use in radar itself has reduced to some extent, as more accurate signals have generally been needed and developers have moved to klystron and traveling-wave tube systems for accurate frequencies.

REFERENCES

Samuel Y. Liao, “Microwave Devices and Circuit”, 3rd , Pearson Education.

R. E. Collin, “Fundamental for Microwave Engineering”, 2nd Ed., John Wiley India.

THANK YOU