MICROWAVE DEVICES SYSTEMS BY Mr V Sudheer Raja

MICROWAVE DEVICES & SYSTEMS BY Mr. V. Sudheer Raja, M. Tech Assistant professor , Department of Electrical Engineering Adama Science and Technology University E-Mail : Sudheerrajasanju@gmail. com Mr. V. Sudheer Raja

Chapter 1 Microwave Transmission lines Ø Microwave Spectrum and bands Ø Microwave Properties Ø Advantages & Limitations Ø Applications Ø Rectangular Waveguides Ø Circular waveguides Mr. V. Sudheer Raja

Electro Magnetic Spectrum Mr. V. Sudheer Raja

Microwaves ØMicrowaves are electromagnetic waves whose frequencies range from about 1 GHz – 300 GHz and wavelengths in air ranging from nearly 30 cm – 1 mm. ØThe word Microwave means very short wave, which is the shortest wavelength region of the radio spectrum and a part of the electromagnetic spectrum. ØMicrowave really indicates the wavelength in micron ranges. Mr. V. Sudheer Raja

Properties of Microwaves 1. 2. 3. 4. 5. Microwave is an electromagnetic radiation of short wavelength. They can reflect by conducting surfaces just like optical waves since they travel in straight line. Microwave currents flow through a thin outer layer of an ordinary cable. Microwaves are easily attenuated within short distances. They are not reflected by ionosphere Mr. V. Sudheer Raja

Advantages and Limitations Ø Ø 1. Increased bandwidth availability: Microwaves have large bandwidths compared to the common bands like short waves (SW), ultrahigh frequency (UHF) waves, etc. For example, the microwaves extending from = 1 cm - = 10 cm (i. e) from 30, 000 MHz – 3000 MHz, this region has a bandwidth of 27, 000 MHz. 2. Improved directive properties: Ø The second advantage of microwaves is their ability to use high gain directive antennas, any EM wave can be focused in a specified direction (Just as the focusing of light rays with lenses or reflectors) Mr. V. Sudheer Raja

Advantages and Limitations 3. Fading effect and reliability: Fading effect due to the variation in the transmission medium is more effective at low frequency. Ø Due to the Line of Sight (LOS) propagation and high frequencies, there is less fading effect and hence microwave communication is more reliable. Ø 4. Power requirements: Ø Transmitter / receiver power requirements are pretty low at microwave frequencies compared to that at short wave band. Mr. V. Sudheer Raja

Advantages and Limitations 5. Transparency property of microwaves: Ø Microwave frequency band ranging from 300 MHz – 10 GHz are capable of freely propagating through the atmosphere. Ø The presence of such a transparent window in a microwave band facilitates the study of microwave radiation from the sun and stars in radio astronomical research of space. Mr. V. Sudheer Raja

Applications Ø Microwaves have a wide range of applications in modern technology, which are listed below 1. Telecommunication: Intercontinental Telephone and TV, space communication (Earth – to – space and space – to – Earth), telemetry communication link for railways etc. Radars: detect aircraft, track / guide supersonic missiles, observe and track weather patterns, air traffic control (ATC), burglar alarms, garage door openers, police speed detectors etc. 2. Mr. V. Sudheer Raja

3. Commercial and industrial applications Ø Ø Ø Ø Microwave oven Drying machines – textile, food and paper industry for drying clothes, potato chips, printed matters etc. Food process industry – Precooling / cooking, pasteurization / sterility, hat frozen / refrigerated precooled meats, roasting of food grains / beans. Rubber industry / plastics / chemical / forest product industries Mining / public works, breaking rocks, tunnel boring, drying / breaking up concrete, breaking up coal seams, curing of cement. Drying inks / drying textiles, drying / sterilizing grains, drying / sterilizing pharmaceuticals, leather, tobacco, power transmission. Biomedical Applications ( diagnostic / therapeutic ) – diathermy for localized superficial heating, deep electromagnetic heating for treatment of cancer, hyperthermia ( local, regional or whole body for cancer therapy). Mr. V. Sudheer Raja

Other Applications 4. Identifying objects or personnel by non – contact method. 5. Light generated charge carriers in a microwave semiconductor make it possible to create a whole new world of microwave devices, fast jitter free switches, phase shifters, HF generators, etc. Mr. V. Sudheer Raja

THE MICROWAVE SYSTEM Mainly consists of two subsystems. ØTransmitter Subsystem: Oscillator, wave guide, Transmitting antenna ØReceiver Subsystem : Receiving antenna, wave guide, microwave amplifier & receiver. Mr. V. Sudheer Raja

MKS units and Physical constants used for Microwaves are: Mr. V. Sudheer Raja

Mr. V. Sudheer Raja

Waveguides Ø Wave guide Ø Basic features Ø Rectangular Wave guide Ø Circular Wave guide Ø Applications Mr. V. Sudheer Raja

Waveguides Introduction Ø At frequencies higher than 3 GHz, transmission of electromagnetic energy along the transmission lines and cables becomes difficult. Ø This is due to the losses that occur both in the solid dielectric needed to support the conductor and in the conductors themselves. ØA metallic tube can be used to transmit electromagnetic wave at the above frequencies Mr. V. Sudheer Raja

Wave guide Definition Ø A Hollow metallic tube of uniform cross section for transmitting electromagnetic waves by successive reflections from the inner walls of the tube is called waveguide. Mr. V. Sudheer Raja

Basic features ØWaveguides may be used to carry energy between pieces of equipment or over longer distances to carry transmitter power to an antenna or microwave signals from an antenna to a receiver ØIn Waveguides the electric and magnetic fields are confined to the space with in the guides. The electric and magnetic fields associated with the signal bounce off the inside walls back and forth as it progresses down the waveguide. ØA waveguide has a definite cutoff frequency for each allowed mode. ØThe dominant mode in a particular guide is the mode having lowest cut off frequency. ØIf the frequency of the impressed signal is greater that the cutoff frequency for a given mode, The electromagnetic energy will be transmitted through the waveguide without attenuation, else if the frequency is less than cutoff frequency, signal is attenuated with in the short distance. ØWaveguides are made from copper, aluminum or brass. These metals are extruded into long rectangular. Mr. V. or circular pipes. Sudheer Raja

Mr. V. Sudheer Raja

Electromagnetic Wave Equation --The Electric & Magnetic wave equations can be basically derived from Maxwell’s equations. They are expressed as follows in time domain Where, Mr. V. Sudheer Raja

--The Maxwell’s equations expressed in frequency domain as, Where, Mr. V. Sudheer Raja

Components of Electric and Magnetic Field Intensities in an EM wave Mr. V. Sudheer Raja

EM field configuration within the waveguide ØIn order to determine the EM field configuration within the waveguide, Maxwell’s equations should be solved subject to appropriate boundary conditions at the walls of the guide. ØSuch solutions give rise to a number of field configurations. Each configuration is known as a mode. The following are the different modes possible in a waveguide system Mr. V. Sudheer Raja

Possible Types of modes 1. Transverse Electro Magnetic (TEM) wave: Here both electric and magnetic fields are directed components. (i. e. ) E z = 0 and Hz = 0 2. Transverse Electric (TE) wave: Here only the electric field is purely transverse to the direction of propagation and the magnetic field is not purely transverse. (i. e. ) E z = 0, Hz ≠ 0 Mr. V. Sudheer Raja

Possible Types of modes 3. Transverse Magnetic (TM) wave: Here only magnetic field is transverse to the direction of propagation and the electric field is not purely transverse. (i. e. ) E z ≠ 0, Hz = 0. 4. Hybrid (HE) wave: Here neither electric nor magnetic fields are purely transverse to the direction of propagation. (i. e. ) E z ≠ 0, Hz ≠ 0. Mr. V. Sudheer Raja

Mr. V. Sudheer Raja

Rectangular Waveguides Ø Any shape of cross section of a waveguide can support electromagnetic waves of which rectangular and circular waveguides have become more common. Ø A hallow metallic tube having rectangular cross section is known as Rectangular waveguide Mr. V. Sudheer Raja

Dimensions of the waveguide which determines the operating frequency range: 1. The size of the waveguide determines its operating frequency range. 2. The frequency of operation is determined by the dimension ‘a’. 3. This dimension is usually made equal to one – half the wavelength at the lowest frequency of operation, this frequency is known as the waveguide cutoff frequency. 4. At the cutoff frequency and below, the waveguide will not transmit energy. At frequencies above the cutoff frequency, the waveguide will propagate energy. Mr. V. Sudheer Raja

Wave paths in a waveguide at various frequencies (a)At high frequency (b) At medium frequency ( c ) At low frequency (d) At cutoff frequency Mr. V. Sudheer Raja

Wave propagation When a probe launches energy into the waveguide, the electromagnetic fields bounce off the side walls of the waveguide as shown in the above diagram. Ø The angles of incidence and reflection depend upon the operating frequency. At high frequencies, the angles are large and therefore, the path between the opposite walls is relatively long as shown in Fig a. Ø At lower frequency, the angles decrease and the path between the sides shortens. Ø When the operating frequency is reaches the cutoff frequency of the waveguide, the signal simply bounces back and forth directly between the side walls of the waveguide and has no forward motion. Ø At cut off frequency and below, no energy will propagate. Ø Mr. V. Sudheer Raja

Cut off frequency Ø The exact size of the wave guide is selected based on the desired operating frequency. Ø The size of the waveguide is chosen so that its rectangular width is greater than one – half the wavelength but less than the one wavelength at the operating frequency. Ø This gives a cutoff frequency that is below the operating frequency, thereby ensuring that the signal will be propagated down the line. Mr. V. Sudheer Raja

Representation of modes Ø The general symbol of representation will be TE or TM m, n where the subscript m indicates the number of half wave variations of the electric field or magnetic intensity along the a ( wide) dimension of the waveguide i. e. xdirection. Ø The second subscript n indicates the number of half wave variations of the electric field or magnetic field in the b (narrow) dimension of the guide i. e. y- direction. If the propagation is in the direction of positive z direction. Ø The TE 1, 0 mode has the longest operating wavelength and is designated as the dominant mode. It is the mode for the lowest frequency that can be propagated in a. Mr. V. waveguide. Sudheer Raja m, n

Expression for cut off wavelength Ø For a standard rectangular waveguide, the cutoff wavelength is given by, Ø Where a and b are measured in centimeters Mr. V. Sudheer Raja

Circular wave guide A Hollow metallic tube of uniform circular cross section for transmitting electromagnetic waves by successive reflections from the inner walls of the tube is called Circular waveguide. Mr. V. Sudheer Raja

Circular wave guide Ø The circular waveguide is used in many special applications in microwave techniques. Ø It has the advantage of greater power – handling capacity and lower attenuation for a given cutoff wavelength. However, the disadvantage of somewhat greater size and weight. Ø The polarization of the transmitted wave can be altered due to the minor irregularities of the wall surface of the circular guide, whereas the rectangular wave guide the polarization is fixed Mr. V. Sudheer Raja

Mr. V. Sudheer Raja

Description Ø The wave of lowest frequency or the dominant mode in the circular waveguide is the TE 11 mode. Ø The first subscript m indicates the number of full – wave variations of the radial component of the electric field around the circumference of the waveguide. Ø The second subscript n indicates the number of half – wave variations across the diameter. Ø The field configurations of TE 11 mode in the circular waveguide is shown in the diagram below Mr. V. Sudheer Raja

Expression for cutoff frequencies for circular wave guide: --Cutoff frequency of circular wave guide depends upon the mode of propagation --The cut off frequency for TE modes of propagation is given as, Where can be found from the following table, Mr. V. Sudheer Raja

--The cut off frequency for TM modes of propagation is given as, Where can be found from the following table, Mr. V. Sudheer Raja

Cut off wavelengths for dominant modes: Ø The cutoff wavelength for dominant mode of propagation TE 11 in circular waveguide of radius ‘a’ is given by ØThe cutoff wavelength for dominant mode of propagation TM 01 in circular waveguide of radius ‘a’ is given by Mr. V. Sudheer Raja

Applications of Circular waveguide Ø Ø Ø Rotating joints in radars to connect the horn antenna feeding a parabolic reflector (which must rotate for tracking) TE 01 mode suitable for long distance waveguide transmission above 10 GHz. Short and medium distance broad band communication (could replace / share coaxial and microwave links) Mr. V. Sudheer Raja

Worked Example 1: Ø The dimensions of the waveguide are 2. 5 cm 1 cm. The frequency is 8. 6 GHz. Find (i) possible modes and (ii) cut – off frequency for TE waves. Solution: Given a = 2. 5 cm , b = 1 cm and f = 8. 6 GHz Free space wavelength Mr. V. Sudheer Raja

Solution The condition for the wave to propagate is that λC > λ 0 For TE 01 mode Since λC < λ 0, TE 01 does not propagate Mr. V. Sudheer Raja

For TE 10 mode, λC = 2 a = 2 2. 5 = 5 cm Ø Since λC > λ 0 , TE 10 mode is a possible mode. Ø Cut – off frequency = Cut-off = wavelength for TE 11 mode For TE 11 λC < λ 0 , TE 11 is not possible. ØThe possible mode is TE 10 mode. ØThe cut – off frequency = 6 GHz Mr. V. Sudheer Raja

Worked Example 2: Ø For the dominant mode propagated in an air filled circular waveguide, the cut – off wavelength is 10 cm. Find (i) the required size or cross sectional area of the guide and (ii) the frequencies that can be used for this mode of propagation The cut – off wavelength = λC = 10 cm The radius of the circular waveguide , r = Mr. V. Sudheer Raja

Solution Area of cross section = The cut – off frequency = = 3 GHz ØTherefore the frequency above 3 GHz can be propagated through the waveguide. Area of cross section = 26. 97 cm 2 Cut – off frequency = 3 GHz Mr. V. Sudheer Raja

Exercise Problem 1: ØA rectangular waveguide has a = 4 cm and b = 3 cm as its sectional dimensions. Find all the modes which will propagate at 5000 MHz. Hint: The condition for the wave to propagate is that λC > λ 0 Here λ 0 = 6 cm ; λC for TE 01 mode = 6 cm Hence λC is not greater than free space wavelength λ 0. TE 01 mode is not possible. Mr. V. Sudheer Raja

Exercise problem 2: Ø For the dominant mode of operation is an air filled circular waveguide of inner diameter 4 cm. Find (i) cut – off wavelength and (ii) cut – off frequency. Hint: λC = 6. 8148 cm and fc = 4. 395 GHz Mr. V. Sudheer Raja