Basic Principles of Microwave Engineering

7/29/2019 Basic Principles of Microwave Engineering

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Introduction

This chapter is intended to provide basic principles of microwave engineering which is treated ingreater detail in the text.

1.1 MICROWAVE SPECTRUM AND BANDS

The whole electromagnetic-spectrum (Table 1.1) that extends from d.c. to -rays and beyond is broadlydivided into two regions, namely, the radio-spectrum from d.c. to 300 GHz and the optical spectrum

extending from 300 GHz to infinity. The term microwave* is commonly used to designate frequencies

ranging from 300 MHz to 300 GHz or having wavelengths ranging from 100 cm to 1 mm in radio-spectrum.

The microwave region has been further divided into three regions, namely, Ultrashort wave (Ultra high

frequency-UHF), Supershort wave (Super high frequency-SHF) and Extreme shortwave (Extreme high

Table 1.1 The electromagnetic spectrum

Frequency limits (GHz) Wavelength limits (cm)Region

Minimum Maximum Maximum MinimumRemarks

(a) Radio-spectrum d.c. 300 Infinity 0.1 VLF/LF/MF/HF/

VHF/UHF/SHF etc.

(b) Optical spectrum

(i) Infrared 300 375 103 0.1 8 105 Heat and invisible light

(ii) Visible 375 103 790 103 8 105 38 106 Light-red to violet

(iii) Ultraviolet 790 103 225 105 38 106 12 107 Chemically invisible

(iv) X-rays 225 105 450 108 12 107 6 1010

(v) -rays 450 108 270 109 6 1010 l 1010

(vi) Cosmic rays 270 109 Indefinite l 1010 Indefinite

(smaller than

1012 cm)

* Corrara in 1932 first used the term microwaves to designate electromagnetic waves of 30 cm wavelength, which is

also the term appeared for the first time in the proceedings of the IRE (Now it is IEEE).

CHAPTER

1


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2 BASIC MICROWAVE ENGINEERING

frequency-EHF) according to the recommendations of the International Radio Consultative Committee.

The position of microwave bands in entire radio-spectrum is shown in Table 1.2. The region of micro-

waves has been pushed into wavelengths lower than 1 mm and are known as submillimetre waves

(> 300 GHz).

Table 1.2 Position of microwave bands in the entire radio-spectrum

S. No. Frequency band Wavelength band Designation (I.R.C.C. band) Remarks

1. 0 to 30 kHz Infinity to 104 m Very low frequency (VLF)/Very long waves

2. 30 to 300 kHz 104 to 103 m Low frequency (LF)/Long waves

3. 300 to 3000 kHz 103 to 102 m Medium frequency (MF)/Medium waves

4. 3 to 30 MHz 100 to 10 m High frequency (HF)/Short waves

5. 30 to 300 MHz 10 to 1 m Very high frequency (VHF)/

Very short waves

6. 0.3 to 3 GHz 1 to 0.1 m Ultra high frequency (UHF)/ Microwaves

Ultrashort waves

7. 3 to 30 GHz 10 to 1 cm Super high frequency (SHF)/ Microwaves

Supershort waves

8. 30 to 300 GHz 10 to 1 mm Extreme high frequency (EHF)/

Extreme short-waves Microwaves

9. 300 to 3000 GHz 1 to 0.1 mm Submillimetre

The Institute of Electrical and Electronics Engineers (IEEE) recommended microwave bands

designations as listed in Table 1.3.

Table 1.3 IEEE microwave frequency bands

Frequency range (GHz) Approximate band designation

0.003 to 0.030 HF

0.030 to 0.300 VHF

0.300 to 1.000 UHF

1.000 to 2.000 L band

2.000 to 4.000 S band

4.000 to 8.000 C band

8.000 to 12.000 X band

12.000 to 18.000 Ku band

18.000 to 26.000 K band

26.000 to 40.000 Ka band

40.000 to 300.000 Millimetre

>300.00 Submillimetre

The radar band classification, as listed in Table 1.4, came into use during World War II and is still

in use today even though the new military band designation, as listed in Table 1.5 has been adopted by

the U.S. Department of Defence since 1970.


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INTRODUCTION 3

Table 1.4 U.S. Military microwave bands (Radar bands)

Frequency range (GHz) Designation

0.225 to 0.390 P band

0.390 to 1.550 L band

1.550 to 3.900 S band

3.900 to 6.200 C band

6.200 to 10.900 X band

10.900 to 36.000 K band

36.000 to 46.000 Q band

46.000 to 56.000 V band

56.000 to 100.000 W band

Table 1.5 U.S. New military microwave bands (New radar bands)

Frequency range in (GHz) Designation Frequency range in (GHz) Designation

0.100 to 0.250 A band 6.000 to 8.000 H band

0.250 to 0.500 B band 8.000 to 10.000 I band

0.500 to 1.000 C band 10.000 to 20.000 J band

1.000 to 2.000 D band 20.000 to 40.000 K band

2.000 to 3.000 E band 40.000 to 60.000 L band

3.000 to 4.000 F band 60.000 to 100.000 M band

4.000 to 6.000 G band

1.2 UNIQUE CHARACTERISTICS OF MICROWAVESThe microwaves display certain unique characteristics that make them distinct from the waves of other

adjacent bands, namely, (1) Short wavelengths, (2) More bandwidth (information carrying capacity).

For instance, a frequency band 109 to 1012 Hz (microwave region) contains thousand sections such as

the entire frequency band from 0 to 109 Hz. Then any one of these thousand sections may be used to

transmit all radio, television or other communication that is presently transmitted by 0 to 10 9 Hz band.

A 10% bandwidth at 4 GHz carrier is 400 MHz which can carry 1,00,000 voice channels (bandwidth

4 kHz) or 66 television channels (bandwidth 6 MHz), or 8000 266 digital data channels (50 kHz 1.5 MHz bandwidth). (3) Small antenna size with large antenna gain (narrow beam antenna), (4) Travel

by line of sight propagation through ionosphere with negligible absorption and reflection. For instance,

a 140 cm diameter parabolic reflector type antenna produces a beam of 1 beam width (angular beam

width = 140/(D/); whereD is diameter of the parabola and is the wavelength) at 30 GHz (= 1 cm).

At 300 MHz (= 100 cm) the diameter of the parabolic antenna becomes 140 m to provide a beam of1 beam width. The size is too large to be carried aboard in an aeroplane. (5) Reflection from metallic

surfaces, (6) Microwave heating, (7) Molecular, atomic and nuclear resonance, etc. These features have

provided unique opportunities for several useful applications. Some important and typical applications

of microwaves are briefly described here under.


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1.3 APPLICATIONS OF MICROWAVES

1.3.1 Microwave Communication Systems

Because of the increase in bandwidth microwaves are extensively used to carry voice (4 kHz bandwidth),

digital data (50 kHz 1.5 MHz bandwidth), television signals (6 MHz bandwidth) or telephonic traffic

etc., over long distances links on the ground (ground communication) to deep-space spacecraft (space

communication). Broadly, microwave communication systems are of two types, namely, ( i) ground-

wave system where signal is transmitted over a low loss cable or waveguide. With the commercial

availability of high power lasers, fiber optical cables are increasingly used for long-distance telephonic

traffic, as they are characterised by low-loss, very high bandwidth and a high degree of ruggedness, and

(ii) Radio-links where the signal propagates through space. This is because of the fact that microwave

signals travel by line of sight and are not bent/reflected by the ionosphere. However, for long distance

links on the ground, repeater stations are required at frequent intervals, to receive and retransmit the

signal. The newer systems, like (i) direct broadcast satellite television (DBST), (ii) personal

communication systems (PCSs), (iii) wireless local area computer networks (WLANs), (iv) cellular

video (CV) systems and (v) global positioning satellite (GPS) systems operate in microwave band.

1.3.2 Radar Systems and Countermeasures

Microwaves are widely used for directive signal transmission (navigation) and locating and ranging

objects in space (radar or radio-detection and ranging). It is because extremely narrow microwave beam

could be produced by the microwave antennas. Many varied forms of radar systems have been

developed and are in use. Besides, military applications of radar systems such as ( i) air and marine

navigation, (ii) detection and tracking of aircraft, missiles, spacecraft, (iii) missile guidance, (iv) fire

control for missiles and artillery, (v) weapon fuses, and (vi) reconnaissance, there are civilian and

scientific applications of radar systems some of which are listed below:

Civilian applications(i) Airport surveillance, (ii) marine navigation, (iii) weather detecting radar,

(iv) altimetry, (v) aircraft landing radar, (vi) automobile speed measuring radar (Police radar) etc.

Scientific applications(i) Remote sensing of natural resources, (ii) mapping and imaging radar,

(iii) astronomy etc.CountermeasuresCountermeasures involve the radiation of noise power in the operating band

of the radar to confuse or deceive the radar as communication system. Electronic support measures

(ESM), Electronic countermeasures (ECM) and Electronic counter-counter-measures (ECCM) have

been developed using microwave technology.

1.3.3 Radiometry/ Radio-astronomical Research

Very sensitive microwave receivers are being used to develop information about a target solely from the

microwave portion of the black body radiations (noise) that it either emits directly or reflects from

surroundings. It is known as radiometry and the device is called radiometer. Radiometry is being used

by radio astronomers in (a) planetary mapping, (b) solar emission mapping, (c) mapping of galactic

objects, and (d) measurement of cosmological background radiation. The technique has been extended

to environment studies (remote sensing) such as (a) measurements of soil moisture, (b) flood mapping,(c) snow cover/ice cover mapping, (c) ocean surface wind speed, and (d) developing atmospheric

temperature and humidity profiles. Radiometry is extensively used in military operations such as

(a) target detection and recognition, (b) surveillance, and (c) mapping. Radio-astronomical applications

of microwaves are due to the fact that ionosphere is a transparent media for propagation of

microwaves.


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INTRODUCTION 5

1.3.4 Industrial Applications

During the last one and half decades, the industrial applications of microwaves have expanded greatly.

Microwave heating has entered into commercial use in the form of microwave ovens, in which cooking

is done quickly and uniformly by waves, inside and outside the food-material simultaneously. To theaverage consumer, the microwave connotes a microwave oven, which is getting increasingly popular

in households for heating food. Microwave drying machines are used in textile and paper industries.

Industrial applications are particularly evident in the food processing industry, the rubber industry and

foundries. Other industrial applications include the central and on-line measurement of expensive

materials during processing procedures in environments hostile to opto-electronic techniques. Recently,

microwaves have also been used for non-destructive testing of metals such as thickness measurements,

and measurement of moisture content in paper and textile industry and in liquids etc.

1.3.5 Microwaves in Basic and Applied Research

Microwaves have provided a very powerful experimental probe for the study of basic properties of

matter as various molecular atomic and nuclear resonance occurs at microwave frequencies. Consequently,

microwaves are capable of energetically interacting with matter. This feature is widely used in microwaveand radio frequency spectroscopy for structural analysis. Microwave absorption spectra provide

information about molecular structure and energy levels. Useful molecular resonances exist at microwave

frequencies in the diodes of certain crystal materials. The resonant interaction of microwaves with

crystals has been used for the generation of microwave power. Impact conjugation avalanche transist

time (IMPATT) device, the non-reciprocal ferrite devices and masers are yet another examples of

microwave resonances in molecules.

The interaction of an electron beam with periodic slow-wave microwave structures has been used

to design high power linear accelerators that are indispensable instruments in nuclear research.

1.3.6 Biomedical Applications

The potential of microwaves for used in medicine is immense. The exact location of deep cancerous

tissue, in particular, can be known by means of microwave radiometers. Microwave diathermymachines are used to remove rheumatic pains by producing heat inside the muscle without affecting the

skin. Patients afflicted with uncontrollable pain or random muscle movements can be treated using

microwave irradiation which creates thermal blocks in the nerve network.

Microwave radiations are being used for cancer therapyhyperthermialocal, regional and whole

body. Electromagnetic transmission through a human body has been used for monitoring heartbeat,

lung-water detection, etc. Biomedical applications of microwaves are ever increasing.

1.3.7 Potential Application in the Field of Energy Transfer

It has a potential application in the field of energy transfer, i.e., electrical energy can be transferred,

without use of transmission lines, by converting it into microwave power and radiating through antenna

in a narrow beam. At the receiving station it could be converted back to electric energy. Such a system

has been proposed in the form of satellite power stations (SPS) as a method of tapping solar energy ona 24 hour-a-day, 365-days-a-year basis for the present century and beyond.

Although the field of microwave engineering is already well developed, the scope of its applica-

tions in communication, industry and basic research is ever increasing. The extension of microwave

techniques into the field of optics is one such example.


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However, varied uses of microwaves have increased microwave pollution and the consequent risk

to health. This has stimulated the study of the biological effects and safety in several large

research projects.

Table 1.6 summarises the typical and unique applications of microwaves. The scope of microwaveapplications in communication industry and basic and applied research is ever increasing. The exten-

sion of microwave techniques into the field of development of optical communication system is one

such example.

Table 1.6

S. No. Applications Frequency band

1. Television, satellite communication, surveillance radar, navigational aids, 0.3 to 3 GHz

food industry (microwave-ovens) point to point communication.

2. Altimeter, air and ship borne radar, microwave links, common-carrier 3 to 30 GHz

land mobile communication, satellite communication, navigation, basic

research-microwave spectroscopy.

3. Radar, radio-astronomy, radio-meteorology, space research, 30 to 300 GHznuclear physics, nucleonics.

1.4 TYPICAL MICROWAVE SYSTEM

Figure 1.1 shows a typical microwave system. It has a transmitter subsystem, which normally consists

of microwave generator, waveguides, wavemeter, attenuator and transmitting antenna, and a receiver

subsystem, which normally consists of receiving antenna, waveguides, a microwave amplifier, detector.

Another typical radar system is shown in Fig. 1.2 which uses single antenna for transmission and

reception.

Therefore, a first course on Microwave Engineering should include four major areas of study,

namely (i) Microwave Transmission Lines Waveguides, (ii) Waveguide Component and Applications,

(iii) Microwave Sources Tubes and Solid State Devices, and ( iv) Microwave Measurements.These topics are discussed in the text keeping a balance between mathematical and physical

approach.

Fig. 1.1 Typical microwave system

Powersupply

Microwavesource

WavemeterCalibratedattenuator

Transmittinghorn antenna

Receiving

horn antenna

Indicating

meter orpower meter

Waveguidetermination

Crystalmount

Stand

Waveguide

Microwavesourcemount


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INTRODUCTION 7

Fig. 1.2 Typical radar system

1.5 BASIC MICROWAVE CONCEPTS(i) Microwave Transmission: The principles of microwave transmission cannot be derived by mere

extensions of either low frequency radio or high frequency optical concepts, although they are all based

upon the fundamental laws of electromagnetism. For instance, if microwave power is fed in a conven-

tional two conductor lines where the longitudinal and transverse dimensions of the line are comparable

to the wavelength of the propagating signal, it leads to a series of interesting effects that fall outside the

scope of problems examined by classical theory of long transmission lines. It turns out that such a line

cannot be used for microwave transmission. One has, therefore, to use hollow metal tubes called

waveguides. The energy propagation in these structures is basically a reflection phenomena. Con-

versely, a hollow-pipe waveguide cannot be treated by the rules of low-frequency electricity, for by

these rules opposite currents cannot flow in the same metallic conductor without coalescing into one net

current, yet we see that opposite currents can flow in the same conductor in waveguides. Another class

of waveguides of most recent origin is called surface waveguides which are absolutely uncommon tolow frequency transmission.

The waveguide transmission of microwaves is associated with a number of interesting problems

such as coupling of power to another system say from generator to line, exciting of waves in a waveguide,

etc. To overcome these, three basic coupling methods, viz. electrical coupling (probe), magnetic

coupling (loop), and aperture coupling (waveguide to waveguide) have been evolved. The basic feature

of these methods is that one can control the amount of coupling because these structures have small

antennas that radiate into the waveguide to be coupled.

(ii) Microwave Circuit Elements: It is a well-established fact that conventional circuit elements such

as resistors, inductors and capacitors do not respond well at microwave frequencies. For instance, a coil

of wire may be an excellent inductor at 1 MHz, but at 500 MHz it may be an equally good capacitor

because of the predominating effect of inter-turn capacitance. However, this does not mean that energy

dissipating (resistors) and storing (capacitors and inductors) elements cannot be constructed at micro-

wave frequencies but their geometrical shape will be quite different. As will be seen, a section of a

microwave line (distributed parameters), offers reactances varying from to + if its length issuitably chosen.

To antenna

Rotaryjoint

Fle

xible

wave

guid

e

Switch

Nois

e

generato

r

Match

edload

Dir

ecti

onal

couple

r

Match

ed

load

Ferrit

ed

uple

xer

ToAF

CTo power

meterM

atch

edload

Circula

tor

Frommicrowave

sourceDirectional

coupler

Ferrite

T-R switch

Match

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load

To receiv

er


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Similarly, conventional resonant and anti-resonant circuits are replaced by resonant microwave line

sections known as resonant cavities. Often resonant cavities are used as circuit elements with varying

properties.

When a number of such microwave circuit elements are,connected together, we have a microwavecircuit. The analysis of a microwave circuit can be carried out either in terms of equivalent transmission

line, voltage and current waves or in terms of amplitudes of the incident and reflected waves. The first

approach is called the conventional equivalent impedance description approach while the second is

known as the scattering matrix approach. The latter approach being closely related to the wave nature

of fields.

(iii) Generation and Amplification of Microwaves: The operation of conventional vacuum tubes and

solid state devices is limited by transit time effects. However, the frequency range of operation of these

devices can be extended to the lower edge of the microwave spectrum at the cost of power output and

noise characteristics. Therefore, the development of new devices was essential to exploit this frequency

region. Fortunately, number of new principles of operation such as velocity modulation, interaction of

space charge waves with electromagnetic fields were proposed. It involves transfer of power from a

source of direct voltage to a source of alternating voltage by means of a density-modulated stream ofelectrons resulting in the development of klystron (in 1939 by R.H. Varian and S.F. Varian [1]), mag-

netron (in 1921 by Hull and an improved model in 1940 by Boot and Randal [2, 3]) and travelling-wave

tube (TWT) (in 1944 by R. Komfner [4].

The interaction of the impact ionization avalanche and the transit time of charge carriers was used

to develop Reed diode in 1958 [5], IMPATT by Johnston et al. in 1965 [6], TRAPATT by Prager et al.

in 1967 [7]. Quantum mechanical tunneling was used to develop Tunnel Diode by Esaki in 1958 [8].

Transferred electron techniques were used to develop transfer electron devices by Ridley et al. in

1961 [9] and Hilsum in 1962 [10]. Gunn oscillator was developed in 1963 [11] which operate simply

by the application of a dc voltage to a bulk. In all solid-state devices the negative resistance character-

istics is exploited for microwave generation and amplification.

REVIEW QUESTIONS

1. Explain how microwave engineering is different from low frequency electronic engineering. (JNTU, 2008)

2. Describe in brief basic microwave concepts.

3. List unique characteristics of microwaves.

4. List the typical application of microwave.

5. Explain with an example how microwave has large information carrying capacity.

6. Explain Microwaves are widely used for directive signal transmission and locating and ranging objects in

space.

SELECTED TITLES

1. Collin, R.E., Foundations for Microwave Engineering, McGraw-Hill Book Company, New York, 1966.

2. Gandhi, O.P., Microwave Engineering and Applications, Pergamon Press, New York, 1981.

3. Liao, S.Y., Microwave Devices and Circuits, Prentice-Hall Inc, Englewood Cliffs, N.J., U.S.A., 1995

[Indian reprint].

4. Soohoo, R.F., Microwave Electronics, Addison-Wesley Publishing Company, Reading, Mass., 1971.


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INTRODUCTION 9

5. Reich, H.J., et al., Microwave Principles, D. Van Nostrand Reinhold Company, New York, 1957 (an East-

West Edition).

6. Reich, H.J., et al., Microwave Theory and Techniques, D. Van Nostrand Reinhold Company, New York,

1953.

7. Brownwell, A.B., and R.E. Beam, Theory and Application of Microwaves, McGraw-Hill Book Co.,New York, 1947.

8. Chatterjee, R., Elements of Microwave Engineering, East-West Press, New Delhi, 1984.

9. Rizzi, P.A., Microwave Engineering: Passive Circuits, Prentice-Hall, Inc., Englewood Cliffs, N.J., 1988.

10. Sisodia, M.L. and G.S. Raghuvanshi, Microwave Circuits and Passive Devices, John Wiley & Sons,

New York, 1987 (Wiley Eastern Limited, New Delhi, 1987, now New Age, 2003).

11. Sisodia, M.L. Microwave Active Devices, Vacuum and Solid State, New Age International Publishers,

New Delhi, 2003.

12. Sisodia, M.L. and G.S. Raghuvanshi, Basic Microwave Techniques and Laboratory Manual, John Wiley &

Sons, 1987. (Wiley Eastern Limited, 2000, New Delhi, now New Age).

13. Sisodia, M.L. and Vijay Laxmi Gupta, Microwaves: Introduction to Circuits, Devices and Antennas,

New Age International Publishers, New Delhi, 2003.

REFERENCES

1. Warnecke, R.R., et al., Velocity Modulated Tubes: The Advances in Electronics, Vol. 3, Academic Press,

New York, 1951.

2. Hull, A.W., Phys. Rev. 18, 31 (1921).

3. Okress, E., Editor, Crossed Field Microwave Devices, Academic Press, New York, Vols. I and II, 1961.

4. Kompfner, R., The Travelling Wave Tube as Amplifier at Microwaves, Pros. IRE 35, 124-127, February 1947.

5. Read, W.T.,A Proposed High Frequency Negative-Resistance Diode, Bell System tech. J., 37, 401-446, 1958.

6. Johnston, R.L., B.C. Deloach, and G.B. Cohen,A Silicon Diode Microwave Oscillator, Bell System Tech., J.,

44, 369-372, February 1965.

7. Prager, H.J., et al.,High-power, High-efficiency Silicon Avalanche Diodes at Ultra High Frequencies, Proc.

IEEE (letters), 55, 586-587, April 1967.

8. Esaki, L.,New Phenomenon in Narrow Ge p-n Junctions, Phys. Rev., 109, 603, 1958.

9. Ridley, B.K. and T.B. Watkins, The Possibility of Negative Resistance Effect in Semiconductors, Proc. Phys.Soc., 78, 293-304, August 1961.

10. Hilsum, C., Transferred Electron Amplifiers and Oscillators, Proc. IEEE., 50, 185-189 February 1962.

11. Gunn, J.B.,Instabilities of Current in III-V Semiconductors, IBM J. Res. Develop., 8,141-159, April 1964.

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Basic Principles of Microwave Engineering