Microwave Engg Notes-Unit 1

8/10/2019 Microwave Engg Notes-Unit 1

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Microwave Engineering Unit No: 1Lecture No: 1

P.Ravi Kumar, Asst. Professor, ECE-GMRIT-R ajam

MICROWAVE ENGINEERING

Introduction to microwaves:

Microwaves As the name implies, are very short waves .In General RF Extends

from dc up to Infrared region and these are forms of electromagnetic energy.A glance look at the

various frequency ranges makes it clear that UHF (Ultra high frequency) & SHF (super high

frequencies) constitutes the Microwave frequency range with wave length ( ) extending from 1

to 100 cm The basic principle of low frequency radio waves and microwaves are the same .Here

the phenomena are readily explained in terms of current flow in a closed electric circuit. At low

frequencies, we talk in terms of lumped circuit elements such as C. L, R which can be easily

identified and located in a circuit. On the other hand in Microwave circuitry, the inductance &

capacitance are assumed to be distributed along a transmission line. Microwaves are

electromagnetic waves whose frequencies range from 1 GHz to 1000 GHz (1 GHz =10 9).

Microwaves so called since they are defined in terms of their wave length, micro in the sense

tininess in wave length, period of cycle (CW wave), is very short. Microwave is a signal that

has a wave length of 1 foot or less 30.5 cm. = 1 foot. F= 984MHz approximately 1 GHz

Microwaves are like rays of light than ordinary waves.

Microwave Region and band Designation

Frequency Band Designation

3Hz30 Hz Ultra low frequency(ULF)30 to 300 Hz Extra low frequency (ELF)

300 to 3000 Hz (3 KHz) Voice frequency, base band / telephony3 KHz to 30 KHz VLF30 to 300 KHz LF300 to 3000 KHz ( 3 MHz) MF3 MHz to 30 MHz HF30 to 300 MHz VHF300 to 3000 MHz (3GHz ) Ultra high frequency (UHF)3 GHz to 30 GHz SHF30 to 300 GHz EHF300 to 3000 GHz(3 THz ), (3 -30 THz,30to3000 T )

Infra red frequencies

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P.Ravi Kumar, Asst. Professor, ECE-GMRIT-R ajam

The Microwave spectrum starting from 300MHz is sub dived into various bands namely L, S, C,X, etc.

Band designation Frequency range (GHz)

UHF 0.3 to 3.0L 1.1 to 1.7S 2.6 to 3.9C 3.9 to 8.0X 8.0 to 12.5Ku 12.5 to 18.0K 18.0 to 26Ka 26 to 40Q 33 to 50U 40 to 60

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Microwave Engineering Unit No:1Lecture No:2

P.Ravi Kumar, Asst. Professor, ECE-GMRIT-RAJAM

Microwave Engineering

Advantages : There are some unique advantages of microwaves over low frequencies.

1) Increased bandwidth availability : Microwaves have large bandwidths (1GHz-1000GHz)

compared to the common band namely UHF, VHF waves. The advantage of large bandwidths is

that the frequency range of information channels will be a small percentage of the carrier

frequency and more information can be transmitted in microwave frequency ranges. Microwave

region is very useful since the lower band of frequency is already crowded. Infact microwave

region (1000GHz) contains thousand sections of the frequency band 0-10 9 Hz and hence any one

of these thousand sections may be used to transmit all the TV, radio and other communicationsthat is presently transmitted by the 0-10 9 Hz band.(Bandwidth of speech is 4KHz; Music=10-

15KHz; T.V.= 5-7 MHz; Telegraph channel=120-240 Hz). It is current trend to use microwaves

more and more in various long distance communication applications such as Telephone

networks, TV network. Space communication, Telemetry, Defence, Railways etc.

2) Improved directive properties: As frequency increases directivity increases and beam

width decreases. Hence the beam width of radiation theta is proportional to (lambda/D). A t low

frequency bands the size of the antenna becomes very large if it is required to get sharp beams of

radiation. However at microwave frequencies antenna size of several wavelengths wide leads to

smaller beam widths and an extremely directed beam, just the same way as an optical lens

focuses light rays. Therefore microwave frequencies are said to posses quasi-optical properties.

As the frequency increases lambda decreases and power radiated and gain increases. As gain is

inversely proportional to (lambda) 2 high gain is achievable at microwave frequencies i.e. high

gain and directive antennas can be designed and fabricated more easily at microwave

frequencies, which is highly impracticable at lower frequency bands.

3) Fading effect and reliability: Fading effect due to variation in the transmission medium is

more effective at low frequency. Due to line of sight(LOS) propagation and high frequencies

there is less fading effect and hence microwave communication is more reliable.

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Microwave Engineering Unit No:1Lecture No:2

P.Ravi Kumar, Asst. Professor, ECE-GMRIT-RAJAM

4) Power requirements: Transmitter/receiver power requirements are pretty low at microwave

frequencies compared to that at short wave band.

5) Transparency property of microwaves: Microwave frequency band ranging from 300MHz-

10GHz are capable of freely propagating through the ionized layers surrounding the earth as well

as through the atmosphere. 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 apace. It also makes it possible for duplex communication and exchange of information

between ground stations and apace vehicles.

Applications: Microwaves have a broad range of applications in modern technology.

Most important among them are in long distance communication systems, radars, radio

astronomy, navigation etc. Broadly the applications can be in the areas listed below.

1) Telecommunications: International Telephone and T.V., space communication,

telemetry communication link for railways etc.

2) Radars: Detect aircraft, track/guide supersonic missiles, observe and track weather

patterns, air traffic control (ATC), burglar alarms, gargage door openers, police speeddetectors etc.

3) Commercial and industrial applications use heat property of microwaves: 1)

microwave Owens (2.45 GHz, 600W). 2) Drying machines- textile, food and paper

industry for drying clothes, potato chips etc. 3) Rubber industry/plastics/chemical

industries etc. 4) Biomedical applications etc.

4) Electronic warfare: ECM/ECCM systems spread spectrum systems.

5) Identifying objects or personnel by non contact method.

6) 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

generation, tuning elements etc.

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Waveguides

Waveguides, like transmission lines, are structures used to guide electromagnetic waves

from point to point. However, the fundamental characteristics of waveguide and

transmission line waves (modes) are quite different. The differences in these modes

result from the basic differences in geometry for a transmission line and a waveguide.

Waveguides can be generally classified as either metal waveguides or dielectric

waveguides. Metal waveguides normally take the form of an enclosed conducting metal

pipe. The waves propagating inside the metal waveguide may be characterized by

reflections from the conducting walls. The dielectric waveguide consists of dielectrics

only and employs reflections from dielectric interfaces to propagate the electromagnetic

wave along the waveguide.

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Comparison of Waveguide and Transmission Line CharacteristicsTransmission line

Two or more conductors separated by some insulating medium (two-wire,

coaxial, microstrip, etc.).

Normal operating mode is the TEM or quasi-TEM mode (can support TE and

TM modes but these modes are typically undesirable).

No cutoff frequency for the TEM mode. Transmission lines can transmit signals

from DC up to high frequency.

Significant signal attenuation at high frequencies due to conductor and dielectric

losses. Small cross-section transmission lines (like coaxial cables) can only transmit low

power levels due to the relatively high fields concentrated at specific locations

within the device (field levels are limited by dielectric breakdown).

Large cross-section transmission lines (like power transmission lines) can

transmit high power levels.

Waveguide Metal waveguides are typically one enclosed conductor filled with an insulating

medium (rectangular, circular) while a dielectric waveguide consists of multiple

dielectrics.

Operating modes are TE or TM modes (cannot support a TEM mode). Must operate the waveguide at a frequency above the respective TE or TM mode

cutoff frequency for that mode to propagate.

Lower signal attenuation at high frequencies than transmission lines. Metal waveguides can transmit high power levels. The fields of the propagating

wave are spread more uniformly over a larger cross-sectional area than the smallcross-section transmission line.

Large cross-section (low frequency) waveguides are impractical due to large size

and high cost.

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Microwave Engineering Lecture No: 5 Unit No: 1

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RECTANGULAR AND CIRCULAR WAVEGUHDES

Hollow-pipe waveguides do not support a TEM wave. In hollow-pipe waveguides the waves are

of the TE and TM variety. The waveguide with a rectangular cross section is the most widely

used one. It is available in sizes for use at frequencies from 320 MHz up to 333 GHz. The WR-

2300 waveguide for use at 320 MHz has internal dimensions of 58.42 in by 29.1 in and is a very

large duct. By contrast, the WR-3 waveguide for use at 333 GHz has internal dimensions of

0.034 in by 0.017 in and is a very miniature structure. The standard WR-90 X-band waveguide

has internal dimensions of 0.9 in by 0.4 in and is used in the frequency range of 8.2 to 12.5 GHz.

The rectangular waveguide is widely used to couple transmitters and receivers the antenna. Forhigh-power applications the waveguide is filled with j inert gas such as nitrogen and pressurized

in order to increase the voltage breakdown rating. Circular waveguides are not as widely used as

rectangular waveguide but are available in diameters of 25.18 in down to 0.239 in to cover tn

frequency range 800 MHz up to 116 GHz.

RECTANGULAR WAVEGUIDE :

The rectangular waveguide with a cross section is an example of a wave guiding

device that will not support a TEM wave. Consequently, it turns out that unique voltage and

current waves do not exist, and the analysis of the waveguide properties has to be carried out as

a field problem rather than as a distributed-parameter-circuit problem. In a hollow cylindrical

waveguide a transverse electric field can exist only if a time-varying axial magnetic held is

present. Similarly, a transverse magnetic field can exist only if either an axial displacement

current or an axial conduction current is present, as Maxwell's equations how. Since a TEM

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wave does not have any axial field components and there is no center conductor on which

conduction current can exist, a TEM wave cannot be propagated in a cylindrical waveguide. The

types of waves that can be supported (propagated) in a hollow empty waveguide are the TE and

TM modes. The essential properties of all hollow cylindrical waveguides are the same, so that an

understanding of the rectangular guide provides insight into the behavior of other types as well.

As for the case of the transmission line, the effect of losses is initially neglected. The attenuation

is computed later by using the perturbation method given earlier, together with the loss-free

solution for the currents on the walls. The essential properties of empty loss-free waveguides,

which the detailed analysis to follow will establish, are that there is a double infinity of possible

solutions for both TE and TM waves. These waves, or modes, may be labeled by two identifyinginteger subscripts n and m, for example, TEnm. The integers n and in pertain to the number of

standing-wave interference maxima occurring in the field solutions that describe the variation of

the fields along the two transverse coordinates. It will be found that each mode has associated

with it a characteristic cutoff frequency f c m below which the mode does not propagate and

above which the mode does propagate. The cutoff frequency is a geometrical parameter

dependent on the waveguide cross-sectional configuration. Another feature common to all

empty uniform waveguides is that the phase velocity v p is greater than the velocity of light c by

the factor A g /A 0. On the other hand, the velocity at which energy and a signal are propagated is

the group velocity v. and is smaller than c by the factor A 0 /A #. Also, since /3, and hence A s, u ,

and vg , are functions of frequency, any signal consisting of several frequencies is dispersed, or

spread out, in both time and space as it propagates along the guide. This dispersion results from

the different velocities at which the different frequency components propagate. If the guide is

very long, considerable signal distortion may take place. With some of the general properties of

waveguides considered, it is now necessary to consider the detailed analysis that will establish

the above properties and that, in addition, will provide the relation between k c and the guide

configuration, the expressions for power and attenuation, etc. The case of TE modes in a loss-

free empty rectangular guide is considered first. If the waveguide walls have finite conductivity,

there will be a continuous loss of power to the walls as the modes propagate along the guide.

Consequently, the phase constant jfi is perturbed and becomes y = a + j/3, where a is an

attenuation constant that gives the rate at which the mode amplitude must decay as the mode

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progresses along the guide. For practical waveguides the losses caused by finite conductivity are

so small that the attenuation constant may be calculated using the perturbation method outlined

in Sec. 3.8 in connection with lossy transmission lines. The method will be illustrated for the

dominant H 10 mode only. For the H nm and also the E nm modes, the calculation differs only in that

somewhat greater algebraic manipulation is required.

CIRCULAR WAVEGUIDES:

Figure illustrates a cylindrical waveguide with a circular cross section of

radius a. In view of the cylindrical geometry involved, cylindrical coordinates are most

appropriate for the analysis to be carried out. Since the general properties of the modes that mayexist are similar to those for rectangular guide, this analysis is not as detailed.

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TE and TM Modes in Ideal Waveguides

Waves propagate along the waveguide (+ z-direction) within the waveguide through the

lossless dielectric. The electric and magnetic fields of the guided waves must satisfy the

source-free Maxwells equations.

Assumptions:

(1) The waveguide is infinitely long, oriented along the z-axis, and uniform along its

length.

(2) The waveguide is constructed from ideal materials [perfectly conductingpipe (PEC) is filled with a perfect insulator (lossless dielectric)].

(3) Fields are time-harmonic.

The cross-sectional size and shape of the waveguide dictates the discrete modes that can

propagate along the waveguide. That is, there are only discrete electric and magnetic

field distributions that will satisfy the appropriate boundary conditions on the surface of

the waveguide conductor. If the single non-zero longitudinal field component associated

with a given waveguide mode can be determined Ez for a TM mode, Hz for a TE mode),

the remaining transverse field components can be found using the general wave equations

for the transverse fields in terms of the longitudinal fields.

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avi uma , ,

General waves in an arbitrary medium:

The longitudinal magnetic field of the TE mode and the longitudinal electric field of the

TM mode are determined by solving the appropriate boundary value problem for the

given waveguide geometry.

Ideal Rectangular Waveguide:

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avi uma , ,

The original second order partial differential equation dependent on two variables has

been separated into two second order ordinary differential equations each dependent on

only one variable. The general solutions to the two separate differential equations are

The resulting longitudinal electric field for a rectangular waveguide TM mode is

The TM boundary conditions for the rectangular waveguide are

The application of the boundary condition yields

The resulting product of the constants A and C can be written as a single constant

(defined as E o). The number of discrete TM modes is infinite based on the possible

values of the indices m and n. An individual TM mode is designated as the TM mn mode.

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avi uma , ,

The longitudinal electric field of the TM mn mode in the rectangular waveguide is given

by

The transverse field components of the TM mn mode are found by differentiating the

longitudinal electric field as defined by the standard TM equations.

In general, the cutoff frequency will increase as the mode index increases. Thus, in

practice, only the lower order modes are important as the waveguide is operated at

frequencies below of the cutoff frequencies of the higher order modes.

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Microwave Engg Notes-Unit 1

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