Transcript 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 : [email protected]
Mr.V. Sudheer Raja
Chapter 1
Microwave Transmission lines
 Microwave
Spectrum and bands
 Microwave Properties
 Advantages & Limitations
 Applications
 Rectangular Waveguides
 Circular waveguides
Mr.V. Sudheer Raja
Electro Magnetic Spectrum
Mr.V. Sudheer Raja
Microwaves
Microwaves
are electromagnetic waves whose
frequencies range from about 1GHz – 300 GHz
and wavelengths in air ranging from nearly 30 cm
– 1 mm.
The word Microwave means very short wave,
which is the shortest wavelength region of the
radio spectrum and a part of the electromagnetic
spectrum.
Microwave 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



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:
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.
 Due to the Line of Sight (LOS) propagation and high
frequencies, there is less fading effect and hence
microwave communication is more reliable.
4. Power requirements:
 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:

Microwave frequency band ranging from 300
MHz – 10 GHz are capable of freely
propagating 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 space.
Mr.V. Sudheer Raja
Applications

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







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.
Transmitter Subsystem: Oscillator, wave guide, Transmitting
antenna
Receiver 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
 Wave
guide
 Basic features
 Rectangular Wave guide
 Circular Wave guide
 Applications
Mr.V. Sudheer Raja
Waveguides
Introduction
 At
frequencies higher than 3 GHz,
transmission of electromagnetic energy along
the transmission lines and cables becomes
difficult.
 This
is due to the losses that occur both in
the solid dielectric needed to support the
conductor and in the conductors themselves.
A
metallic tube can be used to transmit
electromagnetic
wave at the above
frequencies
Mr.V. Sudheer Raja
Wave guide Definition

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
Waveguides
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
In 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.
A waveguide has a definite cutoff frequency for each allowed mode.
The dominant mode in a particular guide is the mode having lowest
cut off frequency.
If 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.
Waveguides are made from copper, aluminum or brass. These metals are
extruded into long rectangular or circular pipes.
Mr.V. 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
Ey, Hy
Y
Ez, H z
O
Z
Ex,
Hx
X
Mr.V. Sudheer Raja
EM field configuration within the
waveguide
In
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.
Such
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
 Any
shape of cross section of a waveguide can
support electromagnetic waves of which
rectangular and circular waveguides have
become more common.
 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
Angle of incidence(A)
Angle of reflection (B)
(A = B)
(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.
 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.
 At lower frequency, the angles decrease and the path
between the sides shortens.
 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.
 At cut off frequency and below, no energy will propagate.
Mr.V. Sudheer Raja
Cut off frequency
 The
exact size of the wave guide is selected
based on the desired operating frequency.
 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.
 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
 The
general symbol of representation will be
TE m, n 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.
 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.
 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 aMr.V.
waveguide
.
Sudheer Raja
Expression for cut off wavelength
 For
a standard rectangular waveguide, the
cutoff wavelength is given by,
c 
2

m


 a 
2
n
 
b
2
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
 The
circular waveguide is used in many
special applications in microwave techniques.
 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.
 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
 The
wave of lowest frequency or the dominant
mode in the circular waveguide is the TE11
mode.
 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.
 The second subscript n indicates the number of
half – wave variations across the diameter.
 The field configurations of TE11 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:
 The
cutoff wavelength for dominant mode of
propagation TE11 in circular waveguide of
radius ‘a’ is given by
2 πa
c 
1.814
The cutoff wavelength for dominant mode of
propagation TM01 in circular waveguide of radius ‘a’ is
given by
c
2πa

2.405
Mr.V. Sudheer Raja
Applications of Circular waveguide



Rotating joints in radars to connect the horn
antenna feeding a parabolic reflector (which
must rotate for tracking)
TE01 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 Example1:

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
C
3  10
0 

 3.488 cm
9
f
8  10
10
Mr.V. Sudheer Raja
Solution
The condition for the wave to propagate is
that λC > λ0
For TE01 mode
C 
2ab
m 2b 2  n 2 a 2

2ab
a2
 2b  2  1  2 cm
Since λC < λ0, TE01 does not propagate
Mr.V. Sudheer Raja
For TE10 mode, λC = 2a = 2  2.5 = 5 cm
 Since λC > λ0 , TE10 mode is a possible mode.
C
3  10 10

 6 GHz
Cut – off frequency f C 
C
5
=
2 ab
Cut-off
=
wavelength
a2  b2
for TE11 mode
2  2.5  1

 1.856 cm
2
2
( 2.5)  (1)

For TE11 λC < λ0 , TE11 is not possible.
The possible mode is TE10 mode.
The cut – off frequency = 6 GHz
Mr.V. Sudheer Raja
Worked Example2:

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 =
10  1.841
 2.93 cm
2
Mr.V. Sudheer Raja
Solution

Area of cross section =
πr   (2.93)  26.97 cm
2
2
The cut – off frequency
=
3 10 = 3 GHz
fc 

c
10
C
10
Therefore the frequency above 3 GHz can be
propagated through the waveguide.
Area of cross section = 26.97 cm2
Cut – off frequency = 3 GHz
Mr.V. Sudheer Raja
2
Exercise Problem1:
A
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 > λ0
Here λ0 = 6 cm ; λC for TE01 mode = 6 cm
Hence λC is not greater than free space wavelength λ0 .
TE01 mode is not possible.
Mr.V. Sudheer Raja
Exercise problem2:

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