UNIT-1

Introduction to Microwaves
and model of Microwave Transmission

By,
Mr. S. Gopala Krishna, Assistant Professor

Electronics and Communication Engineering

Contents:
Introduction to Microwaves:

Microwave spectrum bands

Advantages and applications

Model of Microwave Transmission:

Concept of Mode
TE ,TM and TEM modes
Impossibility of TEM mode
Mode characteristics:
Cut off frequencies
Phase and group velocities
Wavelengths
Impedance relations
Power transmission and losses
MICROWAVE INRODUCTION
 Microwaves are electromagnetic waves (EM) with wavelengths ranging from
10cm to 1mm. The corresponding frequency range is 30Ghz (=109 Hz) to
300Ghz (=1011 Hz) . This means microwave frequencies are upto infrared
and visible-light regions.
 The microwaves frequencies span the following three major bands at the
highest end of RF spectrum.

i)Ultra high frequency (UHF) 0.3 to 3 Ghz

ii)Super high frequency (SHF) 3 to 30 Ghz
iii)Extra high frequency (EHF) 30 to 300 Ghz
 Most application of microwave technology make use of frequencies in the
1 to 40 Ghz range.
 During world war II , microwave engineering became a very essential
consideration for the development of high resolution radars capable of
detecting and locating enemy planes and ships through a Narrow beam of EM
energy.

 The common characteristics of microwave device are the negative resistance

that can be used for microwave oscillation and amplification.
Properties of Microwaves
Following are the main properties of Microwaves.
 Microwaves are the waves that radiate electromagnetic
energy with shorter wavelength.

 Microwaves are not reflected by Ionosphere.

 Microwaves travel in a straight line and are reflected by the

conducting surfaces.

 Microwaves are easily attenuated within shorter distances.

 Microwave currents can flow through a thin layer of a cable.

Microwave spectrum bands
Electromagnetic Spectrum consists of entire range of electromagnetic
radiation. Radiation is the energy that travels and spreads out as it
propagates. The types of electromagnetic radiation that makes the
electromagnetic spectrum is depicted in the following screenshot.
 Advantages of Microwave:

There are many advantages of Microwaves such as the following:

 Supports larger bandwidth and hence more information is transmitted. For this reason, microwaves are
used for point-to-point communications.
 More antenna gain is possible.
 Higher data rates are transmitted as the bandwidth is more.
 Antenna size gets reduced, as the frequencies are higher.
 Low power consumption as the signals are of higher frequencies.
 Effect of fading gets reduced by using line of sight propagation.
 Provides effective reflection area in the radar systems.
 Satellite and terrestrial communications with high capacities are possible.
 Low-cost miniature microwave components can be developed.
 Effective spectrum usage with wide variety of applications in all available frequency ranges of
operation.

Disadvantagesof Microwaves
There are a few disadvantages of Microwaves such as the following:
 Cost of equipment or installation cost is high.
 They are hefty and occupy more space.
 Electromagnetic interference may occur.
 Variations in dielectric properties with temperatures may occur.
 Inherent inefficiency of electric power.
Applicationsof Microwaves
There are a wide variety of applications for Microwaves, which are not possible
for other radiations. They are -

Wireless Communications
 For long distance telephone calls
 Bluetooth
 WIMAX operations
 Outdoor broadcasting transmissions
 Broadcast auxiliary services
 Remote pickup unit
 Studio/transmitter link
 Direct Broadcast Satellite (DBS)
 Personal Communication Systems (PCSs)
 Wireless Local Area Networks (WLANs)
 Cellular Video (CV) systems
 Automobile collision avoidance system
Electronics
 Fast jitter-free switches
 Phase shifters
 HF generation
 Tuning elements
 ECM/ECCM (Electronic Counter Measure) systems
 Spread spectrum systems
Commercial Uses
 Burglar alarms
 Garage door openers
 Police speed detectors
 Identification by non-contact methods
 Cell phones, pagers, wireless LANs
 Satellite television, XM radio
 Motion detectors
 Remote sensing
Navigation
 Global navigation satellite systems
 Global Positioning System (GPS)

Military and Radar

 Radars to detect the range and speed of the target.
 SONAR applications
 Air traffic control
 Weather forecasting
 Navigation of ships
 Minesweeping applications
 Speed limit enforcement
 Military uses microwave frequencies for communications and for mentioned
applications.
Research Applications
 Atomic resonances
 Nuclear resonances

Radio Astronomy
 Mark cosmic microwave background radiation
 Detection of powerful waves in the universe
 Detection of many radiations in the universe and earth’s
atmosphere

Food Industry
 Microwave ovens used for reheating and cooking
 Food processing applications
 Pre-heating applications
 Pre-cooking
 Roasting food grains/beans
 Drying potato chips
 Moisture levelling
 Absorbing water molecules
Industrial Uses
 Vulcanizing rubber
 Analytical chemistry applications
 Drying and reaction processes
 Processing ceramics
 Polymer matrix
 Surface modification
 Chemical vapor processing
 Powder processing
 Sterilizing pharmaceuticals
 Chemical synthesis
 Waste remediation
 Power transmission
Transmission Line
A transmission line is a connector which transmits energy from one point to another. The study of
transmission line theory is helpful in the effective usage of power and equipment.
There are basically four types of transmission lines:
 Two-wire parallel transmission lines
 Coaxial lines
 Strip type substrate transmission lines
 Waveguides

While transmitting or while receiving, the energy transfer has to be done effectively, without the
wastage of power. To achieve this, there are certain important parameters which has to be
considered.
MainParametersofaTransmissionLine
The important parameters of a transmission line are resistance, inductance,
capacitance and conductance.
Resistance and inductance together are called as transmission line

impedance. Capacitance and conductance together are called as

admittance.
Concept of Mode
A wave has both electric and magnetic fields. All transverse components of electric
and magnetic fields are determined from the axial components of electric and
magnetic field, in the z direction. This allows mode formations, such as TE, TM,
TEM and Hybrid in microwaves. Let us have a look at the types of modes.
The direction of the electric and the magnetic field components along three mutually
perpendicular directions x, y, and z are as shown in the following figure.
The direction of the electric and the
magnetic field components along
three mutually perpendicular
directions x, y, and z are as shown in
the following figure.
Microwaves ─ Modes of Propagation

TypesofModes
The modes of propagation of microwaves are -

TEM (Transverse Electromagnetic Wave)

In this mode, both the electric and magnetic fields are purely transverse to the direction of
propagation. There are no components in ‘Z’ direction.
𝑬𝒛 = 𝟎 𝒂𝒏𝒅 𝑯𝒛 = 𝟎

TE (Transverse Electric Wave)

In this mode, the electric field is purely transverse to the direction of
propagation, whereas the magnetic field is not.
𝑬𝒛 = 𝟎 𝒂𝒏𝒅 𝑯𝒛 ≠ 𝟎

TM (Transverse Magnetic Wave)

In this mode, the magnetic field is purely transverse to the direction of propagation, whereas the
electric field is not.
𝑬𝒛 ≠ 𝟎 𝒂𝒏𝒅 𝑯𝒛 = 𝟎
HE (Hybrid Wave)
In this mode, neither the electric nor the magnetic field is purely
transverse to the direction of propagation.
𝑬𝒛 ≠ 𝟎 𝒂𝒏𝒅 𝑯𝒛 ≠ 𝟎

Multi conductor lines normally support TEM mode of propagation, as

the theory of transmission lines is applicable to only those system of
conductors that have a go and return path, i.e., those which can
support a TEM wave.
Waveguides are single conductor lines that allow TE and TM modes but
not TEM mode. Open conductor guides support Hybrid waves.
Types of TransmissionLines

The conventional open-wire transmission lines are not suitable for microwave
transmission, as the radiation losses would be high. At Microwave frequencies, the
transmission lines employed can be broadly classified into three types. They are –
 Multi conductor lines
o Co-axial lines
o Strip lines
o Micro strip lines
o Slot lines
o Coplanar lines, etc.

 Single conductor lines (Waveguides)

o Rectangular waveguides
o Circular waveguides
o Elliptical waveguides
o Single-ridged waveguides
o Double-ridged waveguides, etc.

 Open boundary structures

o Di-electric rods
o Open waveguides, etc.

Multi-conductor Lines
The transmission lines which has more than one conductor are called as Multi-conductor
lines.
Waveguides

Generally, if the frequency of a signal or a particular band of signals is high, the bandwidth
utilization is high as the signal provides more space for other signals to get accumulated.
However, high frequency signals can’t travel longer distances without getting attenuated.
We have studied that transmission lines help the signals to travel longer distances.
Microwaves propagate through microwave circuits, components and devices, which act as
a part of Microwave transmission lines, broadly called as Waveguides.
A hollow metallic tube of uniform cross-section for transmitting electromagnetic waves by
successive reflections from the inner walls of the tube is called as a Waveguide.
The following figure shows an example of a waveguide.
A waveguide is generally preferred in microwave communications. Waveguide is a
special form of transmission line, which is a hollow metal tube. Unlike a
transmission line, a waveguide has no center conductor.
The main characteristics of a Waveguide are -
 The tube wall provides distributed inductance.

 The empty space between the tube walls provide distributed capacitance.

 These are bulky and expensive.

AdvantagesofWaveguides
Following are few advantages of Waveguides.
Waveguides are easy to manufacture.
 They can handle very large power (in kilo watts).

 Power loss is very negligible in waveguides.

 They offer very low loss (low value of alpha-attenuation).

 When microwave energy travels through waveguide, it experiences

Types of Waveguides

There are five types of waveguides.

 Rectangular waveguide
 Circular waveguide
 Elliptical waveguide
 Single-ridged waveguide
 Double-ridged waveguide

The following figures show the types of waveguides.

Rectangular waveguide
A rectangular waveguide is a conducting cylinder of rectangular cross section used to
guide the propagation of waves.
Rectangular waveguide is commonly used for the transport of radio frequency signals at
frequencies in the SHF band (3–30 GHz) and higher.
Circular waveguide
A circular waveguide is a hollow metallic tube with circular cross section for propagating
the electromagnetic waves by continuous reflections from the surfaces or walls of the
guide
Elliptical Waveguides
Elliptical Waveguides are almost twice the size of an electrically equivalent
rectangular type. The decisive advantage of a flexible elliptical waveguide is that larger
lengths can be transported in rolls or on cable drums. Elliptical waveguides allow smaller
bending radii than purely round waveguides.
Single ridge waveguide can be thought of as half of a double ridge waveguide with a
horizontal perfect electrical conductor (PEC) inserted at exactly the middle of the gap
region. This waveguide will have the same cut-off frequency of the fundamental
waveguide mode, however the higher order modes will be pushed further away in
frequency.

Double Ridge Waveguide Double-ridged waveguide can provide more bandwidth than
"normal" rectangular waveguide. A pair of ridges protrude into the center of the
waveguide, parallel to the short wall. This is where the E-field is maximum. By bringing
ground down the ridges, the E-field is further increased.
Applications of waveguide
The applications of Waveguides are:
•Waveguides are used in Optical fiber communication.
•They are used Photonic integrated circuits.
•They are extensively used in Microwave ovens.
•Waveguides are used for broadcasting and radar installations.
•They are used in space crafts.
 2. Microwaves ─ Transmission Lines Microwave Engineering

A transmission line is a connector which transmits energy from one point to another.
The study of transmission line theory is helpful in the effective usage of power and
equipment.

There are basically four types of transmission lines:

 Two-wire parallel transmission lines

 Coaxial lines
 Strip type substrate transmission lines
 Waveguides

While transmitting or while receiving, the energy transfer has to be done effectively,
without the wastage of power. To achieve this, there are certain important parameters
which has to be considered.

MainParametersof a TransmissionLine
The important parameters of a transmission line are resistance, inductance, capacitance
and conductance.
Resistance and inductance together are called as transmission line impedance.

Capacitance and conductance together are called as admittance.

Resistance
The resistance offered by the material out of which the transmission lines are made, will
be of considerable amount, especially for shorter lines. As the line current increases, the
ohmic loss (𝐼2𝑅 𝑙𝑜𝑠𝑠) also increases.

The resistance R of a conductor of length “l” and cross-section “a” is represented as

𝒍
𝑹= 𝝆
𝒂
Where
ρ = resistivity of the conductor material, which is constant.

Temperature and the frequency of the current are the main factors that affect the
resistance of a line. The resistance of a conductor varies linearly with the change in
temperature. Whereas, if the frequency of the current increases, the current density
towards the surface of the conductor also increases. Otherwise, the current density
towards the center of the conductor increases.

This means, more the current flows towards the surface of the conductor, it flows less
towards the center, which is known as the Skin Effect.

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 Microwave Engineering

Inductance
In an AC transmission line, the current flows sinusoidally. This current induces a magnetic
field perpendicular to the electric field, which also varies sinusoidally. This is well known
as Faraday’s law. The fields are depicted in the following figure.

This varying magnetic field induces some EMF into the conductor. Now this induced voltage
or EMF flows in the opposite direction to the current flowing initially. This EMF flowing in
the opposite direction is equivalently shown by a parameter known as Inductance, which
is the property to oppose the shift in the current.

It is denoted by “L”. The unit of measurement is “Henry (H)”.

Conductance
There will be a leakage current between the transmission line and the ground, and also
between the phase conductors. This small amount of leakage current generally flows
through the surface of the insulator. Inverse of this leakage current is termed as
Conductance. It is denoted by “G”.

The flow of line current is associated with inductance and the voltage difference between
the two points is associated with capacitance. Inductance is associated with the magnetic
field, while capacitance is associated with the electric field.

Capacitance
The voltage difference between the Phase conductors gives rise to an electric field
between the conductors. The two conductors are just like parallel plates and the air in
between them becomes dielectric. This pattern gives rise to the capacitance effect between
the conductors.

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 Microwave Engineering

CharacteristicImpedance
If a uniform lossless transmission line is considered, for a wave travelling in one direction,
the ratio of the amplitudes of voltage and current along that line, which has no reflections,
is called as Characteristic impedance.

It is denoted by 𝒁𝟎

𝒗𝒐𝒍𝒕𝒂𝒈𝒆 𝒘𝒂𝒗𝒆 𝒗𝒂𝒍𝒖𝒆

𝒁𝟎 = √
𝒄𝒖𝒓𝒓𝒆𝒏𝒕 𝒘𝒂𝒗𝒆 𝒗𝒂𝒍𝒖𝒆

𝑹 + 𝒋𝒘𝑳
𝒁𝟎 = √
𝑮 + 𝒋𝒘𝑪

For a lossless line, 𝑹 𝟎 = √ 𝑳𝑪

Where L & C are the inductance and capacitance per unit lengths.

ImpedanceMatching
To achieve maximum power transfer to the load, impedance matching has to be done. To
achieve this impedance matching, the following conditions are to be met.

The resistance of the load should be equal to that of the source.

𝑹𝑳 = 𝑹𝑺

The reactance of the load should be equal to that of the source but opposite in sign.

𝑿𝑳 = −𝑿𝑺

Which means, if the source is inductive, the load should be capacitive and vice versa.

ReflectionCo-efficient
The parameter that expresses the amount of reflected energy due to impedance mismatch
in a transmission line is called as Reflection coefficient. It is indicated by ρ (rho).

It can be defined as “the ratio of reflected voltage to the incident voltage at the load
terminals”.
𝒓𝒆𝒇𝒍𝒆𝒄𝒕𝒆𝒅𝒗𝒐𝒍𝒕𝒂𝒈𝒆 𝑽𝒓
𝝆= = 𝒂𝒕 𝒍𝒐𝒂𝒅 𝒕𝒆𝒓𝒎𝒊𝒏𝒂𝒍𝒔
𝒊𝒏𝒄𝒊𝒅𝒆𝒏𝒕 𝒗𝒐𝒍𝒕𝒂𝒈𝒆 𝑽𝒊
If the impedance between the device and the transmission line don’t match with each
other, then the energy gets reflected. The higher the energy gets reflected, the greater
will be the value of ρ reflection coefficient.

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 Microwave Engineering

VoltageStandingWaveRatio (VSWR)
The standing wave is formed when the incident wave gets reflected. The standing wave
which is formed, contains some voltage. The magnitude of standing waves can be
measured in terms of standing wave ratios.

The ratio of maximum voltage to the minimum voltage in a standing wave can be defined
as Voltage Standing Wave Ratio (VSWR). It is denoted by “S”.

|𝑉𝑚𝑎𝑥 |
𝑆= 1≤𝑆≤∞
|𝑉𝑚𝑖𝑛 |

VSWR describes the voltage standing wave pattern that is present in the transmission line
due to phase addition and subtraction of the incident and reflected waves.

Hence, it can also be written as

1+ 𝜌
𝑆=
1− 𝜌

The larger the impedance mismatch, the higher will be the amplitude of the standing wave.
Therefore, if the impedance is matched perfectly,

𝑉𝑚𝑎𝑥 ∶ 𝑉𝑚𝑖𝑛 = 1: 1

Hence, the value for VSWR is unity, which means the transmission is perfect.

Efficiencyof TransmissionLines
The efficiency of transmission lines is defined as the ratio of the output power to the input
power.
𝑷𝒐𝒘𝒆𝒓 𝒅𝒆𝒍𝒊𝒆𝒗𝒆𝒓𝒆𝒅 𝒂𝒕 𝒓𝒆𝒄𝒆𝒑𝒕𝒊𝒐𝒏 𝒆𝒏𝒅
% 𝒆𝒇𝒇𝒊𝒄𝒊𝒆𝒏𝒄𝒚 𝒐𝒇 𝒕𝒓𝒂𝒏𝒔𝒎𝒊𝒔𝒔𝒊𝒐𝒏 𝒍𝒊𝒏𝒆 𝜼 = × 𝟏𝟎𝟎
𝑷𝒐𝒘𝒆𝒓 𝒔𝒆𝒏𝒕 𝒇𝒓𝒐𝒎 𝒕𝒉𝒆 𝒕𝒓𝒂𝒏𝒔𝒎𝒊𝒔𝒔𝒊𝒐𝒏 𝒆𝒏𝒅

VoltageRegulation
Voltage regulation is defined as the change in the magnitude of the voltage between the
sending and receiving ends of the transmission line.

𝒔𝒆𝒏𝒅𝒊𝒏𝒈 𝒆𝒏𝒅 𝒗𝒐𝒍𝒕𝒂𝒈𝒆 − 𝒓𝒆𝒄𝒆𝒊𝒗𝒊𝒏𝒈 𝒆𝒏𝒅 𝒗𝒐𝒍𝒕𝒂𝒈𝒆

%𝒗𝒐𝒍𝒕𝒂𝒈𝒆 𝒓𝒆𝒈𝒖𝒍𝒂𝒕𝒊𝒐𝒏 = × 𝟏𝟎𝟎
𝒔𝒆𝒏𝒅𝒊𝒏𝒈 𝒆𝒏𝒅 𝒗𝒐𝒍𝒕𝒂𝒈𝒆

Lossesdue to ImpedanceMismatch
The transmission line, if not terminated with a matched load, occurs in losses. These losses
are many types such as attenuation loss, reflection loss, transmission loss, return loss,
insertion loss, etc.

Attenuation Loss
The loss that occurs due to the absorption of the signal in the transmission line is termed
as Attenuation loss, which is represented as

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 Microwave Engineering

𝐸𝑖 − 𝐸𝑟
𝐴𝑡𝑡𝑒𝑛𝑢𝑎𝑡𝑖𝑜𝑛 𝑙𝑜𝑠𝑠 (𝑑𝐵) = 10 𝑙𝑜𝑔10 [ ]
𝐸𝑡

Where

 𝐸𝑖 = the input energy

 𝐸𝑟 = the reflected energy from the load to the input
 𝐸𝑡 = the transmitted energy to the load

Reflection Loss
The loss that occurs due to the reflection of the signal due to impedance mismatch of the
transmission line is termed as Reflection loss, which is represented as
𝐸𝑖
𝑅𝑒𝑓𝑙𝑒𝑐𝑡𝑖𝑜𝑛 𝑙𝑜𝑠𝑠 (𝑑𝐵) = 10 log 10 [ ]
𝐸𝑖 − 𝐸𝑟

Where

 𝐸𝑖 = the input energy

 𝐸𝑟 = the reflected energy from the load

Transmission Loss
The loss that occurs while transmission through the transmission line is termed as
Transmission loss, which is represented as

𝐸𝑖
𝑇𝑟𝑎𝑛𝑠𝑚𝑖𝑠𝑠𝑖𝑜𝑛 𝑙𝑜𝑠𝑠 (𝑑𝐵) = 10 log 10
𝐸𝑡

Where

 𝐸𝑖 = the input energy

 𝐸𝑡 = the transmitted energy

Return Loss
The measure of the power reflected by the transmission line is termed as Return loss,
which is represented as

𝐸𝑖
𝑅𝑒𝑡𝑢𝑟𝑛 𝑙𝑜𝑠𝑠 (𝑑𝐵) = 10 log 10
𝐸𝑟

Where

 𝐸𝑖 = the input energy

 𝐸𝑟 = the reflected energy

Insertion Loss
The loss that occurs due to the energy transfer using a transmission line compared to
energy transfer without a transmission line is termed as Insertion loss, which is
represented as

𝐸1
𝐼𝑛𝑠𝑒𝑟𝑡𝑖𝑜𝑛 𝑙𝑜𝑠𝑠 (𝑑𝐵) = 10 log 10
𝐸2

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 Microwave Engineering

The types of waveguides shown above are hollow in the center and made up of copper
walls. These have a thin lining of Au or Ag on the inner surface.

Let us now compare the transmission lines and waveguides.

TransmissionLines Vs Waveguides
The main difference between a transmission line and a wave guide is -

 A two conductor structure that can support a TEM wave is a transmission line.

 A one conductor structure that can support a TE wave or a TM wave but not a
TEM wave is called as a waveguide.

The following table brings out the differences between transmission lines and waveguides.

Transmission Lines Waveguides

Supports TEM wave Cannot support TEM wave

Only the frequencies that are greater than

All frequencies can pass through cut-off frequency can pass through

One conductor transmission Two conductor transmission

Wave travels through reflections from the

Reflections are less walls of waveguide

It has characteristic impedance It has wave impedance

Propagation of waves is according to Propagation of waves is according to “Field

“Circuit theory” theory”

Return conductor is not required as the

It has a return conductor to earth body of the waveguide acts as earth

Bandwidth is not limited Bandwidth is limited

Waves do not disperse Waves get dispersed

PhaseVelocity
Phase Velocity is the rate at which the wave changes its phase in order to undergo a phase
shift of 2π radians. It can be understood as the change in velocity of the wave components
of a sine wave, when modulated.

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 Microwave Engineering

Let us derive an equation for the Phase velocity.

According to the definition, the rate of phase change at 2π radians is to be considered.

Which means, λ / T hence,

𝜆
𝑉=
𝑇
Where,

λ = wavelength and T = time

𝜆
𝑉= = 𝜆𝑓
𝑇

Since 𝑓 = 1𝑇

If we multiply the numerator and denominator by 2π then, we have

2𝜋𝜆𝑓
𝑉 = 𝜆𝑓 =
2𝜋

We know that 𝜔 = 2𝜋𝑓 and 𝛽 = 2𝜋𝑓

The above equation can be written as,

2𝜋𝑓 𝜔
𝑉 = 2𝜋 = 𝛽
𝜆

Hence, the equation for Phase velocity is represented as

𝜔
𝑉𝑝 =
𝛽

Group Velocity
Group Velocity can be defined as the rate at which the wave propagates through the
waveguide. This can be understood as the rate at which a modulated envelope travels
compared to the carrier alone. This modulated wave travels through the waveguide.

The equation of Group Velocity is represented as

𝑑𝜔
𝑉𝑔 =
𝑑𝛽

The velocity of modulated envelope is usually slower than the carrier signal.

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