Introduction to antenna system

COURSE TITLE:- MODERN ANTENNA SYSTEM ENGINEERING
COURSE CODE:- ECENG 6203
Course Coordinator:- Dr. Mulugeta Atlabachew
(Ass. Professor )
JIMMA UNIVERSITY
JIMMA INSTITUTE OF TECHNOLOGY
FACULTY OF ELECTRICAL AND COMPUTER
ENGINEERING

Introduction to Antenna
 Electrical signals are carried between points in one of
two ways:
1) Via Transmission Line (TL)
 By confining the energy of the electromagnetic waves to the region near,
or inside, the transmission line.
 Transmission lines are coaxial cables, parallel-wire lines, and
waveguides.
2) Via Antennas - through empty space
JIT,Faculty of Electrical and Computer Engineering, Jimma University 2

Introduction to Antenna
 Transmission line and antenna have opposite purpose,
 to confine-Transmission Line
 to radiate-Antenna
 It is not possible to imagine Antenna with out
transmission line.
 The IEEE definition for antenna
“part of a transmitting or receiving system that is designed to
radiate or receive electromagnetic waves.”

Types of Antenna
 Wire antennas: (single element)
 Dipole, monopole, loop antenna, helix
 Usually used in personal applications, automobiles, buildings, ships,
aircrafts and spacecraft.
 Aperture antennas:
 horn antennas, waveguide opening
 Usually used in aircrafts and space crafts, because these
antennas can be flush-mounted.
JIT,Faculty of Electrical and Computer Engineering, Jimma University
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Types of Antenna
 Reflector antennas:
 Parabolic reflectors, corner reflectors
 These are high gain antennas usually used in radio astronomy,
microwave communication and satellite tracking.
 Lens antennas:
 Convex-plane, covex-convex , convex-concave and concave-
plane lenses
 These antennas are usually used for very high frequency
applications.
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Types of Antenna
 Microstrip antennas:
 Rectangular, circular etc. shaped metallic patch above a ground plane
 Used in aircraft, spacecraft, satellites, missiles, cars, mobile phones
etc.
 Array antennas:
 Yagi-Uda antenna, microstrip patch array, aperture array, slotted
waveguide array.
 Used for very high gain applications with added advantage, such as,
controllable radiation pattern.
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Aperture Antennas
Horn Antenna
Large Vertical aperture antenna
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Types of Antenna
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Types of Antenna
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Reflector Antenna
 An antenna reflector is a device that reflects electromagnetic
waves.
 It can exist as a standalone device for redirecting RF energy, or
 It can be integrated as part of an antenna assembly to modify the
radiation pattern of the antenna.
 Common standalone reflector types are
 Corner reflector, commonly used in radar.
 Flat reflector, used as a passive repeater.
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Reflector Antenna
 Common integrated reflector types are
 Parabolic reflector
 a passive element slightly longer than and located behind a
radiating dipole element that absorbs and re-radiates the signal
in a directional way as in a Yagi antenna array.
 a flat reflector such as used in a Short backfire antenna or
Sector antenna.
 a corner reflector used in UHF television antennas.
 a cylindrical reflector as used in Cantenna.
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Reflector Antenna
 Simple reflecting antenna consists of reflecting surface and small feed antenna
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Reflector Antenna
 Parabolic reflector, which focuses a beam signal into one point or directs a radiating
signal into a beam
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Reflector Antenna
 a passive radiator or parasitic element is a conductive element, typically a
metal rod, which is not electrically connected to anything else.
 The Yagi-Uda antenna typically consist of a "driven element" which is connected
to the radio receiver or transmitter through a feed line, and parasitic elements,
which are not.

Reflector Antenna
 The purpose of the parasitic elements is to modify the radiation
pattern of the radio waves emitted by the driven element, it makes
more directional and act as resonator.
 The waves from the different antenna elements interfere,
strengthening the antenna's radiation in the desired direction, and
cancelling out the waves in undesired directions.
JIT, Faculty of Electrical and Computer Engineering, Jimma University 15

Reflector Antenna

Corner Reflector Antenna
 A corner reflector is a retroreflector consisting of three mutually
perpendicular, intersecting flat surfaces, which reflects waves back directly
towards the source, but translated.
JIT, Faculty of Electrical and Computer Engineering, Jimma University
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Lens Antenna
 The lens antenna is 3-dimensional electro-magnetic device which has refractive
index other than unity.
 It consists of electro-magnetic lens along with feed. It is similar to glass lens use
in optical domain.
 It has the following functions :
 It generates plane wavefront from spherical.
 It forms incoming wavefront at its focus.
 It generates directional characteristics.
 It is used to collimate electromagnetic rays.
 It controls aperture illumination.
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Lens Antenna
Figure- Operation of lens Antenna

Lens Antenna

Reflector Vs Lens Antenna

Reflector Vs Lens Antenna
 They have the same purpose but lens antenna is more costly.

Microstrip Antenna/Patch Antenna
 Patch antennas are low cost, have a low profile and are easily fabricated.

Array Antenna
 Patch antennas are low cost, have a low
profile and are easily fabricated.

Array Antenna

Antenna Parameters
 Antenna parameters are important to evaluate
the performance of an antenna.
 Radiation pattern,
 directivity,
 gain,
 polarization,
 impedance,
 bandwidth,
 scanning and the likes

Radiation Pattern
 A radiation pattern (antenna pattern pattern/far-field pattern)- is a
graphical/mathematical representation of the far field properties of
an antenna.
 It is a spatial distribution of the radiated energy or received energy
of the antenna as a function of the angular variation (spherical
coordinates).
 It is independent of the direction(transmitting/receiving) but
dependent on operating frequency.

Radiation Pattern
 Common Types of Antenna Patterns
 Power Pattern - normalized power vs. spherical coordinate position.
 Field Pattern - normalized E or H vs. spherical coordinate position.
 Antenna Field Types
 Reactive Field- the portion of the antenna field characterized by
standing (stationary) waves which represent stored energy.
 Radiation Field- the portion of the antenna field characterized by
radiating (propagating) waves which represent transmitted energy.

Radiation Pattern
 Antenna Field Regions
 Reactive Near Field Region - the region immediately surrounding the
antenna where the reactive field (stored energy – standing waves) is
dominant.
 Near-Field (Fresnel) Region - the region between the reactive near-
field and the far-field where the radiation fields are dominant and the
field distribution is dependent on the distance from the antenna.
 Far-Field (Fraunhofer) Region- the region farthest away from the
antenna where the field distribution is essentially independent of the
distance from the antenna (propagating waves).

Antenna Field Regions

Radiation Pattern
 Antenna Pattern Definitions
 Isotropic Pattern - an antenna pattern defined by uniform radiation in
all directions, produced by an isotropic radiator (point source, a non-
physical antenna which is the only non directional antenna).
 Directional Pattern - a pattern characterized by more efficient
radiation in one direction than another (all physically realizable
antennas are directional antennas).
 Omnidirectional Pattern - a pattern which is uniform in a given plane.

Radiation Pattern
 Antenna Pattern Definitions
 Principal Plane Patterns - the E-plane and H-plane patterns of a
linearly polarized antenna.
 E-plane- the plane containing the electric field vector and the direction of
maximum radiation.
 H-plane - the plane containing the magnetic field vector and the direction
of maximum radiation.

Radiation Pattern
 Antenna Pattern Parameters
 Radiation Lobe - a clear peak in the radiation intensity surrounded by
regions of weaker radiation intensity.
 Main Lobe (major lobe, main beam) - radiation lobe in the direction of
maximum radiation.
 Minor Lobe - any radiation lobe other than the main lobe.
 Side Lobe - a radiation lobe in any direction other than the direction(s) of
intended radiation.
 Back Lobe - the radiation lobe opposite to the main lobe.

Radiation Pattern
 Antenna Pattern Parameters
 Half-Power Beamwidth (HPBW) - the angular width of the main beam at
the half-power points.
 First Null Beamwidth (FNBW) - angular width between the first nulls on
either side of the main beam.
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Radiated Power

Radiated Power
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Radiated Power
 Poynting vector, a quantity describing the magnitude and direction
of the flow of energy in electromagnetic waves.
 The Poynting vector represents the directional energy flux (the
energy transfer per unit area per unit time) of an electromagnetic
field.
 The SI unit of the Poynting vector is the watt per square metre
(W/m2).
 It is named after its discoverer John Henry Poynting who first
derived it in 1884.
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Radiated Power
 To determine the average radiated power by an antenna, we start
with the instantaneous Poynting vector (vector power density or the
instantaneous power flow) defined by instantaneous electric and
magnetic fields.
Assume the antenna is enclosed by surface S
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Radiated Power
 The total instantaneous radiated power leaving the surface S is
found by integrating the instantaneous Poynting vector over the
surface.

Radiated Power
 For time-harmonic fields, the time average instantaneous Poynting
vector (time average vector power density) is found by integrating
the instantaneous Poynting vector over one period (T) and dividing
by the period.
 The instantaneous magnetic field may be rewritten as

Radiated Power
 The instantaneous Poynting vector becomes
 The time average power density becomes
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Radiation Intensity
 Radiation intensity is defined as the power per unit solid angle, that is the
power incident on that portion of the surface of a sphere which subtends an
angle of one radian at the center of the sphere in both the horizontal and the
vertical planes.
 Radiation Intensity - radiated power per solid angle (radiated power
normalized to a unit sphere).
 In the far field, the radiation electric and magnetic fields vary as 1/r and the
direction of the vector power density (Pavg) is radially outward.
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Radiation Intensity
 If we assume that the surface S is a sphere of radius r, then the
integral for the total time-average radiated power becomes
Where defines the differential solid Angele.
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Radiation Intensity
 The average radiation intensity is found by dividing the radiation
intensity by the area of the unit sphere (4𝜋) which gives
 The average radiation intensity for a given antenna represents the
radiation intensity of a point source producing the same amount of
radiated power as the antenna.
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Antenna Efficiency
 The total antenna efficiency e0 is used to take into account losses at
the input terminals and within the structure of the antenna. Such
losses may be due to
1. Reflections because of the mismatch between the transmission line and the
antenna
2. I 2R losses (conduction and dielectric)
o In general, the overall efficiency can be written as
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Antenna Efficiency
 Where
 Usually ec and ed are very difficult to compute, but they can be
determined experimentally. Even by measurements they cannot be
separated, and it is usually more convenient to rewrite as
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Antenna Efficiency
 Usually ec and ed are very difficult to compute, but they can be
determined experimentally. Even by measurements they cannot be
separated, and it is usually more convenient to rewrite as
 Where ecd = eced = antenna radiation efficiency, which is used to
relate the gain and directivity.
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Antenna Radiation Efficiency
 The antenna efficiency takes into account the reflection, conduction,
and dielectric losses.
 The conduction and dielectric losses of an antenna are very difficult
to compute and in most cases they are measured.
 Even with measurements, they are difficult to separate and they are
usually lumped together to form the ecd efficiency.
 The resistance RL is used to represent the conduction-dielectric
losses.
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 The conduction-dielectric efficiency ecd is defined as the ratio of the
power delivered to the radiation resistance Rr to the power
delivered to Rr and RL.
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 Antenna efficiency for loss less transmission line is the same as the
antenna radiation efficiency.
 So, When an antenna is driven by a voltage source (generator), the
total power radiated by the antenna will not be the total power
available from the generator.
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Antenna Radiaiton Efficiency
 The antenna efficiency for loss less transmission line (antenna
radiation efficiency or conduction-dielectric efficiency, ecd ) can be
defined as the ratio of the power delivered to the radiation
resistance Rr to the power delivered to Rr and RL.
Or

Directivity
 Directivity ( D ) - the ratio of the radiation intensity in a given
direction from the antenna to the radiation intensity averaged over
all directions.
 The average radiation intensity is equal to the total power radiated
by the antenna divided by 4π. If the direction is not specified, the
direction of maximum radiation intensity is implied.
 Stated more simply, the directivity of a non-isotropic source is
equal to the ratio of its radiation intensity in a given direction over
that of an isotropic source.
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Directivity
 If the direction is not specified, it implies the direction of maximum
radiation intensity (maximum directivity) expressed as
 For a spherical coordinate system, the total maximum directivity for
the orthogonal θ and φ components of an antenna can be written
as
 where
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Directivity
Where

Gain
 Gain of an antenna (in a given direction) is defined as “the ratio of
the intensity, in a given direction, to the radiation intensity that
would be obtained if the power accepted by the antenna were
radiated isotropically.
 The radiation intensity corresponding to the isotropically radiated
power is equal to the power accepted (input) by the antenna divided
by 4π.” In equation form this can be expressed as

Gain
 The total input power can be related with radiated power using the
antenna radiation efficiency by
 The Gain then becomes,
 In terms of Directivity, it becomes
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Gain
 The maximum Gain of the antenna is related with the maximum
Directivity, which is given by
 In decibels
 For spherical coordinate system, the total maximum Gain becomes
 Where
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Input Impedance
 Input impedance is defined as
“the impedance presented by an antenna at its terminals (a-b) or the
ratio of the voltage to current at a pair of terminals or the ratio of the
appropriate components of the electric to magnetic fields at a point.”
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Input Impedance
 Input impedance is defined as
“the impedance presented by an antenna at its terminals (a-b) or the
ratio of the voltage to current at a pair of terminals or the ratio of the
appropriate components of the electric to magnetic fields at a point.”
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Input Impedance
 Using the Thevenin Equivalent, the current developed with in the loop
is given
 Its magnitude becomes, where Vg is peak generator voltage
 Power delivered to the antenna for radiation is given by
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Input Impedance
 The power dissipated as heat due to loss resistance in the Antenna
is given by
 The remaining power will be dissipated as heat in the internal
resistance of the generator, it is given by
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Input Impedance
 The maximum power delivered to the antenna occurs when we have
conjugate matching; that is when

Input Impedance
 For this case
 From the above equations, it is understood that
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Input Impedance
 The power supplied by the generator during conjugate matching is
given by
 During conjugate matching, half of the power supplied by the
generator shall be dissipated as heat in the internal resistance of
the generator. From the remaining some of the power shall be
dissipated a heat in the internal resistance of the antenna and the
remaining shall be radiated the space.
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Input Impedance
 If the antenna is lossless and matched to the transmission line half
of the power shall be radiated to the free space.
 If the transmission line is lossy, additional power shall be dissipated
as heat in the transmission medium which decreases the power to
be transmitted.
 Therefore, impedance matching circuit is very important concept in
the design of an antenna system.
 What would happen when the antenna acts as a receiver? When it
operates in receiving mode.

Antenna Polarization
 A radio wave is made up of both electric and magnetic fields.
 In free space, the electric and magnetic fields are mutually
perpendicular and are also perpendicular to the direction of
propagation.
 The direction of oscillation of the electric field component, when
a radio wave is propagating in a medium, is called the polarization
of the radio wave.

 Polarization of an antenna in a given direction is defined as “the
polarization of the wave transmitted (radiated) by the antenna.
 When the direction is not stated, the polarization is taken to be the
polarization in the direction of maximum gain.
 Polarization of a radiated wave is defined as “the property of an
electromagnetic wave describing the time-varying direction and
relative magnitude of the electric-field vector”
 Antennas are usually developed to receive and transmit radio waves
that are polarized in a specific way.

 Polarization then is the curve traced by the end point of the arrow
(vector) representing the instantaneous electric field.
 According to the shape of the trace, three types of polarization exist
for harmonic fields: linear, circular and elliptical.
 Any polarization can be represented by two orthogonal linear
polarizations, (Ex, Ey), or (EH, EV), whose fields are out of phase by
an angle of δL
 When the electric field is oscillating in the horizontal or vertical
direction, the radio wave is said to be linearly polarized.
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 Polarization then is the curve traced by the end point of the arrow
(vector) representing the instantaneous electric field.
 According to the shape of the trace, three types of polarization exist
for harmonic fields: linear, circular and elliptical. Any polarization
can be represented by two orthogonal linear polarizations, (Ex, Ey),
or (EH, EV), whose
 fields are out of phase by an angle of δL
 When the electric field is oscillating in the horizontal or vertical
direction, the radio wave is said to be linearly polarized.
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 When the electric field oscillates at –45 degrees and +45 degrees
from a reference plane of 0 degrees, the polarization is said to be
slant.
 It is another form of linear polarization, it is equivalent to taking a
linearly polarized radio wave and rotating it 45 degrees.
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 .
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Linear Polarization
 Linear polarization is the most common form of antenna
polarization.
 It is characterized by the fact that all of the radiation is in one plane
 There are two types of linear Polarization
 Horizontal Polarization: This form of antenna polarization has horizontal
elements. It picks up and radiates horizontally polarized signals, i.e.
electromagnetic waves with the electric field in the horizontal plane.
 Vertical Polarization: This form of antenna is typified by the vertical
elements within the antenna. It could be a single vertical element. One of
the reasons for using vertical polarization is that antennas comprising of a
single vertical element can radiate equally around it in the horizontal plane.
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Linear Polarization
 Typically vertically polarized antennas have what is termed a low angle of
radiation enabling a large proportion of their power to be radiated at an
angle close to the earth’s surface.
 Vertically polarized antennas are also very convenient for use with
automobiles.
 Slant Polarization: This is a form of radio antenna polarization that is at
an angle to the horizontal or vertical planes. In this way both vertical and
horizontally polarized antennas are able to receive the signal.
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Circular Polarization
 This has a number of benefits for areas such as satellite
applications where it helps overcome the effects of propagation
anomalies, ground reflections and the effects of the spin that occur
on many satellites.
 A Circularly Polarized signal consists of two perpendicular
electromagnetic plane waves of equal amplitude, which are 90
degree out of phase.
 The tip of the electric field vector will then be seen to trace out a
helix or corkscrew as it travels away from the antenna.
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Circular Polarization
 Circular polarization can be classified as two types: Right Hand
Circular Polarization (RHCP) and Left Hand Circular
Polarization (LHCP).
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Elliptical (Mixed) Polarization
 Elliptical Polarization- Elliptically polarized radio signals consist of
two perpendicular waves of unequal amplitude which differ in phase
by 90°.
 Elliptical polarization is the polarization of electromagnetic
radiation such that the tip of the electric field vector describes an
ellipse in any fixed plane intersecting, and normal to, the direction of
propagation.
 An elliptically polarized wave may be resolved into two linearly
polarized waves in phase quadrature, with their polarization planes
at right angles to each other.

 Since the electric field can rotate clockwise or counterclockwise as
it propagates, elliptically polarized waves exhibit chirality.
 Other forms of polarization, such as circular and linear polarization,
can be considered to be special cases of elliptical polarization.

Elliptical (Mixed) Polarization
 It occurs when there is a mix of linear and circular polarisation.
 It is possible for linearly polarized antennas to receive circularly
polarized signals and vice versa.
 The strength will be equal whether the linearly polarized antenna is
mounted vertically, horizontally or in any other plane but directed
towards the arriving signal.
 There will be some degradation because the signal level will be 3 dB
less than if a circularly polarized antenna of the same sense was
used.
 The same situation exists when a circularly polarized antenna
receives a linearly polarized signal.JIT,Faculty of Electrical and Computer Engineering, Jimma University 80

Advantages of Elliptical Polarization
Reflectivity:
 Radio signals are reflected or absorbed depending on the material
they come in contact with.
 For linear polarized antennas, if the reflecting surface does not
reflect the signal precisely in the same plane, that signal strength
will be lost.
 Since circular polarized antennas send and receive in all planes, the
signal strength is not lost, but is transferred to a different plane and
are still utilized.

Absorption:
 As stated above, radio signal can be absorbed depending on the
material they come in contact with.
 Different materials absorb the signal from different planes.
 As a result, circular polarized antennas give you a higher probability
of a successful link because it is transmitting on all planes.

Phasing Issues:
 High-frequency systems (i.e. 2.4 GHz and higher) that use linear polarization typically
require a clear line-of sight path between the two points in order to operate effectively.
 Such systems have difficulty penetrating obstructions due to reflected signals, which
weaken the propagating signal.
 Reflected linear signals return to the propagating antenna in the opposite phase, thereby
weakening the propagating signal.
 Conversely, circularly-polarized systems also incur reflected signals, but the reflected
signal is returned in the opposite orientation, largely avoiding conflict with the propagating
signal.
 The result is that circularly-polarized signals are much better at penetrating and bending
around obstructions.

Multi-path:
 Multi-path is caused when the primary signal and the reflected signal reach
a receiver at nearly the same time.
 This creates an "out of phase" problem. The receiving radio must spend its
resources to distinguish, sort out, and process the proper signal, thus
degrading performance and speed.
 Linear Polarized antennas are more susceptible to multi-path due to
increased possibility of reflection.
 Out of phase radios can cause dead-spots, decreased throughput, distance
issues and reduce overall performance in a 2.4 GHz system.

Inclement Weather:
 Rain and snow cause a microcosm of conditions explained above (i.e.
reflectivity, absorption, phasing, multi-path and line of sight) Circular
polarization is more resistant to signal degradation due to inclement
weather conditions for all the reason stated above.
Line-of-Sight:
 When a line-of-sight path is impaired by light obstructions (i.e. foliage or
small buildings), circular polarization is much more effective than linear
polarization for establishing and maintaining communication links.

Applications for different types of antenna
Polarization
 Different types of polarization are used in different applications to
enable their advantages to be used.
 Accordingly different forms of polarization are used for different
applications:
 General radio communications:
 Linear polarization is by far the most widely used for most radio
communications applications as the radio antennas are generally simpler
and more straightforward.

Polarization
Mobile phones and short range wireless communications:
 Normally linear polarzation is used for these devices because linearly
polarized antennas are easier to fabricate in these devices, and hence the
base stations need to have a similar polarization.
 Although vertical polarization is often used, many items like Wi-Fi routers
have adjustable antennas.
 Also the fact that these communications often have signal paths that may
reflect from a variety of surfaces, the polarization that reaches the
receiver can be relatively random, and therefore it can be less of an issue.

Polarization
Mobile two way radio communications:
 There are many traditional mobile two way radio communication systems
still in use for everything from the emergency services to a host of private
mobile radio applications where radio transceivers are located in vehicles.
 Vertical polarization is often used for these mobile two way radio
communications. This is because many vertically polarized radio antenna
designs have an omni-directional radiation pattern and it means that the
antennas do not have to be re-orientated as positions as always happens
for mobile radio communications as the vehicle moves.

Polarization
Long distance HF ionospheric communications:
 Both vertical and horizontal polarization are used:
 Horizontal polarization: Wire antennas are widely used for HF
communications. These tend to be more easily erected using two poles
leaving he wire antenna to be suspended between the two. In this way the
antenna is horizontally polarized.
 For large multi-element antenna arrays, mechanical constraints mean that
they can be mounted in a horizontal plane more easily than in the vertical
plane.
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Polarization
 This is because the RF antenna elements are at right angles to the vertical
tower of pole on which they are mounted and therefore by using an antenna
with horizontal elements there is less physical and electrical interference
between the two.
 Vertical polarization: Antennas consisting of a single vertical element
are widely used. The vertically polarized antenna provides a low angle of
radiation which enables it to provide good long distance transmission and
reception.

Polarization
 Medium wave broadcasting:
 Medium wave broadcast stations generally use vertical polarization
because ground wave propagation over the earth is considerably better
using vertical polarization, whereas horizontal polarization shows a
marginal improvement for long distance communications using the
ionosphere.
 A typical medium wave broadcast transmitter antenna is used for relatively
local coverage using ground wave propagation.
 A vertically polarized antenna has the advantage that it will radiate equally
in all directions parallel to the Earth and this has advantages for coverage.
 Additionally a vertical antenna only requires the vertical element - a
horizontally polarized antenna would need two supports.

Polarization
Satellite communications:
 Circular polarization is sometimes used for satellite radio communications
as there are some advantages in terms of propagation and in overcoming
the fading caused if the satellite is changing its orientation.
 As can be seen, each form of radio antenna polarization has its own
advantages which can be utilized to effect in particular instances.
 Selecting the right form of polarization can provide some advantages, and
therefore can be quite important.

Antenna Polarization Loss
 Generally, the polarization of the receiving antenna is not the same
as the polarization of the incident wave. This is called polarization
mismatch.
 The Polarization Loss Factor (PLF) also called Antenna Polarization
Efficiency (APE), characterizes the loss of EM power because of
polarization mismatch:
 The above definition is based on the representation of the incident
field and the antenna polarization by their polarization vectors.

Antenna Polarization Loss
 If the incident field is
 then the field of the same magnitude that would produce maximum
received power at the antenna terminals is
Where Polarization Vector
 If the antenna is polarization matched, then PLF =1, and there is no
polarization power loss. If PLF = 0, then the antenna is incapable of
receiving the signal.

Additional Points on Antenna Polarization
Points for further Reading
 Co-Polarization is defined as the polarization the antenna was meant to
radiate, while
 Cross-Polarization is defined as its orthogonal pair. ...
 Multi-Polarization.
 Polarization Diversity.

Maximum Directivity and Maximum
Effective Area
 .

Effective Area
 The effective areas and directivities of each are designated as At ,
Ar and Dt , Dr . If antenna 1 were isotropic, its radiated power
density at a distance R would be
Where Pt is the total radiated power.
 For an antenna having some directivity, the radiated power density
becomes

Effective Area
 The power collected (received) by the antenna and transferred to
the load would be
Or
 If antenna 2 is used as a transmitter, antenna 1 as a receiver, and
the intervening medium is linear, passive, and isotropic, we can
write that

Effective Area
 Then equating the above two
 Increasing the directivity of an antenna increases its effective area
in direct proportion. Thus, the above equations can be written as
where Atm and Arm (D0t and D0r ) are the maximum effective areas
(directivities) of antennas 1 and 2, respectively.

Effective Area
 If antenna 1 is isotropic, then D0t = 1 and its maximum effective area
can be expressed as
 The above equation states that the maximum effective area of an
isotropic source is equal to the ratio of the maximum effective area
to the maximum directivity of any other source.
 In general then, the maximum effective aperture (Aem) of any
antenna is related to its maximum directivity (D0) by

Friis Transmission Equation
 The Friis Transmission Equation relates the power received to the
power transmitted between two antennas separated by a distance
R > 2D 2/λ, where D is the largest dimension of either antenna.
 Let us assume that the transmitting antenna is initially isotropic.

 If the input power at the terminals of the transmitting antenna is Pt
, then its isotropic power density W0 at distance R from the
antenna is
 For a non-isotropic transmitting antenna, the power density in the
direction θt, φt can be written as

 Where Gt (θt, φt ) is the gainan d Dt (θt, φt ) is the directivity of
the transmitting antenna in the direction θt, φt .
 Since the effective area Ar of the receiving antenna is related to its
efficiency er and directivity Dr by
 the amount of power Pr collected by the receiving antenna can be
written as

 the ratio of the received to the input power as
 Assumes that the transmitting and receiving antennas are matched
to their respective lines or loads (reflection efficiencies are unity)
and the polarization of the receiving antenna is polarization-
matched to the impinging wave (polarization loss factor and
polarization efficiency are unity).

 If these two factors are also included, then the ratio of the received
to the input power of can be represented by and it is called the Friis
Transmission Equation
 For reflection and polarization-matched antennas aligned for
maximum directional radiation and reception, the Friis formula
reduces to
105

Individual Term Paper (30%)
Title
Antennas:- Applications, Challenges and opportunities in the
Past, Present and Future
 Not more than 10 pages.
 Use IEEE conference format to write your term paper.
 Due Date:- June 19, 2020
106

Introduction to antenna system

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