DRDO Science Spectrum, March 2009, pp 66-78 DRDO SCIENCE SCECTRUM 2009
© 2009, DESIDOC
Antennas and its Applications
Pramod Dhande
Armament Research & Development Establishment, Dr Homi Bhabha Rd, Pashan, Pune-411 021
ABSTRACT
In the world of modern wireless communication, engineer who wants to specialize in the communication field needs
to have a basic understanding of the roles of electromagnetic radiation, antennas, and related propagation phenomena.
These papers discuss on the performance, characteristic, testing, measurement and application of antennas in modern
wireless communication systems. Antenna is an important part of any wireless communication system as it converts
the electronic signals (propagating in the RF Transreceiver) into Electromagnetic Waves (Propagating in the free space)
efficiently with minimum loss. We use antennas when nothing else is possible, as in communication with a missile or
over rugged mountain terrain where cables are expensive and take a long time to install. The performance characteristics
of the parent system are heavily influenced by the selection, position and design of the antenna suite. To understand
the concept of antenna one should know the behaviour of Electromagnetic waves in free space. So I am briefly covering
the basics of Electromagnetic waves and its propagation modes in free space. Apart from that I am also covering Antenna
classifications (based on Frequency, aperture, polarization and radiation pattern), its performance parameters (Gain,
Directivity, Beam area and beam efficiency, radiation pattern, VSWR/Return loss, polarization, Efficiency), measurement
techniques (Outdoor and Indoor Testing) and its defence applications (Naval antennas, Airborne Antennas and Earth
Station Antennas). Finally I discuss about Pyramidal horn antennas, Monopole antennas.
Keywords: Antenna, wireless communication, pyramidal horn antennas, monopole antennas
1.
INTRODUCTION
Antennas are basic components of any electric system
and are connecting links between the transmitter and free
space or free space and the receiver. Thus antennas play
very important role in finding the characteristics of the
system in which antennas are employed. Antennas are
employed in different systems in different forms. That is,
in some systems the operational characteristic of the system
are designed around the directional properties of the antennas
or in some others systems, the antennas are used simply
to radiate electromagnetic energy in an omnidirectinal or
finally in some systems for point-to-point communication
purpose in which increased gain and reduced wave interference
are required.
1.2 Antenna Definitions
There are several definitions of antenna, and are as
follows:
•
The IEEE Standard Definitions of Terms (IEEE Std 1451983):
--A means for radiating or receiving radio waves
•
“An antenna is any device that converts electronic
signals to electromagnetic waves (and vice versa)”
effectively with minimum loss of signals as shown in
Fig.1.
66
Figure 1. Wireless communication system.
•
•
•
•
•
An antenna is basically a transforming device that will
convert impedance of transmitter output (50/75 Ohm)
into free space impedance (120pi or 377 Ohm).
Region of transition between guided and free space
propagation
Concentrates incoming wave onto a sensor (receiving
case)
Launches waves from a guiding structure into space
or air (transmitting case)
Often part of a signal transmitting system over some
distance.
DHANDE: ANTENNAS AND ITS APPLICATIONS
Antenna placed at nose of the aircraft is a part of guidance
RADAR system, which will guide the aircraft. Various jamming
antenna are placed on different parts of aircraft for jamming
the enemy signals. Antenna placed at the belly of the
aircraft for data link application. All these antennas are
operated on different frequency bands, so care should be
taken that to avoid the interference of radiation pattern
of all these antennas. Also when these antennas are placed
on the aircraft body, its radiation pattern gets distorted,
so one should design an antenna such that it will meet
our application.
Figure 2. Propagation of EM waves.
1.2.1 Antenna Definitions
•
The radiation pattern and radiation resistance of an
antenna is the same when it transmits and when it
receives, if no non-reciprocal devices are used. So,
Same antenna can be used for Transmission and Reception
of Electromagnetic Waves
•
Does not apply to active antennas.
NB: Antenna is a passive device, it does not amplify
the signals, it only directs the signal energy in a particular
direction in reference with isotropic antenna.
2.
3.
ELECTROMAGNETIC WAVES
Before understanding the concept of antenna one should
know what are Electromagnetic wave and its propagation
modes in free space. The full Electromagnetic spectrum is
shown in Fig.4. Antennas dimensions are dependent on
wavelength of the signal being transmitted. From Fig.4, it
IMPORTANCE OF ANTENNA IN AIRBORNE
APPLICATION
As shown in Fig.3, different frequency band antennas
are placed on aircraft/missile body for different communication.
Figure 4. Electromagnetic spectrum.
is clear that if we move towards high frequency, wavelength
of the signal being smaller (from Equation 1); hence the
dimensions of the antenna and RF component become
smaller. So at higher frequency the size of the wireless
system becomes compact.
f =
(a)
1
λ
(1)
3.1 Electromagnetic (em) Wave in Free Space
Electromagnetic waves are disturbances to the electrical
and magnetic fields. A changing electric disturbance produces
a changing magnetic field at right angle to the electric
field.
(b)
Figure 3. Application of airborne antennas.
Figure 5. EM wave in free space.
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DRDO SCIENCE SCECTRUM 2009
Electromagnetic Wave originates from a point in free
space, spreads out uniformly in all directions and it forms
a spherical wave. An observer, however, at a grate distance
from the source is able to observe only the small part of
the wave in his immediate vicinity and it appears to him
as plane wave just as the ocean appears flat to a person
who can only see a few miles around him. Thus at a large
distance from the source the wave has similar properties
to the plane waves in the strip line and so by analogy of
strip line the properties of EM waves in free space as
follows:
1. At every point in space, the electric vector field E and
the magnetic vector field H are perpendicular to each
other and to the direction of propagation as shown
in Fig.5.
2. Velocity of EM wave in free space is given by
c=1/(μ0å0) 1/2 = 3 × 10 8 m/s
(2)
3. E and H oscillate in phase and ratio of their amplitude
is constant being equal to 120ð or 377 Ohm or (μ0/
å0)12.
4. Whatever may be the frequency, the EM waves travels
in space with the velocity of light.
5. EM wave propagates in free space as Transverse Electro
Magnetic waves (TEM mode).
Equation of EM waves in free space is given by:
∂ 2 Ex
1 ∂ 2 Ex
=
2
∂t
μ0ε 0 ∂z 2
∂2 H y
∂t
ω
f =
2π
2
=
2
1 ∂ Hy
μ0ε 0 ∂z 2
λ=
1
μ 0ε 0 f
Ex = E0 e j (ωt ± β z )
Horizontal Polarization
When E field vector of EM wave is parallel to the
earth, the EM wave said to be Horizontally Polarized.
Figure
3.
7.
Horizontal
polarisation.
Circular Polarization
When E and H field of the EM wave are of same
amplitude and having a phase difference of 90o, wave
is said to be circularly polarised..
(3)
(4)
H y = H 0 e j (ω t ± β z )
Figure 8. Circular polarisation.
β=
2π
λ
E
Z0 = 0
H0
μ0
Z0 =
ε0
3.1.1 Polarization of Electromagnetic Wave
The Polarization of Electromagnetic wave is defined
as the orientation of electric field vector in space with
respect to time. There are three types of EM wave polarization:
1.
2.
Vertical Polarization-
3.1.2 Properties of Electromagnetic Waves
1.
Reflection and Refraction:
EM waves gets affected from Reflection and Refraction
same as that of light wave. Due to Reflection and
Refraction the polarization of the EM wave get changed,
so care should be taken that the designed antenna will
transmit or receive the EM wave of desired polarization.
When E field vector of EM wave is perpendicular to
the earth, the EM wave said to be Vertically Polarized..
Figure 6. Vertical polarisation.
68
Figure 9. Reflection and refraction of EM wave.
DHANDE: ANTENNAS AND ITS APPLICATIONS
•
Reflection,
3.3.2
θ r = θi
Reflection coefficient:
Depends on media, polarisation
E
ρof= r
incident wave and angle
E
of i
incidence.
η1
• Refraction,
sin(θ t ) = sin(θi )
η2
if both media are lossless sin(θ t ) = μμ εε sin(θ i )
1 1
2 2
3.2 Guded Electromagnetic Waves
Electromagnetic Wave also exists in guided structure
like:
Cables
:
Used at frequencies below 35 GHz
Waveguides :
Used between 0.4 GHz to 350 GHz
Quasi-Optical Systems : Used above 30 GHz
In above structures propagating modes of EM wave
gets changed like in waveguide EM wave propagates in
Transverse Electric (TE) and Transverse Magnetic (TM)
modes.
3.3 Launching of EM Waves
EM wave launched into the free space by means of
antennas and the selection of antenna is depending on
the guided media:
3.3.1
4.
Open and flare up wave guide
: Aperture (Horn) antenna
RADIATION PRINCIPLE OF ANTENNA
One of the first questions that may be asked concerning
antennas would be “How are the electromagnetic fields
generated by the source, contained and the guide in the
transmission line and antenna, and finally detached from
the to form a free-space wave? “ The best explanation can
be given as follows.
Let us consider a voltage source connected to a twoconductor transmission line, which is connected to an
antenna as shown in Fig. 11. Applying a voltage source
across the two-conductor transmission line creates an electric
field between the conductors. The electric field associated
with it electric line of force, which is tangent to the electric
field at each point and the strength, is proportional to the
electric field intensity. The electric field forces the charge
carriers to be displaced which constitutes the current and
hence creates magnetic field intensity. Associated with the
magnetic field intensity, the magnetic line of force, which
are tangent to the magnetic field.
Open up the cable and separate wires
: Monopole & Dipole antenna
Figure 11. Launching of EM wave from waveguide through
aperture antenna.
Figure 10. Launching of EM wave from open cable and separated
wires through dipole antenna.
When a.c. signal is applied to the line from source
time varying electric and magnetic fields are created. The
creation of time varying electric and magnetic fields between
the conductors form electromagnetic waves which travel
along the transmission line as shown in Fig. 11. The
electromagnetic waves enter the antenna and have associated
with them electric charges and corresponding currents. If
we remove part of antenna structure as shown in Fig. 11,
free space waves can be formed by connecting the open
ends of the electric lines. The free space waves are also
periodic but a constant phase point moves outwardly with
the speed of light and travels a distance of wavelength/
2 in the time of one half of a period.
Before we attempt to explain how guided waves are
detached from the antenna to create the free space waves,
let us draw a parallel between the guided and free space
waves, and water waves created by the dropping of a
pebble in a calm body of water or initiated in some other
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DRDO SCIENCE SCECTRUM 2009
manner. Once the disturbance in the water has been initiated,
water waves are created which begin to travel outwardly.
If the disturbance has been removed the waves do not
stop or extinguish themselves but continue their course
of travel. If the disturbance persists, new waves are continuously
created which lag in their travel behind the others. The
same is true with the electromagnetic waves created by
an electric disturbance. If the initial electric disturbance
by the source is of short duration, the created electromagnetic
waves will travel inside the transmission line, then into
the a antenna, and finally will be radiated as free space
waves, even if the electric source ceased to exist. If the
electric disturbance is of continuous nature, electromagnetic
waves will exist continuously and follow in their travel
behind the others.
When the electromagnetic waves are within the
transmission line and antenna, their existence is associated
with the presence of the charges inside the conductors.
However, when the waves are radiated, they form closed
loops and there are no charges to sustain their existence.
This leads us to conclude that electric charges are required
to excite the fields but are not needed to sustain them and
may exist in their absence. This is in direct analogy with
water waves.
5.
by
1.
2.
3.
5.2
Follows contour of the earth.
Can propagate considerable distances.
Frequencies up to 2 MHz.
Example
– AM radio
Sky-wave Propagation
The sky waves are of practical importance at medium
and high frequencies for very long distance radio
communications. In this mode of propagation electromagnetic
waves reach the receiving point after reflection from the
ionized region in the upper atmosphere called ionospheresituated between 50Km to 400 Km above earth surfaceunder favorable conditions.
ELECTROMAGNETIC WAVE PROPAG-ATION
MODES:
Electromagnetic wave can propagate into the free space
three modes:
Ground-wave propagation
Sky-wave propagation
Line-of-sight propagation
5.1 Ground-wave propagation
The ground wave is a wave that is guided along the
surface of the earth just as an electromagnetic wave is
guided by a waveguide or transmission line. Surface wave
permits the propagation around the curvature of the earth.
This mode of propagation exists when the transmitting and
receiving antennas are closed to the surface of the earth
and is supported at its lower edge by the presence of the
ground.
Figure 12. Ground wave propagation.
70
•
•
•
•
Figure 13. Sky wave propagation.
•
•
•
•
Signal reflected from ionized layer of atmosphere back
down to earth.
Signal can travel a number of hops, back and forth
between ionosphere and earth’s surface.
Reflection effect caused by refraction.
Frequency: 2-30MHz.
Examples
– Military Comm.
– Amateur radio
5.3 Line-of-sight propagation
In this mode of propagation, electromagnetic waves
from the transmitting antenna reach the receiving antenna
either directly or after reflections from the ground in the
earth’s troposphere region. Troposphere is that portion of
the atmosphere which extends upto 16Km from the earth
surface. Frequency: More then 30MHz
Figure 14. Line of sight propagationa
DHANDE: ANTENNAS AND ITS APPLICATIONS
•
Transmitting and receiving antennas must be within
line of sight
– Satellite communication – signal above 30
MHz not reflected by ionosphere
– Ground communication – antennas within
effective line of site due to refraction
Refraction – bending of microwaves by the atmosphere
– Velocity of electromagnetic wave is a function
of the density of the medium
– When wave changes medium, speed changes
– Wave bends at the boundary between
mediums
Examples:
TV, satellite, optical comm.
•
6.
ANTENNA CLASSSIFICATION
Antenna can be classified on the basis of:
1 Frequency - VLF, LF, HF, VHF, UHF, Microwave,
Millimeter wave antenna
2 Aperture
- Wire, Parabolic Dish, Microstrip
Patch antenna
3. Polarization - Linear (Vertical/Horizontal),
Circular polarization antenna
4. Radiation
- Isotropic, Omnidirectional,
Directional, Hemispherical
antenna
6.1
Frequency Basis
4.
5.
Very High Frequency (VHF) & Ultra High Frequency
(UHF) antennas:
Yagi-Uda antennas, log periodic antennas, Helical antennas,
Panel antennas, Corner reflector antennas, parabolic
antennas, discone antennas,
Super High Frequency (SHF) & Extremely High Frequency
(EHF) antennas:
Parabolic antenna, pyramidal horn antennas, discone
antennas, monopoles and dipoles antennas, Microstrip
patch antennas, fractal antenns.
6.2 Aperture Antennas
its
•
•
•
•
Aperture antennas transmit and receive energy from
aperture.
Wire antennas
Horn Antenna
Parabolic reflective antenna
Cassegrain antenna
6.2.1 Wire Antenna
A wire antenna is simply a straight wire of length ë/
2 (dipole antenna) and ë/4 (monopole antenna), where ë
is the transmitted signal wavelength. A wire antenna can
be a loop antenna such as circular loop, rectangular loop,
etc. Basically all vertical radiators are come in to wire
antenna categories. A whip antenna is the best example
of wire antenna.
6.2.2 Vertical Monopole antenna
Frequency
Band
Designation
Typical service
3-30 KHz
Very Low frequency (VLF)
Navigation, SONAR.
30-300 KHz
Low Frequency (LF)
Radio beacons, Navigational Aids.
300-3000 KHz
Medium Frequency (MF)
AM broadcasting, maritime radio, coast
guard
communication,
direction
finding.
3-30 MHz
High Frequency (HF)
Telephone, Telegraph and Facsimile,
amateur radio, ship-to-coast and shipto-aircraft communication.
30-300 MHz
Very High Frequency (VHF)
Television, FM broadcast, air traffic
control, police, navigational aids.
300-3000 MHz
Ultra High Frequency (UHF)
Television, satellite communication,
radiosonde, surveillance RADAR,
navigational aids.
3-30 GHz
Super High Frequency (SHF)
Airborne RADAR, Microwave Links,
Satellite Communication.
30-300 GHz
Extremely High Frequency
(EHF)
RADAR, Experimental
Examples of Antenna on Frequency basis
1.
2.
3.
Very Low Frequency (VLF) & Low frequency (LF)
antenna:
Vertical Radiators, Top-loaded Monopoles, T and Inverted
L antennas, Triatic antenna, Trideco antenna, Valleyspan antenna.
Medium Frequency (MF) antennas:
Radiators (monopoles and dipoles), directional antennas.
High Frequency (HF) antennas:
Log periodic antenna, conical monopole and Inverted
Cone antennas, Vertical whip antenna, Rhombic antenna,
Fan dipole antenna.
• Length < 0.64l
• Self impedance:
ZS = Z ANT+R GND + R REF
• Efficiency:
η = |Z ANT | /|ZS | η ranges
from < 1% to > 80% depending on antenna length
and ground system
• Efficiency improves as monopole gets longer and
ground losses are reduced
Figure 15.
ë /4 Vertical Monopole: (Fig.16)
Figure 16. ë /4 Vertical monopole
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DRDO SCIENCE SCECTRUM 2009
•
•
•
Length ~ 0.25l
Self impedance: ZS ~ 36 - 70 W
The l /4 vertical requires a ground system, which acts
as a return for ground currents. The “image” of the
monopole in the ground provides the “other half” of
the antenna
•
The length of the radials depends on how many there
are
•
Take off angle ~ 25 deg
ë /4 Vertical Monopole: (Fig.17)
one end and open at the other end. If flaring is done in
one direction, then sectorial horn is produced. Flaring in
the direction of Electric vector and Magnetic vector, the
sectorial E-plane horn and sectorial H-plane Horn are obtained
respectively. If flaring is done along both walls (E and H)
of the rectangular waveguide, then pyramidal horn is obtained.
By flaring the walls of a circular waveguide, a conical horn
is formed.
Figure 19. Corrugated conical
Figure 20. Pyramidal and
horn antenna
Figure 17. ë
•
•
•
•
•
conical horn antennas.
/4 Vertical monopole.
Length is approximately 0.48l
Self impedance ~ 2000 W
Antenna can be matched to 50 ohm coax with a tapped
tank circuit
Take off angle ~ 15 deg
Ground currents at base of antenna are small; radials
are less critical for l/2 vertical
The Rectangular Loop: (Fig.18)
6.2.4 Parabolic Reflective Antenna
A parabola is a two dimensional plane curve. A practical
reflector is a three dimensional curved surface. Therefore
a practical reflector is formed by rotating a parabola about
its axis. The surface so generated is known as “paraboloid”
which is often called as “microwave dish” or “parabolic
reflector”. The paraboloid reflector antenna consists of a
primary antenna such as a dipole or horn situated at the
focal point of a paraboloid reflector. The important practical
implication of this property is that reflector can focus
parallel rays on to the focal point or conversely it can
produce a parallel beam from radiations originating from
the focal point.
6.2.5 Prime Focus Paraboloid Reflector antenna
•
Figure 18.
•
•
•
•
•
•
Rectangular loop.
Shaped reflector: parabolic dish, cylindrical antenna.
–Reflector acts as a large collecting area and
concentrates power onto
–a focal region where the feed is located
The total length is approximately 1.02 l.
The self impedance is 100 - 130 W depending on height.
The Aspect Ratio (A/B) should be between 0.5 and
2 in order to have Zs ~ 120 W.
SWR bandwidth is ~ 4.5% of design frequency.
Directivity is ~2.7 dBi. Note that the radiation pattern
has no nulls. Max radiation is broadside to loop
Antenna can be matched to 50 Wcoax with 75 W l /
4 matching section.
6.2.3 Horn Antennas
A horn antenna maybe regarded as a flared out or
opened out waveguide. A waveguide is capable of radiating
radiation into open space provided the same is excited at
72
Figure 21. Prime focus paraboloid reflector antenna.
DHANDE: ANTENNAS AND ITS APPLICATIONS
6.2.6 Cassegrain Antenna
In cassegrain antenna primary feed radiator is positioned
around an opening near the vertex of the paraboloid instead
of at focus. Cassegrain feed system employs a hyperboloid
secondary reflector whose one of the foci coincides with
the focus of paraboloid. The feed radiator is aimed at the
secondary hyperboloid reflector or sub-reflector. As such,
the radiations emitted from feed radiator are reflected from
cassegrain secondary reflector which illuminates the main
paraboloid reflector as if they had originated from the
focus. Then the paraboloid reflector colliminates the rays
as usual.
plane. The major disadvantages of patch or microstrip antennas
are their inefficiency and very narrow bandwidth which
is typically only a fraction of a percent or at the most a
few percent.
6.3 Antenna Classification on Polarization Basis
Antenna polarization is governed by the polarization
of Electromagnetic waves. Based on that:
1. Linearly (Vertically/Horizontally) Polarized antenna.
2. Circularly Polarized antenna.
6.3.1 Linearly (Vertically/Horizontally) polarized
antenna
If antenna is transmitting/receiving Vertical E field
vector, then antenna is said to be vertically polarized antenna.
If antenna is transmitting/receiving horizontal E field
vector, then antenna is said to be horizontally polarized
antenna.
Figure 22. Cassegrain antenna.
6.2.7 Advantages of cassegrain antenna
•
•
•
•
•
•
•
Less prone to back scatter than simple parabolic antenna
Greater beam steering possibility: secondary mirror
motion amplified by optical system
Much more compact for a given f/D ratio.
Reduction in spill over and minor lobe radiation.
Ability to get an equivalent focal length much greater
than the physical length.
Ability to place the feed in a convenient location.
Capability for scanning or broadening of the beam by
moving one of the reflecting surfaces.
6.2.8 Microstrip Patch Antenna
In spacecraft or aircraft applications, where size, weight,
cost, performance, ease of installation, and aerodynamic
profile are constraints, low profile antennas are required.
In order to meet these specifications Microstrip Patch antennas
are used. These antennas can be flush mounted to metal
or other existing surfaces and they only require space for
the feed line which is normally placed behind the ground
Figure 23. Microstrip patch
antenna.
Figure 25. Examples of linearly polarised antennas.
6.3.2 Circularly Polarized antenna
If the antenna is able to transmit or receive E field
vectors of any orientation, then antenna is said to be
circularly polarized antenna.
Figure 24. Various shapes of
patch antenna.
Figure 26. Examples of circularly polarised antennas.
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DRDO SCIENCE SCECTRUM 2009
6.4 Antenna classification on Radiation Pattern Basis
On the basis of radiation pattern antenna can be classified
as:
1.
2.
3.
4.
Isotropic antenna.
Omnidirectional antenna.
Directional antenna.
Hemispherical antenna.
6.4.1 Isotropic Antenna
An isotropic antenna is a fictitious antenna and is
defined as a antenna which radiates uniformly in all directions.
It is also called as isotropic source or omnidirectional
antenna or simply unipole. An isotropic antenna is a
hypothetical lossless antenna, with which the practical
antennas are compared. Thus an isotropic antenna is used
as reference antenna. Although sometimes, a half-wave
dipole antenna is also used as reference antenna but these
days use of isotropic antenna as reference antenna is preferred.
Let us assume that practical antenna is having a gain of
3 dBi means that gain of practical antenna is three times
more than that of isotropic antenna when both the antenna
are connected with same source.
6.4.4 Hemispherical Antenna
Antenna whose radiation pattern will cover the one
half of the hemisphere either upper hemisphere or lower
hemisphere is said to be antenna with Hemispherical Radiation
pattern. Such types are antennas are implemented on aircraft
body to cover the lower hemisphere for data link purpose.
Examples are all Monopoles antennas with large ground
plane. The radiation pattern of these antennas are shown
below.
Figure 28. Directional radiation pattern.
6.4.2 Omnidirectional Antenna
7.
ANTENNA CHARACTERISTICS
Omnidirectional antennas are those antennas which
will cover equally well in azimuth direction and having
some angle in elevation direction. Basically most of the
wire antennas are having omnidirectional radiation pattern.
Examples are Whip antenna, Dipoles antennas, etc. The
radiation patterns of omnidirectional antennas are shown
below.
Before designing an antenna one should know its
performance parameters or characteristics of antenna for
particular applications. The beam pattern of any antenna
is shown below in Fig.29 and 30.
Figure 29. Upper hemispherical radiation pattern.
Figure
27.
Omnidirectional
antenna.
6.4.3 Directional Antennas
Antennas which directs its energy in one particular
direction is said to be directional antennas. These antennas
are having very high gain and directivity to cover large
wireless distance. Examples are paraboloid reflector antenna,
Yagi-Uda antenna, Log periodic antenna, etc. Radiation
pattern of these antennas are shown below.
74
Figure 30. Antenna pattern showing main beam and side
lobes.
DHANDE: ANTENNAS AND ITS APPLICATIONS
The performance parameters of the antennas are discussed
below:
7.1 Radiation Pattern
The radiation pattern of any antenna determines its
coverage area in free space. The radiation pattern of any
antenna looks like as shown in Fig.31.
Figure 32. Antenna radiating regions.
7.2 Gain (G)
Figure 31. Antenna Parameters definitions are based on the
geometry of the antenna gain pattern.
7.1.1 Properties of Radiation Pattern of antenna
•
Always measured in Far field.
Far field: r > 2
•
•
•
•
D2
λ
D: largest dimension of the antenna
Field intensity decreases with increasing distance, as
1/r .
Radiated power density decreases as 1/r2.
Pattern (shape) independent on distance.
Usually shown only in principal planes.
7.1.2 Antenna Regions
7.3 Directivity (D)
Far-Field (Fraunhoffer) Region r > 2
–
–
–
D2
λ
Where D is the largest linear dimension of the antenna
This is the region where the wavefront becomes
approximately planar
The apparent gain of the antenna is a function only
of the angle (i.e., the antenna pattern is fully formed)
Radiating Near-Field (Transition region)
–
–
–
–
–
–
λ
D2
< r <2
λ
2π
The region between near and far field
E and H are equal, but inverse square law does not
apply
The antenna pattern is not fully formed
Reactive Near-Field r<
Gain of an antenna without involving the efficiency
is defined as “the ratio of maximum radiation intensity in
given direction to the maximum radiation intensity from a
reference antenna produced in the same direction with
same power input”.
Gain is also defined as the increase in signal strength
as the signal is processed by the antenna for a given
incident angle
– Usually expressed in dB
– Can be negative
An isotropic antenna has unity gain
– 0 dB
A general Gain equation is given byG
ç (4ð/ë2) Ap
where
ç – efficiency of the antenna
ð – wavelength in meters
Ap– the physical area of the aperture in m2
λ
2π
Gain is not a meaningful parameter here
E and H are not equal
Reactive components 10% or more of radiating components
may cause error in field measurements
Directivity of an antenna is defined as the ratio of
Maximum radiation intensity to its average radiation intensity.
Relation between Directivity and Gain of antennaG
ç D
where
ç – efficiency of the antenna
7.4 Antenna Efficiency (ç )
The efficiency of antenna is defined as the ration of
power radiated to the total input power supplied to the
antenna and is denoted by ç . Thus,
Antenna Efficiency, ç =Power Radiated/Total Input
Power
In terms of resistances,
ç =
[Rr/(Rr+Rl)] × 100
where, Rr = Radiation resistance; Rl = Ohmic loss resistance
of antenna conductor
7.5 Beam Area and Beam Efficiency
Beam area
:
ΩA = ∫
2π
0
∫
π
0
Pn (θ , φ ) ⋅ sin(θ ) dθ dφ = ∫∫ Pn (θ , φ ) d Ω
4π
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DRDO SCIENCE SCECTRUM 2009
ΩM =
Main Beam area
:
Minor lobes area
: Ωm =
n
Main
beam
∫∫
Pn (θ , φ )d Ω
εM
Ω
= M
ΩA
min or
lobes
Main Beam Efficiency :
range in units of frequency over which the antenna operates
– Often stated in percentage bandwidth
∫∫ P (θ ,φ )d Ω
7.8 Beamwidth (èB, ÖB)
7.6 Effective Aperture and Aperture Efficiency
Effective aperture of the antenna is that aperture that
will actively take part in transmission and reception of
electromagnetic waves. The relation between physical and
effective aperture of the antenna is given byEffective Aperture=K × Physical Aperture, 0< K <1
Receiving antenna extracts power from incident wave: Prec = Sin ⋅ Ae
λ2
Aperture and beam area are linked: Ae = Ω
A
Ae
Aperture efficiency can be defined: ε ap = A
p
7.6.1
7.9
Polarization
The polarization of an antenna defines the orientation
of the E and H waves transmitted or received by the antenna
– Linear polarization includes vertical, horizontal or slant
(any angle)
– Typical non-linear includes right- and left-hand circular
(also elliptical)
7.10 VSWR/Return loss
Radiation Resistance
The radiation resistance is a hypothetical resistance
and does not correspond to a real resistor present in the
antenna but to the resistance of space coupled via the
beam to the antenna terminals.
Antenna presents impedance at its terminals, Z = R + jX
Resistive part is radiation resistance
A
plus loss resistance,
The “n”-db beamwidth (èB, ÖB) of an antenna is the
angle defined by the points either side of boresight at
which the power is reduced by n-dB, for a given plane.
– For example if èB, represents the beamwidth in the
horizontal plane, ÖB represents the beamwidth in the
orthogonal (vertical) plane.
– The 3-dB beamwidth defines the half-power beam.
A
A
R A = RR + RL
VSWR or Return Loss determines the matching properties
of antenna. It indicates that how much efficiently antenna
is transmitting/receiving electromagnetic wave over particular
band of frequencies.
7.11 Impedance
Antenna must be terminating with 50 Ohm impedance
in order to transfer maximum power from transmitter into
free space.
8.
ANTENNA MEASUREMENT
Antenna must be undergoing various measurements
before installing on the system. Basically there are two
types of measurement conducted on antennas:
1. Passive Measurement/Laboratory Measurement
• VSWR/Return Loss
• Impedance Bandwidth
2. Active Measurement
• Radiation Pattern (Elevation And Azimuth)
• Gain
• Directivity
• Half Power Beamwidth
• Cross Polarization
8.1 Passive Measurement/Laboratory Measurement
7.6.2 Frequency Coverage
The frequency coverage of an antenna is the range
of frequencies over which an antenna maintains its parametric
performance
– Antennas are generally rated based upon their stated
centre frequency
– Example: 9.85-10.15 GHz, fc = 10.0 GHz
7.7 Bandwidth (B)
The bandwidth (B) of an antenna is the frequency
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VSWR/Return Loss and Impedance Bandwidth
measurement can be done on Vector Network Analyzer.
Antenna port is connected to one port of the network
Analyzer and can see its VSWR/Return Loss and Impedance
Bandwidth directly on the screen of the Network Analyzer.
8.2 Active Measurement
In active measurement, the following properties of
antenna can be tested:
• Radiation Pattern (Elevation And Azimuth)
• Gain
DHANDE: ANTENNAS AND ITS APPLICATIONS
Figure 34. Set up for measuring VSWR/Return loss and
impedance of antenna using vector network analyser.
•
•
•
Directivity
Half Power Beamwidth
Cross Polarization
8.3 Radiation Pattern Measurement
•
•
Open field
– Outdoor Elevated Range
– Ground Reflection Range
Anechoic chamber
– Rectangular Anechoic Chamber
– Compact Antenna Test Range
Open Field
Radiation Pattern of Mobile antennas
9.
ANTENNA APPLICATIONS
9.1 Astronomical Antenna
Helical
Antenna
1. Highly Directional
Antenna
2. Circularly Polarized
Antenna
3. Use in Radio Astronomy
Anechoic Chamber
9.2 Defence Antennas
Radiation Pattern of Some Antennas
A close-up view of the conical high-frequency Dipole
antenna mounted on the bow of the Ship
Paraboloid
Grid
Reflector Antenna
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DRDO SCIENCE SCECTRUM 2009
A view of the antenna array on the island structure of
the nuclear-powered aircraft USS Theodore Roosevelt
(CVN-71).
A close-up view of the antenna masts and
bridge structure aboard the guided missile
cruiser as seen from off the ship`s starboard
bow.
A view of the AN/SPN-46(V) radar antenna for the
automatic carrier landing system (ACLS) aboard
the nuclear-powered aircraft carrier USS Abraham
Lincoln (CVN-72).
A view of the antenna rig aboard the guided missile
frigate USS DOYLE (FFG-39).
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