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ANTENNAS AND WAVE PROPAGATION
UNIT-4 SPECIAL ANTENNAS
Prepared by : M.Pachiyaannan, AP/ECE, Nehru Institute of Engineering and Technology
Topics
2
Special Antennas
Principle of frequency independent antennasspiral antenna ,Helical antenna ,Log periodic.
Modern Antenna- Reconfigurable antenna ,
Active antenna, Dielectric antenna.
Electronic band gap structure and application
Antenna Measurements:
Anechoic chamber
Directional pattern, Gain, Phase,
Polarization, Impedance, Efficiency.
What is an ‘antenna’?
 An antenna is a device that is made to efficiently
radiate and receive radiated electromagnetic waves.

An antenna is an electrical conductor or system of
conductors.



Transmission - radiates electromagnetic
energy into space
Reception - collects electromagnetic energy
from space.
In two-way communication, the same antenna can be
used for transmission and reception.
Antenna Band







1. L band ( 1-2 GHz)
2. S band (2-4 GHz)
3. C band ( 4- 8 GHz)
4. X band (8-12 GHz)
5. Ku band ( 12-18 GHz)
6. K band (18-26 GHz)
7. Ka band ( 26-40 GHz)
RADIO WAVE PROPAGATION
INTRODUCTION
EM waves travel in straight lines, unless acted upon by some
outside force. They travel faster through a vacuum than through
any other medium.
As EM waves spread out from the point of origin, they decrease in
strength in what is described as an inverse square relationship.
For example: a signal 2 km from its starting point will be only 1/4
as strong as that 1 km from the source. A signal 3 km from the
source will be only 1/9 that at the 1 km point.
RADIO WAVES
x
Electric
Field, E
y
z
Direction of
Propagation
Magnetic
Field, H

Electromagnetic radiation comprises both an Electric and a Magnetic Field.
The two fields are at right-angles to each other and the direction of propagation is at right-angles to both fields.

The Plane of the Electric Field defines the Polarisation of the wave.

POLARIZATION



The polarization of an antenna is the orientation of the
electric field with respect to the Earth's surface and is
determined by the physical structure of the antenna and
by its orientation.
Radio waves from a vertical antenna will usually be
vertically polarized.
Radio waves from a horizontal antenna are usually
horizontally polarized.
Direction of Propagation
Direction of Propagation
Vertically polarized
omnidirectional
dipole antenna
Horizontally polarized directional
yagi antenna
Transmitter
Receiver
Classification of Antennas
Wire-Type Antennas
Antennas
Dipoles
waveguide
Monopoles
Biconical antennas
Loop antennas
Helical antennas
Linearly polarised antennas
antennas
Element antennas
Narrow-band
Transmitting
Aperture-Type
Horn and open
Reflector antennas
Slot antennas
Microstrip antennas
Circularly polarised
Antenna array
Broad-band
Receiving
SPECIAL ANTENNAS
Frequency Independent principle
Rumsey’s principle suggests that the impedance and pattern properties
of an antenna will be frequency independent if the antenna shape is
specified only in terms of angles.
To satisfy the equal-angle requirement, the antenna configuration needs
to be infinite in principle, but is usually truncated in size in practice.
This requirement makes frequency-independent antennas quite large
in terms of wavelength.
Rumsey’s principle has been verified in spiral antennas, conic
spiral antennas and some log periodic antennas
Yagi Uda Antenna


In the frequency range for VHF ( 30 to 300 Mhz)
and UHF (300 Mhz to 3000Mhz ) Yagi Uda antenna,
folded dipole, ground plane corner reflector antenna
are normally used.
Highly directional antennas such as the Yagi-Uda
are commonly referred to as "beam antennas" due to
their high gain. However, the Yagi-Uda design only
achieves this high gain over a rather narrow
bandwidth, making it useful for specific
communications bands.
INTRODUCTION
 In the 1926, Dr. S. Uda and Dr.H.Yagi has invented a
directional antenna system consisting of an array of
coupled parallel dipoles. This is commonly known as
Yagi-uda or simply Yagi antenna.
Yagi-uda antenna is familiar as the commonest kind
of terrestrial television antenna to be found on the
rooftops of houses. It is usually used at frequencies
between 30Mhz and 3Ghz and covers 40 to 60 km.
construction
There are three types of elements:
1) The reflector
2) The driven element
3) The directors
Working
 Reflector here derives it’s main power from a driver ,
it reduces the signal strength in it’s own direction and
thus reflects the radiation towards the driver and
directors.
 The driven element is where the signal is intercepted
by the receiving equipment and has the cable attached
that takes the received signal to the receiver.
 The radiator and driver can be placed more closer to
increase the radiation length towards the directors.
Working
Length >(λ/2) ---inductive effect (current lags voltage )
Length <(λ/2) --capacitive effect (current leads voltage )




One director will excite the another director.
Close spacing is also used for more excite.
By suitable dimension the radiating energy will add up
and backward energy will cancel.
Some times as an driven element folded dipole also
used.
radiation pattern
Design Parameters for 3 elements


reflector ------------------- 0.55 lambda
0.1 lambda
driven --------------- 0.5 lambda
0.1 lambda
director ----------- 0.45 lambda
Spacing between elements are 0.1 λ
jack
Comparison
Design Parameters for 6 elements Yagi
uda antenna

reflector ------------------- 0.476 lambda
0.250 lambda
driven --------------- 0.452 lambda
0.289 lambda
director ----------- 0.436 lambda
0.406 lambda
director ----------- 0.430 lambda
0.323 lambda
director ----------- 0.434 lambda
0.422 lambda
director ----------- 0.430 lambda
Frequency independent antenna



Rumsey’s Principle: Impedance and pattern
property of an antenna will be frequency
independent if the antenna shape is specified in
terms of angles.
Operating Frequency is 10 to 10,000Mhz
Applications: TV, Point to point communication
Feeds for reflector
Explain Rumsey Principle:
Explain Rumsey Principle:
Log Spiral antenna
Equation of log
spiral is
r=aΘ
Where
r= radial distance from point p on
spiral
Θ=angle with respect to x axis
a=constant
One application of spiral
antennas is wideband
communications
Planar Spiral Antenna


Spiral antennas are
usually
circularly
polarized.
The
fractional
Bandwidth can be as
high as 30:1. This
means that if the
lower frequency is 1
GHz, the antenna
could still be in band
at 30 GHz.
Planar spiral cut from large ground plate
Helical Antennas
It may be viewed as a derivative of the dipole or monopole,
but it can also be considered a derivative of a loop.

Normal mode Helix
 It
may be treated as the superposition of n
elements, each consisting of a small loop of
diameter D and a short dipole of length s, thus
the far fields are
 They
are orthogonal and 90 degrees out of
phase
 The combination of them gives a circularly or
elliptically polarised wave.
 The axial ratio:
 When
the circumference is equal to
the axial ratio becomes unity and the radiation is
circularly polarised.

Axial Mode Helix
 The
axial (end-fire) mode occurs when the
circumference of the helix is comparable with
the wavelength (C = pD ≈ l) and the total
length is much greater than the wavelength.
 This has made the helix an extremely popular
circularly-polarised broadband antenna at the
VHF and UHF band frequencies
 The recommended parameters for an optimum
design to achieve circular polarisation are:
 The
normalised radiation pattern:
Half power beamwidth:
1st null beamwidth:
 The
directivity:
 The
axial ratio
 Radiation
resistance
Radiation patterns
Which is better?
Conical antenna
Tapered helical or conical spiral
Conical antenna
Log periodic antenna

τ =periodicity Factor
 The
antenna is divided into the so called active
region and inactive regions.
 The role of a specific dipole element is linked
to the operating frequency: if its length, L, is
around half of the wavelength, it is an active
dipole and within the active region; Otherwise
it is in an inactive region and acts as a
director or reflector as in Yagi-Uda antenna
 The driven element shifts with the frequency –
this is why this antenna can offer a much wider
bandwidth than the Yagi-Uda. A travelling
wave can also be formed in the antenna.
 The highest frequency is basically determined
by the shortest dipole length while the lowest
frequency is determined by the longest dipole
length (L1).
Log periodic antenna
1) In active Region:
 Provide capacitive impedance
 Elements spacing low
 Current in the region is very low.
Log periodic antenna
2) active Region:
 Length is approximately (λ/2)
 Center region
 Maximum radiation take place.
3) In active stop Region:
 Length is greater than (λ/2)
 Provide inductive impedance
 Called reflective region.
 When wavelength λ is high the radiation region will
go in the right side.
Log periodic antenna


Relation between apex angle
length L.
, spacing S and
Log periodic antenna
Antenna design

This seems to have too many variables. In fact, there are only three
independent variables for log-periodic antenna design.
the scaling factor:
the spacing factor:
the apex angle:
In practice, the most likely scenario is that the frequency
range is given from fmin to fmax, the following equations
may be employed for design
Another parameter (such as the directivity or the length of
the antenna) is required to produce an optimised design.
FREQUENCY RECONFIGURABLE
System Block Diagram
Patch 1
(1.575 GHz)
Switches?
On
Off
Inset Feed
Matches to 50
ohm
Output to Coax
Connector
Patch 2
Connected
(1.227 GHz)
Switching Method

MEMS Switch
 RMSW201,
RADANT MEMS
 0.3dB Insertion Loss @ 2GHz
 35dB Isolation Loss @ 2GHz
 1.9mm x 1.85mm package size
RMSW201 MEMS Operation

+/- 90 VGS Actuation Voltage
RMSW201 MEMS Operation

+/- 90 VGS Actuation Voltage
Implementing MEMS


RS = RD = 100kΩ
Stability
Switching Method
+5V Supply
Line
DC-DC
Converter
MEMS Gate
Switching Method

DC-DC Converter: +5V to -90V

R2/R1 = Vout/Vref

R2 = Vout/10uA
+5VDC
-90VDC
Switching Method

DC-DC Converter: +5V to -90V
+5VDC
-90VDC
Switching Method

DC-DC Converter: +5V to -90V
+5VDC
-90VDC
Switching Method

DC-DC Converter: +5V to -90V
+5VDC
-90VDC
Implementing MEMS


Conductive epoxy, double-stick thermal tape
Wire bonding, gold plating
MEMS Evaluation Board
MEMS Evaluation Board
MEMS Evaluation Board
MEMS Evaluation Board
MEMS Evaluation Board
Micro-Circuits, Inc.

Contact: Robert Modica
(630) 628-5764
 [email protected]

Fabricated Antenna System
Slot Antenna




A slot antenna consists of a metal surface, usually a flat
plate, with a hole or slot cut out.
When the plate is driven as an antenna by a driving
frequency, the slot radiates electromagnetic waves in
similar way to a dipole antenna.
Often the radio waves are provided by a waveguide,
and the antenna consists of slots in the waveguide.
Slot antennas are used typically at frequencies between
300 MHz and 24 GHz.
Slot Antenna



Slot antennas are often used
at UHF and microwave frequencies instead of line
antennas when greater control of the radiation
pattern is required.
Widely used in radar antennas, for the sector antennas
used for cell phone base stations.
Often found in standard desktop microwave sources
used for research purposes.
Slot Antenna
Active antenna
Active antenna….




Active antennas are any antennas with integrated signal
amplifiers.
Passive antennas are antennas that have no amplification
stages.
An active antenna is a passive antenna that simply includes an
onboard amplifier. There is no difference between the antenna
element of an active or passive antenna of the same type; the
only difference is whether an amplifier is included.
Active antennas can be used for both receiving and
transmitting applications, but they are most often seen as
receiving antennas. When used to receive signal, the
integrated amp boosts the RF picked up by the antenna and
allows much longer remote cable runs.
Dielectric Antenna
Dielectric Antenna




An antenna in the form of a section of dielectric rod excited by
a radio wave guide or the post of a coaxial cable.
A surface wave is generated in the rod of the antenna and
propagates along the axis of the rod.
Dielectric antennas are essentially traveling-wave antennas,
consisting of elementary electric and magnetic dipoles.
The radiation maximum coincides with the axis of the rod, as
does the maximum of any traveling-wave antenna.




The type of radiation of a dielectric antenna depends on the phase
velocity of propagation of the surface wave, which decreases with an
increase in the diameter of the dielectric rod and in the dielectric
constant of its material.
The lower the phase velocity, the greater the length of the rod.
As the phase velocity decreases, or as it approaches the speed of
light in the surrounding medium (air), the dielectric rod begins to lose
its wave-guide properties.
This leads to an abrupt decrease in the field intensity near the end of
the rod, an increase of radiation into the medium surrounding the
antenna (directly from the open end of the wave guide), and a
decrease in the antenna’s efficiency.



The rods of dielectric antennas are made from
dielectric materials with low attenuation of
electromagnetic waves.
Dielectric antennas are used mainly in aircraft radio
equipment, which operates on centimeter or
decimeter wavelengths.
low cost alternative to free space high gain antenna
designs such as Yagi-Uda and horn antennas, which
are often more difficult to manufacture at these
frequencies
Printed Antenna


Printed antenna technology used for wireless system
Printed antenna application are:
- Arrays for low or medium directivity
- Efficient radiators
- Planar antenna
Printed Antenna

Originated from the use of planar microwave technologies.

The begin antenna printed in the mid 1970.



The layered structure with 2 parallel conductors separated by a
thin dielectric substrate and the lower conductor acting as a
ground plane.
Printed belongs to the class or resonant antennas. Printed
antennas have found use in most classical microwave applications.
Operates typically from 1- 100 GHz.
Printed Antenna
Phase Array Antenna



A phased array is an array of antennas in which the relative
phases of the respective signals feeding the antennas are
varied in such a way that the effective radiation pattern of
the array is reinforced in a desired direction and suppressed
in undesired directions.
An antenna array is a group of multiple active antennas
coupled to a common source or load to produce a directive
radiation pattern.
Usually, the spatial relationship of the individual antennas
also contributes to the directivity of the antenna array.
Phase Array Antenna


Use of the term "active antennas" is intended to
describe elements whose energy output is modified
due to the presence of a source of energy in the
element or an element in which the energy output
from a source of energy is controlled by the signal
input.
One common application of this is with a standard
multiband television antenna, which has multiple
elements coupled together.
Omnidirectional antenna
An omnidirectional antenna is an antenna that has a non-directional pattern
(circular pattern) in a given plane with a directional pattern in any orthogonal
plane.
A omnidirectional antenna is an antenna which radiates radio wave power
uniformly in all directions in one plane, with the radiated power decreasing
with elevation angle above or below the plane, dropping to zero on the
antenna's axis.
Application of omnidirectional antenna






Cell phones
Fm radio
Walkie talkie
Wireless computer network.
Cordless phone.
Gps
Operation of omnidirectional antenna.
The omnidirectional antenna radiates or receives equally well in all directions. It is also
called the "non-directional" antenna because it does not favor any particular
direction.
The pattern for an omnidirectional antenna, with the four cardinal signals. This type of
pattern is commonly associated with verticals, ground planes and other antenna types
in which the radiator element is vertical with respect
to the Earth's surface.
Radiation pattern omnidirectional antenna
Omnidirectional antennas have a similar radiation pattern. These antennas provide
a 360 degree horizontal radiation pattern. These are used when coverage is
required in all directions (horizontally) from the antenna with varying degrees of
vertical coverage. Polarization is the physical orientation of the element on the
antenna that actually emits the RF energy. An omnidirectional antenna, for
example, is usually a vertical polarized antenna.
Advantage of omnidirectional antenna
advantage

An omni directional antenne receives or transmits signals from all directions as in
360 degrees.
disadvantage
Signal strength is uniform with omnidirectional antennas.
If you use an omnidirectional antenna all signals are noise sources look into the
same antenna gain. There is neither increase of the desired signal, nor
suppression of undesired signals. On a crowded band, the omnidirectional
antenna confers no SNR.
Electronic band gap structure
What are Microwave Band Gap Structures?

The structures having periodic arrangement of dielectric or magnetic
materials that result in the formation of stop bands in the microwave
frequency region are called Microwave band gap structures. In general,
these structures are called Electromagnetic band gap (EBG) structures or
Photonic band gap (PBG) structures.
Why such stop bands appear?

Explanation - 1: The electromagnetic waves traveling through such structures
experience a periodic variation of dielectric permittivity or magnetic
permeability similar to the periodic potential energy of an electron in an
atomic crystal. Therefore, like the electronic state in an atomic crystal, the
photonic state in a photonic crystal can be classified into bands and gaps,
the frequency range over which the photons are allowed or forbidden
respectively to propagate in the medium.
Cont..


Explanation - 2: The electromagnetic waves scattered by the
materials form secondary sources. The waves from these secondary
sources interfere destructively at the receiving antenna for certain
frequency region.
What are the parameters that control the stop band?
1. Periodicity of the geometric arrangement
2. Dielectric/ impedance/ effective refractive index contrast
3. Filling fraction
4. Geometry of the structure
ANTENNA MEASUREMENTS
Topics:
Anechoic chamber
Directional pattern, Gain, Phase,
Polarization, Impedance, Efficiency.
Gain, directivity and efficiency
 Gain
and directivity are quantities which define
the ability to concentrate energy in a particular
direction and are directly related to the antenna
radiation pattern.
 It
includes all ohmic and dissipative losses arising
from conductivity of metal and dielectric loss.
 Antenna
efficiency is a coefficient that accounts
for all the different losses present in an antenna
system.
Antenna Gain
• The fields across the aperture of the parabolic
reflector is responsible for this antenna's
radiation.
• Directivity or gain is the ratio of the power
radiated by an antenna in its direction of
maximum radiation to the power radiated by a
reference antenna in the same direction.
• Is measured in dBi (dB referenced to an
isotropic antenna) or dBd (dB referenced to a
half wavelength dipole).
Antenna Gain

Parabolic Antenna Gain,
G = 6D2/l2
where D = diameter

Horn Antenna Gain
G = 10A/l2
A =flange area
Antenna Efficiency
Antenna efficiency is affected by:
1. The sub reflector and supporting structure
blockage.
2. The main reflector rms surface deviation.
3. Illumination efficiency, which accounts for the
non uniformity of the illumination, phase
distribution across the antenna surface, and
power radiated in the side lobes.
4. The power that is radiated in the side lobes.
Radiation Efficiency



The radiation efficiency is the usual efficiency that
deals with ohmic losses.
Horn antennas are often used as feeds, and these
have very little loss.
Parabolic reflector is typically metallic with a very
high conductivity, this efficiency is typically close to
1 and can be neglected.
 The effective antenna aperture is the ratio of the available
power at the terminals of the antenna to the power flux
density of a plane wave incident upon the antenna, which
is polarization matched to the antenna.
Effective Aperture
 If there is no specific direction chosen, the direction of
maximum radiation intensity is implied.
Antenna Measurements
95
It is usually most convenient to perform antenna
measurements with the test antenna in its receiving mode. If
the test
antenna is
reciprocal,
the receiving
mode
characteristics (gain, radiation pattern, etc.)are identical to
those transmitted by the antenna.
Gain Measurements
96




The most important figure of merit that describes
the performance of a radiator is the gain
The choice of either depends largely on the
frequency of operation
Antenna gains are not usually measured at
frequencies below 1 MHz
Usually there are two basic methods that can be used
to measure the gain of an electromagnetic radiator
 Absolute-gain
 Gain-transfer or gain-comparison
The absolute-gain method is used to calibrate antennas
that can then be used as standards for gain measurements,
and it requires no a priori knowledge of the gains of the
antennas.
Gain-transfer methods must be used in conjunction with
standard gain antennas to determine the absolute gain of the
antenna under test
The two antennas that are most widely used and universally
accepted as gain standards are
Resonant λ/2 dipole
Pyramidal horn antenna
97
Absolute-Gain Measurements
98

Two-Antenna Method

Three-Antenna Method

Extrapolation Method

Ground-Reflection Range Method
99
Two/Three-Antenna Method –
Single Frequency
100
Two/Three-Antenna Method –
Swept Frequency
Two-Antenna Method
101
where
(G0t )dB = gain of the transmitting antenna (dB)
(G0r )dB = gain of the receiving antenna (dB)
Pr = received power (W)
Pt = transmitted power (W)
R = antenna separation (m)
λ = operating wavelength (m)
If the transmitting and receiving antennas are identical (G0t = G0r )
Three-Antenna Method
102
The two- and three-antenna methods are both subject to errors.
Care must be utilized so
1. the system is frequency stable
2. the antennas meet the far-field criteria
3. the antennas are aligned for boresight radiation
4. all the components are impedance and polarization matched
5. there is a minimum of proximity effects and multipath
103
interference
Extrapolation Method
104

Suitable for circularly polarized antennas

The method requires both amplitude and phase
measurements when the gain and the polarization
of the antennas are to be determined

For the determination of gains, amplitude
measurements are sufficient
105
Ground-Reflection Range
Method
106
Gain-Transfer (Gain-Comparison)
Measurements
where
(GT )dB Test Antenna Gain
(GS )dB Standard Antenna Gain
PT Transmit Power
PS Receive Power
Impedance Measurements
107

Associated with an antenna, there are two types of
impedances: a self and a mutual impedance

To attain maximum power transfer between a source or a
source-transmission line and an antenna (or between an
antenna and a receiver or transmission line-receiver), a
conjugate match is usually desired
If conjugate matching does not exist, the power lost can be
computed using
where
Zant = input impedance of the antenna
Zcct = input impedance of the circuits which are connected to the
antenna at its input terminals
108
In a mismatched system, the degree of mismatch determines the
amount of incident or available power which is reflected at the
input antenna terminals into the line
where
 = |  |ejγ = voltage reflection coefficient at the antenna
input terminals
VSWR = voltage standing wave ratio at the antenna
input terminals
Zc = characteristic impedance of the transmission line
109
The phase γ of the reflection coefficient is then computed using
where
n = the voltage minimum from the input terminals
xn = distance from the input terminals to the nth voltage minimum
λg = wavelength measured inside the input transmission line
Once the reflection coefficient is completely described by
its magnitude and phase, it can be used to determine the
antenna impedance by
110
Impedance Measurements
111
To determine the antenna impedance other methods
Utilizing impedance bridges
Slotted lines
Broadband swept-frequency network analyzers
Antenna Ranges
112
The testing and evaluation of antennas are performed in
antenna ranges. Antenna facilities are categorized as outdoor
and indoor ranges
Reflection Ranges
Free-Space Ranges
Elevated Ranges
Slant Ranges
Anechoic Chambers
Compact Ranges
Near-Field/Far-Field Methods
Reflection Ranges
113
Elevated Ranges
114
Slant Ranges
115
Anechoic Chambers
116
Rectangular Chamber
Tapered
Chamber
Compact Ranges
117
CATR-Knife-edge
118
CATR- Rolled-edge
119
Near-Field/Far-Field Methods
120
Cylindrical Scanning
Planar Scanning
Spherical Scanning
Setup
121
Near Field
Far Field
122
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