-
Degree Project
Muhammad Kamran Khattak Osama Siddique Waqar Ahmed
2011-05-17
Subject: Master Thesis
Level: Second
Course code: 5ED06E
Design and Simulation of Microstrip Phase Array Antenna using
ADS
Supervisor: Prof. Sven-Erik Sandstrm
Department of Computer Science, Physics and Mathematics
Submitted for the degree of Master in Electrical Engineering
Specialized in Signal Processing and Wave Propagation
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Acknowledgement
We would like to thank our supervisor Dr. Sven-Erik Sandstrm for
his kind support, patient
guidance and co-operation in making this work possible. We would
like to thank the Department
of Computer Science, Physics and Mathematics, Linnaeus
University, Sweden, for providing
best educational facilities. We would like to thank the Swedish
government for providing free
education, education facilities and co-operation. We would like
to thank all our friends and
siblings whose company will always be cherished. And last, we
would like to thank our parents
who are the source of our very existence. Without their support,
accomplishing this goal was
never possible. We thank our parents from heart and soul.
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Abstract
The aim of this project is to design a microstrip phase array
antenna in ADS (Advance Design
System) Momentum. The resonant frequency of which is 10 GHz. Two
circular patches with a
radius of 5.83 mm each are used in designing the array antenna.
RT-DURROID 5880 is used as a
substrate for this microstrip patch array design. These circular
patches are excited using coaxial
probe feed and transmission lines of particular lengths and
widths. These transmission lines
perfectly match the impedance of the circular patches. Various
parameters, for example the S-
parameters, two dimensional and three dimensional radiation
patterns, excitation models, gain,
directivity and efficiency of the designed antenna are obtained
from ADS Momentum.
Key words: Microstrip phase array antenna, Circular patch
antenna, ADS Momentum.
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Table of Contents
1 Introduction
................................................................................................................................................................
1
1.1 Thesis Approach
..................................................................................................................................................
1
1.2 Objective
.............................................................................................................................................................
1
1.3 Thesis Organization
.............................................................................................................................................
2
2. Literature Review
....................................................................................................................................................
3
2.1 Basic Antenna Terminology
................................................................................................................................
3
2.1.1 Radiation Pattern
..........................................................................................................................................
3
2.1.2 Radiation Pattern of a dipole antenna
..........................................................................................................
4
2.1.3 Directivity
....................................................................................................................................................
4
2.1.4 Gain
..............................................................................................................................................................
4
2.1.5 Aperture Efficiency
......................................................................................................................................
5
2.1.6 Beamwidth
...................................................................................................................................................
5
2.1.7 Input
Impedance...........................................................................................................................................
6
2.1.8 Polarization
..................................................................................................................................................
6
2.1.9 Antenna Efficiency
......................................................................................................................................
6
2.1.10 Beam Efficiency
.........................................................................................................................................
6
2.1.11 Bandwidth
..................................................................................................................................................
7
2.1.12 Antenna Radiation Efficiency
....................................................................................................................
7
2.1.13 Return Loss
................................................................................................................................................
7
2.2 Basics of Transmission Line Theory
...................................................................................................................
7
2.2.1 Wave propagation on a transmission line
....................................................................................................
8
2.2.2 Phase velocity
..............................................................................................................................................
8
2.2.3 Voltage reflection coefficient ()
.................................................................................................................
8
2.2.4 Standing wave ratio (VSWR)
......................................................................................................................
8
2.2.5 Transmission lines with some special
lengths..............................................................................................
9
2.2.6 Charactereristic Impedence
..........................................................................................................................
9
2.2.7 The Smith Chart
.........................................................................................................................................
10
2.2.8 S-parameters
..............................................................................................................................................
11
2.4 Antenna Arrays
.................................................................................................................................................
12
2.4.1 Broadside Array
........................................................................................................................................
13
2.4.2 End-Fire Array
...........................................................................................................................................
13
2.5 Mutual Coupling in Antenna Array
..............................................................................................................
14
2.6 Microstrip Antennas
..........................................................................................................................................
15
2.6.1 Introduction
................................................................................................................................................
15
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2.6.2 Rectangular Patch
......................................................................................................................................
16
2.6.3 Feed Models
...............................................................................................................................................
18
2.6.4 Microstrip Line Feed
..................................................................................................................................
18
2.6.5 Coaxial Probe Feed
....................................................................................................................................
18
2.6.6 Aperture- coupled Feed
..............................................................................................................................
19
2.7 Photonic crystals in microstrip antenna substrates
............................................................................................
22
3. ADS Momentum Overview
.....................................................................................................................................
23
3.1 Introductions to ADS Momentum
.....................................................................................................................
23
3.2 Applications of Momentum
...............................................................................................................................
23
3.3 Method of Calculation
.......................................................................................................................................
24
3.4 Working with ADS Momentum
........................................................................................................................
25
3.5 Theory of Operation for Momentum
.................................................................................................................
27
3.6 Method of Moment Technology
........................................................................................................................
28
3.7 Simulation Techniques Used in ADS
................................................................................................................
31
3.8 Block Diagram of ADS Momentum Simulation
...............................................................................................
32
4. Design and Analysis
................................................................................................................................................
33
4.1 Design of a Rectangular Patch Antenna
............................................................................................................
33
4.2 Gain and Directivity
..........................................................................................................................................
35
4.3 Design of the Circular
Patch..............................................................................................................................
37
4.3.1 Resonant Frequency
...................................................................................................................................
37
4.3.2 Radius of the Patch
....................................................................................................................................
38
4.3.3 Feed Point Location
...................................................................................................................................
38
4.4 Proposed Design of a Single Circular Patch Antenna
.......................................................................................
39
4.4.1 Gain and Directivity:
..................................................................................................................................
40
4.4.2 S11 Parameters:
...........................................................................................................................................
41
4.4.3 Efficiency
...................................................................................................................................................
43
4.5 Proposed Design for the Circular Patch Array Antenna
....................................................................................
43
4.5.1 Directivity and Gain
...................................................................................................................................
45
4.5.2 S11 Parameters
............................................................................................................................................
47
5. Conclusion
...............................................................................................................................................................
50
5.1 Conclusion Summary
........................................................................................................................................
50
5.2 Future work
.......................................................................................................................................................
50
References
...................................................................................................................................................................
51
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Table of Figures
Figure 1: Radiation pattern of a Dipole antenna.
............................................................................
3
Figure 2: Antenna Beamwidth.
.......................................................................................................
5
Figure 3: The Smith chart.
............................................................................................................
10
Figure 4: Mutual Coupling Mechanism.
.......................................................................................
14
Figure 5: Different shapes of microstrip patch.
............................................................................
16
Figure 6: Rectangular microstrip patch antenna.
..........................................................................
16
Figure 7: Fringing effects in the microstrip patch antenna.
.......................................................... 17
Figure 8: Microstrip feed line designed in ADS.
..........................................................................
18
Figure 9: Coaxial probe
feed.........................................................................................................
19
Figure 10: Aperture-coupled feed.
................................................................................................
20
Figure 11: Proximity-coupled feed.
..............................................................................................
20
Figure 12: Types of feed.
..............................................................................................................
21
Figure 13: Stepwise simulation of ADS Momentum.
...................................................................
24
Figure 14: Layout window of ADS Momentum.
..........................................................................
25
Figure 15: Different parameters in ADS Momentum.
..................................................................
26
Figure 16: Output S-parameter curves.
.........................................................................................
27
Figure 17: Discretization of the surface current using rooftop
basis function. ............................. 29
Figure 18: Mesh representation in the form of L and C.
..............................................................
30
Figure 19: Block diagram of ADS Momentum simulation.
......................................................... 32
Figure 20: Rectangular patch designed in ADS Momentum layout.
............................................ 33
Figure 21: Magnitude of S11 in dB.
...............................................................................................
34
Figure 22: S11-parameter shown in Smith chart
............................................................................
35
Figure 23: Gain and Directivity of the rectangular patch.
............................................................ 36
Figure 24: 3D graph of the far field radiation.
..............................................................................
36
Figure 25: Design of single circular patch antenna in ADS
Momentum. ..................................... 39
Figure 26: Excitation of circular patch antenna in ADS Momentum.
.......................................... 40
Figure 27: Gain and Directivity of single circular patch antenna
in ADS Momentum. ............... 40
Figure 28: 3D view of the directivity of the single circular
patch simulated in ADS Momentum.
.......................................................................................................................................................
41
Figure 29: Magnitude vs Frequency graph of input reflection
coefficient. .................................. 42
Figure 30: S11-parameter of a single circular patch antenna on a
Smith chart. ............................ 42
Figure 31: Efficiency of single circular patch antenna simulated
by ADS Momentum. .............. 43
Figure 32: Circular patch phase array antenna designed in ADS
Momentum. ............................ 43
Figure 33: 3D view of circular patch microstrip phase array
antenna. ......................................... 45
Figure 34: Gain and Directivity graphs of circular patch
microstrip phase array antenna. .......... 45
Figure 35: 3D Directivity of circular patch microstrip phase
array antenna. ............................... 46
Figure 36: 3D radiation pattern shown by EMDS.
.......................................................................
47
Figure 37: Magnitude vs Frequency graph of S11 parameter.
....................................................... 47
Figure 38: Phase vs Frequency graphs of S11 parameter.
.............................................................
48
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vii
Figure 39: S11 parameter plotted on the Smith
chart.....................................................................
48
Figure 40: Efficiency of the circular patch microstrip phase
array antenna. ................................ 49
Figure 41: Radiated power of the circular patch microstrip phase
array antenna. ....................... 49
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1 Introduction
1.1 Thesis Approach
The thesis comprises design of a microstrip phase array antenna
using circular patches. This
antenna will have the main beam in the broadside direction with
a specified beam width. The
designed antenna consists of an array antenna with two circular
patches. These two circular
patches are connected to the quarter-wave transmission line
through two transmission lines with
specified length and width depending on the impedance of the
circular patches. A coaxial probe
is connected to the quarter-wave transmission line which will
excite the system i.e. the antenna.
The design is implemented and analyzed in ADS Momentum. ADS
Momentum is a 2.5D
simulator which is used to solve complex electromagnetic
circuits. It can build passive
electromagnetic circuits and the simulation shows the
S-parameters of the designed system. ADS
Momentum takes care of the electromagnetic coupling effect. It
also provides 2D and 3D visuals
of output parameters, for example the radiation pattern and the
directivity of the antenna.
1.2 Objective
Design and simulate a microstrip phase array antenna in ADS
Momentum with a main
beam in the broadside direction with specified beam width. It
operates at 10 GHz
(Resonant Frequency) with RT-DURROID 5880 as a substrate.
The array antenna will consist of two circular patches in a
linear fashion, having radius of
5.49 mm each. The height of each of the patch is 17.4 m. The
thickness of the substrate
is 0.787 mm for both the patches.
Two transmission lines are used to connect these patches to
quarter-wave transmission
lines. The impedance of each transmission line is required to be
200 . The length of
each transmission line is 462 mil (11.72 mm) and the width of
each transmission line is
2.39 mil (0.06095 mm). The electrical length of each line is
180.
A quarter-wave transmission line is used to match the impedance
of the system. The
impedance of the line is 50 . The length calculated to get 50
impedance is 5.428 mm
(213.70 mil) and the width is 2.419 mm (95.23 mil). The
electrical length of the quarter
wave transmission line is 90.
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2
This corporate feed network is excited by coaxial probe feed. A
50 coaxial probe is
connected to the quarter wave transmission line.
The antenna is designed and simulated in ADS Momentum.
1.3 Thesis Organization
Chapter 1 consists of an introduction and the objectives of the
thesis. Chapter 2 represents a
literature review and the prerequisite knowledge required in the
design and simulation of the
antenna. Chapter 3 gives a short overview of ADS Momentum.
Chapter 4 is related to the design
of the microstrip phase array antenna. Chapter 5 concludes and
suggests future work that can be
done in this field.
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2. Literature Review
2.1 Basic Antenna Terminology
Antennas radiate and receive electromagnetic waves which are
converted into current after
reception. Some of the basic characteristics of antennas are
discussed below.
2.1.1 Radiation Pattern
The antenna radiation pattern, or antenna pattern, is defined as
``a mathematical function or a
graphical representation of the radiation properties of antenna
as a function of space
coordinates. Radiation properties include power flux density,
radiation intensity, field strength,
directivity, phase or polarization. A trace of received electric
or magnetic field at a constant
radius is called amplitude patten. A graph of the spatial
variation of the power density along a
constant radius is called an amplitude power pattern [1]. The
radiation pattern can be presented
in two forms :
Azimuth Pattern
Elevation Pattern
The top view of the energy radiated by an antenna is known as
Azimuth Pattern while the
graphical side view is called an Elevation. The combination of
these two terms is known as 2D
pattern of the field produced [1]. The basic radiation pattern
of a dipola antenna is shown in Fig.
1.
Figure 1: Radiation pattern of a Dipole antenna.
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2.1.2 Radiation Pattern of a dipole antenna
Following are some important terms for an antenna [1]:
Field Pattern (in linear scale) represents a plot of the
magnitude of the electric or
magnetic field as a function of angular space.
Power Pattern (in linear) typically represents a plot of the
square of the magnitude of the
elecrtric or magnetic field as a function of the angular
space.
Power Pattern (in decibels) represents the magnitude of the
electric or magnetic field in
decibels, as sa function of the angular space.
2.1.3 Directivity
The ratio of the radiation intensity in a given direction to the
radiation intensity avreaged over all
directions [1]. Mathematically directivity can be expressed
as
(1)
The directivity of a non-isotropic source is equal to the ratio
of the its radiation intensity in a
given direction over that of isotropic source
(2)
The partial directivity of an antenna for a given polarization
is, the part of the radiation intensity
corresponding to that polarization, divided by the total
radiation intensity averaged over all
directions.
2.1.4 Gain
The gain of the antenna is related to the directivity of the
antenna. Gain takes into account the
directional capabilities as well as the efficiency of the
antenna [1].
The 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 result if the
power fed to the antenna were radiated
isotropically. The radiation intensity corresponding to the
isotropically radiated power is equal
to the power from the generator, to the antenna divided by 4
[1].
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5
Mathematically this can be expressed as
Gain = 4 radiation intensity
total input (accepted)power = 4
( )
(dimensionless) (3)
2.1.5 Aperture Efficiency
The ratio of the maximum effective area to the physical
area.
2.1.6 Beamwidth
The beamwidth involves a trade-off because the side lobe level
increases as the beamwidth
decreases and vice versa. The beamwidth is also used to describe
the capability of the antenna to
distinguish between two adjacent radiating sources or radar
targets [1].
The beamwidth of an antenna is defined as the angular separation
between two identical points
on opposite sides of the pattern maximum. There are a number of
beamwidths in the antenna
pattern. One of the most widely used beamwidths is the
Half-Power Beamwidth (HPBW) [1].
Figure 2: Antenna Beamwidth.
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2.1.7 Input Impedance
The impedance presented by an antenna at its terminals or the
ratio of the voltage to current at a
pair of terminals [1].
ZA = RA + jXA (5)
RA is the real part and XA is the imaginary part. The resistive
part relates to the power
dissipation, while the imaginary (reactive) part relates to
power stored in the near field of the
antenna.
2.1.8 Polarization
The polarization is the orientation of the electric field far
from the source [2]. It describes the
time-varying direction and relative magnitude of the electric
field vector. Polarization for an
antenna in a given direction is defined as the polarization of
the E-field 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. The polarization of a wave radiated
by an antenna, in a specified
direction, at a point in the far field, is defined as the
polarization of the plane wave which is used
to represent the radiated wave at that point [1]. Polarization
may be classified as linear, circular,
elliptical, circular left hand, circular right hand, elliptical
right and elliptical left hand.
2.1.9 Antenna Efficiency
The total antenna efficiency eo is used to take into account
losses at the input terminals of the
antenna. Such losses may be caused by:
Reflections because of the mismatch between transmission line
and antenna.
I2R losses (conductive and dielectric).
In general, overall efficiency can be written as:
(6)
eo = total efficiency, er = reflection mismatch efficiency, ec =
conduction efficiency,
ed = dielectric efficiency.
2.1.10 Beam Efficiency
A parameter used to judge the quality of transmitting and
receiving antennas is the beam
efficiency [1].
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7
( )
( ) (7)
1 is the half angle of the cone in the above equation.
2.1.11 Bandwidth
The bandwidth of an antenna is defined as the range of
frequencies within which the
performance of the antenna, with respect to some
characteristics, conforms to a specified
standard. The bandwidth can be considered to be the range of
frequencies on either side of the
center frquency where the antenna characteristics are close to
those at the center frequency [1].
2.1.12 Antenna Radiation Efficiency
The conduction dielectric efficiency is defined as the ratio of
the power delivered to the radiation
resistance Rr, to the power delivered to Rr and RL. The
resistance RL is used to represent the
conduction-dielectric losses [1].
2.1.13 Return Loss
The characterization of the input and output signal can be shown
in a more convenient way in the
form of return loss when a load is mismatched [3]. This means
that all the source power is not
delivered to the load. This loss of power is known as return
loss and can be represented as:
| | ( ) (8a)
Where | |
(8b)
| |= Magnitude of reflection coefficient, Vo = Reflected
voltage, Vin
= Incident voltage, ZL and
Zo are the load and characteristic impedances.
2.2 Basics of Transmission Line Theory
Transmission lines and waveguides are conduits for transporting
RF signals between elements of
a system. For example transmission lines are used between an
exciter output and transmitter
input, between the transmitter input and its output and between
the transmitter output and the
antenna [4]. Transmission lines are complex networks containing
the equivalent of all the three
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basic electrical components: resistance, capacitance, and
inductance. Hence, transmission lines
must be analyzed in terms of an RLC network [4].
2.2.1 Wave propagation on a transmission line
The wavelength of the travelling waves is defined as the
distance between two successive points
of equal phase on the line at a fixed instant of time [4].
(9)
2.2.2 Phase velocity
The phase velocity of the wave is defined as the speed at which
a constant phase point travels
down the line.
(10)
2.2.3 Voltage reflection coefficient ()
The amplitude of the reflected voltage wave, normalized to the
amplitude of the incident voltage
wave, is defined as the voltage reflection coefficient [1].
(11)
The average power flow is constant at along the line and the
total power delivered to the load is
the difference between incident power and the reflected power.
If =0, maximum power is
delivered to the load while no power is delivered for =1 (all
the incident power is reflected
back from the load) [1].
2.2.4 Standing wave ratio (VSWR)
When a tranmission line is not matched to its load some of the
energy is absorbed by the load
and some is reflected back down the line towards the source. The
intereference of the incident
and reflected wave creates standing waves on the transmission
line [4]. As the magnitude of the
reflection coefficient increases, the ratio of Vmax to Vmin also
increases. A measure of the
mismatch of the line is called the standing wave ration (SWR),
also know as the volatage
standing wave ratio (VSWR) [1].
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9
(12)
One has, 1R
(14b)
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10
2.2.7 The Smith Chart
The mathematics of transmission lines becomes cumbersome at
times, especially when dealing
with complex impedances and nonstandard situations. In 1939,
Phillip H. Smith published a
graphical device for solving these problems, the Smith Chart. It
consists of a series of
overlapping orthogonal circles that intersect each other at
right angles. These sets of orthogonal
circles make up the basic structure of the Smith chart and are
shown in Fig. 3. The following is a
brief description of the Smith chart and how it works [4].
Figure 3: The Smith chart.
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11
2.2.7.1 The normalized impedance line
A baseline bisects the Smith chart outer circle and forms the
reference of measurements made on
the chart. The Complex impedance contains both resistance and
reactance and is expressed
mathematically as:
Z= R jX (15)
R is the resistive component of the impedance and X is the
reactive component of the impedance
[4].
The pure resistance circle represents the situation where X=0,
and the impedance is therefore
equal to the resistive component only. To make the Smith chart
universal, the impedances along
the pure resistance line are normalized with reference to system
impedance (Z0). The actual
impedance it is divided by the system impedance. The pure
resistance line is structured such that
the system standard impedance is at the center of the chart and
has a normalized value of 1.0 [4].
2.2.7.2 The constant resistance circles
The isoresistance circles, also called the constant resistance
circles represent points of equal
resistance. These circles are all tangent to the point at the
right hand extreme of the pure
resistance line and are bisected by that line.
2.2.7.3 The constant reactance circles
The circles above the pure resistance line represent the
inductive reactance (+X) while the circles
below the pure resistance line represent capacitive reactance
(-X). The outermost circle is called
the pure reactance circle. Points along this circle represent
reactance only.
2.2.8 S-parameters
The S-parameters are very important in microwave design for
describing the behavior of
electrical devices. Most of the electrical properties i.e. gain,
return loss, power, VSWR etc relates
to the S-parameters. The S-parameters can be observed by sending
a signal through an input port
and observing the response on an output port. The term impedance
is of great importance while
calculating the S-parameters because the system should be
matched properly, otherwise
reflection which will give rise to standing waves and the system
will not produce the desired
output. The S-parameters S11 and S22 represent input and output
reflection while S21 is the
forward transmission coefficient (gain) and S12 is the reverse
transmission coefficient (isolation).
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12
2.4 Antenna Arrays
Usually the radiation pattern of a single element is relatively
wide, and each element provides
low values of directivity (gain). In many applications, it is
necessary to design antennas with
very high directive characterristics (very high gain) to meet
the demands of long distance
communication. This can only be accomplished by increasing the
electrical size of the antenna
[1]. Enlarging the dimensions of single elements often leads to
more directive characteristics.
Another way to enlarge the dimensions of the antenna, without
increasing the size of individual
element, is to form an assembly of radiating elements in an
electrical and geometrical
configuration. This new antenna antenna formed is referred to as
an array [1].
The antenna arrays are of vast importance and are widely used
nowadays for various purposes
like military, missiles and satellite communication. There are
different forms of antenna arrays
linear, circular, planar etc. The radiation pattern of an array
antenna is mostly considered in the
far field, where the field depends on two parameters. One is the
distance r of the reciever and the
other deals with the spherical coordinates and . The radiation
pattern of an antenna can be
calculated by :
Array Pattern = Array element pattern * Array factor(AF)
(16)
The array factor determines the overall radiation pattern of the
array while the element pattern
describes radiation pattern of the individual element [5]. The
array factor can also be defined as
The function of the total number of elements, their spacing and
the phase difference between
each element [6]. The array factor for a uniform antenna can be
written mathematicaly as:
(17)
One may normalise the array factor so that the maximum value is
equal to unity.
( )
(18)
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13
Antenna array design involves two broader concepts:
Broadside Array
End-Fire Array
2.4.1 Broadside Array
In a broadside array, the radiators are along a straight line
producing a beam perpendicular to the
line [5]. For an optimal design, the maxima of the single
element as well as of the array should
be directed toward = 900 and the phase angle is zero.
( )
( )
( ) (19)
Where and
The requirements for the single element can be met by a
judicious choice of the radiators, and
those of the array factor by the proper separation and
excitation of the individual radiators [1].
2.4.2 End-Fire Array
A linear array whose direction of maximum radiation is along the
axis of the array. It may either
be unidirectional or birectional. The main beam will either be
at o= 0o or 180
o.
(20)
For o= 0o or 180
o
(21)
Which gives
( ) (22)
( )
( ( )
( ( ) (23)
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14
2.5 Mutual Coupling in Antenna Array
One of the basic characteristics of an antenna array appears
when two or more elements are
located near to each other and effect each other [1]. The amount
of coupling depends on the
following:
Radiation characteristics.
Actual separation between elements .
Relative orientation of elements.
The mutual coupling between two radiating elements depends upon
the distance between them.
If they are close to each other the mutual coupling will be
greater. Thus energy is transferred
between elements and this is called mutual coupling. One can say
that the electromagnetic
coupling between the elements is mutual [7].
The transmitting mode coupling can be shown with the help of
Fig. 4. Two antennas, A and B
are placed relative to each other. Antenna A is excited by a
source and radiates. When this
radiation reaches antenna B, it excites antenna B and
rescatteres some the energy back to antenna
A. Antenna A recieves the energy again and so on. The total
contribution that an element makes
to the far field pattern does not depend on its own excitation
from the generator only, but also
upon the total parasitic excitation due to which coupling is
introduced to other generators [8].
The mutual coupling phenomenon is reciprocal in nature. If one
antenna is used as a transmitter
and the other as a reciever or vice versa. Both is the same.
A B
Figure 4: Mutual Coupling Mechanism.
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15
Since mutual coupling in phased array antennas can affect the
radiation pattern so consideration
should be given to this mechanism.
2.6 Microstrip Antennas
2.6.1 Introduction
For applications where size, weight, cost, performance, ease of
installation and aerodynamics are
constraints, low profile antennas are needed. Aircraft,
spacecraft, satellite and missile
applications and recently mobile radio and wireless
communications demands this [1]. To meet
these requirements microstrip antennas can be used. These
antennas are low profile, suited to
planar and non planar surfaces, simple and inexpensive to
manufacture, mechanically robust
when mounted on rigid surfaces, compatible with MMIC designs,
and when the particular patch
shape and mode are selected, they are very versatile in terms of
resonant frequency, polarization,
pattern and impedance [1].
Major operational disadvantages of microstrip antennas are their
low efficiency, low power, high
Q, poor polarization purity, poor scan performance, spurious
feed radiation and narrow
frequency bandwidth which is typically only a fraction of a
percent or at most, a few percent [1].
Microstrip antennas also exhibit large electromagnetic
signatures at certain frequencies outside
the operating band and are rather large physically at VHF and
possibly UHF frequencies. In large
arrays there is a trade-off between bandwidth and scan volume
[1].
The idea of the microstrip antenna was introduced in 1953 by G.A
Deschamps and it received
considerable attention by 1973. In 1970, Howell and Munson
defined a transmission model for
microstrip antennas. Microstrip antenna patch elements are the
most common form of printed
antennas. These antennas are quite cheap, light weight and give
good results. The microstrip
patch can have different shapes like circular, rectangular or
square as shown in Fig. 5.
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16
Figure 5: Different shapes of microstrip patch.
2.6.2 Rectangular Patch
The rectangular patch is by far the most widely used
configuration. A basic form of rectangular
patch is shown in the Fig. 6.
Figure 6: Rectangular microstrip patch antenna.
The patch of a microstrip antenna is usually made of a
conducting material. The patch is parallel
to the ground plane. In between the patch and the ground plane
there is substrate with a dielectric
constant whose value depends on the substrate used. The inside
of the rectangular patch is shown
in Fig. 7.
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17
Figure 7: Fringing effects in the microstrip patch antenna.
Because the dimensions of the patch are finite, the fields at
the edges of the patch undergo
fringing [1]. The amount of fringing is a function of the
dimensions of the patch and the height
of the substrate. For the principal E-plane (xy-plane), fringing
is a function of the ratio of the
length of the patch L to the height h of the substrate (L/h),
and the dielectric constant r of the
substrate [1]. Most of the electric field lines reside in the
substrate and parts of some line exist in
air. As W/h >> 1 and r >> 1, the electric field
lines concentrate to the substrate. Fringing in this
case makes the microstrip line look wider electrically compared
to its physical dimensions [1].
The resonant length can be calculated using:
L is resonant lenght, is the wavelength in printed circuit
board, is wavelenght in free space
and r is the dielectric constant. The effective dielectric
constant can be calculated by the
formula:
*
+
(22)
Some of the electric field rests inside the substrate while some
extends outwards due to fringing.
Because of the fringing field between the edge of the patch and
the ground plane, the patch
radiates. To make antennas efficient, thick dielectric
substrates with low dielectric constant are
suitable. This gives larger bandwidth efficiency and desirable
radiation. Due to large bandwidth,
the size of the antenna will be very large, which is not wanted.
To get rid of this problem, a thin
dielectric substrate with high reduces the bandwidth but a
trade-off has to be made.
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18
2.6.3 Feed Models
There are many configurations that can be used to feed
microstrip antennas. The four most
popular feed models are microstrip line, coaxial probe, aperture
coupling and proximity coupling
[1].
2.6.4 Microstrip Line Feed
The microstrip feed line is also a conducting strip, usually of
much smaller width compared to
the patch. The microstrip feed line is easy to fabricate, simple
to match by controlling the inset
position and rather simple to model [1]. However, as the
substrate thickness increases, surface
waves and spurious feed radiation increases which for practical
designs limit the bandwidth [1].
Figure 8: Microstrip feed line designed in ADS.
2.6.5 Coaxial Probe Feed
The inner conductor of the coax is attached to the radiation
patch and the outer conductor is
connected to the ground plane [1]. The coaxial probe feed is
also easy to fabricate and match,
and has low spurious radiation. However, it also has narrow
bandwidth and it is more difficult to
model, especially for thick substrates [1].
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19
Figure 9: Coaxial probe feed.
2.6.6 Aperture- coupled Feed
The most difficult technique to fabricate is the aperture
coupled feed. Having a narrow
bandwidth, it is however somewhat easier to model and has
moderate spurious radiation [1]. The
aperture coupling consists of two substrates separated by a
ground plane. On the bottom side of
the lower substrate there is a microstrip feed line whose energy
is coupled to the patch through a
slot in the ground plane separating the two substrates. This
arrangement allows independent
optimization of the feed mechanism and the radiating element.
Typically a high dielectric
material is used for the bottom substrate, and a thick low
dielectric constant material is used for
the top substrate [1].
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20
Figure 10: Aperture-coupled feed.
The main disadvantage of such a design is that it requires
complex multiple layers.
2.6.7 Proximity-coupled Feed
Proximity-coupled Feed is sometimes called an electromagnetic
coupling scheme. It consists of
two layers on top of each other. There is no ground plane in
such an antenna. The microstrip feed
line is in between the two substrates and the radiation patch is
on the top of the substrate as show
in Fig. 11. Of the four feeding models, the proximity coupling
has the largest bandwidth and it is
fairly easy to model, having low spurious radiation [1].
However, its fabrication is more difficult.
The length of the feeding stub and the width- to-line ratio of
the patch can be used to control the
match [1].
Figure 11: Proximity-coupled feed.
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21
2.6.8 Arrays and Feed Networks
Arrays are very versatile and are used, among other things, to
synthesize a required pattern that
cannot be achieved with a single element. In addition, they are
used to scan the beam of an
antenna system, increase directivity, and perform various other
functions which would be
difficult with any one single element [1]. The elements can be
fed by a single line called the
series-feed network or by multiple lines called corporate-feed
network [1]. Among all the
feeding techniques, corporate feed is mostly used in scanning,
phased multiple beam or shaped-
beam arrays. With this method, the designer has more control of
the feed of each element
(amplitude and phase) and it is ideal for scanning phased
arrays, multiple beam arrays, or
shaped-beam arrays [1]. While designing an array, the feed point
and the distance between each
patch is kept constant in order to provide equal phase patch
excitation. A series feed network is
easy to fabricate and implement as compared to corporate feed
network. The disadvantage of
using series feed is that it gives phase delay and hence it is
not preferred for the phase scanning
arrays [9]. These phase shifts are frequency dependant due to
which beam scanning is dependent
on the frequency [9]. Corporate feed networks provide flexible
phase control of each array
element. It is suitable for phase scanning as it is less
affected by the frequency scan [9]. The most
common form of corporate feed network is the Wilkinson Power
divider rule.
(a) Series Feed. (b) Corporate Feed.
Figure 12: Types of feed.
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22
2.7 Photonic crystals in microstrip antenna substrates
During the past decade, a new technology has emerged which has
become the key to developing
ultra-wideband microstrip antennas. This technology manipulates
the substrate in such a way that
the surface waves are completely forbidden from forming, hence
resulting in improvements in
the antenna efficiency and bandwidth, while reducing the side
lobes and electromagnetic
interference levels. These substrates contain so called photonic
crystals [10].
The patch antennas on high substrates are not efficient
radiators due to losses. The patch
antenna having a narrow frequency bandwidth results in reduced
gain and efficiency at high
frequencies. Patch antennas also have an unacceptably high level
of cross polarization and
mutual coupling within the array environment. Therefore, much
effort has been made recently to
realize high efficiency patch antennas on high permittivity
substrates at high frequencies [11].
A PGB crystal is a periodic structure that forbids the
propagation of electromagnetic waves
within a particular frequency band, called the band gap, thus
permitting controls of the behavior
of the electromagnetic waves other than the conventional guiding
and filtering structures [10].
The photonic crystals are a class of periodic metallic,
dielectric or composite structures that
exhibit a forbidden band (band gap) of frequencies in which
waves, incident at various directions
interfere destructively and thus are unable to propagate [12].
Based on the spatial periodicity of
the crystal structure, the band gaps can be in one, two or
three-dimensional planes, with a level
of complexity that increases with the number of dimensions. The
three-dimensional nature of
the band gap rejects incident energy from all directions around
a unite sphere like a high
efficiency reflector or mirror. In a 2-D photonic crystal fiber
the band gap exists only within a
plane, thereby allowing propagation along one axis only. This is
the ideal scenario for microstrip
antenna design, since the rejection plane could be in the plane
of the patch and thus prevent
surface wave formation [12].
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23
3. ADS Momentum Overview
3.1 Introductions to ADS Momentum
Momentum is a part of Advance Design System and it provides the
simulation tools required to
evaluate and design products of modern communication systems.
Momentum is an
electromagnetic solver in the form of a simulator that computes
the S-parameters for general
planar circuits which includes microstrip, slotline, stripline,
coplanar waveguides and many other
topologies. Multilayer communication circuits and printed
circuit boards can also be simulated in
ADS Momentum with accurate results. Momentum is a complete tool
for prediction of the
performance of high frequency circuit boards, antennas and
integrated circuits [13].
The ADS Momentum optimization tool extends Momentum capability
to a real design
automation tool. The Momentum Optimization process varies
geometry parameters
automatically to help in achieving the optimal structure that
for the circuit or device performance
goals. Momentum optimizations can be done by using layout
components (parameterized) from
the schematic page.
One of the great advantages that Momentum possesses is the
3-dimensional interface that it
provides for the user during simulations and results. Momentum
is a 2.5D solver that can do both
2D and 3D computations. For example while computing the antenna
parameters, Momentum
provides both 2D and 3D graphs of the directivity and the
far-field radiation patterns of the
antenna.
3.2 Applications of Momentum
ADS Momentum can be used as follows [13].
ADS Momentum is applicable when no analytical model exists for
the circuit.
Momentum co-simulates with ADS and performs the required
tasks.
ADS Momentum can be used to determine coupling effects.
ADS Momentum can calculate narrow resonances within the circuit
model which cannot
be found with analytical models.
-
24
ADS Momentum can be used to display the radiation patterns and
far field radiation plots
for antennas etc.
ADS Momentum can show the current pattern and current densities
within the circuit.
Momentum can be used for the CPW (Co Planar Waveguides) results
with no slot mode.
Momentum can be used to optimize or modify the geometry of the
passive layouts to
achieve the desired results.
3.3 Method of Calculation
The method of simulation that is used by ADS Momentum is called
the Method of Moments
which is based on the integral formulation of Maxwells
equations, simulating the circuit
with matrix equations. Fig. 13 shows the stepwise simulation of
a circuit by ADS
Momentum. Where a known circuit is first simulated and then
divided into mesh strip, wires
with rectangles and triangles (arbitrary surface meshes). The
next step is to model the surface
current in each current cell i.e. linear distribution. The final
step is to solve a mesh matrix
equation and calculate S-parameters.
Figure 13: Stepwise simulation of ADS Momentum.
-
25
3.4 Working with ADS Momentum
A short literature on how one should start using ADS Momentum is
given below. As with every
simulator, working with the ADS Momentum is a stepwise process.
Some of the steps are given
below.
Step 1:
Step 1 shows the startup of the ADS Momentum. Momentum starts in
the ADS layout window
as shown in Fig. 14.
Figure 14: Layout window of ADS Momentum.
A number of options can be seen on the task and menu bar. We can
use different kinds of
microwave components depending on our requirement. Fig. 14 shows
the mapping of microstrip
patches of varying lengths in the ADS layout window. Ports are
connected on both sides of the
circuit, making it a two port network for the S-parameter
calculation.
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26
Step 2:
The Microstrip patch antennas cannot be designed without a
substrate. In order to define a
particular substrate we can create our own substrate or we can
use the built in substrates defined
in ADS momentum.
Figure 15: Different parameters in ADS Momentum.
Fig. 15 shows the different parameters which need to be set
before simulating any design in the
ADS Momentum layout. After substrate definition, we will use
ports calibration. Port is
necessary in the optimization of any design because it serves as
an input to the system. The next
parameter in the list is Mesh setting. We can change the mesh
frequency in order to synchronize
it with the input resonant frequency. Design cannot work if the
mesh frequency is not
synchronized with the input resonant frequency.
Step 3:
The final step is to calculate the S-parameters. Depending on
the number of ports used in the
network, ADS Momentum will provide the related S-parameters.
Fig. 16 shows an example of
output curves of S-parameters calculated by ADS Momentum.
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27
Figure 16: Output S-parameter curves.
3.5 Theory of Operation for Momentum
Momentum is based on a numerical discretization technique called
the Method of Moments. This
technique is used to solve the Maxwell equations for planar
structures embedded in multilayer
dielectric substrates. Momentum uses two different modes of
simulation which are based on the
Method of Moments. The first one is the microwave, or full wave,
mode of simulation and the
second one is the RF, or quasi-static, mode of simulation. The
application and formulation of the
Greens function is the main difference between these two
methods.
Momentum, or the full wave simulation mode, uses the full wave
Greens function. The Full
wave Greens function is frequency dependant and it fully
characterizes the substrate without
making any further approximations. This formulation results in
the L and C elements that are
complex and frequency dependant as shown in Fig. 13. The RF, or
quasi-static, mode uses a
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28
frequency independent Greens function which results in L and C
elements which are complex
but frequency independent. As this mode is not frequency
dependant, the quasi-static mode only
approximates the solution of the network (L and C) for the first
frequency simulation point and
hence the RF mode runs much faster than the Momentum mode. The
simulations also show that
the quasi-static mode should be used for structures that are
smaller than half a wavelength [13].
Both the engine modes use the star-loop basis function that
ensures a stable solution at all
frequencies. Both the modes use the mesh reduction algorithm
which helps in reducing the
number of unknowns when dividing the design into polygonal
meshes. This function can be
turned on or off [13].
Excitation of the networks is fed through the input port. The
currents in the equivalent network
model are given by unknown amplitudes in the rooftop expansion
model. The amplitudes are
obtained by solving for the unknowns in the rooftop expansion.
The S-parameters are extracted
with the help of the port calibration process.
3.6 Method of Moment Technology
The method of moments (MoM) was first applied by R.F. Harrington
who worked extensively on
the method and successfully applied it to electromagnetic field
problems. It is based on the
theory of weighted residuals and variational calculus. In the
MoM, Maxwells equations are
transformed into integral equations before discretization.
Momentum uses the mixed potential integral equation (MPIE)
formulation [14]. This method
expresses the electric and magnetic fields with a combination of
the scalar and the vector
potential. The electric and magnetic surface currents in the
design network are the unknowns in
the planar circuit. From electromagnetics one has the integral
equation,
( ) ( ) ( ) (23)
-
29
Here, ( ) represents the unknown surface current and ( )
represents the known excitation of
the problem. The Greens dyadic of the layered medium acts as an
integral kernel. The unknown
surface currents are discretized by meshing the planar
metallization pattern and applying an
expansion in a finite number of sub-sectional basis functions
B1( )., BN( ) [14]:
( ) = ( ) (24)
The rooftop functions are used in planar EM simulators. These
standard basis functions are
defined over rectangular, triangular and polygonal cells in the
mesh. Each rooftop is associated
with one edge of the mesh and represents current with continuous
density as shown in Fig. 17. Ij,
determine the current elements that corresponds to the edges of
the mesh [14].
Figure 17: Discretization of the surface current using rooftop
basis functions.
Eq. 23 is discretized by inserting the rooftop expansion of Eq.
24. We can write:
-
30
= j(r) or ZI = V (25)
( ) ( ) ( ) (26)
( ) ( ) (27)
The matrix Z is known as the interaction matrix since the
elements in this matrix describe the
electromagnetic interaction between the rooftop basis functions.
The vector V represents the
discretized contribution of the excitation applied at the ports
of the circuit [14].
The final values of L and C in the network can be written
as:
( ) ( ) ( )
(28)
( )
( ) ( ) (29)
Eq. (28) and Eq. (29) gives a physical interpretation to the
interaction matrix, as shown in Fig.
18.
Figure 18: Mesh representation in the form of L and C.
The whole discussion is explained diagrammatically in Fig.
17.
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31
3.7 Simulation Techniques Used in ADS
In addition to the auto-select mode, ADS uses three different
matrix solution techniques which are explained as following.
1. Direct Dense. 2. Iterative Dense. 3. Direct Compressed.
3.7.1 Direct Dense Method
In this method, the matrix N is stored in a dense matrix format
which requires memory space of
order N2. This matrix is then solved by the direct matrix
factorization technique. This method
requires N3 order for solution (computer time). The direct dense
matrix solver has a
predetermined number of operations. The main disadvantage of
this method is that it requires
cubic computer time to solve dense matrices of the order N.
Hence it requires larger time for
complex problems [13].
3.7.2 Iterative Dense Method
In this method, the matrix N is stored in a dense matrix format
which requires N2 memory space
while the matrix N is solved using iterative matrix solver
technology. This solution method
requires N2 order to solve the matrix, hence scaling the
computer time to quadratic (N
2) from
cubic (in the direct dense method). This yields shorter
simulation time for larger problem sizes
[13].
Convergence of the iterative technique is the main drawback of
this method of simulation
because iterative methods do not converge quickly in large and
complex problems. ADS
monitors the convergence rate and automatically jumps to the
direct dense method of simulation
when it detects stagnancy in the convergence [13].
3.7.3 Direct Compressed Method
Direct compressed method (DCM) is one of the latest techniques
used for matrix solution. DCM
is also known as the FMM (Fast Multipole method) and is
considered to be one of the top ten
-
32
algorithms of the 20th
century. In this method, the matrix is stored in a compressed
matrix form
which requires NlogN memory space while it is solved using
direct compressed matrix
factorization technique. The direct compressed factorization
technique requires (NlogN)1.5
computer time [20].
The computer time that this method requires is linear
logarithmic with matrix size N which
makes this method most useful for the solution of large and
complex problems. This method
reduces both the simulation speed and the memory allocation
space required for the simulation
[13].
All these three type of methods are available in ADS. Generally
the default settings of the ADS
are set to auto-mode but user can change the type of simulation
manually. ADS is sensitive to the
type of problem and chooses proper simulation method according
to the problem, so the
preferable way to use ADS is to keep the settings on
auto-mode.
3.8 Block Diagram of ADS Momentum Simulation
The Block diagram in Fig. 20 shows how ADS Momentum simulates
its designs and provides
the outputs.
Figure 19: Block diagram of ADS Momentum simulation.
-
33
4. Design and Analysis Before going into the details of the
project, it is useful to perform a simple test with the ADS
Momentum to validate the simulations of ADS Momentum with known
results. For this purpose,
an example from the book of Antenna Theory [1] was simulated in
ADS momentum. All the
results were calculated mathematically and then fed to the ADS
Momentum design guide to get
the visuals of the far field and other graphical results.
4.1 Design of a Rectangular Patch Antenna
A rectangular patch with TMX
010 mode is designed in ADS Momentum. The length of the
patch
is 0.906 cm, the width of the patch is 1.186 cm and the height
of the patch is 0.1588 cm.
Permittivity of the substrate is 2.2 and the resonance frequency
is 10 GHz. RT Durroid 5880 is
used as a substrate with a substrate height of 0.787 m. The
rectangular patch was energized
using coaxial probe feed. The mathematical solution is available
in [1].
This patch was designed in ADS Momentum. After design, the patch
was simulated in ADS
Momentum to get the directivity, the gain curves along with the
3D visuals of the far field
radiation and the 3D view of the designed antenna patch. Fig. 20
shows the design of the single
rectangular patch in ADS Momentum environment.
Figure 20: Rectangular patch designed in ADS Momentum
layout.
-
34
Fig. 21 and Fig. 22 show the shows the simulation results of the
rectangular patch in ADS
Momentum. Fig. 21 shows the behavior of the S11 parameter or the
input reflection coefficient
over a range of frequencies. It is clear from the figure that
the patch resonates at 10 GHz and has
minimum loss at the resonant frequency i.e. -3 dB.
Figure 21: Magnitude of S11 in dB.
Fig. 22 shows the same input reflection coefficient (S11) result
in the Smith chart. We can see
from the Smith chart that the impedance of the system is also
resistive at the input. The marker
shows the impedance of the system at the resonant frequency.
2 4 6 8 10 12 140 16
-3
-2
-1
-4
0
Frequency
Mag. [d
B]
Readout
m1
S11
m1freq=dB(example_14_1_mom_a..S(1,1))=-3.077Min
10.35GHz
-
35
Figure 22: S11-parameter shown in Smith chart
4.2 Gain and Directivity
One of the main features of the ADS Momentum is that it can give
us both the 2D and 3D graphs
of the gain and directivity of the system. Fig. 23 shows the
gain and directivity of the rectangular
patch simulated in ADS Momentum. The lower graph line, the Gain
of the system is
approximately -20 dB while the upper line, the directivity of
the system, is approximately 10 dB.
-
36
Figure 23: Gain and Directivity of the rectangular patch.
Similarly ADS Momentum simulates the three dimensional view of
the directivity, or the far
field radiation pattern, of the rectangular patch microstrip
antenna as shown in Fig. 24. It can be
seen that the far field radiation is not broad side but almost
isotropic. As it is a single patch
antenna, it does not possess great directivity.
Figure 24: 3D graph of the far field radiation.
PowerGain Directivity
-80
-60
-40
-20
0 20 40 60 80-100
100
-40
-20
0
-60
20
THETA
Mag
. [dB
]
-
37
4.3 Design of the Circular Patch
The design of the circular patch requires some constraints.
Following are some design
parameters which should be calculated while designing the
circular patch.
4.3.1 Resonant Frequency
The resonant frequency of the circular patch can be analyzed
with the cavity model. The cavity
model consists of the electrical conductors above and below the
cavity while a perfect magnetic
conductor having cylindrical shape and radius a in between the
two electrical conductors
represents the value of the cavity [1, 9, 15].
The resonant frequency for TMzmn0 mode is:
(fr)mn0 =
( ) (
) (30)
in Eq. (30) is the n
th zero of the Bessel function Jm(X) which determines the
resonant
frequency that is different for different modes of operation.
Following are values of the [1].
= 1.1841
= 3.0542
= 3.8318
= 4.2012
With the values of the zeros of the Bessel function in Eq. (30)
we can show,
(fr)mn0 =
(31)
For the mode that is used in the design one has,
(fr)110 =
(32)
Eq. (32) gives the resonant frequency for the circular patch in
the cavity model. Here ae is the
radius of the patch while c is the velocity of light in
vacuum.
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38
4.3.2 Radius of the Patch
In designing a rectangular patch, we account for the length and
width of the patch. Changing
these two parameters will change the mode of operation of the
rectangular patch. In the circular
patch, we have only one degree of freedom and that is the radius
of the patch. To calculate the
radius of the patch we have to include fringing in the circular
patch. Fringing is the effect which
makes the patch electrically larger than geometrical patch. Due
to this phenomenon, the effective
radius ae is introduced. Eq. 33 shows the expression for
effective radius for the circular patch [1,
2, 15].
ae = a,
* (
) +- (33)
Here the original radius a is given by
a =
,
* (
) +-
(34)
F =
(35)
4.3.3 Feed Point Location
In the design and excitation of the circular patch, the feed
point location is one of the most
important parameters [15]. The impedance of the circular patch
antenna is almost zero at the
center of the patch while it is about 200-300 ohms at the edges
of the patch. Impedance matching
can only be obtained by locating the feed point so that the
overall system impedance equals 50
ohms. According to Karmakar [9], the mathematical expression for
the feed point location for the
TM110 is given below:
= a/3 (36)
Eq. (36) is the location of the feed point.
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39
4.4 Proposed Design of a Single Circular Patch Antenna
To design a circular patch antenna in ADS Momentum, we need to
know all the parameter
values [15, 16, 17]. All the needed design parameters were
calculated using the known equations.
RT DURROID 5880 was used as a substrate with relative
permittivity equal to 2.2. The height
of the substrate H is equal to 0.787m. The loss tangent for this
substrate is equal to 0.0009.
The radius of the circular patch calculated using Eq. 33 was
equal to 5.83 mm. The feed point
distance from the center of the circle is equal to 1.83 mm. We
used coaxial probe feed to excite
the circular patch antenna with the input impedance equal to 50
ohms. Fig. 25 shows the design
of this circular patch in ADS Momentum.
Figure 25: Design of single circular patch antenna in ADS
Momentum.
After designing the circular patch in the ADS Momentum
environment, the patch was simulated
to check the performance of the patch. Figures (26a) and (26b)
shows the three dimensional
design of the circular patch antenna before and after
excitation, respectively.
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40
(a): Circular Patch before excitation (b) Circular patch after
excitation
Figure 26: Excitation of circular patch antenna in ADS
Momentum.
4.4.1 Gain and Directivity:
Fig. 27 shows the gain and directivity curves of the single
circular patch antenna simulated by
ADS momentum.
Figure 27: Gain and Directivity of single circular patch antenna
in ADS Momentum.
The lower curve in Fig. 27 represents gain while the upper curve
represents the directivity of the
patch. As visible from the figure, the gain of the single
circular patch antenna is 5 dB
approximately while the directivity of the patch is
approximately 8 dB.
-
41
ADS Momentum can also simulate the three-dimensional graph of
the single circular patch
antenna along with two dimensional graphs as shown in Fig. 27.
Fig. 28 shows the 3D graph of
the far field radiation of the antenna.
Figure 28: 3D view of the directivity of the single circular
patch simulated in ADS Momentum.
From Fig. 28, it is clearly visible that the single circular
patch antenna has the main beam in the
90 to 270 range i.e. Perpendicular to the axis of the patch. The
single circular patch antenna is
not an efficient antenna so the main lobe is wide.
4.4.2 S11 Parameters:
ADS Momentum simulations help us in gathering information about
the reflection coefficients of
the antenna. We are using only one probe, so all the
coefficients of the S-matrix will be zero
accept the S11 parameter which is the input reflection
coefficient [1]. We can easily understand
the performance of the circular patch antenna from the S11
Parameter graph. Fig. 29 shows the
graph of S11 parameter simulated by ADS Momentum.
-
42
Figure 29: Magnitude vs Frequency graph of input reflection
coefficient.
Fig. 29 clearly indicates that the single circular patch antenna
resonates at 9.875 GHz having a
minimum magnitude of approximately -5 dB.
ADS Momentum also simulates the graph for the input reflection
coefficient on the smith chart
as shown in Fig. 30. The single circular patch antenna resonates
at 10 GHz with minimum
impedance at that particular point which is also indicated by
the m2 marker on the Smith chart.
Figure 30: S11-parameter of a single circular patch antenna on a
Smith chart.
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43
4.4.3 Efficiency
Fig. 31 shows the efficiency of the single circular patch
antenna simulated by ADS.
Figure 31: Efficiency of single circular patch antenna simulated
by ADS Momentum.
4.5 Proposed Design for the Circular Patch Array Antenna
Fig. 32 shows our final project design. The array consists of
two circular patches excited with
coaxial probe feed, designed and simulated in ADS Momentum.
Figure 32: Circular patch phase array antenna designed in ADS
Momentum.
-
44
Each of the circular patches has the radius of 5.83 mm. The
separation between the two circular
patches is 12 mm from center to center. The impedance of the
circular patch is 200-300 ohms at
the edge of the patch while this impedance decreases to zero
towards the center of the patch. The
two circular patches are excited with a coaxial probe feed
through the corporate feed method i.e.
The Wilkinson power divider rule. The patches are connected to
the coaxial probe via two
transmission lines. These two transmission lines are terminated
by a quarter wave transmission
line.
In order to make the circular patch array antenna radiate, the
impedance of the system should be
matched and it should not exceed 50 ohms [13, 15]. For this
purpose the length and width of the
transmission lines were calculated with the software called Line
Calc, which is available in ADS.
For a 200 ohms transmission line, the length is equal to 11.72
mm and the width is equal to
0.06096 mm. Both these transmission lines are terminated on the
100 ohms termination point on
the quarter wave transmission line. The length of the quarter
wave transmission line is equal to
5.428 mm and the width of the quarter wave transmission line is
equal to 2.419 mm. The 50
ohms coaxial probe is connected to the other termination point
of the quarter wave transmission
line. With these arrangements, the impedance of the system is
matched to approximately 50
ohms.
RT-Durroid 5880 is used as the substrate for the circular patch
microstrip phase array antenna.
This substrate is used worldwide for the design of the
microstrip phase array antenna. The
thickness of the substrate is very important in the design of
the microstrip antenna because the
beamwidth changes with the thickness of the substrate [16, 18,
19]. For a 10 GHz resonating
frequency, the thickness of the substrate is 0.787 mm while the
thickness of the circular patches
is 17.8 m. The thickness of the patch should not exceed the
thickness of the substrate.
The microstrip phase array antenna was simulated after design in
ADS Momentum. Following
were the different simulation results of the proposed
design.
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45
The 3D view of the design simulated by the ADS Momentum is shown
in Fig. 33.
Figure 33: 3D view of circular patch microstrip phase array
antenna.
4.5.1 Directivity and Gain
ADS Momentum provides both the 2D and 3D graphs of the gain and
directivity of the
microstrip phase array antenna. The 2D graphs are shown in Fig.
34.
Figure 34: Gain and Directivity graphs of circular patch
microstrip phase array antenna.
From Fig. 34, it is clear that the magnitude of the gain and
directivity of the system is
approximately 5 dB. Maximum results are obtained from 60 to
120.
The 3D graph of the radiation pattern of the designed antenna is
shown in Fig. 35.
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46
Figure 35: 3D Directivity of circular patch microstrip phase
array antenna.
Fig. 35 shows an accurate three dimensional graph of the
radiation pattern of the microstrip
phase array antenna. The antenna is radiating broadside i.e.
Perpendicular to the axis of the
patch. The main beam is sharp in between 90 and 270. This graph
shows that the design is
working well and that it has achieved the desired results.
A more advanced 3D curve is obtained from the ADS Momentum using
a feature called the
Electro Magnetic Design Solver showing a different presentation
method of the 3D curves. The
Graph obtained by EMDS is shown in Fig. 36.
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47
Figure 36: 3D radiation pattern shown by EMDS.
EMDS in Fig. 36 shows main lobes on 90 and 270 with nulls at 0
and 180 .
4.5.2 S11 Parameters
As we have used only one probe feed in our design, we will find
only the input reflection
coefficients or the S11 parameters of the micro strip phase
array antenna. The simulation results
of the ADS Momentum are shown in Fig. 37.
Figure 37: Magnitude vs Frequency graph of S11 parameter.
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48
Figure 38: Phase vs Frequency graphs of S11 parameter.
Figure 39: S11 parameter plotted on the Smith chart.
Figures 37, 38, and 39 give us clear information about the
performance of our design. In Fig. 37
we can see that the designed antenna resonates at the desired
frequency which is 10 GHz. At the
resonant frequency the input reflection coefficient has the
minimum magnitude which is about
-16 dB.
In Fig. 39, the input reflection coefficient is shown on the
Smith chart where the marker clearly
indicates that the microstrip phase array antenna resonates at
10 GHz having the minimum
impedance over the straight resistance line at the resonating
frequency. The graphs in the three
figures verify the performance of the designed antenna to a
great extent.
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49
The efficiency of this antenna is shown in the Fig. 40. The
efficiency of array antenna is far
better than the efficiency of single circular patch antenna. The
efficiency of the array antenna is
90% while the efficiency of single circular patch antenna was
60%.
Figure 40: Efficiency of the circular patch microstrip phase
array antenna.
The power radiated by the designed antenna is shown in Fig.
41.
Figure 41: Radiated power of the circular patch microstrip phase
array antenna.
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50
5. Conclusion
5.1 Conclusion Summary
All the simulation results show that the microstrip phase array
antenna performs better then the
single circular patch or the single rectangular patch antenna.
The radiation pattern of the
microstrip array antenna is far better than the single circular
or the single rectangular patch. The
main beam is in the broadside direction between 90 and 270 with
nulls at 0 and 180. Similarly
the efficiency, directivity and gain of the array patch antenna
is better than the single patch. All
these simulations lead to the conclusion that the number of
patches in an array is directly
proportional to the efficiency, directivity and gain of the
antenna. If we increase the number of
elements in the array, the radiation pattern will improve
further.
Another important conclusion that was deduced from all the
experiments that were made during
the design of this antenna was that impedance matching is very
important. Effic ient results were
only obtained when the impedance of the system was perfectly
matched to 50 .
5.2 Future work
Antenna technology is a vast field. Every day new research is
published. A few design
parameters were taken into consideration while designing this
antenna. Further improvements
can be made in the following areas.
The gain, directivity, radiation pattern and efficiency can be
improved by using 2n array
elements in the microstrip phase array antenna. We have used
only two circular patches.
The beam of the circular patch phase array antenna can be
steered using phase shifters in
the design.
Recent research involves the use of photonic band gap crystals
in the substrate to improve
the band width of the antenna.
Instead of the ADS Momentum, the HFSS simulator can be used for
design and
simulation.
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51
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