rectangulat patch antenna
DESCRIPTION
Design of rectangular patch antenna and parametersTRANSCRIPT
ABSTRACT
With the modern Electronic wireless communication and Radar development, an antenna is a critical item. Every electronic equipment either transmits or receivers or both, an antenna is indispensible.
Hence we propose to design an aero space Micro strip Antenna in s-band for telemetry or commanding application by using conventional and classical method for the design of dimension for rectangular type. This will be fabricated by using printed circuit technique. A micro strip structure is structured by using a dielectric material at the top and bottom. Bottom is used as ground plane and top is etched by standard techniques. We will fabricating this by taking the mask ant he screen printing will done. We propose to feed the antenna by using the available coaxial RF connector for input/output of SMA type.
This will be tested after connecting the connector for its electrical characteristics like VSWR, input impedance, Return losses on a smith chart. After satisfying and iterating for correct frequency, we will finalize the feed point location.
We propose to design a VSWR of 2:1 ratio and return loss of more than or equal to 15db at a particular frequency in s-band and hence we get the antenna band width from this measurement. We propose to carry out the antenna radiation to find E & H plane beam width of antenna pattern coverage and gain.
Since the above design is only for a particular frequency, for a band of frequencies or at any frequency a software program with mat lab is complied for various thickness and various dielectric.
Finally results will be compared with expected, measured and simulate one and their variations and analyzed.
INDEX
ABSTRACT Page no:
LIST OF SYMBOLS
LIST OF FIGURES
LIST OF TABLES
CHAPTER–1 INTRODUCTION
1.1 Introduction 1
1.2 Definition Of Antena 1
1.3 Origin of Antenna 2
1.4 History of Antenna Technology 3
1.5 Basic Antenna Characteristics 4
1.5.1 Radiation pattern 4
1.5.2 Gain 4
1.5.3 Directivity 5
1.5.4 Polarization 5
1.5.5 VSWR 5
1.5.6 Reflection Coefficient and Return Loss 6
1.5.7 Bandwidth 6
1.5.8 Beam width 6
1.6 Types of Antenna 7
1.7 Aim and Objective of the Project 7
CHAPTER– 2 OVERVIEW OF THE MICROSTRIP ANTENNA
2.1 History Of Micro Strip Antenna 9
2.2 Definition Of Micro Strip Antenna 9
2.3 Advantages And Disadvantages 10
2.4 Radiation Mehcanism 11
2.5 Various Micro Strip Antenna Configurations 13
2.5.1 Micro strip patch antenna 13
2.5.2 Micro strip or Printed Dipole Antenna 15
CHAPTER–3 HARDWARE IMPLIMENTATION OF PROJECT
3.1 Introduction 16
3.2 Basic Principles of Operation 17
3.3 Resonant Frequency 18
3.4 Radiation Patterns 18
3.5 Radiation Efficiency 20
3.6 Bandwidth 22
3.7 Input Impedance 23
3.8 Feed Techniques 24
3.8.1 Micro strip Line Feed 24
3.8.2 Coaxial Feed 25
3.8.3 Aperture Coupled Feed 26
3.8.4 Proximity Coupled Feed 27
3.9 Methods of Analysis 29
3.9.1 Analytical Models 29
3.9.2 Transmission Line Model 31
CHAPTER – 4 IMPLEMENTATION & FABRICATION
4.1 Selection of Substrate 35
4.2 Design procedure for Rectangular Micro strip Antenna 38
4.2.1 Considered Values 38
4.2.2 Initial Design Values 38
4.3 Microwave Co-axial Connector 44
4.4 Fabrication Procedure 47
4.5 Step By Step Design Procedure 49
CHAPTER-5 MEASUREMENTS, TESTING & RESULT ANALYSIS
5.1 MESUREMENTS 53
5.2 TESTING 53
5.2.1 Network Analyzer 53
5.2.2 Elements of Network Analyzer 54
5.2.3 Reflection Measurement 58
5.2.4 RADIATION PATTERN MEASUREMENTS 63
5.2.5 Gain Measurement 67
5.3 ANALYSIS 69
CHAPTER-6 CONCLUSIONS 71
CHAPTER-7 FUTURE SCOPE 72
CHAPTER-8 BIBLOGRAPHY 73
CHAPTER-9 MAT LAB PROGRAM 75
LIST OF SYMBOLS
B Band Width of the Micro Strip Antenna
C Velocity of Antenna
E Electric Field Vector
I Feed Current
K Magnetic Current Line Source
L Length of Rectangular Patch
S Voltage Standing Wave Ratio
V Feed Voltage
W Width of Rectangular Patch
Z Input Impedance
εr Relative Di-electric Constant
εeff Effective Di-electric Constant
Vp Phase Velocity
Rr Radiation Resistance
Fr Resonant Frequency
Λo Free Space Wavelength
∆ Skin Depth
δ Loss Tangent
Λ Wavelength in Di-electric Substrate
η Free Space Impedance
σ Conductivity of Metal
µo Permeability of Free Space
εo Permittivity of Free Space
R, θ, Ф Spherical Co-ordinates
Rin Input Resistance
Q, Qt Total Q Factor
Qs Associated Q Factor of the Surface Wave Loss
Qr Associated Q Factor of the Radiation Loss
Qd Associated Q Factor of the Di-electric Loss
Qc Associated Q Factor of the Conductor Loss
Psw Power Lost in Surface Wave Generation
Prad, P Power Radiated
Ko Wave Number
Js Electric Current Vector
h Thickness of Substrate
LIST OF FIGURES
Figure 1.1 Schematic of an Antenna System
Figure 1.2 Electromagnetic spectrum
Figure 2.1 Structure of a Microstrip Patch Antenna
Figure 2.2 Electric field distributions in microstrip cavity
Figure 2.3 Charge distribution and current density on a microstrip antenna
Figure 2.4 Microstrip patch antenna shapes commonly used in practice
Figure 2.5 Other possible geometries of Microstrip patches
Figure 3.1 Rectangular Patch Antennas
Figure 3.2 Circular Patch Antennas
Figure 3.3 Electric & Magnetic Current Distributions
Figure 3.4 Simulated Radiation Pattern (E & H plane) polar plot
Figure 3.5 Radiation Efficiency for a rectangular patch Antenna
Figure 3.6 Calculated & Measured Bandwidth
Figure 3.7 Equivalent Circuit of Patch Antenna
Figure 3.8 Microstrip Line Feed
Figure 3.9 Probe fed Rectangular Microstrip Patch Antenna
Figure 3.10 Aperture-coupled feed
Figure 3.11 Proximity-coupled Feed
Figure 3.12 Microstrip Line
Figure 3.13 Electric Field Lines
Figure 3.14 Microstrip Patch Antenna proximity feed
Figure 3.15 Top View of Antenna
Figure 3.16 Side View of Antenna
Figure 4.1 Variation of Width with Frequency
Figure 4.2 Variation of Length with the Frequency
Figure 4.3 Variation of Gain with the Frequency
Figure 4.4 Variation of Bandwidth with Frequency for different dielectric substrate antennas
Figure 4.5 APC-7 Connector
Figure 4.6 BNC Connector
Figure 4.7 SMA Connector
Figure 4.8 SMC Connector
Figure 4.9 TNC Connector
Figure 4.10 Type N Connector
Figure 4.11 Flow chart showing the fabrication process
Figure 4.12 Photographic Negative of ground plane Used for Fabrication
Figure 4.13 Photographic Negative of patch Used for Fabrication
Figure 5.1 Major elements of Network Analyzer
Figure 5.2 Vector Network Analyzer used for testing of our antenna
Figure 5.3 Plot of our antenna Return Loss measurement for resonant frequency
Figure 5.4 Plot of our antenna SWR for resonant frequency
Figure 5.5 Plot of our antenna Impedance on a Smith Chart
Figure 5.6 Experimental Set Up For Plotting Radiation Pattern
Figure 5.7 Anechoic Chambers with Free Space Environment
Figure 5.8 Anechoic Chamber when Our Antenna is being Tested
Figure 5.9 Plot of Our antenna Radiation pattern in E and H plane
Figure 5.10 Bottom (ground plane) view of Our Antenna
Figure 5.11 Top view (patch) of Our Antenna
LIST OF TABLES
Table 3.1 Characteristics of the Different Feed Techniques
Table 4.1 Thickness of Cladding for Different Materials
Table 4.2 Dielectric and Loss Tangent for Different Materials
Table 4.3 Basic Features of the Most Common Connector Series
Table 5.1 Specifications of Network Analyzer
Table 5.2 Gain Measurement
Table 5.3 Comparison of calculated and measured values
CHAPTER 1
INTRODUCTION
1.1 Introduction
In high performance aircrafts, spacecrafts, satellites, missiles and other aerospace
applications where size, weight, performance, ease of installation and aerodynamics
profile are the constraints, a low or flat/conformal profile antenna may be required. In
recent years various types of flat profile printed antennas have been developed such as
Microstrip antenna (MSA), strip line, slot antenna, cavity backed printed antenna and
printed dipole antenna. When the characteristics of these antenna types are compared, the
micro strip antenna is found to be more advantageous.
Microstrip antenna are conformable to planar or non planar surface, simple and
inexpensive to manufacture, cost effective compatible with Monolithic Microwave
Integrated Circuits (MMIC) designs and when a particular patch shape like rectangular,
circular, triangular etc., And excitation modes like TM01, TM10 are selected; they are very
versatile in terms of resonant frequency, polarization, radiation patterns and impedance.
In this Project work Design, Fabrication and Testing of linear polarized co-axial
fed microstrip rectangular patch antenna in S-band at 2250 for aerospace applications is
presented. Microstrip antennas have several advantages compared to conventional
microwave antennas and therefore have many applications over the broad frequency
range from 100MHz to 50GHz.
1.2 Definition of Antenna
An antenna (or aerial) is a transducer designed to transmit or receive
electromagnetic waves. In other words, antennas convert electromagnetic waves into
electrical currents and vice-versa. They are used with waves in the radio part of the
electromagnetic spectrum, that is, radio waves, and are a necessary part of all radio
equipment. Antenna has many uses: communication, radar, telemetry, navigation etc.
Figure 1.1 shows the output from a coherent source (e.g. an oscillator) is directed
out into free space using an antenna. The signal source is linked to the antenna by some
kind of transmission line (like open wire), co-axial cable, waveguide, strip lines and
microstrip lines.
The antenna acts as a sort of impedance (50ohm of transmitter impedance to free
space impedance of 377ohm vice-versa) transformer. It takes the electromagnetic field
pattern, moving along the guide and transforms it into some other pattern, which is
radiated out into free space.
Figure 1.1 Schematic of an antenna system
Using this simple picture one can establish two basic properties of any antenna:
Firstly, the antenna doesn't itself generate any power. So, unless the antenna is
imperfect and dissipates some power, the total powers carried by the guide and
free space fields must be the same. (In reality, all practical antennas tend to be
slightly resistive so some power is normally lost, but for now one can assume any
loss is small enough to ignore.)
Secondly, the antenna is a reciprocal device — i.e. it behaves in the same way
irrespective of which way it pass signal power through it. This reciprocal behavior
is a useful feature of a coherent antenna. It means that, in principle, the only real
difference between a ‘transmitting’ and a ‘receiving’ antenna is the direction one
has chosen to pass signals through it.
1.3 Origin of Antennas
Communication is the process of transferring information from one entity to
another. Communication has existed since the beginning of human beings, but it was not
until the 20th century that people began to study the process. At first this was achieved by
sound through voice. As the distance of communicating increased, various devices were
introduced, such as drums, horns and so forth and for even greater distances visual
methods were introduced such as signal flags and smoke signals in the daytime and
fireworks at night. These optical communication devices, of course, utilize the light
portion of electromagnetic spectrum. It has only been recently in human history that the
electromagnetic spectrum outside the visible region has been employed for
communication, through the use of radio.
Figure 1.2 Electromagnetic spectrum
The antenna is an essential component in any radio system which provides a
means for radiating or receiving radio waves that is it provides a transition from a guided
wave on a transmission line to a free-space wave.
1.4 History of Antenna Technology
The theoretical foundations for antennas rest on Maxwell’s equations. James
Clark Maxwell in 1864 presented his results before Royal Society, which showed that
light and electromagnetics were one in physical phenomenon and also predicted that light
and electromagnetic disturbances both can be explained by waves travelling at the same
speed. And in 1886 Heinrich Hertz verified the above and discovered that the electrical
disturbances could detected with a secondary circuit of proper dimensions for resonance
and containing an air gap for sparks to occur.
Guglielmo Marconi built a microwave parabolic cylinder at a wavelength of 25
cm for his original code transmission and worked at longer wavelengths for improved
communication range. Marconi is considered as the father of amateur radio. Antenna
developments in the early years were limited by the availability signal generators. About
1920 resonant length antennas were possible after the De Forest triode tube was used to
produce continuous wave signals up to 1MHz.
At these higher frequencies antennas could be built with a physical size in
resonant region. Just before World War II microwave (about 1 GHz) klystron and
magnetron signal generators were developed along with hollow pipe waveguides. These
lead to the development of horn antennas, although Jagadish Chandra Bose in India
produced the first electromagnetic horn antenna many years earlier. The first commercial
microwave radiotelephone system in 1934 was between England and France and operated
at 1.8G Hz. During the war an intensive development effort primarily detected toward
radar, spawned many modern antenna types, such as large reflectors, lenses and
waveguide slot arrays.
1.5 Basic Antenna Characteristics
An antenna is a structure that is made to efficiently radiate and receive radiated
electromagnetic waves. There are several important antenna characteristics that should be
considered when choosing an antenna for application such as Gain, radiation pattern,
bandwidth, beam width etc., are as follows:
1.5.1 Radiation pattern
Practically any antenna cannot radiate energy with same strength uniformly in all
directions. The radiation from antenna in any direction is measured in terms of field
strength at a point located at a particular distance from antenna. Radiation pattern of an
antenna indicates the distribution of energy radiated by the antenna in the free space. In
general radiation pattern is a graph which shows the variation of actual field strength of
electromagnetic field of all the points equidistant from antenna. The two basic radiation
patterns are field strength radiation pattern which is expressed in terms of field strength
E (in V/m) and power radiation pattern expressed in terms of power per unit solid angle.
Field radiation pattern is a 3-dimensional pattern. To achieve this it requires
representing the radiation for all angles of Φ and θ which give E-plane (vertical plane)
and H-plane (horizontal plane) pattern respectively.
1.5.2 Gain
Antenna gain relates the intensity of an antenna in a given direction to the
intensity that would be produced by a hypothetical ideal antenna that radiates equally in
all directions (isotropically) and has no losses. Since the radiation intensity from a
lossless isotropic antenna equals the power into the antenna divided by a solid angle of 4π
steridians, we can write the following equation:
Gain = 4π * Radiation Intensity/Antenna Input Power
1.5.3 Directivity
The directive gain of the antenna is the measure of the concentration of radiated
power in a particular direction. It may be regarded as the ability of the antenna to direct
radiated power in a given direction. It is usually a ratio of radiation intensity in a given
direction to the average radiation intensity. Generally D > 1, except in the case of an
isotropic antenna for which D = 1. An antenna with directivity D >> 1 is directive
antenna.
1.5.4 Polarization
Polarization is the orientation of the electromagnetic waves far from the source.
There are several types of polarization that apply to antennas. They are Linear (which
comprises vertical and horizontal), oblique, Elliptical (left hand and right hand
polarizations), circular (left hand and right hand) polarizations.
1.5.5 VSWR
VSWR is the ratio of the maximum to minimum values of the “voltage standing
wave" pattern that is created when signals are reflected on a transmission line. This
measurement can be taken using a "slotted line" apparatus that allows the user to measure
the field strength in a transmission line at different distances along the line.
The voltage standing wave ratio is a measure of how well a load is impedance-
matched to a source. The value of VSWR is always expressed as a ratio with 1 in the
denominator (2:1, 3:1, etc.) It is a scalar measurement only (no angle), so although they
reflect waves oppositely, a short circuit and an open circuit have the same VSWR value
(infinity:1). A perfect impedance match corresponds to a VSWR 1:1, but in practice you
will never achieve it. Impedance matching means you will get maximum power transfer
from source to load.
1.5.6 Reflection Coefficient and Return Loss
Reflection coefficient shows what fraction of an incident signal is reflected when
a source drives a load. A reflection coefficient magnitude of zero is a perfect match, a
value of one is perfect reflection. The symbol for reflection coefficient is uppercase
Greek letter gamma (Γ). Note that the reflection coefficient is a vector, so it includes an
angle. Unlike VSWR, the reflection coefficient can distinguish between short and open
circuits. A short circuit has a value of -1 (1 at an angle of 180 degrees), while an open
circuit is one at an angle of 0 degrees. Quite often we refer to only the magnitude of the
reflection coefficient.
Return Loss shows the level of the reflected signal with respect to the incident
signal in dB. The negative sign is dropped from the return loss value, so a large value for
return loss indicates a small reflected signal. The return loss of a load is merely the
magnitude of the reflection coefficient expressed in decibels. The correct equation for
return loss is:
Return loss = -20 x log [mag (Γ)]
1.5.7 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
specific standard.
The reason for this qualitative definition is that all the antenna parameters are
changed with frequency and the importance of the different parameters as gain, return
loss, beam width, side-lobe level etc., much depends on the frequency band.
The bandwidth of an antenna for gain (-3dB from the maximum) is defined as
Bandwidth (%) = (fv-fl)*100 fc
Where fv is the upper frequency, fl is the lower frequency, and fc is the centre
frequency.
1.5.8 Beam width
Antenna beam width is defined as the angle between half power point on the main
beam. In case that we have a logarithm radiation power pattern in [dB] units, it means
that we measure the angle between two 3dB points.
1.6 Types of Antennas
There are two fundamental types of antenna directional patterns, which, with
reference to a specific two dimensional plane (usually horizontal [parallel to the ground]
or [vertical perpendicular to the ground]), are either:
1. Omni-directional (radiates equally in all directions), such as a vertical rod (in the
horizontal plane) or
2. Directional (radiates more in one direction than in the other).
In colloquial usage "omni directional" usually refers to all horizontal directions
with reception above and below the antenna being reduced in favor of better reception
near the horizon. A directional antenna usually refers to one focusing a narrow beam in a
single specific direction such as a telescope or satellite dish, or, at least, focusing in a
sector such as a 120° horizontal fan pattern in the case of a panel antenna at a cell site.
The present antenna in this project i.e., Microstrip antenna is an antenna which
radiates normal to the patch surface into the hemisphere (180° in elevation plane).
1.7 Aim and Objective of the Project:
1. The main aim of the project is to design an aerospace wide beam width
rectangular micro strip antenna for an aerospace vehicle such as a missile,
satellite, aircraft etc., by using available Microstrip substrate (printed circuit board
of type FR4 with dielectric constant of 4.4 and loss tangent of 0.02 and thickness
of 1.6mmof double clad copper), calculated the dimensions of the patch W
(width) and L (length) and also theoretically calculated the antenna bandwidth for
VSWR of ≤2:1 at a frequency of 2250MHz in s-band (2-4 GHz) frequency. And
then we calculated the 3-dB beam width in principle E-plane and H-plane.
2. The Micro strip antenna is carried out for fabrication by using the AutoCAD
software on a PC of size 13.5cm × 13.5cm and h=1.6mm (thickness). The
fabrication process has been done with help of M/s Sravanthi Electronic Industry
by using the standard PCB techniques. After the fabrication, the feed point at 1/3
of half distance is drilled with 1.3mm hole and then connected sub miniature type-
A (SMA) female RF connector of type radial R12540300 with the centre
conductor of diameter 1.28mm. This has been soldered on the Microstrip patch at
a point where 50Ω’s impedance is achieved at 2250MHz. The ground plane is
also soldered with the outer conductor of coaxial connector.
3. Then the centre conductor is checked, to not have short circuit with the ground
plane by an ohm meter and it is found that there was no short circuit. The antenna
has been tested by using an automatic vector network analyzer of type R&S ZVL
at M/s Advanced Communication Division, Charlapally, Hyderabad, a sister
concern of Advanced Radio Mast (ARM). The test has been conducted for the
following:
1. VSWR
2. Return Loss
3. Impedance by smith chart
4. Radiation pattern in E-plan and H-plane
5. Gain
CHAPTER 2
OVERVIEW OF MICROSTRIP ANTENNA
2.1 History of Microstrip Antenna
The concept of microstrip radiators was first proposed by Deschamps as early as
1953. The first practical antennas were developed in the early 1970’s by Howell and
Munson. Since then, extensive research and development of microstrip antennas and
arrays, exploiting the new advantages such as light weight, low volume, low cost, low
cost, compatible with integrated circuits, etc., have led to the diversified applications and
to the establishment of the topic as a separate entity within the broad field of microwave
antennas.
2.2 Definition of Microstrip Antenna
A microstrip antenna in its simplest configuration consists of a radiating patch on
one side of a dielectric substrate (εr ≤ 10), which has a ground plane on the other side.
The patch conductors, normally of copper and gold, can assume virtually any shape, but
conventional shapes are generally used to simplify analysis and performance prediction.
A patch antenna is a narrowband, wide-beam antenna. Feeding in microstrip is achieved
through use of coaxial line with an inner conductor that terminates on the patch. The
placement of the feed is important for proper operation of the antenna.
Figure 2.1 Structure of a Microstrip Patch Antenna
2.3 Advantages and Disadvantages of Microstrip Antenna
Microstrip antennas have several advantages compared to conventional
microwave antennas and therefore many applications over the broad frequency range
from 100MHz to 50GHz. Some of the principle advantages are:
Light weight, low volume, low profile planar configurations which can be made
conformal:
Low fabrication cost ; readily amenable to mass production;
Can be made thin ; hence, they do not perturb the aerodynamics of host aerospace
vehicles;
The antennas can be easily mounted on missiles, rockets and satellites without
major alterations;
These antennas have low scattering cross section;
Linear, circular (left hand or right hand) polarizations are possible with simple
changes in the feed positions;
Dual frequency and dual polarization antennas can be easily made;
No cavity backing required;
Can be easily integrated with microwave integrated circuits;
Microstrip antennas are compatible with modular designs (solid state devices such
as oscillators, amplifiers, variable attenuators, switches, modulators, mixers etc.
can be added directly to the antenna substrate board);
Feed lines and matching networks are fabricated simultaneously with the antenna
structure;
However, Microstrip antennas also have some disadvantages compared to
conventional microwave antennas are:
Narrow bandwidth and associated tolerance problems;
Loss, hence somewhat lower gain(~ 6dB);
Large ohmic loss in the feed structure of arrays;
Complex feed structure required for high performance arrays;
Polarization purity is difficult to achieve;
Extraneous radiation from feeds and junctions;
Low power handling capability
Excitation of surface waves
Reduced gain and efficiency as well as unacceptably high levels of cross-
polarization and mutual coupling within an array element at high frequencies
There are ways to minimize the effect of some of the limitations. For example,
bandwidth can be increased to more than 60%by usage of special techniques;
lower gain and lower power handling limitations can be overcome through an
array configuration;
surface wave associated limitations poor efficiency, increased mutual coupling,
reduced gain and radiation pattern degradation can be overcome by the use of
photonic band gap structures;
2.4 Radiation Mechanism of Microstrip Antenna
The radiation from a Microstrip line, a structure similar to Microstrip antenna,
can be reduced considerably if the substrate employed is thin and has a higher relative
dielectric constant. Radiation from Microstrip antenna, on the other hand, is encouraged
for better radiation efficiency. Therefore, thick substrates with low permittivity are used
in Microstrip antennas. Radiation from Microstrip antenna can be determined from the
field distribution between patch metallization and the ground plane.
Alternatively, radiation pattern can be described in terms of surface current
distribution on the patch metallization. An accurate calculation of the field or current
distribution of the patch is very complicated. However, crude approximations and simple
arguments can be used to develop a workable model for a Microstrip antenna. Consider a
Microstrip antenna that has been connected to a microwave source. The energization of
the patch will establish a charge distribution on upper and lower surfaces of the patch, as
well as on the surface of the ground plane as shown in figure below:
Figure 2.2 Electric field distributions in microstrip cavity
The –ve and +ve nature of the charge distribution arises because the patch is about a half-
wave long at the dominant mode. The repulsive forces between like charges on the
bottom surface, around its edges, to its top surface.
This movement of charge creates corresponding current densities and at the
bottom and top surface of the patch as shown in figure below:
Figure 2.3 Charge distribution and current density on a microstrip antenna
For most microstrip antennas, the ratio h/W is very small. Therefore, the attractive
force between the charges dominates and most of the charge concentration and the
current flow remain underneath the patch. A small amount of current flows around the
edges the edges of the patch to its top surface and are responsible for weak magnetic field
tangential to the edges. Hence, we can make a simple approximation that the magnetic
field is zero and one can place magnetic walls all around the periphery of the patch. This
assumption has the greater validity for thin substrates with high εr. Also, since the
substrate used is very thin compared to the wavelength (h<<λ) in the dielectric, the field
variations along the height can be considered to be constant and electric field nearly
normal to the surface of the patch.
Consequently, the patch can be modeled as a cavity with electric walls (because
the electric field is near normal to the patch surface) at the top and below and four
magnetic walls along the edges of the patch (because the tangential magnetic field is very
weak). Only TM modes are possible in this cavity.
2.5 Various Micro strip Antenna Configurations:
Microstrip antennas are characterized by large number of physical parameters
than are conventional microstrip antennas. They can be designed to have many
geometrical shapes and dimensions. All Microstrip antennas can be divided into four
basic categories:
1. Microstrip patch antennas
2. Microstrip dipoles
3. Printed slot antennas
4. Microstrip travelling-wave antennas.
2.5.1 Microstrip patch antenna
A Microstrip patch antenna (MPA) consists of a conducting patch of any planar
geometry on one side of dielectric substrate backed by a ground plane on the other side.
There are virtually an unlimited number of patch patterns for which radiation
characteristics may be calculated. The basic configurations used in practice are:
Figure 2.4 Microstrip patch antenna shapes commonly used in practice
Figure 2.5 Other possible geometries of Microstrip patches
2.5.2 Microstrip or Printed Dipole Antennas
Microstrip or printed dipole differs geometrically from rectangular patch antennas
in their length-to-width ratio. The width of a dipole is typically less than 0.05λo. The
radiation patterns of the dipole and patch are similar owing to similar longitudinal current
distributions. However, the radiation resistance, bandwidth, and cross-polar radiation
differ widely. These are well suited for higher frequencies for which the substrate can be
electrically thick and therefore can attain significant bandwidth.
CHAPTER 3
RECTANGULAR PATCH ANTENNA
3.1 Introduction
Microstrip antennas are among the most widely used types of antennas in the
microwave frequency range, and they are often used in the millimeter-wave frequency
range as well (Below approximately 1 GHz, the size of a microstrip antenna is usually too
large to be practical, and other types of antennas such as wire antennas dominate) also
called patch antennas. Microstrip patch antennas consist of a metallic patch of metal that
is on top of a grounded dielectric substrate of thickness h, with relative permittivity and
permeability εr and μr as shown in Figure 3.1 (usually μr = 1). The metallic patch may be
of various shapes, with rectangular and circular being the most common, as shown in
Figure 3.1& 3.2.
Figure 3.1 Rectangular Patch Antennas
Figure 3.2 Circular Patch Antennas
Most of the discussion in this section will be limited to the rectangular patch,
although the basic principles are the same for the circular patch. (Many of the CAD
formulas presented will apply approximately for the circular patch if the circular patch is
modeled as a square patch of the same area). Various methods may be used to feed the
patch, as discussed below. One advantage of the microstrip antenna is that it is usually
low profile, in the sense that the substrate is fairly thin.
If the substrate is thin enough, the antenna actually becomes “conformal,”
meaning that the substrate can be bent to conform to a curved surface (e.g., a cylindrical
structure). A typical substrate thickness is about 0.02 λ0. The metallic patch is usually
fabricated by a photolithographic etching process or a mechanical milling process,
making the construction relatively easy and inexpensive (the cost is mainly that of the
substrate material).
Other advantages include the fact that the microstrip antenna is usually
lightweight (for thin substrates) and durable. Disadvantages of the microstrip antenna
include the fact that it is usually narrowband, with bandwidths of a few percent being
typical. Some methods for enhancing bandwidth are discussed later. Also, the radiation
efficiency of the patch antenna tends to be lower than some other types of antennas, with
efficiencies between 70% and 90% being typical.
3.2 Basic Principles of Operation
The metallic patch essentially creates a resonant cavity, where the patch is the top
of the cavity, the ground plane is the bottom of the cavity, and the edges of the patch
form the sides of the cavity. The edges of the patch act approximately as an open-circuit
boundary condition. Hence, the patch acts approximately as a cavity with perfect electric
conductor on the top and bottom surfaces, and a perfect “magnetic conductor” on the
sides. This point of view is very useful in analyzing the patch antenna, as well as in
understanding its behavior. Inside the patch cavity the electric field is essentially z
directed and independent of the z coordinate. Hence, the patch cavity modes are
described by a double index (m, n). For the (m, n) cavity mode of the rectangular patch
the electric field has the form
…………………..(3.1)
Where L is the patch length and W is the patch width. The patch is usually
operated in the(1,0) mode, so that L is the resonant dimension, and the field is essentially
constant in the y direction.
3.3 Resonant Frequency
The resonance frequency for the (1, 0) mode is given by
…………………………..……………….(3.2)
Where c is the speed of light in vacuum. To account for the fringing of the cavity
fields at the edges of the patch, the length, the effective length Le is chosen as
Le= L + 2ΔL
The Hammerstad formula for the fringing extension is [1]
………………………..(3.3)
Where,
………………………(3.4
3.4 Radiation Patterns
The radiation field of the microstrip antenna may be determined using either an
“electric current model” or a “magnetic current model”. In the electric current model, the
current is used directly to find the far-field radiation pattern. The electric current for the
(1, 0) patch mode. If the substrate is neglected (replaced by air) for the calculation of the
radiation pattern, the pattern may be found directly from image theory. If the substrate is
accounted for, and is assumed infinite, the reciprocity method may be used to determine
the far-field pattern.
(a)Electric Current for (1, 0) patch
(b) Magnetic Current for (1, 0) patch
Figure 3.3 Electric & Magnetic Current Distributions
In the magnetic current model, the equivalence principle is used to replace the
patch by a magnetic surface current that flows on the perimeter of the patch. The
magnetic surface current is given by:
……………………………..(3.5)
Where E is the electric field of the cavity mode at the edge of the patch and n is
the outward pointing unit-normal vector at the patch boundary. The far-field pattern may
once again be determined by image theory or reciprocity, depending on whether the
substrate is neglected or not [4]. The dominant part of the radiation field comes from the
“radiating edges” at x = 0 and x = L. The two non-radiating edges do not affect the pattern
in the principle planes (the E plane at φ = 0 and the H plane at φ = π/2), and have a small
effect for other planes.
It can be shown that the electric and magnetic current models yield exactly the
same result for the far-field pattern, provided the pattern of each current is calculated in
the presence of the substrate at the resonant frequency of the patch cavity mode [5]. If the
substrate is neglected, the agreement is only approximate, with the largest difference
being near the horizon.
The patch is resonant with W/ L = 1.5. Note that the E-plane pattern is broader
than the H-plane pattern.
Figure 3.4 Simulated Radiation Pattern (E & H plane) polar plot
3.5 Radiation Efficiency
The radiation efficiency of the patch antenna is affected not only by conductor
and dielectric losses, but also by surface-wave excitation - since the dominant TM mode
of the grounded substrate will be excited by the patch. As the substrate thickness
decreases, the effect of the conductor and dielectric losses becomes more severe, limiting
the efficiency. On the other hand, as the substrate thickness increases, the surface-wave
power increases, thus limiting the efficiency. Surface-wave excitation is undesirable for
other reasons as well, since surface waves contribute to mutual coupling between
elements in an array, and also cause undesirable edge diffraction at the edges of the
ground plane or substrate, which often contributes to distortions in the pattern and to back
radiation.
For an air (or foam) substrate there is no surface-wave excitation. In this case,
higher efficiency is obtained by making the substrate thicker, to minimize conductor and
dielectric losses (making the substrate too thick may lead to difficulty in matching,
however, as discussed above). For a substrate with a moderate relative permittivity such
as εr = 2.2, the efficiency will be maximum when the substrate thickness is approximately
λ0 = 0.02. The radiation efficiency is defined as
Where Psp is the power radiated into space, and the total input power Ptotal is given
as the sum of Pc - the power dissipated by conductor loss, Pd- the power dissipated by
dielectric loss, and Psw - the surface-wave power. The efficiency may also be expressed in
terms of the corresponding Q factors as
A plot of radiation efficiency for a resonant rectangular patch antenna with W / L
= 1.5 on a substrate of relative permittivity εr = 2.2 or εr = 10.8 is shown in Figure 3.4.
The result is plotted efficiency versus normalized (electrical) thickness of the substrate,
which does not involve frequency.
The conductivity of the copper patch and ground plane is assumed to be ζ =
3.0×107 [S/m] and the dielectric loss tangent is taken as tanδd = 0.001. The resonance
frequency is 5 GHz. However, a specified frequency is necessary to determine conductor
loss. For h / λ0 < 0.02, the conductor and dielectric losses dominate, while for h /λ0 >
0.02, the surface-wave losses dominate. (If there were no conductor or dielectric losses,
the efficiency would approach 100% as the substrate thickness approaches zero.
Figure 3.5 Radiation Efficiency for a rectangular patch Antenna
3.6 Bandwidth
The bandwidth increases as the substrate thickness increases (the bandwidth is
directly proportional to h if conductor, dielectric, and surface-wave losses are ignored).
However, increasing the substrate thickness lowers the Q of the cavity, which increases
spurious radiation from the feed, as well as from higher-order modes in the patch cavity.
Also, the patch typically becomes difficult to match as the substrate thickness increases
beyond a certain point (typically about 0.05 λ0). This is especially true when feeding with
a coaxial probe, since a thicker substrate results in a larger probe inductance appearing in
series with the patch impedance. However, in recent years considerable effort has been
spent to improve the bandwidth of the microstrip antenna, in part by using alternative
feeding schemes. The aperture-coupled feed of is one scheme that overcomes the
problem of probe inductance, at the cost of increased complexity.
Lowering the substrate permittivity also increases the bandwidth of the patch
antenna. However, this has the disadvantage of making the patch larger. Also, because of
the patch cavity is lowered, there will usually be increased radiation from higher-order
modes, degrading the polarization purity of the radiation.
By using a combination of aperture-coupled feeding and a low-permittivity foam
substrate, bandwidths exceeding 25% have been obtained. The use of stacked patches (a
parasitic patch located above the primary driven patch) can also be used to increase
bandwidth even further, by increasing the effective height of the structure and by creating
a double-tuned resonance effect.
Figure 3.6 Calculated & Measured Bandwidth
Figure 3.6 shows calculated and measured bandwidth for the same patch. It is
seen that bandwidth is improved by using a lower substrate permittivity, and by making
the substrate thicker.
3.7 Input Impedance
A variety of approximate models have been proposed for the calculation of input
impedance for a probe-fed patch. These include the transmission line method, the cavity
model, and the spectral-domain method. These models usually work well for thin
substrates, typically giving reliable results for h / λ0 < 0.02.
The cavity model has the advantage of allowing for a simple physical CAD model
of the patch to be developed, as shown in Figure 3.7
In this model the patch cavity is modeled as a parallel RLC circuit, while the
probe inductance is modeled as a series inductor. The input impedance of this circuit is
approximately described by
Figure 3.7 Equivalent Circuit of Patch Antenna
3.8 Feed Techniques
Microstrip patch antennas can be fed by a variety of methods. These methods can
be classified into two categories- contacting and non-contacting. In the contacting
method, the RF power is fed directly to the radiating patch using a connecting element
such as a microstrip line. In the non-contacting scheme, electromagnetic field coupling is
done to transfer power between the microstrip line and the radiating patch. The four most
popular feed techniques used are the microstrip line, coaxial probe (both contacting
schemes), aperture coupling and proximity coupling (both non-contacting schemes).
3.8.1 Microstrip Line Feed
In this type of feed technique, a conducting strip is connected directly to the edge
of the Microstrip patch as shown in Figure 3.8. The conducting strip is smaller in width
as compared to the patch and this kind of feed arrangement has the advantage that the
feed can be etched on the same substrate to provide a planar structure.
Figure 3.8 Microstrip Line Feed
The purpose of the inset cut in the patch is to match the impedance of the feed
line to the patch without the need for any additional matching element. This is achieved
by properly controlling the inset position. Hence this is an easy feeding scheme, since it
provides ease of fabrication and simplicity in modeling as well as impedance matching.
However as the thickness of the dielectric substrate being used, increases, surface waves
and spurious feed radiation also increases, which hampers the bandwidth of the antenna.
The feed radiation also leads to undesired cross polarized radiation.
3.8.2 Coaxial Feed
The Coaxial feed or probe feed is a very common technique used for feeding
Microstrip patch antennas. As seen from Figure 3.8, the inner conductor of the coaxial
connector extends through the dielectric and is soldered to the radiating patch, while the
outer conductor is connected to the ground plane. The main advantage of this type of
feeding scheme is that the feed can be placed at any desired location inside the patch in
order to match with its input impedance.
Figure 3.9 Probe fed Rectangular Microstrip Patch Antenna
This feed method is easy to fabricate and has low spurious radiation. However, a
major disadvantage is that it provides narrow bandwidth and is difficult to model since a
hole has to be drilled in the substrate and the connector protrudes outside the ground
plane, thus not making it completely planar for thick substrates (h > 0.02λo). Also, for
thicker substrates, the increased probe length makes the input impedance more inductive,
leading to matching problems. It is seen above that for a thick dielectric substrate, which
provides broad bandwidth, the microstrip line feed and the coaxial feed suffer from
numerous disadvantages. The non-contacting feed techniques which have been discussed
below, solve these issues.
3.8.3 Aperture Coupled Feed
In this type of feed technique, the radiating patch and the microstrip feed line are
separated by the ground plane as shown in Figure 3.9. Coupling between the patch and
the feed line is made through a slot or an aperture in the ground plane. The coupling
aperture is usually centered under the patch, leading to lower cross-polarization due to
symmetry of the configuration. The amount of coupling from the feed line to the patch is
determined by the shape, size and location of the aperture
Figure 3.10 Aperture-coupled feed
. Since the ground plane separates the patch and the feed line, spurious radiation is
minimized. Generally, a high dielectric material is used for bottom substrate and a thick,
low dielectric constant material is used for the top substrate to optimize radiation from
the patch. The major disadvantage of this feed technique is that it is difficult to fabricate
due to multiple layers, which also increases the antenna thickness. This feeding scheme
also provides narrow bandwidth.
3.8.4 Proximity Coupled Feed
This type of feed technique is also called as the electromagnetic coupling scheme.
As shown in Figure 3.11, two dielectric substrates are used such that the feed line is
between the two substrates and the radiating patch is on top of the upper substrate. The
main advantage of this feed technique is that it eliminates spurious feed radiation and
provides very high bandwidth (as high as 13%) , due to overall increase in the thickness
of the microstrip patch antenna. This scheme also provides choices between two different
dielectric media, one for the patch and one for the feed line to optimize the individual
performances.
Figure 3.11 Proximity-coupled Feed
Matching can be achieved by controlling the length of the feed line and the width-
to-line ratio of the patch. The major disadvantage of this feed scheme is that it is difficult
to fabricate because of the two dielectric layers which need proper alignment. Also, there
is an increase in the overall thickness of the antenna.
Table 3.1 Characteristics of the different feed techniques.
3.9 Methods of Analysis
The preferred models for the analysis of Microstrip patch antennas are the
transmission line model, cavity model, and full wave model (which include primarily
integral equations/Moment Method). The transmission line model is the simplest of all
and it gives good physical insight but it is less accurate. The cavity model is more
accurate and gives good physical insight but is complex in nature. The full wave models
are extremely accurate, versatile and can treat single elements, finite and infinite arrays,
stacked elements, arbitrary shaped elements and coupling. These give less insight as
compared to the two models mentioned above and are far more complex in nature.
3.9.1 Analytical Models
There are many methods of analysis and are divided into two types-
1. Model – Based Analysis Technique
2. Full – Wave Analysis Technique
The various model – based and full – wave analysis techniques that have been used for the analysis of the Microstrip Antenna are:
Wire Grid Model Cavity Model
Modal Dispersion Model
Transmission Line Model
Integral Equation Method
Vector Potential Approach
Dyadic Green’s Function Technique
Radiating Aperture Method
In Wire Grid Model the antenna is modeled as a fine grid of wire segments. The
currents on the wire segments are solved using the Richmond’s reaction theorem to get
all the antenna characteristics of interest.
The Cavity Model offers both simplicity and physical insight. In this model the
antenna is treated as a cavity whose fields are computed using the full model expansions.
The importance of this model is that it includes the effects of non resonant modes.
The Modal Expansion Method is similar to cavity model but differs in
impedance boundary conditions that are imposed at the four radiating walls to obtain a
solution. Though the method does not lead to an exact solution, it provides a good insight
into the physics of antenna.
The Transmission Line Model considers the antenna as two radiating slots
perpendicular to the feed line of length L. This model is easy to analyze due to its
simplicity but suffers from some disadvantages. This model is limited to square and
rectangular geometries.
The Integral equation method is general method and can treat patches of
arbitrary shapes including those with thick substrate. The method requires considerable
analytical and computational efforts and provides little physical insight.
In Vector Potential Approach, the field produced by a horizontal electric dipole
is determined and the antenna characteristics are then evaluated by numerical techniques.
Though the solution obtained is rigorous, it is less attractive due to lack of closed form
expressions.
In Dyadic Green’s Function Method the characteristics of the micro strip
antenna are evaluated and the field from an arbitrary source distribution may be found by
means of a super position integral.
In Radiating aperture method the Vector Kirchoff relation is used. This method
is mathematically precise if the aperture fields are known exactly.
Transmission model is adapted in this work for the analysis of the rectangular
microstrip antennas and is explained in detail below.
3.9.2 Transmission Line Model
This model represents the microstrip antenna by two slots of width W and height
h, separated by a transmission line of length L. The microstrip is essentially a non-
homogeneous line of two dielectrics, typically the substrate and air. Hence, as seen from
Figure 3.12, most of the electric field lines reside in the substrate and parts of some lines
in air.
Figure 3.12 Microstrip Line Figure 3.13 Electric Field
Lines
As a result, this transmission line cannot support pure transverse-electromagnetic
(TEM) mode of transmission, since the phase velocities would be different in the air and
the substrate. Instead, the dominant mode of propagation would be the quasi-TEM mode.
Hence, an effective dielectric constant (εreff) must be obtained in order to account for the
fringing and the wave propagation in the line. The value of εreff is slightly less then εr
because the fringing fields around the periphery of the patch are not confined in the
dielectric substrate but are also spread in the air as shown in Figure 3.13 above. The
expression for εreff is given by Balanis:
Where εreff = Effective dielectric constant
εr = Dielectric constant of substrate
h = Height of dielectric substrate
W = Width of the patch
Consider Figure 3.14below, which shows a rectangular microstrip patch antenna
of length L, width W resting on a substrate of height h. The co-ordinate axis is selected
such that the length is along the x direction, width is along the y direction and the height
is along the z direction.
Figure 3.14 Microstrip Patch Antenna proximity feed
In order to operate in the fundamental TM10 mode, the length of the patch must be
slightly less than λ/2 where λ is the wavelength in the dielectric medium and is equal to
λo/√εreff where λo is the free space wavelength. The TM10 mode implies that the field
varies one λ/2 cycle along the length, and there is no variation along the width of the
patch. In the Figure 3.14 shown below, the microstrip patch antenna is represented by
two slots, separated by a transmission line of length L and open circuited at both the ends.
Along the width of the patch, the voltage is maximum and current is minimum due to the
open ends. The fields at the edges can be resolved into normal and tangential components
with respect to the ground plane.
Figure 3.15 Top View of Antenna Figure 3.16 Side View of
Antenna
It is seen from Figure 3.15 that the normal components of the electric field at the
two edges along the width are in opposite directions and thus out of phase since the patch
is λ/2 long and hence they cancel each other in the broadside direction. The tangential
components which are in phase, means that the resulting fields combine to give
maximum radiated field normal to the surface of the structure. Hence the edges along the
width can be represented as two radiating slots, which are λ/2 apart and excited in phase
and radiating in the half space above the ground plane. The fringing fields along the
width can be modeled as radiating slots and electrically the patch of the microstrip
antenna looks greater than its physical dimensions. The dimensions of the patch along its
length have now been extended on each end by a distance ΔL, which is given empirically
by Hammerstad:
The effective length of the patch is given by:
For a given resonance frequency fo, the effective length is given by:
For a rectangular Microstrip antenna, the resonant frequency for any TMmn is given by
James and Hall:
When m and n are modes along L and W respectively.
For efficient radiation, the width is given by:
CHAPTER 4
IMPLEMENTATION & FABRICATION
This chapter deals with the procedure for practically designing a rectangular
microstrip antenna. The overall goal of a design is to achieve specific performance
characteristics at a stipulated operating frequency. The design of a rectangular microstrip
antenna involves the following process:
1. Selection of substrate and
2. Calculating length, width and feed point of the patch
3. Selection of connector
4.1 Selection of Substrate
The selection of a substrate material is a balance between the required electrical,
mechanical and environmental performance required by a design versus economic
constraints. Generally, if one has the available design volume to use air as a substrate for
a Microstrip antenna, this is a good choice. The antenna efficiency is high, the gain is
maximized as is the impedance bandwidth of a conventional Microstrip antenna. The
surface wave loss when air is used as a substrate is minimal.
When a dielectric substrate is selected, one is interested in a material with the
lowest tangent (tan δ) available. The loss tangent is a metric of the quantity of electrical
energy which is converted to heat by a dielectric. The lowest possible loss tangent
maximizes the antenna efficiency (decreases the losses).
The relative dielectric constant εr of the substrate determines the physical size of a
patch antenna. The larger the dielectric constant the smaller the element size, but also the
smaller the impedance, bandwidth and directivity and the surface wave loss increases.
The use of the substrates with higher dielectric constants also tightens fabrication
tolerances. The tolerance of the dielectric value is also of significant importance in
manufacturing yield.
A Monte-Carlo type analysis using the cavity model is a good method of
estimating antenna manufacturing yield for a rectangular Microstrip antenna when an
etching tolerance, substrate thickness tolerance, feed point location tolerance and
dielectric tolerances are known.
Substrate electrical and physical parameters also vary with temperature. Recent
work by Kabacik and Bialkowski indicates that Teflon/Fiberglass substrates can have a
significant variation of dielectric constant for many airborne and space borne
applications. The dielectric constant and loss tangent of Teflon fiberglass often differed
from what was quoted by manufacturers in their datasheets compared with measurements
and were valid over a much narrower temperature range that encountered in many
aerospace applications. The performance variations are due to changes in the material
dielectric properties–thermal expansion had a minor effect on Microstrip antenna
performance.
Generally the metal cladding to the dielectric substrate material is copper. Two
types of copper foil are used as cladding, rolled foil and electrodeposited foil. Rolled foil
is passed through a rolling mill a number of times until the desired physical dimensions
are obtained and bonded the substrate. Rolled copper has a polished mirror-like
appearance. Electrodeposited foil is created by electrodeposition of copper onto an inert
form. A thin layer of copper is continuously removed from the form then bonded to the
substrate.
The computation of characteristic impedance and losses of a Microstrip
transmission line depend on the copper foil thickness. The copper cladding is described in
terms of weight per square yard.
The thickness of the cladding may then be derived and is listed in the table below:
Foil weight Foil thickness
½ oz (14gms) 0.0007 in (0.01778mm)
1 oz (28gms) 0.0014 in (0.03556mm)
2 oz (57gms) 0.0028 in (0.07112mm)
4 oz (142gms) 0.0056 in (0.14224mm)
Table 4.1 Thickness of cladding for different materials
Material Εr Tan δ
Teflon (PTFE) 2.1 0.0005
Rexolite 1422 2.55 0.0007
Noryl 2.6 0.0011
FR4 4.4 0.02
Alumina 9.8 0.0003
Table 4.2 Dielectric and Loss tangent for different materials
Generally, dielectric constant εr and loss tangent tanδ increase with temperature.
In space applications moisture outgassing produces a lower dielectric constant and loss
tangent.
Teflon (Polytetrafluoroethylene) has very desirable electrical qualities but is not
recommended for many space applications. An extensive discussion of PTFE substrates
and their fabrication may be found in the literature.
Rexolite is a very good material for space applications and has many desirable
mechanical properties. Rexolite is easily machined and its dielectric constant remains
stable up to 100 GHz.
Noryl is suitable for many commercial microwave applications. It has a much
lower loss than FR4 and is relatively cost effective, but it is soft and melts at a relatively
low temperature which can create soldering complications, and sometimes has unsuitable
mechanical properties for some applications.
FR4 is inexpensive and find use in many commercial applications below 1 GHz.
The material can be used for some wireless applications, but great care must be taken to
budget and minimize the losses when it is used as a substrate of PTFE and Epoxy glass
(FR4) which has the desirable properties of FR4 with lower loss.
Alumina has desirable microwave properties for applications which require a
relatively high dielectric constant εr ~ 10.0 and low loss tangent. Its drawbacks are the
difficulty involved in machining it and its brittleness. Alumina has good thermal
conductivity and in some aerospace applications it more readily dissipates heat and
remains cooler than other common microwave substrates. In some missile applications
where high temperatures may compromise solder joints alumina is a viable option for the
dissipation of heat. Alumina’s dielectric constant is very sensitive to the processing used
to produce the alumina.
All substrates and laminates have different requirements for the processing.
Details of fabrication issues and methods may be found in the literature and directly from
manufacturers. Other fabrication options such as screen printing conductive inks directly
on substrates have also been investigated.
4.2 Design procedure for Rectangular Microstrip Antenna
4.2.1 Considered Values
The three essential parameters for the design of a rectangular Microstrip Patch
Antenna:
Frequency of operation (fo): The resonant frequency of the antenna must be
selected appropriately. Since we developing antenna for microwave applications
we choose design an antenna in s-band which ranges from 2 GHz to 4 GHz. We
designed microstrip antenna at 2.25 GHz..
Dielectric constant of the substrate (εr): There are many dielectric substrates
available in the market having different dielectric constant and thickness. Of them
RT Duroidd provides the best results but is highly costly and hence the dielectric
material selected for our design is FR4 (Fiber-reinforced plastic) which has a
dielectric constant of 4.4. This substrate is selected since it can obtain better
results and is cost effective.
Height of dielectric substrate (h): The height of the selected dielectric material
is 1.6mm which is optimal for having maximum radiation and has less leaky
waves. This provides a balance between conductor and dielectric loss and hence
we choose FR4 material dielectric substrate with 1.6mm thickness.
4.2.2 Initial Design Values
There are many analysis methods for the design of antenna which are discussed
later. From them we use transmission line analysis method for our antenna.
Step 1: Calculation of the Width (W)
The width of the Microstrip patch antenna is given as:
…………………………… (4.1)
Where,c is velocity of light
fo is Resonant Frequency
εr is Relative Dielectric Constant
Figure 4.1 Variation of Width with Frequency
Of course other widths may be chosen but for widths smaller than those selected
according to equation (4.1), radiator efficiency is lower while for larger widths, the
efficiency are greater but for higher modes may result, causing field distortion. As a
result design aid, equation (4.1) is plotted for the common dielectric substrates. If other
materials are employed equation (4.1) should be used with appropriate value of ε r. In this
work upon Substituting c=3.0×10^(11)mm/s, εr = 4.4 and fo = 2.25 GHz, we get:
W = 38.5 mm
Step 2: Calculating the Length (L)
Effective dielectric constant (εeff)
Once W is known, the next step is the calculation of the length which involves
several other computations; the first would be the effective dielectric constant. The
dielectric constant of the substrate is much greater than the unity, the effective value of
εeff will be closer to the value of the actual dielectric constant ε r of the substrate. The
effective dielectric constant is also a function of frequency.
As the frequency of operation increases the effective dielectric constant
approaches the value of the dielectric constant of the substrate is given by:
…………………..
(4.2)
In our design for the above mentioned values the effective dielectric is found to be
εeff = 4.100
Effective length ( Leff)
The effective length is:
……………………….. (4.3)
Which is found to be Leff = 32.92mm
Length Extension (∆L)
Because of fringing effects, electrically the micro strip antenna looks larger than
its actual physical dimensions. For the principle E – plane (x-y plane), where the
dimensions of the path along its length have been extended on each by a distance, ∆L,
which is a function of the effective dielectric constant and the width-to-height ratio
(W/h).The length extension is:
..….……………………… (4.4)
Substituting εeff = 4.4, W = 40.57 mm and h = 1.6 mm we get:
∆L = 0.739 mm
Calculation of actual length of patch (L)
Because of inherent narrow bandwidth of the resonant element, the length is a
critical parameter and the above equations are used to obtain an accurate value for the
patch length L.
Figure 4.2 Variation of Length with the Frequency
Fig 4.2 which is a plot of L versus frequency for the various substrates and for
chosen substrate may then be used to verify the design.
The actual length is obtained by:
………………………….. (4.5)
Substituting Leff = 32.92 mm and ∆L = 0.7391 mm we get:
L = 31.44mm
Step 3: Calculation of the Gain (G)
The gain of the micro strip antenna is given by the following formula
G = 4 πA
λg2 ………………..……………………. (4.6)
where A = L*W = 31.44*40.57 =1275.6556
λg=λ0
√ε r ………..……………………..…. (4.7)
= 133.33
√4.4 = 63.56 mm
By substituting the above values we get
G = 4 dB
Figure 4.3 Variation of Gain with the Frequency
Step 4: Calculation of the Beam Width (θ)
The beam width of a micro strip element can be increased by choosing a smaller
element, thus reducing W and L. For a given resonant frequency, these dimensions may
be changed by selecting a substrate having a higher relative permittivity. In many
applications, a decrease in physical size is desirable.
Beam Width in H-Plane
θBH=2 cos−1( 1
21+W K 0
2 )1 /2
……………………....... (4.8)
θBH-Beam Width in H- Plane
Substituting W = 40.57 mm and Ko=0.047 we get:
θBH = 89.29 degrees
Beam Width in E-Plane
θBE = 2cos−1( 7.033K 0
2W 2+K 02h2 )
1/2
……………………… (4.9)
θBE-Beam Width in E- Plane
Substituting W = 40.57 mm, h=1.6mm and Ko=0.047 we get:
θBE = 77.25 degrees
As beam width increases, element gain and consequently directivity decrease,
however the antenna efficiency remains unaffected.
Step 5: Calculation of the Band Width Percentage (BW %)
The bandwidth of the microstrip antenna gives the range of frequencies for which
the microstrip antenna works that is either transmits or receive and it s given by the
following equation:
BW = 100(S−1)
√S8
3 εr
hλ0
………………………………… (4.10)
Substituting λ0=133.33 mm, h=1.6mm and S=2:1, εr = 4.4 we get:
BW = 0.514%
Figure 4.4 Variation of Bandwidth with Frequency for different dielectric substrate
antennas
4.3 Microwave Co-axial Connector
For high frequency operation the average circumference of a coaxial cable must
be limited to about one wavelength, in order to reduce multimodal propagation and
eliminate erotic reflection coefficients, power losses and signal distortion. The
standardization of coaxial connectors during World War II was mandatory for microwave
operation to maintain a low reflection coefficient or a low voltage standing wave ratio
(VSWR). Since that time many modifications and new designs for microwave connectors
have been proposed and developed. Seven types of microwave coaxial connectors are
described below.
APC-3.5: The APC-3.5 (Amphenol Precision Connector-3.5mm) was originally
developed by Hewlett-Packard, but is now manufactured by Amphenol. The connector
provides the repeatable connections and has very low voltage standing-wave ratio
(VSWR). Either the male or female end of this 50Ω connector can mate with the opposite
type of SMA type connector. The APC-3.5 connector can work at frequencies up to 34
GHz.
APC-7: The APC-7 (Amphenol Precision Connector-7mm) was also developed by
Hewlett-Packard in the mid 1960s, but it was recently improved and is now manufactured
by Amphenol. The connector provides a coupling mechanism without male or female
distinction and is the most repeatable connecting device used for very accurate 50Ω
measurement applications. Its VSWR is extremely low, in the range of 1.02 to 18 GHz.
Figure 4.5 APC-7 Connector
BNC: The BNC (Bayonet Navy Connector) was originally designed for military system
applications during World War II. The connector operates very well at frequencies up to
about 4GHz, beyond that it tends to radiate electromagnetic energy. The BNC can accept
flexible cables with diameters of up to 6.35mm (0.25inches) and characteristic impedance
of 50 to 75Ω. It is now the most commonly used connector for frequencies under 1 GHz.
Figure 4.6 BNC Connector
SMA: The SMA (Sub-Miniature A) was originally by Bendix Scintilla Corporation, but
it has been manufactured by Omni-Spectra Inc. (as the OSM connector) and many other
electronic companies. The main application of SMA connector is on component for
microwave systems.
Figure 4.7 SMA Connector
SMC: The SMC (Sub Miniature C) is a 50Ω connector that is smaller than the SMA. The
connector is manufactured by Sealectro Corporation and can accept flexible cables with
diameters of up to 3.17mm (0.125 inches) for a frequency range of up to 7 GHz.
Figure 4.8 SMC Connector
TNC: The TNC (Threaded Navy Connector) is merely a thread BNC. The function of
thread is to stop radiation at higher frequencies, so that the connector can work at
frequencies up to 12GHz.
Figure 4.9 TNC Connector
Type N: The Type N (Navy) connector was originally designed or military systems
during World War II and is the most popular measurement connector for the frequency
range of 1 to 18GHz. It is 50 or 75Ω connector and its VSWR is extremely low, less than
1.02.
Figure 4.10 Type N Connector
Size Series Coupling Impedance ()
Frequency (GHz)
VSWR (max)
Voltage (V)
Subminiature
Miniature
Medium
Large
SMA
SMB
SMC
BNC
TNC
SHV
BN
MC
C
N
NC
QM
QL
Screw
Snap on
Screw
Bayonet
Screw
Bayonet
Screw
Screw
Bayonet
Screw
Screw
Screw
Screw
50
50
50
50
50
NC
50
50
50
50
50
50
50
12.4/18
4
10
4
11
NA
0.2
0.5
11
11
11
4
5
1.3
1.41
1.6
1.3
1.3
1.3
1.3
1.3
1.35
1.3
1.3
1.3
1.3
500
500
500
500
500
5000
200
200
1500
1000
1000
5000
5000
Table 4.3 Basic Features Of the most Common Connector Series
4.4 FABRICATION PROCEDURE
The first step in the fabrication process is to generate the art work from drawings.
Accuracy is vital at this stage and depending on the complexity and dimensions of the
antenna; either full or enlarged scale artwork should be prepared on Stabiline or Rubilith
film. Using the precision cutting blade of a manually operated coordinagraph, the opaque
layer of the Stabiline or Rubylith film is cut to the proper geometry and can be removed
to produce either a positive or negative representation of the Microstrip antenna. The
design dimensions and tolerances are verified on a Cordax measuring instruments using
optical scanning.
Enlarged artwork should be photo reduced using high precision camera to
produce a high resolution negative, which is later used for exposing the photo resist. The
laminate should be cleaned using the substrate manufacturer recommended procedure to
insure proper adhesion of the photo resist and the necessary resolution in the photo
development process. The photo resist is now applied to both sides of the laminate using
laminator. Afterwards, the laminate is allowed to stand to normalize to room temperature
prior to exposure and development.
The photographic negative must be now held in very close contact with the
polyethylene cover sheet of the applied photo resist using a vacuum frame copy board or
other technique, to assure the fine line resolution required. With exposure to the proper
wavelength light, a polymerization of the exposed photo resist occurs, making it insoluble
in the developer solution. The backside of the antenna is exposed completely without a
mask, since the copper foil is retained to act as a ground plane.
The protective polythene cover sheet of the photo resist is removed and the
antenna is now developed in a developer which removes the soluble photo resist material.
Visual inspection is used to assure proper development. When these steps have been
completed, the antenna is now ready for etching. This is a critical step and requires
considerable care so the proper etch rates are achieved.
After etching, the excess photo resist is removed using a stripping solution. Visual
and optical inspections should be carried out to insure a good product and to insure
conformance with dimensional tolerances, with final acceptance or rejection being based
on resonant frequency, radiation pattern and impedance measurement. For acceptable
units the edges are smoothened and the antenna is rinsed in water and dried.
If desired, a thermal cover bonding may be applied by placing a bonding film
between the laminates to be bonded and placing these between tooling plates. Dowel pins
can be used for alignment and the assembly is then heated under pressure until the bond
line temperature is reached. The assembly is allowed to cool under pressure below the
melting point of the
4.5 STEP BY STEP DESIGN PROCEDURE
DESIGN
MASTER DRAWING
ART WORK LAY OUT
PHOTO REDUCTION
NEGATIVE DEVELOPMENT
LAMINATE CLEANING
RESIST APPLICATION
RESIST EXPOSURE
RESIST DEVELOPMENT
ETCHING
BONDING
FINISHING
INSPECTION
DESIGN
MASTER DRAWING
Figure 4.11Flow chart showing the fabrication process
bonding film and the laminate is then removed for inspection. The above procedure
comprises the general steps necessary in producing a Microstrip antenna. The substances
used for the various processes example cleaning, etching, etc., are the tools used for
machining, etc., depending on the substrate chosen. Most manufacturers provide
informative brochures on the appropriate choice of chemicals, cleaners, etchants, etc., for
their substrates.
INSPECTION
Drilling hole of diameter 1.3mm by using precision drilling machine
SOLDERING
Checking with ohm meter for the patch & centre conductor continuity
Visual inspection of solder point which should be blister
Figure 4.12 Photographic Negative of ground plane Used for Fabrication
Figure 4.13 Photographic Negative of patch Used for Fabrication
CHAPTER 5
MEASUREMENTS, TESTING & RESULT ANALYSIS
5.1 MESUREMENTS
Testing of antenna involves measurement of electrical and electromagnetic
parameters. Electrical parameters involve measurement of Return loss or VSWR,
Impedance and electromagnetic parameters involves the measurement of radiation
pattern in E-plane and H-plane and gain . These measurements have been carried out
for the designed microstrip antenna.
Network Analyzer has been used to measure the return loss, VSWR and
impedance shown in figures 5.3,5.4 & 5.5. Radiation patterns and gain of the antenna at
the designed frequency are done in an anechoic chamber at ACD, Hyderabad .
5.2 TESTING
Here is a description of some of the components used to test various antenna
parameters Return Loss, VSWR, impedance measurements using Smith Chart has been
obtained using the Vector Network Analyzer. Radiation Patterns can be obtained using
the experimental set up containing Anechoic Chamber.
5.2.1 Network Analyzer
The testing of antenna is done using R&S ZVL which is a Two Port Vector
Network Analyzer. R&S ZVL vector network analyzer provides the best combination of
speed and accuracy for measuring multi-port and balanced components such as filters,
duplexers and RF modules up to 6GHz. A vector analyzer provides simple and complete
vector network measurements in a compact, fully integrated RF network. R&S ZVL
vector network analyzer offers built-in source, receiver and s-parameter test set covering
frequencies from 10 MHz to 6 GHz.
The R&S ZVL automatic port extension feature automatically measures and
corrects for fixtures, making measurements of in-fixture devices simple and accurate. The
configurable test set provides access to the signal path between the internal source and
the analyzer's test ports. This option provides the capability to improve instrument
sensitivity for measuring low-level signals, to reverse the directional coupler to achieve
even more dynamic range or to add components or other peripheral instruments for a
variety of applications such as high-power measurements. The extended power range
adds a 60 dB step attenuator internally to the RF source path. This attenuator extends the
source output power range to over 80 dB, allowing for maximum flexibility when
stimulating the device under test.
5.2.2 Elements of Network Analyzer
Figure 5.1 Major elements of Network Analyzer
A Network analyzer measurement system consists of four major parts: a signal
source providing the incident signal, signal separation devices to separate the incident,
reflected and transmitted signals, a receiver to convert the microwave signals to a lower
intermediate frequency (IF) signal, and a signal processor and display section to process
the IF signals and display detected information. The receiver performs the full S-
parameters.
Signal Source: The signal source (RF or microwave) produces the incident signal used to
stimulate device under test (DUT). The DUT responds by reflecting part of the incident
energy and transmitting the remaining part. By sweeping the frequency of the source the
frequency response of the device can be determined. Frequency range, frequency
stability, signal purity and output power level and level control are factors which may
affect the accuracy of a measurement. The source used for network analyzer
measurements is a synthesizer, which is characterized by stable amplitude frequency and
high frequency resolution (less than 100 Hz at microwave range).
Signal Separation: The next step in the measurement process is to separate the incident,
reflected and transmitted signals. Once separated, their individual magnitude and/or
phase differences can be measured. This can be accomplished through the use of
wideband directional couplers, bridges, power splitters.
A directional coupler is a device that consists of two transmission lines that are
configured to couple energy to an auxiliary port if it goes through the main port in one
direction and not in the opposite direction. Directional couplers usually have relatively
low loss in the mainline path and present little loss to the incident power. In a directional
couple structure the coupled arm samples a signal travelling in one direction only. The
coupled signal is at a reduced level and the relative amount of reduced level is called the
coupling factor. For instance a 20 dB directional coupler means that the coupled port
power level is 20 dB below the input, which is equivalent to 1 percent of the incident
power. The remaining 99 percent travels through the main arm. The other key
characteristic of a directional coupler is directivity. Directivity is defined as the
difference between a signal detected in the forward direction and the signal detected in
the reverse direction (isolation between the forward and reverse signals).
The two resistor power splitter is used to sample either the incident or transmitted
signal. The input signal is split equally between the two arms, with the output signal
(power) from each arm being 6 dB below the input. A primary application of the power
splitter is for producing a measurement with a very good source match. If one side of the
splitter output is taken to a reference detector and the other side goes through the device
under test to a transmission detector, a ratio display of transmitted to incident has the
effect of making the resistor in the power splitter determine the equivalent source match
of the measurement. Power splitters are very broadband, have excellent frequency
response and present a good match at the test device input requires a directional device.
Separation of the incident and reflected signals can be accomplished using either a dual
directional coupler or Splitter.
Figure 5.2 Vector Network Analyzer used for testing of our antenna
Receiver: The receiver provides the means for converting and detecting the RF or
Microwave signals to a lower IF or DC signal. There are basically two receiver
techniques used in network analysis. The receivers are broadband tuned receivers that use
either a fundamental mixing or harmonic mixing input structure to convert RF signal to a
lower frequency IF signal. The tuned receivers provide a Narrowband pass IF filter to
reject spurious signals and minimized the noise floor of the receiver. The vector
measurement systems (tuned receivers) have the highest dynamic ranges are less suspect
from harmonic and spurious responses, they can measure phase relationships of input
signals and provide the ability to make complex calibrations that lead to more accurate
measurements.
Table 5.1 specifications of network analyzer
5.2.3 Reflection Measurement
The return loss is the measure of power reflected and is related to the reflection
coefficient ‘Γ’ given by
Return Loss in dB = -20 log Γ
The relation between reflection coefficient and VSWR is given by
VSWR (S) = 1+Γ 1-Γ
Network Analyzer Calibration:
An Agilent R&S ZVL vector network analyzer is employed in the present
measurements. Before measuring the return loss of the antenna, the network analyzer
should be calibrated as explained below:
1. The terminal at the test port at which the test antenna is to be mounted is short
circuited. Now the power fed to the test port travels back through the short
circuits so that there will be no radiation at all. The reflected power will be equal
to the incident power and so the reflection coefficient is equal to 1, which in turn
leads to a return loss of zero dB, therefore, when the test port terminals are short
circuited, we must get a zero dB line on the display.
2. The terminals at the test port are now open circuited. The power fed to the test
port cannot be radiated because there is no load. So all the power reflects back.
The reflection coefficient is 1 and therefore leads to a return loss of 0 dB. Hence
when the terminals at the test port are open circuited the screen should display a 0
dB line.
During short circuit of test port terminals the power reflects back with phase
reversal. During the open circuit the reflected power is in- phase with respect to the
incident power. These two settings are stored in memory and the setup is ready for
practical measurements. The antenna is then connected at the test port and the observed
plot is the return loss of the antenna. The percentage bandwidth at -10dB return loss is%
Bandwidth = (f2-f1)/fr × 100
Where (f2-f1) is the frequency band for which the return loss is less than 10 dB.
Reflection Measurement
Under Reflection measurement we measured Return Loss, VSWR and
impedance.
1. Press Begin, filter and Reflection, the return loss of the antenna is displayed.
2. Press freq and then start 2.1 GHz to 2.3 GHz, scale, Auto scale reflection
coefficient in dB as a function of frequency is displayed. You can save and print
the data observed.
3. Press Format, Line Mag, to get the absolute value of reflection coefficient as a
function of frequency is displayed.
Standing Wave Ratio and Impedance
1. Press Format, and SWR. The SWR as a function of frequency is displayed. one
can save and print the data.
2. Press Format, More formats, Impedance Magnitude to get Z0as a function of
frequency. Save and print the data.
3. Press Format and Smith Chart for getting display of the real and imaginary values
of the impedance of the impedance as a function of frequency. Set the start
frequency to 2.1 GHz and stop frequency to 2.3 GHz, the impedance is about
50Ω’s in the pass band and then save and print the data.
Figure 5.3 Plot of our antenna Return Loss measurement for resonant frequency
Figure 5.4 Plot of our antenna SWR for resonant frequency
Figure 5.5 Plot of our antenna Impedance on a Smith Chart
5.2.4 RADIATION PATTERN MEASUREMENTS
The radiation patterns of an antenna are usually represented graphically by
plotting the electric field of the antenna as a function of direction. This electric field
strength is expressed as volts per meter or normalized field in dB.
A complete radiation pattern comprises the radiation for all the angles of and
and really requires three dimensional presentations. This is quite complicated. For the
practical purposes, the pattern is measured in planes of interest. Cross sections in which
the radiation patterns are the most frequently taken are the horizontal (=90 degrees) and
vertical (=constant) planes. These are called the horizontal patterns and vertical patterns
respectively. The terms commonly used are the E- plane and H-plane and they are the
planes passing through the antenna in the direction of beam maximum and parallel to the
far-field E and H vectors. These patterns are known as the ‘Principal Planes’ patterns.
The radiation patterns of the antenna are measured with the scientific Atlanta
instrumentation in an anechoic chamber. The instrumentation consists of the following
four major parts as shown in below figure.
1. Transmitting System
2. Positioning and Controlling System
3. Receiving System
4. Recording System
Transmitting System:
The transmitting or source instrumentation consists primarily of the RF signal
source and associated transmitting antenna.
Signal Source: The model 2150 signal source provides RF power in the 0.1 to 18 GHz
frequency range. The control unit is located near the operator’s console. The RF
oscillators are installed in the main frame assembly which is mounted near the source
antenna.
Source Antenna: Several types of antennas designed especially for the antenna test
range can be used. These include standard gain horns, dipoles, parabolic reflector
antennas, log periodic arrays and circularly polarized antennas depending upon the
requirement.
Positioning & Controlling System:
The antenna to be tested is mounted on the turntable of the antenna test positioner.
The speed and direction of the rotation of the test antenna can be controlled from the
operator’s console by a direct current motor. A synchro transmitter is mechanically
coupled to the positioner turntable and electrically to a position indicator. The antenna
test positioner is controlled by the series 4100 positioner control unit. Electrical cables
are used to supply power from control system to test positioner.
Indicator system: A position indicator allows remote angle read out of the test
positioner. The synchro transmitter in the test positioner provides the position data to
operate the position indicator.
Receiving System:
The antenna under test usually tested in the receive mode. Therefore a receiving
or detecting system must be connected to the test antenna to convert RF signals to a low
frequency signals which can drive the pen system of pattern recorder. Thus the antenna
must receive an RF signal i.e modulated with an audio signal. The model 2150 signal
source has an audio oscillator as a standard feature. The two types of detectors commonly
used for making antenna measurements are crystal detector and Bolometer. Scientific
Atlanta antenna pattern recorders will operate crystal detectors or Bolometer detectors
directly.
Antenna Pattern Recorder:
The radiation patterns of the antenna are recorded as relative amplitude and / or
phase as a function of the position (or angle). The synchro position data from the test
positioner is connected to the recorder’s chart servo system. The resultant graph is a plot
of the relative amplitude of the received signal as a function of the antenna position (or
angle).
Polarization positioner Azimuth positioner
SIGNAL SOURCE
Source control SA 2150
Remote Positioner Control Unit SA 4110-10
Position indicator
INDICATOR
Position Control Unit SA 4100
Pattern recorder
RECEIVER
ANECHOIC CHAMBER
Figure 5.6 Experimental Set Up For Plotting Radiation Pattern
Figure 5.7 Anechoic chambers with free space environment
Figure 5.8 Anechoic Chamber when our antenna is being tested
Figure 5.9 Plot of our antenna Radiation pattern in E and H plane
5.2.5 Gain Measurement
The setup used for measurement of gain is the same as that used for radiation
pattern measurement given in table (5.2). The gain of the antenna is measured by
replacing the test antenna with a standard antenna (horn antenna in this case) and taking
the pattern of the same. The gain is then calculated by comparing the power level
differences of the test antenna with that of the standard antenna.
Table 5.2 Gain Measurement
Figure 5.10 Bottom (ground plane) view of our antenna
Figure 5.11 Top view (patch) of our antenna
5.3 ANALYSIS
This section deals with the comparing the calculated values with the measured
values. Thus we can analyze the differences between them. The comparison is as follows:
ANTENNA PARAMETE
RS
CALCULATED
EQUATIONS
MEASURED
FIGURE/
TABLE
Length 31.44 mm 4.5 32.09 mm Fig 5.11
Width 40.57 mm 4.1 41.15 mm Fig 5.11
Thickness 1.6 mm - 1.6 mm -
frequency 2250MHz - 2200MHzFig 5.4
Bandwidth 51.556 MHz 4.10 56 MHz Fig 5.3
Beam Width E- plane
76.5(degrees) 4.975.6(degrees
)Fig 5.9
Beam Width H- Plane
89.29 (degrees) 4.881.9(degrees
)Fig 5.10
Gain 6.13 dB 4.9 3.94 dB Tab 5.2
Table 5.3 Comparison of calculated and measured values
From the above we finally conclude that the measured values and the obtained
values are approximately equal. Thus this project has been carried out successfully. The
changes in the measured values are due to the variation of dielectric constant of FR4
material, from actual value at our antenna operating frequency. And also due to slight
changes in dimensions of the patch in the fabrication process which was done at M/S
Sravanthi electronics at UPPAL industries. For Aerospace vehicles smaller bandwidth is
required which have been seen in the Microstrip Antenna.
CONCLUSIONS
A rectangular micro strip antenna is designed using the appropriate design
formulae and is fabricated using the PCB fabrication procedure and is tested by using the
vector network analyzer R&S ZVL. The antenna is designed at frequency 2250MHz
frequency with FR4 (εr=4.4),h=1.6mm,tan =0.02. Even though the antenna is desired to
operate at this frequency, when tested practically it is found that, it is resonating at
2200MHz.
The dielectric constant plays a major role in the overall performance of a patch
antenna. It affects both the width, in turn the characteristic impedance and the length
resulting in an altered resonant frequency. We have used the fiber glass substrate but the
permittivity (εr) alters from batch to batch some times even between different sheets of
substrates. In addition FRP-4 has a high loss tangent and is highly frequency dependent..
And also manufacture recommends this FR4 for use up to 1 GHz only with Eeff 4.36
The bandwidth of the patch antenna depends largely on the permittivity (εr) and
thickness of the dielectric substrate. Ideally a thick dielectric lower permittivity (εr) low
insertion loss is preferred for broad band applications.
From the result 1 observed that the band width of the micro strip element can be
increased by choosing a smaller element, thus reducing W and L. For the given resonant
frequency these dimensions will be changed by selecting a substrate having a higher
relative permittivity. The advantages of the micro strip antenna are that they are low cost,
conformable, light weight and low profile, while both linear and circular polarization is
easily achieved.
This antenna material is also ideal for antenna arrays. Longer ranges, larger areas,
faster assembly line speeds will all benefit from the focused energy and directionality
available through antenna array beam forming. The print and etch process of printed
circuit board is very repeatable and highly cost effective. It eliminates the labor and the
technician work required to insure proper phase matching between elements. It also
reduces energy requirements of the system. The reduced side lobe emissions reduce false
alarms, reduce interference between other antennas and minimize emission in unwanted
directions.
FUTURE SCOPE
The project provides the complete overview of Rectangular Microstrip antenna and also
provides the necessary equations to design a rectangular Microstrip antenna and also
provides the fabrication process of a rectangular Microstrip antenna. This also gives the
necessary information for choosing substrate and their properties for getting better
results.
Future challenges of a Microstrip antenna are:
Bandwidth Extension Techniques
Control of Radiation Patterns
Reducing Losses / increasing efficiency
Improving feed networks
Size reduction techniques
The band width can be increased as follows
By increasing the thickness of the substrate
By use of high dielectric constant of the substrate so that physical dimensions
of the parallel plate transmission line decreases.
By increasing the inductance of the micro strip by cutting holes or slots in it.
By adding reactive components to reduce the VSWR
In order to increase the directivity of the micro strip antennas multiple micro strip
radiators are used to cascade to form an array.
REFERENCES
Books
[1] R. Garg, P. Bhartia, I. Bahl, and A. Ittipiboon, Microstrip Antenna Design Handbook,
ArtechHouse, 2001.
[2] K. F. Lee, Ed., Advances in Microstrip and Printed Antennas, John Wiley, 1997.
[3] D. M. Pozar and D. H. Schaubert, Microstrip Antennas: The Analysis and Design of
Microstrip Antennas and Arrays, IEEE Press, 1995.
[4] F. E. Gardiol, “Broadband Patch Antennas,” Artech House.
[5] S K Behera, “Novel Tuned Rectangular Patch Antenna As a Load for Phase Power
Combining” Ph.D Thesis, Jadavpur University, Kolkata.
[6] D. R. Jackson and J. T. Williams, “A comparison of CAD models for radiation from
rectangular microstrip patches,” Intl. Journal of Microwave and Millimeter-Wave
Computer Aided Design, Vol. 1, No. 2, pp. 236-248, April 1991.
[7] D. R. Jackson, S. A. Long, J. T. Williams, and V. B. Davis, “Computer- aided design
of rectangular microstrip antennas”, ch. 5 of Advances in Microstrip and Printed
Antennas, K. F. Lee, Editor, John Wiley, 1997.
[8] D. M. Pozar, “A reciprocity method of analysis for printed slot and slot- coupled
microstrip antennas,” IEEE Trans. Antennas and Propagation, vol. AP-34, pp. 1439-
1446, Dec. 1986.
Websites
[9] Over view of microstrip antenna, ”httpwww.ecs.umass.edu/ece/pozar/aperture.pdf”
APPENDICES
Basic Models of Antennas
There are many variations of antennas. Below are a few basic models.
The isotropic radiator is a purely theoretical antenna that radiates equally in all
directions. It is considered to be a point in space with no dimensions and no mass.
This antenna cannot physically exist, but is useful as a theoretical model for
comparison with all other antennas. Most antennas' gains are measured with
reference to an isotropic radiator, and are rated in dBi (decibels with respect to an
isotropic radiator).
The dipole antenna is simply two wires pointed in opposite directions arranged
either horizontally or vertically, with one end of each wire connected to the radio
and the other end hanging free in space. Since this is the simplest practical
antenna, it is also used as a reference model for other antennas; gain with respect
to a dipole is labeled as dBd.
The Yagi-Uda antenna is a directional variation of the dipole with parasitic
elements added which are functionality similar to adding a reflector and lenses
(directors) to focus a filament light bulb.
The random wire antenna is simply a very long (at least one quarter wavelength)
wire with one end connected to the radio and the other in free space, arranged in
any way most convenient for the space available. Folding will reduce
effectiveness and make theoretical analysis extremely difficult.
The horn is used where high gain is needed, the wavelength is short (microwave)
and space is not an issue. Horns can be narrowband or wideband, depending on
their shape. A horn can be built for any frequency, but horns for lower frequencies
are typically impractical. Horns are also frequently used as reference antennas.
The parabolic antenna consists of an active element at the focus of a parabolic
reflector to reflect the waves into a plane wave. Like the horn it is used for high
gain, microwave applications, such as satellite dishes.
The patch antenna consists mainly of a square conductor mounted over a ground
plane. Another example of a planar antenna is the tapered slot antenna (TSA), as
the Vivaldi-antenna.
PROGRAM IN MATLAB
Merits of Programming
The design of the microstrip antenna involves many lengthy and tedious
calculations such as width, length, feed locations, and dimensions of the feed. As these
calculations are cumber some and time consuming when done by hand a computer
programming approach is adopted to simplify the task.
Program to find Width, Length & Feed Point
The width and length of the micro strip antenna are to be calculated from the
corresponding equations as given in chapter 4. The next parameter to be found is the feed
point location. In the project, the coaxial type of feed is chosen to feed the antenna. The
impedance of the feed is 50Ώ. Hence in the program the importance of the antenna is
found at every point along the length of the antenna according to the standard formulae
given in the chapter 4 and the point of feed is hence found.
Thus the program in MATLAB to find the length, width of the micro strip
antenna and also the feed location is given below. It takes the input as frequency of
operation(GHz), substrate thickness (in cm) and dielectric constant.
MATLAB Program
clear allclear k0 f1 f2c=3e11
er=input('enter the er value');h=input('enter the value of h');fr=input('enter the resonant frequency');% width calculationw=c/(2*fr*sqrt((er+1)/2));disp('width=')disp(w)% effective lengthp1=(er+1)/2;p2=(er-1)/2;p3=1/(sqrt(1+((12*h)/w)));eeff=p1+(p2*p3);disp('eeff=')disp(eeff)%delta L calculationp4=h*0.412*(eeff+0.3)*((w/h)+ 0.264);p5=(eeff-0.258)*((w/h)+0.8);dl=p4/p5;disp('del L=')disp(dl)% length calculationp6=c/(2*fr*sqrt(eeff));L=p6-(2*dl);disp('Length=')disp(L)% Area calculationA=L*w;disp('Area=')disp(A)% Gain calculationlg=133.33/sqrt(er);G=(4*pi*A)/(lg*lg);disp('Gain=')disp(G)% Beam Width% H planek0=0.047;p7=1+(w*k0/2);p8=sqrt(1/(2*p7));BWH1=2*(acos(p8));BWH=BWH1*57.18;disp('BEAM WIDTH IN H-PLANE')disp(BWH)% E planep9=(3*k0*k0*w*w)+(k0*k0*h*h);p10=sqrt(7.03/p9);
BWE1=2*(acos(p10));BWE= BWE1*57.18;disp('BEAM WIDTH IN E-PLANE')disp(BWE)% BAND WIDTH CALCULATIONs=2;p11=(8*h)/(3*er*133.33);p12=(100*(s-1))/sqrt(s);BWP = p11*p12;disp('BAND WIDTH')disp(BWP)BW=BWP*2250;disp(BW)OUTPUT OF PROGRAM