DBDP MSA

A

PROJECT REPORT

ON

Design of Dual Frequency and Dual Polarized

Microstrip Antenna

Submitted by

AMIT JAYANT NAIK

Roll No. MT2008310

Under the guidance of

Prof. A. B. NANDGAONKAR

In the partial fulfillment of M. Tech. in Electronics & Telecommunication

Engineering course of Dr. Babasaheb Ambedkar Technological University,

Lonere (Dist. Raigad) in the academic year 2010-2011

Department of Electronics & Telecommunication Engineering

Dr. Babasaheb Ambedkar Technological University, Lonere

Lonere-402103

2008 - 2011

A

PROJECT REPORT

ON

Design of Dual Frequency and Dual Polarized

Microstrip Antenna

Submitted by

AMIT JAYANT NAIK

Roll No. MT2008310

Under the guidance of

Prof. A. B. NANDGAONKAR

Department of Electronics & Telecommunication Engineering

Dr. Babasaheb Ambedkar Technological University, Lonere

Lonere-402103

2008 - 2011

CERTIFICATE

This is to certify that the project entitled ‘Design of Dual Frequency & Dual

Polarized Microstrip Antenna’ submitted by Mr. Amit Jayant Naik (Roll No:

MT2008310) is a record of bonafide work carried out by him under my guidance in the

partial fulfillment the requirement for the award of Degree of M. Tech. in Electronics &

Telecommunication Engineering course of Dr. Babasaheb Ambedkar Technological

University, Lonere (Dist. Raigad) in the academic year 2010-2011.

Prof. A. B. Nandgaonkar Dr. S. L. Nalbalwar

Guide Professor and Head Department of Electronics &

Telecommunication Engineering

Dr. Babasaheb Ambedkar Technological

University, Lonere – 402103

Date: December , 2010

Place : Lonere, Dist.: Raigad.

ACKNOWLEDGEMENT

Words are inadequate to express the overwhelming sense of gratitude and humble

regards to my guide supervisor Prof. A. B. Nandgaonkar of the Department of Electronics

and Communication Engineering for his constant motivation, support, expert guidance,

constant supervision and constructive suggestion for the submission of my progress report of

project work “Design of Dual Frequency & Dual Polarized Microstrip Antenna”.

I express my gratitude to Prof. Dr. S. L. Nalbalwar Professor and Head of the

Department of Electronics and Communication Engineering for his valuable suggestions and

constant encouragement all through the project work.

I also thank all the teaching and non-teaching staff for their nice cooperation to us.

This report would have been impossible if not for the perpetual moral support from

my family members, and my friends. I would like to thank them all.

Amit Jayant Naik

M. Tech Electronics & Telecommunication

Roll No. MT2008310

Project Overview

The aim of this project is to design a dual frequency and dual polarized Microstrip

antenna using single coaxial feed. The frequency bands selected for design are, UMTS-I

(1.92GHz-2.17GHz) & UMTS-II (2.5GHz - 2.69GHz). The dual band square Microstrip

antenna is operates at 1.81GHz & 2.6 GHz frequencies, with linear polarization at 1.81GHz

& circular polarization at 2.6GHz. The thesis covers three aspects of Microstrip antenna

design. The first is the introduction to Microstrip antenna. The Second part covers design of

square Microstrip antenna which operates at the central frequency of 2.6 GHz. First, the

design parameters for single element of square patch antenna have been calculated from the

transmission line model equation and then extended the antenna design to Dual Band Square

Microstrip patch antenna using the slot at the center & shorting pins. To enhance the

bandwidth at central frequency suspended Microstrip antenna technique is used. The antenna

has been modeled, designed and simulated. The simulation process has been done through

Ansoft’s HFSS 11.1v electromagnetic software which is based on Finite Element Method

(FEM). For fabrication of this Dual band & Dual polarized Microstrip antenna FR-4 substrate

is used, which is having a dielectric constant 4.4 with loss tangent of 0.02 and the substrate

height is 1.6 mm. The last part covers analysis of antenna for bandwidth, S-Parameter, axial

ratio, impedance plot. The results for fabricated antenna are found by Agilent’s network

analyzer E5062A. It has been investigated and compared between different optimization

scheme and theoretical results. Also application & conclusion has been discussed.

Contents

List of Abbreviations I

List of Figures III

List of Tables V

1. Chapter 1: Introduction to Antennas 1 - 18

1.1 Antenna Types 2

1.2 Antenna Fundamentals 4

1.2.1 Antenna Definition 4

1.2.2 Antenna Radiation 4

1.2.3 Radiation Pattern 6

1.2.4 Directivity 6

1.2.5 Reflection Coefficient 7

1.2.6 Voltage Standing Wave Ratio (VSWR) 7

1.2.7 Antenna Gain 8

1.2.8 Polarization 8

1.2.9 Bandwidth 17

1.2.10 Reciprocity 17

2. Chapter 2: Microstrip Patch Antenna 19 – 31

2.1 Introduction 19

2.2 Radiation Mechanism of Microstrip Antenna 20

2.3 Methods of Analysis 22

2.3.1 Transmission Line Model 22

2.3.2 Cavity Model 23

2.4 Feeding Techniques 26

2.4.1 Microstrip Line Feed 26

2.4.2 Coaxial Feed 27

2.4.3 Aperture Coupled Feed 27

2.4.4 Proximity Coupled Feed 28

2.4.5 Coplanar Waveguide Feed 29

2.5 Advantages and Disadvantages 30

2.6 Surface Waves 31

3. Chapter 3: Parametric Study of Microstrip Antenna 32 – 36

3.1 Techniques to Increase Bandwidth 32

3.2 Techniques to Achieve Dual Frequency Operation 33

3.3 Techniques to Achieve Circular Polarization 34

4. Chapter 4: Design of Microstrip Antenna 37 - 41

4.1 Design Parameters 37

4.2 Calculation 37

5. Chapter 5: Results & Discussion 42 – 51

5.1 Return Loss 42

5.2 VSWR 44

5.3 Frequency vs. Gain 45

5.4 Axial ratio vs. Frequency 47

5.5 Smith Chart 48

6. Chapter 6: Conclusion & Future Scope 52

References 53

Project report on Design of Dual Frequency and Dual Polarized Microstrip Antenna

Dr. Babsaheb Ambedkar Technological University, Lonere

Department of Electronics & Telecommunication Engineering I

List of Abbreviations

IEEE Institute of Electrical & Electronics Engineers

FEM Finite Element Method

HF High Frequency

HFSS High Frequency Structure Simulator

DBDP Dual Band and Dual Polarized

UMTS Universal Mobile Telecommunication Systems

MSA Microstrip Antenna

RL Return Loss

HPBW Half Power Beam Width

dBi Gain of an antenna in decibel compared with isotropic antenna

dB Decibel

VSWR Voltage Standing Wave Ratio

AR Axial ratio

CW Clockwise

CCW Counter clockwise

CP Circular Polarization

LHCP Left Handed Circular Polarization

RHCP Right Handed Circular Polarization

2D 2- Dimensional

3D 3- Dimensional

MNM Multi-port network model

MoM Method of moments

FDTD Finite-difference time domain



Department of Electronics & Telecommunication Engineering II

SDT Spatial domain technique

RF Radio frequency

TM Transverse Magnetic

MICs Microwave Integrated Circuits

Q Quality Factor

GHz Giga Hertz

MHz Mega Hertz

εr Dielectric constant

εreff Effective dielectric constant

h Height of the substrate

fr Resonant frequency

W Width of the patch

L Length of the patch

c Velocity of light

g Height above ground plane

Lg Length of ground plane

Wg Width of ground plane



Department of Electronics & Telecommunication Engineering III

List of Figures

Figure

no.

Name of figure Page

no.

1.1 Transmission system 1

1.2. Dipole antenna 2

1.3 Rectangular horn antenna 2

1.4 Parabolic reflector with front end 2

1.5 Lens antennas 3

1.6 Microstrip patch antenna 3

1.7 Microstrip patch antenna array 4

1.8 Free-space wave generation 5

1.9 Only accelerating charges produce radiation 5

1.10 Radiation pattern of directional antenna 6

1.11 Polarization schemes

8

1.12 Linear polarization

9

1.13 Wave propagation in Z-direction

9

1.14 Elliptical polarization

10

1.15 Circular polarization 10

1.16 Elliptical polarization 11

1.17 Linear polarized wave at 450 14

1.18 Rotation of electric field around axis 15

1.19 3D view for circular polarization 16

1.20 Reciprocity theorem 17

2.1 Various shapes for patch. 19

2.2 Operation of Microstrip antenna 20



Department of Electronics & Telecommunication Engineering IV

2.3 Radiation from the patch 21

2.4 Electric field lines 22

2.5 Distribution of electric field lines 23

2.6 Charge distribution & current density creation on the patch 24

2.7 Microstrip line feed 26

2.8 Coaxial feed 27

2.9 Aperture coupled feed 28

2.10 Proximity-coupled Feed 29

2.11 Coplanar waveguide Feed 29

3.1 Various single fed circularly polarized patch antennas 34

3.2 Responses of diagonally fed nearly square MSA for different L1/L2 35

3.3 Dual feed circularly polarized patch with power divider & 900 hybrid 36

4.1 Top view & side view for MSA from HFSS 40

4.2 Top view for fabricated DBDP MSA 41

4.3 Side view for fabricated DBDP MSA 41

5.1 Experimental set up for testing of antenna 42

5.2 Experimental & simulated results for return loss 43

5.3 Experimental & simulated results for VSWR 45

5.4 Simulated result for Frequency vs. Gain 46

5.5 Simulated gain pattern for frequency 2.61GHz 47

5.6 Simulated gain pattern for frequency 1.81GHz 48

5.7 Simulation result for frequency vs. axial ratio 49

5.8 Simulated radiation pattern for frequency 2.61GHz 50

5.9 Simulation result for smith chart 51



Department of Electronics & Telecommunication Engineering V

List of Tables

Table

no.

Name Page

no.

1.1 Polarization of wave 15

4.1 Parameters for design of antenna 39



Department of Electronics & Telecommunication Engineering 1

Chapter 1

Introduction to Antennas

The term Antenna in zoology refers to feeler i.e. part of insects or organ for touch [1].

In Radio sense it is a metallic device used to send or receive radio waves so it is also called as

aerial or radiator. An antenna is defined by Webster’s Dictionary as “a usually metallic

device for radiating or receiving radio waves.” i.e. any metallic body can act as an antenna. It

just differs on either radiating or receiving [3].

The IEEE Standard Definitions of terms for Antennas (IEEE Std 145–1983) defines

the antenna or aerial as “a means for radiating or receiving radio waves” [3]. In other words

the antenna is the transitional structure between free-space and a guiding device, Antenna is

used as an impedance matching device between free-space has impedance of 120Π with that

of source cable or waveguide having impedance of 50Ω.

Figure.1.1. Transmission system

If the impedance of transmission line (cable) is matched with antenna maximum

power will be delivered from source to antenna or can be received from the antenna. If the

impedance is not matched with that of antenna a standing wave pattern is appeared on the

transmission line which out of phase with that of incident wave leading to considerable

energy loss in the signal, as shown in Figure 1.1. So, instead of guiding element antenna will

act as storage element. This loss can also be avoided either by using infinite length

transmission line. Antenna in advanced receiver and transmitter wireless system is used to

concentrate or to catch electromagnetic waves & to suppress it in other direction.




1.1 Antenna Types [3]:-

Antennas are categorized as:

1) Wire antennas:- These antennas can be found everywhere & they can have any

shape such as straight (e.g. Dipole), circular, helix, loop (either rectangular, square

or ellipse) .

Figure.1.2. Dipole antenna

2) Aperture Antennas:- These antennas are used for HF application & they are

having conical, pyramidal structures. e.g. Horn antennas (Pyramidal horn or

Conical horn or rectangular waveguide).

Figure.1.3. Rectangular horn antenna

3) Reflector Antennas:- When there is a need to communicate over millions of miles

long distance these antennas are preferred. Antennas of this type can have

diameter of around 305 meters, such high diameter are used to achieve high gain.

e.g. Parabolic reflector (Dish antennas) & Corner reflector.

Figure.1.4. Parabolic reflector with front end




4) Lens Antennas:- These antennas are used to concentrate incident divergent energy

to prevent it from spreading in undesired directions. They transform various type

of divergent energy into plane waves.

Figure.1.5. Lens antennas

5) Microstrip Antennas:- This type of antennas are very much popular from 1970’s

due to their small size, ease in fabrication, low profile, easily attached to planar &

non-planar surfaces, simple, inexpensive & mechanically robust. It is consist of a

metallic patch on a grounded substrate. The metallic patch can take any shape

such as rectangular, triangular, square or circular. These antennas are also

versatile in terms of resonant frequency, polarization, radiation pattern &

impedance bandwidth.

Figure.1.6. Microstrip patch antenna




6) Array antennas:- In many applications radiation energy required is not achievable

using single element. So this antenna uses multiple elements in definite pattern

leads to increase in radiation energy. This increase in radiation energy is due to

the addition of radiation energy from individual element.

Figure.1.7. Microstrip patch antenna array

In Microstrip array antennas different diversities are used,

a) Spatial diversity:- It means MSA elements are separated by their position

in the array.

b) Polarization diversity:- It means MSA elements are separated by using

different polarization schemes to avoid cross-polarization or distortion.

c) Beam diversity:- In any given direction antennas have different gains. This

results in a different weighting of multipath components of the desired

signal being delivered by each antenna.

1.2 Antenna Fundamentals:

1.2.1. Antenna Definition:-

An antenna is basically a transducer that converts electrical alternating current

oscillations at a radio frequency to an electromagnetic wave of the same frequency or vice

versa..

1.2.2. Antenna Radiation:-

When a sinusoidal voltage source is applied across a transmission line the electric

field is created between two conductors which in turn provides magnetic field. Due to time

varying electric and magnetic fields electromagnetic waves are created and travel through the

transmission line to the antenna and radiate in free space.




Figure.1.8. Free-space wave generation

Radiation mechanism: To create radiation there must be time varying current i.e.

acceleration or deceleration of charge & to create acceleration or deceleration of charge a

wire must be bent, curved, discontinuous or terminated. A group of charges in uniform

motion or stationary do not produce radiation as in figure1.9.1. In figure.1.9.2 – 1.9.4,

however, radiation does occur, because the velocity of the charges is changing in time. In

figure 1.9.2 the charges are reaching the end of the wire and reversing direction, producing

radiation. In figure 1.9.3 the speed of the charges remains constant, but their direction is

changing, thereby creating radiation. Finally, in figure 1.9.4, the charges are oscillating in

periodic motion, causing a continuous stream of radiation. This is the practical case, where

the periodic motion is excited by a sinusoidal transmitter. Antennas can therefore be seen as

devices which cause charges to be accelerated in ways which produce radiation with desired

characteristics.

Figure.1.9. Only accelerating charges produce radiation




Also rapid changes in direction of structures which are designed to guide waves may

produce undesired radiation.

1.2.3. Radiation Pattern:-

It is defined as radiation pattern is the representation of graph which describes the

radiation properties of antenna as a function of space coordinate. Radiation pattern are

described with reference to isotropic antenna. Plot of directional antenna is shown in figure

1.10; typically directional antenna has a more power in particular direction as compared to

isotropic antenna.

Figure.1.10. Radiation pattern of directional antenna

Half power beamwidth (HPBW):- In a plane containing the direction of maximum of

beam, the angle between two directions in which the radiation intensity is one half

value of beam.

Main lobe:- Lobe having maximum radiation in particular direction.

Side lobe:- Lobes other than main lobe is called side lobe. These lobes are unwanted

and degrade the antenna performance.

Back lobe:- This is the minor lobe which is in opposite direction of main lobe.

Front-back ratio:- It is the ratio between the peak amplitudes of the main and back

lobes, usually expressed in decibels.

1.2.4. Directivity (D):-

The directivity (D) of an antenna, a function of direction, is defined as measure of a

maximum power density radiated by the practical antenna relative to the power density

radiated by an ideal isotropic antenna




antenna isotropic ideal ofintensity Radiation

φ),(direction in antenna ofintensity Radiation D

Directivity of antenna is generally expressed in dBi. Where dBi is a logarithmic unit

relative to the gain of an isotropic antenna. Antenna having a narrow main lobe would have

better directivity.

1.2.5. Reflection Coefficient:-

As shown in figure 1.1, if a transmission line is not terminated properly a reflected

wave get produced. This wave will be in out of phase to that of incident wave and forms

standing wave structure. Reflection coefficient is the measure of how much energy is lost, as

it is the ratio of reflected wave’s peak voltage to the incident wave’s peak voltage. It is

represented by Γ.

Poweror oltageIncident v

Poweror voltageReflectedΓ

0L

0L

ZZ

Z-Z

The reflection coefficient of a line in impedance terms as the ratio of the power

reflected back from the line to the power transmitted into the line. As given in the second

equation is the load impedance and is characteristic impedance of the transmission line.

The value for reflection coefficient lies in the range from 0 to +1. Γ = 0 means load is said to

be matched to the line & there is no reflection of the incident wave. If the antenna is

mismatched, then, not all the available power from the transmission line is delivered to the

antenna. This loss is called Return Loss and is defined as

(dB) log 20- lossReturn

For maximum power transfer the return loss should be as large number as possible.

1.2.6. Voltage Standing Wave Ratio (VSWR):-

A standing wave in a transmission line is a wave in which the distribution of current,

voltage or field strength is formed by the superimposition of two waves of same frequency

propagating in opposite direction. Then the voltage along the line produces a series of nodes

and antinodes at fixed positions.

-1

+1=

V

V=VSWR

min

max




The value of VSWR should be between 1 and 2 for efficient performance of an antenna.

1.2.7. Antenna Gain:-

Gain of antenna is product of efficiency and directivity when efficiency is 100% then

gain is equal to directivity. When direction is not stated power gain is normally taken in

direction of maximum radiation.

Gain is given by,

G = η × D (dBi)

1.2.8. Polarization [1]:-

Polarization of uniform wave means the time varying behavior of the electric field

intensity vector at some fixed point in space. It is also defined as the property of an

electromagnetic wave describing the time varying direction and relative magnitude of the

electric filed vector. The direction or position of the electric field with respect to the ground

gives the wave polarization.

The common types of the polarization are circular and linear. The linear includes

horizontal and vertical and the circular polarization includes right hand polarization and left

hand polarization.

It is said to be linearly polarized when the path of the electric field vector is back and

forth along the line.

Figure.1.11. Polarization schemes

It can be seen that the circular polarization has the electric field vector’s length

constant but rotates in a circular path.




As this thesis is about circular polarization we concentrate on it, in detail. Now, to

understand wave polarization; consider a wave is travelling in Z-direction.

Figure.1.12. Linear polarization

In linear polarization the electric field vector always remain in one direction i.e. Y-

direction as shown above figure or in linear polarization the electric field vector always varies

with only one direction (Y-direction) & propagates in other (Z-direction).

Figure.1.13. Wave propagation in Z-direction

δβz-ωtsin EE 2y

In general, if a wave is progressing in positive direction may have both x & y

components, then this wave termed as “Elliptically Polarized”.




Figure.1.14. Elliptical polarization

At fixed value z, the electric field vector E rotates as a function of time. This is also

called as “Polarization Ellipse” as in figure 1.14.

Axial ratio:- It’s the most basic term used to understand polarization or used simply to

differentiate linear from circular & can be defined as ratio of major axis to minor axis in

polarization ellipse.

axisMinor

axisMajor

E

E(AR) ratio Axial

1

2

For linear polarization,

E1 = 0,

0

1 AR

Figure.1.15. Circular polarization




And, for circular polarization,

E1 = E2;

So, AR = 1.

It is a special case of elliptical polarization as can be seen from figure 1.15 both

electric field vectors are having same magnitude so it forms a circle. Also axial ratio will be

1.

In elliptical polarization the polarization may assume any orientation. Elliptical

polarized wave is a combination or resultant of two linearly polarized waves of same

frequency.

Figure.1.16. Elliptical polarization [3]

Let,

Ex = Electric field component of horizontally polarized wave & E1 is maximum amplitude of

it.

Ey = Electric field component of vertically polarized wave & E2 is maximum amplitude of it.

So,

.....(1) βz-ωtsin EE 1x

.....(2) δβz-ωtsin EE 2y

Where E1 & E2 are amplitudes of x & y . δ is the phase angle by which Ey leads Ex.

.....(3) EaEaE yyxx




From equation 1 & 2

δ)βztsin( Eaβzωtsin EaE 2y1x

Consider the wave is at initial point where Z = 0, above equation becomes

δ)tsin( Eaωtsin EaE 2y1x

Substituting Z = 0 in equation 1 & 2 we get

1

x1x

E

Eωtsin ωtsin EE

δωtsin EE 2y

δωt δ ωt EE y sincoscossin2

Now,

ωt sin - 1 ωt cos 2

E

E - 1 ωt cos

2

1

x

So,

sinδ E

E - 1 cosδ

E

E

E

E2

1

x

1

x

2

y

sinδ E

E - 1 cosδ

E

E

E

E2

1

x

1

x

2

y

Sqauring both sides we get,

δsin E

E - 1 cosδ

E

E

E

E2

2

1

x

2

1

x

2

y




δsinE

E - δsin cosδ

E

E cosδ

E E

E E 2 -

E

E2

2

1

x2

2

1

x

21

yx

2

2

y

cosδE

E δsin

E

E - δsin cosδ

E E

E E 2 -

E

E2

1

x2

2

1

x2

21

yx

2

2

y

1δcos δsin δ.....(4)sin E

E cosδ

E E

E E 2 -

E

E 222

2

1

x

21

yx

2

2

y

1 sinδ E

E cosδ

δsinE E

E E 2 -

sinδ E

E2

1

x

2

21

yx

2

2

y

1E cEE bE a2

yyx

2

x

Where, δsin E

1c ;

δsinE E

cosδ 2 b ;

δsin E

1a

22

1

2

2122

2

For polarization, Axial ratio must be in the range

AR 1

Case I:- Let Ey be in phase or 1800 out of phase with Ex, then δ = KA

K = 0, 1, 2, 3….

So equation 4 becomes,

1- πcos 1, 0 cos

0 sin π 0sin 0

E

E

E E

E E 2

E

E2

1

x

21

yx

2

2

y

0 E

E

E

E2

1

x

2

y

....(i)E E

E E x

1

2y

This is the equation of straight line with slope E2/E1.

Therefore, when two linearly polarized waves are in phase or out of phase, the

resultant wave will be a linearly polarized wave. Now, if E1 = 0, the wave will be polarized in




Y-direction & when E2 = 0 the wave is polarized in X-direction & if E1 = E2 & δ = 0, the

resultant wave is still linearly polarized but at 450 as shown in figure 1.17.

Figure.1.17. Linear polarized wave at 450

Case II:- Let E1 = E2 & δ = ± 900

So equation 4 becomes,

0 90cos 1, 90sin 1 E

E

E

E2

1

x

2

2

y

Since, E1 = E2

2

1

2

y

2

x E EE

2

2

2

y

2

x E EE

These are the equations of circle. Therefore, when two linearly polarized waves of

equal magnitude but with phase difference of 900 produces a circularly polarized wave.

So, E1 = E2 & δ = ± 900

the wave is circularly polarized wave.

If, δ = 900, the wave is said to be Left circularly polarized.

δ = - 900, the wave is said to be Right circularly polarized.

From equation 3 we have

yyxx EaEaE




δ)βztsin( Eaβzωtsin EaE 2y1x

Figure.1.18. Rotation of electric field around axis

Thus, when δ = 900 & Z = 0 & ωt = 0.

yyEaE

After one cycle, Z = 0 & ωt = 900

xxEaE

At fixed point electric field vector rotates in clockwise direction (As seen wave

approaching) this is called as Left circular polarization. Also when δ = - 900 corresponds to

right circular polarization.

So to conclude if a wave is viewed from receding end then, the clockwise rotation

(CW) of electric field vector is described as right handed polarization & counter clockwise

(CCW) rotation of electric field vector is described as left handed polarization.

i.e. when E1 = E2 & wave viewed from receding end (From negative Z-direction)

Table.1.1 Polarization of wave

δ Rotation of wave Circular Polarization

+900 Clockwise Right Handed [RHCP]

- 900 Counter clockwise Left Handed [LHCP]

A wave to be said as linearly or circularly polarized the phase differences between

two electric field vectors must be,

y2x1 a tz,Ea t)(z,Etz,E




x11 δ βz ωt cos E t)(z,E

y22 δ βz ωt cos E t)(z, E

Now, for Linear polarization the phase difference

0,1,2,3..n here Wnπδδ δ xy

So, the phase difference for linear polarization should be multiple of Π or 1800 or

3600…

For Circular polarization

This can be achieved when magnitudes of two components must be same & phase

difference between them should be odd multiples of Π/2.

21 EE

.0,1,2,3...n e Wher

CCWfor π2n2

1-

CWfor π2n2

1

δδ δ xy

Figure.1.19. 3D view for circular polarization

As the wave progress in positive Z-direction the combination of two electric field

vectors Ex & Ey produces a circularly polarized wave that can be clearly seen in 3D view in

above figure. Circular polarization is used where rotational or moving behavior of transmitter

& receiver are required. The circular polarization wave reverses its sense of rotation from




right to left or vice-versa after reflection. So it may cause interference in receiver system due

to the reception of direct path signal & reflected signal. In practical applications it is

admirable to achieve 3-6dB axial ratio bandwidth.

Where as in linear polarization the receiver having weak reception only when

transmitter & receiver are orthogonal to each other.

1.2.9 Bandwidth:-

Bandwidth can be said as the frequencies on both the sides of the centre frequency in

which the characteristics of antenna such as the input impedance, polarization, beam width,

radiation pattern etc are almost close to that of this value. As the definition suggest, the range

of suitable frequencies within which the performance of the antenna, with respect to some

characteristic, conforms to a specific standard.

The bandwidth is the ratio of the upper and lower frequencies of an operation.

L

H

f

f )(BroadbandBandwidth

(%) 100 f

ff d)(NarrowbanBandwidth

C

LH

If L

H

f

f ≥ 2 then, we can easily say it’s a broadband antenna.

1.2.10 Reciprocity:-

In order to study antennas behavior in receiving mode, it can be stated using the

reciprocity theorem.

Figure.1.20. Reciprocity theorem

The thermo is illustrated in figure 1.20 and it states If a voltage is applied to the




terminals of an antenna A and the current measured at the terminals of another antenna B

then an equal current will be obtained at the terminals of antenna A if the same voltage is

applied to the terminals of antenna B.

Thus, in figure 1.20, if Va = Vb then the reciprocity theorem states that Ia = Ib and

can be extended to show that Va / Ia = Vb / Ib. But a consequence of this theorem is that the

antenna gain must be the same whether used for receiving or transmitting. The reciprocity

theorem holds for any linear time-invariant medium. The reciprocity theorem does not state

that the current distribution on the two antennas will be the same when receiving or

transmitting, or the way in which the field changes with respect to time or space at the two

antennas will be the same.




Chapter 2

Microstrip Patch Antenna

In 1953, Deschamps first proposed the concept of printed Microstrip patch antenna.

But later it takes around two decades to develop a practical Microstrip antenna. It was

designed in 1970s by Munson & Howell. In the last few years printed antennas have been

largely studied due to their advantages over other radiating systems in applications such as

aircraft, spacecraft, satellite and missiles, where size, weight, cost, performance, ease of

installation and aerodynamic profile are major constraint.

2.1 Introduction:-

In simplest form of Microstrip antenna, a dielectric substrate is sandwiched between

ground plane and a conducting patch, as shown in figure 1.6. The patch is generally made of

conducting material such as copper or gold and can take any possible shape such rectangular,

triangular, square, circular etc. Most basic shape for the patch is rectangular type. Various

patch types are shown in figure 2.1.

Figure.2.1. Various shapes for patch.

Microstrip patch antennas radiate primarily because of the fringing fields between the

patch edge and the ground plane. For good antenna performance, a thick dielectric substrate

having a low dielectric constant is desirable since this provides better efficiency, larger

bandwidth and better radiation. However, such a configuration leads to a larger antenna size.

In order to design a compact Microstrip patch antenna, substrates with higher dielectric

constants must be used which are less efficient and result in narrower bandwidth. Hence a

trade-off must be realized between the antenna dimensions and antenna performance. Also

increasing the height of substrate introduces surface waves which are undesirable. Surface




waves travel within the substrate and scattered on the edges of patch which causes poor

polarization & radiation pattern called the firing effect.

2.2 Radiation Mechanism of Microstrip Antenna [2]:-

Microstrip antennas radiate primarily because of the fringing fields between the patch

edge and the ground plane. The patch length is approximately half the wavelength (λ/2) in

conventional Microstrip patches to generate fundamental TM10 mode.

reffε

λλ

Where λ0 is the free space wavelength & is the effective dielectric constant which can be

found by following equation,

0.5

rrreff

w

12h1

2

1ε

2

1εε

The value of is slightly less than 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 in figure 2.2. Also the figure shows that if there are no variations of electric field along the

width and the thickness of the patch, then the electric field configuration along the length.

Figure.2.2. Operation of Microstrip antenna

If the fringing fields are resolved into its parallel and tangential components with

respect to the ground plane, the normal components will be out of phase with each other and

would cancel out each other. The tangential components are in phase with each other

therefore the resulting tangential field components would combine to give maximum radiated

field in a direction normal to the patch i.e., the broadside direction.




Figure.2.3. Radiation from the patch

The main challenge in the design for patch of Microstrip antennas is with loosely

bound fields extending into space while keeping the fields tightly bound to the feeding

circuitry. This has to be accomplished with high radiation efficiency and with the desired

polarization, impedance and bandwidth. The patch is usually operated near resonance in order

to obtain real-valued input impedance. If the substrate parameters are specified, there are

three design parameters; the patch length, the patch width and the feed point that controls the

resonant frequency and the resonant resistance. The parameters can be easily acquired by

analyzing the Microstrip antenna. The preferred models for the analysis of Microstrip patch

antennas are the transmission line model, cavity model, and full wave model.

Methods for analysis of Microstrip antenna are as follows:

As Microstrip antennas patch is a 2D component, therefore they may be categorized

as 2D planar component for analysis.

1. First group:- These methods are based on measure of equivalent magnetic current

distribution around patch edges. They are as follows

a. Transmission line model

b. Cavity model

c. Multi-port network model (MNM)

2. Second group:- These methods are based on measure of electric current distribution

on the patch and the ground plane. They are as follows

a. Method of moments (MoM)

b. Finite element method (FEM)

c. Finite-difference time domain (FDTD) method




d. Spatial domain technique (SDT)

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.

2.3 Methods of Analysis:-

2.3.1Transmission Line Model:-

In this model a fringe effect is created at the edges of the patch which cause radiation

from the patch. Fringe is an effect which is situated on the edge or away from the centre of

something. Fringing effect is also explained as the amount of fringing is a function of the

dimensions of the patch and height of the substrate. Due to limitation of patch dimensions,

fields at the edges of patch produce fringing effect. For principle of E-Plane fringe effect is

the function of the ratio of the length of the patch L to the height h of the substrate (L/h) and

of substrate.

Figure.2.4. Electric field lines

In figure 2.4 typical electrical fields lines are situated within the substrate and some in

air. The electric field distribution at the centre is zero and maximum to positive on one side

and max to the negative on the opposite side. For an applied signal it has to see to it that the

maximum and minimum change continuously are maintained according to the instantaneous

phase.




Figure.2.5. Distribution of electric field lines

The fundamental TM10 mode implies that the field varies one half cycle along the

length, and there is no variation along the width of the patch. Along the width of the patch,

the voltage is maximum and current is minimum due to the open end. It can be observed from

figure 2.5 that the vertical components of the electric field (E-field) at the two edges along

the width are in opposite directions and hence cancel one another in the broadside direction,

whereas the horizontal components are in same direction and hence combine in the broadside

direction. Therefore, the edges along the width are termed as radiating edges. The fields due

to the sinusoidal distribution along the length cancel in the broadside direction, and hence the

edges along the length are known as non-radiating edges. So the Microstrip antenna operating

at TM10 mode can be visualized as a transmission line, because the field is uniform along the

width and varies sinusoidally along the length.

2.3.2 Cavity Model:-

Although the transmission line model discussed in the previous section is easy to use,

it has some inherent disadvantages. Specifically, it is useful for patches of rectangular design

and it ignores field variations along the radiating edges. These disadvantages can be

overcome by using the cavity model. A brief overview of this model is given below.

In this model, the interior region of the dielectric substrate is modeled as a cavity

bounded by electric walls on the top and bottom. The basis for this assumption is the

following observations for thin substrates (h << λ).

• Since the substrate is thin, the fields in the interior region do not vary much in the z

direction, i.e. normal to the patch.

• The electric field is z directed only, and the magnetic field has only the transverse

components Hx and Hy in the region bounded by the patch metallization and the

ground plane. This observation provides for the electric walls at the top and the

bottom.




Figure.2.6. Charge distribution & current density creation on the patch

Consider Figure 2.6 shown above. When the Microstrip patch is provided power, a

charge distribution is seen on the upper and lower surfaces of the patch and at the bottom of

the ground plane. This charge distribution is controlled by two mechanisms-an attractive

mechanism and a repulsive mechanism.

The attractive mechanism is between the opposite charges on the bottom side of the

patch and the ground plane, which helps in keeping the charge concentration intact at the

bottom of the patch. The repulsive mechanism is between the like charges on the bottom

surface of the patch, which causes pushing of some charges from the bottom, to the top of the

patch. As a result of this charge movement, currents flow at the top and bottom surface of the

patch. The cavity model assumes that the height to width ratio (h/W) is very small and as a

result of this the attractive mechanism dominates and causes most of the charge concentration

and the current to be below the patch surface. Much less current would flow on the top

surface of the patch and as the height to width ratio further decreases, the current on the top

surface of the patch would be almost equal to zero, which would not allow the creation of any

tangential magnetic field components to the patch edges. Hence, the four sidewalls could be

modeled as perfectly magnetic conducting surfaces. This implies that the magnetic fields and

the electric field distribution beneath the patch would not be disturbed. However, in practice,

a finite width to height ratio would be there and this would not make the tangential magnetic

fields to be completely zero, but they being very small, the side walls could be approximated

to be perfectly magnetic conducting.

Since the walls of the cavity, as well as the material within it are lossless, the cavity

would not radiate and its input impedance would be purely reactive. Hence, in order to

account for radiation and a loss mechanism, one must introduce a radiation resistance RR and




a loss resistance RL. A lossy cavity would now represent an antenna and the loss is taken into

account by the effective loss tangent δeff which is given as:

T

effQ

1δ

Where QT is the total antenna factor & can be expressed as;

rcdT Q

1

Q

1

Q

1

Q

1

Qd represents quality factor of dielectric material & is given as;

tanδ

1

P

WωQ

d

Trd

Where, is the angular resonant frequency.

is the total energy stored in the patch at resonance

is the dielectric loss

is the loss tangent of dielectric substrate

Qc represents quality factor of conductor & is given as

Δ

h

P

WωQ

c

Trc

Where, Pc is conductor loss

Δ is the skin depth of the conductor

h is the height of the substrate

Qr represents quality factor of radiation & is given as;

r

Trr

P

WωQ

Where, Pr is the power radiated from the patch.

Tr

reff

Wω

P

h

Δtanδδ

The above equation gives the total effective loss tangent for the Microstrip patch antenna.

The cavity model method is applicable to antenna in which ground plane & patch

having same dimensions. To overcome this drawback an extension of cavity model, a Multi-

port network model (MNM) is used. In this model the area except patch is considered as




combination of multiple numbers of ports. So the resistant for individual port is calculated &

combined with cavity model to find total radiation pattern.

2.4 Feeding Techniques [3]:-

Microstrip patch antennas can be fed by a variety of methods. These methods can be

classified into two categories- Direct /Contacting and Indirect/ Non-contacting method. In the

direct method, the RF power is fed directly to the radiating patch using a connecting element

such as a Microstrip line. In indirect scheme, electromagnetic field coupling is done to

transfer power between the Microstrip line and the radiating patch.

1. Direct feeding schemes:-

a) Microstrip line feed

b) Coaxial/ Probe feed

2. Indirect feeding schemes:-

a. Electromagnetic coupling /Proximity coupling

b. Aperture coupling

c. Coplanar waveguide

2.4.1 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 2.7. 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.2.7. 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.

2.4.2 Coaxial Feed:-

The Coaxial feed or probe feed is a very common technique used for feeding

Microstrip patch antennas. As can be seen in figure 2.8, 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.

Figure.2.8. Coaxial feed

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. 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. 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. So the non-contacting feed techniques

solve these issues.

2.4.3. Aperture Coupled Feed:-

In this type of feeding technique, the radiating patch and the Microstrip feed line are

separated by the ground plane as shown in Figure 2.9. Coupling between the patch and the

feed line is made through a slot or an aperture in the ground plane.




Figure.2.9. Aperture coupled feed

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. 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.

2.4.4. Proximity Coupled Feed:-

This type of feed technique is also called as the electromagnetic coupling scheme. As

shown in figure 2.10, 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, 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.2.10. 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.

2.4.5 Coplanar Waveguide Feed:-

Figure.2.11. Coplanar waveguide Feed

In this method, the coplanar waveguide is etched on the ground plane of the MSA.

The line is excited by a coaxial feed and is terminated by a slot, whose length is chosen to be

between 0.25 and 0.29 of the slot wavelength. The main disadvantage of this method is the

high radiation from the rather longer slot, leading to the poor front to back ratio. The front-to-

back ratio is improved by reducing the slot dimension and modifying its shape in the form of

a loop.




2.5 Advantages and Disadvantages [3]:-

Microstrip patch antennas are increasing in popularity for use in wireless applications

due to their low-profile structure. Therefore they are extremely compatible for embedded

antennas in handheld wireless devices such as cellular phones, pagers etc. The telemetry and

communication antennas on missiles need to be thin and conformal and are often in the form

of Microstrip patch antennas. Another area where they have been used successfully is in

Satellite communication. Some of their principal advantages are given below:

Light weight and low volume.

Low profile planar configuration which can be easily made conformal to host surface.

Low fabrication cost, hence can be manufactured in large quantities.

Supports both, linear as well as circular polarization.

Can be easily integrated with microwave integrated circuits (MICs).

Capable of dual and triple frequency operations.

Mechanically robust when mounted on rigid surfaces.

Microstrip patch antennas suffer from more drawbacks as compared to conventional

antennas. Some of their major disadvantages are given below:

Narrow bandwidth

Low efficiency

Low Gain

Extraneous radiation from feeds and junctions

Poor end fire radiator except tapered slot antennas

Low power handling capacity.

Surface wave excitation

Microstrip patch antennas have a very high antenna quality factor (Q). It represents

the losses associated with the antenna where a large Q leads to narrow bandwidth and low

efficiency. Q can be reduced by increasing the thickness of the dielectric substrate. But as the

thickness increases, an increasing fraction of the total power delivered by the source goes into

a surface wave. This surface wave contribution can be counted as an unwanted power loss

since it is ultimately scattered at the dielectric bends and causes degradation of the antenna

characteristics. Other problems such as lower gain and lower power handling capacity can be

overcome by using an array configuration for the elements.




2.6 Surface Waves:-

To increase radiation from antenna either height of the substrate or length of the

radiating edge must be increased. While doing so the radiation intensity within the patch

increases, it helps in generation of Surface waves. Now, surface waves are defined as the

modes of propagation supported by the grounded waves.

In antennas, Surface waves spread out in a cylindrical manner, around the feeding

point. The field amplitudes of these waves decreases with distance (r) or more precisely .

When surface waves meet the boundary of dielectric substrate & ground plane, they get

reflected back within the dielectric substrate & propagate within the same. As the surface

waves take up a part of energy from feeded signal, thus decreasing the amplitude of feeding

signal. So, ultimately the efficiency of the antenna decreases.

When the surface waves reach the boundary between dielectric material & air they get

reflected & diffracted by the edges. These diffracted waves causing degradation in the

antenna pattern by producing ripples & also raises side lobes & cross polarization.

To avoid surface waves,

1. Height of the dielectric material should be less than given by following,

r0 ε2π

0.3

λ

h

2. Overall dimension of antenna must be increased (i.e. Use of infinite ground plane).

The second option is impractical as we concentrate on compactness of Microstrip

antenna. So height must be maintained as per above equation.




Chapter 3

Parametric Study of Microstrip Antenna

As basic aim of this project is to achieve broadbanding, dual frequency as well as dual

polarization in a single element probe fed Microstrip antenna. In this chapter certain

techniques to achieve these features are discussed below:

3.1 Techniques to Increase Bandwidth [2]:-

The bandwidth can be defined as frequency range over which Microstrip antenna is

matched with that of the feed line within specified limits.

Q

1Bandwidth

L

W

ελ

hA %Bandwidth

r0

Where, A = 180 for 0.045ελ

h

r0

A = 200 for 075.0ελ

h0.045

r0

A = 220 for 075.0ελ

h

r0

1) Use of thicker substrate with lower dielectric constant:- As bandwidth is directly

proportional to the height of the substrate & inversely proportional to the dielectric

constant it helps in enhancing the bandwidth.

r

hBandwidth

But we have limitation on increasing height as it leads to generate surface

waves which degrades performance of the antenna. Also compactness vanishes.

2) Increasing the length of patch’s radiating edge:- Increasing the length increases

radiation from the antenna which helps in increasing the bandwidth.

3) Using modified shape of the patches:- By converting regular circular or rectangular

shaped patches to circular ring or rectangular ring structures. It enhances the




bandwidth because it decreases quality factor (Q) of the antenna.

The quality factor (Q) of the antenna decreases because less energy is stored

beneath the patch due to reduction in the area of the patch, so higher will be radiation.

4) Use of planar multi-resonator configuration:- Use of Staggered tuned or gap coupled

Microstrip antennas helps in enhancing the bandwidth. In these types of antennas each

element has resonant frequency close to the resonant frequency of main patch.

5) Use of multilayer configuration:- Same as planar multi-resonator configuration but

instead of planar patches are stacked. The stacked patches can be coupled through

electromagnetic or aperture coupled techniques.

6) Impedance matching network

7) Log-Periodic Microstrip antenna configurations:- These configurations are used to

obtain multi-octave bandwidth.

8) Ferrite substrate based Microstrip antenna

9) Cutting slot in Microstrip antennas

10) Use of suspended Microstrip antenna:- In this type of antenna patch with dielectric

material is placed well above ground plane. So air will be acting as another dielectric

material and ultimately effective dielectric constant decreases & bandwidth increases.

The spacing between ground plane & Patch-dielectric can be given by [10],

r0 εh0.16λg

3.2 Techniques to Achieve Dual Frequency Operation [2]:-

Generally dual frequency operation can be achieved using dual feed or single feed

technique. One of the generated frequencies can be used for transmission & other is used for

reception. So isolation becomes matter of concern in antennas. Dual feed antenna uses

separate feed point for each frequency, so better isolation can be achieved. But in single feed

to isolate between transmitter & receiver a diplexer or circulator is required.

1. Use of dual feeding scheme

2. Placing a slot near radiating edge

3. Use of shorting pins or shorting posts (wall):- Use of pins or posts also helps in

generation of multiple frequencies. But it reduces gain of the antenna.




4. Use of varactor diodes or optically tuned diodes.

5. Stacked patch configuration:- In stacked patch configuration lower patch is generally

fed by probe feed method while upper patch can be electromagnetically coupled or

aperture coupled can be used. By adjusting the height of upper substrate frequency

ratio can be increased or decreased.

6. Use of planar multi-resonator configuration

3.3 Techniques to Achieve Circular Polarization:-

Patches such as square, circular, pentagonal, equilateral triangular, ring and elliptical

are capable of generating circular polarization. Circular polarization can be obtained if two

orthogonal modes are excited with a 900 time-phase difference between them. This can be

accomplished by adjusting the physical dimensions of the patch and using either single, or

two, or more feeds.

1. Single feed circularly polarized Microstrip antenna [2]:-

Figure.3.1. Various single fed circularly polarized patch antennas

In single fed circularly polarized Microstrip antenna axial ratio bandwidth is

generally low but VSWR bandwidth is quite large. A best way to recognize whether

the given dimensions are optimum for best AR is, to look in the impedance plot of the

antenna.

If in impedance plot there is kink (knot or extremely small loop), it

corresponds to excitation of two orthogonal modes & yields best AR at that

frequency. Where as if there is absence of kink in impedance plot it leads to poor AR.

As shown in figure 3.2. To achieve better axial ratio L1/L2 in square patch is adjusted.

The results of a diagonally single fed CP MSA are discussed for understanding




importance of kink in impedance plot.

Figure.3.2. Responses of diagonally fed nearly square MSA for different L1/L2

In above figure we can observe different shapes of loops for different

dimension of L1, while L2 kept constant. As L1/L2 increases the loop squeezes

becomes kink, and for that particular frequency axial ratio bandwidth increases

(shown by dark line). Axial ratio decreases to smallest possible value but VSWR

bandwidth for same dimension decreases as compared to its previous value for greater

loop size (shown by dashed line). If we further increase the ratio the kink starts to

disappear & it becomes a straight line (shown by dot-dash line), and its AR bandwidth

decreases. So smaller the loop in impedance plot greater will be AR bandwidth.

In CP square MSA with modified corners, the chopping of two diagonally

opposite corners makes the resonance frequency of mode along this diagonal to be

higher than that for the mode along un-chopped diagonal.

In CP square MSA with diagonal slot, the difference in frequency can be

obtained by using rectangular slot. It increases the path length for other mode, so as

path increases frequency decreases.

CP square MSA with short or chip resistor loaded, in this both AR & VSWR

bandwidth can be increased by replacing shorting posts by chip resistors of 4.7Ω. But

it decreases gain of the antenna.

Square MSA with slits at the edges can also be used for generating CP. Also

parasitic patches can be used to achieve broader bandwidth in CP MSA.

2. Dual fed circularly polarized patch antenna [3]:- Dual feed MSA configuration yields

a wider axial ratio bandwidth & narrower VSWR bandwidth as compared to single

feed MSA configuration.




For a square patch element, the easiest way to excite ideally circular

polarization is to feed the element at two adjacent edges, as shown in Figures 3.3, to

excite the two orthogonal modes. The quadrature phase difference is obtained by

feeding the by feeding the element with a 900

power divider or 90◦ hybrid.

Figure.3.3. Dual feed circularly polarized patch with power divider & 900 hybrid.

While for achieving better AR bandwidth it can be seen from impedance plot

whether to increase or decrease the patch dimension.

i. If a loop is present in impedance plot, it indicates separation between two

orthogonal modes is large & it is to be reduced to obtain better axial ratio

bandwidth.

ii. If there is slight bend in impedance plot without any kink or loop then separation

between two modes needs to be increased for this slot size must be increased.




Chapter 4

Design of Microstrip Antenna

The transmission line model can be further extended to find out length, width,

effective dielectric constant & resonant frequency. We are provided with dielectric material

Epoxy resin i.e. FR-4, which is having dielectric constant (εr) of 4.4. The height of the

substrate is maintained at 1.6mm. As we have seen in previous section for generating circular

polarization square Microstrip antennas are preferred, so we start our design procedure for

square Microstrip antenna.

4.1 Design Parameters:-

Input parameters

a. Substrate material (εr):-

For fabrication Epoxy resin i.e. FR-4 material is used, which is having dielectric

constant (εr) of 4.4.

b. Height of the substrate(h):-

The height of substrate is kept at 1.6mm.

c. Resonant frequency(fr):-

Antenna is designed for UMTS-I (1920 GHz - 2170 GHz) & UMTS-II i.e. 3G (2500

GHz - 2690 GHz) with center frequency 1.81GHz & 2.6GHz. The patch is designed

with center frequency (fr1) of 2.6 GHz & then modified to achieve the other lower

frequency (fr2) of 1.81 GHz.

d. Probe radius of 0.65mm is used.

e. Coax of radius 1.5mm is used.

4.2 Calculation [2]:-

By using transmission model we have,

Width (W) of the patch,

35.11mm1ε

2

2f

cW

rr

As we are designing square patch length & width will be the same.

Length or width of ground plane:-

When the ground plane dimensions are six times of substrate thickness greater than

dimension of the patch, it gives same result as that of infinite ground plane.




45mm44.71W6hWL gg

So ground plane of 45mm × 45mm with patch size of 35.11mm × 35.11mm is used

for fabrication. The patch is feeded with a coaxial probe because of simplicity it provides in

design. The coaxial probe used having a radius of 0.65mm. The design concentrates on 2.6

GHz frequency which a 3G band requires approximately 200MHz bandwidth. To achieve

such a large bandwidth, suspended Microstrip antenna concept is used. The amount of air gap

for suspended Microstrip antenna can be found by following formula [10],

r0 εh0.16λg

So after substituting known parameters we get,

15mm 15.105mmg

But as probe length increases it becomes more inductive & impedance locus shifts

towards right in smith chart. To nullify inductive nature of probe, either we have to add a

capacitance in series with it or increase the diameter of the probe. If we increase of the probe

diameter the cost for SMA connector will be more & due to series capacitance complexity of

the antenna increases. Our basic aim focuses on design of a compact MSA, so we restrict the

height of antenna (g) above ground plane to 3mm. Now to achieve the same bandwidth we

use another broadbanding technique as discussed previously in chapter 3, in this technique

square or circular antennas are converted into square ring or circular ring. So a square slot of

16mm × 16mm is added to enhance bandwidth. Now we can easily achieve the required

bandwidth by combining the two techniques. After adding slot we are getting the required

bandwidth but the radiation pattern for the antenna is not in broad sense. We are getting peak

gain at an angle 300. So after rotating the center slot by 45

0 we can get a broadside radiation

from antenna [8].

The radiation lobe achieved in broad side still suffers from discontinuities due to

unwanted radiation from some antenna edges. To suppress unwanted radiation from antenna

we added a shorting pin of 1mm diameter at (14,-14), which directly ground the waves

directed from that edge. The shorting pin helps in achieving pure broadside radiation, but the

property of shorting pin to ground the waves reduces gain of the antenna [5, 9].

Now the design of antenna is for dual frequency operation, so to generate another

frequency we add a shorting pin at diagonally opposite position (-14,-14) to that of previous

pin [5, 9]. It helps in generating another frequency but not at second center frequency (fr2)




1.81GHz & also it shifts fr1. To shift these two bands to their required center frequency we

have added two corner square shaped slots. By properly adjusting dimensions for these two

corner slots we shifted the bands to the required center frequencies.

The first part in designing is over, still dual polarization not yet achieved. We want

linear polarization for band having center frequency fr2 1.81GHz & circular polarization for

other band. For generating circular polarization axial ratio for that particular band must be

below 2dB. To achieve circular polarization we have added four rectangular slots at radiating

edge.

By using a trial & error method the feeding point is selected for maximum return loss,

and input impedance closer to 50Ω can be achieved.

Table.4.1. Parameters for design of antenna

1 Dielectric material(εr) FR-4 (εr = 4.4)

2. Height of the substrate(h) 1.6mm

2 Center frequency(fr1) 2.6 GHz

3 Center frequency (fr2) 1.81 GHz

4 Ground plane(Lg × Wg ) 45mm × 45mm

5 Width (W) for patch 35.11mm

6 Length (L) for patch 35.11mm

7 Height above ground plane (g) 3mm

8 Center slot (Ls× Ws) 16mm × 16mm

9 Slots at radiating edges (Lss× Wss) 20mm × 2mm

10 Corner slot (Lcs× Wcs)

a. Corner slot1 (Lcs1 × Wcs1)

b. Corner slot2 (Lcs2 × Wcs2)

1mm × 1mm

1.5mm × 1.5mm

11 Shorting pin

a. Pin1

b. Pin 2

Diameter = 1mm,

Length = 4.6 mm

(14,-14)

(-14,-14)

12 Probe Diameter = 1.3mm

Length = 4.6 mm

(-12,-2)




Figure.4.1. Top view & side view for MSA from HFSS [11]




Figure.4.2. Top view for fabricated DBDP MSA

Figure.4.3. Side view for fabricated DBDP MSA




Chapter 5

Results & Discussion

The antenna design focuses on two bands UMTS-I (1.92GHz - 2.17GHz) & UMTS-II

(2.5GHz - 2.69GHz) with different polarization for both bands. The antenna is simulated &

designed using Ansoft’s HFSS 11.1v. HFSS stands for High Frequency Structure Simulator.

The software is based on FEM method, in this method large structures are converted into

number of triangular, pyramidal or trapezoidal shape structure for ease of analysis. The

results for fabricated antenna are found by Agilent’s network analyzer E5062A. The

simulated & measured results are compared below

Figure.5.1. Experimental set up for testing of antenna

5.1 Return Loss:-

The return loss for experimental & simulated can be seen from Figure 5.2 below. The

return loss can be given by,

a. Using simulated results:- For center frequency(fr1) 2.61Ghz it is found to be -36.76dB

& for center frequency(fr2) 1.81Ghz it is found to be -28.02dB.

b. Using experimental results:- Shift in bands is obtained due suspended structure &

uneven ground. So center frequency (fr1) shift to 2.64GHz with return loss of -

34.71dB & center frequency (fr2) shifts to 1.83GHz with return loss of -25.70dB.

The return loss bandwidth can be found for 10dB below frequency response in graph.




The return loss bandwidth can be given by,

100 f

ff (%)bandwidth lossRetun

C

LH

Frequency (GHz)

1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

Re

turn

lo

ss (

dB

)

-40

-30

-20

-10

0

Simulated

Measured

Figure.5.2. Experimental & simulated results for return loss [11, 12]

By using above equation the return loss bandwidth

a. Using simulated results

i. For band with center frequency (fr1) 2.61 GHz

100 2.61

2.4 2.82

% 16.09

ii. For band with frequency (fr2) 1.81 GHz

100 1.81

1.695 1.965

% 14.92




b. Using measured results

i. For band with center frequency (fr1) 2.64 GHz

100 2.64

2.445 2.835

% 14.77

ii. For band with center frequency (fr2) 1.83 GHz

100 1.83

1.725 1.95

% 12.3

Results are quite satisfactory with slight shifts in center frequencies for both the

bands. Generally, the thickness of ground plane is neglected in simulation results. But it

affects in experimental results. Also the ground is uneven & fragile, because its thickness is

very less & no supporting structure is present, can be seen from fabricated antenna figure 4.3.

The antenna is suspended above ground plane, so for supporting purpose three Teflon pins of

1mm are added which also changes effective dielectric constant. We can see from figure 5.2,

the shift in center frequencies of measured & simulated results.

5.2 VSWR:-

As we have seen in return loss graph, the measured response of two frequency bands

also get shifted in VSWR plot.

The VSWR using simulation result is 1.08 & 1.03 for center frequency 1.81GHz &

2.61GHz respectively. The experimental results show 1.31 & 1.38 for center frequency

1.81GHz & 2.61GHz respectively. As the bands shifted for new center frequency 1.83GHz &

2.64GHz the VSWR found to be 1.11 & 1.33.

The VSWR bandwidth is measured for 2dB below frequency response of an antenna.

Using simulation result we have 310MHz & 480 MHz VSWR bandwidth for center

frequency 1.81GHz & 2.61GHz respectively. Where as from experimental set up we have

225MHz & 315MHz VSWR bandwidth for center frequencies 1.83GHz & 2.64GHz

respectively. Figure 5.3 shows comparison of simulated & experimental results for MSA.




Frequency (GHz)

1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

VS

WR

0

1

2

3

4

5

Simulated

Measured

Figure.5.3. Experimental & simulated results for VSWR [11, 12].

5.3 Frequency vs. Gain:-

The Gain of the antenna found to be -0.85dB for 2.61GHz from following graph. The

gain is found to be low due to the use of two shorting pins at opposite corners which

suppresses modes & directly




Frequency (GHz)

1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

Ga

in (

dB

)

-25

-20

-15

-10

-5

0

Figure.5.4. Simulated result for Frequency vs. Gain [11]

grounding the waves. Much of the waves are directly grounded, less energy getting radiated

from antenna & thus gain reduces. The gain pattern for operating frequencies are shown in

figure 5.5 & 5.6 below.




-10 -8 -6 -4 -2

-10

-8

-6

-4

-2

-10-8-6-4-2

-10

-8

-6

-4

-2

0

30

60

90

120

150

180

210

240

270

300

330 Phi = 0 Deg

Phi = 90 Deg

Figure.5.5. Simulated radiation pattern for frequency 2.61GHz [11].




-24 -22 -20 -18 -16 -14 -12 -10 -8

-24

-22

-20

-18

-16

-14

-12

-10

-8

-24-22-20-18-16-14-12-10-8 -24

-22

-20

-18

-16

-14

-12

-10

-8

0

30

60

90

120

150

180

210

240

270

300

330Phi = 0 Deg

Phi = 90 Deg


5.4 Axial ratio vs. Frequency:-

The simulation results found using HFSS are shown in figure 5.7.

As the antenna design focuses on generating dual polarization. We generate a linear

polarization at 1.81 GHz with axial ratio of 17.61dB & circular polarization with axial ratio

of 1.4dB at 2.61GHz.

The axial ratio bandwidth is found for 3dB below frequency response from simulation

result graph. The bandwidth found to be 160MHz. In practical applications it is admirable to

have 3-6dB below frequency response. So for 6dB below frequency response, 340MHz axial

ratio bandwidth can be achieved.




Frequency (GHz)

1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

Axia

l ra

tio

(dB

)

0

2

4

6

8

10

Figure.5.7. Simulation result for frequency vs. axial ratio [11].

Figure 5.8 shows simulated results of LHCP & RHCP patterns for antenna in dB. It

shows LHCP is maximum at Ө = 00

& RHCP is maximum at Ө = 900.




-50 -40 -30 -20 -10 0

-50

-40

-30

-20

-10

0

-50-40-30-20-100-50

-40

-30

-20

-10

0

0

30

60

90

120

150

180

210

240

270

300

330 LHCP (dB)

RHCP (dB)


5.5 Smith Chart:-

In smith chart we can see that whether the antenna feed is perfectly matched or there

is reflection of incident wave. If feed is not perfectly matched energy will be lost & less

power will be delivered to antenna. The input impedance of antenna depends on feeding

location, probe diameter, probe length.




Figure.5.9. simulation result for smith chart [11].

From simulation result we get 51Ω input impedance for our antenna at 2.6GHz.

Ideally 50Ω impedance is required to avoid losses.




Chapter 6

Conclusion & Future Scope

As we have studied various techniques for achieving broad banding, multiple

frequency & polarization schemes using MSA during the scope of the project. We use some

technique for achieving dual frequency, and some of circular polarization technique &

combine them in a compact MSA. But the simulation results generated using HFSS are not

perfectly matched with experimental result, due to the problem in ground plane. Also

combining different technique together generates problem, such as for achieving broad band

center slot is placed in patch but the orthogonal feed location required for achieving circular

polarization is unable to achieve. So axial ratio bandwidth achieved are not fulfilling the

requirement. Use of shorting pins helps in achieving dual frequency operation but it reduces

gain of the antenna to a very low level. Corner chopping helps in shifting the two generated

bands to desired position, simply by varying their dimensions. The slots placed at radiating

edges helps in reducing axial ratio for the antenna. The suspended structure of antenna

requires supporting structure which increases the cost for antenna.

The experimental & simulated results varied not much but in practice 225MHz &

390MHz bandwidths for different frequencies can be obtained using single feed MSA. Also

VSWR bandwidths of 225MHz & 315MHz are achieved.

Future scope:-

As the axial ratio achieved is very less & force antenna to be impractical to use for

mobile communication system, so work can be done to improve it. Corner chopping with

triangular shape can be tried for improving axial ratio bandwidth. Also gain of antenna can be

increased by avoiding use of the shorting pins, so to suppress unwanted waves something else

can be used.




References

[1] K. D. Prasad, “Antenna Wave and Propagation”, Satya Prakashan, New Delhi, 1995

[2] G. Kumar and K. P. Ray, “Broadband Microstrip Antennas”, Artech House, 1992

[3] C. A. Balanis, “Antenna Theory Analysis and Design”, 3rd

Edition, John Wiley and

Sons, New York, 1997.

[4] Kin-Lu Wong, “Compact and Broadband Microstrip”, John Wiley and Sons, 2002

[5] Ramesh Gerg, Prakash Bhartia, Indar Bhal & Apisak Ittipiboon, “Microstrip Antenna

Design Handbook”, Artech House, London, 2001

[6] R. B. Waterhouse, “Microstrip Patch Antenna, A Designer’s Guide”, Kluwer

Academic Publishers, London, 2003

[7] Guntur Kompa, “Practical Microstrip Design and Applications”, Artech House,

London, 2005

[8] V. P. Sarin, M. S. Nishamol, Gijo Augustin, P. Mohanan, C. K. Aanandan, and K.

Vasudevan, “An Electromagnetically Coupled Dual-Band Dual-Polarized Microstrip

Antenna for WLAN Applications”, Microwave And Optical Technology Letters, Vol.

50, No. 7, July 2008, pp-1867-1870.

[9] J.-S. Row and K.-W. Lin, “Low-Profile Design of Dual-Frequency and Dual-

Polarised Triangular Microstrip Antennas”, IEEE 2004, Vol. 40 No. 3.

[10] V. G. Kasbegoudar and K. G. Vinoy, “Broadband Suspended Microstrip Antenna for

Circular Polrization”, PIER 90, pp-353-368, 2009

[11] Ansoft’s HFSS 11.1v

[12] Sigma Plot for Windows version 11.0

DBDP MSA

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