design and equivalent circuit modeling of miniature...

Design and Equivalent Circuit Modeling of Miniature Slotted RFID

Tag Antennas for Metallic Applications

By

Apoorva Sharma

200932002

Communications Research Center

International Institute of Information Technology Hyderabad

June 2011

Design and Equivalent Circuit Model of Miniature Slotted RFID

Tag Antenna for Metallic Applications

A Thesis Submitted

In Partial Fulfillment of the Requirements

For the degree of

Master of Science (by Research)

By

Apoorva Sharma

200932002

[email protected]


International Institute of Information Technology Hyderabad

Gachibowli, Hyderabad, A.P., INDIA - 500032

June, 2011

mailto:[email protected]

Certificate

International Institute of Information Technology, Hyderabad

It is certified that the work contained in this thesis, titled “Design and Equivalent Circuit Model of RFID Tag Antenna for Metallic Objects” by Apoorva Sharma has been carried out under our supervision and it is fully adequate in scope and quality as a dissertation for the degree of Master of Science

Date Dr.Syed Azeemuddin


International Institute of Information Technlogy,

Hyderabad

Date Dr. A.R.Harish

Department of Electrical Engineering

Indian Institute of Technology Kanpur

Dedicated to my Parents

Table of Contents:

Contents List of Figures……………………………………………………………………………………………………………………………….. i

List of Tables………………………………………………………………………………………………………………………………… ii

Acknowledgement………………………………………………………………………………………………………………………. iii

Abstract………………………………………………………………………………………………………………………………………..ix

Chapter 1 ............................................................................................................................................ 1

Introduction ........................................................................................................................................ 1

1.1 Motivation .......................................................................................................................... 1

1.2 Objectives of the Study ....................................................................................................... 4

1.3 Overview of RFID System .................................................................................................... 5

1.4 Advantages of RFID ............................................................................................................. 8

1.5 Organization of Thesis ........................................................................................................ 8

1.6 Key Contributions: .............................................................................................................. 9

Chapter 2 .......................................................................................................................................... 10

Slotted RFID Tag Antenna Design and Analysis ................................................................................ 10

2.1 Structural Design of Slotted RFID Tag Antenna .......................................................................... 11

2.2. Effect of Via, Slots and Floating Plate on tag Antenna .............................................................. 12

2.2.1. Role of Via ..................................................................................................................... 12

2.2.2. Role of Slots .................................................................................................................. 17

2.2.3.1 Analysis of Unslotted Antenna Design ...................................................................... 18

2.2.3.2 Analysis of Slotted antenna Design .......................................................................... 22

2.2.3. Role of floating plate..................................................................................................... 27

2.3 Different Designs with Floating Plate, Conducting Vias and Slots .............................................. 27

2.3.1. Design 1: Slotted RFID Tag Antenna with One Floating Plate....................................... 28

2.3.1.1 Resistance vs. Reactance Plot for Unslotted Antenna and Slotted Antenna ........... 28

2.3.2. Design 2: Slots on Floating Plate ................................................................................... 30

2.4 Effect of floating plate on Slotted Antenna ................................................................................ 31

2.5 Effect on Gain due to Floating Plates ......................................................................................... 33

2.6 Modified Slotted RFID Tag Antenna ........................................................................................... 35

2.7 Analysis of Study ......................................................................................................................... 38

Chapter 3 .......................................................................................................................................... 40

3. Simulation and Fabrication Results .......................................................................................... 40

3.1 Slotted RFID tag Antenna ............................................................................................................ 40

3.1.1 Reflection Coefficient Plot for two tag ICs .................................................................... 42

3.1.2 Interference Effect due to Metallic sheet ..................................................................... 43

3.1.3 Fabrication results ........................................................................................................ 45

3.2 Modified Slotted RFID Tag Antenna ........................................................................................... 45

3.2.1 Simulation Results: .............................................................................................................. 46

3.2.2 Fabrication Results ........................................................................................................ 47

3.3 Comparision between different antennas .................................................................................. 49

CHAPTER 4 ........................................................................................................................................ 52

4. Equivalent Circuit Model .......................................................................................................... 52

4.1 Circuit Model of Unslotted Antenna ..................................................................................... 52

4.2 Circuit model of slotted RFID Antenna ............................................................................. 56

4.3 Regression Technique ....................................................................................................... 56

4.4 Modified Inductance Equation ......................................................................................... 57

4.4 Modified Capacitance Value ................................................................................................... 60

4.5 Circuit Values of RFID tag antenna ................................................................................... 61

4.6 Findings ............................................................................................................................. 65

Chapter 5 ........................................................................................................................................... 66

Conclusion......................................................................................................................................... 66

Related Publications: ........................................................................................................................ 68

References ........................................................................................................................................ 69

List of Figures

Figure 1. 1 RFID System ...................................................................................................................... 6

Figure 1. 2 RFID Tag Picture Showing its Components ....................................................................... 6

Figure 1. 3 RFID Tree ........................................................................................................................... 6

Figure 2. 1 Antenna Characteristics .................................................................................................. 10

Figure 2. 2 Layers of Antenna ........................................................................................................... 12

Figure 2. 3 Position in Antenna ......................................................................................................... 12

Figure 2. 4 a) Design with No Via b) Design with Two Vias ...................... 13

Figure 2. 5 a) Radiation Pattern for Case 1 (antenna without via) b) Radiation Pattern for case 2

(antenna with via) ............................................................................................................................. 14

Figure 2. 6 Current Distribution on Coplanar Patches ...................................................................... 15

Figure 2. 7 Current Distribution on Ground Plane ............................................................................ 15

Figure 2. 8 Current Distribution on Coplanar Patches ...................................................................... 16

Figure 2. 9 Current Distribution on Ground Plane ............................................................................ 16

Figure 2. 10 Current Density Representations ................................................................................. 18

Figure 2. 11 Unslotted Antenna Design ............................................................................................ 19

Figure 2. 12 Radiation Pattern of Unslotted Antenna ...................................................................... 21

Figure 2. 13 Reflection Coefficient Plot ............................................................................................ 22

Figure 2. 14 Slotted Antenna Design ................................................................................................ 23

Figure 2. 15 Radiation Pattern of Slotted Antenna .......................................................................... 25

Figure 2. 16 Reflection Coefficient Plot ............................................................................................ 26

Figure 2. 17 R-X plot Unslotted Antenna with One Floating plate ................................................... 29

Figure 2. 18 R-X Plot of Slotted Antenna with One Floating plate ................................................... 30

Figure 2. 19 Four Slots on floating Plate- RX plot ............................................................................. 31

Figure 2. 20 Designs upto 5 Floating Plates ...................................................................................... 32

Figure 2. 21 Comparison Plot between Frequency and Reflection Coefficient ................................ 33

Figure 2. 22 Gain Patterns ................................................................................................................ 34

Figure 2. 23 Objectives ..................................................................................................................... 35

Figure 2. 24 Modified Slotted Antenna ............................................................................................ 36

Figure 2. 25 R- X Plot of Modified RFID Tag Antenna ....................................................................... 37

Figure 3. 1 Design of Miniature Slotted RFID Tag Antenna ............................................................. 42

Figure 3. 2 Reflection Coefficient of Miniature Slotted RFID Tag Antenna ..................................... 43

Figure 3. 3 Interference Effect ......................................................................................................... 44

Figure 3. 4 Fabricated Antenna ........................................................................................................ 45

Figure 3. 5 Radiation Pattern of Modified RFID Tag Antenna ......................................................... 47

Figure 3. 6 Radiation Pattern of Modified RFID Tag Antenna ......................................................... 47

Figure 3. 7 Fabricated Antenna ........................................................................................................ 48

Figure 3. 8 Experiment setup in Anechonic Chamber ..................................................................... 49

Figure 4. 1 RFID tag antenna ............................................................................................................. 53

Figure 4. 2 RLC circuit ....................................................................................................................... 54

Figure 4. 3 Equivalent Circuit ............................................................................................................ 56



Figure 4. 6 Unslotted RFID Tag Antenna ........................................................................................... 63

Figure 4. 7 Slotted RFID Tag Antenna ............................................................................................... 64

List of Tables:

Table 1.1 Comparisons between Active, Semi-Active and Passive Tags ............................................ 7

Table 2. 1 Design Parameters ........................................................................................................... 13

Table 2. 2 Unslottted Antenna Design Parameters .......................................................................... 20

Table 2. 3 Design Parameters of Slotted Antenna ............................................................................ 23

Table 2. 4 Comparison Table ............................................................................................................ 26

Table 2. 5 No. of Floating Plates, Resonant Frequency and Bandwidth ........................................... 32

Table 2. 6 Gain Table ........................................................................................................................ 34


Table 3. 1 Design Parameters of Slotted Antenna with One Floating Plate ..................................... 41

Table 3. 2 Experimental Results ........................................................................................................ 44

Table 3. 3 Design Parameters of Modified Slotted RFID Tag Antenna ............................................. 46

Table 3. 4 Read Range of Modified Slotted Antenna ....................................................................... 48


Table 4. 1 Range Of Parameters Taken for Regression Analysis ...................................................... 57

Table 4. 2 Unslotted RFID Tag Antenna (55 x 18 x 3.2 mm3) .......................................................... 63

Table 4. 3 Slotted RFID Tag Antenna (49.4 X 18 X 3.2 mm3) ........................................................... 64

Acknowledgement

The present thesis is the result of a collective effort of many people involved at

different stages. I was inspired by their kind cooperation and support. I would like to

express my gratitude to all of them.

First and foremost I take this opportunity to express my sincere gratitude to my guide

Dr. Syed Azeemuddin whose able guidance helped me in completing my thesis. I am

indebted to him for providing the supportive climate to allow me to carry out the

research work effectively. His scholarly advice helped me at each and every stage of

the thesis. This works owe a lot to him.

I also acknowledge my gratitude to my co-guide Dr. A. R. Harish for channeling my

ideas towards my objectives and sharing his valuable comments and suggestions,

allowing me to work in IITK RF and microwave lab. He demonstrated amazing

commitment and inspired me throughout in framing, compilation and presentation of

contents.

I am grateful to Mr. Ankesh Garg and Mr. Raghvendra who provided me useful

suggestions from time to time. I would like to thank Sri. S.K.Kohli for helping me in

fabrication of antennas.

I thank Prof. Rajiv Sangal, Director, IIITH, and all the members of CRC IIITH and

Department of Electrical Engineering IITK for their direct and indirect contribution

towards providing an atmosphere which helped in promoting my personal and

professional growth. I owe my special thanks to my seniors Mr. Sai Sandeep, Mr.

Sandeep Kausik who has consistently supported me at different stages. I am deeply

thankful to my friends Mr. Varun Chawla, Ms. Goral Maheshwari, Mr. Aditya

Gautam for their support and for making my stay wonderful.

I feel deep sense of gratitude to my parents, without whose emotional and social

support it might have been difficult to finish this study.

Apoorva Sharma

Abstract

Designing Radio Frequency Identification (RFID) tag antenna for metallic objects is a

challenging task. The reason is that the antenna parameters like gain and radiation pattern

are highly affected by metallic surface. This thesis presents two different designs of

miniature RFID tag antenna for metallic objects. They can be used for different tag

Integrated Circuits (ICs) having different input impedances simply by varying slot length

and keeping other dimensions intact. The proposed antennas are designed specifically to

eliminate interference effect caused due to metallic objects. By eliminating this limitation

the tag proves to be a uniquely good solution that enables the use of UHF RFID efficiently

for metallic applications like aerospace or automobile industry. In this thesis firstly a

slotted RFID tag antenna is designed whose size is 33 mm x 16 mm x 3.2 mm, and

secondly a modified slotted RFID tag antenna is designed whose size is 64 mm x 24 mm x

1.6 mm.

The proposed slotted RFID tag antenna design contains three metallic layers. Top layer

consists of two metallic rectangular patches which are electrically connected to the ground

plane through copper vias. Multiple slots are created on metallic patches and a non-

connected metallic plate is placed between the ground and the patches. The antenna design

is simulated, fabricated and its performance is analyzed in an Anechonic Chamber. The

read range of the proposed slotted RFID tag antenna for a slot length of 7 mm, mounted on

the metallic surface is approximately 80 cm when.

Next we presented a modified slotted RFID tag antenna with lower antenna thickness.

This design contains two metallic layers. Top layer consists of two coplanar metallic

patches with two slots on each patches and bottom layer is a ground layer. In this design

copper vias are placed at extreme end corners of the patches. Experimental results show

that the same antenna can be used for metallic as well as for non-metallic cases because the

read range of the proposed design is 1.7 m when the tag is mounted on a metallic sheet and

1.1 m without any metal sheet.

In this work we also proposed a mathematical model for slotted as well as for non-

slotted RFID tag antenna to enhance the computation speed of antenna design. RLC circuit

model proposed in this thesis can be used to develop slotted as well as for unslotted RFID

tag antenna for various frequencies and for different tag ICs having different input

impedances. This work is expected to contribute to improvement in antenna design

automation.

1

Chapter 1 Introduction

This chapter contains motivation of the work followed by an overview and organization

of the thesis.

1.1 Motivation

Rapid advances in wireless applications have remarkably increased the usage of Radio

Frequency Identification (RFID) technique which is used for identifying and tracking

objects wirelessly through radio waves. It is gaining popularity in several industries, e.g.,

automotives, aerospace, chemical, health care, power plants and transportation.

Globally, the Ultra High Frequency (UHF) band for RFID system ranges between 840-

960 MHz, within which each region/country is allotted a unique range. For example, UHF

band allotted for RFID applications in Europe is 866-869 MHz and that for India is 865-

867 MHz band [1 −2].

RFID tag is usually attached to different kinds of objects like wood, plastic, metal, etc.

which have different properties. When RFID is attached to metallic objects it develops

interference as metal is an electromagnetic reflector and radio signals cannot penetrate

through it. Hence metallic objects strongly affect the performance of antenna like radiation

pattern, gain, etc. [3]. Due to this limitation it is essential to design an efficient RFID tag

2

antenna which has minimum interference effects due to metallic objects to which it is

attached.

Further, different applications demand that RFID antenna be designed in such a

way that it should have following features:

a) Small size;

b) Proper impedance matching between tag IC and antenna;

c) Low interference effect, due to material in which tag is placed;

d) High gain;

e) Good read range; and

f) Low fabrication cost [4].

Several designs have been proposed in the literature for RFID tag antenna mountable on

metallic objects. To name a few are: dipole antenna, loop antenna, microstrip patch and

planar inverted F-antenna (PIFA). Most common UHF tags are dipole or its variant due to

simple design and cost effectiveness. However, Patch antenna and PIFA (Printed Inverted

F-Antenna) are sometimes a better choice than dipole antenna or wire antenna because of

the following disadvantages of dipole antenna and wire antenna:

(a) Efficiency of dipole antenna gets reduced in close proximity with a metallic sheet

[5];

(b) Resonant frequency of a typical dipole antenna changes when it is located near

metallic surfaces;

(c) Wire antenna consumes a lot of space [6].

3

Fractals are also used in the design of RFID tag antennas to reduce antenna size. One of

the advantages of fractal antennas is their low resonant frequency [7- 8]. By altering the

antenna geometry one can lower the resonant frequency with acceptable radiation pattern.

It has been found that larger the antenna perimeter, lower is the resonant frequency. Hence

the aim for antenna design should be to completely utilize the available sheet space [9].

Another advantage of fractal antennas is its multi-band nature [10]. But these structures are

highly complex to fabricate and are costly.

To reduce interference effect caused due to metallic objects there are two approaches of

designing RFID metallic tags. First is the insertion of high permittivity substrate or by

embedding a High Impedance Surface (HIS) ground plane [11] and second is use of antenna

designs like PIFA and IFA. However, half power bandwidth of PIFA is narrow, in free

space as well as when placed on metallic objects [12-13].

One of the challenges for antenna designer is to design an antenna with low

thickness because several applications such as notebooks, aluminium cans etc. require

antennas with low profile structures having low thickness (around 1mm) [3]. Literature

shows that using High-Impedance Surface (HIS) structure we can design antennas with

lower thickness. Sung Lin Chen and Ken Huang Lin proposed a slim RFID tag antenna

operating at 925 MHz (European RFID band) for metallic objects which is based on a unit

cell structure of HIS whose overall size is 65 mm x 20 mm x 1.5 mm. It is thinner than

inverted-F antenna (IFA), planar inverted-F antenna (PIFA), or patch-type antennas for

metallic objects [14].

Another challenge for antenna designers is to design small sized tag antenna which

can be achieved by inserting a non-connected conductive layer between patch and ground

4

plane, which leads to the increase in overall capacitance [15]. As conductive layer is

introduced between patch and ground plane the overall size is reduced to 32 mm x 18 mm x

3.2 mm from 65 mm x 20 mm x 1.5 mm at 925 MHz [15].

However, we noticed during our

study that the introduction of more than one conductive layer leads to decrease in antenna

gain and bandwidth (discussed in Chapter 2).

One can further miniaturize the antenna with the introduction of slots. To accomplish

this problem we propose a miniature slotted RFID tag antenna with one non-connected

conductive layer and two slots in each patch whose overall size is 33 mm x 16 mm x 3.2

mm at 865 MHz [16].

We also focused on the usability of tag antenna for other tag chips. There are wide

ranges of tag Integrated Circuits (ICs) available in the market with different input

impedances. We know that for maximum power transfer between tag IC and an antenna,

input impedance of antenna has to be a complex conjugate of tag IC‟s input impedance.

One way to vary antenna input impedance is to change overall dimensions like length,

width or by changing dielectric material. The main objective behind this study is to

conceive the geometry of the tag antenna such that the same antenna with same dimensions

can be used for a large range of tag ICs. As a conclusion it is proposed that the slotted RFID

tag antenna can be reused for different tag chips having different input impedances simply

by varying slot length without changing any other antenna dimensions.

1.2 Objectives of the Study

Objectives of this work are as follows:

5

1. In this thesis miniature slotted RFID tag antenna is presented for metallic object

which has low interference effect due to metallic objects. Proposed antenna has

high read range and a good gain. One of the main features of proposed antenna is

that it can be matched to a large range of tag ICs just by varying slot length and

keeping all other dimensions same.

2. In this work we also designed an equivalent lumped model of a proposed RFID

tag antenna such that the tag antenna can be directly synthesized from the

equivalent circuit instead of designing antenna in simulation software which will

enhance computation speed. With the help of equivalent circuits we can easily

estimate resonant frequency, S - parameters, and bandwidth etc.

1.3 Overview of RFID System

RFID system consists of a reader, a tag and an interface software. Schematic diagram of

RFID system is shown in Fig. 1.1. RFID tag can be subdivided into a tag IC and an

antenna (Fig. 1.2). Tag IC holds the unique identification data (ID) of the object such as

ISBN number of the book or title of the book, etc.

6

RFID Reader RFID Tag

Reader Antenna Tag Antenna

Data

Management

System

Figure 1. 1 RFID System

Figure 1. 2 RFID Tag Picture Showing its Components

Figure 1. 3 RFID Tree

RFID System

Passive

LF HF UHF Microwave

Active

Sensor Tags

Other Active Tags

Semi-Passive

7

There are several categories of RFID tags, namely passive, semi-passive or active tags,

each of which has certain advantages and certain disadvantages. Passive tags do not have a

battery. Thus for the system to operate, the tag needs to receive enough power to excite a

tag IC. Moreover, the modulated backscatter signal from RFID tag needs to be correctly

received and decoded by the reader. Semi-passive tags have a tag power source but no

active transmitter. Active tags have both on tag power source and active transmitter. Active

tags have higher read range capability as compared to passive tags. Passive tags have range

of less than 6 meters while active tags can have range in kilometers. Yet one disadvantage

of active tags is that they are more costly than the passive tags. Brief comparisons between

different passive, semi-active and active tags are shown in Table 1.1.

Tags can also be classified as read only and read-write tags [17]. As we want tag IC

with minimal cost and which can store sufficient identification number so for our study we

are using Alien passive RFID tag ICs.

Table 1.1 Comparisons between Active, Semi-Active and Passive Tags

Type Power Supply Size Cost Range

Passive

No battery

present

Small, thin and

light Cheap Short (<6m)

Semi-active Has battery

Large, thick and

heavy Intermediate Upto 100 m

Active Has battery

Large, thick and

heavy Expensive Long(>100m)

8

1.4 Advantages of RFID

RFID tag has many advantages and, therefore, it is increasingly replacing the barcode

technology. Characteristics that make RFID better than the barcode are as follows:

Read range of the RFID tag is greater than that of the bar code;

A large number of tags can be read at the same time;

RFID tags have read/write capacity which is not limited by the line of sight

propagation, as used in barcode.

However, the RFID tags are more expensive than the barcodes [18].

1.5 Organization of Thesis

This thesis is organized as follows:

Thesis consists of five chapters.

Chapter 2 deals with the design of RFID tag antenna for metallic objects. It

describes a brief procedure to make the initial design.

Chapter 3 presents the simulated results and measured results of RFID tag

antenna.

Chapter 4 contains equivalent circuit model of antenna designed for RFID tags. It

shows the various steps of the method used for finding the circuit element values.

This chapter also discusses the ways of modifying the existing equivalent circuit

for HIS and compares simulation and circuit model responses.

9

Chapter 5 contains the conclusion of the work. It also suggests the limitations of

the thesis and in what direction further work need to be carried out.

1.6 Key Contributions:

Designed antennas for RFID tag antenna for metallic applications which can be

used for non-metallic applications also.

Fabrication cost is low.

Simple antenna design.

Antenna can be attached directly to the metallic object without providing spacer.

Antenna can be used for other tag ICs without changing overall dimensions or

without inserting extra matching circuit.

We also proposed a circuit model for tag antenna which is simple and can be

used for any tag IC and at any frequency and has similar response with wave

solver method.

10

Chapter 2 Slotted RFID Tag Antenna Design and Analysis

This chapter discusses design methodology of slotted RFID tag antenna mounted on

metallic objects with a brief description of all the elements involved in the proposed

antenna. Tag antenna in this work is designed to serve the following properties as shown in

Fig. 2.1. In chapter 3 we will discuss the simulation and fabrication results of proposed

antennas.

Figure 2. 1 Antenna Characteristics

TAG Antenna

Small Size High GainGood Read

Range

Usability of tag antenna for

various tag chips

11

2.1 Structural Design of Slotted RFID Tag Antenna

In this chapter we propose a slotted RFID tag antenna mountable on metallic objects

operating in the UHF band of India (865-867 MHz). For this study we have considered

Alien 9440 RFID tag IC whose input impedance is 6-j*125 ohm at 865 MHz. For

maximum power transfer, antenna‟s input impedance has to be complex conjugate of the

input impedance of the tag ICs which are in generally capacitive in nature, hence for

proper impedance matching antenna has to be inductive. To match slotted RFID tag

antenna with Alien 9440 IC, the input impedance of a tag antenna should be 6+j125 Ω at

865 MHz.

Structure of slotted RFID tag antenna consists of three metallic layers, as shown in Fig.

2.2. The top layer contains two rectangular symmetrical metallic patches separated by 1

mm gap and the bottom layer is a ground plane. The tag IC (1 mm x 1 mm) is placed

across the gap between two coplanar patches. In between ground and patch, a non-

connected metallic plate called as „floating plate‟ is present. On the top layer (metallic

patches) multiple slots are created. The top layer is electrically connected to ground plane

through a copper via (vertical post). In this design the insulation between metallic layers

and ground plane is provided by FR4 substrate whose relative permittivity is 4.4, relative

permeability is 1 and dielectric loss tangent is 0.02. Antenna is excited at the position of

tag IC using lumped port excitation which is shown in Fig. 2.3 Proposed design is

simulated in Ansoft HFSS simulator. Advantages of having vias, slots and floating plate in

a tag antenna are explained in next sections.

12

Figure 2. 2 Layers of Antenna

Figure 2. 3 Position in Antenna

2.2. Effect of Via, Slots and Floating Plate on tag Antenna

2.2.1. Role of Via

Conducting via (vertical post) is used to connect two different metallic layers

electrically. In proposed slotted RFID tag antenna, ground plane is electrically connected

to metallic coplanar patches through conducting vias. In this section effect of conducting

via on an antenna is studied. For this study we considered two cases operating at 865 MHz

whose antenna parameters are given in Table 2.1. These cases are:

13

Case 1: Design with no via (Fig. 2.4 (a))

Case 2: Design with two vias (Fig. 2.4 (b))

Ground Plane

Tag ChipMetallic

Patch

Via 1

G = 1 mm

LT = 55 mm

µr, εr

H =3.2 mm

W = 18 mm

a) Top View

b) Side View

LP = 27 mm

X

Y

Z

Via 2

D = 2 mmA =52 mm

Ground Plane

Tag ChipMetallic

Patch

G = 1 mm

LT = 55 mm

H =3.2 mm

W = 18 mm

a) Top View

b) Side View

LP = 27 mm

X

Y

Z

µr, εr

Figure 2. 4 a) Design with No Via b) Design with Two Vias

Table 2. 1 Design Parameters

Parameters Case 1 Case 2

Resonant frequency 865 MHz 865 MHz

Patch length (LP) 27 mm 27 mm

Patch width (W) 18 mm 18 mm

Height (H) 3.2 mm 3.2 mm

Total length (LT) 55 mm 55 mm

Via radius 2 mm 2 mm

14

Center coordinates of Via

1 from bottom -

x= 9 mm, y= 2 mm,

z= 0 mm

Center coordinates of Via

2 from bottom -

x= 9 mm ,y= 53

mm, 0 mm

Via height - 3.2 mm

Size of metallic object 20 x 20 cm2 20 cm x 20 cm

Simulation results show that antenna without via (case 1) has gain -7.6 dB at 865 MHz

(Fig. 2.5 (a)) and antenna with via (case 2) has gain -2.8 dB at 865 MHz (Fig. 2.5 (b)).

Hence gain of an antenna containing via is higher than the antenna without via. The reason

behind this behavior can be explained as follows:

Figure 2. 5 a) Radiation Pattern for Case 1 (antenna without via) b) Radiation Pattern for case 2 (antenna with

via)

Looking at the current distribution on coplanar patches and ground plane we conclude

the following (Fig. 2.6- 2.9):

15

a) For antenna without via the current distribution on coplanar patches is triangular

which means that the current is maximum at the center (where antenna is fed) and

current decreases linearly towards the ends of the antenna (Fig. 2.6).

Figure 2. 6 Current Distribution on Coplanar Patches

Figure 2. 7 Current Distribution on Ground Plane

b) In case of via the current distribution is almost uniform. Maximum peak on coplanar

patches is at center and also at via positions (Fig. 2.8). Also current is distributed

uniformly on the ground plane and peak exists on vias (Fig. 2.9).

16

Figure 2. 8 Current Distribution on Coplanar Patches

Figure 2. 9 Current Distribution on Ground Plane

We can relate the above two cases with short dipole and half wave dipole. A Short dipole

antenna is formed by two conducting rods with a total length (L/2) which is much smaller

than the operating wavelength (λ). A dipole antenna is formed by two quarter wavelength

conductors placed back to back for a total length λ/2 and this antenna is longer than the

short dipole. In both the cases antenna is centrally fed. Experiments show that the current

on a short dipole antenna has a triangular distribution with maxima at the center which

17

linearly tapers off to zero at the ends [19]. As the length of the dipole approaches a

significant fraction of the wavelength, it is found that the current distribution is closer to a

sinusoidal distribution rather that triangular distribution. However, in case of half wave

dipole the current distribution is almost uniform with a node at the corners and anti-node at

center. We can conclude that as the length of the antenna increases, the beam becomes

narrower and as a result, the directivity also increases [20] for e.g. gain of the short dipole

antenna is 1.7 dB and that of half wave dipole is 2.15 dB [21]. From the above arguments

we infer as the electrical length is higher in case of an antenna containing vias compared to

electrical length in case of antenna without via that is why antenna gain is more in case of

antenna containing via.

2.2.2. Role of Slots

In this section unslotted as well as slotted designs are simulated in Ansoft HFSS to

understand the behavior of slots on coplanar patches. In a slotted design two slots are

inserted on each coplanar patches. Fig.2.10 (a) and 2.10 (b) show the current density vector

on coplanar patches of unslotted and slotted antenna respectively and we can conclude that

in case of slotted antenna design the slots cut off the flow path of the current which makes

the current to flow around the slots and this leads to increase in equivalent current path

[22]. As current path is increased, overall inductance is increased which is dependent on

slot length and slot width. Hence by adjusting slot length, antenna inductance can be

increased or decreased. Inductance developed from slots is dependent on slot length and

slot width [23]. This means that if a tag IC is highly capacitive then slot length has to be

18

large and if tag IC‟s capacitance is low then slot length has to be small. Hence by adjusting

slot length, antenna can be matched to any tag IC without changing overall dimensions.

a) Current Density in Unslotted Design

b) Current Density in slotted Design

Figure 2. 10 Current Density Representations

2.2.3.1 Analysis of Unslotted Antenna Design

In this section, a study is done for the comparison of unslotted antenna with slotted

antenna in terms of bandwidth, gain, antenna dimensions, etc. Both antennas are designed

in HFSS such that both can operate at 865 MHz. Structural design of an unslotted tag

antenna consists of a two metallic layers. The top layer contains two coplanar metallic

19

patches which are separated by a gap G and bottom layer is a ground layer. Length of each

patch is LP and width is W (Fig. 2.11). Metallic patches are electrically connected to the

ground plane through conducting vias (vertical post). In between metallic patch and ground

plane, there is a dielectric substrate whose thickness is H and relative permeability is µr

and relative permittivity is εr and a tag IC is placed across the gap between two coplanar

patches.

Ground Plane

Tag ChipMetallic

Patch

Via 1

G = 1 mm

LT = 55 mm

µr, εr

H =3.2 mm

W = 18 mm

a) Top View

b) Side View

LP = 27 mm

X

Y

Z

Via 2

D = 2 mmA =52 mm

Figure 2. 11 Unslotted Antenna Design

20

Geometrical parameters of an unslotted antenna are shown in Table 2.2 which are

optimized to a tag IC whose input impedance is 6-j125Ω at 865 MHz. Dielectric layer FR4

(εr is 4.4 and µr is 1) is filled between coplanar patches and the ground plane. Simulations

are carried out considering that the tag is placed horizontally on a metallic object.

Table 2. 2 Unslottted Antenna Design Parameters

Resonant frequency 865 MHz

Patch Length (LP) 27 mm

Width (W) 18 mm

Height (H) 3.2 mm

Total length (LT) 55 mm

Metallic object 20 x 20 cm2

Via Radius 1 mm

Center Coordinates of

Via 1 on ground plane

9 mm, 2 mm, 0 mm

Center Coordinates of

Via 2 on ground plane

9 mm , 53 mm, 0 mm

Via height 3.2 mm

We observe that the resonant frequency of unslotted antenna is 865 MHz. Antenna gain

is -2.8 dB (Fig.2.12), reflection coefficient is -42.7 dB (Fig. 2.13), and bandwidth is 305

MHz at the 865 MHz.

21

a) Phi plane

b) Theta Plane

Figure 2. 12 Radiation Pattern of Unslotted Antenna

-16.50

-13.00

-9.50

-6.00

90

60

30

0

-30

-60

-90

-120

-150

-180

150

120

Ansoft Corporation HFSSDesign1Radiation Pattern 5

Curve Info

dB(GainPhi)

Setup1 : LastAdaptive

-24.00

-18.00

-12.00

-6.00

90

60

30

0

-30

-60

-90

-120

-150

-180

150

120


Curve Info

dB(GainTheta)


22

Figure 2. 13 Reflection Coefficient Plot

2.2.3.2 Analysis of Slotted antenna Design

In slotted design two identical slots are created in each coplanar metallic patches

(Fig.2.14). These slots behave as an inductor where slot inductance is proportional to the

slot length and slot width [22-23].

0.6 0.7 0.8 0.9 10.65 0.75 0.85 0.950.55-45

-40

-35

-30

-25

-20

-15

-10

Frequency (GHz)

Reflection C

oeffic

ient (d

B)

23

G = 1 mm

LT = 49.4 mm

µr, εr

H = 3.2 mm

W = 18

mm

LP = 24.2 mm

LS = 7 mm

G = 1 mm

S1 = ( 18 mm, 12.35 mm, 3.2 mm)

S3 = ( 18 mm, 30.87 mm, 3.2 mm)

S2 = ( 0 mm,

18.37 mm, 3.2 mm)

S4 = ( 0 mm,

37.05 mm, 3.2 mm)

Y

X

Z

Via 1 Via 2

A = 46.4 mm D =2 mm

Figure 2. 14 Slotted Antenna Design

In this section proposed slotted antenna is simulated at 865 MHz. Design parameters are

shown in Table 2.3. Dielectric layer of FR4 is filled between coplanar patches and the

ground plane.

Table 2. 3 Design Parameters of Slotted Antenna


Total Length (LT) 49.4 mm

Width (W) 18 mm

Height (H) 3.2 mm

24

Slot length (LS) 7 mm

Slots in each patch 2

Slot width (WS) 1 mm

Slot S1 location 18 mm, 12.35 mm , 3.2

mm


mm


mm


mm

Via Diameter 2 mm

Via 1 center coordinate x= 9mm, y= 2 mm, z= 0

mm

Via 2 center coordinate x= 9mm, y= 47.4 mm, z=

0 mm

Via Height 3.2 mm

Ground Plane 20 x 20 mm2

Simulation results show that antenna resonated at 865 MHz. At the resonating frequency

antenna gain is -3.4 dB (Fig. 2.15) and reflection coefficient is -40 dB (Fig. 2.16), and the

bandwidth is 325 MHz.

25

a) Theta Plane

b) Phi Plane

Figure 2. 15 Radiation Pattern of Slotted Antenna

-24.00

-18.00

-12.00

-6.00

90

60

30

0

-30

-60

-90

-120

-150

-180

150

120


Curve Info

dB(GainTheta)


-18.50

-14.50

-10.50

-6.50

90

60

30

0

-30

-60

-90

-120

-150

-180

150

120


Curve Info

dB(GainPhi)


26

Figure 2. 16 Reflection Coefficient Plot

From the above study we conclude that the introduction of slot has resulted in reduction

in the size of the tag, but we also observe a slightly lower gain and broader bandwidth

(Table 2.4).

Table 2. 4 Comparison Table

Dimension

(mm3)

Resonant

Frequency

(MHz)

Gain

(dB)

Bandwidth

(MHz)

Reflection

Coefficient

(dB)

Unslotted

Antenna

55 x 18 x

3.2

865 -2.8 305 -42.7

Slotted

Antenna

49.4 x 18 x

3.2

865 -3.4 325 -40

0.6 0.7 0.8 0.9 10.65 0.75 0.85 0.950.55-45

-40

-35

-30

-25

-20

-15

-10

Frequency (GHz)

Reflection C

oeffic

ient (d

B)

27

2.2.3. Role of floating plate

Resonant frequency is inversely proportional to product of inductance and capacitance

value. In notations (Eqn. 1):

Where, ω is resonant frequency, L is inductance and C is capacitance. Sung Lin Chen

proposed [15] a method to increase the capacitance of an antenna with the decrease in the

resonant frequency keeping other dimensions intact. Design contains conductive layer

(floating plate) between coplanar patches and the ground plane, which leads to the increase

in overall capacitance and lowers resonant frequency without changing other antenna

dimensions. Therefore, as the number of floating plate increases, the capacitance of

antenna also increases. Further, the increase of slot length also results in the increase of

inductance. Thus for a given antenna size, the resonant frequency can be decreased either

by introducing more floating plates or by increasing slot length in the upper patches.

2.3 Different Designs with Floating Plate, Conducting Vias and Slots

It can be noticed from the above study that the length of the current path increases due to

slots and this results in a larger inductance as compared to unslotted antenna. Also it is

28

possible to adjust the inductance by changing only the slot length and keeping patch size

unchanged. Floating plates also helps in size reduction. As tag ICs are capacitive in nature

hence for proper matching between antenna and tag IC, antenna has to be inductive. Now

antenna can be designed by creating slots on three metal layers: a) floating plate, b)

coplanar patches c) ground plane. In this section we are trying to find best position of slots

based on reactance vs. resistance plot.

2.3.1. Design 1: Slotted RFID Tag Antenna with One Floating Plate

The proposed slotted RFID tag antenna contains three metallic layers: top layer is

coplanar patches, middle layer is floating plate and bottom layer is a ground. The upper

layer is electrically connected to the ground plane through a copper via. In this design two

identical slots are created in each coplanar metallic patches.

2.3.1.1 Resistance vs. Reactance Plot for Unslotted Antenna and Slotted Antenna

In this section unslotted antenna and slotted antenna are simulated for various lengths

and widths to check the range of inductance and resistance so that the same design can be

used for other tag ICs having different resistance and capacitance values. For an unslotted

antenna, simulations were carried out by varying antenna width from 13 mm to 23 mm and

antenna length from 25 mm to 35 mm. For slotted RFID antenna, simulations were carried

out for slot length 1 mm to 15 mm, antenna width from 13 mm to 23 mm, and antenna

length from 25 mm to 35 mm. Plots in Fig.2.1 7 & 2.18 shows the Reactance vs. Resistance

Plot for the unslotted antenna and slotted antenna respectively. It is clear from scatter plots

that the tag antenna without slots is capable of achieving only selected combinations of

resistance and reactance. It can be observed from the plot of unslotted design in Fig. 2.17

29

that at a lower resistance the corresponding reactance is also lower whereas for higher

resistance the resultant reactance is also higher. There is almost a linear relationship

between resistance and reactance. Hence the unslotted design will not work for tag ICs

having high capacitance and low resistance for e.g. NXP RFID IC SL3S10 01 FTT UCODE

EPC G2 TSSOP8 package whose input impedance is 22+j*404 ohm. To overcome this

problem, slots are created on the antenna. Therefore, by adjusting slot length inductance

value of an antenna can be increased or decreased. It is clear that this design can give low as

well as high impedance, (for example 22 + j*350 and 22 + j*150), by simply adjusting the

slot length.

Figure 2. 17 R-X plot Unslotted Antenna with One Floating plate

30

Figure 2. 18 R-X Plot of Slotted Antenna with One Floating plate

2.3.2. Design 2: Slots on Floating Plate

In this case four slots are inserted on floating plate instead of coplanar patches and slot

length is varied from 1 mm to 15 mm, antenna width from 13 mm to 23 mm, and antenna

length from 25 mm to 35 mm. Plot in Fig. 2.19 shows the Reactance vs. Resistance Plot. It

is clear that this design can give low as well as high impedance, by simply adjusting the slot

length but there is a gap between area covered in plot. Hence it is a not a good idea to insert

slots on floating plate.

31

Figure 2. 19 Four Slots on floating Plate- RX plot

2.4 Effect of floating plate on Slotted Antenna

Now in this section, a brief study is done on the effect of increasing the number of

floating plates on the performance of the antenna. In all the proposed designs, antenna

dimensions are same (33 mm x 16 mm x 3.2 mm). We used upto five floating plates (Fig.

2.20) and studied the effect on performance. It is found that an increase in the number of

floating plates between ground and patches has the following consequences (Table 2.5 &

Fig. 2.21):

a) Decrease in bandwidth: In the absence of floating plate, the bandwidth is 404 MHz

whereas in the presence of six floating plates the bandwidth decreases to 70 MHz.

b) Decrease in resonant frequency: In the absence of a floating plate resonant frequency

is 1.14 GHz whereas in case of six floating plates it is 525 MHz.

0 10 20 30 40 50 60 70 80 90 1000

50

100

150

200

250

300

350

Resistance

Re

acta

nce

32

c) As the number of floating plate increases the gap between the adjacent resonating

frequencies decreases. Hence we can conclude that by inserting more plates the antenna

size can be reduced but with some trade-off.

Figure 2. 20 Designs upto 5 Floating Plates

Table 2. 5 No. of Floating Plates, Resonant Frequency and Bandwidth

No. of Floating Plate Resonating Frequency

(MHz) Bandwidth (MHz)

Zero 1140 440

One 870 245

Two 745 165

Three 665 120

Four 605 105

Five 565 85

Six 525 70

33

Figure 2. 21 Comparison Plot between Frequency and Reflection Coefficient

2.5 Effect on Gain due to Floating Plates

It is clear from Table 2.6 and Fig. 2.22 that gain of the antenna at the resonant

frequency decreases as the number of floating plate increases. Antenna gain is more in the

absence of floating plate.

34

Figure 2. 22 Gain Patterns

Table 2. 6 Gain Table

Number of floating Plates Gain (dB)

Zero -3

One -6

Two -8

Three -9

Four -9.9

Five -10

Six -11

35

2.6 Modified Slotted RFID Tag Antenna

In this section slotted RFID tag antenna is modified for the following objectives:

a) to improve gain and read range of the antenna;

b) to decrease antenna thickness ;

c) to broaden reactance vs. resistance plot such that the same design with different

dimensions can be used for more number of tag chips. All objectives are given in

Fig. 2.23.

Figure 2. 23 Objectives

Design of modified slotted RFID tag antenna is also based on double mushroom like

structure and contains two metallic layers. Top layer contains two rectangular patches and

bottom layer is a ground layer. These two layers are electrically connected through

Modified TAG Antenna

Low antenna thickness

Small Size High GainGood Read

Range Broader R-X

Plot

36

conducting vias. In the two patches two vias are placed in the extreme end corners: a) in

the first patch it is put in the top left position b) in the second patch it is put in the right

bottom position (Fig. 2.24). The reason to place two vias in extreme end corners is that

now distance between vias is increased which leads to larger current path. As current path

is increased overall inductance is also increased. Hence to have more inductance for small

structure we placed two vias at extreme end corners. In this design we used Duroid (εr is

2.2 and µr is 1) substrate instead of FR4 because FR4 is a lossy dielectric as compared to

Duroid dielectric. The only disadvantage with Duroid is that it is costlier than FR4.

Figure 2. 24 Modified Slotted Antenna

G

LT

LP

LS

Y

X

Z

LS1

LS2LS3

LS4

LW

µr, εr H

Via 1 Via 2

A D

Ground Plane

Tag ChipMetallic

Patcha) Top View

b) Side View

Via

W

Y

37

We now simulate the modified slotted RFID antenna with different length, width and

slot length values for the reactance vs. resistance plot (R-X plot). Here slot length is varied

from 1 mm to 16 mm, antenna width from 10 mm to 30 mm, and antenna length from 45

mm to 65 mm, via 1 position from 2 mm to width/2 mm, via 2 position from width/2 to

width-2 mm. It is clear from Fig. 2.20 that this design can give low as well as high

impedance by simply adjusting antenna dimensions and this design has more covering area

as compared to all designs given above (Fig. 2.13- 2.15). Hence this design is better than

above all designs.

Figure 2. 25 R- X Plot of Modified RFID Tag Antenna

38

2.7 Analysis of Study

From the study we can conclude that:

a) We can improve radiation pattern and gain by connecting ground plane through

patches.

b) Floating plate decreases the resonant frequency. With this we can achieve

miniaturized antenna.

c) With the help of slots we can increase and decrease antenna inductance by

changing slot length without disturbing antenna dimensions.

d) On increasing the number of floating plates it was found that:

1) Antenna gain decreases as the number of floating plate increases; and

2) Antenna bandwidth and resonate frequency decreases with increase in floating

plates.

e) Modified Slotted RFID tag antenna has broader R-X plot as compared to all other

designs proposed in this paper.

f) All proposed antennas with their dimensions, gain are given in Table 2.7. Based on

the table slotted antenna with one floating plate is a good option because its size is

least and modified slotted antenna is another good option because its thickness is

low and its gain is best among all designs. Hence we will focus on these two

designs only. Next chapter contains simulation and fabrication results of slotted

antenna with one floating plate and modified slotted antenna with one floating

plate.

39


Antenna Type

Size (mm x

mm x mm)

Resonant

Frequency

(MHz)

Gain (dB)

Case 1: No Floating Plate

Antenna without via 58 x 18x 3.2 865 -7.6

Antenna with via 55 x 18x 3.2 865 -2.6

Slotted antenna with

vias

49.4 x 18x 3.2 865 -3.4

Case 2: One Floating Plate

Unslotted antenna 42 x 16 x 3.2 865 -7.2

Slotted antenna 33 x 18 x 3.2 865 -6

Modified Slotted

antenna without

metallic object

66 x 24 x 1.6 865 0.5

Modified Slotted

antenna with 180 cm x

120 cm metallic object

66 x 24 x 1.6 865 0.8

40

Chapter 3 3. Simulation and Fabrication Results

This chapter contains simulation and fabrication results of two antennas:

a) Slotted RFID tag with one floating plate

b) Modified slotted RFID tag Antenna

3.1 Slotted RFID tag Antenna

In this work proposed slotted RFID tag Antenna is designed for following

characteristics:

(a) Miniature size (33 mm x 16 mm x 3.2 mm);

(b) Usability of antenna with different tag ICs without changing antenna size; and

(c) Reduction of interference due to metallic object for the given design.

Design parameters are shown in Table 3.1 which is simulated for two different tag ICs

namely, Alien 9440 IC [24] and NXP RFID IC SL3S10 01 FTT UCODE EPC G2 TSSOP8

package [25] by putting their input impedances and different slot lengths in the design.

Here we matched our antenna to the tag IC by taking 22 – j*404 ohm as the IC‟s input

impedance of NXP RFID IC and 6 – j*125 ohm as the IC‟s input impedance of Alien 9440

IC at 865 MHz. To check antenna performance in the presence of metallic object, a 20 cm

x 20 cm metallic sheet is placed below the slotted antenna at a gap of 0.2 mm.

41

Table 3. 1 Design Parameters of Slotted Antenna with One Floating Plate

Parameter Alien 9440 IC NXP RFID IC

Input impedance of a

tag IC

6-j*125 ohm 22 – j*404 ohm

Patch Length (LP) 16 mm 16 mm

Width (W) 16 mm 16 mm

Total Length (LT) 33 mm 33 mm

Height (H) 3.2 mm 3.2 mm

Slot length (LS) 7 mm 13 mm

Slots in each patch 2 2

Slot width (WS) 1 mm 1 mm

Slot S1 location 16 mm, 8.25 mm , 3.2 mm 16 mm, 8.25 mm , 3.2 mm




Via Diameter 2 mm 2 mm

Via 1 center coordinate 8 mm, 2 mm, 0 mm 8 mm, 2 mm, 0 mm

Via 2 center coordinate 8mm, 31 mm, 0 mm 8mm, 31 mm, 0 mm

Via Height 3.2 mm 3.2 mm

Ground Plane 20 x 20 mm2 20 mm x 20 mm

Gap between metallic

object and antenna

0.2 mm 0.2 mm

42

G = 1 mm

LT = 33 mm

H = 3.2 mm

W = 16

mm

LP = 16 mm

LS

G = 1 mm

S1 = ( 16 mm, 8.25 mm, 3.2 mm)

S3 = ( 16 mm, 20.625

mm, 3.2 mm)

S2 = ( 0 mm,

12.3 mm, 3.2 mm)

S4 = ( 0 mm,

24.75 mm, 3.2 mm)

Y

X

Z

Via 1 Via 2

A = 30 mm D =2 mm

Ground Plane

Floating Plane

a) Top View

b) Side View

Figure 3. 1 Design of Miniature Slotted RFID Tag Antenna

3.1.1 Reflection Coefficient Plot for two tag ICs

To match slotted RFID tag antenna with two different tag ICs, we considered two

antennas with the same dimensions (33 mm x 16 mm x 3.2 mm) but different slot lengths.

For perfect matching the reflection coefficient has to be greater than -10 dB. Simulation

results show that the reflection coefficient is -38 dB when the slot length is 13 mm at 865

MHz and the antenna is matched to NXP RFID IC. For the second case the reflection

coefficient is -34 dB when the slot length is 7 mm at 865 MHz and the antenna is matched

to Alien 9440 IC (Fig 3.2). It can be concluded that the same design can be used for

43

different tag ICs just by adjusting slot length without changing its dimensions, i.e., length,

width, height or other parameters.

.

Figure 3. 2 Reflection Coefficient of Miniature Slotted RFID Tag Antenna

3.1.2 Interference Effect due to Metallic sheet

To explore the interference effect due to metallic objects, slotted RFID tag antenna was

simulated by taking metal sheets of different sizes and keeping a constant distance of 0.2

mm between the antenna and the metallic sheet. In this study, the metallic sheet size is

varied from 3 cm to 39 cm. Since the shift in resonant frequency due to different sizes of

metallic sheet is very low, we can conclude that the interference effect is low (Table 3.2).

As the antenna performance does not change by varying metallic sheet dimensions and

44

also this antenna is a wideband antenna, hence this antenna can be attached to smaller as

well as larger metallic sheets.

Figure 3. 3 Interference Effect

Table 3. 2 Experimental Results

Metallic Object Size (mm3 ) Resonant Frequency (MHz)

3 x 3 853

15 x 15 869

24 x 24 870

33 x 33 869

39 x 39 868

45

3.1.3 Fabrication results

Slotted antenna (33 mm x 16 mm x 3.2 mm) with slot length 7 mm was fabricated in

IIT Kanpur PCB lab. Substrate used in the fabrication is FR4 and Alien 9440 IC was

attached to the antenna in between the two patches as shown in Fig 3.4. To verify the read

range performance of the slotted tag antenna, RFID reader Alien 8800 was setup at 865-

867 MHz. Maximum radiation power was 2 W EIRP. The maximum read range was

found to be 80 cm when a metallic plate having size 20 cm x 10 cm was placed below

antenna. This measurement was conducted inside an Anechonic Chamber.

Figure 3. 4 Fabricated Antenna

3.2 Modified Slotted RFID Tag Antenna

Modified slotted RFID tag antenna discussed in Sec. 2.6 is now simulated in Ansoft

HFSS for dimensions shown in Table 3.3.

46

Table 3. 3 Design Parameters of Modified Slotted RFID Tag Antenna


Patch Length (LP) 32.5 mm

Width (W) 24 mm

Total Length (LT) 66 mm

Height (H) 1.6 mm

Slot length (LS) 7 mm

Slots in each patch 2

Slot width (WS) 1 mm

Via Diameter 2 mm

3.2.1 Simulation Results:

We observe that the resonant frequency of modified slotted RFID tag antenna is 865

MHz. Antenna gain is 0.3 dB (Fig.3.5), and reflection coefficient is -32 dB (Fig. 3.6) at 865

MHz.

47

Figure 3. 5 Radiation Pattern of Modified RFID Tag Antenna

Figure 3. 6 Radiation Pattern of Modified RFID Tag Antenna

3.2.2 Fabrication Results

Slotted antenna (66 mm x 24 mm x 1.6 mm) with slot length 7 mm was fabricated.

Substrate used in the fabrication is Duroid and Alien IC was attached to the antenna in

between the two patches as shown in Fig 3.7. To verify the read range performance of the

slotted tag antenna, RFID reader Alien 8800 was setup at 865-867 MHz. Maximum

0.50 0.60 0.70 0.80 0.90 1.00Freq [GHz]

-35.00

-30.00

-25.00

-20.00

-15.00

-10.00

-5.00

dB

(S(L

um

pP

ort

1,L

um

pP

ort

1))

Ansoft Corporation HFSSDesign1XY Plot 3

48

radiation power was 2 W EIRP. Measurement results are given inTable 3.4.The maximum

read range was found to be 1.7 m when a metallic plate having size 180 cm x 120 cm was

placed below the antenna while 1.1 m read range was found without any metal sheet. This

measurement was conducted inside an Anechonic Chamber (Fig. 3.8). From this

experiment it is clear that modified design works for non-metallic as well as for metallic

sheets and read range of tag antenna increases with increase in metal plate size.

Table 3. 4 Read Range of Modified Slotted Antenna

Metal Size (cm2) Read Range (m)

Antenna without metal 1.1

30 x 20 1.44

50 x 50 1.5

180 x 120 1.7

Figure 3. 7 Fabricated Antenna

49

a) Setup of Alien reader

b) Antenna placed on metallic sheet

Figure 3. 8 Experiment setup in Anechonic Chamber

3.3 Comparision between different antennas

A brief comparison of antennas proposed in this thesis is now compared with other

RFID tag antennas for metallic objects in Table 3.5.

50


Paper Name

Band-

width

(MHz)

Size

(mm3)

Gain

(dB) Design Design Description Comment

Miniature Slotted

RFID Tag Antenna for

Metallic Objects

(Design which is proposed in this

thesis) [16]

650-950 33x 16 x 3.2

-6 at

865

MHz

Design contains

three metallic layer

Top layer and

bottom layer is interconnected

with each other

through vias and slots are

introduced on top layer

Middle layer is a

floating layer which is used to

decrease antenna

size

Small size Broadband

antenna

80 cm read range Low interference

due to metallic

objects This antenna can

be used for wide range of tag ICs

keeping antenna

dimesnions same just by chaning

slot length

Modified Slotted RFID Tag

Antenna for

Metallic Objects (Design which is

proposed in this

thesis and to be published)

710-970 66 x 24 x 1.6

0.3 dB

at 865

MHz

Design contains two metallic

layer

Slots are introduced on top

layer and vias are

placed at extreme end corners

Thickness is low High gain

Broad band

1.7 m read range Low interference

due to metallic

objects This antenna can

be used for wide

range of tag ICs keeping antenna

dimesnions same

just by changing slot length

A Slim RFID Tag Antenna for

Metallic

Applications [14]

902-960 65 x 20

x 1.5

Design contains

two metallic layers which are

interconnected

through copper vias

Low antenna

thickness

Good read range

Antenna designed for one

tag IC.

A Miniature RFID Tag Antenna

Design for

Metallic Applications [15]

890-945 32 x 18 x 3.2

Designs contains three layer

Bottom most

layer is used for size reduction

Small size Good design for

future research

Read range 1.5 m Antenna

designed for one

tag IC.

Broadband

Capacitively Coupled Patch

Antenna

for RFID Tag Mountable on

Metallic

Objects[29]

889-932 83 x 35

x 0.8

-6 at 915

MHz

Capacitively

coupled

technique is used for broadband

operation

Long patch is bend into four

radiating parts

where each part is one fourth of

wavelengtht

which makes surface current in

phase in all

elements

Big size Broadband

Antenna

High read range Antenna

designed for one

tag IC.

51

Low-Profile Broadband RFID

Tag antennas

mountable on metallic

objects[30]

840-940 85.5 x

43 x 1.6

-1 at 865

MHz

Parasitic patch

excites another

resonant mode at the frequency

nearby the

fundamental resonant

frequency of driven patch,

which increases

antenna bandwidth

Big size

Broadband

antenna

High gain Antenna

designed for one tag IC.

A Compact UHF

RFID Tag

Antenna Design for Metallic

Objects[31]

898–

930

36 × 36

x 0.8

Structure

contains two L-shaped strips and

two shorting pin

L shaped stip is used to control

reactance of the

antenna

Shorting pins is

used to reduce influence of

metallic object

L shaped strip increases antenna

bandwidth

Complex

Structure

Small Size

1.2 m read range Antenna

designed for one tag IC.

A Low-Profile

Broadband RFID

Tag Antenna for Metallic

Objects[32]

840-960 50 x 98

x 1.6

-5 at

867 MHz

Dual frequency

microstrip antenna is

achieved by

inserting t shaped microstrip line in

one of the

radiating patch

By changing size

of T-shaped

microstrp, resonant

frequency can be

made nearby the resonant

frequency of

main patch. Which results in

increase in

antenna bandwidth.

Large size

High read range Antenna

designed for one

tag IC.

Planar inverted-E antenna for UHF

RFID tag

on metallic objects with

bandwidth

enhancement[33]

857–

980

75 x 22

x 1.6

Stub line is shorted through

via

Simple design with 2.5 m read

range

Antenna designed for one

tag IC.

A Long Read

Range Rfid Tag

Design For Metallic

Objects[34]

922–

928

70 x 24

x 3

Simple design designed for one

tag IC.

52

CHAPTER 4 4. Equivalent Circuit Model

Equivalent circuit model either in form of transmission line model or in form of lumped

circuit model of antennas is widely used to enhance the computation speed of antenna

designs as simulation software sometimes take lot of time to simulate. Circuit parameters

can be determined either from curve fitting method, by experiments or from software results

[26]. With the help of equivalent circuits, parameters like resonance frequency, S -

parameters and bandwidth etc. can be easily estimated. This chapter focuses on RLC circuit

model of slotted as well as of unslotted RFID tag antenna without any floating plate.

In this chapter regression analysis is used to determine the element values of the

equivalent circuit. The advantage of the modified equations is that these equations can be

used for any tag chip and at any frequency. Proposed antenna can be directly synthesized

from the equivalent circuit instead of designing through simulation software [27].

4.1 Circuit Model of Unslotted Antenna

In this section, equivalent circuit model of mushroom shaped unslotted RFID tag

antenna shown in Fig. 4.1 is discussed. This is based on High Impedance Surface (HIS)

structure. This structure is a unit cell of HIS model. The circuit contains two metallic

layers which are electrically interconnected through copper vias.

53

Ground Plane

Tag Chip

Via 1

G

LT

µr, εr

H

W

a) Top View

b) Side View

LP

X

Z

Via 2

D A

Figure 4. 1 RFID tag antenna

This structure can be described as LC lumped circuit model in which inductor is parallel

to capacitor. More than ten years ago Sievenpiper proposed a RLC circuit model for high-

impedance surface (HIS) shown in Fig. 4.2 [11]. HIS structure contains infinitely long

ground plane and an array of infinite number identical rectangular patches. These patches

are electrically connected with ground plane.

In this circuit C is the fringing capacitance, L is a loop inductance, and R is the overall

resistance. This circuit model is based on two principles. Firstly, the electric field is

localized between two coplanar patches which generates fringing capacitance between

them. Fringing capacitance is affected by the separation between coplanar patches which

increases as the separation between the patches is increased. Secondly, in this model the

electric current traverses in a loop pattern starting from the metallic patch to the ground

plane through conducting vias, and then again to the metallic patch. This current path

54

provides inductance which includes patch inductance, via inductance and ground plane

inductance. Sievenpiper also proposed the following equations for inductance, capacitance

of the circuit.

C

L R

A B

Figure 4. 2 RLC circuit

To calculate capacitance, firstly the sheet capacitance is calculated between two

coplanar patches. Then the sheet capacitance is multiplied with geometrical factor, overall

fringing. Here sheet capacitance can be derived using conformal mapping, in which two

semi-infinite plates are separated from each other by gap G and voltage V is applied across

the gap [27]. Sheet capacitance derived from conformal mapping is given by the following

equations [11].

Where Csheet is the sheet capacitance, LP is unit patch length, G is the gap between two

coplanar patches and A is the distance between centers of two vias, εo is the permittivity of

55

vacuum, εr is the relative permittivity of substrate. To get the overall capacitance, sheet

capacitance is multiplied with the geometrical factor which is w/LT [11].

Total inductance L can be calculated by considering geometry as a solenoid, in which

current flows across solenoid in the form of a loop. Solenoid inductance is given as [11]:

Where L is the total inductance, µr is the relative magnetic permeability of the dielectric

material, µo is the vacuum, permeability. LT is the antenna length, W is the antenna width,

and H is the substrate thickness.

In our study the proposed tag antenna is based on a unit cell of High Impedance Surface

instead of array of similar cells. Thus the above L, C equations, based on an infinite array

of cells, need to be modified for single unit cell. As we are considering one unit cell only,

rather than the infinite array, we have to include overhanging capacitance in the circuit

(Fig. 4.3). Overhanging capacitance is the capacitance formed between portion of patch

which is not involved in loop and the ground plane. In case of an HIS we can ignore the

overhanging capacitance because this overhanging part is connected to another unit cell.

In this chapter existing L, C equations of HIS is modified for a unit cell based on

regression analysis which can be applied for slotted case also.

56

Loop Inductance

Fringing Capacitance

between patches Overhanging A B

Figure 4. 3 Equivalent Circuit

4.2 Circuit model of slotted RFID Antenna

For modeling of an equivalent RLC circuit of RFID tag antenna with inductive slots here

we considered two main capacitances: a) fringing capacitance; b) overhanging capacitance

and we considered two main inductances: a) slot inductance; b) loop inductance. Here slot

inductances and loop inductance are in series. On simplification of this circuit we can

realize it as a simple RLC circuit in which overall inductor is parallel to the overall

capacitor similar to circuit shown in Fig. 4.2.

4.3 Regression Technique

In this study an attempt is made to improve the predictability of the equation using

multiple linear regression analysis. Multiple regression analysis is a technique for modeling

and analyzing relationship of one dependent variable on a number of independent variables.

It helps in estimating what part of the variability in the dependent variable can be explained

with the help of the independent variables and which independent variables determine the

57

values of the dependent variable more. More specifically, regression analysis helps us to

examine how the value of the dependent variable changes when any one of the independent

variables is varied by one unit, while the other independent variables are held fixed. Here

we are considering least square method for fitting [28].

On the basis of least square method here we added some correction factor based on

regression analysis to the equations given in [11] and Eqn. 3 & 4 such that results obtained

from modified equation matches with the results generated from HFSS in terms of

reflection coefficient, bandwidth and resonant frequency. Regression is carried out for the

following range of antenna parameters (Table 4.1):

Table 4. 1 Range Of Parameters Taken for Regression Analysis

Parameters Values

Slot width 1 mm

Number of slots on each patch 2 or 0

Slot length 0 mm to 12 mm

Via radius 1 mm

Number of vias 2

H 1.6 mm to 3.2 mm

LT 17 mm to 129 mm

W 16 mm to 64 mm

Gap between two patches 1 mm

4.4 Modified Inductance Equation

Here a correction factor based on regression analysis is added to the Eqn. 4 in such a way

that its response matches with the antenna designed in HFSS with best correlation between

modified equation and desired inductance. We also considered the impact of inductive slots

on antenna. Modified equation is regressed by considering slot length, antenna width,

58

antenna length, antenna height as independent variables. Modified equation can be written

as:

In the above equations A is a correction factor which is linearly regressed from slot

length, antenna width and antenna length. D is a regression coefficient (B value) of original

inductance equation (Eqn. 4), which is estimated from regression analysis. Regression

coefficients for all independent variables are estimated from Statistical Analysis Software

SPSS. Regression line A is given below:

Here, LT is total antenna length, LS is slot length, W is antenna width. Constants of each

variable are estimated from B values estimated from regression analysis.

Here desired inductance is a dependent variable, which is obtained manually for

matching HFSS results and RLC circuit results. Slot length, width, height and total length

are independent variables. Regression analysis showed that slot length has the maximum

effect on the inductance. Antenna width has the minimum effect which is statistically

insignificant at 5% level of significance. Hence slot length is the most important

determinant of inductance.

59

R square value depicts the correlation between outcomes and the predicted value. If R

square is equal to one which means that the fitted curve is 100 % correlated with the desired

values. In our case regression model explains 97.9 percent of the correlation between the

outcomes and the desired inductance values. One of the advantages of modified inductance

equation (Eqn. 3) is that we can use single equation for slotted as well as for unslotted RFID

tag antenna. In case of an antenna without inductive slots consider slot length (LS) to be

zero.


In Fig. 4.4 axis contains different combinations of antenna length, width, antenna height

and slot length. We can easily conclude that the inductance values obtained from my

equation based on regression analysis are very close to the desired inductance values.

0

0.5

1

1.5

2

2.5x 10

-8

Various combinations of length,width,height and slot length

Ind

uc

tan

ce

(H

)

Desired Inductance

Inductance calculated from paper

Inductance calculated from modified formula

From Proposed Equations

From HIS equations

From Wave Solver

60

4.4 Modified Capacitance Value

Now a correction factor based on regression analysis is added to the Eqn. 3 (original

capacitance equation). Modified capacitance equation can be written as:

Here B is a correction factor which is regressed from antenna width and E is a correlation

constant related with original capacitance equation. Correction factor B is given below:

We noticed that Beta value of width square is highest as compared to other independent

variables. Hence square of width is the most important determinant of capacitance. In this

case regression explains 99.8 percent of correlation between the outcomes and the desired

capacitance values.

61


In Fig. 4.5, X axis contains different combinations of length, width, antenna height and

slot length. It can be concluded that the capacitance values obtained from my equation

based on regression are very close to the desired capacitance values.

4.5 Circuit Values of RFID tag antenna

In this section reflection coefficient is calculated for RFID tag antenna (Fig. 4.2). It

serves the following three objectives: a) to check the resonant frequency of circuit; b) to

check the matching of RLC lumped circuit with IC‟s input impedance; and c) to check

accuracy of these equations with respect to the high frequency model designed in HFSS.

Formula of reflection coefficient is given as:

0

2

4

6

8

10

x 10-12

Various Combinations of antenna height, width, length and slot length

Ca

pa

cit

an

ce

(F

)

Modified Capacitance

Capacitance Calculated from paper

Desired Capacitance

From Proposed Equations

From HIS Equations

From Wave Solver

62

Here, Zl is the load impedance and Zo is the characteristic impedance. Load impedance

is the equivalent impedance of RLC circuit shown in Fig. 4.2 which is given as below.

In this study reflection coefficient plots obtained from HFSS and from modified

equations are compared with each other for slotted RFID tag antenna (49.4 x 18 x 3.2 mm3

and slot length is 7 mm) and unslotted RFID tag antenna (55 x 18 x 3.2 mm3). The value of

capacitor calculated from modified capacitance equation for unslotted RFID tag antenna is

is equal to 2.67 pF and value of capacitor for slotted RFID tag antenna is 2.58 pF.

Inductance calculated for unslotted RFID tag antenna is 8.19 nH from modified inductance

equation and for slotted RFID tag antenna is 8.39 nH. If we calculate reflection coefficient

for both circuits from modified equations (Table 4.2 & 4.3) then both circuit resonates at

865 MHz. and both circuits have same bandwidth as compared to the antenna bandwidth

obtained from HFSS.

63

From HFSS

From Modified

Equations

Re

fle

ctio

n C

oe

ffic

ien

t (d

B)

Freq(Hz)

Figure 4. 6 Unslotted RFID Tag Antenna

Table 4. 2 Unslotted RFID Tag Antenna (55 x 18 x 3.2 mm3)

Resistance

Inductance

(nH)

Capacitance

(pF)

Freq.

(MHz)

Modified

Eqn. 0.7 8.19 2.67 865

HFSS - - - 865

64

Re

fle

ctio

n C

oe

ffic

ien

t (d

B)

Freq(Hz)

From Modified

Equations

HFSS

Figure 4. 7 Slotted RFID Tag Antenna

Table 4. 3 Slotted RFID Tag Antenna (49.4 X 18 X 3.2 mm3)

Resistance Inductance (nH) Capacitance (pF) Freq. (MHz)

Modified Eqn. 0.6 8.39 2.58 865

HFSS - - - 865

Hence it can be concluded that reflection coefficient plot generated from modified

equations is very much similar to the plot generated in HFSS. The problem with this model

is that modified equations don‟t give higher order resonating modes as HFSS gives another

resonating mode at 1.8 GHz (Fig. 4.6 & 4.7). Hence these modified equations are valid for

estimation of first resonating mode.

65

4.6 Findings

In this work inductance and capacitance equations are modified using regression

analysis. We can conclude from whole study:

RLC circuit based on modified equations has same resonant frequency as that of

RFID antenna designed in HFSS.

Slot length is the most important determinant of the antenna inductance.

Square of antenna width is the most important determinant of antenna capacitance.

Linear regression equation of capacitance has 99.8 % correlation.

Linear equation of inductance has 97.9% correlation.

This study is helpful to the RFID engineers because designers can design slotted RFID

tag antenna at any frequency and for any tag chip using modified formulae and unslotted

RFID antenna can be designed from modified formulae by putting slot length as to be zero.

66

Chapter 5

Conclusion

In this thesis we presented two different designs of miniature sized slotted RFID tag

antenna for metallic objects. The proposed antennas were designed in such a way that they

can used for other tag ICs having different input impedances just by changing slot length

and keeping other dimensions same. Also antennas are designed to eliminate interference

problem cause due to metallic objects. By eliminating this limitation the tag proves to be a

uniquely good solution that enables the use of UHF RFID efficiently for metallic objects

like in metal container, weaponry tracking.

We showed that the antenna is small in size which is achieved by inserting a floating

layer between patch and ground. By inserting slots and increasing slot length, we achieve

an increase in the inductance which results in low resonant frequency and which leads to

further decrement in size. We also showed that with the help of slots we can achieve high

reactance impedance for such a small design.

Various effects on increasing the number of floating plates were studied and it was

found that: (a) antenna gain decreases as the number of floating plate increases; and (b)

antenna bandwidth and resonating frequency decrease with increase in floating plates. It is

also shown that the same design can be used with wide range of tag ICs by varying the slot

length for example NXP RFID IC SL3S10 01 FTT UCODE EPC G2 TSSOP8 package

when slot length is 13 mm and Alien 9440 IC when slot length is 7 mm.

The experimental results show that the maximum read range of the prototype placed on

a 20 cm x 10 cm metallic object is 80 cm which is matched to Alien 9440 IC. To enhance

67

the computational design of slotted RFID tag antenna we also proposed a circuit model

which is based on regression analysis. Results calculated from circuit model equations give

very similar performance as compared to antenna designed in Ansoft HFSS. One of the

limitations of this circuit model is that it cannot give higher order resonant modes.

We also proposed modified RFID Tag Antenna for metallic objects whose maximum

read range is 1.7 m when the prototype tag is placed on 180 cm x 120 cm metallic object

and 1.1 m without metal sheet. This modified RFID tag can work for metallic as well for

non-metallic applications.

Future Work:

Considering previous studies and the work in this thesis there are possible future works

on RFID antenna design. One area of future work will be to design of RFID antennas in

such a way that the input impedance of antenna and radiation properties remain unchanged

when tag is attached to different materials such as wood, plastic, metal etc.

Another area of future work will to design circuit model in such a way that it should

give other higher resonant modes similar to higher modes given by HFSS. A more analysis

of different slot patterns are required to improve antenna gain and read range.

68

Related Publications:

“Slotted RFID Tag Antenna for Metallic Objects” in ICCSP 2011 (International

Conference on Communication and Signal Processing) [Accepted]

“Non-linearity Between Frequency Bands and Segmentations of Meander Antennas” in

SIBIRCON 2010 (IEEE Region8 Conference on Computational Technologies in

Electrical and Electronics Engineering) [Published]

“Slotted Slim RFID Tag Antenna for Metallic Objects” in IEEE Indian Antenna

Week 2012 [Submitted]

69

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