Design and Equivalent Circuit Modeling of Miniature Slotted RFID Tag Antennas for Metallic Applications. By Apoorva Sharma

Transcription

1 Design and Equivalent Circuit Modeling of Miniature Slotted RFID Tag Antennas for Metallic Applications By Apoorva Sharma Communications Research Center International Institute of Information Technology Hyderabad June 2011

2 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 apoorva.sharma@research.iiit.ac.in Communications Research Center International Institute of Information Technology Hyderabad Gachibowli, Hyderabad, A.P., INDIA June, 2011

3 Copyright 2011 Apoorva Sharma All Rights Reserved

4 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 Communications Research Center International Institute of Information Technlogy, Hyderabad Date Dr. A.R.Harish Department of Electrical Engineering Indian Institute of Technology Kanpur

5 Dedicated to my Parents

6 Table of Contents: Contents List of Figures.. i List of Tables ii Acknowledgement. iii Abstract..ix Chapter Introduction Motivation Objectives of the Study Overview of RFID System Advantages of RFID Organization of Thesis Key Contributions:... 9 Chapter Slotted RFID Tag Antenna Design and Analysis Structural Design of Slotted RFID Tag Antenna Effect of Via, Slots and Floating Plate on tag Antenna Role of Via Role of Slots Analysis of Unslotted Antenna Design Analysis of Slotted antenna Design Role of floating plate Different Designs with Floating Plate, Conducting Vias and Slots Design 1: Slotted RFID Tag Antenna with One Floating Plate Resistance vs. Reactance Plot for Unslotted Antenna and Slotted Antenna Design 2: Slots on Floating Plate Effect of floating plate on Slotted Antenna Effect on Gain due to Floating Plates... 33

7 2.6 Modified Slotted RFID Tag Antenna Analysis of Study Chapter Simulation and Fabrication Results Slotted RFID tag Antenna Reflection Coefficient Plot for two tag ICs Interference Effect due to Metallic sheet Fabrication results Modified Slotted RFID Tag Antenna Simulation Results: Fabrication Results Comparision between different antennas CHAPTER Equivalent Circuit Model Circuit Model of Unslotted Antenna Circuit model of slotted RFID Antenna Regression Technique Modified Inductance Equation Modified Capacitance Value Circuit Values of RFID tag antenna Findings Chapter Conclusion Related Publications: References... 69

8 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 Figure 2. 2 Layers of Antenna Figure 2. 3 Position in Antenna Figure 2. 4 a) Design with No Via b) Design with Two Vias Figure 2. 5 a) Radiation Pattern for Case 1 (antenna without via) b) Radiation Pattern for case 2 (antenna with via) Figure 2. 6 Current Distribution on Coplanar Patches Figure 2. 7 Current Distribution on Ground Plane Figure 2. 8 Current Distribution on Coplanar Patches Figure 2. 9 Current Distribution on Ground Plane Figure Current Density Representations Figure Unslotted Antenna Design Figure Radiation Pattern of Unslotted Antenna Figure Reflection Coefficient Plot Figure Slotted Antenna Design Figure Radiation Pattern of Slotted Antenna Figure Reflection Coefficient Plot Figure R-X plot Unslotted Antenna with One Floating plate Figure R-X Plot of Slotted Antenna with One Floating plate Figure Four Slots on floating Plate- RX plot Figure Designs upto 5 Floating Plates Figure Comparison Plot between Frequency and Reflection Coefficient Figure Gain Patterns Figure Objectives Figure Modified Slotted Antenna Figure R- X Plot of Modified RFID Tag Antenna Figure 3. 1 Design of Miniature Slotted RFID Tag Antenna Figure 3. 2 Reflection Coefficient of Miniature Slotted RFID Tag Antenna Figure 3. 3 Interference Effect Figure 3. 4 Fabricated Antenna Figure 3. 5 Radiation Pattern of Modified RFID Tag Antenna... 47

9 Figure 3. 6 Radiation Pattern of Modified RFID Tag Antenna Figure 3. 7 Fabricated Antenna Figure 3. 8 Experiment setup in Anechonic Chamber Figure 4. 1 RFID tag antenna Figure 4. 2 RLC circuit Figure 4. 3 Equivalent Circuit Figure 4. 4 Equivalent Circuit Figure 4. 5 Equivalent Circuit Figure 4. 6 Unslotted RFID Tag Antenna Figure 4. 7 Slotted RFID Tag Antenna List of Tables: Table 1.1 Comparisons between Active, Semi-Active and Passive Tags... 7 Table 2. 1 Design Parameters Table 2. 2 Unslottted Antenna Design Parameters Table 2. 3 Design Parameters of Slotted Antenna Table 2. 4 Comparison Table Table 2. 5 No. of Floating Plates, Resonant Frequency and Bandwidth Table 2. 6 Gain Table Table 2. 7 Comparison Table Table 3. 1 Design Parameters of Slotted Antenna with One Floating Plate Table 3. 2 Experimental Results Table 3. 3 Design Parameters of Modified Slotted RFID Tag Antenna Table 3. 4 Read Range of Modified Slotted Antenna Table 3. 5 Comparison Table Table 4. 1 Range Of Parameters Taken for Regression Analysis Table 4. 2 Unslotted RFID Tag Antenna (55 x 18 x 3.2 mm 3 ) Table 4. 3 Slotted RFID Tag Antenna (49.4 X 18 X 3.2 mm3)... 64

10 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

11 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 nonconnected 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

12 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 nonslotted 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.

13 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 MHz, within which each region/country is allotted a unique range. For example, UHF band allotted for RFID applications in Europe is MHz and that for India is 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 1

14 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: [5]; (a) Efficiency of dipole antenna gets reduced in close proximity with a metallic sheet (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]. 2

15 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 3

16 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: 4

17 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 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. 5

18 Reader Antenna Tag Antenna RFID Reader RFID Tag Data Management System Figure 1. 1 RFID System Figure 1. 2 RFID Tag Picture Showing its Components RFID System Passive Semi- Passive Active LF HF UHF Microwave Sensor Tags Other Active Tags Figure 1. 3 RFID Tree 6

19 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 Semi-active No battery present Has battery Small, thin and light Cheap Short (<6m) Large, thick and heavy Intermediate Upto 100 m Active Has battery Large, thick and heavy Expensive Long(>100m) 7

20 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. 8

21 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. 9

22 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 In chapter 3 we will discuss the simulation and fabrication results of proposed antennas. TAG Antenna Small Size High Gain Good Read Range Usability of tag antenna for various tag chips Figure 2. 1 Antenna Characteristics 10

23 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 ( 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 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 nonconnected 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 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. 11

24 Figure 2. 2 Layers of Antenna Figure 2. 3 Position in Antenna 2.2. Effect of Via, Slots and Floating Plate on tag Antenna 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: 12

25 Case 1: Design with no via (Fig. 2.4 (a)) Case 2: Design with two vias (Fig. 2.4 (b)) L P = 27 mm L T = 55 mm G = 1 mm L P = 27 mm L T = 55 mm G = 1 mm Y Y W = 18 mm W = 18 mm X a) Top View Tag Chip Metallic Patch X a) Top View Tag Chip Metallic Patch Z Z A =52 mm D = 2 mm µ r, ε r µ r, ε r H =3.2 mm b) Side View Ground Plane H =3.2 mm Via 1 b) Side View Ground Plane Via 2 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 (L P ) 27 mm 27 mm Patch width (W) 18 mm 18 mm Height (H) 3.2 mm 3.2 mm Total length (L T ) 55 mm 55 mm Via radius 2 mm 2 mm 13

26 Center coordinates of Via 1 from bottom - Center coordinates of Via 2 from bottom - x= 9 mm, y= 2 mm, z= 0 mm x= 9 mm,y= 53 mm, 0 mm Via height 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 ): 14

27 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). 15

28 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 16

29 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 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 17

30 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 Current Density Representations 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 18

31 patches which are separated by a gap G and bottom layer is a ground layer. Length of each patch is L P 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 µ r and relative permittivity is ε r and a tag IC is placed across the gap between two coplanar patches. L P = 27 mm L T = 55 mm G = 1 mm Y W = 18 mm X a) Top View Tag Chip Metallic Patch Z A =52 mm D = 2 mm µ r, ε r H =3.2 mm Via 1 Ground Plane Via 2 b) Side View Figure Unslotted Antenna Design 19

32 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 (ε r is 4.4 and µ r 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 Patch Length (L P ) Width (W) Height (H) Total length (L T ) 865 MHz 27 mm 18 mm 3.2 mm 55 mm Metallic object 20 x 20 cm 2 Via Radius Center Coordinates of Via 1 on ground plane Center Coordinates of Via 2 on ground plane Via height 1 mm 9 mm, 2 mm, 0 mm 9 mm, 53 mm, 0 mm 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 db (Fig. 2.13), and bandwidth is 305 MHz at the 865 MHz. 20

33 0 Curve Info db(gainphi) Setup1 : LastAdap a) Phi plane Corporation Radiation Pattern 6 HFSSDe 0 Curve Info db(gainthe Setup1 : LastAdap b) Theta Plane Figure Radiation Pattern of Unslotted Antenna 21

34 Reflection Coefficient (db) Frequency (GHz) Figure Reflection Coefficient Plot 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]. 22

35 L T = 49.4 mm L P = 24.2 mm S 2 = ( 0 mm, mm, 3.2 mm) G = 1 mm S 4 = ( 0 mm, mm, 3.2 mm) Y W = 18 mm G = 1 mm L S = 7 mm X Z S 1 = ( 18 mm, mm, 3.2 mm) S 3 = ( 18 mm, mm, 3.2 mm) A = 46.4 mm D =2 mm H = 3.2 mm µ r, ε r Via 1 Via 2 Figure 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 Resonant frequency Total Length (L T ) Width (W) Height (H) 865 MHz 49.4 mm 18 mm 3.2 mm 23

36 Slot length (L S ) 7 mm Slots in each patch 2 Slot width (W S ) 1 mm Slot S1 location 18 mm, mm, 3.2 mm Slot S2 location 0 mm, mm, 3.2 mm Slot S3 location 18 mm, mm, 3.2 mm Slot S4 location 0 mm, mm, 3.2 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 mm 2 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. 24

37 0 Curve Inf db(gaint Setup1 : LastA Corporation Radiation a) Theta Pattern Plane 15 HFSSD 0 Curve Info db(gainph Setup1 : LastAda b) Phi Plane Figure Radiation Pattern of Slotted Antenna 25

38 Reflection Coefficient (db) Frequency (GHz) Figure 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 Slotted Antenna 55 x 18 x x 18 x

39 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 27

40 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 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 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 & 2.18 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

41 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 R-X plot Unslotted Antenna with One Floating plate 29

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

43 Reactance Resistance Figure 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. 31

44 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 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 One Two Three Four Five Six

45 Figure 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 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. 33

46 Figure 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

47 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 Modified TAG Antenna Low antenna thickness Small Size High Gain Good Read Range Broader R-X Plot Figure 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 35

48 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 (ε r is 2.2 and µ r 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. L T L P G L S1 L S4 Y W L W L S X L S2 Tag Chip a) Top View Via L S3 Metallic Patch Z A D H µ r, ε r Via 1 Via 2 Ground Plane b) Side View Y Figure Modified Slotted Antenna 36

49 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 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 ). Hence this design is better than above all designs. Figure R- X Plot of Modified RFID Tag Antenna 37

50 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. 38

51 Antenna Type Table 2. 7 Comparison Table Size (mm x mm x mm) Resonant Frequency (MHz) Case 1: No Floating Plate Antenna without via 58 x 18x Antenna with via 55 x 18x Slotted antenna with vias 49.4 x 18x Case 2: One Floating Plate Unslotted antenna 42 x 16 x Slotted antenna 33 x 18 x Modified Slotted antenna without metallic object Modified Slotted antenna with 180 cm x 120 cm metallic object 66 x 24 x x 24 x Gain (db) 39

52 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. 40

53 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 (L P ) 16 mm 16 mm Width (W) 16 mm 16 mm Total Length (L T ) 33 mm 33 mm Height (H) 3.2 mm 3.2 mm Slot length (L S ) 7 mm 13 mm Slots in each patch 2 2 Slot width (W S ) 1 mm 1 mm Slot S1 location 16 mm, 8.25 mm, 3.2 mm 16 mm, 8.25 mm, 3.2 mm Slot S2 location 0 mm, 12.3 mm, 3.2 mm 0 mm, 12.3 mm, 3.2 mm Slot S3 location 16 mm, mm, 3.2 mm 16 mm, mm, 3.2 mm Slot S4 location 0 mm, mm, 3.2 mm 0 mm, 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 mm 2 20 mm x 20 mm Gap between metallic object and antenna 0.2 mm 0.2 mm 41

54 L T = 33 mm L P = 16 mm S 2 = ( 0 mm, 12.3 mm, 3.2 mm) G = 1 mm S 4 = ( 0 mm, mm, 3.2 mm) Y W = 16 mm G = 1 mm L S X Z S 1 = ( 16 mm, 8.25 mm, 3.2 mm) a) Top View A = 30 mm S 3 = ( 16 mm, mm, 3.2 mm) D =2 mm H = 3.2 mm Via 1 Via 2 Floating Plane Ground Plane b) Side View Figure 3. 1 Design of Miniature Slotted RFID Tag Antenna 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 42

55 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 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 43

56 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 (mm 3 ) Resonant Frequency (MHz) 3 x x x x x

57 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 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

58 Table 3. 3 Design Parameters of Modified Slotted RFID Tag Antenna Resonant frequency Patch Length (L P ) Width (W) Total Length (L T ) Height (H) Slot length (L S ) 865 MHz 32.5 mm 24 mm 66 mm 1.6 mm 7 mm Slots in each patch 2 Slot width (W S ) Via Diameter 1 mm 2 mm 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. 46

59 db(s(lumpport1,lumpport1)) Figure 3. 5 Radiation Pattern of Modified RFID Tag Antenna Freq [GHz] Fabrication Results Figure 3. 6 Radiation Pattern of Modified RFID Tag Antenna 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 MHz. Maximum 47

60 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 (cm 2 ) Read Range (m) Antenna without metal x x x Figure 3. 7 Fabricated Antenna 48

61 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

62 Table 3. 5 Comparison Table Paper Name Miniature Slotted RFID Tag Antenna for Metallic Objects (Design which is proposed in this thesis) [16] Modified Slotted RFID Tag Antenna for Metallic Objects (Design which is proposed in this thesis and to be published) Bandwidth (MHz) Size (mm 3 ) 33x 16 x x 24 x 1.6 Gain (db) -6 at 865 MHz 0.3 db at 865 MHz Design Design Description Comment 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 Design contains two metallic layer Slots are introduced on top layer and vias are placed at extreme end corners 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 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] 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] 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] x 35 x 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. 50

63 Low-Profile Broadband RFID Tag antennas mountable on metallic objects[30] x 43 x 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] 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] x 98 x 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] 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] x 24 x 3 Simple design designed for one tag IC. 51

64 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. 52

65 L P L T G W X a) Top View Tag Chip Z A D H Via 1 b) Side View µ r, ε r Ground Plane Via 2 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 highimpedance 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 53

66 provides inductance which includes patch inductance, via inductance and ground plane inductance. Sievenpiper also proposed the following equations for inductance, capacitance of the circuit. A B C L R 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 C sheet is the sheet capacitance, L P is unit patch length, G is the gap between two coplanar patches and A is the distance between centers of two vias, ε o is the permittivity of 54

67 vacuum, ε r is the relative permittivity of substrate. To get the overall capacitance, sheet capacitance is multiplied with the geometrical factor which is w/l T [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, µ r is the relative magnetic permeability of the dielectric material, µ o is the vacuum, permeability. L T 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. 55

68 Overhanging A B Fringing Capacitance between patches Loop Inductance 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 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 56

69 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 L T 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, 57

70 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, L T is total antenna length, L S 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. 58

71 Inductance (H) 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 (L S ) to be zero. 2.5 x Desired From Proposed Inductance Equations Inductance From HIS equations calculated from paper Inductance From Wave calculated Solver from modified formula Various combinations of length,width,height and slot length Figure 4. 4 Equivalent Circuit 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. 59

72 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. 60

73 Capacitance (F) x Modified From Proposed Capacitance Equations Capacitance From HIS Equations Calculated from paper Desired From Wave Capacitance Solver Various Combinations of antenna height, width, length and slot length Figure 4. 5 Equivalent Circuit 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: 61

74 Here, Z l is the load impedance and Z o 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 mm 3 and slot length is 7 mm) and unslotted RFID tag antenna (55 x 18 x 3.2 mm 3 ). 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. 62

75 Reflection Coefficient (db) From Modified Equations From HFSS Freq(Hz) Figure 4. 6 Unslotted RFID Tag Antenna Table 4. 2 Unslotted RFID Tag Antenna (55 x 18 x 3.2 mm 3 ) Resistance Inductance (nh) Capacitance (pf) Freq. (MHz) Modified Eqn HFSS

76 Reflection Coefficient (db) From Modified Equations HFSS Freq(Hz) 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 HFSS 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. 64