by
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D" artment of Electrical Engineering and Computer Science
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MASSACHUSETTS INSI
OF TECHNOLOGY
E
AUG 1 4 2006

Submitted to the Department of Electrical Engineering and Computer Science on April 7, 2006, in partial fulfillment of the requirements for the degree of
Master of Engineering in Electrical Engineering
In this thesis, we present the design, simulation, and empirical evaluation of two novel multi-dimensional ultra-high frequency (UHF) passive radio frequency identification
(RFID) tag antennas, the Albano-Dipole antenna and the Albano-Patch antenna, that provide omnidirectional communication capabilities. The performance of a passive UHF RFID tag is highly dependent upon the tag's antenna design, the tag's placement on an item, the materials in the item, and the item's surrounding environment. The majority of existing commercial tag antennas are two-dimensional making the tags a) orientation-sensitive, working well in some directions and not at all in others, and b) susceptible to communication interference from the contents of the tagged object. The Albano antenna designs are three-dimensional, affording the tags to be minimally affected by object material while maintaining near omnidirectional performance. The Albano antenna designs provide significantly improved orientation insensitivity compared with existing widely deployed commercial tag antenna designs.
Thesis Supervisor: Daniel W. Engels
Title: Research Scientist

First and foremost, I'd like to thank my advisor Dr. Daniel W. Engels for his incredible guidance, friendly nature, and ludicrous deadlines. Without Dan's daily reminders to get my thesis done, I would have undoubtedly procrastinated until the extended August deadline to hand in my thesis rather than get it done early. Dan is a brilliant resource with an extensive RFID network and creative and unconventional ideas about antenna design. Without him and our afternoon visits to Starbucks, this thesis could not have happened.
I want to thank Leena Ukkonen from Tampere University of Technology in Finland for her collaborative work on the Albano-Patch design and her patience in answering any questions I had about antenna design and testing. As a female engineer and an
RFID antenna expert, Leena has been a role model for me throughout this whole process.
I'd also like to extend a huge thank you to Silvio Albano of the Gillette Company for both developing the original concept of a 3D RFID tag and for being so helpful and supportive during the dynamic testing at the Gillette RFID facilities.
I would like to thank everybody involved with the MIT Auto-ID Labs- John
Williams, Steve Miles, Rich Fletcher, and Ed Schuster, for inviting me into the exciting RFID community and affording me this opportunity to do what I love: play with electromagnetic waves. In particular, I want to thank my lab mate Sivaram
Cheekiralla for keeping me from talking to myself in the lab: thanks Sivaram for the help with LaTeX, the Doritos, and all the Indian treats.
Lastly, I want to thank my roommates, my friends, and my family for their support this past year. What could have been an awkward, lonely, and transitional year has instead turned out to be one of the most fulfilling and enjoyable times of my life.
Mom and dad, when you suggested back in September that I stay for my PhD, I thought there was absolutely no way I could continue school for another five years. If however, I knew that every year of PhD would be as enjoyable as this year, I definitely would have decided to stay. Thanks everybody.

1 Introduction
1.1 The RFID System ............................
15
16
1.2 Active, Semi-Active, and Passive Tags . . . . . . . . . . . . . . . . . 16
1.3 Passive RFID in Supply Chain Management . . . . . . . . . . . . . . 17
1.4 RFID Frequencies ............................. 18
20 1.5 Ultra-High Frequency Adoption .....................
1.6 Commercial Tag Inlays ..........................
1.7 Thesis Overview ..............................
20
21
2 Electromagnetics and Antenna Theory
2.1 Maxwell's Equations ................... ........
25
25
2.2 Electromagnetic Waves .......................... 27
2.3 Sum m ary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
29
3 Antenna Theory
3.1 The Hertzian Dipole ...........................
3.1.1 The Near-Field Region ......................
3.1.2 The Far-Field Region .......................
3.2 Antenna Parameters ...........................
3.2.1 G ain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
35
3.2.2 Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
35
3.2.3 Resonant Frequency and Bandwidth . . . . . . . . . . . . . . 35
3.2.4 Radiation Pattern ......................... 36
31
32
33
34
34
7

3.2.5 Polarization ............................
3.2.6 Input Impedance .........................
3.3 Sum m ary ............. . ....... ............
4 Transmission and Reception of Power in a Passive RFID System 39
4.1 Reader to Tag Transmission ....................... 39
4.2 Tag to Reader Transmission ....................... 41
4.2.1 Scattering and Radar Cross Section . . . . . . . . . . . . . . . 41
4.2.2 Modulated Backscatter ...................... 42
4.3 Sum m ary . .. . . . .. . . .. . . . . . . . . . . . . . .. . .. . . .
42
36
37
38
5 Challenges in Designing Effective Tag Antennas
5.1 Orientation Sensitivity ..........................
43
43
5.2 Conductive Material Sensitivity . . . . . . . . . . . . . . . . . . . . . 44
5.2.1 Reflections ...... .... .... .. ............. 44
5.2.2 Mirror-Image Charges and Currents . . . . . . . . . . . . . . . 45
5.2.3 Effects of Metal on Antenna Parameters . . . . . . . . . . . . 46
5.3 Environmental Effects ..........................
5.3.1 Temperature and Humidity ...................
5.3.2 Multipath Propagation ......................
5.3.3 Other Antennas .......................... 47
5.4 Sum m ary . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
48
46
46
47
6 The Albano Antenna Designs
6.1 The Albano-Dipole Design .... ..... .. .
....... . .....
49
49
6.1.1 Dipole Antennas ......................... 49
6.1.2 The Three-Dipole Design . . . . . . . . . . . . . . . . . . . . . 52
6.1.3 The Around-the-Corner Design . . . . . . . . . . . . . . . . . 52
6.2 The Albano-Patch Design ........................ 55
6.2.1 The Microstrip Patch Antenna . . . . . . . . . . . . . . . . . . 56
6.2.2 The Albano-Patch Design . . . . . . . . . . . . . . . . . . . . 57
8

6.3 Sum m ary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7 Static Testing of the Albano Tags
7.1 The Albano-Dipole Tag ..........................
7.2 The Albano-Patch Tag ..........................
7.3 Sum m ary .......... ... .............. ......
8 Dynamic Testing of the Albano Tags
8.1 The Albano-Dipole Tag ..........................
8.2 The Albano-Patch tag ..........................
67
67
70
8.3 Sum m ary . . . .. .. . . . . . . . . . . . . . . . . . . . . . . . . .. 71
61
62
63
66
9 Conclusions 73

1-1 The RFID system and its basic components and operating principles. 16
1-2 The 96-bit EPC contains a header and the manufacturer, SKU, and a unique serial number of the identified object . . . . . . . . . . . . . 18
1-3 The UHF frequency ranges for RFID in different regions around the w orld . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
1-4 Commercial passive UHF RFID tags manufactured by Alien Technology and Avery Dennison. ........................
1-5 The Albano tag looks like an "L" in its two-dimensional state. It expands into its three-dimensional state to fit nicely around the corner
21 of a box . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22
2-1 A point charge q has a radially symmetric electric field, E . . . . . . 26
2-2 A current I creates a magnetic field Ht that circles the current-carrying w ire. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27
2-3 A time-varying electric field and a time-varying magnetic field that are coupled and propagating orthogonal to each other, produce a propagating electromagnetic wave. ...................... 28
3-1 The Dish antenna and the Yagi- Uda antenna are two popular antenna designs [14]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32
3-2 The field components of radiating elements are often expressed in spherical coordinates (r, 0, ¢) rather than cartesian coordinates (x, y, z).
33
3-3 The radiation patterns of dipole antennas with lengths A/2 and 5A/4. 36

3-4 Linear and circular polarization [30] . . . . . . . . . . . . . . . . . . 37
5-1 A 915Mhz half-wave dipole simulated in HFSS and its doughnut-shaped radiation pattern .............................. 44
5-2 A point charge placed near to a metal surface creates an image charge at twice the distance from the metal surface . . . . . . . . . . . . . . 45
5-3 Multipath propagation can completely attenuate a signal if two waves add destructively. ............................. 47
6-1 The current distributions of center-fed linear antennas with lengths
A/2, A, and 3A/2 . . . . . . .. .. . .. . . . . . . . . . . . . . . . .. 50
6-2 A center-fed folded half-wave dipole . . . . . . . . . . . . . . . . . . 50
6-3 The original three-dipole Albano-Dipole design, its resonant frequency at 915 Mhz and its radiation pattern as shown in HFSS . . . . . . . 52
6-4 An Optimetric sweep of the inner leg-length shows that the antenna resonates at 915 Mhz when the inner leg-length is equal to 70mm.
..
53
6-5 The final Albano-Dipole design and its dimensions . . . . . . . . . . 54
6-6 The Albano-Dipole's resonant frequency and bandwidth, and its threedimensional polar radiation pattern as shown in HFSS . . . . . . . . 54
6-7 The Albano-Dipole antenna attached to a foil-filled box and the antenna's distorted three-dimensional polar radiation pattern as shown in HFSS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
55
6-8 A microstrip patch antenna is composed of a radiating patch on one side of a dielectric substrate, a ground plane on the other side, and a microstrip feed that connects the radiating patch and the ground plane. 56
6-9 The Albano-Patch antenna and its physical dimensions . . . . . . . . 57
6-10 Directivity patterns (E-plane, o -, H-plane ] -, < = 400 -1-) of the Albano-Patch antenna attached to a) the foiled box, and b) the six-pack of metallic cans .......................... 58
12

7-1 The static testing was conducted in a long, empty hallway to minimize external effects such as metal cabinets and other tags . . . . . . . . .
7-2 Alien Technology's I and Squiggle tags . . . . . . . . . . . . . . . . .
7-3 Orientations A, B, and C. Orientation D is opposite orientation A, orientation E is opposite side B, and orientation F is opposite side C.
7-4 The maximum read ranges of the Albano-Dipole, I, and Squiggle tags in six different orientations when attached to an air-filled box.....
7-5 The maximum read ranges of the Albano-Dipole, I, and Squiggle tags in six different orientations when attached to a foil-filled box . . . . .
7-6 The maximum read ranges of the Albano-Patch, I, and Squiggle tags in six different orientations when attached to an air-filled box .....
7-7 The maximum read ranges of the Albano-Patch, I, and Squiggle tags in six different orientations when attached to a foil-filled box.....
8-1 The dynamic testing setup . ..........................
8-2 The average read rates of the Albano-Dipole, the I, and the Squiggle tags attached to an airbox in six different orientations on a 2.7
feet/second conveyor belt. ........................
8-3 The average read rates of the Albano-Dipole, I, and Squiggle tags when attached to a foil-filled box and placed on two moving conveyor belts.
8-4 The average read rates of the Albano-Patch, I, and Squiggle tags when attached to an airbox. .............................
8-5 The average read rates of the Albano-Patch, I, and Squiggle tags when attached to afoil-filled box and placed on two moving conveyor belts.

Radio Frequency IDentification (RFID) is an enabling technology that allows for the wireless retrieval of unique information about an object. The first RFID-like system appeared in World War II during which the British army attached transponders to their military aircraft to identify and distinguish their aircraft from their enemies'
[24, p. 44] .
Since then, RFID systems have been widely used in a broad range of closed-loop applications, such as animal tracking, highway toll collection systems, and access control systems. Even more recently, emerging interest in RFID has appeared in various global applications. The US Department of Homeland Security has established a $10 billion program to secure the US borders using RFID [25]. Hospitals are investigating RFID to improve the safety of blood transfusion [6]. RFID is seen as a solution to the $500 billion losses in counterfeit products [11]. Supermarkets are interested in using RFID for quality sensing of their perishable products [18]. The benefits of RFID have been realized around the world and across industries, establishing RFID as a permanent technology. This thesis focuses on the application of
RFID in supply chain management, looking specifically at ultra-high frequency tag antennas for use in a packaging environment.

A basic RFID system consists of a reader, a set of tags affixed to objects of interest, and a data management system. The transpoder, or tag, is composed of an antenna and a microchip and attaches to an item X. The tag's microchip contains an identifier that stores information about X. The interrogator, or reader, has antennas that transmit electromagnetic energy to communicate with multiple tags. The reader then interfaces with a data management system that processes and organizes information about the tagged objects. Figure 1-1 presents the basic components and operating principles of an RFID system.
Reader transmits electromagnetic energy to tag
Tag antenna backscatters tag's ID back to reader
Figure 1-1: The RFID system and its basic components and operating principles.
RFID tags are classified as either active, semi-active, or passive depending on their source of power. An active tag has an internal power source, allowing it to generate its own signal and communicate at long distances on the order of kilometers. Technologies such as Bluetooth and Zigbee use active tags to track large assets and create sensor networks. For instance, an active tag placed on a golf course could sense rain and in effect, adjust the automatic sprinkler system to delay a scheduled watering session. The high performance of an active tag however, comes at a high cost. Active

tags today range from $10-50 [8].
Semi-active tags, known also as semi-passive tags, contain a small power source for powering the tag's microchip, yet rely on the reader's energy to backscatter a signal. The performance of a semi-active tag surpasses the performance of a passive tag, but again, at an added cost.
A passive tag has no on-board power source, and instead relies entirely on the energy transmitted by the reader in order to power its microchip and backscatter a signal. Passive tags have read ranges of less than ten meters but the absence of an internal power source allows a passive tag to be small and disposable. The potential of passive tags for extreme low cost make them very attractive for supply chain management. A common price goal for item-level packaging is 5 cents per tag
[13, p. 22]. Currently, Rafsec, a Finnish tag manufacturer announced that its inlays
(the tag antenna antenna and chip without the label) are just under 10 cents per inlay with a minimum purchase of 50,000 inlays [5]. Avery Dennison's AD-220 basic tag inlay is priced at 7.9 cents per tag on a purchase of 1 million tags [5]. Clearly, passive RFID is much more economically feasible for retail management than either active or semi-active RFID systems.
Passive RFID has gained significant popularity in supply chain applications since
June 2003 when Wal-Mart mandated its top 100 suppliers to tag all cases and pallets with RFID transponders [22, p. 3]. RFID provides considerable advantages over bar code scanning, the primary technology used to track objects throughout the supply chain today. Unlike bar codes that require physical line-of-sight communication, passive RFID systems allow simultaneous recognition of multiple items through visible obstructions. Additionally, Wal-Mart mandated a 96-bit electronic product code
(EPC) identification scheme allowing for 1029 unique identifiers [4, p. 6] providing absolute unique tracking not available with the 14-digit Global Trade Item Number
(GTIN) family of identifiers [24, p. 39]. Figure 1-2 shows an example of a 96-

bit Class 1 Gen. 2 electronic product code, which includes information about the identified object such as its manufacturer, stock keeping unit (SKU) or product type, and unique serial number.
O23..*547.!L14=51
.... ....
l
734
Figure 1-2: The 96-bit EPC contains a header and the manufacturer, SKU, and a unique serial number of the identified object.
Passive RFID provides the benefits of real-time inventory visibility, mass serialization, and simultaneous tag recognition, all at a feasible cost.
Passive RFID systems operate at different frequencies depending on the applications they are being used for and the countries in which they are being deployed. Low frequency (LF) RFID systems operate globally at 125-134 Khz and read just beyond actual contact. LF RFID applications include access control and payment technologies and are rarely used for tagging objects. High frequency (HF) RFID tags are globally used at 13.56 Mhz and they can be read less than a meter away from the reader. HF RFID systems are beneficial for tagging items at close range and items which contain liquids. The pharmaceutical industry has taken particular interest in
HF RFID because of its efficiency in the presence of liquids. Pfizer for instance, is tagging individual bottles of Viagra with HF tags to track and authenticate the highly-conterfeited drug [19]. Unlike LF and HF RFID which have standard global spectrum allotments, Ultra-high frequency (UHF) RFID functions at different frequency ranges depending on the country in which it is being used. In the United
States, UHF RFID readers use a frequency hopping scheme between 902 Mhz and
928Mhz, a range often referred to by its center frequency, 915Mhz [12, p. 4]. Figure

1-3 displays a chart comparing frequency ranges used for UHF RFID in different parts of the world.
/
UHF
Figure 1-3: The UHF frequency ranges for RFID in different regions around the world.
UHF RFID systems provide read ranges up to ten meters, and are primarily used in supply chain applications. In December 2004, EPCglobal, a global body for creating industry standards in RFID technology, approved the second generation
EPC air interface protocol [21]. Among many specifications such as the identification scheme shown in Figure 1-2, the Gen 2 standard requires UHF tags to operate globally between 860-960 Mhz, such that a tagged product made in China can be shipped to the United States and continue to be identified and tracked.
Finally, microwave RFID is being newly explored for its high data read rates.
Microwave RFID operates at either 2.45 Ghz or 5.8Ghz and has a range limited to 50 cm. Microwave-based systems in general are highly sensitive to liquid and conductive materials, and this type of RFID is currently restricted to specialized applications such as airline baggage tracking.
Due to their low read ranges, LF and microwave RFID systems are not suitable for supply chain tracking. HF and UHF systems however, each offer their own advantages, water-penetration and long range respectively, and are both being tested and used for various applications.

Ultra-high frequency (UHF) RFID has been globally adopted as the standard RFID system in the distribution sector of the supply chain with items tagged at the case and pallet levels.
UHF systems operate in the far-field region where energy is transferred through electromagnetic propagation. High frequency (HF) systems function in the near-field region where energy is transferred through magnetic coupling [20, p. 22-28]. UHF tags are attractive because of their small size, faster response rates, and longer read ranges as compared to common HF RFID tags. However, UHF tag antennas have increased sensitivity when placed close to certain materials. Radiolucent materials such as paper, cardboard, and fabrics have little effect on the penetration of UHF waves, whereas metals, liquids, and moisture-containing materials reflect and attenuate electromagnetic waves and cause performance degradation in the UHF RFID systems [29, p. 1].
In addition to their high sensitivity to conductive and absorbing materials, UHF tag antennas are orientation-limited [7, p. 1]. A true isotropic antenna, one that radiates equally in all directions, is simply not realizable for passive UHF tags [15, p.
23].
Alien Technology, Avery Dennison, and Rafsec are three of many companies manufacturing UHF RFID tag inlays. These companies develop creative antenna designs that are optimized for either range, bandwidth, size, or the ability to work in the presence of metals. For instance, Alien's "2x2" tag is a designed for packages that require small labels [1]. Avery Dennison's "AD-612" is a bow-tie antenna that operates globally.
Figure 1-4 shows various examples of tags designed by Alien Technology and Avery
Dennison.
Irrespective of the creativity and intricacy involved in these designs, they all share

Alien's "2x2" Alien's "Squiggle"
"I"
"AD-220"
Figure 1-4: Commercial passive UHF RFID tags manufactured by Alien Technology and Avery Dennison.
a common limiting feature; they are two-dimensional.
The two-dimensional structure of existing tags allows for a simple "slap and stick labelling process, but prevents radiation in certain directions. This orientation limitation prompted the idea of an L-shaped antenna configuration, the Albano antenna, that is placed on a flat box such that it forms a three-dimensional antenna when the box is folded into shape. Figure 1-5 presents the Albano-Dipole antenna and its three-dimensional transformation.
We have designed two versions of the Albano tag, a folded-dipole version and a folded microstrip patch version, that have omnidirectional readability with equal communication range in all directions. The Albano-Dipole antenna and the Albano-
Patch antenna were simulated using Ansoft's High Frequency Structure Simulator
(HFSS). They were then tested in both static and dynamic environments and the performance of the Albano designs were measured and compared to the performance

Figure 1-5: The Albano tag looks like an "L" in its two-dimensional state. It expands into its three-dimensional state to fit nicely around the corner of a box.
of Alien Technology's planar "I" and "Squiggle" tags. The static testing showed that while the Alien tags perform extremely well in some orientations and'poorly in others, the Albano tags radiate consistently in all directions. Furthermore, both Albano tags outperfrom the Alien tags when placed next to conductive material, and the Albano-
Patch particularly maintains full omnidirectionality in the presence of metal where the Alien tags communicated in only two of six orientations.
The Albano tags were then tested on moving conveyor belts and again they displayed high performance by communicating in at least two more directions than the Alien tags in every instance. In particular, the Albano-Dipole performed omnidirectionally next to metal on a moving 2.7 feet/second conveyor belt, where the performances of the I and Squiggle tags were greatly degraded.
The Albano-Dipole antenna and the Albano-Patch antenna are two novel threedimensional tag antenna designs that provide improved orientation insensitivity over existing commercial tag antenna designs.
Before a detailed discussion of the tags is presented, we introduce basic electromagnetic concepts in Chapter 2 to conceptualize the propagation and energy storage of electromagnetic waves. In Chapter 3, we discuss antenna theory to understand how antennas work and the characteristics that designers use to describe and optimize antennas. We then explain the power transfer that occurs between the reader antenna and the tag antenna in Chapter 4. In Chapter 5, we survey the challenges that antenna designers face in UHF RFID tag antenna design. In Chapter 6, we

present the design and simulations of the Albano-Dipole and Albano-Patch antennas.
We empirically evaluate the performances of the Albano tags in static and dynamic environments in chapters 7 and 8. Finally, we discuss relevant conclusions and future work in Chapter 9.

In 1864, James Clerk Maxwell recognized a symmetric relationship between electric and magnetic fields and by joining four simple equations, he established the basis of electromagnetism [16, p. 13]. In differential form, Maxwell's equations are:
-
-0B dt
(2.1)
Vx I = j+ ad dt
V-D = p
(2.2)
V-B = 0 (2.4) where

E = electric field (V/m)
H magnetic field (A/m)
B
= magnetic flux density (T)
D
= electric displacement (C/m 2
)
J = electric current density(A/m 2
)
p = electric charge density (C/m 3
)
Coulomb originally showed that a static electric charge q creates an electric field
E that radiates omnidirectionally from the point charge:
A ff
Figure 2-1: A point charge q has a radially symmetric electric field, E.
The strength of the electric field decays as 1/r
2 where r is the radial distance from the point charge:
q 2 F(2.5)
4w7re
A magnetic field H however, stems from the movement of charge, I as shown by the Biot-Savart Law:
:
H= I xir (2.6)
In 1831, Faraday experimentally demonstrated that a changing magnetic field produces an electric field, a phenomena that we refer to as Faraday's Law and constitutes the first of Maxwell's four equations. Even earlier in 1820, Ampere had experimentally shown that a current on a wire produces a magnetic field that circles the wire,

Qldt
Figure 2-2: A current I creates a magnetic field H that circles the current-carrying wire.
leading to Ampere's Law which is Maxwell's second of four equations and is consistent with the Biot-Savart Law. Figure 2-2 shows a visual representation of Ampere's Law.
Maxwell appended Ampere's Law to include a changing electric field, &. By showing that a changing electric field acts as a current and induces a magnetic field, Maxwell made a symmetrical relationship between electric and magnetic fields that has forever shaped our understanding of electromagnetics.
From Maxwell's equations, we know that a time-varying electric field induces a timevarying magnetic field which then induces a changing electric field and so on. If a time-varying electric field E, and a time-varying magnetic field H, are coupled and propagating orthogonal to each other, they produce an energy-storing electromagnetic
wave which travels orthogonal to both of the fields. Figure 2-3 displays a propagating electromagnetic wave and its field components.
Electromagnetic waves travel at the speed of light, c = 3 x 108 m/s. They are characterized by their wavelength A (m) and their frequency f (1/s) which are inversely proportional to each other:

16
Figure 2-3: A time-varying electric field and a time-varying magnetic field that are coupled and propagating orthogonal to each other, produce a propagating electromagnetic wave.
(2.7)
The energy of an electromagnetic wave is stored in the electric and magnetic fields and the wave has an "energy flux" at a particular time and space proportional to the magnitudes of its corresponding electric and magnetic fields:
S = Re(E x IH*) (W/m
2
) (2.8) where H* is the complex conjugate of H. Because E and H are time-varying, we define the Poynting vector S as the wave's time averaged power per unit area through a surface which is normal to it. S is also known as the power density and has units of W/m
2
The field strengths of E and H are related by the permnittivity e and the permeability p of the medium through which the electromagnetic wave is traveling. In free space,
= = nlo (Q)
(2.9) where

Eo
= free space permittivity = 8.8542x 10-12 (F/m)
Ao= free space permeability = 47r x 10-' (H/m) and rio is the impedance of free space and equals 377 Q.
Another important wave parameter is the spatial frequency k, which characterizes the variation of an electromagnetic wave in space [16, p. 8]: w 27r
where w and Ao are the radial frequency and wavelength of the traveling wave respectively, and c is the speed of light.
In this chapter, we showed how charges and the movement of charges create electric and magnetic fields. We also demonstrated how coupled electric and magnetic fields can generate energy-storing propagating waves. Propagating electromagnetic waves are the basis for all existing wireless technologies. In the following chapter, we will use this understanding to discuss the role of an antenna as it provides an interface between information-carrying guided waves and electromagnetic waves.

An antenna is a "transition device, or transducer, between a guided wave and a freespace wave, or vice-versa" [15, p. 12]. Using Ampere's Law, we illustrate that a current-carrying element or antenna creates a magnetic field which then creates an electric field and so forth as shown in Figure 2-3. When the antenna is attached to a load, it radiates the load's information in an energy storing electromagnetic wave. The reciprocity of Maxwell's equations demonstrate that an antenna can equally receive electromagnetic energy and translate the information back into a guided wave.
All wireless technologies today- radio, 802.11g, cellular, RFID, depend on the existence and efficiency of well-designed antennas to successfully communicate information through the air. Antennas come in various shapes and sizes depending on factors such as the attached load, the operating frequency, and the desire directionality. For instance, the parabolic-shaped Dish antenna (Figure 3-1) that was popularized in
1997's Contact has a high gain and is used for radio and television communication
[15]. The Yagi-Uda (Figure 3-1), developed in the late 1920s, is a highly effective antenna noted for its simplicity and directivity [10, p. 127]. The antenna consists of a an array elements: a driven dipole, a reflector, and parasitic elements.
The Dish and Yagi-Uda antennas are two of the many antenna designs that people have developed over the last century. Antenna design has evolved in a creative manner; while simple wire antennas are fairly easy to analyze mathematically, added complexity and arrays of antennas are nearly impossibly to mathematically analyze.

Figure 3-1: The Dish antenna and the Yagi- Uda antenna are two popular antenna designs [14].
Instead, antenna designers rely on intuition and creativity to develop antennas and turn to simulation software to optimize their designs. In the remainder of this chapter, we will discuss basic antenna theory and parameters that designers use to characterize antennas.
The Hertzian dipole, the most fundamental radiating element, is defined as an infinitesimal wire carrying current with uniform amplitude and phase over its length
[16, p. 515].
In spherical coordinates, the field components of an oscillating Hertzian dipole of length dl, carrying current 10, and lying on the z-axis as shown in Figure 3-4, are:
Eo -
Er -
Iodl 21 dl " 2
2cos0
47
1
(1
+
' ejkr-
(jkr)
3
Iodl
1
0
o k sinO
47 k
1 1)
+ jkr (jkr)
2
+ r)3 e-k
(jkr)
3
HO dl 1i( 1) k
2 sinO
47
(1 + jkr (jkr)
2 e-jk
(3.1)
(3.2)
(3.3)

Figure 3-2: The field components of radiating elements are often expressed in spherical coordinates (r, 9, q) rather than cartesian coordinates (x, y, z).
The distance rcutoff = 1/k = A/21r determines the nature of the surrounding fields and distinguishes the near-field region from the far-field region. For distances r < of the magnetic field H dominates, and e
- j kr approaches 1:
Enear- field
Iodl d
47rkr
3
(2cosO + sinO0 (3.4) lodl
Hnear- field = 2 sinO (3.5)
This region for distances r < rcutoff is called the near-field region. The strength of the field components E and A decay as 1/r
3 and 1/r
2 respectively, signifying the short range of the near-field from the radiating object. Also, evaluating the Poynting vector with the given field components indicates that no real power flow exists in the near-field. Rather, because the electric field and the magnetic field are 900 out of

phase with each other, energy is simply being stored and released between the two fields. High-frequency (HF) antennas operate in the near-field region where energy is transferred at close range and using magnetic coupling.
3.1.2 The Far-Field Region
When r > r,&toff, the first order terms of the field equations dominate:
Efar-field = j
Iodl
4xr t1k e
krsin
O 9 (3.6)
Hfar-field =- j
Iodl k e
- j krsin
O 0
4wr
(3.7)
We define this region for distances r > r.toff as the far-field region. Looking at the equations 3.6 and 3.7, we see that the E and A fields are in phase, orthogonal, and decay similarly as 1/r. Evaluating the Poynting vector in the far-field region results in a real power density signifying wave propagation. Ultra-high frequency
(UHF) antennas operate in the far-field region.
From now on, we focus our discussion on UHF antennas, which operate in the far-field region. Regardless of the antenna's complexity, a UHF antenna exhibits the same 1/r
2 power loss that the Hertzian dipole demonstrated above. In the following section, we will describe antenna characteristics that can be optimized to maximize the efficiency of an antenna and its range of radiation.
3.2 Antenna Parameters
In a passive UHF RFID system, the most important and differentiating performance characteristic of a tag is its read range. To afford maximum read range for a passive
RFID tag, antenna designers must optimize antenna parameters such as gain and impedance to create a highly efficient and lossless tag antenna. The parameters described below are dependent on the antenna's geometry. In Chapter 6, we present two common RFID antennas, the dipole antenna and the microstrip antenna, and

their special geometries that make them effective RFID tag antennas.
3.2.1 Gain
The gain of an antenna is defined as the ratio of the maximum power density to its average value over a sphere and it is often expressed in dBi, where i represents the gain with respect to an isotropic antenna [15, p. 23]. An isotropic antenna radiates equally in all directions and therefore, has a gain of 1. For example, the common half-wave dipole has a gain of 2.15 dBi. ThingMagic's Mercury4 reader antenna has a gain of 4.6 dBi. High gain antennas such as the Dish antenna have gains on the order of 20 dBi.
3.2.2 Power
Just as gain is often measured in reference to some standard antenna, i.e. an isotropic resonator, so too is the radiated power. An RFID reader's radiated power is stated in terms of effective (or equivalent) isotropically radiated power, EIRP, which is defined as its input power multiplied by its gain relative to an isotropic antenna.
EIRP = GtPt (3.8)
3.2.3 Resonant Frequency and Bandwidth
UHF RFID antennas typically "resonate" or operate at a particular frequency and are size proportionally to the wavelength of the operating electromagnetic wave. For example, a half-wave dipole antenna that operates at 915 Mhz has a length of fifteen centimeters, which is half operating wave's wavelength.
An antenna's bandwidth is defined as the range of frequencies at which the antennas gain is less than 3dB of its peak gain. With the Gen. 2 requirement for UHF tags to operate from 860 960 Mhz, a larger bandwidth for the tags is desired. However, increasing bandwidth reduces antenna gain.

3.2.4 Radiation Pattern
An antenna's radiation pattern is a graphical representation of the strength of the antenna's power density in space. An isotropic antenna in theory has a spherical radiation pattern but physically does not exist. A half-wave dipole has a doughnutshaped radiation pattern, radiating in all directions except for the direction of the axis on which it lies. Figure 3-3 presents the three-dimensional polar radiation patterns of a half-wavelength dipole and a 5/4-wavelength dipole.
VI
Figure 3-3: The 3D polar radiation patterns of dipole antennas with lengths A/2 and
5A/4.
3.2.5 Polarization
The polarization of antenna is determined by the electric field of the wave emitted by the antenna. Specifically, the magnitude and phase of the electric field E dictate the antenna's polarization. If the magnitudes and phases of the electric field components are equal, the antenna is linearly polarized. If the magnitudes are equal, but the phases differ by 900, the antenna is circularly polarized.
The electric field components of a linearly-polarized wave project a line onto the plane where the electric field components of a circularly polarized wave project a circle as shown in Figure 3-4. In order for two linearly polarized antennas to communicate with each other, their projected electric fields must be aligned. A circularly polarized

K-~.
E
Figure 3-4: Linear and circular polarization [30].
antenna however, can communicate with any linear antenna regardless of its orientation. Each polarization type has its advantage; where a circular antenna is orientation insensitive, a linear antenna radiates higher power because all the power is directed in one direction as opposed to being split among the two components. Depending on the application, a reader antenna is either linear or circular, and ideally, the tag antenna should be circularly polarized such that it can be read from any orientation.
3.2.6 Input Impedance
The input impedance of an antenna is defined as the ratio of the voltage to current at the antennas terminals. The length and size of the antenna determine its input impedance. The impedance Z has a real portion, which includes the antenna's radiation resistance Rrad and its ohmic losses, and a reactive portion which contains energy from the fields surrounding the antenna.
Z = Rrad + Rohmic + jX (3.9)
For instance, a half-wave dipole has an input impedance of 73 ohms + j42.5 0 [3, p. 460].
When electromagnetic energy is transferred from one medium to another, say from an antenna to a microchip, the absorption of energy depends on the relative

impedances of the two media. For maximum power transfer between the antenna and its attached load, the impedances of the antenna and the load must be conjugate matches. Their real components should be equal, while their reactive components should be equal and opposite.
More specifically, the reflection coefficient F, is a measure of how much of the transferred energy is reflected back into the original source [17, p. 112]:
SZIZa
Equation 3.10 shows that when the load impedance of the microchip Z, is open or shorted, the reflection coefficient F equals 1 and all the energy is reflected back into the antenna. If however, the impedance of the microchip Z, and the impedance of the antenna Za are equal, F equals 0 and all the energy is absorbed by the microchip.
With power levels so low in passive RFID, impedance matching between the antenna and the microchip is extremely important to minimize unnecessary losses.
The design of a UHF antenna requires the manipulation of the antennas shape and size to achieve optimal antenna characteristics. An ideal passive UHF RFID tag antenna should be designed such that its gain is maximized, its bandwidth is 860-960 Mhz, its radiation pattern is isotropic, its polarization is circular, and its impedance is matched to the microchips impedance. However, antenna parameters are not independent of each other and improving one characteristic, say gain, can negatively affect another, say bandwidth. The fine balance in optimizing antenna parameters makes antenna design extremely difficult and somewhat of an art.
With a basic understanding of electromagnetism and antenna theory, we will now provide a detailed description of the power transmission and reception that occurs in a passive UHF RFID system.

In Passive UHF RFID systems, tags are powered by the energy they receive from the reader. In the United States, the output power level of the reader is limited by the Federal Communications Commission (FCC) to 1 Watt [9]. In order to maximize read range, the power transfer between the reader and the tag must be understood to determine which antenna parameters can be optimized to reduce the amount of loss that occurs in the communication.
The transaction between the reader and the tag begins when the reader transmits power Pi to the tag. The receiving tag antenna "wakes up" because the electric field
Er of the incident electromagnetic wave from the reader antenna induces a voltage Vt across the tag antenna,
Vt = Er h (4.1) where ht is the vector effective height of the tag antenna, a parameter related to the antenna's aperture; h* is its complex conjugate. The power density available

at the tag antenna, a factor of Er, must be sufficient to induces this voltage Vt.
Otherwise, the tag will not have enough energy to power its microchip and thus, fail to backscatter information to the reader.
In order to determine the power received by the tag antenna P
2
, we must define several factors: first, the magnitude of the Poynting vector or power density S expressed in equation 2.8, can also be defined in terms of the reader antenna's output power P
1
, its gain G
1
, and the distance R between the reader and the tag:
PIG1 (W/m
=4rR2
2
) (4.2)
The tag antenna also has an effective area Ae, which is a measure of the antennas wavelength A
0 and gain G
2 as displayed in Equation 4.3 [10, p. 124].
Ae = 47 2
(oG ) (4.3)
Finally, we must consider polarization mismatches that might exist between the two communicating antennas. We define a unitless polarization mismatch factor p,
P = E
- h|
I Er
121 ht 12
(4.4) to account for any power loss that may occur due to polarization mismatches [23, p.
30]. When there is no polarization mismatch, p is 1.
Equation 4.5 shows the power received by the tag antenna, P
2 and its relationship to the power density, S , the antennas effective area, Ae, and the polarization mismatch factor, p.
P
2
= pAeS = p (4R
G1G2
(47rR)
2
2
(4.5)
In passive RFID systems, where factors such as wavelength and reader power levels can not be controlled, the tag antenna should have a high gain G
2 and an input impedance Za matched to the impedance of the microchip, Z
1
. This maximizes the tag's received power P2 and in essence, allows for a long read range, R from the RFID

readers antenna.
Once the passive UHF RFID tag is powered by the reader, it then backscatters its data back to the reader. In a tag-talk-first communication scheme, the tag attempts to communicate with the reader when it has sufficient operating power. In a readtalk-first communication scheme, the tag communicates with the reader only when commanded to by the reader.
When an electromagnetic wave interacts with an irregularity in a medium, the wave becomes dispersive and may scatter. Scatter occurs when an electromagnetic wave approaches an object and induces currents within the object, resulting in new electromagnetic fields. The amount and behavior of the scatter depends on the size, shape, and composition of the object, the incident waveform, and the direction of wave's arrival. An object's radar cross section (RCS), a, measures an object's ability to reflect waves. For an RFID tag, the antenna's radar cross section, a depends on the antenna's effective area, Ae and how well the antenna is matched to its corresponding microchip. We discussed earlier that if the antenna and the microchip have well-matched impedances, the microchip will absorb all the energy from the antenna and no energy will be reflected. If however, the microchip has a load impedance that is either shorted or infinite, as discussed in section 3.2.6 (impedance matching), the microchip will not absorb any energy from the antenna, and instead, all the energy will be reflected back into the antenna: amax = Ae I Ri 0, Ri -- +
00
(4.6)
amin = 0 I R = Ra

where R, is the load resistance and Ra is the input resistance of the antenna.
In an RFID tag, the dependence of the RCS on the microchip's load resistor R, is exploited to send information back to the reader. R, is short-circuited in an on-off pattern, dictated by the data stream it wants to communicate back to the reader
[10, p. 123]. As a result, the tag's radar cross section a modulates according to the changing load resistor R
1
. The reader then receives a signal from the tag proportional to the tag's a, and ultimately obtains the tag's identification information. Equation
4.8 shows the power P
4
, received by the reader from the tag.
P4
1
A 2
(4r)
3
R a
4
(4.8)
The power received by the reader P
4
, is inversely proportional to the fourth power of the distance R between the reader and the tag. In effect, small increases in distance will greatly reduce the power received by the reader. The most important and differentiating performance characteristic of a passive UHF RFID tag is its read range. As we have shown that RFID systems are exceptionally sensitive to increases in distance in equation 4.8, it is imperative that tag antenna parameters such as gain, polarization, and input impedance are optimized to ensure the only losses in the system are due to distance attenuation.
However, even if the antenna and system designs are optimized, extrinsic factors in the environment severely affect the efficiency of a UHF RFID system. In the next section, we present these challenges; and following the survey of challenges, we focus on two limitations, orientation sensitivity and metal sensitivity, and how the Albano-
Dipole and the Albano-Patch antennas are designed to overcome these limitations that afflict existing planar RFID tags.

UHF passive RFID tags provide significant advantages over HF tags. These advantages include smaller dimensions, faster read rates, and longer read ranges. However, they exhibit increased sensitivities, both inherently and externally, that prevent them from working with 100% efficiency in practical applications. In this chapter, we discuss some of these factors.
A true isotropic antenna, one that radiates equally in all directions, is not realizable for passive UHF antennas. Figure 5-1 presents the commonly used half-wave dipole antenna positioned along the y-axis and its doughnut-shaped radiation pattern. The antenna radiates in all directions except for the direction of the axis on which it lies.
A reader antenna position on the y-axis would be unable to communicate with this dipole antenna.
Regardless of the complexity of the design, a two-dimensional dipole-like antenna can not radiate omnidirectionally.

Figure 5-1: A 915Mhz half-wave dipole simulated in HFSS and its doughnut-shaped radiation pattern.
A metallic surface placed next to a UHF dipole-like antenna negatively affects the antenna's radiation pattern, resonant frequency, and input impedance, and ultimately degrades the overall efficiency of the antenna [27].
5.2.1 Reflections
Conductive materials reflect incident electromagnetic waves, both from the reader and from the tag's backscattered energy. The boundary conditions for a perfect conductor, derived from Maxwell's equations, show that magnetic fields can only be parallel to a perfect conductor and electric fields can only be perpendicular: ft-B3 = 0 (5.1)
.-D = p
,hx A = J
(5.4)

Inside a conductor, E= R = 0 and no electromagnetic waves can exist. In order to satisfy the boundary conditions shown in equations 5.1-5.4, such that Ell and H± are equal to zero at a conductive boundary, a new electromagnetic wave is created that travels in the opposite direction, hence is "reflected", from the conducting surface, with an electric field that cancels the incident electric field, and a magnetic field that creates surface currents on the metallic surface.
Another way to understand the reflective property of metals is to evaluate the reflection coefficient as defined in equation 3.10 for metal surfaces. For instance, copper has a conductivity of 5.8 x 107 S/m. Conductivity is the inverse of resistance, and thus, the "resistance" of copper is 1.72 x 10
-
1 Q. The copper's resistance is the load impedance Z, in equation 3.10 and as it approaches zero, the reflection coefficient
F at a copper surface is practically 1 and the incident electromagnetic wave is fully reflected away from the surface.
When a dipole-like antenna is placed next to a metal surface, "mirror-image" charges and currents are created on the surface. Image theory states that a single charge at a distance d from a conductive surface produces what appears to be a duplicate mirror image charge a distance 2d away from the original charge and opposite sign [26, p.
62].
Bectric Field
---------
2d
Conductive surface
Electhic Field
Figure 5-2: A point charge placed near to a metal surface creates an image charge at twice the distance from the metal surface.

This equivalence continues as more charges are placed near the metal surface.
Also, moving charges, such as a current on a wire, can create associated mirror currents on the metal surface.
In the context of RFID, a metallic surface near a tag antenna reflects incident electromagnetic waves from both the reader and the chip's backscatter, affecting the radiation pattern of the tag antenna. The resulting changes depend on the size and shape of the metal and the antenna's distance from the metal.
Also, the image currents that a dipole-antenna induces on a nearby conductive surface create new E and H fields near the tag antenna that interfere with the antenna's fields and ultimately disrupt the antenna's radiation pattern and overall efficiency.
The presence of metal also affects the tag antenna's input impedance, reduces its matching with the microchip's impedance and shifts or "detunes" the antenna's resonant frequency, such that the antenna will no longer operate at its labeled frequency.
Finally, if an antenna is directly attached to a metal surface, it short-circuits and no energy reaches the antenna's microchip, as discussed in sections 3.2.6 (impedance matching) and 4.2.1 (radar cross section).
In UHF RFID systems, where tags and readers communicate over distances on the order of a few meters, environmental factors can interrupt a system's efficiency.
The ambient temperature can influence the matching circuits between the tag antenna and its microchip and subsequently, reduce the efficiency of the tag. Also, humidity can affect the efficiency of an RFID system. Water absorbs electromagnetic waves.
Like metal, water next to a tag antenna can greatly affect the efficiency of the antenna

simply by absorbing incident waves. Similarly, gases in the air, especially water vapor, will absorb propagating electromagnetic waves. A humid environment causes an RFID system to be less efficient than a dry environment.
5.3.2 Multipath Propagation
Objects in any environment cause electromagnetic waves to scatter, reflect, and diffract and consequently lead to multipath propagation. For instance, if a reader transmits energy to a tag near a metal door, two waves will reach the tag: a directpath wave and a reflected wave. These two phasors will add constructively or destructively, either enhancing the transmitted signal or completely diminishing it.
Reader
Figure 5-3: Multipath propagation can completely attenuate a signal if two waves add destructively.
One of the major advantages that an RFID system has over bar code technology is its ability simultaneously recognize multiple tags in a shared interrogation area using anti-collision algorithms. However, when a tag antenna is at close range with another,

coupling between the two antennas can cause detuning, distorted radiation patterns, and overall performance degradation.
Despite the growing interest in UHF RFID systems for tracking objects throughout the supply chain, we see that physical limitations of tag antennas create many obstacles for antenna designers. UHF antennas are orientation sensitive, material sensitive, and are greatly affected by environmental factors. In the next chapter, we present two three-dimensional box corner tag antennas, the Albano-Dipole antenna and the
Albano-Patch antenna, which are designed to perform omnidirectionally and in the presence of conductive material.

The three-dimensional Albano antenna design radiates omnidirectionally without complicating the labelling process of the tag. The Albano label is shaped like an
"L" such that it easily attaches to a box in its flat two-dimensional state. Figure 1-5 shows that when the box expands into its three-dimensional state, the label likewise becomes three-dimensional.
Simulations of both the Albano-Dipole antenna design and the Albano-Patch antenna design were conducted using Ansoft's High Frequency Structure Simulator
(HFSS), a software program based on Finite Element Methods (FEM) [2].
The development of the Albano-Dipole design was a combination of exploiting basic dipole antenna principles and performing numerous simulations to optimize the design.
A thin linear antenna, called a dipole antenna was invented by Heinrich Rudolph
Hertz around 1886, and is considered the simplest practical antenna. A dipole is symmetrically fed at the center by a two-wire transmission line and has a sinusoidal

current distribution. Typically, dipole antennas are most effective or resonant at some ratio of the desired wavelength, with the half-wave dipole being the shortest and simplest resonant antenna. Figure 6-1 shows the current distributions of dipole antennas with lengths A/2, A, and 3A/2.
D
V2
()
X12
31/2
Figure 6-1: The current distributions of center-fed linear antennas
A, and 3A/2.
with lengths A/2,
For reference, a half-wave dipole has a input impedance of 73 + j45 Q and a gain
2.15 dBi. A variation on the half-wave dipole is the folded-dipole which is defined two wires with lengths A/2 separated by a small distance d < .05A.
/12
Figure 6-2: A center-fed folded half-wave dipole.
No
The folded-dipole is often desired for matching loads with higher impedances because the added "legs" increase the antenna's impedance without affecting its resonant frequency. In principal, a folded-dipole acts as a step-up impedance transformer. The input power P for any center-fed dipole antenna is a relationship of the current on the antenna, I and the antenna's impedance, Z:

1
2
(6.1)
For a folded dipole, the currents, If on the two legs add to equal the single current distribution Id on the regular half-wave dipole:
2If = Id (6.2) is applied to a half-wave dipole and a folded dipole, equations 6.3 through 6.5 demonstrate that due to the current relationship between the two antennas, the impedance of the folded-dipole Zf will be four times the impedance of the regular dipole:
Pd
=1
2
Pf =
1
2
(6.3)
21f = Id (6.4) therefore,
Zf = 4Zd (6.5)
A folded half-wave dipole has an impedance Zf P 300Q, which is four times the impedance of a regular half-wave dipole Zd = 73Q. Increasing the number of "folded" wires increases the antenna's impedance without affecting its operating frequency.
Manipulating the "step-up impedance transformer" property of folded-dipoles is an example of a common technique used in antenna design.
With this basic understanding of dipole antennas, we discuss the design and development of the Albano-Dipole antenna.

6.1.2 The Three-Dipole Design
The original design of the Abano-Dipole antenna was a simple three-dimensional wire antenna composed of three quarter-wavelength (75 mm) dipoles conjoining at the corner of the box as shown in Figure 6-3.
4
*bm
V
Figure 6-3: The original three-dipole Albano-Dipole design, its resonant frequency at
915 Mhz and its radiation pattern as shown in HFSS.
By manipulating the size and shape of the antenna in HFSS, we created an optimized antenna that radiates at 915Mhz. Figure 6-3 shows the antenna's resonant frequency and radiation pattern as they are displayed in HFSS.
Despite the antennas simplicity and desirable characteristics, we dismissed the design after realizing the practical infeasibility of having an antenna intersect exactly on the corner of the box. Such placement of the antenna makes it highly vulnerable to physical impact. Instead, we designed an antenna that circumvents the corner of the box.
6.1.3 The Around-the-Corner Design
The initial design of the around-the-corner idea was a simple folded dipole bent into the shape of an "L". Again, our main design goals for the antenna were omnidirectionality, a 915 Mhz resonant frequency, and an input impedance to match the microchips impedance, 1200-j145 Q. We manipulated the antennas physical parameters- length, width, and spacing, to optimize the antenna, but met much difficulty as improving the resonant frequency negatively affected the impedance and vice versa. We then in-

creased the number of folded wires to increase the impedance of the antenna without affecting the resonant frequency, using the step-up impedance transformer property of folded dipoles mentioned in section 6.1.1.
A three-leg design was ultimately chosen to optimize both the resonant frequency and the impedance of the antenna. Physical parameters such as leg-length, legwidth, and spacing were manipulated using HFSS's Optimetrics tool which allows the designer to sweep a particular parameter to achieve desired behavior. For example, we executed an Optimetric sweep of the inner leg-length from 60 mm to 75 mm with increments of 5 mm. When the inner leg-length is equal to 70 mm, the antenna resonates at 915 Mhz. Figure 6-4 depicts the resonant frequencies of each of the tested lengths.
01 May2006 18:03:08 Ansft Corporation
XYPIot I
MFSSModel
·- · ·-. -
-10-
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I,
40.00
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Figure 6-4: An Optimetric sweep of the inner leg-length shows that the antenna resonates at 915 Mhz when the inner leg-length is equal to 70mm.
The antenna's resulting dimensions were 70 mm for the inner legs- 15mm on the conjoined box side and 55 mm on the other sides. Each leg was 2mm thick and the spacings between the legs were 8mm such that each outer leg was 90 mm long total.
Figure 6-5 displays the Albano-Dipole design in HFSS.
The bandwidth of the antenna is 30 Mhz and its gain is 1.6485 dBi. Figure 6-6 gives a visual representation of the antenna's parameters as shown in HFSS.
The major design criteria of the Albano-Dipole design was omnidirectionality,

Figure 6-5: The final Albano-Dipole design and its dimensions.
1
Figure 6-6: The Albano-Dipole's resonant frequency and bandwidth, dimensional polar radiation pattern as shown in HFSS.
and its three-

which in UHF antenna design, is not physically realizable. However, the placement of the Albano-Dipole tag's toroidal radiation pattern gives the semblance of omnidirectionality when attached to the corner of a box. The low-gain axis of the radiation pattern is shifted so that it lies tangential to the box corner, as opposed to parallel to any one of the planes of the box. This radiation pattern allows the Albano-Dipole tag to communicate with the reader regardless of which side of the box is facing the reader.
We also used HFSS to simulate the effect of metal product next to the Albano-
Dipole design. The Albano-Dipole was simulated next to a cardboard box filled with conductive material a = 58 - 106 S/m. Figure 6-7 displays the detrimental effect of conductive material on the antenna's radiation pattern.
Figure 6-7: The Albano-Dipole antenna attached to a foil-filled box and the antenna's distorted three-dimensional polar radiation pattern as shown in HFSS.
Up until now, our discussion about antennas has focused on dipole antennas. In the following section, we present a new type of antenna, the microstrip patch antenna and our implementation of a three-dimensional box corner version of a patch antenna.

6.2.1 The Microstrip Patch Antenna
In Chapter 5, we discussed the detrimental effect of metallic materials on the efficiency of an antenna. The microstrip patch antenna however, can exploit nearby metal to act as part of the antenna. A microstrip patch antenna is simply a radiating patch on one side of a dielectric substrate and a ground plane on the other side [29, p. 12].
The load attaches to a microstrip which connects the antenna's radiating patch to its ground plane. Microstrip patch antennas radiate due to the fringing fields created between the patch and the ground plane and resonate according to the dimensions of the radiating patch. Figure 6-8 displays top and side views of a typical microstrip patch antenna.
L
L - ak/2-h
; ;`; :;o
Radiating Patch
I
Radiating Patch
Microstrip w!
/,
/
Ground plane
Figure 6-8: A microstrip patch antenna is composed of a radiating patch on one side of a dielectric substrate, a ground plane on the other side, and a microstrip feed that connects the radiating patch and the ground plane.
Specifically, the length of the radiating patch determines the resonant frequency as follows:
A
L = - - h
2
(6.6) where L is the length of the patch, A is the resonant frequency, and h is the thickness of the dielectric. The width W of the patch affects the frequency only slightly, but greatly impacts the impedance of the antenna. The microstrip patch antenna is used in many wireless devices such as cellphones and GPS receivers, and

in the context of RFID, the microstrip patch antenna offers the following specific advantages:
* low-profile configuration
* can be designed to be linearly or circularly polarized
* capable of multiple frequency operations
* mechanically robust and most importantly, it works next to a metal surface which can act as the
"ground plane" of the antenna. The Albano-Patch antenna, discussed below, is designed to work omnidirectionally and in the presence of metals.
6.2.2 The Albano-Patch Design
Using HFSS, the physical dimensions of the Albano-Patch antenna were simulated and optimized to radiate at 915Mhz and have a high input impedance to match the microchips 1200 Q impedance. Figure 6-9 shows the tag in its two dimensional state and its dimensions.
.. 3- In ..
.
61
Microstr'p feed
W= 1 5 mm t W"
48 MM
48 rmm
180 mm
Figure 6-9: The Albano-Patch antenna and its physical dimensions.

HFSS was also used to simulate the effects of certain materials on the radiation pattern of the Albano-Patch design. We simulated the Albano-Patch antenna attached to a cardboard box with conductive lining (conductivity a = 58. 10
6
S/m ) and then attached to a six-pack of aluminium cans (a = 38. 10 6
S/m). The dimensions of the cardboard box in the simulation models were 230x100x100 mm. The cardboards dielectric constant, er was 1.5 and its thickness was 2mm. The simulated aluminium cans were 67 mm and 88 mm in diameter and height respectively. The cans were placed inside a cardboard container with dimensions 198x133x89 mm.
Figure 6-10 present the E-plane (phi = 00) and H-plane (phi = 900) of the two simulated models. In both simulation models, the radiating patch of the antenna design is in the xy-plane. In Figure 6-10 where the antenna is attached to metal-lined box, the antennas radiation pattern is directed out of the box corner. When the antenna is attached to the six-pack of metallic cans, the antenna radiates directly upward from the radiating patch.
-leo
Figure 6-10: Directivity patterns (E-plane, o -, H-plane -
-,
= 400 of the Albano-Patch antenna attached to a) the foiled box, and b) the six-pack of
metallic cans.
so

After completing the simulations of the Albano-Dipole and the Albano-Patch antenna designs, we built copper prototypes of the simulated antenna designs using PCB
Design and Make with a Roland Camm-1 Vinyl Cutter. Each antenna was attached to a 96-bit EPC Gen. 1 Lepton strap manufactured by Alien Technology. The copper antennas were then mounted onto a paper substrate with a permittivity ep = 2. We discuss the details of the static and dynamic experiments in the next two chapters.

The Albano-Dipole antenna and the Albano-Patch antenna were designed to attach to corners of a box and thus, were tested statically to determine their read ranges in six different box orientations. Read ranges were measured using a ThingMagic
Mercury4 reader with circularly polarized reader antennas. The output power and gain of the reader antenna are 1 W and 4.6 dBi respectively [28, p. 13-15]. Figure
7-1 shows a picture of the static testing setup.
Figure 7-1: The static testing was conducted in a long, empty hallway to minimize external effects such as metal cabinets and other tags.
The maximum read range was determined to be the longest distance at which the tag was reading at a consistent rate of 1 reads/second for thirty seconds [7, p.

3]. We also tested two high-performance commercially available tags, Alien Technology's I and Squiggle tags, shown in Figure 7-2, to compare their performances to the performances of the Albano designs.
Figure 7-2: Alien Technology's I and Squiggle tags.
Each tag was placed on the corner of a box and measured in six different orientations, orientations A-F as show in Figure 7-3. Initially, we tested and measured read ranges of each tag attached to an airbox. Subsequently, to examine the effect of conductive material, we measured the read ranges of the each tag attached to a box containing foiled product, Oral-B Whitening Strips.
Figure 7-3: Orientations A, B, and C. Orientation D is opposite orientation A, orientation E is opposite side B, and orientation F is opposite side C.
In the static aribox testing of the Albano-Dipole tag, the results show that the I and Squiggle tags perform better than the Albano-Dipole tag in some directions, reaching read ranges of almost six meters, but barely read in other directions. The

Albano-Dipole tag however, had a consistent maximum read range, 2-3 meters in all six orientations. Figure 7-4 displays the results of all three tags.
6.
5
/
E -43
2
1
0
A
Static Read Ranges of Tags in Six Different
Orientations When Attached to Ar-Filled Box
8 C D
Orientation
E F
*AItbano Dipole a gSqtggile
Figure 7-4: The maximum read ranges of the Albano-Dipole, I, and Squiggle tags in six different orientations when attached to an air-filled box.
The I and Squiggle tags fail to perform in the B and E orientations, namely because the reader antenna is positioned in the nulls of their radiations patterns, and little power can be transferred or received in those directions.
After experimentally establishing the Albano-Dipole tag's omnidirectionality in the static airbox testing, we tested the effect of metal on the tags performance. The same static experiments were conducted, this time affixing the tags to a box containing foiled packaging of Oral-B Whitening Strips. As expected, the Albano-Dipole, the
I and the Squiggle tags all performed much worse in this setting, but where the I and Squiggle tags were read in two out of six orientations, the Albano-Dipole tag successfully performed in five out of six directions. Figure 7-5 shows the results of the foil-filled testing.
The same static read range measurements were conducted with the Albano-Patch tag. The Albano tags were tested on different days, leading to possible variances in temperature and humidity that could affect the performances of the tags. However,

25
Static Road Ranges of Tags in Six Different
Orientations When Attached to a Foil-Filled Box
Le 2.5
S9 tag
0ý5
S
0s
A B C D
Orientation
E F
Figure 7-5: The maximum read ranges of the Albano-Dipole, I, and Squiggle tags in six different orientations when attached to a foil-filled box.
the consistency in performance of the I and Squiggle tags lends to the assumption that atmospheric variations were negligible and the results of the two Albano tags can be reliably compared. Figures 7-6 and 7-7 show the read range measurements of the Albano-Patch tag attached to an air-filled box and a foil-filled box.
Similar to the Albano-Dipole tag, the Albano-Patch tag performed omnidirectionally when attached to an airbox. Furthermore, it continued to radiate omnidirectionally even in the presence of metallic material. The Albano-Patch tag proves to be a high-performance tag where in the airbox testing, its maximum read range approaches six meters and next to metal, it performs over two and a half meters away from the reader.
Looking at Figure 7-7, we see that the Albano-Patch tag worked particularly well in orientations A, B, and C, directions where the Albano-Patch antenna was directly facing the reader. The Albano design has the natural advantage over two-dimensional tags in that it lies on three sides of the box as opposed to just one. Where material in the box may obstruct a two-dimensional tags direct line of sight with the reader in five directions out of six, it obstructs the Albano tags line of sight in only three directions.

7
Es5
52
0
A
Static Read Ranges of Tags Attached to Air-Flied
Box in Six Different Orientations
C D
Orientation l Albano Patch Tag
SI Tag
O Squiggle Tag
Figure 7-6: The maximum read ranges of the Albano-Patch, I, and Squiggle tags in six different orientations when attached to an air-filled box.
11.5
3
2,5
Static Read Ranges of Tags Attached to Foil-
Filled Box in Six Different Orientations
I0
Albano-Patch Tag
El
Tag
SSquiggle Tag
0.5-
0-
A
8
C 0
Orientation
E F
Figure 7-7: The maximum read ranges of the Albano-Patch, I, and Squiggle tags in six different orientations when attached to a foil-filled box.

Overall, static testing of the Albano designs demonstrated their omnidirectional behaviors when attached to air-filled boxes. Both tags then continued to perform considerably better than the I and Squiggle tags in the presence of metal, and in particular, the Albano-Patch tag continued to radiate omnidirectionally. In the following chapter, we present the dynamic testing results of the Albano tags.

Beyond static testing, dynamic testing of RFID tags is necessary to fully assess the performance of tags in warehouse environments. As part of its initiative, Wal-Mart requires 100 % tag read performance regardless of orientation with tagged cased on a conveyor belt [22, p. 4]. The tags were attached to both air-filled and foil-filled boxes and tested on various conveyor belts.
Initially, the Albano-Dipole tag was attached to an airbox and placed on a conveyor belt moving at 2.7 feet/second. An Alien reader with a circularly polarized antenna positioned eight inches away from the box was used to measure the tags read rates
(reads per minute). Figure 8-1 displays the dynamic testing setup.
The read rates of the Albano-Dipole, I, and Squiggle tags were measured in six different box orientations. In each orientation, the read rates were recorded three times and averaged. The three tags performed considerably well in all orientations and the Albano-Dipole tag demonstrated no advantage over the I and Squiggle tags.
Figure 8-2 presents the read rate results of the Albano-Dipole, the I, and the Squiggle tags.

Figure 8-1: The dynamic testing setup.
Figure 8-2: The average read rates of the Albano-Dipole, the I, and the Squiggle tags attached to an airbox in six different orientations on a 2.7 feet/second conveyor belt.

The I and Squiggle tags unexpectedly demonstrated omnidirectional performance despite the nulls in their radiation patterns. This improved performance can be explained by a) the proximity of the tags to the reader, and b) the movement of the tags. Because the tags were only 8 inches away from the reader antennas, the tag antenna's gain in its weak orientations, though minimal, was sufficient to communicate with the reader antenna. Also, the movement of the tag antenna shifted the tag antenna's radiation pattern making it more accessible to the reader antennas and thus, facilitated the communication between the reader antennas and the tag antennas.
Tag movement always enhances the tag's performance.
Using the same experimental setup, we then tested the three tags attached to a box filled with foiled packaging of Oral-B Whitening Strips. We also performed the conductive material testing on a faster 5 feet/second conveyor belt, eight inches away from a linear Alien reader antenna. Figure 8-3 shows the results of the two experiments.
C 40
0
Average Read Rates of Tags Attached to Foil-Filled
Box in Different Orientations Running on 2.7
feet/second Conveyor Belt
20a
C rA D
Orientation
F
Different Orientations Running on S feet/second Conveyor
20u
100
1
80
60
40
20
0
A B C D
Orientation
E F
U
Adbano
Figure 8-3: The average read rates of the Albano-Dipole, I, and Squiggle tags when attached to a foil-filled box and placed on two moving conveyor belts.
The presence of metal degraded the performance of all three tags. However the
Albano-Dipole tag performed considerably better than the I and Squiggle tags, especially on the slower conveyor belt where it performed in all six directions. In both cases, the Albano-Dipole tag performed in at least two more directions than the other tags.

The same experiments were run with the Albano-Patch tag, differing only in that circularly-polarized reader antennas were used in every set up. Figure 8-4 shows the read rate results of the tags when attached to an airbox.
Average Read Rates of Tags Attached to Air-Filled
Box in Different Orientations Running on 2.7
feet/second Conveyor Belt
400
350
300
S250
A 200
150
100
50
0
A B C D
Orientation
E F
EIAlbano-Patch Tag
SI Tag
OSquiggle Tag
Figure 8-4: The average attached to an airbox.
read rates of the Albano-Patch, I, and Squiggle tags when
Again, all three tags performed exceptionally well and the Albano-Patch displayed no advantage over the I and Squiggle tags. We then conducted the experiments with the box filled with Oral-B Whitening Strips the results of which are shown in Figure
8-5.
250 a oo
1
150
Average Read Rates of Tags Attached to Foil-Filled
Box in Different Orientations Running on 2.7
feet/second Conveyor Belt
100
4 so
0
A C D
Orientation
F
Average Read Rates of Tags Attached to Foil-Filled
Box in Different Orientations Running on 5 feet/second Conveyor Belt
* U
-;II-· :- I S
AlbanoPatch Tag
N1 IT"g
A C D
Orientation
E F
Figure 8-5: The average read rates of the Albano-Patch, I, and Squiggle tags when attached to afoil-filled box and placed on two moving conveyor belts.

The conductive material severely affected the performance of all three tags. Regardless, the Albano-Patch tag still performed in at least two more directions than the other tags on both conveyor belts.
As with the static testing, the Albano-Patch tag was tested on a different day from
Albano-Dipole tag. Drastic differences in the performances of the I and Squiggle tags suggest that changes in environmental conditions affected the performances of all the tags. On the first day of testing, the I tag performed next to metal in four out of six directions on the 2.7 feet/second conveyor belt. On the second day however, it only performed in two directions. In this regard, the Albano-Dipole tag and the
Albano-Patch tag cannot be directly compared.
Though the Albano-Patch tag does not radiate omnidirectionally next to metals, it still outperforms the I and Squiggle tags by performing in at least two directions more than the other tags.
Dynamic testing supplements static testing in providing a comprehensive analysis of the Albano tags in warehouse environments. The omnidirectional advantage that the Albano tags presented in the static testing was not as considerable in the dynamic testing. Nonetheless, for applications involving conductive materials where orientation can not be controlled, the Albano-Dipole tag and the Albano-Patch tag proved to be far more efficient than Alien's I and Squiggle tags. In the next chapter, we conclude the discussion of the Albano antennas and consider opportunities for future work.

Despite the appeal of passive UHF RFID systems to enhance object identification in packaging environments, physical limitations of passive UHF RFID tag antennas prevent these systems from working with 100% efficiency. Existing two-dimensional
UHF RFID tag antennas have natural orientation limitations and material sensitivities. This thesis addresses these challenges and presents two three-dimensional antenna designs, the Albano-Dipole antenna and the Albano-Patch antenna, that are designed to perform omnidirectionally and next to conductive materials.
The three-dimensional Albano design is feasible in practice because it attaches to a package when it is flat. When the flat package is folded into a carton, the Albano tag assumes its three-dimensional shape. We are currently applying for a patent for this novel three-dimensional antenna design.
The two Albano antennas were tested in both static and dynamic environments and they outperformed Alien Technology's I and Squiggle tags in all the scenarios.
In the static testing, the Albano-Patch tag radiated omnidirectionally in the presence of metal, where the I and Squiggle tags were read in at most two directions. Equally impressive, the Albano-Dipole tag performed omnidirectionally next to metal on a moving 2.7 feet/second conveyor belt, where the performances of the I and Squiggle tags were greatly degraded.
As part of future work, we plan to make further optimizations to improve the
Albano-Dipole antenna and the Albano-Patch antenna. Specifically, we plan to make

the Albano tags operate globally in the 860-960 Mhz range as specified by the EPCglobal Gen 2/future ISO 18000-6c specs. This reqiures increasing the bandwidth of both antennas and improving the impedance matching to further reduce losses. Additionally, we plan to scale down the size of the Albano-Patch tag to a more reasonable size and cost for packaging applications.
We are currently working on a new bowtie Albano antenna design. Because of their shape, bowtie antennas are naturally more broadband than their dipole and patch counterparts as shown in Figure 9-1.
04 May AMftCorpor tion
Xy Plot I
HF'SSIMO60le
14:2206
1
-4
840 - 990 Mhz
BW= Mhz
-1040
.12.60
iiU
OM
91OMhz /
I UN
L
1A0
Figure 9-1: A wire bowtie antenna simulated in HFSS has a bandwidth of 150 Mhz ranging from 840-990 Mhz.
We are designing an Albano bowtie antenna that is both omnidirectional and broadband and we plan to conduct physical testing in static and dynamic environments as was done with the Albano-Dipole and the Albano-Patch antennas.
Another interesting approach to designing antennas is using genetic algorithms.
A genetic algorithm (GA) is a technique for solving search and optimization problems through evolutionary biology processes. A GA begins with a population of solutions and through mutations, recombination, and "survival of the fittest", the best solution survives. Where traditional antenna design relies largely on intuition and empirical experiments that can lead to approximate results, genetic algorithms present the potential for entirely new and more precise antenna designs. We plan to compare

the bandwidth efficiency of GA antennas versus commercial broadband antennas and ultimately, build and test physical prototypes of GA RFID tag antennas.
Additionally, we plan to examine the effects of other materials, such as liquidbased materials, on the antennas, to create new three-dimensional designs that accommodate the materials. A fellow researcher is designing a three-dimensional bowtie antenna that wraps around a water bottle and performs well in the presence of water.
Finally, we are examining packaging aspect of the tags, the plastic deformation that occurs as the tags fold over corners of the box, and any effects the mechanical packaging aspect of the antenna design will have on the antennas.
The Albano-Dipole tag and the Albano-Patch tag are two three-dimensional RFID tag designs that perform better than existing two-dimensional tags in packaging environments.

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