**2. Introduction to RFID systems**

RFID technology has drawn great attention in the past decade. Recently it has been used in in‐ ventory control, managing large volumes of books in libraries and tracking of products in the retail supply chain [14,15]. Its usage is growing and replacing the bar code technology used for the purpose of object identification and recognition. A bar code requires a clear line of sight and a small distance between the object and the laser bar code scanner (which is a limitation) where‐ as RFID works at microwave frequencies so it can identify the object from a distance, it does not require line of sight for its operation and unlike bar codes it can also store some additional in‐ formation which makes it very attractive as compared to bar codes [1].

**Figure 1.** Overview of RFID System

A RFID system consists of a RFID reader and a RFID tag. An overview of a typical RFID system is shown in Fig. 1. RFID systems comprise of RFID tags or transponders which are fairly simple, small and inexpensive devices at one end and a reader which is relatively complex and a bigger device on the other end. Application Specific Integrated Circuits (ASICs) are attached to the tag antenna and are used for sensor applications, to harvest ener‐ gy, communicate and store information for later recovery. The reader emits an electromag‐ netic field which contains power and timing information for use by the passive RFID. If a RFID tag comes within the range (also known as the interrogation zone [1]) it receives the information which is fed to the ASIC and in response the ASIC switches its impedance states between a lower and higher value in a predetermined fashion as shown in Fig. 2. By chang‐ ing the impedance states the ASIC changes the radar cross-section (RCS) of the tag antenna thus changing the backscattered power. This backscattered power is collected at the reader and is used for tag identification and information. The maximum distance for which a read‐ er can successfully identify a tag is known as max read range.

RFID tags are usually classified into three categories: active tags, semi-passive tags and pas‐ sive tags [1]. An active tag has a dedicated power supply for operation on the tag. A semipassive tag has an integrated power supply attached to it and it only starts working when electromagnetic power transmitted by the reader is incident on the tag. This feature enhan‐ ces the maximum read range of the tag [1] because less power is required from the incoming incident field from the reader. A passive tag has no power source attached to it and it har‐ vests power for its operation from the incident electromagnetic field transmitted by the reader.

**Figure 2.** Thevenin equivalent circuit of RFID tag

This chapter will focus on the design of ZOR antennas for passive UHF RFID tags. First, a brief introduction and working principles of RFID systems is presented using Friis's trans‐ mission equation. Then, the characteristics of CRLH transmission lines will be discussed and its Bloch impedance will be derived to introduce the ZOR concept. Then coplanar-wave‐ guides (CPW) and its characteristics are presented. Then the design of a capacitive loaded CPW based ZOR antenna for passive UHF RFID tag is discussed. Finally, future work and

RFID technology has drawn great attention in the past decade. Recently it has been used in in‐ ventory control, managing large volumes of books in libraries and tracking of products in the retail supply chain [14,15]. Its usage is growing and replacing the bar code technology used for the purpose of object identification and recognition. A bar code requires a clear line of sight and a small distance between the object and the laser bar code scanner (which is a limitation) where‐ as RFID works at microwave frequencies so it can identify the object from a distance, it does not require line of sight for its operation and unlike bar codes it can also store some additional in‐

RFID Tag

Backscattered electromagnetic field carrying data via changes in RCS

Digital information stored in RFID Tag ASIC

Region

Max read range

formation which makes it very attractive as compared to bar codes [1].

Incident Field

Region Far Field

conclusion about this chapter is presented.

132 Radio Frequency Identification from System to Applications

**2. Introduction to RFID systems**

RFID Reader

**Figure 1.** Overview of RFID System

Near Field

RFID Antenna

A common method to describe the RFID wireless communication system is the following Friis transmission equation [16]:

$$P\_r = P\_t \frac{G\_r G\_t \lambda^2}{(4\pi R)^2} q$$

where Pr is the power received by RFID tag, Pt is power transmitted by RFID reader, Gr is the gain of tag antenna, Gt is the gain of reader, λ is free space wavelength of the operating frequency of reader, R is distance between reader and tag and q is impedance mismatch fac‐ tor (0 ≤ q ≤ 1) between impedance of the antenna on the tag and the input impedance of the ASIC on the tag. Equation (1) assumes a perfect polarization match between the antenna on the reader and the antenna on the RFID tag. Reorganizing (1) and solving for R, the follow‐ ing equation for determining the read rang of a tag can be derived [17,18] as:

$$R = \frac{\lambda}{4\pi} \sqrt{\frac{qG\_tG\_rP\_t}{P\_r}}\tag{2}$$

capacitance represents the capacitance between the printed signal conductors on one side of the board and the reference or ground plane. In fact, this inductance and capacitance exists on every printed TL (traditional or CRLH) because in the propagating band current is trav‐ elling down the TL and there is always capacitance between the conductors supporting this current and a reference conductor. When introducing the CRLH-TL, this series inductance and shunt capacitance is referred to as the parasitic values and are denoted in Fig. 3 as LR

Design of a Zeroth Order Resonator UHF RFID Passive Tag Antenna with Capacitive Loaded Coplanar…

http://dx.doi.org/10.5772/53284

135

Next, to support LH-propagation, a series capacitance and a shunt inductance is introduced. These values are shown in Fig. 3 and are denoted CL and LL, respectively. The subscript L stands for left-handed propagation. More particularly, the series capacitance is in series with the inductance and the shunt inductance is in parallel with the shunt capacitance. Therefore, to achieve LH-propagation, CL and LL should dominate over the values of LR and CR. Closer observation of the equivalent circuit in Fig. 3 shows that the LH-values will only dominate over a certain band which is called the LH-propagating band. When the RH-values of LR and CR are dominant, this is called the RH-propagating band. When both the RH- and LHvalues are equal; this is called the transition frequency between the RH- and LH-propagat‐ ing bands or simply the transition frequency. In practice, the series capacitance is usually introduced by defining interdigital capacitors down the length of the TL [10]. The shunt in‐ ductance has been introduced in many different ways such as split ring resonators and

A CRLH-TL has several unique properties as a result of the introduction of CL and LL. The property used in this work is the sign change associated with the phase constant. The phase constant on a CRLH-TL is opposite to the phase constant on conventional RH-TL. This phase advance feature can be very useful for antenna designers and will be used in the next

The term "Coplanar" means sharing the same plane and this is the type of transmission line where the reference conductors are in the same plane as of signal carrying conductor. The signal carrying conductor is placed in the middle with a reference plane conductor on either side as shown in Fig. 4. The advantage of having both conductors in the same plane lies in the fact that it is easier to mount lumped components between the two planes and it is easier to realize shunt and series configurations. The CPW was first proposed by Wen [19] and

The disadvantage of CPW is that it can be difficult to maintain the same potential between the reference and signal conductors throughout the signal trace. Nevertheless many advan‐ ces have been made by using CPW such as novel filters [22] and right/left handed propaga‐

since then have been used extensively in wireless communications [20,21].

and CR. The subscript R stands for right-handed (RH) propagation.

few sections to introduce the idea of ZOR antennas.

**4. Coplanar-waveguide structures**

shunt stubs [10].

tion on CPW lines [23].

If the minimum power required for tag operation is Pth then Equation (2) can be written as

$$R\_{max} = \frac{\lambda}{4\pi} \sqrt{\frac{q\_i G\_t G\_r P\_t}{P\_{th}}} \tag{3}$$

Equation (3) is useful for designers to determine the maximum operating range of the tag. Typically the approach by a designer is to maximize the Rmax. One way of achieving this is to minimize the mismatch between tag antenna and ASIC impedances or design a receive an‐ tenna on the RFID tag with a maximized gain Gr.

**Figure 3.** Reconfigurable CRLH-TL

#### **3. Introduction to left-handed propagation**

To help illustrate the use of ZOR properties to improve the gain and matching of a compact antenna on a passive UHF RFID tag, several properties of left-handed (LH) propagation will be introduced and summarized here. It is well known that the equivalent circuit of a tradi‐ tional printed microstrip TL consists of a series inductance and a shunt capacitance. The ser‐ ies inductance is caused by the current travelling down the printed TL and the shunt capacitance represents the capacitance between the printed signal conductors on one side of the board and the reference or ground plane. In fact, this inductance and capacitance exists on every printed TL (traditional or CRLH) because in the propagating band current is trav‐ elling down the TL and there is always capacitance between the conductors supporting this current and a reference conductor. When introducing the CRLH-TL, this series inductance and shunt capacitance is referred to as the parasitic values and are denoted in Fig. 3 as LR and CR. The subscript R stands for right-handed (RH) propagation.

Next, to support LH-propagation, a series capacitance and a shunt inductance is introduced. These values are shown in Fig. 3 and are denoted CL and LL, respectively. The subscript L stands for left-handed propagation. More particularly, the series capacitance is in series with the inductance and the shunt inductance is in parallel with the shunt capacitance. Therefore, to achieve LH-propagation, CL and LL should dominate over the values of LR and CR. Closer observation of the equivalent circuit in Fig. 3 shows that the LH-values will only dominate over a certain band which is called the LH-propagating band. When the RH-values of LR and CR are dominant, this is called the RH-propagating band. When both the RH- and LHvalues are equal; this is called the transition frequency between the RH- and LH-propagat‐ ing bands or simply the transition frequency. In practice, the series capacitance is usually introduced by defining interdigital capacitors down the length of the TL [10]. The shunt in‐ ductance has been introduced in many different ways such as split ring resonators and shunt stubs [10].

A CRLH-TL has several unique properties as a result of the introduction of CL and LL. The property used in this work is the sign change associated with the phase constant. The phase constant on a CRLH-TL is opposite to the phase constant on conventional RH-TL. This phase advance feature can be very useful for antenna designers and will be used in the next few sections to introduce the idea of ZOR antennas.
