Radio Frequency Identification (RFID) tags are finding increasing use in many applications, such as logistics, security system, transportation and manufacturing process control. An RFID system consists of a reader, a tag (transponder) and a computer connected to the reader.

Recently there has been considerable research into the design of RFID tag antennas in the UHF band. Most applications require that the tags be small as well as inexpensive. Realizing a good impedance match between the antenna and the chip is of paramount importance in RFID designs. Since tailoring new IC designs to suit a tag is a rather costly venture, we attempt, instead, to design RFID tag antennas for a specific ASIC chip that is readily available in the market. Also, adding an external matching network with lumped elements in RFID tags is usually prohibitive because of cost and fabrication issues. In light of this situation, it is desirable to explore designs in which the antenna is directly matched to the ASIC, which has a complex impedance. In the present study, for instance, we seek to match the input impedance of the antenna to an IC chip (Philips RFID/ASIC), whose impedance was Zc ~ 12- j 300 Ù at 900 MHz.

In this presentation we will discuss a number of UHF/RFID tag designs, including the hybrid loop and the dual crossed-meandering-dipoles, shown in Fig. 1. We have carried out extensive parametric studies in the process of analyzing the characteristics of the above antennas and the results will be summarized in the presentation. A design methodology based on the Genetic Algorithm (GA) will be discussed for the optimization of conformal antennas with Electromagnetic Bandgap (EBG) surfaces to improve the antenna performance. Some representative results of our study are presented in Figs. 2 through 4, and an objective evaluation of these results will be included in the presentation.

Fig.1 Top View of the dual cross dipole antenna (normalized).

Fig.2 Power delivered to the Length of the antenna: 46.8 x 46.8 mm

Fig.3 Max. Directivity vs. Frequency MHz.

Fig.4 Normalized Pattern at 910

The fast development of wireless communications and mobile systems in last decade has stimulated the market demand for novel antenna designs for handsets. Consequently, a lot of investigation has been focused on small antennas, resulting in a great variety of compact wideband or multiband designs. However, the performance of all these antennas is subject to the well-known small antennas fundamental limits. Hence, new antenna design strategies may be explored if an increase on the radiation efficiency of handset antennas is desired. An example of innovative design concept is one that considers the Printed Circuit Board (PCB) of the mobile as part of the antenna. Since the mobile PCB, which acts as the antenna ground plane, presents resonant dimension at mobile frequencies, its shape and size affect the antenna performance in significant way. In fact, at lowest frequencies the PCB is the main radiator, while the antenna only works as a probe to excite the PCB current modes. Obviously, to design an antenna from this new perspective, an in-depth knowledge of the current modes of the structure is needed. These modes can be computed using the Theory of Characteristic Modes (TCM) that can be applied to conducting structures of arbitrary shape. TCM not only brings information about the resonant frequency and radiation properties of each current mode, but also helps to choose the appropriate location for the antenna to excite the desired PCB current modes. Moreover, TCM models the coupling between the antenna and the PCB, as it treats both elements as a whole radiating structure.

The purpose of this paper is to describe the procedure carried out to design a handset antenna using the Theory of Characteristic Modes. The antenna, which is based on the PCB resonance design concept, is shown in Figure 1, and it presents excellent wideband performance for S11<-6dB and omnidirecctional radiation patterns. It consists of a folded slotted PCB that is excited by means of a planar square monopole. Using TCM it can be verified that the longitudinal current modes of the PCB exhibit broadband radiating characteristic. So, to properly excite these modes, a wide-band feeding element, like a square monopole, may be employed. This feeding monopole acts as a wideband impedance transformer between the feed port and the ground plane, providing better performance than the classical coaxial probe. However, the return losses depicted in Figure 2 show that even using a wide-band feeding configuration if no slot is made in the ground plane, poor impedance matching is achieved at lowest frequencies. This happens because without a slot, the first radiating mode of the PCB, which resonates at 1.25GHz, is weakly coupled to the excitation, and a strong antiresonance at 1.1 GHz, result of the combination of various modes, outshines its radiation. The TCM demonstrate that the insertion of a slot near the feed point of the PCB, not only produces a meandering effect that reduces the resonant frequencies of longitudinal modes, but also changes the current distribution of these modes and favours its excitation. TCM also reveals that when the PCB is slotted, the unwanted antiresonance at 1.1 GHz moves to 0.9GHz, and softens. Thus, the antiresonance reduces its effects over the radiation of the first radiating mode, and the bandwidth at lowest frequencies improves.
Figure 1. Antenna geometry.
Figure 2. Return losses for the antenna with and without slot.

Although the possibility to realize compact microstrip antennas employing the concept of periodically loaded lines has been demonstrated, their ultimate performance limits are not sufficiently known, yet. Therefore, in this work the performance limitations as a consequence of size reduction are analyzed employing the software tool CST Microwave Studio. Focal parameter is the unloaded Q-factor or the available bandwidth. Additionally, aspects of pattern, input impedance and gain are addressed.

Analyzed Structures

The operation principle of a periodically LC loaded patch antenna (shown schematically in Fig. 1) is similar to a conventional patch antenna. The fringing fields of an open-ended microstrip line resonator lead to a radiation perpendicular to the substrate surface. At resonance the electrical length of the line is pi for the fundamental mode.

Within this work several patch configurations are investigated. Two of them are exemplarily shown in Fig. 1. A one cell structure, with two inductors and one capacitor is shown in Fig. 1a), whereas Fig. 1b) reveals a sketch of a three cell antenna with four inductors, each of them realized with five parallel wires.

Simulation

For the parametric analyses of the structures CST Microwave Studio has been employed. Although, as a time domain solver, this tool seems principally unsuitable to analyze high-Q structures, the auto-regressive filter option allows accurate calculations on a short time scale. Consequently a large data base has been generated by a systematic variation of parameters such as: cell number, number of parallel wires, capacitance value (influenced by gap in the microstrip line and the dielectric constant of the substrate), and the ratio between length, width and height. Additionally, for a real assessment of the properties, small loop and dipole antennas have been analyzed.

Results

Fig. 2 shows a comparison of the calculated unloaded Q factors of a three cell compact microstrip antenna (CPSM) with a small loop and a dipole antenna. The loop and the dipole are brought to resonance with and ideal lumped reactive element connected in series to the feeding port. For the CSMA configurations with one, three, and five wires in parallel are compared. The following results can be summarized:

- The Q-performance of the 3Cell CSMA is comparable with the dipole, however exhibits at resonance input impedances much closer to 50Ω.
- The 3Cell CSMA with one parallel wire can significantly be improved by replacing it with three or five wires in parallel.
- The Q performance of the loop is better than the CSMA. The factor of 3 times higher bandwidth from Fig.2 can be reduced to a factor of 2 by structural modifications. However the drawback of a loop is the very low resonance input impedance The performance of a loop as well as of a dipole antenna suffers significantly when brought close to an electric wall as shown in Fig. 3. As expected the CSMA performance is improved under the same circumstances. Fig. 3 gives a comparison for these three types of antennas each with a radius of approx. Lambda /20. Further results will be discussed in the full paper.

Due to the high data rates required in modern personal communications, one solution consists in enhancing the capacity of the radio-frequency channel between the base station and the mobile terminal. This can be achieved by increasing the number of handsets’ radiating elements. This is not an easy task, especially at frequencies where the currents flowing on the Printed Circuit Board (PCB) of the phone contribute to the whole radiation mechanism of the structure. In this integration process, the main goal is to keep a high isolation between these radiators because if not, the diversity gain of the receiver and the channel capacity of the whole system can be drastically reduced [1]. Recently, Diallo et al. [2] demonstrated the possibility to integrate two closely spaced quarter-wavelength resonators on a small PCB, with high antenna's isolation and high antenna's efficiencies if using a neutralization line. However, these two Planar Inverted-F Antennas (PIFAs) were not operating at the same frequency.

This work has been further extended to the integration of several UMTS antennas on a small PCB for diversity and MIMO purposes. An optimized structure with two neutralized PIFAs positioned at the top edge of a PCB is shown in Fig. 1. The simulated (IE3D Zeland) and measured S-parameters are compared in Figure 2. A good agreement is observed. Due to the neutralization line, we maintain a low insertion loss in the overall UMTS bandwidth. The maximum total efficiency of both PIFAs has been measured to be as high as 95%. In addition, the diversity performance in a rich multi-path environment can be described by the envelope correlation coefficient. Theoretical and measured envelope correlation coefficients extracted from the S-parameters formula given in [3] are shown in Figure 3. They are found to be largely below -10 dB in the bandwidth of interest which is commonly accepted as a good enough value to provide sufficient diversity performance.

This neutralization technique has also been used when integrating four UMTS PIFAs on a small terminal (Fig. 4). We present in Fig. 5-6, the simulated and measured isolation parameters between the antennas of this system. These curves are in a very good agreement with all these parameters always below -15 dB in the frequency bandwidth: this structure has a strong potential for being efficient in diversity and MIMO systems.
In the final paper, all the envelope correlation coefficients will be presented [3] and we’ll achieve a discussion about the importance of having both high efficiency antennas and low correlation coefficients when integrating several antennas on a small handset terminal for diversity applications.

[1] A. Derneryd and G. Kristensson, "Signal correlation including antenna coupling", Electronics Lett.,Vol. 40, N° 3, 5th February 2004, pp. 157-159.
[2] A. Diallo, C. Luxey, P. Le Thuc, R. Staraj, G. Kossiavas, "Study and reduction of the mutual coupling between two mobile phone PIFAs operating in the DCS1800 and UMTS bands", submitted to IEEE Trans. on Ant. and Prop.
[3] J. Thaysen and K. B. Jakobsen, "Envelope correlation in (N, N) MIMO antenna array from scattering parameters", Microwave and Opt. Tech. Lett.,Vol. 48, N° 5, May 2006, pp. 832-834.

Several multi-antenna handsets have been designed and fabricated at the LEAT of the University of Nice for diversity and MIMO purposes (Fig. 1).These systems include up to four antennas operating in the UMTS band (1920-2170 MHz). Particularly, a neutralization effect has been used to achieve high isolation between the antennas [1]. These prototypes have been characterized in terms of matching and insertion losses (Fig. 2), total efficiencies, radiation patterns and antenna envelope correlation coefficient (Fig. 3) [2]. All these measurements (particularly high efficiency and very low correlation coefficients) have proven that these structures have a strong potential for an efficient implementation of diversity scheme in mobile terminals. However, the fully characterization of these prototypes in uniform multi-path propagation environments, needs some particular facilities and the associated expertise [3]. Chalmers Institute of Technology already possesses these capabilities through the Bluetest reverberation chamber [4].

This paper results from a short-term COST 284 mission where the small antennas designing skills of the LEAT have been gathered with the reverberation chamber measurement system of Chalmers Institute of Technology. Several prototypes are currently being characterized at Chalmers in terms of total efficiency, diversity gain and channel capacity (Fig. 4). The efficiency results will be compared with the measurements done with a Wheeler Cap set-up at the LEAT. The diversity gain as well as the channel capacity and the cumulative probability distribution function results will also be presented [3, 5]. It is also planned to repeat those measurements when the structure is located in different talk positions relative to a head phantom. The performances of different systems with and without the neutralization line will be compared. Particularly, we’ll achieve a discussion about the benefit of using our neutralization solution and the importance of optimizing the antenna’s isolation for diversity and MIMO purposes.

[1] A. Diallo, C. Luxey, P. Le Thuc, R. Staraj, G. Kossiavas, "Study and reduction of the mutual coupling between two mobile phone PIFAs operating in the DCS1800 and UMTS bands ", submitted to IEEE Trans. on Ant. and Prop.

[2] Z. Ying and D. Zhang, "Study of the Mutual Coupling, Correlations and Efficiency of Two PIFA Antennas on a Small Ground Plane", IEEE Antennas and Propagation Society International Symp., Washington (USA), July 2005.

[3] P-S. Kildal and K. Rosengren, "Correlation and Capacity of MIMO Systems and Mutual Coupling, Radiation Efficiency, and Diversity Gain of their Antennas: Simulations and Measurements in a Reverberation Chamber", IEEE Communications Magazine, Vol. 42, No. 12, pp. 104-112, December 2004.

[4] www.bluetest.se

[5] T. Bolin, A. Derneryd, G. Kristensson, V. Plicanic and Z. Ying, "Two-antenna receive diversity performance in indoor environment", Electronics Letters, Vol. 41, No 22, 27th October 2005, pp. 1205-1206.

We will present the results showing performance enhancement of microstrip antennas acquired by using partial dielectric filling. The results are obtained with an analytical model and verified both with simulations and measurements. The optimal partial filling pattern of the microstrip antennas can be found using the analytical model. In the optimal configuration the radiation efficiency is maximized while the patch size is considerably reduced.

The analytical model for the radiation quality factor of partially filled microstrip antennas is derived from the current and voltage distributions which can be analytically solved from a transmission-line model of the antenna. The stored electromagnetic energy can be calculated via the current and voltage distributions in the quasi-static regime. Studying the voltage and current distributions of the antenna as well as the analytical expression for the radiation quality factor for the antenna, proper filling pattern is found. The analytical model is also applicable for other types of substrates than non-dispersive dielectrics.

The analytical results show that the optimal way to fill microstrip antennas using dielectric substrate is to fill the antenna volume near the radiating edge of the patch. By increasing the permittivity of the substrate the volume filling ratio can be lowered. The results show that the radiation quality factor can be decreased this way more than 10 percent. The radiation efficiency of the antenna is studied with a numerical model. The studies show that the radiation efficiency of the antenna is also enhanced when filling the microstrip antenna according to the optimum filling pattern.

The analytical results are verified with two different simulations software packages, with a method-of-moments based and a finite-difference-time-domain based software. Both simulations results agree with the analytical results quite accurately. Measured results will be presented as well.

I. INTRODUCTION

The idea of using side-channels as a source of information to attack cryptographic devices was first introduced by Kocher [1]. He measured the timing characteristics and the power consumption of a crypto chip and analyzed it to find the secret information used by the cryptographic algorithm. In 2001, Gemplus [2] and Quisquater and Samyde [3] published the first papers about electromagnetic (EM) analysis. For those measurements, simple handmade coils were used. This research tries to look for an in-depth approach. In a first stage we made different magnetic probes and looked at their matching. II. PROBES

A probe type commonly used in Electromagnetic Compatibility studies is the shielded loop. Ideal for its rejection of electric fields and good sensitivity to magnetic fields, it is often used to measure H. For our application a knowledge of magnetic field strength is not an issue, as statistics will retrieve any information from the measured signal. Four types of loops, based on the types described in [4] were compared. As can be seen on figure 1, RG-58 coaxial cable was used. One type, the symmetric loop was also compared with a commercially available design [5].

The four loop types. From left to right: unshielded, symmetric, balanced and two moebius types. The upper one is a symmetric type as sold by EMCO.

One of the major problems in EM side channel analysis is that the currents, as they are small in magnitude, give raise to a small signal picked up by the probes. Therefore it is best to use an oscilloscope with high input impedance Zin, resulting in larger amplitude. As RG-58 coaxial cable used for the sensors, has a characteristic impedance Zc of 50 ohm, an oscilloscope at 1M ohm would cause reflections at the oscilloscope's port. These reflections are not of a problem, as the aim is to obtain a large signal at the oscilloscope, instead of transferring power, which is typically he case in antenna applications. It should however be avoided that these reflections are bounced back to the oscilloscope by a mismatch between the coaxial cable an its sensor at the end. Therefore matching of the loops is of paramount importance. Only the moebius shielded loop without shorting is matched with a reflection of less than -10dB over the entire frequency band up to 1GHz, except for the frequency where the capacitance between the two adjacent cables becomes a short causing the reflection to go up to -3dB. It is however expected that this mismatch will disappear with a well designed distance between the cables. This type will thus preferably be used for cryptographic analysis.
REFERENCES

[1] P. Kocher. Timing attacks on implementations of Diffie-Hellman, RSA, DSS and other systems. In CRYPTO, 1996.
[2] K. Gandolfi, C. Mourtel, and F. Olivier. Electromagnetic analysis: Concrete results. In C. K. Koc, D. Naccache, and C. Paar, editors, CHES, 2001.
[3] J.-J. Quisquater and D. Samyde. Electromagnetic analysis (EMA): Measures and counter-measures for smard cards. In E-smart, 2001.
[4] Roy Ediss. Probing the magnetic field probe. EMC & Compliance Journal, July 2003.
[5] EMCO near field probe set model 7405

Radio Frequency IDentification (RFID) of objects and remote control of devices has become very popular in logistics, inventory management and bio-engineering applications. Various kinds of data can be contactless transferred to a local querying system (reader) from a remote transponder (tag) including the antenna and a microchip transmitter. A new frontier is the wireless biometry of people within Mobile Healthcare Services [C. Liu et al., IEEE Int. Conf. Networking, Sensing and Control, pp.1014-1019, 2004]. An RFID system could provide real-time bio-monitoring and localization of patients inside hospitals or domestic environments, as well as inside a space capsule. In such cases the tag should be placed on the human body and equipped with bio-sensors (temperature, blood pressure, glucose content) and, when activated by the reader, tag ID and bio-signals could be transferred to a remote units and then stored and processed. The tag antenna plays a key role in the RFID system performances such as the reading range and the compatibility with the human body [Marrocco, IEEE MGWL-2, pp.302-305, 2003]. Conventional general-purpose tags are designed in the free space, but when adopted in on-body applications, the pattern distortion and the efficiency loss, produced by the human body dissipation and scattering, need to be taken into account in the first stage of the design as in the case of antennas on mobile platforms. This contribution investigates on the coverage properties of proper designed 869MHz planar miniaturized antenna for general purpose on-body passive tags, e.g. such to be directly energized by the reader radiation itself. These antennas could be used as a pendant on the human chest, eventually integrating sensors and electronics. The design includes the presence of the human body through layered cylindrical models with elliptical cross-section. Different antenna shapes are evaluated with the purpose to maximize the radiation pattern in the front and in the back of the chest, while keeping small the required space. Fig.1left shows an example of S-shaped dipole laying on the body through a silicone layer to improve tissue compatibility and matching. The tag microchip will be connected at the dipole centre. Under the hypothesis of perfect conjugate matching between the antenna and the microchip, it has been verified that the reading range corresponding to the maximum EIRP allowed for the reader (3.2W in Europe) is about 15m in front of the tag and 5m in the opposite direction (see Fig.1right). There results promise a feasible use of a RFID system in small environments and even in large hospital wings, provided that multiple reading points are deployed.

In future UWB communication devices an antenna should be not only very well matched (i.e. achieve a VSWR<2) within the entire operational frequency band to the feeding circuits, but also closely integrated with a generator or input circuits of a receiver. The optimal solution would be to have antennas integrated on printed circuit boards. Close integration with RF circuits gives additional freedom in antenna design as the antenna input impedance is not limited to the characteristic impedance of the transmission lines (50Ù). Furthermore, for balanced antenna feeding can be realized without a balun by using a differential amplifier in the receiver and an impulse generator with a differential output in the transmitter. The co-design of an impulse generator on chip and miniaturized UWB antenna is the focus of this work.

The main goals of the study are to find out values for impedance matching between a UWB antenna and an impulse generator, a proper design for the antenna feeding and minimize influence of the chip and chip feeding lines on antenna performance. The output of this study should be a preferable chip location with respect to the antenna and PCB design. We have selected an elliptically shaped dipole printed on a dielectric substrate (without a ground plane) as a transmit antenna because of its planar structure, the input impedance, which is constant over a large frequency band, and compact size.

In the antenna design and optimization we have used a theoretical model which is based on the mixed-potential form of the IE (MPIE) formulation as implemented in commercially available EM simulator FEKO. Three scenarios for antenna integration with an impulse generator on the same PCB with chip feeding lines lying in H-plane of the antenna have been defined and simulated. In the first scenario is the chip is located between antenna flairs. In this scenario generator feeding lines are located on both sides from the antenna flairs. Simulations show that the antenna bandwidth is too narrow, the reflection coefficient and antenna gain are very far from acceptable values. In the second scenario the chip is shifted in H-plane from the antenna feed point. Since the generator has a differential output, two microstrips come from a chip directly to the antenna flairs. The distance between antenna flairs is reduced to 1mm. The feeding line "chip - antenna" and all electronic circuits are mounted on the same side of the dielectric substrate as the antenna flairs. It is very good because we can suppress influence from this feed line. Due to presence of a short transmission line between the antenna and the generator, a mismatch between the antenna and generator happens and results in decrease of the antenna bandwidth. Finally, in third scenario the generator is located on the other side of antenna substrate above the feed point and is connected to antenna through a pair of short pins (Fig. 1). Optimization of the antenna and the distance between the chip and the antenna results in broadband matching between the antenna and generator and good antenna radiation performance. Experimental results show a radiated waveform without substantial ringing (Fig. 2).

Fig.1 Fig.2

Radio Frequency Identification (RFID) is a recent technology which found applications in many practical areas such as automatic identification and track of objects, supply chain integration, security access to controlled areas, and so on. Passive identification systems are of great importance in all those applications that require large scale-low cost tagging. To meet the market requests, there has been intense research on RFID tag antennas, especially in UHF band, where the possibility to obtain middle to long range wireless links is joined with a good reliability of the communication links.

Passive RFID at UHF band (866-869 MHz in Europe) uses the modulated scattered technique, where the reflected signal from the tag is modulated by a microchip (IC) connected directly to the antenna. As a consequence, RFID transponder performances are strongly affected by the frequency-dependent impedance match between antenna and IC. Because of small size, cost consideration and low power of the transponder, a matching network cannot be used. Instead, depending from the applications, the antenna must be designed for conjugate-matching to the specific IC input impedance or to maximize the efficiency and power transfer of the tag. Passive ICs are intrinsically highly reactive because of the necessary power to bias the IC front-end, which is delivered through electromagnetic coupling. In this case, due to the low input resistance and high capacitive reactance of the IC, antenna with a conjugate-match to the IC have to be designed with a low resistive and high inductive input impedance.

This paper presents a load-bars meander line dipole RFID tag antenna operating at European UHF Band. The meandered line dipole has been a natural choice in order to provide small size, proper bandwith and omnidirectional pattern in a plane perpendicular to the antenna plane. In order to allows a better control of the input impedance of the tag antenna, three inductively coupled load bars has been introduced. The antenna has been designed and optimized using an electromagnetic modelling software. An optimum match with the input impedance of a EM Microelectronic EM4444 UHF IC has been obtained by drawing and examining the antenna electrical parameter curves. It is found that the key parameters of the antenna such as loading bar spacing and width and meander step geometrical dimensions, can be easily adjusted to obtain a very good match between antenna and IC.

The obtained antenna size is smaller than 40x54 mm. The finalized designs were built on a 0,15 mm thick polyester film using silver ink for the printed antenna, to which the IC were soldered using a flip-chip attachment process. Several tag prototypes has been tested with a UHF reader and compared with simulated theoretical curves for the read/write range. It is found that theoretical and experimental data are in good agreement.