Metamaterial‐based, miniaturised circularly polarised antennas for RFID application

Yuandan Dong, School of Electronic Engineering, University of Electronic Science and Technology of China, Chengdu, China. Email: ydong@uestc.edu.cn Abstract Miniaturised circularly polarised (CP) antennas based on dispersion‐engineered metamaterial transmission lines (TLs) are proposed and developed for RFID applications. It is first studied and developed based on the equivalent circuit and dispersion curves of composite right/left‐handed TL (CRLH‐TL), which is featured by a novel/low‐cost double‐layered 3D structure, including metallic screws, one ground layer and metal‐ insulator‐metal (MIM) capacitor loaded on the top layer. CP radiation is achieved by two crossed CRLH‐TLs with a microstrip‐slot coupling excitation. One CP antenna and one polarisation reconfigurable antenna, which utilise CRLH‐TLs are then implemented, respectively. Compared with the conventional right‐handed (RH) TL‐based antennas (positive resonance‐based antennas), miniaturisation is achieved by pushing down the dispersion curve to a lower frequency using larger LH capacitance or/and inductance. Negative‐order resonances could be successfully excited by cascading π‐type unit‐cells. It shows similar radiation characteristics and additional size reduction as the positive resonance‐based antennas. This antenna shows an electrical size of 0.24 λ0 � 0.24 λ0 � 0.03 λ0, a −10 dB bandwidth of 4.8%, and a peak gain of 4.2 dBic. It demonstrates advantages in terms of flexible size reduction, low cost, easy manufacturing, and good radiation performance, which are very suitable for 902–928 MHz UHF band RFID application.


| INTRODUCTION
Engineering the dispersion curve to explore and utilise the unusual and exotic electromagnetic properties has been a research interest on metamaterials over the past two decades [1][2][3][4][5][6][7]. Among all the approaches, transmission line (TL) theory appears as one of the most useful tools to extract and analyse the dispersion relation. Composite right/left-handed (CRLH) TL metamaterial with left-handed (LH) passband in lower band and right-handed (RH) passband in higher band simultaneously was initially proposed in [1]. Metamaterials include not only CRLH materials, but also other artificially engineered structures showing novel features, such as negative or low values of permittivity (ε), permeability (μ), or zero index media.
Circularly polarised (CP) antennas are widely used in the radio frequency identification (RFID), satellite communication, global positioning system (GPS), and wireless local area network [28][29][30][31]. To meet the application demands of miniaturised devices and different scenarios in RFID application, a compact CP UHF-band RFID reader antenna with good bandwidth and radiation performance, and low cost is urgently needed [29,30]. One wideband CP antenna with a two-port feeding network was proposed in [31], but it has a high profile and extra feeding circuits. The work in [17,32] proposed two miniaturised metamaterial-based CP antennas, respectively. However, both suffer from a narrow bandwidth and large loss. Therefore, it is a big challenge to design a miniaturised CP antenna with good bandwidth and radiation performance for RFID reader applications.
A novel miniaturised CP antenna for RFID applications is presented herein. By engineering the dispersion curve of the CRLH-TL metamaterials, the -first-order resonance mode is implemented. The equivalent circuits and dispersion relations of CRLH-TLs are first analysed and discussed. The CP operation is achieved by applying two crossed CRLH-TLs with proper feeding. Their resonance frequencies are slightly separated, leading to two orthogonal electric components with 90°p hase difference [28]. In addition, a CRLH-TL-based polarisation reconfigurable antenna is developed with four PIN diodes applied on the two coupling slots. Miniaturisation could be easily realised by using the negative-order resonance mode [14]. Moreover, the substrate of the antenna can be considered as the quasi-air substrate, which significantly reduces the dielectric loss, resulting in good efficiency. Low cost is also clearly achievable by using the standard metallic screws and thin dielectric substrate. Figure 1 shows the equivalent circuits of five different forms of TLs. The conventional RH-TL is represented by a series RH inductor (L R ) and a shunt distributed capacitor (C R ), as show in Figure 1a. The equivalent circuit of LH-TL is shown in Figure 1b, which consists of a series LH capacitor (C L ) and a shunt LH inductor (L L ), but the LH-TL is only an ideal mode. Figure 1c-e shows three forms of metamaterial TLs, namely LH capacitor-loaded TL (CL-TL), LH inductor-loaded TL (IL-TL) and CRLH-TL, respectively.

| DISPERSION ANALYSIS
The equivalent circuit of CL-TL is featured as a π-type model, as shown in Figure 1c. For CL-TL configuration, series C L is inserted into the RH-TL. Figure 2 shows the dispersion curves for this form obtained from a circuit simulation with ADS software. There is one ZOR frequency, which comes from the series resonance of the equivalent circuit. It is defined by the following equation [5]: ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi The resonance frequencies of different-order modes for an N-stages CL-TL can be observed on the dispersion curve when the following condition satisfies [5] 8 < : where p is defined as the length of periodic unit-cell, and N is the number of unit-cells. By cascading more unit-cells, the +first-order mode would shift to a lower frequency as explained by Equation (2). When C L is increased, f se shifts to a lower frequency, and the slope of this dispersion curve would increase significantly. As a result, the bandwidth of the +firstorder resonance becomes wider. Its resonance frequency (f +1 ) also shifts to a lower region, as indicated by Figure 2. Note that its miniaturisation is limited compared with CRLH-TL. Thus, here, an analysis is shown. Figure 3 shows the dispersion curve of the LH IL-TL, which is shown in Figure 1d. For this IL-TL configuration, the shunt LH inductor is loaded to the conventional RH-TL. Similarly, there is one ZOR frequency, which comes from the shunt resonance, as defined by the following equation [5]: ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi L L C R p ð3Þ when L L goes up, f sh is pushed down to a lower frequency.
Note that under this scenario the whole dispersion curve is shifted down with a relatively stable slope rate. All the RH resonances, including the +first-order resonance frequency (f +1 ), would decrease by increasing L L . Thus, miniaturisation then could be achieved by using the +first-order resonance mode. Compared with the CRLH-TL, the bandwidth of the antenna would not increase. Therefore, only an analysis is presented for this metamaterial TL. Figure 4 shows a dispersion curve analysis for CRLH-TL, using the equivalent circuit shown in Figure 1e. For this configuration, the series C L and shunt L L are both applied in the conventional RH-TL. There are two ZOR frequencies given by the following equations which come from the series and shunt resonant tanks, respectively [5]: ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi In the balanced condition (f se = f sh ), they equal each other and the bandgap vanishes. This equivalent circuit belongs to the π-type model. The resulting resonance frequencies of different modes for an N-stage CRLH-TL can be observed on the dispersion curve when satisfying the following conditions [5]: where p is defined as the length of periodic unit, and N is the number of cascaded unit cells. The negative order resonance (NOR) modes can be excited when the π-type mode CRLH-TL is effectively built and driven. Note that the first-order resonance frequency ( f -1 ) would shift to a lower region if more periodic units are used (N > 1). The field distribution and radiation character of the NOR modes are similar to their corresponding positive-order modes. As indicated by its dispersion curve, when C L is increased, both f se and the LH resonance frequencies would shift to lower frequency. The RH resonance frequencies also move to the lower side. The f -1 shifts to the lower region resulting in size miniaturisation. Figure 4a shows that the slope of the dispersion curve in the LH region becomes larger by increasing C L , therefore an antenna based on the negative-order resonance with a large C L could not only achieve size reduction but also certain bandwidth enhancement. When L L is increased, the ZOR frequency ( f sh ) and the LH resonance frequencies would shift to a lower frequency. So f -1 would move to a lower region resulting in miniaturisation, but the slope of the dispersion curve in the LH region actually decreases, as shown in Figure 4b. Three miniaturised antennas based on CRLH-TL are proposed in Section 3. They are implemented on -first-order resonance mode, with similar radiation properties of the conventional half-wavelength patch antennas. Figure 5 shows the configuration and geometry of the proposed miniaturised LP antenna based on CRLH-TL metamaterial. In the top layer, the metallic strips are printed on both sides of the F4BM (ε r = 2.2, tan δ = 0.001) substrate with a thickness of 0.254 mm using PCB technology, which builds series metal-insulator-metal (MIM) capacitors (C L ). The MIM capacitor works as an LH capacitor (C L ) as well as a radiator. The thickness of ground PCB board is 0.508 mm. Metallic screws with a diameter of 3 mm are directly used to connect the top PCB layer and the ground layer. The metallic screws not only provide structural support but also work as a shunt LH inductor (L L ). Every unit-cell is configured with two screws, which could be used to obtain a suitable L L and a balanced support. The coupled feeding is implemented by a rectangle slot and a 50 Ω microstrip line in the ground layer.

| LP CRLH-TL metamaterial-inspired antenna
Here, a multilayered 3D CRLH-TL-based antenna is realised using the PCB technology and mechanical assembling method, which not only has good performance but also provides a novel design guideline/example for metamaterial application. The CRLH-TL is set to a bowtie shape to facilitate a crossed RFID CP antenna design. Here, both C L and L L are loaded onto the conventional RH-TL, resulting in successful implementation of a CRLH metamaterial TL. The -first-order resonance mode is selected to achieve a maximum boresight radiation. Due to the -first-order resonance the mode moves to higher frequency by cascading multiple unit-cells, so only four unit-cells are used in this design to achieve a balance of miniaturisation and gain. It corresponds to an x-axis value of 0.25 on the dispersion curve shown in Figure 6 for this four- -551 F I G U R E 6 Dispersion curves for the unit-cell shown in Figure 10 4) CRLH-TL. The electric size of this LP antenna is 0.24 λ 0 � 0.24 λ 0 � 0.03 λ 0 , where λ 0 is the free-space wavelength at 900 MHz. It is clearly observed that this antenna is much more compact than the conventional patch antenna. It is noted that an NOR-based antenna could be arbitrarily miniaturised by engineering the dispersion curve through L L and C L . Figure 6 compares the dispersion curve of the proposed CRLH-TL unit-cell obtained from the full-wave and equivalent circuit simulation. Both the circuit model and the simulated physical structure of the unit-cell belong to the 1-stage π-type model, where both the +firstand -first-order resonance modes can be excited. The +first-order resonance mode is located around 2.85 GHz, while the -first-order resonance mode is at 700 MHz. By using a four-stage CRLH-TL, thefirst-order resonance mode goes up to 0.89 GHz. The -firstorder resonance frequency after optimisation is around 915 MHz and its −10 dB bandwidth is 903-927 MHz (2.62 %) in Figure 7. Note that the simulated results agree with the dispersion curve shown in Figures 6 and 7. The small difference results from the varied excitation and boundary condition, as well as the small size difference. The simulated peak gain is over 5 dBi across the passband region and its radiation patterns are similar to the conventional patch antenna. Figure 8 shows a parametric study for the proposed LP antenna. When C L is increased, the resonance frequency shifts to the lower frequency region and its bandwidth is slightly enhanced, which agrees with the equivalent circuit analysis shown in Figure 4a. When the diameter of the metallic screw is increased, L L is reduced and the frequency of the proposed LP antenna moves higher, with a relatively fixed bandwidth. Figure 9 shows the configuration, geometrical parameters, and a photograph of the fabricated CP antenna using CRLH-TLs. Based on a previously proposed LP antenna, two CRLH-TLs (TLA and TLB) are placed perpendicularly and a frequency perturbation is introduced by adjusting either the series LH capacitor or the shunt LH inductor. In practice, the screws' position, quantity, and diameter could be adjusted to tune the shunt LH inductance. However, it is relatively easier to adjust the resonance frequency of negative-order resonance modes by controlling the LH capacitor. Two rectangular coupling slots are orthogonally etched on the top layer of the ground board and a 45°microstrip feeding line is printed on the other side of the ground PCB board. They are used to excite the top TLs in the related direction. The length of the feeding microstrip line (c) is used to improve the matching. With ±45°phase delay travelling along the opposite directions, 90°phase shift between the two crossed -first-order modes could be obtained. Therefore, two orthogonal electric field (E-field) components with 90°phase difference are obtained, resulting in a CP  [28,30]. To simplify the analysis and design, the size of two crossed coupling slots is kept the same in this design. Note that the CP polarisation type could be determined by the frequency perturbation. When the -firstmode resonance frequency of the TLA is slightly lower than that from TLB, LHCP radiation is observed. In contrast, RHCP radiation could be realised when the LH capacitor of the TLA is slightly smaller than that of the TLB. A metallic frame (a thin aluminium sheet with a thickness of 1 mm) is also applied for better assembling reasons. In Figure 9d, this CP antenna shows a flexible method of packaging, which is easy to widely manufacture. Figure 10 shows the E-field distribution and the 3D radiation pattern for the CP antenna at 915 MHz. The E-field is mainly distributed around the capacitive gaps of CRLH TLs. A left-handed rotated field distribution can be observed in a full cycle for two first-order resonance modes. The radiation pattern for this negative-order resonance antenna is very similar to the conventional patch antenna. Figures 11 and 12 show the measured and simulated |S 11 | and AR response. The measured −10 dB bandwidth and 3 dB AR bandwidth are 4.81% (893-937 MHz) and 1.21% (902-913 MHz), respectively. Two resonance modes are observed in the passband, which are generated by the -first-order resonance frequencies of the two crossed CRLH TLs. This CP antenna covers well the 902-928 MHz UHF band, which is suitable for RFID reader application. The measured frequency response slightly shifts to the low-frequency region, which is mainly caused by the fabrication and assembling tolerance. Figure 13 shows the measured and simulated LHCP peak gain and radiation efficiency. The measured peak gain and  60 dB (69.2%), respectively. The difference between simulation and measurement mainly comes from the loss of the stainless steel screws and the SMA connector. Note that copper screws are selected in HFSS simulation, but replaced with stainless steel screws in fabrication, so a larger conductor loss occurs. In practice, this issue can be well solved by selecting the brass screw or aluminium screw. Figure 14 shows the measured and simulated radiation patterns at 915 MHz. LHCP radiation is implemented and the RHCP levels (cross polarisation levels) have been well suppressed below −18 dB in the main radiation direction. In addition, a wide beam-width is also clearly observed, which makes this CP antenna well suitable for RFID application with the wide-angle reading region. Figure 15 shows the configuration, geometrical parameters, and a photograph of the fabricated polarisation reconfigurable antenna using CRLH-TLs. Based on the above-proposed CRLH-TL CP antenna, four PIN diodes (SMP1345) are soldered and applied across the two cross-coupling slots (two each). The switches are used to control the shorting/opening states of the two coupling slots. Large-value inductors (L = 82 nH) and capacitors (C = 100 pF) are used to isolate the RF signal and DC source [33]. The resistors (R = 100 Ω) are used to limit the current level of the bias circuits. The detailed circuit information is presented in Figure 15b,c. To simplify the configuration and avoid disturbance, the bias circuits of the four switches are the same. The equivalent circuits of the PIN diode are shown in Figure 15d, and are used in HFSS simulation.

| Polarisation reconfigurable antenna based on CRLH-TL
Three polarisation states are achieved by switching the diodes, namely LHCP, LP (in x-axis), and LP (in y-axis). Table 1 shows the states of the switches with different polarisation types. When D1and D3 are ON and D2 and D4 are OFF, the coupling slot in the y-axis direction is shorted and the polarisation of the antenna is LP along the x-axis. In contrast, LP along the y-axis could be obtained by exchanging the states of these switches. When all of the four switches are OFF, two crossed coupling slots are all open and the LHCP is obtained. Figure 16 shows the measured and simulated |S 11 | response for the three reconfigured states, including the lower  Figure 18. The reconfigurable antenna uses four PIN diodes, resulting in additional loss from the parasitic resistance of the PIN diodes. The radiation gain is therefore slightly lower than the proposed CP antenna.

| RFID validation: reading range test
To validate the actual performance of a dispersion-engineered CP antenna for RFID reader application, a reading range test was performed using the authors' CP antenna and a typical inlay RFID tag, as shown in Figure 19. All parameters in the measurement setup and performance results, including the calculated and measured reading distance (d ), are illustrated in Table 2. The calculated distance is obtained from the Friis transmission equation [28]. It demonstrates a good transmitting range and radiation performance.

| Comparison with previous works
To highlight the advantages of the proposed RFID reader antenna, a comparison with previous works is summarised in Table 3. The antennas in [34][35][36][37][38] are all for RFID reader applications. In detail, a small CP patch antenna using a high-ε r F I G U R E 1 8 Measured and simulated peak gain for the proposed polarisation reconfigurable antenna in three polarisation states F I G U R E 1 9 The RFID setup for reading range test using the authors' proposed CP antenna WANG ET AL.
substrate was proposed in [34], but it suffers from a narrow bandwidth and large dielectric loss. In [35,36], two RFID reader antennas with an air substrate were developed using a PIFA-type structure, but their profile is higher. Two shared-aperture LP and CP UHF reader antennas using a hybrid mode structure were implemented in [37,38], respectively. However, they have large dimensions and complex structures. The metamaterial-based antennas using reactive impedance surface [39] and metasurface [40] are able to achieve a wideband operation, but they have a large size and high cost. A compact CP antenna loaded with LH structures was studied in [32]. However, its radiation performance was not very good and its bandwidth was narrower.
Overall, the authors' proposed metamaterial-based CP antenna demonstrates good potential in UHF RFID reader applications and other Internet of Things (IoTs) devices because of its promising features such as compact size, low profile, low cost and low loss, good radiation performance, and impedance bandwidth.

| CONCLUSIONS
A miniaturised metamaterial-based CP antenna with an electric size of 0.24 λ 0 � 0.24 λ 0 � 0.03 λ 0 and a polarisation reconfigurable antenna for RFID applications is proposed and developed herein. For the CRLH-TL-based antenna, the -first-order resonance mode is excited. Its resonance frequency is much smaller than the frequency of its corresponding +first-order resonance mode, especially with larger value LH components (C L and L L ). Compared with the conventional half-wavelength patch antennas, it shows similar radiation characteristics, but exhibits a much more compact size. By using the metallic screws and a thin substrate, low loss and low cost are realised simultaneously. Compact size, broad bandwidth, and excellent radiation performance make them good candidates for RFID application and other IoT systems. Besides, a good engineering application example for dispersion-guided, high-performance metamaterial-based antennas has been demonstrated.