Near-Field HF-RFID and CMA-Based Circularly Polarized Far-Field UHF-RFID Integrated Tag Antenna

+is research proposes an integrated high-frequency (HF) and ultrahigh-frequency (UHF) passive radio frequency identification (RFID) tag antenna for near-field (13.56MHz) and far-field (920–925MHz) communication.+is tag antenna is advantageous for the applications with lossy material in the near-field communication and mitigates polarization loss in the far-field communications. +e HF-RFID tag antenna is of square spiral structure, and the circularly polarized UHF-RFID structure consists of a square loop radiator with cascading loop feeding and shorted stub. +e structure of HF-RFID tag antenna situated inside the circularly polarized UHF-RFID tag can avoid the significant effect of the near-field magnetic coupling from the square loop. +e UHF-RFID tag antenna is realized by using characteristic mode analysis for wideband circular polarization. +e HF-RFID structure is conjugate-matched with NXP NT3H2111 chip, and the UHF-RFID structure is conjugate-matched with NXP G2X chip. Simulations were carried out, and an antenna prototype was fabricated. +e experimental results reveal that the radiation pattern of UHF-RFID tag antenna is bidirectional with a gain of 0.31 dBic. +e impedance bandwidth covers the frequency range of 903–944MHz, and the axial ratio in boresight direction at 922.5MHz is 1.67 dB, with the axial ratio bandwidth over 863–938MHz. +e maximum near-field and far-field reading ranges are 4.9 cm and 8.7m. +e proposed integrated dual-band passive tag antenna is operationally ideal for HF-RFID and UHF-RFID applications.


Introduction
Radio frequency identification (RFID) technology has been employed in various applications, including object or person identification and tracking and toll collection [1]. ere are two classifications of RFID: near-field and far-field communication.
e operating frequency of RFID near-field communication is high frequency (HF) of 13.56 MHz, and those of RFID far-field communication are ultrahigh frequency (UHF), 2.4 GHz and 5.8 GHz bands. Allocation of UHF-RFID frequency (860 MHz-960 MHz) varies from country to country. In this research, the UHF-RFID operating frequency is 920 MHz-925 MHz (center frequency of 922.5 MHz), which is the frequency allocation in China (upper band), Hong Kong, Singapore, ailand, and Vietnam [2].
In RFID tag antenna, the communication distance and frequency band are subject to application, reader capability, and implementation cost. For specific applications, RFID tag antennas with near-field and far-field communication capability are required, such as in highway tracking and toll collection. e RFID near-field communication in HF band has the benefit that the prominent magnetic field can penetrate the lossy materials and other dielectric media such as glass or other components of vehicles. e larger memory in HF-RFID tag is capable of storing the important information such as car owner name, registration information, and others. is is very convenient for officers to inspect some information for special circumstance. For RFID farfield communication using electromagnetic radiation in the UHF band, the tag antenna can be operated in long distance that can provide the positioning and tracking for various vehicle types. is fact necessitates integration of near-field and far-field RFID antennas in HF band and UHF band into one single structure.
In the literature, the research on antenna design for RFID near-field and far-field communication has been presented for either single band or dual-band operation. e target of greater performance with compact dimension is desirable. e RFID near-field and far-field antennas operating in dual HF band and UHF band have been presented in [3][4][5][6][7]. To conserve the antenna size, the HF-RFID and UHF-RFID tag antennas were designed in proximity with good isolation [3][4][5]. Alternatively, the design of the dualband RFID tag antennas is to accommodate the UHF-RFID tag antenna into the HF-RFID tag antenna because the typical HF-RFID tag antenna is relatively larger due to lower frequency operation [6,7]. Specifically, the near-field RFID capability is realized by using planar coil structures, and the far-field UHF-RFID capability is achieved using Hilbert curve [3], C-shaped radiator [4], bent rectangular patch [5], meander line [6], and S-dipole [7]. ese antennas [4][5][6][7] can be communicated with the RFID reader antenna using magnetic coupling in HF band and electromagnetic radiation in UHF band. e far-field tag antennas of linearly polarized radiation are prone to polarization mismatch with reader antenna. As a result, far-field tag antennas with circularly polarized (CP) radiation are more operationally desirable. In [3], the idea of designing the integrated nearfield tag antenna with circularly polarized far-field tag antenna using Hilbert curve structure is proposed. However, the main beam is shifted from the boresight direction, and the difference between impedance of the antenna and that of the chip is pronounced.
is is impractical for the implementation.
To realize CP radiation, most tag antennas [8][9][10][11][12][13][14][15][16][17][18][19][20] have been designed only for far-field radiation in UHF band. e CP tag antennas for UHF-RFID band with modified slit [8][9][10] and slot [11,12] are proposed. e inverted-Z geometry [13] and meandered loop [14] are utilized to generate CP radiation. In [15][16][17], both meandered and orthogonal techniques are used to realize CP radiation. e simple loop structure is implemented to generate CP radiation [18][19][20]. However, these circularly polarized tag antennas [8][9][10][11][12][13][14][15][16][17][18][19][20] suffer from relatively narrow axial ratio bandwidth (ARBW). To realize CP radiation with enhanced ARBW, characteristic mode analysis (CMA) is utilized in the UHF-RFID tag antenna design. CMA characterizes the modes of current on perfect electric conductor (PEC) of arbitrary conductor object to obtain characteristic modes that generate circular polarization. CMA [21][22][23] has been applied to optimize the antenna excitation locations [24,25] to design tilted-up radiation pattern [26] and to enhance impedance bandwidth [27,28]. e crucial challenge of this research is to design the integrated dual-band passive HF-RFID and UHF-RFID tag antenna for near-field and far-field communication with great performance, compact structure, and cost effectiveness. e integrated structure is made up of the HF-RFID tag antenna situated inside the circularly polarized UHF-RFID tag. is model is introduced to avoid the significant effect of the near-field magnetic coupling from the portion of UHF-RFID tag antenna to the HF-RFID tag antenna. e nearfield HF-RFID communication can be efficiently performed with lossy materials and other dielectric media. Furthermore, the design goal of the far-field UHF-RFID tag antenna is to achieve circularly polarized radiation for mitigation of polarization loss in some scenarios such as misalignment with reader antenna and multipath propagation environment. With limitation of structural compactness, the design of circularly polarized UHF-RFID tag antenna with enhanced axial ratio bandwidth (ARBW) is advantageous. e compact HF-RFID tag antenna of square spiral structure is initially designed based on modified Wheeler formula to conjugately match with NT3H2111 chip at 13.56 MHz. en, the UHF-RFID square loop tag antenna, relying on the CMA principle using characteristic angle difference, is realized to surround HF-RFID tag antenna with achievable wideband CP radiation. e inductive coupling element (ICE) technique has been applied for designing the proper CP excitation location. e UHF-RFID tag antenna with a square loop radiator with cascading square loop feeding structure is conjugate-matched with NXP G2X chip at 922.5 MHz. Simulations were carried out, and an antenna prototype was fabricated. e proposed integrated HF-RFID and UHF-RFID tag antenna is operable over 13.56 MHz and 920-925 MHz frequency range.

Antenna Configuration and Design
To design the integrated dual-band HF-RFID and UHF-RFID tag antenna for near-field and far-field communication, the antenna characteristics must meet the system requirements. e antenna size and its performance must be traded off. Usually, the antenna size should be less than or equal to a quarter wavelength of its lowest operating frequency. e antenna configurations should be conjugatematched with integrated circuit (IC) chips of each band. e HF-RFID tag antenna is miniaturized to locate inside UHF-RFID tag antenna. e far-field UHF-RFID tag antenna encompasses the HF-RFID tag antenna and radiates circularly polarized directional pattern. e antenna gain is closed to that of the dipole antenna with equivalent electrical length (1-2 dBic). e axial ratio (AR) must be less than 3 dB over the entire bandwidth that is widened to cover regulated frequency bands in various countries. e IC chips of NXP NT3H2111 and NXP G2X are used in the design of the HF-RFID and UHF-RFID passive tag antenna. e dual-band tag antenna is conjugate-matched with the IC chips to maximize power transfer between the IC chips and the antenna. Table 1 tabulates the characteristics of the RFID chips.
In conjugate matching, the impedance of IC chip is first determined. In Table 1, the input capacitance (C) of NXP NT3H2111 chip at 13.56 MHz is 50 pF, and the corresponding capacitive reactance is 234.73 Ω. International Journal of Antennas and Propagation impedance of 15.87 + j148.73 Ω so as to conjugate match with the NXP G2X chip whose impedance is 15.87 -j148.73 Ω.

HF-RFID Tag Antenna.
Given the input capacitance of NT3H2111 chip at 13.56 MHz of 50 pF, the inductance of the HF-RFID tag antenna should be 2,755 nH, corresponding to its inductive reactance of 234.73 Ω. To realize the inductance, a square spiral inductor is used in the HF-RFID tag antenna due to high immunity to a wide range of process variations. Figure 1 illustrates the configuration of square spiral HF-RFID tag antenna with 5 mm access line. e square spiral inductor is printed on FR4 substrate whose dielectric constant and thickness are 4.3 and 0.8 mm. e inner and outer dimensions of the square spiral structure are represented by d in and d out . e turn width (W) and the turn spacing are identical. e initial dimension of the square spiral structure is approximated by modified Wheeler formula (MWF) [29,30]. In theory, the inductance of the square spiral structure ( Figure 1) is a function of permeability, turn width, turn spacing, and the number of turns. e inductance (L) of the square spiral structure can be expressed as where μ 0 is free-space permeability, n is the number of turns, d out is the outer dimension, d in is the inner dimension, and ρ is the fill ratio. Due to fabrication constraints, the turn width (W) and turn spacing remain constant at 1 mm. e initial d out of the square spiral structure is 50 mm, which is the width of typical key card. Figure 2 illustrates the inductive reactance of the square spiral structure under variable numbers of turns (n) and d out , i.e., (4, 80), (5, 64), (6, 55), (7, 50), (8, 48), (9, 47), (10,46), and (11,46), with the resulting inductive reactance of 228.96 Ω -238.17 Ω. Ideally, the inductive reactance of the square spiral structure should be 234.73 Ω (L � 2,755 nH). As a result, the square spiral structure contains nine turns (n � 9) and is 47 mm in outer dimension (d out � 47 mm), which is subsequently mounted at the center of the UHF-RFID square loop tag antenna.
In the near-field communication of HF-RFID system, the magnetic field coupling plays a vital role in the connection between the reader and tag. To assess the communication performance, the magnetic field intensity of the HF-RFID tag antenna is simulated by CST Studio Suite full-wave simulation software [31]. e reading range of the HF-RFID communication system primarily depends on the magnetic field [32]. However, the estimation of reading range cannot be exactly determined because the certain sensitivity of the HF-RFID tag is not available. Nevertheless, the simulated total magnetic field distributions of the HF-RFID tag antenna for various distances, from the tag surface to 6 cm, are depicted in Figure 3. In principle, the intensity of total magnetic field distribution roughly decays with distance (d) to the power of three (H ∼ 1/d 3 ) [33]. It is obvious that the magnetic field distribution in Figures 3(a)-3(d) are strong at the surface and near the tag of around 4 cm. When the distance is 5 cm, the intensity of magnetic field is relatively weak as illustrated in Figure 3(e). From the distance of 6 cm and further, the magnetic field strength is very low as shown in Figure 3(f ). e measured reading range associated with this simulated magnetic field distribution will be mentioned in Section 3.

UHF-RFID Tag Antenna.
In the design of the circularly polarized UHF-RFID tag antenna, a square loop radiator is introduced to surround the HF-RFID square spiral structure. In the design of the UHF-RFID square loop tag antenna, characteristic mode analysis (CMA) is utilized to realize circularly polarized radiation. CMA characterizes the modes of current on perfect electric conductor (PEC) of arbitrary conductor object and its radiation property. e modal solution of CMA (equation (2)) is used to investigate the dynamic behavior (i.e., surface current) of the square loop structure to obtain characteristic modes that generate circular polarization. e current on conductor surface (J) is the summation of mode currents and can be expressed as

International Journal of Antennas and Propagation
where α n is the modal weighting coefficient and J n is the eigencurrent. e modal weighting coefficient can be expressed as where λ n is the eigenvalue and v i n is the modal excitation coefficient which is defined by where E i is incident electric field and S is the surface of a conductor. In CMA, the characteristic modes are determined by the eigenvalue (λ n ) or characteristic angle (CA), which is represented by β and expressed in equation (5). λ n is zero and β is 180°if J n is resonant.
In realizing the circularly polarized UHF-RFID tag antenna based on CMA, the square loop radiator is initially simulated without excitation. e initial width (W 1 ) and length (L 1 ) of the square loop radiator are 3 mm and 81 mm (quarter wavelength at the center frequency of 922.5 MHz), as shown in Figure 4.
In circular polarization (CP) theory, the phase difference of two orthogonal electric field components is 90°with identical magnitude. Figure 5 illustrates the simulated farfield patterns of the first six modes (J 1 -J 6 ) at 922.5 MHz of the CMA-based square loop radiator. In Figures 5(a) and 5(b), the far-field patterns of modes J 1 and J 2 are of nearomnidirectional radiation and orthogonal to each other, rendering them ideal for excitation of CP radiation.
To generate CP radiation, modes J 1 and J 2 are simultaneously excited at optimal excitation locations whose characteristic currents are high. e optimal excitation locations can be determined by inductive coupling elements (ICEs) or capacitive coupling elements (CCEs) [26,34]. In this research, ICE is used to optimize the excitation locations.
Figures 6(a) and 6(b), respectively, illustrate the simulated characteristic currents of modes J 1 and J 2 . e current is highly concentrated around regions A 1 and A 2 for mode J 1 and regions B 1 and B 2 for mode J 2 . As a result, either region A 1 or A 2 for mode J 1 and either region B 1 or B 2 for mode J 2 of the square loop radiator are excited. A 2 and B 2 are selected for the optimal excitation locations, giving rise to a cascading square loop feeding structure as illustrated in Figure 6(c). Figure 7 shows the eigenvalues of modes J 1 and J 2 (λ 1 and λ 2 ) and the characteristic angle difference of the square loop radiator with cascading square loop feeding structure over 860-960 MHz. At 922.5 MHz, λ 1 and λ 2 are 0.64 and − 1.71, which are close to zero, indicating near-resonant condition.
eoretically, CP radiation is induced when the phase difference between two orthogonal electric fields is 90°. In Figure 7, the characteristic angle difference between modes J 1 and J 2 at 922.5 MHz is 92.51°, which is approximately quadrature phase difference for CP radiation.
To realize the UHF-RFID tag antenna, the square loop radiator with cascading square loop feeding structure is conjugate-matched with NXP G2X chip. A shorted stub is thus introduced into the cascading square loop feeding structure for conjugate matching, as shown in Figure 8.
Given the length of the square loop radiator (L 1 ) of 81 mm, the initial length of cascading square loop feeding (L 2 ) is 20 mm. e initial width (W c ) and total length of the shorted stub (L c + 2H c ) are 3 mm and 39 mm. e square loop radiator with cascading loop feeding is printed on FR4 substrate whose dielectric constant and thickness are 4.3 and 0.8 mm.
e initial dimension of the substrate is 101 mm × 101 mm. e optimization of UHF-RFID tag antenna parameters is carried out using CST Studio Suite, given the conjugate matching between the UHF-RFID tag antenna and the NXP G2X chip and AR < 3 dB. Table 2 tabulates the initial and optimal UHF-RFID tag antenna parameters. Given the optimal parameters, the axial ratio of the UHF-RFID tag antenna in boresight direction is 1.16 dB. Figure 11 shows the simulated far-field electric fields under variable time periods (T) in boresight direction at 922.5 MHz. e electric field is rotated counterclockwise with respect to T, resulting in left-hand circular polarization (LHCP).

Antenna Prototype and Results
In the prototype fabrication, the HF-RFID and UHF-RFID structures are integrated on one single FR4 substrate for an integrated HF-RFID and UHF-RFID tag antenna as shown in Figure 12(a). e proposed HF-RFID structure is located inside the square loop radiator of the UHF-RFID structure.
is arrangement yields the magnetic field distribution along the square spiral antenna in HF-RFID band. e magnetic field intensity has very weak coupling on the portion of square loop radiator with cascading square loop feeding structure as illustrated in Figure 12(b). Figure 12(c) International Journal of Antennas and Propagation depicts the fabricated prototype antenna that is used for the measurement.
Since the impedance of the HF-RFID and UHF-RFID structures (in the absence of IC chips) are j234.73 Ω and 14.39 + j147.94 Ω, it is impractical to determine the impedance using 50 Ω network analyzer. As a result, a differential probe is deployed [35], as illustrated in Figure 13. e network analyzer is calibrated in full two-port network mode. e scattering parameters (S ij ) are used to    6 International Journal of Antennas and Propagation calculate the impedance (Z Tag ) and |S 11 | of the HF-RFID and UHF-RFID structures in the following equations: S 11 (dB) � 20 log 10 (Γ). (8) Figure 14 compares the simulated and measured impedance of HF-RFID and UHF-RFID structures (without IC chips). e simulated impedance of the HF-RFID and UHF-RFID structures is 6.08 + j236.76 Ω at 13.56 MHz and 14.39 + j147.94 Ω at 922.5 MHz. e corresponding measured impedance is 5.81 + j236.31 Ω and 17.84 + j147.51 Ω. e simulated and measured results are in good agreement. Figure 15 compares the simulated and measured |S 11 | of the UHF-RFID structure. Given |S 11 | ≤ -10 dB, the simulation result reveals that the UHF-RFID tag antenna is operable over 894 MHz-947 MHz (53 MHz bandwidth). e measured result shows the operating frequency of 903 MHz-944 MHz (41 MHz bandwidth). e simulated and measured results are in reasonable agreement.
Since the feature of the proposed tag antenna has the circularly polarized radiation in UHF-RFID band, the properties of circular polarization where the axial ratio (AR) of equal or less than 3 dB in terms of AR bandwidth, AR pattern, and tag antenna gain (in dBic) are investigated. e CP properties of the UHF-RFID tag antenna prototype are measured in an anechoic chamber, as shown in Characteristic angle difference (degree) Eigenvalue   International Journal of Antennas and Propagation      International Journal of Antennas and Propagation respectively, obtained in case that the polarizations between the transmitting and receiving antennas are the same sense (E co-pol ) and opposite sense (E x-pol ). e AR can be subsequently determined from [36] AR(dB) � 20 log     In the far-field communication of UHF-RFID system, the radiation pattern including antenna beamwidth is important in practical application. For the proposed tag antenna in UHF-RFID band radiating circular polarization, the so-called AR pattern that superimposes the normalized pattern with the AR as function of angle will be shown. e AR pattern is inscribed by inner and outer envelopes. For specified angle, the ratio of the outer and inner envelopes represents the AR. e ideal CP is corresponding to zero dB difference of outer and inner envelopes. In addition, the beamwidth of AR pattern can be considered from angular separation between two points with − 6 dB of the ideal CP [37]. Figure 18 compares the simulated and measured xzplane and yz-plane of normalized AR patterns at 922.5 MHz of the UHF-RFID tag antenna. e simulated and measured axial ratio beamwidths in boresight direction in xz-plane are 104°and 106°and those in yz-plane are 106°and 101°. e realized gain is the significant property of the antenna. For the circularly polarized antenna, the antenna gain is considered from the co-polarized radiation, where the unit of its gain is dBic. e standard circularly polarized antenna model ALR9610BC (Alien Technology) with left-handed CP is used as the transmitting antenna. Figure 19 compares the simulated and measured antenna gains over 860 MHz-960 MHz. e simulated and measured maximum gains are 2.45 dBic and 1.08 dBic at 897 MHz and 884 MHz, respectively. At the center frequency of 922.5 MHz, the simulated and measured antenna gains are 2.08 dBic and 0.31 dBic.
e large discrepancy could be attributed to impedance mismatch and fabrication loss.
For near-field and far-field communication of HF-RFID and UHF-RFID tag antenna, the reading range must be examined. e reading range of the proposed tag antenna in HF-RFID band is mainly influenced from the magnetic field distribution as simulated in Section 2.1. e measurement of magnetic field strength of the proposed tag is difficult. erefore, the measurement of the reading range will be performed instead. e Arduino Nano and Elechouse NFC module V3 with built-in PCB antenna are used as the reader for the HF-RFID tag antenna. e distance between the reader and the tag antenna is varied, and the maximum reading range is determined. e maximum reading range of the HF-RFID tag antenna is 4.9 cm. is measured reading range coincides with the distance of simulated magnetic field intensity in Figure 3.
For the UHF-RFID tag antenna, the far-field communication based on Friis transmission equation is evaluated. e maximum reading range (R max ) can be estimated from the effective isotropic radiated power (EIRP) of the UHF-RFID reader and the characteristics of the tag antenna. e calculation of the reading range can be determined from [19].
where Γ � 10 (|S 11 |(dB)/20) . e characteristics of the tag antenna are the gain of tag antenna (G tag ), reflection coefficient (Γ), polarization loss factor (PLF), and minimum required power for the tag (P tag ). e maximum reading range of 10.08 m is calculated under the given EIRP of 3 W, polarization loss factor of unity, and minimum required power for tag of − 15 dBm, respectively. In the measurement, the RFID reader model ALR9780 and standard CP reader antenna model ALR9610BC (Alien Technology) are used as the far-field   -10dB e measured maximum reading distance of the UHF-RFID tag antenna is 8.7 m. e measured maximum reading range is slightly lower than the calculated result because the measured gain is lower than the simulated one.
A comprehensive comparison of the proposed integrated HF-RFID and UHF-RFID tag antenna with some current state of the art in RFID tag antennas is tabulated in Table 3. e dimension and performance, i.e., single/dual operation band, polarization type of UHF band, and fractional bandwidths (|S 11 | ≤ − 10 dB and AR ≤ 3 dB), are compared under the fair-basis condition. e sizes of these antennas are approximately the quarter wavelength of their lowest operating frequencies.
From Table 3, the previously published papers of integrated dual-band HF-RFID and UHF-RFID tag antennas [4][5][6][7] were presented with linear polarization in UHF band. ese antennas suffer from the polarization loss from misalignment with reader antenna and multipath propagation environment. In [8][9][10][11][12][13][14][15][16][17][18][19][20], the presented circularly polarized tag antennas were operated in single band of UHF-RFID band. ese antennas have immunity from the polarization loss, but they are not suitable for the applications with the lossy material and some dielectric objects due to the natural limitations of UHF band. Apparently, the proposed antenna achieves the wide ARBW of 6.87%, which is relatively large for the compact antenna size.

Conclusion
e integrated HF-RFID and UHF-RFID tag antenna has been proposed for lossy dielectric and other materials in near-field communication and mitigates polarization loss in far-field communication.
e HF-RFID tag antenna of square spiral structure with measured impedance of 5.81 + j236.31 Ω is conjugate-matched with NT3H2111 chip at 13.56 MHz, resulting in the maximum reading range of 4.9 cm. e circularly polarized UHF-RFID tag antenna using a square loop radiator with cascading square loop feeding structure and shorted stub is realized by the CMA principle to achieve the measured impedance of 17.84 + j147.51 Ω at 922.5 MHz. With the enhanced techniques, the measured AR bandwidth is 75 MHz (863-938 MHz) which widely covers regulated frequency bands in various countries. e gain of the UHF-RFID tag antenna at 922.5 MHz is 0.31 dBic, with the maximum reading range of 8.7 m. Essentially, the proposed integrated dual-band HF-RFID and UHF-RFID passive tag antenna is suitable for near-field and far-field communication such as in highway tracking and toll collection applications.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper. - [5] 49 × 83 × 0.7 HF/UHF 0.12 Linear pol.