Compact Circularly Polarized Microstrip Ring Antenna Using Capacitive Coupling Structure for RFID Readers

This paper presents a compact circularly polarized microstrip antenna for radio frequency identification (RFID) readers. The dimensions of the proposed antenna are reduced by etching a cross slot in the radiation patch and loading four grounded coupling patches on the four corners of the radiation patch. An inverted Z-shaped coupling feedline is used to realize circularly polarized operation. The measured −10 dB <inline-formula> <tex-math notation="LaTeX">$\vert \text{S}_{11}\vert $ </tex-math></inline-formula> bandwidth, 3-dB axial ratio bandwidth and maximum gain are 872–1095 MHz, 888–933 MHz, and 5.52 dBi, respectively, at 922 MHz frequency. The overall dimension of the proposed antenna is <inline-formula> <tex-math notation="LaTeX">$0.36\,\,\lambda _{\mathrm {g}} \times 0.36\,\,\lambda _{\mathrm {g}} \times 0.05\,\,\lambda _{\mathrm {g}}$ </tex-math></inline-formula> (<inline-formula> <tex-math notation="LaTeX">$\lambda _{\mathrm {g}}$ </tex-math></inline-formula> is the guide wavelength at the center frequency of 915 MHz). The proposed antenna has small size with good overall performance and is suitable for compact RFID devices.


I. INTRODUCTION
Radio frequency identification (RFID) is a wireless communication technology that uses electromagnetic waves to transmit and exchange information, and has been widely used in tracking of goods, manufacturing, supply chain, industry, and commerce. An RFID system typically includes a tag with product information and a reader receiving the information. Hence, the performance of reader antenna directly affects the RFID system [1].
Compact planar microstrip patch antennas are widely used in RFID systems because of their low profile, easy construction, and simple structure. Many techniques, such as using a substrate with high dielectric permittivity [2], slotting the radiation patch [3], and loading shorting probe [4], [5], have been developed to miniaturize microstrip antennas. Using a high dielectric permittivity substrate may be the effective means to reduce the size of antennas. However, these methods frequently lead to some negative effects, such as narrow bandwidth, high cost, and distorted antenna patterns. Circularly polarized reader antennas are usually required The associate editor coordinating the review of this manuscript and approving it for publication was Qammer Hussain Abbasi .
in RFID systems to avoid polarization mismatch between tags and readers, because RFID tag antennas are usually linearly polarized and randomly placed [6]. Theoretically, circularly polarized radiation can be obtained by exciting two orthogonal electric field components with equal amplitude and phase difference (PD) of 90 • . Commonly used methods to obtain circularly polarized radiation mainly include multifed and single-fed methods. Multifed methods, such as 3-dB orthogonal branch coupler [7], [8], Wilkinson power splitter [9], 180 • PD power splitter [10] and balun bandpass filter [11], can easily achieve circular polarization but need complicated feed networks. Single-fed techniques [12], [13] can also achieve circular polarization, although their design is difficult to implement because of their complex antenna structure with limited bandwidth.
In addition to miniaturization and circular polarization, high forward gain in certain environment and unidirectional radiation patterns are required in RFID reader antennas. A conducting plane can be placed as a reflector at a quarter wavelength distance from the radiator to achieve good unidirectional radiation for avoiding destructive interference between reflected and radiated waves [14]. However, the required quarter-wavelength profile height and large VOLUME 8, 2020 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see http://creativecommons.org/licenses/by/4.0/ reflector will result in a large antenna dimension. Many techniques have been proposed to reduce the distance between the radiator and the reflector for obtaining a unidirectional antenna with compact dimension. An artificial magnetic conductor (AMC) structure can reflect 0 • phase over a finite frequency range, thereby reducing the profile height [15]. An electromagnetic bandgap structure provides a good directional radiation characteristic by suppressing surface wave propagation [16]. Frequency selective surface layers offer reflective phase coherence in broadband to achieve good antenna performance [17]. These techniques provide unidirectional radiation patterns although they experience the same problem, that is, the reflector structure is complex with extremely large size. Compared with [15]- [17], a simple resonance-based reflector structure is proposed to provide unidirectional radiation in broadband, but it cannot reduce the profile height [18], [19].
In this paper, a compact circular polarization unidirectional antenna for RFID readers is proposed. The antenna dimensions are reduced by etching a cross slot in the radiation patch to form a ring antenna and loading four grounded coupling patches on the four corners of the radiation patch. Circular polarization and wide bandwidth are realized using an inverted Z-shaped coupling feedline. The proposed antenna has smaller size, lower axial ratio (AR), and wider bandwidth compared with other microstrip structures with air gap [20], [21].

II. ANTENNA CONFIGURATION
As shown in Fig. 1(a), the proposed antenna consists of a radiation patch, a feedline, a ground plane and four small grounded coupling patches on the four corners, where the radiation patch and the feedline are printed on the upper and lower surfaces of the dielectric substrate. The dielectric substrate has a relative dielectric constant of 3.5, loss tangent of 0.0025, and thickness of 0.8 mm. The raw material of the ground and the four grounded patches are brass plate with a thickness of 0.3 mm. The upper dielectric substrate and the ground are separated by air with the height of h 1 . As shown in Fig. 1(b), the square radiation patch is etched with a cross slot. The inverted Z-shaped coupling feedline is directly connected by the coaxial probe. As shown in Fig. 1(c), the four small grounded square patches are connected to the ground plane through the side grounding wall with the height of h 2 .
The parameters of the proposed antenna are optimized on ANSYS HFSS 18. The optimized dimensions of the proposed antenna are listed in Table 1. The relevant design considerations and processes are discussed in detail.   because of the lower profile height between the radiation patch and the ground plane. On the basis of Ant.1, a cross slot is etched in the center area of the radiation patch leading to a ring-shaped radiation patch of Ant.2. Thus, the resonant frequency of Ant.2 significantly decreases to 1793 MHz compared with that of Ant.1. This condition is because the side length of a ring patch antenna is approximately a quarter wavelength, which is half of the wavelength required by a conventional microstrip antenna. Four small grounded patches are symmetrically placed on the four corners to form Ant.3. The resonant frequency is decreased to 906 MHz compared with that of Ant.2 because of the introduction of coupling capacitance between the ground plane and the  ring patch. In other words, the introduction of a cross slot and four grounded patches can reduce the dimension of the antenna when the resonant frequency is to be maintained at 906 MHz, and Ant.3 has the smallest size among the three antennas. It can be seen in Fig. 2, the antenna size is reduced, and its impedance bandwidth is improved from Ants.1 to 3. Fig. 3(a) shows the equivalent inductance and capacitance of the proposed antenna, where L F and C FP represent the inductance produced by the coaxial probe and the capacitance between the lower inverted Z-shaped coupling feedline and the upper radiation patch, respectively. C GP and C FG denote the capacitance between the ground plane and the radiation patch and between the radiation patch and the four small grounded coupling patches, respectively. R total represent the total resistance including radiation dielectric, and equivalent resistance of the surface wave. Fig. 3(b) shows the equivalent circuit model of the proposed antenna. As described in [5], the total equivalent capacitance of the proposed structure by adding parallel C FP and C FG to the circuit model, can be expressed using the following equation:

III. ANTENNA DESIGN AND ANALYSIS
Equation (1) indicates that the introduction of the capacitors of C FP and C FG can increase the total equivant capacitance of the proposed antenna, thereby reducing the resonant frequency on the basis of Cuircuit theory.

B. CIRCULAR POLARIZATION ANALYSIS
Circular polarization can be only achieved when two field components (i.e., |E y | and |E x |) have equal in magnitude and their time PDs are odd multiple of 90 • . Three feed structures are considered, namely Feed-1, Feed-2, and Feed-3, respectively, to explain the generation of circular polarization of the proposed antenna, as shown in Fig. 4. Feed-1 is a rectangular patch and symmetric about the x-axis. On the basis of Feed-1, a small rectangular patch is introduced and connected to the bottom left end of Feed-1 to form Feed-2. Another small rectangular patch is introduced and connected to the top right end of Feed-2 to form the Feed-3, thereby forming an inverted Z-shaped coupling feedline used in the proposed antenna. Fig. 4(a) shows the two orthogonal far-field E y and E x amplitude ratios |E y /E x | on the z-axis for the three feed structures. At the frequency range of 902−928 MHz, |E y /E x | of Feed-1 is close to 0, which is a typical value of a linearly polarized antenna. This condition is because the antenna configuration is perfectly symmetric about Feed-1 in the x-axis direction, indicating that |E y /E x | should be 0. |E y /E x | of Feed-2 slightly deviates from 0 because the symmetry is destroyed by the connected small rectangular patch on the bottom left end of Feed-1. For Feed-3, |E y /E x | in the frequency range is close to 1, indicating that the two far-field electric field components E y and E x are substantially equal in magnitude. Fig. 4(b) shows two orthogonal far-field E y and E x PDs for the three feed structures on the z-axis. At the frequency range of 902−928 MHz, the PDs of Feed-1 and Feed-2 are far from 90 • , whereas Feed-3 PD is extremely close to 90 • . The reason is that their varying lengths of W l2 , L l2 , W l3 , and L l3 produce different capacitance and control phases of the x-directed and y-directed current on the ring patch. As shown in Fig. 4(c), the AR of Feed-3 is less than 3 dB at the center frequency of 915 MHz.
To verify that the proposed antenna can generate circularly polarized radiation, the current density distributions on the ring-shaped radiation patch at different time instants are simulated and studied with the increase in time t from ωt = 0 • to ωt = 270 • , as shown in Fig. 5. The resultant current at the frequency of 915 MHz points to the +x-direction, -y-direction, -x-direction and +y-direction, when ωt = 0 • , 90 • , 180 • and 270 • , respectively. Thus, the resultant current vector sum flows in a clockwise direction. Therefore, the surface current distribution as a function of time mainly results in left-handed circular polarization (LHCP) radiation in the z > 0 half-space.

C. PARAMETRIC STUDIES
This section discusses some key parameters to clearly describe the antenna performance in terms of |S 11 | and AR. Only one geometric parameter is changed at a time unless VOLUME 8, 2020 otherwise specified, and the remaining geometric parameters remain unchanged. Fig. 6 shows the effects of height (h 2 ) of the four grounded coupling patches on |S 11 | and AR. The resonant frequency shifts from the lower frequency to the higher one, and the impedance matching effectively increases first and then rapidly degrades when h 2 increases from 14 mm to 16 mm at steps of 1 mm. Similarly, AR is significantly improved first, and then the lowest value of AR shifts to the high frequency. This phenomenon occurs because the equivalent   capacitance between the four grounded patches and the ringshaped radiation patch is reduced with the increase in h 2 .
The effects of side length (f g ) of the grounded coupling patch on |S 11 | and AR is shown in Fig. 7. The resonant frequency and the lowest values of AR shift to a lower frequency   when f g increases from 14 mm to 16 mm at steps of 1 mm. This finding is because the overlap area between the grounded coupling patches and the radiation patch increases with the increase in f g , thereby increasing the equivalent capacitance between them.
As shown in Fig. 8, the effects of width (L u3 ) of the cross slot in the radiation patch on |S 11 | and AR is studied. |S 11 | and AR become better and then worse when L u3 increases from 16 mm to 20 mm at steps of 2 mm. That is because  the change in L u3 affects the coupling strength between the radiation patch and the inverted Z-shaped coupling feedline, resulting in significant variations of current distribution on the radiation patch.

IV. EXPERIMENTAL RESULTS AND DISCUSSIONS
The proposed antenna is fabricated on the basis of the final optimized antenna geometry parameters in Table 1 Fig. 11 illustrates the simulated and measured curves of AR and gain at the boresight. The measured 3-dB AR bandwidth is 888−933 MHz, whereas the simulated AR bandwidth is 896−931 MHz. The measured results are in good agreement with the simulated results. The measured peak gain in the RFID operating band is 5.52 dBi. Fig. 12(a-c) shows the measured and simulated radiation patterns in xoz and yoz planes at the frequencies of 902, 915, and 928 MHz, respectively. The measured gains at 902, 915, and 928 MHz are 5.22, 5.33, and 5.19 dBi, respectively. The simulated results are in reasonable agreement with the measured results. The difference between them is because of the manufacturing tolerances and errors between measurement and simulation. Table 2 provides a comparison of antenna dimensions and other performances of the proposed antenna and previously published studies. It shows that the proposed antenna has wide impedance bandwidth and small electrical size.

V. CONCLUSION
This paper presents a compact circular polarization unidirectional antenna for RFID readers. The radiation patch of the proposed antenna is etched with a cross slot to form a ring antenna for reducing the antenna size. Four grounded coupling patches are placed on the four corners of the radiation patch to generate equivalent coupling capacitive reactance for further reducing the antenna size. Circular polarization and wide bandwidth are realized using the coupling feed of an inverted Z-shaped feedline. The measured −10 dB |S 11 | bandwidth, 3-dB AR bandwidth, and maximum gain in the operating frequency band of UHF RFID are 872-1095 MHz, 888-933 MHz and 5.52 dBi. The proposed antenna has the smallest size and maintains high unidirectional radiation gain compared with existing antennas, making it suitable for RFID readers.