A Compact Dual-Band Wideband Circularly Polarized Microstrip Antenna for Sub-6G Application

A dual-band wideband circularly polarized (CP) microstrip antenna is proposed for sub-6G application. The antenna consists of an upper L-shaped radiator and two circular strips on the ground. This produces the right-handed circular polarization (RHCP) in the Wi-Fi (2.4–2.48GHz) and n77 (3.34.2GHz) band with the help of two circular strips at the left and right corners on the lower ground. The antenna occupies a small radiating area of 45× 45× 1.0mm3. The measured results show wide −10 dB reflection coefficient bandwidths of 46.4% (1.82–2.92GHz) and 40.5% (3.15–4.75GHz). The 3-dB axial ratio bandwidths of the antenna are 25.1% (1.88–2.42GHz) and 40.6% (3.20–4.83GHz). The measured peak gains are 4.8 and 7.5 dBi at the lower and higher bands, respectively. Therefore, the proposed antenna in this study is suitable for the dual-band wideband CP antenna as a reference.


INTRODUCTION
Circular polarization (CP) of radio waves has special advantages in communication application. For example, CP antenna avoids polarization mismatch and improves communications capacity and quality [1,2]. Since various modern wireless communication systems require wideband and high-speed information processing capabilities, antennas are also required to have wideband or multi-band operating characteristics. Besides, the demand for microstrip antennas with small size and high gain operating frequencies is increasing [3]. CP is an ideal characteristic of wideband antennas. In view of this, compact wideband multi-frequency antennas with CP characteristics have become a research hotspot in the industry [4,5].
In recent years, many scholars have done a lot of researches on dual-band CP microstrip antennas. In [6], a dual-band single-feed CP microstrip antenna is composed of two layers of microstrip metal patches. The CP radiation is realized by adjusting the position of feed points and its CP radiation at 3.5 and 5.18 GHz. The antenna is designed to realize dual-band CP radiation by placing the rectangular ring antenna at the first and second layers, respectively, and the thickness of the antenna is 20 mm [7]. In [8], the mentioned antenna achieves dual-band CP radiation by cutting two circular slots in a noncentral structure, and the antenna works at 2.1 and 3.6 GHz with 3-dB AR bandwidths (ARBW) of 9 and 21 MHz. The proposed single-feed dual-band CP designs are achieved by inserting four Y-shaped slits at the patch corners of a square microstrip antenna [9]. In [10], dual-band CP characteristic is achieved by combining the slot mode working in the GPS L1 band and the patch mode operating in the GPS L2 band. In [11], lower band right-handed circular polarization (RHCP) is mainly realized by the inverted L-shaped strip (ILSS), and the upper band left-handed circularly polarized (LHCP) is mainly achieved by the offset feed structure (OFS). The designed antenna is based on a single-layer and single-feed configuration [12]. However, in the case of low profile and compact size, it is rare for CP microstrip antennas to achieve both impedance and AR wideband in both operating bands.
In this paper, a novel embedded circular strips method is proposed for dual-band wideband CP microstrip antenna applications. At the same time, the designed antenna uses a non-centered L-shaped radiator to generate the lower and higher bands instead of the traditional central feeding mode. By adding two circular strips at the left and right corners on the lower ground, the 3-dB ARBWs of the antenna and 25.1% and 40.6% at the lower and higher bands, respectively.  Table 1.

Antenna Analysis
The design procedure of the dual-band wideband CP antenna structure is presented in Figure 2. As displayed in Figure 2(a) (Step-1), a wide circular slot is etched from the square-shaped patch, which is Step-1 exhibits linear polarization (LP) characteristics, and the S 11 performance is bad almost within the whole bandwidth. After an L-shaped radiator is formed, the lower ARBW of Step-2 is improved. The S 11 performance and ARBW of Step-3 have been greatly improved by adding two circular strips at the left and right corners on the lower ground. The proposed antenna achieves wideband CP characteristics in the lower and higher bands, respectively.   Step-1) of the antenna is a conventional printed line-polarized antenna that has only one rectangular radiator (L 1 × W 1 ). It can be seen from Figure 3(a) that the resonant path (L f + L 1 ) is about one quarter of the medium wavelength, so the lowest resonant frequency of the Step-1 antenna is about 2.4 GHz. To enable the antenna to operate at high frequency bands, another rectangular radiator (L 2 × W 2 ) is added to the beginning of the rectangular radiator (L 1 × W 1 ). At the same time, the non-central position of the radiator breaks the symmetry of LP and achieves the circular polarization effect of low frequency, but the ARBW is very narrow. To achieve the characteristics of high frequency circular polarization and broaden the ARBW, circular matching strips (R 2 , R 3 ) are used. Two circular strips added to the ground and an asymmetric feed structure provide a 90 • phase difference to achieve the high frequency circular polarization characteristics of the antenna. Adjusting the dimensions and center position of the two circular strips on the ground can improve the impedance matching and broaden bandwidth of the antenna [14,15].

Parameter Study
In order to obtain the optimal antenna dimensions, parameters of the two circular strips (R 2 , R 3 ) on the antenna ground are compared and analyzed, as shown in Figure 4. It can be seen from Figure 4(a) that R 2 and R 3 are the key parameters affecting the antenna impedance matching. Figure 4(b) shows the change of ARBW with R 2 and R 3 . Changes in R 2 and R 3 affect the surface current amplitudes of the two circular stripes. The adjustment of the current amplitude of each structure results in the change of superposition current direction at different phases. Therefore, the high-frequency ARBW becomes narrower.

Surface Current Distributions
The surface current distribution of the proposed antenna is studied to explain the dual-band wideband CP mechanism. Figure 5 shows the surface current distribution of the antenna at 2.0 GHz and 3.6 GHz. As shown in Figure 5(a), the surface current (J = 0 • ) at the ωt = 0 phase is mainly formed by the surface currents on the L-shaped radiator (J 8 and J 9 ) and the lower ground (J 1 , J 2 , J 3 , J 4 , J 5 , J 6 , and J 7 ). Obviously, the superposition current in the −x direction (J 1 + J 2 ) is stronger than the superposition current in the +x direction (J 5 + J 6 ), and the superposition current in the −y direction (J 3 + J 4 ) is stronger than the superposition current in the +y direction (J 7 + J 8 ). Thus, the amplitude of superimposed J = 0 • current is mainly controlled by the L-shaped radiator and the square-shaped ground with wide circular slot. After the currents in opposite directions cancel each other, the vector of the J = 0 • superimposed current goes along the −x axis direction with φ about 45 • . For the superimposed surface current distribution at the ωt = T /4 (J = 90 • ) phase, the current distribution is also mainly produced by the surface currents on the L-shaped radiator (J 8 andJ 9 ) and the lower ground (J 1 , J 2 , J 3 , J 4 , J 5 , J 6 , and J 7 ). Actually, the superposition current in the +x direction (J 1 + J 2 + J 3 ) is stronger than the superposition current in the −x direction (J 8 ), and the superposition current in the −y direction(J 4 + J 5 + J 6 + J 7 ) is stronger than the superposition current in the +y direction (J 9 ). Thus, the amplitude of superimposed J = 90 • current is mainly controlled by the two circular strips added to the lower ground. After the currents in opposite directions cancel each other, the vector of the J = 90 • superimposed current goes along the −x axis direction with φ about 135 • . Therefore, the

RESULTS AND DISCUSSION
According to the optimized size, the dual-band wideband CP antenna system has been successfully fabricated and measured. The proposed antenna prototype (top and bottom view) is presented in Figure 6(a). The S-parameters of the antenna were measured using the Agilent E5071C Network Analyzer. Besides, the far field measurement setup inside a Satimo-SG64 Anechoic Chamber system is shown in Figure 6(b). Figure 7 shows the simulated and measured S 11 of the antenna varied with frequency. It can be seen that the −10 dB impedance bandwidths of the antenna are 46.4% (1.82-2.92 GHz) and 40.5% . However, the antenna has frequency deviation in the lower frequency, which may be caused by dimensional deviation of the antenna model.     of the antenna tested in the +z direction is about 15.6 dB. Similarly, Figure 9(b) shows that the main polarization of the antenna radiated at 3.6 GHz is an RHCP wave in the +z direction. The difference between the RHCP wave and LHCP wave tested by the antenna in the +z direction is about 19.7 dB. The measured and simulated radiation patterns are basically the same.
Compared with other studies on dual-band CP antennas as in Table 2 [9][10][11][12], the proposed antenna system has certain advantages over other studies in not only bandwidth but also antenna size. The two circular strips at the left and right corners on the lower ground greatly expand the impedance bandwidth and ARBW of the antennas. Apparently, the proposed antenna also has high peak gain at the higher band. Therefore, the proposed antenna in this study is suitable for the dual-band wideband CP antenna as a reference.

CONCLUSION
This paper proposes a compact dual-band wideband CP microstrip antenna for Sub-6G application. The dual-frequency radiation of the antenna is realized through a non-centered L-shaped radiator. The characteristic of dual-band wideband CP is achieved by adding two circular strips at the left and right corners on the lower ground. The −10 dB impedance bandwidths are 46.4% and 40.5%. Meanwhile, the measured antenna 3-dB ARBWs are 25.1% and 40.6%. Consequently, the proposed antenna has good application value for Sub-6G.