Dual-Band Microstrip Antenna Based on Polarization Conversion Metasurface Structure

A dual-band microstrip patch antenna (MPA) based on a polarization conversion metasurface structure was designed. By etching the complementary split ring resonator (CSRR) on the ground plane, a new resonance frequency is generated. The proposed antenna is obtained through optimizing the structural parameters of CSRR. Compared with the antenna without CSRR, the return loss of the proposed antenna increases by ~40% at the original resonance frequency. The measured results are similar to the simulated results, verifying the reliability of the antenna. This work introduces a new way of designing multi-band antenna.


INTRODUCTION
An antenna plays an important role in modern wireless communication systems. In recent years, the miniaturization and multi-functionalization of communication devices require that the internal antenna has strong integration capabilities [1,2]. Meanwhile, owing to the lack of wireless spectrum resources, the demand for multi-band antenna has increased [3][4][5]. Microstrip patch antenna (MPA) are extensively used due to their advantages of small size, simple structure, low cost, and ease of integration [6,7]. Many methods have been studied to obtain multi-band antenna, such as coupling feed technologies [8], slot-loaded technologies [9], and reconfigurable technologies [10,11]. Unfortunately, these methods require complex calculation, and the antenna structures are difficult to manufacture. Thus, new designs should be explored to simplify the structure and theoretical analysis.
Metasurface, due to its extraordinary electromagnetic properties, is widely used in antenna design [12][13][14], namely in the realization of multi-band [15,16], ultra-wide-band [17][18][19], and high-gain antenna [20,21]. In most cases, the metasurface structure is loaded on the antenna as a radiating element. The capability of a multi-band operation is not currently apparent. Many efforts have been made to solve this problem, such as etching a complementary split ring resonator (CSRR) on the ground plane. The CSRR unit cell can resonate with an electromagnetic wave, resulting in new resonance peaks. Ali et al. [22] created a triple-band antenna by etching rectangular and circular CSRRs. Zhou et al. [23] fabricated a composite right/left-handed structure as the radiating element and employed a square CSRR to get a dual-band antenna. Xu et al. [24] used a simple square split ring resonator to design an ultra-broad-band linear polarization converter, which provided a way to change the antenna polarization mode. We have recently obtained a broadband microstrip patch antenna by using a complementary rhombus resonator [25].
In this work, we designed a dual-band MPA loaded with a CSRR structure. The CSRR serves as a resonator. From the simulated results, it is found that the MPA exhibits excellent performance within the dual-band resonance in the range of 3.0-7.5 GHz. The measured results are similar to the simulated ones, which proves that the proposed antenna can be well-applied to physical devices.

ANTENNA DESIGN
C-band antenna have been widely used in satellite communication and navigation [26]. Therefore, we designed an antenna operating at 6.9 GHz. The top and back view of the antenna are, respectively, shown in Figures 1A,B. The upper layer of the antenna consists of a rectangular patch, and two different-width feedlines. The middle layer is an FR4 substrate with a relative dielectric constant of 2.65 and the lower layer is a metal ground plane. In order to realize the design of the dual-band antenna, the ground plane is modified. As shown in Figure 1C, CSRR was etched on the ground plane which is used to generating new resonance frequency. To further improve the electromagnetic performance of the antenna, a series of parameter optimization was carried out, and the ground plane of the proposed antenna is shown in Figure 1D. The simulated results of the proposed antenna are shown in Figure 2, with a series of side length r, split width d, rotation angle θ , and distance h. It can be seen that the original resonance frequency is affected by length r, split width d, and rotation angle θ , while the new resonance frequency is mainly tuned by distance h. According to Figure 2A, as r increases from 15.0 to 16.5 mm, the original resonance frequency moves to the lower frequency. When r = 16.0 mm, the return loss at the original resonance frequency is maximum. Figure 2B demonstrates that, as d increases from 0 to 4.5 mm, the original resonance frequency moves to the higher frequency. When d = 0, which means CSRR is a closed ring, the original resonance disappears. At this time, the antenna only works in S-band. As displayed in Figure 2C, when θ increases from 0 to 45 • , the original resonance frequency moves to the lower frequency. Moreover, when θ = 30 • , the return loss at the original resonance frequency reaches maximum. Figure 2D depicts that, as h increases from 24.0 to 25.5 mm, the new resonance frequency moves to the lower frequency. When h = 24.5 mm, the return loss at new resonance point is at its maximum, now the centers of CSRR and the substrate coincide. Considering the performance of antenna comprehensively, we designed the ground plane of the proposed antenna as Figure 1D. The parameters of the proposed antenna are listed in Table 1. To intuitively illustrate the effect of CSRR on the antenna frequency band, the return loss of the original antenna and the proposed antenna are compared. As shown in Figure 3, it can be inferred that the proposed antenna has a new resonance peak centered at 3.54 GHz with a return loss of 21.4 dB. The original resonance frequency shifts from 6.87 to 6.72 GHz, and its return loss increases from 22.1 to 31.2 dB.

RESULTS AND DISCUSSION
In order to explain the work principle of CSRR, the surface current distribution of the antenna is observed. Figure 4 shows the surface current distribution of the antenna without CSRR at 6.87 and 5.00 GHz. From Figures 4A,B, we can see that the current density at 6.87 GHz is higher than 5.00 GHz. It can be deduced that the electromagnetic wave of a specific frequency is excited by the rectangular patch and the feedlines, resulting in resonance at 6.87 GHz. Figure 5 shows the surface current distribution of the antenna with CSRR at two resonance frequencies. It can be observed that the current is no longer limited between the radiation patch and feedlines. It also distributes around CSRR, which demonstrates that the CSRR can act as a resonator to generate new resonance peak. Obviously, etching CSRR damages the structure of the ground plane, so that the current distribution is changed. Moreover, the current shocks back and forth in the CSRR, radiating a specific frequency of electromagnetic wave. However, there are some differences in the surface current distribution of Figures 5A,B. At the frequency of 3.54 GHz, the surface current mainly distributes around the CSRR. Thus, the new generated resonance peak is mainly affected by the parameters of the CSRR. Similarly, the resonance peak centered at 6.72 GHz is modulated by the rectangle patch and CSRR.
In order to figure out why CSRR can be used as a resonator, a few more details are exhibited. Comparing the results shown in Figure 5, it can be observed that the surface current distributes around the whole CSRR at 3.54 GHz, while only the lower half of CSRR has a current distribution at 6.72 GHz. It can be inferred that CSRR serves as a frequency-controlled switch which only opens at specific frequencies. When the switch is closed, the current in the CSRR shocks back and forth, resulting in new resonance within the antenna at 3.54 GHz. When the switch is on, the electromagnetic wave is only generated by the lower section of CSRR and it couples with the antenna radiating element, resulting in the original resonance point shifting by ∼0.15 GHz. Combining with the return loss diagram in Figure 3, it can be concluded that the return loss at the resonance frequency increases by ∼40%.
CSRR can not only produce resonance peaks as a resonator, but also affect the radiation characteristics of the antenna. To analyze the effect of CSRR on the polarization mode, the     polarization direction of the antenna with and without CSRR are shown in Figure 6. Figure 6A shows that the polarization direction of the antenna without CSRR is symmetrical at 6.87 GHz, while Figure 6B indicates that the symmetry of the antenna with CSRR is broken at 6.72 GHz. It can be interpreted that the polarization mode of antenna is highly related to its structure and changes within the structure. In order to better evaluate the antenna performance, the radiation patterns of the proposed antenna at resonance points are investigated as shown in Figure 7. It can be seen that the radiation patterns in H-Plane are almost omnidirectional and the radiation patterns in E-Plane are monopole-like. In addition, the distortion of the radiation patterns is extremely slight, which proves that the introduction of CSRR has almost no effect on the performance of the antenna.
To verify the simulated results, the original antenna and the proposed antenna are fabricated, measured, and compared. A circuit board engraving machine is used to fabricate the antenna and a vector network analyzer (VNA) is used to measure the electromagnetic properties of the antenna. The fabricated antenna are shown in Figure 8. The simulated and measured return loss of the original antenna and the proposed antenna are shown in Figure 9. From Figure 9A, the two curves have a nearly consistent trend and the resonance peaks almost coincide. As depicted in Figure 9B, the two measured resonance frequencies move from 3.54 and 6.72 GHz to 3.62 and 6.58 GHz, respectively. Compared with the simulated curves, the return loss decreases by 3-4 dB. Considering the inevitable errors in the process of antenna manufacturing, welding, and measuring, the above deviation can be ignored. Similarly, the fact that the bandwidth of measured curves is broader than the simulated curves is also due to fabrication and measurement errors. Therefore, the measured results correlate with the simulated ones, which proves the reliability of the antenna.
Based on the above analysis, it can be summarized that an antenna etched with CSRR performs excellently despite being small in size and containing a simple structure. Furthermore, we compared the dual-band antenna designed in this work with other literature [8,14,22] in terms of size, number of resonance peaks, operating frequency bands, and peak gain, as listed in Table 2. It is observed that, in other works, multi-band antenna are larger, while small size antenna operate at single-band. The design of our work realizes both miniaturization and multi-band in an antenna, which widens the possibilities for multi-band antenna design.

CONCLUSION
In conclusion, a dual-band MPA etched with CSRR was designed. The proposed antenna with high performance was obtained by optimizing the structural parameters of CSRR. Based on the simulated results of surface current distribution, it was found that the current shocks back and forth in the CSRR, generating a new resonance peak. By adjusting the rotation angle of the CSRR, an obvious resonance frequency appears. Moreover, the consistency between the simulation and the recorded measurement demonstrates the validity of the design. This work provides a new way to design multi-band antenna.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.