I. Introduction
The increasing demand for high-speed transmission and high-volume data transfer in contemporary communication technology has resulted in rapid advancements in high-gain broadband antennas [1–7]. The types of high-gain antennas include, but are not limited to, waveguide feed antennas [8, 9], lens antennas [10–12], two-dimensional planar antennas [13, 14], and so on. However, waveguide feed antennas and lens antennas usually have large sizes, which poses certain challenges for miniaturization. In contrast, planar two-dimensional antennas have a compact structure, are easy to integrate, and have low manufacturing costs, owing to which they have been widely adopted in various wireless transmission systems. Patch antennas, in particular, are widely used in wireless monitoring systems, data transmission systems, and radar detection systems due to their low-profile design [15]. However, despite their numerous advantages, these antennas have an inherent limitation—since high quality factors lead to relatively narrow operating bandwidths, expanding the frequency bandwidth while maintaining a low profile is a significant technical challenge.
Many techniques for enhancing antenna bandwidth have been reported in the literature, including capacitive coupling feeding [16], stacked patch structures [17], using thick air dielectrics [18], introducing parasitic resonators [19], and applying reactive slot loading, among others [20]. However, each of these techniques has limitations—they either necessitate an increase in the antenna size or significantly complicate the analysis as the number of parasitic resonators and irregular slot loads increases.
In recent years, metasurface (MTS) antennas have demonstrated superior performance to other antennas, particularly in reducing antenna size, expanding the operating frequency band, and enhancing radiation capability [21–23]. For instance, Yan et al. [23] proposed a MTS array antenna that uses periodic units as radiators to achieve a low-profile effect and excites adjacent unit gaps as radiating slots by etching parallel slots on the ground plane, achieving broadband impedance matching and consistent radiation. MTS has gained considerable attention in recent years. Researchers have proposed broadband directional MTS with aperture-coupled feeding [24]. Notably, an antenna composed of mushroom units achieved a 25% impedance bandwidth, attaining a maximum gain of 9.9 dBi within the passband range [25]. Moreover, the impedance bandwidth of the periodically arranged patch unit layer was 28%, and it achieved a peak gain of 9.8 dBi within the passband. The results attained using the abovementioned MTSs indicate that an antenna generally has an operating bandwidth of less than 30% at −10 dB. To address this limitation, a low-profile dual-layer loaded broadband antenna was introduced to achieve a 44% bandwidth (4.08–6.38 GHz) at −10 dB [26]. However, the gain within the passband exhibited certain instabilities.
Based on the above considerations, a broadband antenna array based on MTS is proposed and examined in detail in this study. The proposed array facilitates additional increases in the effective radiating aperture by improving the periodic MTS antenna unit, thereby achieving a very high directional gain. In addition, this paper provides a detailed introduction to and analysis of the operating mechanism of MTS using characteristic mode analysis (CMA). Overall, a novel low-profile MTS array antenna is designed in this study with the ultimate goal of further improving the operating frequency bandwidth. The simulation results demonstrate that the MTS antenna attained a working bandwidth of 61%, ranging from 4.74 GHz to 8.4 GHz, at −10 dB.
In Section II, the proposed MTS antenna unit is introduced. Section III presents an analysis of the CMA results. In Section IV, details pertaining to the parametric study conducted on the MTS are described. In Section V, the MTS antenna array is tested, and its measurement results are compared with the simulation results. Finally, Section VI presents the conclusions of the study.
II. The Unit Design
The proposed MTS antenna array was designed for the C-band. Fig. 1 depicts the geometric structure of the array unit, which consists of three metal plates arranged from top to bottom as follows: the MTS, ground plane, and microstrip line. The dielectric substrate utilized is Rogers RO4003C material, with thicknesses of h1 = 3.5 mm and h2 = 0.5 mm. The MTS comprises subwavelength square patches of widths p and (p – g)/2, with the edge width being g. The length of the square ground plane is l. A coupling slot of length ls and width ws is incorporated at the center of the ground plane, along with two diagonal cross slots of length a2 and width a1 on either side. The bottom layer comprises a virtual short circuit with an opening of 2 × o1 and a radius of ra to achieve sufficient impedance matching. The size of the MTS antenna unit is shown in Fig. 1. Notably, the S-parameters and directivity of the structure in Fig. 1 were analyzed using a High Frequency Structure Simulator (HFSS). The results presented in Fig. 2 show an impedance bandwidth of 60% (4.74–8.34 GHz) at |S11| = −10 dB and a directivity range of 10 dBi.
To further examine the working principle and characteristic modes (CMs) of the MTS radiating patch, we resorted to characteristic mode theory (CMT). The next section provides a detailed introduction to the proposed MTS’s CMs.
III. Operational Mechanism and Characteristic Mode Analysis
1. Theory of Characteristic Mode
The theory of characteristic mode (TCM) was first introduced in 1965 and later redefined in 1971 [27], serving as a technique for calculating the CMs of elements made of perfect electric conductors (PECs) [28, 29]. This fundamental concept was subsequently broadened to encompass dielectric and magnetic materials [30, 31]. TCM has found widespread application in antenna engineering. Significant advancements brought about by TCM include the use of Poggio-Miller-Chang-Harrington-Wu-Tsai (PMCHWT) surface integral equations to model the CMs of dielectric interfaces. Furthermore, mixed potential integral equations have been employed to analyze metallic patches buried in multilayer dielectric structures. The E-PMCHW method, introduced in previous studies, is used for deriving the CMs of composite metal–dielectric configurations, such as microstrip patch antennas. In this method, electric field integral equations are applied to investigate metal surfaces, while PMCHWT formulas are used to analyze metal–dielectric interfaces [32, 33].
In TCM analysis, the current J flowing on an object is considered the sum of the normalized characteristic currents Jn, expressed as J = Σ αnJn. Here, αn refers to the weighting coefficient of the n-th mode current Jn, which is a combination of the individual mode currents to form a complete and systematic orthogonal set of mode currents. When the value of |λn| equals 0, the corresponding mode resonates, achieving maximum radiation efficiency. Notably, a more convenient measure of resonance frequency and mode radiation is the mode significance (MS), expressed as (MS) = |1/(1 + jλn)|, which ranges from 0 to 1. When MS = 1, resonance occurs, and the radiation efficiency reaches its peak.
2. CMA of Metasurface without Coupling Slots
To further investigate the proposed antenna and its working mechanism, a detailed study of changes in the MTS’s modal behavior was conducted. We employed the commercial simulation software CST and a method of moments-based CMA to simulate and analyze the designed antenna element. In this calculation, we accounted for the infinite extension of the ground plane and dielectric layer in the x and y directions. A thorough simulation analysis was conducted, and the corresponding results were obtained.
The structure in Fig. 3(a) depicts the MTS surrounded by open boundaries in every direction. In the case of the MTS structure that does not have coupling slots, a PEC boundary is added along the −z axis to form a closed region within the unit, with the mesh solely applied to the MTS patches to resolve the current on the patches. In Fig. 3(b), a grounded plane with a coupling slot is added to the structure. The MTS equipped with this plane was reanalyzed. Assuming that the dielectric material was lossless, the geometric dimensions remained unchanged. CMA was conducted for both structures (with and without the coupling slot), with J1 considered the modal current of the lowest order or the dominant mode in the CM.
Fig. 4(a) shows the top layer of the dielectric substrate used in the MTS unit structure, while Fig. 4(b) presents the modal significance of the first four CMs of the MTS unit. Figs. 4 and 5 show that J1 and J2, which exhibit a common resonant frequency of 6.6 GHz, form two orthogonal modes with the same modal significance. In contrast, J3 and J4 share a common resonant frequency and exhibit consistent modal characteristics despite changes in frequency.
Fig. 5 illustrates the obtained modal currents, with the back arrows indicating the direction of the current’s path. Notably, unless specified otherwise, all radiation patterns and currents presented in this paper were obtained at 6 GHz. The results show that the current J1 is in phase across all MTS patches, flowing in the direction of the y-axis. J2 is similar to J1, except that its direction is rotated by 90° on the MTS plane. Thus, as shown in Fig. 5(a) and 5(b), J1 and J2 form two orthogonal modes owing to the symmetry of the MTS. Since these currents remain in phase, they produce a wide beam radiation pattern. Meanwhile, as shown in Fig. 5(c) and 5(d), currents J3 and J4 are symmetric to the coordinate axes, but the direction of the currents is toward each other for J3 and opposite to each other for J4. These opposing currents result in a radiation null along the axis view. Based on these findings, J1 and J2 were considered the ideal operating modes for achieving broadside radiation, since exciting either of these modes would generate excellent linear polarization, resulting in outstanding radiation characteristics.
3. CMA of the Metasurface with Coupling Slots
The coupling structure was designed based on two important factors—the type and position of the source current. To reduce the profile, magnetic currents (typically represented by narrow slots) were utilized. The magnetoelectric current aligned with the H-field direction of the desired modes—J1 or J2. The coupling slot was positioned at the maximum H-field location to achieve optimal effective coupling. In this study, we used J1 for further analysis. Notably, choosing J2 would yield the same results, but the antenna would have to be rotated 90° along the z-axis, as demonstrated in Fig. 5(a) and 5(b).
Fig. 5(a) shows that the maximum H-field for J1 and J2 is observed on the middle four patches, while the H-field of J3 and J4 is maximum on the surrounding patches. Additionally, to better excite the desired CM, we designed a coupling slot at the center of the ground plane, as shown in Fig. 7(a). However, given the importance of the coupling slot in this experiment, we had to reanalyze its impact on the CMs, since it is characterized by a sufficiently large electrical size that could introduce potential new modes.
First, the mode significance of the coupling slot was analyzed. The curves for the half-wavelength and full-wavelength modes were acquired, as shown in Fig. 7. In Fig. 7(b), Modes 1 and 2 correspond to the half-wavelength and full-wavelength modes of the coupling slot, respectively. Fig. 7(c) shows that the modal current is primarily concentrated in the central rectangular region for the half-wavelength mode, with the current direction above the region remaining consistent, thereby exhibiting wide-edge radiation characteristics. Meanwhile, the modal current in the full-wavelength mode is symmetric but in opposite directions. This led to a split in the radiation pattern, resulting in the appearance of sidelobes, as shown in Fig. 7(d). These results indicate that when designing the length of the coupling slot, the half-wavelength mode, which exhibits better radiation characteristics, should be prioritized. Furthermore, as shown in Fig. 7(b), the MS peak frequency of the half-wavelength mode is 4.6 GHz—slightly below the target frequency range. Below 7 GHz, the curve of the half-wavelength mode is higher than that of the full-wavelength mode, whereas in the 7–8 GHz frequency range, the higher-order modes are excited. Overall, despite the generation of some sidelobes, good radiation characteristics were maintained in the main radiation direction. Therefore, it was established that the design of the coupling slots met the performance requirements of the antenna.
The geometric structure of the MTS unit with the coupling slot is shown in Fig. 3(b), where open boundaries are applied in all directions. The first four CMs of the MTS with coupling slots—termed J1m, J2m, J3m, and J4m—are presented in Fig. 8(b). It is observed that the resonant frequencies of modes J1m, J2m, J3m, and J4m are at 6.9 GHz, 6.9 GHz, 7.4 GHz, and 6.3 GHz, respectively. Fig. 8(c)–8(f) display the current direction and radiation patterns of these four modes. The results are consistent with the findings obtained using the MTS without coupling slots, which implies that although the addition of the coupling slots altered the MS of each mode, it had little effect on their current distributions and radiation patterns. Furthermore, by adding two cross-shaped coupling slots on either side of the coupling slot, higher-order modes were excited, resulting in the appearance of some sidelobes in the radiation pattern. However, the directional radiation characteristics remained satisfactory. Based on these findings, we deduced that the proposed MTS unit exhibits favorable radiation characteristics in the 5–8.5 GHz range, which makes it suitable for use in array formation.
IV. Optimization of Parameters
Parameter studies are crucial for conventional MTS antenna design. The parameter studies typically depend on the feeding source and feeding structures. Moreover, parametric studies based on CMA significantly aid in optimizing antenna performance. In this study, four parameters— p, g, ls, and ws— were sequentially studied based on the MTS structure and the size of the coupling slots.
The results of the parametric study conducted through time-domain analysis are shown in Fig. 9, clarifying that the working bandwidth increases with a decrease in g and an increase in p and ls. However, the operating frequency did not change with variations in ws. Moreover, parameters g and ls perform the crucial function of adjusting impedance matching. It is also observed that the directivity of the MTS unit is mainly influenced by the structure of its radiating patch, and the width of the coupling slot does not significantly affect the antenna’s directivity.
In conclusion, it is established that high-frequency directivity is influenced by the dimensions of the MTS, while matching at low frequencies is mainly affected by the length and width of the coupling slots.
V. Antenna Array and Analysis
To verify the wideband radiation characteristics of the proposed antenna array, we first analyzed the radiation patterns of the MTS antenna unit. Fig. 10(a)–10(c) present the radiation patterns of the MTS unit at 5.5 GHz, 6.5 GHz, and 7.5 GHz, respectively. The results indicate that the antenna unit exhibits outstanding radiation characteristics across the entire operating frequency band. Additionally, Fig. 10(d) depicts the structural diagram of the dual-port antenna unit. During the simulation, both ports were simultaneously fed to obtain the S-parameters of the structure, as shown in Fig. 10(e). The S-parameter curves confirm that the proposed antenna has a wide impedance bandwidth. Moreover, the |S21| parameter remains below −35 dB, indicating very weak coupling between the two units. In conclusion, the MTS unit exhibited excellent radiation characteristics in the 5–8.5 GHz frequency range, implying that it is highly suitable for antenna array design.
We also designed a 1 × 2 antenna array based on the already designed MTS antenna unit and examined its performance. As shown in Fig. 11(a), the optimized distance between the elements was set to q = 55 mm, while the width was 110 mm (2.2λ0 at 6 GHz). Fig. 11(b)–11(c) present the back and front views of the antenna prototype. The antenna was tested in a microwave anechoic chamber to ensure minimal error, thereby verifying the effectiveness of the design concept. Fig. 12(b)–12(d) present a comparison of the simulated and measured S-parameters, voltage standing wave ratio (VSWR), and gain. It is observed that when |S11| = −10 dB, the measured impedance bandwidth is 61% (4.74–8.4 GHz), while the gain ranges from 9 to 12.68 dBi within the frequency band. The simulation and measurement results show good agreement at multiple frequencies. Furthermore, Fig. 13 depicts the radiation patterns at 5.5 GHz, 6.5 GHz, and 7.5 GHz, where the simulation and measurement results exhibit good consistency. Moreover, across the entire operating bandwidth, the axial cross-polarization of the radiation pattern remained below −20 dB.
Table 1 presents a comparative analysis of the advantages offered by the designed MTS antenna and similar antennas previously reported in reputed journals, especially with regard to the impedance bandwidth and gain [4, 34–39]. It is observed that the proposed antenna achieves a wider impedance bandwidth and higher gain. Furthermore, the radiating portion of the MTS antenna comprises multiple simple square patches, which offers advantages such as a simple structure and low manufacturing costs. The proposed MTS antenna also covers a large portion of the C-band, which significantly enhances its applicability in the C-band, while also possessing a high impedance bandwidth. Overall, these features serve to improve the practical value of the antenna.
Conclusion
In this study, a low-profile broadband MTS array antenna is designed based on CMA. An in-depth analysis was conducted to examine the antenna’s design process, and its practical performance was validated through fabrication. CMA was conducted, and coupled slots were employed as the primary radiating elements. The antenna’s broadband radiation characteristics were achieved by modifying the dimensions of the MTS periodic patches.
This study proves that CMA is an effective method for designing structures and analyzing characteristics, since it not only revealed the operating mechanism of the proposed antenna but also helped to reasonably arrange the position of the coupled slots and optimize the analysis parameters, enabling us to achieve our design goals more precisely.
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