Microstrip antenna loaded with focusing metasurface for high-gain dual-polarization and bidirectional radiation

The advancing wireless communication system necessitates the development features including miniaturization of size, heightened data transmission rates, expanded communication ranges, and preservation of polarization purity.¹ Notably, a single antenna that offers dual-polarization and multifunctionality, capable of producing both circular polarization (CP) and linear polarization (LP) radiation, is highly sought-after for applications such as WLAN, WiMAX, vehicle-to-vehicle, and vehicle-to-mobile tower communications.^2–11 For instance, Lin et al. introduced a low-profile dual-polarization antenna utilizing an artificial magnetic conductor (AMC),⁵ demonstrating a peak gain exceeding 2 dBi for LP omnidirectional radiation and 7 dBic for CP unidirectional radiation, respectively. Yun et al. designed a miniaturized wideband dual-polarization microstrip antenna that employs two coupled lines and transition lines, interconnected through probes.⁷ Wang and Wong proposed a wideband patch antenna capable of polarization reconfiguration between CP and LP waves, achieved through the utilization of a switchable cross-feeding probe and positive-intrinsic-negative (PIN)-diodes.¹⁰ Despite their ability to achieve diverse polarization operating states, the low gain and radiation efficiency of these works restrict their widespread adoption in applications such as mobile base stations, satellite communications, radar systems, and other similar fields.

Metasurfaces (MSs), which are two-dimensional (2D) planar metamaterials (MMs) comprising spatially arranged subwavelength resonator structures, have provided unprecedented capabilities in manipulating electromagnetic (EM) waves and demonstrated unique properties that are not found in nature.^12–15 Over the past decade, MSs have undergone rapid development, and their applications have become increasingly widespread,^12–25 including wavefront manipulation,^13–17 hologram imaging,^18–20 polarization conversion,^21–23 and absorption.^24–28 As the application scope broadens, multifunctional MSs have garnered considerable attention and established novel frameworks for designing cutting-edge antennas, primarily attributed to their exceptional performance.^29–43 For instance, Fan et al. proposed a MS antenna that utilizes split-ring resonators (SRRs), achieving highly efficient CP radiation with a peak gain of 17.9 dB and a reduced radar cross section (RCS).³² Zhou et al. proposed a reflective CP focusing MS antenna, which enhanced the average gain by over 6 dBi across a broadband range.³⁵ Additionally, Jash et al. introduced a dual-band MS-based microstrip antenna that demonstrated a peak gain of 4 dBi in the lower band for LP radiation and 7.1 dBic in the higher band for CP radiation.³⁷ Furthermore, Yang et al. presented a broadband high-efficiency transmit-reflect-array antenna (TRA) capable of simultaneously achieving left-handed circularly polarized (LCP) radiation in the transmission mode and right-handed circularly polarized (RCP) wave in the reflection mode.⁴² Despite the exceptional performance of currently reported MS-based antennas in achieving diverse EM radiation functions, including high gain, broadband capabilities, dual-polarization, and full-space coverage, the development of multifunctional microstrip antennas remains critically important and highly sought-after for the advancement and application of wireless communication systems in the future.

In this study, we introduce an innovative design of a dual-polarization and bidirectional focusing MS integrated with a coaxial-fed laminated microstrip antenna. This design facilitates simultaneous and independent high-gain CP radiation in the transmission mode within the lower frequency band, as well as LP radiation in the reflection mode within the higher frequency band. The remainder of this article is structured as follows: First, we propose and numerically demonstrate a dual-dual MS capable of achieving simultaneous transmitted CP focusing at a lower frequency band and reflected LP focusing effect at a higher frequency band. Second, design and fabricate a coaxial-fed laminated microstrip antenna with dual-polarization capabilities, which is integrated into the aforementioned MS to form a dual-polarization and bidirectional MS antenna. Third, we conduct an in-depth investigation and analysis of the radiation performance of the coaxial-fed antenna, both with and without the MS, by examining return loss (S₁₁), surface current distributions, axial ratio (AR), gain, and radiation patterns. The simulation results reveal that the proposed MS antenna achieves CP radiation in transmission at 8.2 GHz with a peak gain of 13.1 dBic and LP radiation in reflection at 16.2 GHz with a peak gain of 14.6 dBi. The final experimental results of the designed dual-polarization and bidirectional MS antenna demonstrate reasonable agreement with the numerical simulations and exhibit superior performance compared to the initial design of the microstrip antenna.

In this section, we mainly study the MS design that incorporates dual-polarization and bidirectional focusing capabilities, enabling simultaneous transmission of CP focusing effect and reflection of LP focusing effect at two distinct frequency bands. Figures 1(a)–1(c) illustrate the perspective, front, and back views, respectively, of the unit-cell structure of the designed MS. This MS has been designed to incorporate a bi-layer metal resonator structure, which is positioned on both the front and back layers of a dielectric substrate. The middle dielectric layer is made of FR-4, which has a relative permittivity of 4.4 and a loss tangent of 0.025. The outer bi-layered split-ring resonators (SRRs) constitute the exterior component of the metallic structure, employed to adjust the amplitude and phase of the transmitted orthogonal CP waves within the lower frequency band, analogous to prior designs.^38,44 Meanwhile, the arc-shaped and disk patch resonators serve as the internal component of the metallic structure, tasked with modulating the amplitude and phase of the reflected cross-polarized LP waves at the higher frequency band.

To ascertain the feasibility and efficacy of the designed MS, a comprehensive 3D full-wave electromagnetic (EM) simulation was executed utilizing the CST Microwave Studio (MWS), grounded in the finite integration technique (FIT). During the simulation, periodic boundary conditions were applied along both the x- and y-axis directions, while Floquet port excitation was utilized to ascertain the scattering parameters (S-parameters) of the proposed unit-cell. The final optimized geometric parameters of the unit-cell are outlined as follows: t = 2.5 mm, p = 10 mm, r₁ = 4.8 mm, w₁ = 0.5 mm, r₂ = 2.5 mm, w₂ = 1 mm, and r₃ = 3.5 mm. The simulated amplitude and phase of the transmitted orthogonally CP wave, as well as the reflected cross-polarized LP wave, for the designed MS structure, is illustrated in Figs. 1(d)–1(f). As shown in Fig. 1(a), along the z-axis direction, the rotation angle of the outer bi-layered SRRs relative to the x-axis direction is defined as θ₁. The opening angle and the orientation angle of the opening of the inner arc-shaped structure, positioned in the front layer of the unit-cell, relative to the x-axis direction are denoted as θ₂ and θ₃, respectively. Utilizing the Pancharatnam Berry (PB) principle,^44–46 the rotation of the SRR structure within the unit cell of the designed MS facilitates the achievement of an arbitrary phase shift φ = 2θ₁ for the transmitted orthogonally CP wave. By adjusting the rotation angle (θ₁) of the SRR structure within the proposed MS unit-cell, the phase shift within the lower frequency band can be readily tuned to span the entire 0 to 2π range. The phase shift of the reflected cross-polarized LP wave is governed by the θ₂ and θ₃ of the arc-shaped structure located on the front layer of the unit-cell. In accordance with the transmission phase principle,^11,12 adjusting θ₂ and θ₃ facilitates straightforward modulation of the phase of the reflected cross LP wave at a higher frequency, enabling coverage of the entire 0 to 2π range.

Upon optimizing and finalizing the geometric parameters of the unit-cell, the amplitude and resonance frequencies of both the transmitted orthogonally CP wave and the reflected cross-polarized LP wave are established. By adjusting the θ₁, θ₂, and θ₃, the corresponding transmission and reflection phases can be controlled effectively. Figures 1(d) and 1(f) depict the amplitude and phase of the transmitted orthogonally CP wave for the MS structure, with θ₁ varying from 0° to 150° in increments of 30°. It is evident that by tuning θ₁, the transmission coefficient of the orthogonal CP wave can be sustained above 0.6 in the vicinity of 8.2 GHz, while achieving phase coverage from 0 to 2π through consistent phase variations. Figures 1(e) and 1(g) exhibit the amplitude and phase of the reflected cross LP wave for the MS structure, with various values of θ₂ and θ₃. By adjusting these angles, the reflection coefficient of the cross-LP wave can be maintained above 0.5 around 16.2 GHz, while achieving complete phase coverage from 0 to 2π in the reflection mode. It is important to note that the amplitudes of both the transmitted orthogonally CP wave and the reflected cross-polarized LP wave should ideally be maximized to near unity. However, the observed amplitudes are somewhat limited due to the dielectric losses associated with the FR-4 substrate. Noted that the amplitude of both reflection and transmission for this designed MS in the two operating frequencies can be kept about 0.6 in a wide-angle range (<60°). It can be expected that the designed MS has a certain wide angle stable of the oblique incidence.

To illustrate the dual-polarization and bidirectional focusing performance of the designed MS, as shown in Figs. 3(a), 3(b), 3(d), and 3(e), we present the electric field distributions of the transmitted orthogonal CP wave and reflected cross LP waves along the z-axis direction in xoz and xoy planes at 8.2 and 16.2 GHz, respectively. Figures 3(a) and 3(d) demonstrate that the simulated focal lengths of the transmitted orthogonal CP wave and the reflected cross-polarized LP wave for the designed MS are approximately 41.6 and 40.5 mm, respectively, at frequencies of 8.2 and 16.2 GHz. These values align closely with the preset theoretical focal length of 40 mm. It can be further observed that a sustained range of energy accumulation persists along the focal length for both the transmitted CP wave and the reflected LP wave across various frequency bands. This phenomenon presents an opportunity to augment the gain by incorporating the initial microstrip antenna. As Figs. 3(b) and 3(e) unequivocally illustrate, the normally incident CP and LP waves, upon traversing the designed MS, undergo a transformation of their orthogonal components. Consequently, focused spot points are achieved after transmission at 8.2 GHz and reflection at 16.2 GHz, respectively. This substantiates the dual-polarization and bidirectional focusing capability of the proposed MS for each operational mode. Figures 3(c) and 3(f) display the normalized intensity of the transmitted RCP wave at 8.2 GHz and the reflected LP_y wave at 16.2 GHz, respectively. These findings reveal full width at half maximum (FWHM) values of approximately 20.1 and 9.6 mm, respectively, further showcasing an exceptional subwavelength and dual-polarization focusing effect. To assess the practical application, a custom-designed microstrip antenna was positioned at the preset focal length of the designed MS, aiming to enhance its radiation performance. Additional simulations and experimental investigations will be conducted in the future to further explore this phenomenon.

Once the focusing efficiency of the previously designed MS has been verified, the foundation can be utilized to proceed with the development of a high-performance, bidirectional, dual-polarization focusing MS antenna system. Figures 4(a)–4(d) illustrate the design and geometry of the custom-crafted microstrip feed antenna, which comprises three layers of a 0.035 mm thick metal copper pattern and two layers of a FR-4 dielectric substrate, each with distinct thicknesses. As depicted in Fig. 4(d), this tailored microstrip antenna employs a coaxial back-feed design, akin to those utilized in prior works.^11,38,46

As illustrated in Fig. 4(a), the copper pattern on the uppermost layer consists of a compact rectangular patch structure, measuring l_p1 × w_p1, which is directly coupled to the coaxial probe to produce LP radiation within the lower frequency band.¹¹ Concurrently, as depicted in Fig. 4(b), the copper pattern on the intermediate layer showcases a relatively larger truncated corner square patch, sized at l_p2 × l_p2, which although not directly attached to the coaxial probe, is coupled with the top rectangular patch to facilitate the radiation of CP waves within the higher frequency band.^11,34,47,48 As shown in Fig. 4(c), the continuous copper film at the bottom serves as the ground plane for the middle layer, enhancing the CP radiation of the designed microstrip feed antenna.

After simulation optimization, the geometric parameters of the designed feed antenna are given as t₁ = 0.4 mm, t₂ = 1.6 mm, l = 20 mm, l_p1 = 6.5 mm, w_p1 = 3.8 mm, l_p2 = 7.2 mm, a = 2.1 mm, D_v = 0.5 mm, D_p = 2 mm, and D_c = 1 mm. As shown in Fig. 4(e), the designed microstrip feed antenna is positioned at the focal point of the proposed focusing MS, forming a high-gain bidirectional and dual-polarization radiation antenna system. To ascertain their efficiency and radiation performance, the custom-crafted microstrip feed antenna and the focusing MS were produced using traditional PCB technology. Figures 4(f) and 4(g) present the front and rear views of the fabricated focusing MS, respectively, while Fig. 4(h) showcases the prototype and practical arrangement of the ultimate MS antenna system. The radiation capabilities of the custom-designed microstrip feed antenna, both standalone and integrated with the MS, were assessed using a vector network analyzer (Agilent E8362B) in conjunction with a standard antenna testing setup.

Figure 5 presents the simulated and measured return loss (S₁₁) of the custom-designed microstrip feed antenna without and with the proposed MS. As illustrated in Figs. 5(a) and 5(b), the impedance bandwidth range of the custom-crafted microstrip feed antenna, simulated at −10 dB, extends approximately from 8.01 to 8.44 GHz and from 15 to 17 GHz, closely correlating with the experimental results. This suggests that the feed antenna is capable of operating efficiently within these two frequency bands. Moreover, the minima of the return loss (S₁₁) are observed at approximately −10.65 dB at 8.22 GHz and −17.09 dB at 15.95 GHz, respectively. As depicted in Figs. 5(c) and 5(d), similar simulated and measured results for the return loss (S₁₁) are obtained for the feed antenna integrated with the MS. Specifically, the S₁₁ values are below −10 dB at approximately 8.2 and 16.2 GHz, respectively. It is noteworthy that, in comparison to the S₁₁ of the standalone feed antenna, the S₁₁ of the MS antenna system undergoes minor variations, primarily attributed to the parasitic coupling and interference effects between the feed antenna and the MS.

To demonstrate the radiation characteristics of the custom-designed microstrip feed antenna, as depicted in Fig. 6, we present the surface currents on both the middle and top layers of the antenna structure at frequencies of 8.2 and 16.2 GHz, respectively. As Fig. 6(a) clearly illustrates, the induced surface vector current on the middle layer of the feed antenna undergoes a rotational pattern with advancing phase, with clockwise rotations observable at the lower frequency of 8.2 GHz, indicative of CP wave radiation. Conversely, Fig. 6(b) shows that at the higher frequency of 16.2 GHz, the direction of the induced surface current vector on the rectangular patch structure of the top layer remains aligned along the ±y-axis direction as the phase changes, revealing a significant LP radiation. This demonstrates that the custom-designed microstrip feed antenna is capable of effectively radiating both LP and CP waves at the aforementioned two distinct frequencies, respectively.

To provide further insight into the radiation characteristics of the antenna, as depicted in Fig. 7, we present the axial ratio (AR) and gain of the custom-designed microstrip feed antenna, both with and without the incorporated focusing MS. As Figs 7(a) and 7(b) clearly demonstrate, the AR values for both the MS antenna system and the custom-designed microstrip feed antenna remain below 3 dB in the vicinity of the lower frequency of 8.2 GHz, whereas at the higher frequency of 16.2 GHz, these values significantly exceed 3 dB, indicating effective radiation of CP and LP waves at these respective frequencies. It confirms that the addition of the focusing MS does not significantly affect the polarization characteristic of the custom-designed microstrip feed antenna. As illustrated in Figs. 7(c) and 7(d), the gain of the custom-designed microstrip feed antenna is approximately 5.08 dBic at 8.2 GHz and 4.92 dBi at 16.2 GHz. At these frequencies, the gain of the designed MS system reaches up to 13.1 dBic and 14.6 dBi. This suggests that the incorporation of the focusing MS has resulted in a gain enhancement of approximately 8 dBic for CP radiation at 8.2 GHz, and an increase of approximately 10 dBi for LP radiation at 16.2 GHz. The focusing MS has effectively contributed to a significant gain amplification of the proposed antenna system at both frequencies.

To offer a more vivid and comprehensive visualization of the high-gain, bidirectional, and dual-polarization radiation performance of the designed MS antenna system, Fig. 8 presents the 3D radiation patterns of the custom-designed microstrip feed antenna, both in the absence and in the presence of the designed MS, at the aforementioned frequencies of 8.2 and 16.2 GHz. As depicted in Figs. 8(a) and 8(b), the radiation direction of the custom-designed microstrip feed antenna consistently aligns with the positive z-axis direction at both the aforementioned frequencies of 8.2 and 16.2 GHz, for both CP and LP waves, albeit with a relatively lower gain value. As illustrated in Figs. 8(c) and 8(d), the radiation direction of the designed MS antenna system aligns with the positive z-axis direction at the lower frequency of 8.2 GHz, and with the negative z-axis direction at the higher frequency of 16.2 GHz. This indicates that the designed MS antenna system exhibits a comparatively high gain in the propagation direction for both transmission and reflection modes at these two frequencies. Noted that the incident angle plays a crucial role in the performance of a MS antenna design, particularly given the specified dimensions of the MS (130 mm) and focal length (40 mm). It can be expected that by carefully considering and optimizing the MS's response to a range of incident angles, we can achieve more robust and versatile antenna systems capable of operating effectively in diverse environments and applications.

To provide additional insights into the radiation performance of the antenna, Fig. 9 displays the 2D radiation patterns of the custom-designed microstrip feed antenna, both with and without the designed focusing MS. Although there are minor differences between the simulated and measured results, the overall trends of the radiation patterns remain consistent. These slight discrepancies may be attributed to various factors, including the test environment, fabrication techniques, interference from test equipment, and other potential variables. The radiation patterns of the E-plane and H-plane of the custom-designed microstrip feed antenna, both with and without the designed focusing MS, are distinctly illustrated in Fig. 9 at frequencies of 8.2 and 16.2 GHz, respectively. As evident from Fig. 9, the custom-designed microstrip feed antenna, equipped with the designed focusing MS, exhibits significant gain amplification, dual-polarization, and bidirectional radiation performance at these two frequencies. The experimental results further validate that the proposed focusing MS is capable of achieving microwave focusing in transmission mode for circularly polarized CP waves at the lower frequency and in reflection mode for LP waves at the higher frequency. This presents a promising methodology for dual-polarization and bidirectional radiation in communication systems. For a more comprehensive understanding of the advantages proffered by the proposed MS antenna, a comparison is drawn with other extant antennas loaded with focusing MS in Table I. In contrast to the previous designs,^34,35,38,49 the proposed MS antenna is capable of attaining two polarization states without the incorporation of nonlinear elements and exhibits a high gain at both frequencies. There is no doubt that our proposed focusing MS antenna design remarkably outperforms its counterparts in terms of dual-polarization radiation and the realization of high gain, thereby delivering overall preeminent performance.

In conclusion, we have introduced and demonstrated a bidirectional and dual-polarization focusing MS that significantly enhances the radiation performance of the custom-designed microstrip feed antenna. The designed MS antenna system is capable of achieving the transmitted CP radiation with a peak gain of 13.1 dBic at 8.2 GHz, and reflection LP radiation with a peak gain of 14.6 dBi at 16.2 GHz. When compared to the custom-designed microstrip feed antenna alone, the incorporation of the focusing MS results in an approximately 8 dBic increase in gain for the transmitted CP radiation at 8.2 GHz and an approximately 10 dBi increase in gain for the reflected LP radiation at 16.2 GHz. The proposed MS antenna system demonstrates a high-gain bidirectional and dual-polarization radiation characteristic, providing an effective solution for bidirectional and dual-polarization radiation and high-speed information transmission in communication systems.

We acknowledge the financial support from the Nation Natural Science Foundation of China (Grant Nos. 52304410 and 51972242), the Science Fund for Creative Research Groups of the National Natural Science Foundation of Hubei Province (Grant No. 2020CFA038), the Key Research and Development Project of Hubei Province (Grant No. 2020BAA028), and Major Project of Hubei Province (Grant No. 2023BAA003).

The authors have no conflicts to disclose.

Yibo Sun and Lingling Yang contributed equally to this paper.

Yibo Sun: Data curation (equal); Investigation (equal); Methodology (equal); Validation (equal); Visualization (equal); Writing – original draft (equal). Lingling Yang: Conceptualization (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Validation (equal); Visualization (equal). Jinxiu Wang: Conceptualization (equal); Investigation (equal); Methodology (equal). Yongzhi Cheng: Conceptualization (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Project administration (equal); Supervision (equal); Visualization (equal); Writing – review & editing (equal). Hui Luo: Investigation (supporting); Methodology (supporting); Validation (supporting); Visualization (supporting). Fu Chen: Investigation (supporting); Methodology (supporting); Validation (supporting). Xiangcheng Li: Conceptualization (equal); Funding acquisition (equal); Project administration (equal); Software (equal); Supervision (equal); Visualization (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.