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Abstract
In this paper, we introduce a novel technique that utilizes randomly rotated elements (RREs) for the cross-polarization and axial ratio (AR) control of a circularly polarized programmable metasurface (CPPMS). We evaluate the CPPMS performance by comparing RREs layout with uniform elements (UEs) layout, and analyze far-field radiation parameters for 50 groups of CPPMS with different RREs layouts. Simulation results demonstrate consistent and improved performance across various RREs layouts, showcasing reduced cross-polarization and enhanced AR beamwidth. To validate these findings, we design a 1-bit CPPMS in Ku-band comprising 20 × 20 elements with the optimal RREs layout, and conduct measurements in an anechoic chamber. The CPPMS prototype achieves high gain (22.34 dBi), low cross-polarization (-20.5 dB), and a narrow 3 dB AR beamwidth (8.93°). Notably, it offers wide-angle beam scanning capabilities of up to ±60°. The gain bandwidth at -3 dB ranges from 14.54 to 16.65 GHz, with a relative bandwidth of 7.3%, while the 3 dB AR bandwidth extends from 14.24 to 16.07 GHz. Consequently, the proposed 1-bit CPPMS exhibits high-performance two-dimensional AR beam scanning, presenting promising applications in satellite communications.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The programmable metasurface (PMS) is one of the most advanced structures in the metasurface family, which has good real-time beam-forming and beam-scanning capabilities [1]. The PMS is composed of programmable element, each of which has an adjustable spatial phase shift function through the PIN diode [2], varactor diodes [3], graphene [4], phase changing materials [5], liquid crystals [6], or vanadium-dioxide [7]. Therefore, there is no expensive phase shifter and complex feed network in PMS, resulting in lower cost and lighter equipment. By using a field-programmable gate array (FPGA) to regulate PMS, the multi-purpose electromagnetic (EM) beam mode is related to the bit stream in FPGA. PMS shows good real-time control ability for EM wave formation [8–12], computational imaging [13–16], novel communication systems [17–20]. Therefore, PMS can be used as a multi-functional platform and has great prospects to replace the traditional phased array. However, most current PMS control linearly polarized waves [21,22], and there are fewer ways to dynamically control circularly polarized (CP) waves due to the complexity of controlling the rotation of the geometric phase element [23–27].
The method of using physical or geometric rotation of element to achieve CP phase regulation will result in the use of many PIN diodes, further increasing design complexity, manufacturing costs, and element insertion losses. On the other hand, in addition to rotation technology, some works use magnetoelectric dipole technology to achieve symmetry in the field distribution by manipulating the basic mode of the patch structure, to achieve a 1-bit circularly polarized programmable metasurface (CPPMS) element [28,29]. However, this method also requires many PIN diodes to achieve wideband phase response, gain performance, and high axial ratio (AR) performance. Therefore, we hope to develop a new design to implement 1-bit CPPMS element that can reduce the number of PIN diodes while possessing high AR performance and gain performance.
In this paper, a 1-bit reflective CPPMS is proposed by only using a single PIN diode. The element consists of an asymmetric U-slot patch, a ground plane, and a 1-bit reflection-type phase shifter (RTPS), which establishes a receiving - phase shifter - transmitting topology. The inspiration for the randomly rotated elements (RREs) layout is derived from the sequential rotation technology (SRT) applied to CP array antennas [30]. In the SRT circularly polarized array antenna configuration, each 2 × 2 subarray sequentially rotates elements at 0°, 90°, 180°, and 270° to enhance the AR, bandwidth, and other performance. It is worth noting that the radiating elements employed in the SRT array are primarily linearly polarized elements. The primary objective of our work is to control cross-polarization effects. Therefore, applying random rotations to each element in the array is advantageous for disrupting the spatial distribution of electric fields that cause cross-polarization, thereby achieving high AR performance. Additionally, in our work, we apply equiprobable random rotations to the elements at angles of 0°, 45°, 90°, and 135°, respectively. The reason for setting the rotation angles to half of the waveguide of the array antenna is that, for reflective metasurface, the rotational phase is twice the berry phase [19]. The proposed CPPMS element only uses single PIN diode and has the capability to control CP waves achieving two-dimensionally high AR performance beam scanning, which will be beneficial for CP beam control applications in satellite communications.
2. CPPMS modeling with different layout modes
We compared and analyzed the far-field radiation performance of uniform elements (UEs) layout and RRE layout using the CPPMS numerical analysis model established based on modified plane-wave angular spectrum (MPWAS) approach. This analysis establishes the performance advantage of RRE layout. Lastly, considering both beam scanning capability and high AR performance as key factors, we determined the optimal configuration of RREs layout to achieve high AR performance in two-dimensional beam scanning. Refer to the supporting information for a detailed MPWAS approach of the numerical analysis model (Supplement 1).
2.1 Verification with two different layouts
Utilizing the proposed model for far-field calculations, an analysis is conducted on the far-field characteristics of two distinct CPPMS element layouts. Specifically, a CPPMS operating in the Ku-band has been designed, and its element design will be expounded upon in Section 3. The CPPMS comprises a grid of 20 × 20 elements, with an element spacing of 0.4 λ0 at a frequency of 15 GHz. The conventional CPPMS with UEs layout is shown in Fig. 1(a), while Fig. 1(b) shows the array with RREs layout. The Right-handed Circularly Polarized (RCP) conical horn with a standard gain is employed as the feed. The distance from the surface of the array is set to F = 147 mm, corresponding to a focal-to-diameter ratio (F/D, D is the side length of the CPPMS) of 0.85. This configuration strikes a favorable balance between spillover loss and the gradually diminishing amplitude of the reflective array surface, thereby achieving optimal gain. The detailed simulation values for optimizing the feed position are shown in Fig. S2 (Supplement 1). Next, we will analyze and compare the two array layouts. It is worth noting that the beam direction we compare is set to (θ, φ) = (30°, 45°) because its beam scanning ability and beam scanning quality are better compared in the diagonal plane.
To begin with, the phase distribution of the incident RCP feed onto the two different layouts is established, as shown in Fig. 2(a). For the RREs layout, the random rotation angle of each element is shown in Fig. 2(e). The CPPMS coding arrangement of the two different layouts can be calculated in Fig. 2(b) and Fig. 2(f), enabling the realization of high-gain beams in two distinct modes. Figure 2(c) and 2(d), and Fig. 2(g) and 2(h) illustrated the analytical calculation of the three-dimensional normalized radiation maps for RCP and Left-handed Circularly Polarized (LCP) waves in the uv-plane at a frequency of 15 GHz for the two CPPMS configurations, respectively. It is observed that both layouts generate identical co-polarization (RCP) beams. However, in terms of cross-polarization (LCP), the RREs layout significantly reduces the cross-polarization levels compared to the UEs layout within the direction of the main beam. Figure 2(i) shows the two-dimensional radiation of the v = u (diagonal plane) plane. The maximum gain of the main beam of the two layouts is 23.6 dBi, and 23.4 dBi, the level next to the peak is 3.5 dB and 4.8 dB, and the half power beamwidth (HPBW) is 10.4° and 7.3°, respectively. However, as shown in Fig. 2(j), the AR performance of the UEs layout is seriously deteriorated, while the array with RREs layout has wide AR beamwidth.
Fig. 2. The far-field characteristics of two distinct CPPMS distributions. (a) The phase distribution of the incident E-field. (b) The coding of UEs layout. (c) The co- and (d) cross-polarization E-field of UEs layout. (e) Random rotation angle of each element. (f) The coding of RREs layout. (g) The co- and (h) cross-polarization E-field of RREs layout. Comparison of (i) gain and (j) AR between two layouts on the D-plane.
2.2 Verification with different random layouts
To demonstrate the broad applicability of the RREs layout in enhancing the performance of the metasurface, a total of 50 CPPMS with the different RREs layouts were selected. Each layout adheres to the same F/D, and an equal number of rotation angles are employed. By employing the MPWAS approach for analysis, the outcomes were obtained. As shown in Fig. 3(a), among these 50 RREs layouts, we obtained a maximum gain of 23 ± 1 dBi in co-polarization. These findings highlight the consistency and effectiveness of utilizing the RREs layout in improving the performance of the metasurface.
Fig. 3. Comparison of different RREs layouts. (a) Gain and beam-width ratio. Beam scanning performance at (b) xoz plane, (c) yoz plane, and (d) diagonal plane (φ = 45°).
Furthermore, we introduce the ratio of the 3 dB gain beamwidth to the 3 dB AR beamwidth, denoted as AR beam quality, ρ, as a crucial metric for evaluating beam performance. In practical applications, it is desirable for this ratio to be below 1. As shown in Fig. 3(a), the ratio is consistently maintained at approximately 0.6 ± 0.1, indicating excellent CP performance of the beam. The universality of the RREs layout becomes evident, showcasing notable advantages in enhancing the performance of CP beams.
Lastly, to validate the capability of CPPMS with RREs layout to achieve two-dimensional high AR performance beam scanning, we select four relatively optimal RREs layouts (index 9, index 13, index 25, and index 41), enabling beam scanning by adjusting the CPPMS coding state. The gain and AR for different scanning beams of CPMMS with four RREs layouts are given in Fig. 3(b), Fig. 3(c), and Fig. 3(d), respectively. The results demonstrate that the proposed CPPMS with RREs layout possesses two-dimensional high AR performance within the scanning range. The four RRE layouts achieve commendable wide-angle beam scanning within the ±60° scanning range. Even when the beam points towards ±60°, the relative gain reduction at 0° is a mere 3.5 dB. Additionally, the AR consistently maintains favorable characteristics below 3 dB. Consequently, the utilization of RREs layout effectively enhances the performance of CP beams.
3. Design and realization of the 1-bit CPPMS
3.1 Design of 1-bit CPPMS
The configuration of the proposed CPPMS element that uses the guided wave approach to realize the phase response is depicted in Fig. 4. The element consists of two-layer substrates, an asymmetric U-slot microstrip patch, a ground plane, and a 1-bit RTPS loaded by a PIN diode as shown in Fig. 4(a). The RCP incident wave is first coupled to a guided wave by a U-slot microstrip patch printed on the GF265 (εr = 2.65, h1 = 0.7 mm) substrate, produced by Sytech Technology. Then, a 1-bit RTPS, which is supported by the S7136 H (εr = 3.5, h2 = 0.28 mm) substrate, controlled by a single PIN diode is used for phase shifting of the guided wave. When controlling the “on” or “off” of the PIN diode to cause reflection of the guided wave at different positions, the 1-bit phase change of the guided wave is achieved after optimizing the length of pl1. Finally, the guided wave is reflected and re-radiated, resulting in a receiving - phase shifter - transmitting topology. A quarter-wavelength radial stub is designed in the biasing line to choke the RF signal at the direct current (DC) biasing point. The PIN diode is selected as a high-frequency PIN diode MACOM MADP- 000907-14020, validated in our previous study [31].
Fig. 4. The geometry of the designed CPPMS element. (a) Total geometry, (b) top layer, and (c) bottom layer of the element. (d) Reflect-phase responses and (e) reflect-amplitude responses of the element under the normal incident angle, and the oblique incident wave of ±15°, ± 30°.The specific dimensions of the element after optimization are as follows (in mm): Lx = Ly = 8.8, P = 5, Lw = 2.1, Ll = 3.1, Ls = 2.1, Lk = 1.7, ws = 0.4, w2 = 0.52, pl1 = 4.1, dcl1 = 1.5, r = 2.2.
The proposed CPPMS element is modeled and simulated with Ansys HFSS under a periodic boundary condition. The PIN diode is mounted on the 1-bit RTPS, and the PIN diode is modeled as lumped elements: R = 7.8 Ω and L = 25 pH in series for the “on” state and C = 0.025 pF and L = 25 pH in series for “off” state. The working state of the PIN diode can be changed by controlling the bias voltage. Under the RCP normal incidence wave and ±15°, ± 30° oblique incident wave, the simulated element phase, and amplitude responses are plotted in Fig. 4(d) and Fig. 4(e). As can be seen, the element phase differences of the two states are 180°±20° from 14.5 GHz to 16.2 GHz under the normal incident angle. At the oblique incident angle of ±15° and ±30°, the reflective phase error is in the range of ±5°, which shows the stable phase distribution. It is noteworthy that predominant illumination of the edge elements occurs at larger incident angles, resulting in relatively lower received energy. Consequently, the phase error at oblique angles has a marginal impact on the overall performance of the CPPMS.
The reflection loss of the co-polarization is less than 2 dB in the on state. Besides, the magnitude error is less than 0.3 dB under different incident angles, which means the reflective magnitude for each element is almost equal. During the on state, specifically within the frequency range of 14.14 GHz to 15.31 GHz, the co-polarization exhibits a reflection loss of less than 1.5 dB, while the cross-polarization levels remain below -10 dB. However, in the off state, the frequency band where the reflection loss of the co-polarization remains below 1 dB and the cross-polarization levels remain below -10 dB from 14.86 GHz to 15.9 GHz.
3.2 Design of the 1-bit CPPMS array and control board
Based on the proposed 1-bit element, a 20 × 20 square aperture the 1-bit CPPMS was designed and implemented to validate the proposed concept, as shown in Fig. 5. The size of CPPMS is 240 mm × 240 mm. As previously mentioned, each of the 400 CPPMS elements was equipped with only one diode, which was driven through a single biasing line. All DC biasing lines are arranged on the back of CPPMS. The CPPMS is cleverly divided into two parts by column. The DC biasing lines of columns 1-10 are guided to the lower part of the CPPMS, and columns 11-20 are guided to the upper. Every two rows of DC bias lines pass through 1 × 24 sockets converge, as shown in Fig. 5(b). Each socket has 20 DC bias line ports and 4 ground ports. The CPPMS is regulated by MCU boards, each of which could output 10 × 20 DC channels. The CPPMS connects to the MCU boards through 20 ribbon cables. Each port can output a 10-mA current to drive the PIN diode to work normally.
Fig. 5. Prototype of the 1-bit CPPMS (a) top view and (b) bottom view, and control board (c)top view, and (d) bottom view.
The control board is an important part of CPPMS, which controls the DC bias voltage on each PIN diode, achieving electronic reconfigurability. The control board design is shown in Fig. 5(c) and 5(d). The control chip of the control board adopts the STM32F103ZET6 chip. The entire CPPMS board has 400 channels for control. We use the 74HC164 8-bit shift register to expand the MCU IO port and the 74HC245 8-way transceiver to increase the circuit load capacity. To facilitate debugging and enable visual monitoring of the CPPMS operational status, a light-emitting diode (LED) has been incorporated into the DC line of each element, as shown in Fig. 5(c). A series configuration is employed to connect the PIN diode and LED. At one end of the LED, a DC voltage is supplied to power the diodes and facilitate the modification of the element's operational state. The LED on and off is used to easily distinguish between the two operating states of the component.
4. Experiment results and discussion
The measurement of the CPPMS was conducted inside a standard microwave anechoic chamber as shown in Fig. 6. Both co-polarization and cross-polarization were tested during wide-angle beam scanning. The co- and cross-polarization patterns at broadside (θ = 0°, φ = 0°) were simulated and measured at 15 GHz, and the results are shown in Fig. 7(a). The simulation and measurement results exhibited excellent concurrence, particularly in the crucial main beam region. Specifically, the HPBW was measured to be 6.83° and simulated to be 6.17°, respectively, demonstrating a high level of consistency. It is important to note that the measured sidelobe level was observed to be -10.7 dB, slightly higher than the simulated value of -15.1 dB. This discrepancy is primarily attributed to cable and bracket blockage during the processing and manufacturing stages. The sidelobe level can be improved through the implementation of a larger aperture or a multi-bit phase quantization design [32]. Furthermore, it is worth emphasizing that all measured cross-polarization levels were below -20 dB, indicating the effective suppression and improvement of cross-polarization in each component of the CPPMS. Moreover, the AR results of the CPPMS are presented in Fig. 7(b) through both simulation and measurement. The simulation and measurement outcomes are in good agreement within the main beam direction range, with an AR of less than 3 dB. The calculated 3 dB AR beamwidths are 10.54° and 8.93° for simulation and measurement, respectively. As a result, the AR beam qualities can be determined as ρ = 0.58 and ρ = 0.76, respectively.
Fig. 6. (a) Experimental measurement scene of the CPPMS and configurations of beam scanning in (b) xoz-plane, (c) yoz-plane, (d) diagonal plane (φ = 45°), and (e) diagonal plane (φ = -45°).
Fig. 7. Comparison of simulated and measured broadside beam performance. (a) the radiation patterns, (b) the AR. (c) Measured and simulated gain and aperture efficiency as a function of frequency. (d) Measured and simulated AR versus frequency and evaluation of AR quality.
To investigate the bandwidth characteristics of the CPPMS, we conducted measurements of the broadside gain over frequency and subsequently calculated its aperture efficiency, as shown in Fig. 7(c). The measured 3 dB gain bandwidth of the CPPMS was found to be 14.54 - 15.65 GHz, representing a relative bandwidth of 7.3%. Furthermore, the maximum measured gain reached 22.34 dBi, corresponding to an aperture efficiency of 18.01%. Furthermore, Fig. 7(d) presents the measured and simulated AR at various frequencies. It is observed that the measured 3 dB AR value slightly surpasses the simulated value. The simulated 3 dB AR bandwidth is 12.1% (14.24 - 16.07 GHz), whereas the measured AR bandwidth covers 10.1% within the frequency range of 14.44 -15.98 GHz. Additionally, we performed a comprehensive evaluation of the ρ through measurements and simulations at different frequencies. Remarkably, the results from both measurements and simulations exhibit consistent values, all of which are less than 0.9 across the effective bandwidth.
It is worth noting that the deviation in gain, 3 dB gain bandwidth, AR, and 3 dB AR bandwidth, can be primarily attributed to various factors. These factors include feed blockage resulting from the coaxial cable, edge diffraction, and inherent fabrication errors. Moreover, it is essential to consider the impact of certain errors related to the PIN diode equivalent circuit model and the welding process of devices. These factors could also contribute to the discrepancy in the CPPMS gain. Specifically, the simulation did not account for the coupling effect between the physical structures caused by the diode and capacitor welding, further contributing to the variance in the gain characteristics. Despite these small deviations, the overall agreement between measurement and simulation results in terms of AR and ρ reaffirms the reliability and effectiveness of the designed CPPMS.
The 2-D beam steering functionality of the proposed antenna is effectively demonstrated through the design and realization of multiple pencil beams, each possessing distinct steering angles within the four main planes, namely the xoz-plane, yoz-plane, and two orthogonal diagonal planes. At the frequency of 15 GHz, the normalized radiation pattern of the antenna is measured, covering scanning angles ranging from -60° to 60°, as shown in Fig. 8. Notably, the antenna exhibits well-defined circularly polarized beam scanning capabilities in all four planes. Specifically, the measurement results of the antenna operating under RCP excitation within the xoz-plane are shown in Fig. 8(a). The observations reveal wide-angle beam scanning enabled through code-switching. Even as the scanning angle increases, the main lobe gain remains consistently high, with a maximum gain loss of less than 4.7 dB throughout the scanning range. Furthermore, by modifying the phase distribution of the metasurface, wide-angle beam scanning is achieved in the yoz-plane as well, preserving the antenna's optimal gain characteristics. This outcome effectively verifies the antenna's beam control ability in both azimuth and elevation directions as shown in Fig. 8(b). Similarly, the diagonal planes demonstrate impressive beam-scanning performance, achieved by encoding the corresponding phase contour, as shown in Fig. 8(c) and 8(d).
Fig. 8. Measured radiation patterns of the proposed CPPMS in (a) xoz plane, (b) yoz plane, (c) diagonal plane (φ= 45°), and (d) diagonal plane (φ = -45°).
In addition to the radiation plot demonstrating the beam scanning capabilities in the four main planes, we provide a comprehensive analysis of the corresponding beams, including AR and ρ. The achieved beam scanning in each plane demonstrates high polarization performance. It should be noted that we only presented performance analysis in the diagonal plane (φ = 45°). As shown in Fig. 9, the measured ARs are consistently maintained below 3 dB, with all ρ values remaining below 0.9. It indicates that it exhibits good circular polarization AR performance across the HPBW. These results affirm that the 1-bit CPPMS can achieve two-dimensionally high AR performance beam scanning.
Table 1 summarizes the comparison of the performance of the proposed design with those in some representative references. It has the same or better performance as other CPPMS antennas mentioned in the literature in terms of wideband, gain, number of PIN diodes used, etc. There is currently limited research literature on programmable reflective circularly polarized metasurfaces. Although excellent beam scanning performance and wideband performance have been demonstrated, such as [23] - [26], [28] - [29], the analysis of AR characteristics is relatively lacking. These studies are limited in several aspects. Firstly, existing research is unable to maintain stable AR performance in wide-angle beam scanning, and there is no detailed exploration of the quality of AR characteristics. Secondly, the widespread use of lumped components (such as PIN diodes and varactors) in existing literature has led to increased high-frequency losses and increased complexity in element design. For example, the wideband 1-bit circularly polarized programmable metasurface based on magnetoelectric dipoles introduced in [29], although it has a relative bandwidth of up to 32%, requires many PIN diodes, and its AR performance is only excellent at limited angles. Once the beam is deflected, the AR will rapidly deteriorate. This is mainly attributed to insufficient suppression of cross-polarization. Compared with existing methods, this study adopts a random rotation layout of the array, which effectively suppresses cross-polarization issues and maintains good suppression effects at different scanning angles, thereby improving AR performance. Therefore, the solution proposed in this work based on 1-bit circularly polarized programmable metasurfaces performs better in high AR performance beam scanning issues and is more suitable for practical scenarios such as satellite communication.
Table 1. Comparison between the proposed CPPMS in this paper and other relevant reported works
5. Conclusion
A circularly polarized programmable metasurface has been successfully designed, fabricated, and measured. The presented technique employs randomly rotated elements to achieve cross-polarization and AR control of CP beams. Notably, the proposed metasurface element accomplishes CP beam modulation using a single PIN diode, contributing to a straightforward and robust design. The resulting metasurface demonstrates low design complexity and insertion loss, making it a lightweight, cost-effective, and low-loss solution for large-scale 2-D programmable circularly polarized metasurface design. The results illustrate the robustness and versatility of the proposed antenna design, achieving two-dimensionally high AR performance beam scanning. The measured parameters, including gain, efficiency, AR, and AR beam quality, consistently demonstrate the exceptional performance of CPPMS. These valuable insights offer practical implications for the implementation of CPPMS in diverse applications, including wireless communication, satellite communication systems, and beamforming technologies.
Funding
National Key Research and Development Program of China (2023YFB3811503); National Natural Science Foundation of China (62001342, 62288101); Key Research and Development Projects of Shaanxi Province (2021TD-07).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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