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Abstract
In this paper, a novel design strategy for a circularly polarized metasurface antenna array that integrates low scattering and well-behaved radiation is proposed. Firstly, based on the Jerusalem cross-shaped patch, a metasurface element with 180° reflection phase difference under orthogonal normal incident waves is designed. The characteristic mode theory is utilized to analyze the metasurface element mode to excite the linear polarization radiation. Then, the metasurface antenna elements are arranged orthogonally in a 2×2 subarray and fed with a 90° phase difference, which achieves high port isolation, better circular polarization radiation and low scattering performance. Finally, the 4×4 array is fabricated and measured, and the good agreement between simulation and measurement verifies the effectiveness of the design method.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
As a periodic or quasi-periodic artificial structure, the electromagnetic metasurface has been widely applied in the field of controlling electromagnetic waves with its excellent performance, realizing supernatural phenomena [1–3]. Many functions can be achieved by using metasurfaces to control the phase [4–6], polarization [7–10], and amplitude [11–13] of electromagnetic waves. In [4], the reflection phase of the metasurface is controlled to have a phase difference of 180° in the orthogonal direction, the orthogonal arrangement of the metasurface to achieve Radar Cross Section (RCS) reduction, and excellent circular polarization radiation performance is achieved in a large angle range. In [7], by encoding the unit cells with designed coding matrices (both 1-bit and 2-bit matrices), anomalous reflection or diffusion of two circular polarizations can be realized independently by controlling the polarization of electromagnetic waves. In [11], a novel metasurface is constructed by introducing the periodic arrangement of cells to control the amplitude to achieve high gain. The cells are composed of a C-shaped pattern patch at the center surrounded by a pair of L-shaped patches. Besides, absorbing metasurfaces are also widely applied in electromagnetic amplitude control [14–16]. In [14], a broadband infrared metasurface absorber made of Indium Tin Oxide using an asymmetric Fabry-Perot cavity is proposed. By tuning the structure of the metasurface layer and the thickness of the cavity, an absorptivity of over 80% was achieved from λ = 4 to 16 μm. In [15], the usage intercalated few-layer graphene for the development of an optically tunable absorbing metasurface is proposed, which achieves an optically controllable absorption in terahertz imaging and selective absorption. In [16], based on the theoretical concept of uniaxial perfectly matched layer in terahertz range, a wide-angle, broadband and polarization insensitive metamaterial absorber is proposed.
The successful application of metasurfaces has brought a lot of opportunities, but there are still few reports of directly using metasurfaces as radiators. In [17], an anisotropic metasurface is adopted to substitute for the conventional patch, which acts as a radiator directly and retains the radiation performance, but only reduces the in-band radar cross section. In [18], the metasurface antenna is composed of nine elements arranged in a 3 × 3 layout. The four slits are etched to form two orthogonal currents with a phase difference of 90°. Therefore, the circular polarization field is excited. In [19], the proposed antenna consists of a 4×4 unequal size metasurface elements fed by a microstrip feed network in the form of a thin strip cross dipole. The improved boresight gain is realized due to the relatively large lateral size that supports in-phase currents.
The characteristic mode theory (CMT) based on the method of moments (MoM) defines mutually orthogonal characteristic modes (CMs) on the conductor surface, which provides a profound physical operation principle for changing the current path, modifying the modes, and regulating the electromagnetic wave [20–24]. In [25], by analyzing the characteristics of scattering modal current of T-shaped slot antenna, slotting and loading inductance component are carried out to suppress and remove scattering modal current out of the operating band respectively, so as to reduce the radar cross section in the operating band. In [26], by analyzing the characteristic modes and characteristic parameters of the metasurface, adding slots and metallized vias, the high-order modes are suppressed and the radiation performance is improved. In [27], two characteristic modes with the same resonant frequency and orthogonal current distribution are selected as the operation modes. In addition, a hybrid feed system consisting of a cross slot and a microstrip line is utilized to excite two orthogonal modes with a phase difference of 90° to obtain circularly polarized radiation.
This paper utilizes the phase modulation function of the metasurface to design a metasurface with a 180° reflection phase difference under orthogonal normal incident wave. Linearly polarized radiation is excited by characteristic mode analysis of metasurface elements. A 4 × 4 array is fabricated and measured without introducing additional structure and load, and the simulation and measurement results showed excellent circularly polarized radiation and low scattering performance. It is worth mentioning that the proposed method can not only be applied to the microwave field, but also can be extended to the optical frequency band.
2. Design and analysis of the low scattering circularly polarized array
2.1 Metasurface element design
Figure 1(a) shows the Jerusalem cross-shaped patch (E1), which has a symmetrical structure, and its reflection phase is equal under the x- and y-polarized incident waves, as shown in Fig. 2(a). If the reflection amplitudes are equal under the normal incident wave and the reflection phase difference between two adjacent elements meets 180° (±37°), a 10 dB RCS reduction can be obtained. Therefore, the structure of two ‘H’ patches is optimized by compressing or extending them respectively to form the structure shown in Fig. 1(b). The reflection phase difference is close to meeting the conditions required for 10 dB RCS reduction under x- and y-polarized incident waves within 5.3-6.4 GHz, as shown in Fig. 2(b). The proposed metasurface element is obtained by etching four slots on the patch, as shown in Fig. 1(c). The four slots are utilized to adjust the reflection phase to meet the 180° (±37°) under x- and y-polarized incident waves within 5.2-6.4 GHz, and reflection amplitude is almost total reflection, as shown in Figs. 2(c) and (d). The configuration of the element is shown in Figs. 1(d) and (e). The slot on the ground is ignored in the metasurface element, but considered in the antenna. From the top to the bottom, it is composed of metal patch, a middle F4B substrate (εr = 2.65 and tan δ=0.0007), and a bottom ground.Therefore, the phase difference can be used to achieve RCS reduction by arranging the elements orthogonally.
Fig. 1. Evolution process of proposed element. (a) Element 1. (b) Element 2. (c) Element 3. (d) Top view of the proposed element. (e) Perspective of the proposed element. P = 25 mm, l1 = 4 mm, w1 = 16 mm, l2 = 12 mm, w2 = 8 mm, sl = 14 mm, sw = 0.3 mm, sd = 1 mm, h = 3 mm.
Fig. 2. Reflection phase and amplitude. (a) Element 1. (b) Element 2. (c) Reflection phase and (d) reflection amplitude and phase difference of the proposed element.
2.2 Characteristic mode analysis and design
The characteristic modes theory [20] was developed to analyze the radiation and scattering properties of dielectric material bodies based on the method of moment. A generalized eigenvalue equation of multilayered medium can be written
where the real and imaginary parts of the impedance matrix Z are R and X, respectively. λn is the real eigenvalue associated with each characteristic current Jn. The total current can be expressed as a linear weighted superposition of these mode currents where αn is the modal weighting coefficient (MWC) of the n-th modal current. The modal significance (MS) is only related to eigenvalues and is defined asThe eigenvalue has a large variation range. When the eigenvalue is close to 0, it indicates that the mode is easy to be excited. When λn is positive, it behaves inductive, on the contrary, it is capacitive.
The characteristic mode analysis of the element is performed to stimulate its potential radiation performance. Characteristic modes of the element are obtained using the MoM-based characteristic mode analysis (CMA) tool in CST 2020. The dielectric loss is neglected in CMA but considered in the time-domain analysis. The first four modal current and radiation patterns of the proposed element are shown in Fig. 3. It can be seen that J1 is the strongest in the middle, J2 is polarized along x axis. J1 and J1 are linearly polarized along x axis with broadside radiation, while J4 is linearly polarized radiation along the y axis. J3 is rotationally symmetrical, and the modal radiation pattern is double beam. As can be seen, J1, J2 and J4 are the desired modes, J3 is an undesirable radiation pattern. From the eigenvalues, J4 is difficult to be excited, and J1 is the easiest to be excited. There are generally two mode excitation methods, namely capacitive coupling excitation and inductive coupling excitation [28,29]. The corresponding antenna element is called capacitive coupling element (CCE) and inductive coupling element (ICE). Since these excitation elements are electrically small, they have little effect on radiation. The position of excitation is very significant. In order to excite a mode, ICE should be placed at the maximum value of modal current distribution (or the minimum value of modal electric field), while CCE should be placed at the minimum value of modal current distribution (or the maximum value of modal electric field). Therefore, the feeding position is at the maximum modal current, as shown by the black circle in Fig. 3(a). As shown in Fig. 4(d), it can also be seen from the Smith chart that it is inductive within the matching bandwidth. It can be seen from modal weighting coefficient that J1 is dominant, and J2 is also partially excited in the higher frequency band. The excited modes are linearly polarized along x-axis, and the axial ratios are all above 65 dB, as shown in Fig. 4(e). Therefore, it can be applied to form a circular polarization array. The impedance bandwidth of the proposed antenna element is 4.8-5.38 GHz with the gain of 5.23-5.96 dBi over the operating bandwidth. As shown in Figs. 1(d) and (e), the slot on the ground is only utilized to adjust impedance matching. After etching four slots on the patch, the structure of the proposed antenna has changed fundamentally, which can be regarded as composed of main driven patch and parasitically coupled patches. Then, four parasitically coupled patches are removed, the antenna bandwidth is only shifted to the high frequency by 20 MHz, and the gain has no variety. Moreover, it can also be seen from J1 that the modal current on the main driven patch is the strongest. Therefore, the four slots on the patch are utilized to adjust impedance matching and reflection phase.
Fig. 3. The modal current and radiation patterns of the proposed element at 5 GHz. (a) J1 at 5 GHz. (b) J2 at 5 GHz. (c) J3 at 5 GHz. (d) J4 at 5 GHz.
Fig. 4. Characteristic mode parameters and radiation performance of antenna element. (a) Eigenvalue. (b) Modal weighting coefficient. (c) Impedance bandwidth and gain of the proposed antenna element. (d) Smith chart. (e) The axial ratio of antenna element.
2.3 Low-scattering circularly polarized array design
In order to reduce scattering, the elements are arranged orthogonally so that the antenna array can achieve low RCS. The element itself is a radiator, which reduces the RCS by orthogonal arrangement without introducing additional structure and lumped loads.
The general far-field radiation expression of the antenna element is:
The proposed combined subarray is shown in Fig. 5. The polarization directions of two adjacent elements are orthogonal to each other, and four ports are fed with equal power of 0°, 90°, 0°, 90° to generate right-handed circularly polarized (RHCP) waves. The performance parameters of the subarray are shown in Fig. 6. The achieved impedance bandwidth for -10 dB is 4.76-5.38 GHz with the gain of 7-7.9 dBi over the operating bandwidth, and the port isolation is less than -19 dB. Moreover, the axial ratio (AR) of subarray is very excellent in the normal direction, and it satisfies circular polarization in the ±25° spatial range over the operating bandwidth. In order to evaluate its scattering performance, a traditional square patch antenna is designed as a reference antenna. The dielectric thickness and dielectric constant are the same as those of the proposed antenna. The same 2×2 subarray layout is performed on the reference antennas, where the impedance bandwidth is 4.81-5.14 GHz with a patch of 16.5 mm. As can be seen from Fig. 6 (f), the monostatic RCS achieves a maximum reduction of 20 dB within 4.5-7 GHz. The scattering patterns at 5.5 GHz and 6.3 GHz have four beams as predicted by theory. Due to the rotational symmetry of the subarray, the RCS reduction is the identical for x- and y-polarized incident waves.
Fig. 6. The performance comparison between the proposed array and the reference array. (a) Impedance matching. (b) Port isolation. (c) Axial ratio. (d) The radiation pattern at 5 GHz. (e) The performance of reference antenna element. (f) The monostatic RCS and the scattering pattern at 5.5 GHz and 6.3 GHz.
3. Fabrication and experiments
Considering the actual application scenario, four 2×2 element subarrays are integrated into a 4×4 array. The photograph of the proposed antenna, the reference antenna and anechoic chamber measurement environment is shown in Fig. 7, and its total area is 100×100×3 mm3. The antenna array is fed by a power divider network when measuring its impedance bandwidth. The measured impedance bandwidth for |S11| below -10 dB is 13.2% (4.73-5.4 GHz), within which the boresight gain varies from 10 to 14.5 dBi. Moreover, the axial ratio in the operating bandwidth is less than 0.5 dB, showing excellent circular polarization performance. The phase shift of the two lengths of coaxial lines is 90°, and the phase difference between two adjacent elements is utilized to measure the right-hand circular polarization (RHCP) and the left-hand circular polarization (LHCP) radiation pattern. Figures 8(c)-(f) shows the excellent agreement between the simulated and measured normalized radiation patterns at 5 GHz. Figure 8(h) shows the monostatic RCS reduction measured for the proposed and reference antenna arrays under the x-polarization normal incident wave. The measurement result is consistent with the change trend of the simulation result. Figure 8(i) shows that for bistatic RCS under TM-polarized incident wave, the larger the incident angle is, the smaller the reduction effect is. The performance comparison between the proposed antenna and other low-scattering circularly polarized antennas is shown in Table 1. It can be seen that the proposed method has excellent circular polarization radiation performance (AR is less than 0.5 dB), and achieves the in-band and out-of-band RCS reduction.
Fig. 8. Comparison of measurement and simulation. (a) Measured return loss. (b) Measured and simulated gain and axial ratio. Normalized radiation pattern of right-handed circular polarization at (c) 5 GHz for x - z plane and (d) 5 GHz for y - z plane. Normalized radiation pattern of left-handed circular polarization at (e) 5 GHz for x - z plane and (f) 5 GHz for y - z plane. (h) Measured and simulated monostatic RCS reduction. (i) Simulated bistatic RCS reduction.
Table 1. Comparison of the low-RCS antennas and the proposed antenna
4. Conclusion
This paper presents a novel low-scattering circularly polarized antenna design method. The classic Jerusalem cross-shaped patch is optimized to obtain a metasurface element with a 180° reflection phase difference under orthogonal normal incident waves. With the guidance of characteristic mode theory, the metasurface element is excited by inductive feeding to linearly polarize radiation. Then, the metasurface antenna elements are arranged orthogonally, and fed with 90° phase difference. The simulation and measurement results show excellent radiation performance and low scattering characteristics. Although the idea is checked in the radio band, the proposed method is scalable and can be applied to optical frequencies. It is worth noting that the possible limitations of the proposed antenna come from fabrication tolerances and losses at higher frequencies.
Funding
National Postdoctoral Program for Innovative Talents (No.2019M653960, No.BX20180375); Natural Science Basic Research Program of Shaanxi Province (No.20200108, No.2020022, No.2020JM-350, No.20210110); National Natural Science Foundation of China (No.61801508, No.62171460).
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.
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