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Abstract
A specially designed dumbbell type polarization conversion metasurface (PCM) is proposed. The designed PCM achieves line-to-line polarization conversion in ultra-wideband (UWB) from 7.63 GHz to 18.80 GHz. A low-profile metasurface antenna composed of PCM and slot feed is proposed based on characteristic mode analysis (CMA), which realizes the integrated design of radiation and scattering. Because of the checkerboard scattering properties, low-radar cross section (RCS) low-profile multiple-input-multiple-output (MIMO) antenna and array antenna are designed with PCM. The low-RCS high-isolation low-profile MIMO antenna with size of $1.27 \times 1.27 \times 0.07{\lambda _0}^3$ (λ0: the free-space wavelength corresponding to the center frequency point) operating at 5.8 GHz consists of four orthogonal arranged metasurface antennas. The isolation is greater than 26 dB with impedance bandwidth from 5.51 GHz to 6.06 GHz. In addition, the low-RCS high-gain low-profile array antenna with size of $2.55 \times 2.55 \times 0.07{\lambda _0}^3$ is also designed. The operating band covers from 5.63 GHz to 6.12 GHz with the gain of 12-15.6 dBi. The RCS reduction of the two antennas for normal incidence is between 6 dB and 23 dB in both X- and Ku-bands. The measured results of the antennas agree with the simulated results, which shows that they have potential application value in 5.8 GHz WLAN wireless communication.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Today's development belongs to the information age, everything is connected. As a device for receiving and transmitting electromagnetic waves, antenna plays an irreplaceable role in the process of information transmission [1–4]. However, with the continuous improvement of information transmission rate, capacity, quality and other aspects, the performance of traditional antennas at this stage can no longer meet the needs of scientific and technological development. In order to increase the channel capacity of the communication system, multiple-input multiple-output (MIMO) antenna is designed [5–9]. Its principle is to increase the number of transceiver antennas without taking up additional spectrum resources. Different from MIMO antenna to increase the channel capacity of communication system, array antenna can effectively increase the radiation performance of antenna [10–13]. It usually has a certain number of antenna elements in a certain arrangement, and the radiation performance of the antenna is the vector sum of the radiation field of each element. Because the antenna contains metal materials, in addition to considering the radiation characteristics, the scattering characteristics of the antenna are also particularly important in the stealth of the target.
The radar cross section (RCS) measures the stealth capability of a target in radar detection [14–18]. The RCS of MIMO antenna and array antenna with large size is disadvantageous to target stealth. It becomes more difficult to use coating materials and deformation techniques to reduce the RCS of antenna without compromising antenna radiation performance. The rapid development of electromagnetic metasurface brings a new idea to the design of low-RCS antenna. Using the metamaterial absorber is one way to design the low-RCS antenna [19–22]. The absorber is loaded around the antenna and the electromagnetic (EM) wave can be absorbed rather than reflected in other directions for the purpose of reducing the RCS. Frequency selective surface (FSS) is used to replace the metal ground plane of the antenna for low RCS [23–26]. EM waves in the target band pass through the antenna, which weakens the monostatic RCS. The checkerboard configuration with a pair of artificial magnetic conductors (AMCs) satisfying the 180° (±30°) reflection phase difference are also used to design low-RCS antenna [27–29]. Similar to the AMC, the polarization conversion metasurface (PCM) has a 180° phase difference in the reflection of EM waves from its mirror structure, and it is also used in the design of low-RCS antenna [30–35]. Transmitarray antennas and reflectarray antennas with low-scattering characteristic have been proposed in [36–41]. They make the RCS significantly lower with out-of-band, but seriously increase the overall structural profile of the antenna. The antenna array with C-shaped elements obtain low-RCS features without extra RCS reduction structures by phase cancellation [42]. The design of low RCS antenna needs to consider both the radiation and scattering of antenna, and reduce the scattering intensity on the premise of ensuring the radiation performance. Future research should focus on the integrated design of antenna radiation and scattering. The reason why the metasurface antenna is chosen as the research object is that the metasurface can be excited to emit radiation and achieve broadband low scattering. At the same time, the metasurface antenna is usually planar structure with low profile.
In order to take into account the radiation and scattering characteristics of the antenna, a metasurface antenna with the frequency of 5.54-6.16 GHz is proposed on characteristic mode analysis (CMA). The top part of the antenna is composed of PCM, which can not only be excited to radiate, but also generate polarization conversion of EM waves. The PCM achieves line-to-line polarization conversion in UWB from 7.63 GHz to 18.80 GHz, which has application potential to reduce the RCS of antenna in X- and Ku-bands. On this basis, the low-RCS low-profile 4-port MIMO antenna with isolation greater than 26 dB and the low-RCS low-profile array antenna with gain of 12-15.6 dBi are designed and manufactured. And the antenna profile is only 0.07λ0. The proposed antennas have the potential in 5.8 GHz WLAN wireless communication.
This paper is organized as follows. Section 2 discusses the metasurface antenna design, including UWB PCM, checkerboard metasurface and antenna element. Section 3 presents the MIMO antenna using PCM. Section 4 presents the array antenna using PCM. A conclusion is made in section 5.
2. Metasurface antenna
In this section, three aspects of the design of metasurface antenna are discussed, namely, UWB PCM, checkerboard metasurface and single element metasurface antenna. Surface current and vector decomposition are used to analyse the working mechanism of PCM. The reason of scattering weakening of checkboard metasurface is explained by the bistatic scattered fields. The CMA is used to design and analyse the metasurface antenna. All simulation analyses covered in this article are based on CST Microwave Studio Suite.
2.1 UWB PCM
The geometry of the PCM unit is shown in Fig. 1. The dumbbell shaped sheet is printed on a 3 mm backing copper-covered F4B substrate with the dielectric constant of $\varepsilon = 2.65 + 0.002i$. To create resonance for horizontally and vertically polarized EM waves, the patches are arranged along diagonal lines. The detail of the PCM unit are: $l1\textrm{ = 1}\textrm{.74 mm}$, $l2\textrm{ = 1}\textrm{.39 mm}$, $l3\textrm{ = 4}\textrm{.86 mm}$, $w1\textrm{ = 3 mm}$, $w2\textrm{ = 2}\textrm{.5 mm}$, $h1\textrm{ = 3 mm}$, $p1\textrm{ = 11 mm}$. In the CST simulation environment, the x and y directions were set as unit cell boundary conditions, and the z direction was set as open (and space) boundary conditions to simulate the infinite periodic structure.
Figure 2(a) shows the reflection coefficient of the PCM unit for normal incidence of x- and y-polarized EM waves. The reflected EM wave is divided into two polarizations, which are co-polarization and cross-polarization. The PCM realizes UWB line-to-line polarization conversion from 7.63 GHz to 18.80 GHz. In this frequency band, the co-polarization below -10 dB and cross-polarization is close to 0 dB. For x- or y-polarized wave incidence, the reflected wave is transformed into the corresponding cross-polarized wave, and polarization conversion rate (PCR) above 90%, as shown in Fig. 2(b). Because of the symmetry of the structure, the PCR values for x and y polarization remained consistent.
The PCR curve shows three peak frequency points, which are 8.25 GHz, 12.31 GHz and 17.89 GHz. Next, the current distribution at the three peaks of PCR is discussed, as shown in Fig. 3. Under the normal incidence of y-polarized wave, the current distribution of top and bottom metal shows different resonance characteristics. At 8.25 GHz, the current on the top metal is evenly distributed in the middle of the structure, and the current flow direction is opposite to that on the bottom, resulting in magnetic resonance. At 12.31 GHz, the current flow direction of the top layer and the bottom layer is opposite, also forming magnetic resonance, but the current of the top layer is mainly distributed in the short side of the structure. With the increase of frequency, the top current flow accumulates on both sides of the long side, and the top current flows in the same direction as the bottom current flow, forming electrical resonance. The special structure produces different resonant types at different and adjacent frequency points to achieve UWB characteristics.
Fig. 3. Surface current distributions of the PCM unit at (a) 8.25 GHz, (b) 12.31 GHz, (c) 17.89 GHz.
To further explain the polarization conversion principle, the horizontal and vertical electric fields are decomposed into the u- and v-axes as shown in Fig. 4(a). The electric field of the x-polarized normal incident EM wave and the corresponding reflected EM wave are decomposed, as shown below:
2.2 Checkerboard metasurface
The checkerboard shape metasurface element consists of four 3 × 3 PCM arrays, and the adjacent arrays are mirror images of each other. Figure 5 shows the diagram of checkerboard metasurface and electric field vector. Assume that the incident electric field vector is along the x-axis, as shown by the real line. From our previous analysis, the x-polarized wave is transformed into y-polarized with PCM, as shown by the dotted line. In the checkerboard surface, the reflected electric field vectors between adjacent arrays are opposite in direction, and the EM waves cancel each other out. Finally, the total reflected wave shows a weak level in the incident direction, achieving a low RCS characteristic. Figure 6 shows the simulated bistatic RCS patterns of PEC and checkerboard PCM for normal incidence of x-polarized wave at 8 GHz, 13 GHz and 18 GHz. In the operating band of PCM, the scattering field in the main direction is obviously weakened. The energy is dispersed in other directions, and the target monostatic RCS is effectively weakened.
Fig. 6. Simulated bistatic RCS patterns of PEC and checkerboard PCM for normal incidence of x-polarized wave at (a) 8 GHz, (b) 13 GHz, (c) 18 GHz.
2.3 Antenna element
In order to design PCM into a metasurface antenna, the metasurface of the 3 × 3 PCM periodic structure is analyzed by CMA. The multilayer solver is used to simulate the infinite dielectric plate. Open boundary antennas are set in the x- and y-directions, radiation boundary conditions are set in the + z-direction, and perfect electric conductor boundary conditions are set in the -z-direction. Figure 7 shows the geometry of metasurface and the corresponding modal significances of the first four CMs. Meanwhile, the modal currents and modal radiation patterns of metasurface are shown in Fig. 8. As can be seen, J1 and J2 are both in phase across the metasurface, but they are orthogonal to each other and produce broadside radiation patterns. Because of the difference in equivalent electrical length, mode 4 has a higher resonant frequency than mode 1. However, the current distribution of J2 and J3 has reverse current, and the directions null appear at + z-direction. Therefore, modes 1 and 4 that satisfy the radiation characteristics are the desired modes.
For metasurface antennas, the metasurface element is usually excited by slot-coupled feeding. The size of the slot theoretically determines the operating frequency band of the antenna. The modal significances and magnetic currents of the slot are obtained by the CMA, as shown in the Fig. 9. Mode 1 and mode 2 correspond to the half-wavelength mode and full-wavelength mode of the slot, respectively. In half-wavelength mode, the magnetic current is in phase. But in full-wavelength, there is a reverse current. In order to achieve the purpose of microstrip-to-slotline transition, the potential radiation bandwidth of the antenna is between the corresponding frequencies of the half-wavelength mode and full-wavelength mode.
Fig. 9. (a) Metal slot, $lf = 29mm$, $\textrm{wf = 2mm}$, (b) modal significances, (c) half-wavelength mode 1, (d) full-wavelength mode 2.
Modal significances, modal currents and modal radiation patterns are given in Fig. 10 and Fig. 11. Only the expected mode is discussed. Due to the introduction of the slot, the resonant frequency point of mode 2 moves to the lower frequency, and the MS of mode 3 increases. The potential operating band of the antenna lies in the overlap of the two modes.
Fig. 11. Modal currents and modal radiation patterns of metasurface with slot. (a) J1, (b) J2, (c) J3, (d) J4.
The designed metasurface antenna is divided into two parts, namely PCM and coupling slot feeding layer, as shown in Fig. 12(a). Rogers 4003C with the dielectric constant of $\varepsilon = 3.55 + 0.0027i$ is selected as the medium layer for the slotted ground and transmission line in the lower layer. The energy is coupled to the slot by a 50 Ω microstrip line and a short circuit microstrip line. Using Rogers 4003C rather than F4B, the antenna thickness and microstrip line width can be further reduced, which is conducive to the realization of antenna miniaturization. After parameter scanning optimization, the detailed parameters are as follows: $h2\textrm{ = 0}\textrm{.8 mm}$, $p2\textrm{ = 33 mm}$, $f1\textrm{ = 3 mm}$, $f2\textrm{ = 1}\textrm{.7 mm}$, $f3\textrm{ = 15 mm}$, $f4\textrm{ = 9}\textrm{.5 mm}$.The metasurface antenna has a bandwidth from 5.54 GHz to 6.16 GHz with $S11< - 10dB$ and good forward radiation characteristics, as shown in the Fig. 12(b) and Fig. 12(c). Figure 12(b) shows the process of parameter scanning in feeder design about f1 and f3. Subsequent antenna designs are based on this antenna design. Metasurface can not only produce radiation, but also change the polarization state of incoming waves. The design of metasurface antenna realizes the integrated design of radiation and scattering.
Fig. 12. (a) Geometry of the metasurface antenna, (b) reflection coefficient, (c) radiation patterns.
3. MIMO antenna using PCM
The MIMO antenna consists of a metasurface antenna that is rotated 90° four times, as shown in the Fig. 13. The top layer is a checkerboard composed of PCM, which can reduce the forward RCS according to the phase cancellation principle. Under the incident of x- or y-polarized wave, the adjacent PCM unit generates reverse polarized wave. The middle layer is four radiating slots on the ground that transmit the underlying EM waves from microstrip feeds. The four ports are ports 1, 2, 3, and 4 in sequence. Fabricated prototype antenna and test results are presented to verify the accuracy of the design. Figure 14 shows the photographs of the proposed MIMO antenna. Figure 14(a) shows a PCM checkerboard made on F4B substrate with the size of $\textrm{66} \times \textrm{66} \times \textrm{3}\textrm{.8m}{\textrm{m}^\textrm{3}}$. The middle layer with four radiating slots is shown in Fig. 14(b). The mutually orthogonal microstrip lines are made on 0.8 mm Rogers 4003C, as shown in the Fig. 14(c). The proposed MIMO antenna is measured by only exciting port 1, while port 2, 3, 4 are connected to 50 Ω matched load in a microwave anechoic environment. Subsequently, the proposed MIMO antenna performance will be verified by the radiation characteristics and scattering characteristics.
The simulated and measured S-parameters of the proposed MIMO antenna is shown in Fig. 15. It is obvious that the measured results agree well with the simulated results within the margin of error. The measured results show that the MIMO antenna works from 5.51 GHz to 6.06 GHz with the $\textrm{S11 }< - \textrm{10 dB}$, and at the same time, the isolation between the antenna elements is greater than 26 dB.
In order to quantitatively measure the coupling of MIMO antennas, envelope correlation coefficient (ECC) can be calculated from S-parameters in Fig. 16(a). Correspondingly, another important parameter diversity gain (DG) used to measure the impact of diversity is also drawn in Fig. 16(b). It is observed that ECC below 0.001 and DG above 9.995 dB in the antenna operating band, which means the good performance of the MIMO system.
Figure 17(a) shows the far-field measurement setup in microwave anechoic chamber. The radiation patterns of the MIMO antenna at 5.8 GHz in xoz-plane and yoz-plane are given in Fig. 17(b). The antenna has relatively stable radiation characteristics in the z direction, which is suitable for wireless communication system.
The simulated bistatic RCS patterns for normal incidence of x-polarized wave are illustrated in Fig. 18. The scattering patterns of PEC and MIMO antenna with checkerboard PCM at 8 GHz, 13 GHz and 18 GHz are compared and analyzed. Based on phase cancellation principle, the scattering main lobe of the antenna is significantly weakened compared with PEC of the same size.
Fig. 18. Simulated bistatic RCS patterns of PEC and MIMO antenna for normal incidence of x-polarized wave at (a) 8 GHz, (b) 13 GHz, (c) 18 GHz.
Figure 19(a) shows the RCS measurement setup in an anechoic chamber with two horns as transmitting and receiving antennas, and the antenna sample to be measured at the other end. The simulated and measured monostatic RCS reduction for normal incidence of x-polarized wave is plotted in Fig. 19(b). It is obvious that the RCS reduction is at least 6 dB and by up to 23 dB in the entire X- and Ku-bands. The RCS reduced frequency band corresponded to the operating frequency band of PCM, and the PCR value is proportional to the RCS reduction. The monostatic RCS is effectively reduced by the checkerboard PCM.
In this section, a novel low-RCS high-isolation 4-port MIMO antenna with impedance bandwidth from 5.51 GHz to 6.06 GHz is proposed. The checkerboard arrangement and orthogonal polarization arrangement are introduced for adjacent orthogonal metasurface antennas. The checkerboard shape of PCM can effectively reduce the RCS of the antenna more than 6 dBi in X- and Ku-bands. Orthogonal arrangement can effectively increase the isolation more than 26 dB between antenna ports. The proposed MIMO antenna has excellent properties, such as low RCS, high isolation and low profile, which has the potential in 5.8 GHz WLAN wireless communication.
4. Array antenna using PCM
A 4 × 4 elements array antenna with the size of $\textrm{132} \times 132 \times \textrm{3}\textrm{.8m}{\textrm{m}^\textrm{3}}$ is designed, as shown in the Fig. 20. The top layer of the antenna array is a checkerboard PCM, the middle layer is a 16-slot structure, and the bottom layer is a feed network. Figure 21 shows the fabricated prototype, including checkerboard PCM, slots and microstrip line. The feed network is composed of 1-16 constant amplitude in-phase power divider, which is realized by quarter-impedance transformation. The microstrip line is connected to the 50 Ω coaxial line. The 50 Ω microstrip line width is 1.7 mm, and the 70.7 Ω microstrip line width is 0.91 mm.
The return loss of the antenna is measured by vector network analyzer. Simulated and measured S-parameters of the array antenna have a good agreement, as shown in Fig. 22. The antenna works from 5.63 GHz to 6.12 GHz with $\textrm{S11 }< - \textrm{10 dB}$, which covers the 5.8 GHz WLAN band.
Figure 23(a) gives the measured radiation patterns of the array antenna in xoz-plane and yoz-plane. The maximum radiation direction of the antenna is perpendicular to the radiation plane, and the sidelobe levels (SLL) in the yoz-plane reaches -16 dB. In the operating band of the antenna, the gain is always higher than 12 dBi, as shown in the Fig. 23(b).
The simulated and measured monostatic RCS reduction for normal incidence of x-polarized wave is plotted in Fig. 24. The monostatic RCS is effectively reduced by 6-23 dB according to the phase cancellation principle. The bistatic RCS patterns of the array antenna is similar to Fig. 6. The energy is dispersed in other directions, and the target monostatic RCS is effectively weakened.
In this section, a novel low-RCS high-gain 4 × 4 array antenna with impedance bandwidth from 5.63 GHz to 6.12 GHz with the gain of 12-15.6 dBi is proposed. The metasurface not only generates electromagnetic radiation, but also reduces the RCS of the antenna. The checkerboard shape of PCM can effectively reduce the RCS of the antenna more than 6 dB in X- and Ku-bands. The proposed array antenna has excellent properties, such as low RCS, high gain and low profile, which has the potential in 5.8 GHz WLAN wireless communication.
Table 1 lists the comparison between the previously reported low-RCS antenna and this work. Compared with Refs. [23,31–33], the proposed MIMO antenna has higher isolation and wider RCS reduction bandwidth. Compared with Refs. [35,42], the proposed array antenna has higher gain and wider RCS reduction bandwidth. The RCS reduction bandwidth of Ref. [34] is wider, but the gain is lower. References [36,37] achieve higher gain, but the antenna does not have a low profile. The MIMO antenna and array antenna designed based on the metasurface antenna in this paper maintain good size, isolation and gain while maintaining good RCS reduction characteristics.
Table 1. Comparison of proposed antenna with previous research work
5. Conclusion
In this paper, a specially designed dumbbell type UWB PCM that achieves line-to-line polarization conversion from 7.63 GHz to 18.80 GHz is proposed. Based on phase cancellation theory, a checkerboard PCM with low scattering characteristics is designed. A low-profile metasurface antenna composed of PCM and slot feed is proposed based on CMA, which realizes the integrated design of radiation and scattering. Considering the integrated design of radiation and scattering, the MIMO antenna and array antenna are cleverly proposed. The isolation of the MIMO antenna is greater than 26 dB with impedance bandwidth from 5.51 GHz to 6.06 GHz. The operating band of the array antenna covers from 5.63 GHz to 6.12 GHz with the gain of 12-15.6 dBi. The RCS reduction of the two antennas for normal incidence is between 6 dB and 23 dB in both X- and Ku-bands. The measured results of the antennas agree with the simulated results, which shows that they have potential application value in 5.8 GHz WLAN wireless communication.
Funding
National Natural Science Foundation of China (62301229); Fundamental Research Funds for the Central Universities (CCNU22JC018); Knowledge Innovation Program of Wuhan-Shuguang Project (2022010801020290); Natural Science Foundation of Hubei Province (2023AFB320); Ministry of Education Equipment Pre-research Joint Fund (8091B032227).
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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