ZHENG Yuejun, DING Liang, CHEN Qiang, GUO Min, and FU Yunqi

College of Electronic Science and Technology, National University of Defense Technology, Changsha 410073, China

Abstract: The electromagnetic surface antenna array (EMSAA)has been proposed for obtaining reflection suppression and excellent radiation simultaneously. The antenna with rectangular radiation patch is used to design anisotropic electromagnetic surface. Preternatural reflection characteristics of the element antenna can be tailored depending on the incident polarizations.EMSAA can be constructed by using single structured element antenna with 90° rotation and orthometric arrangement. This orthometric arrangement of EMSAA is helpful to achieve reflection suppression and excellent radiation. The simulated results show that the reflection of EMSAA is suppressed from 5.0 GHz to 8.0 GHz with peak reduction of 12.3 dB. The linear- and circularpolarized radiation properties of EMSAA are obtained and the maximum gain is 14.3 dBi. The measured results are consistent with the simulation results. The results demonstrate that the reflection suppression and excellent radiation are achieved simultaneously. Such design of EMSAA will open the path for integrating antenna fields and electromagnetic surface (EMS) fields.

Keywords: electromagnetic surface (EMS), antenna array, radiation, reflection.

1. Introduction

Electromagnetic surfaces (EMSs) are two-dimensional planar artificial surfaces with rationally designed shapes,sizes, and compositions. The EMSs provide exceptional capabilities for manipulating the magnitudes, phases, polarizations of electromagnetic waves [1-4]. Thus, the EMSs attract focus investigations and extensive applications [5-8], such as reflection suppression [7,8]. Recently, various techniques have been reported to suppress reflection by utilizing the EMSs [9-17], among which a representative method was constructing artificial magnetic surface by combining the surface structure with perfect electric surface in a chessboard configuration [9].This method takes advantage of the reflection and provides flexibility to suppress reflection. Subsequently,more and more excellent works were reported to obtain wideband reflection suppression [10-15] or multifunctional performance [16,17].

As a promising application, this kind of reflective EMS is applied to suppress reflection of a single antenna[18-20] or an antenna array [21-26]. This method is useful for the single antenna. Although the reflection of the antenna array is suppressed by utilizing this method, challenges still exist. For example, the design of the EMS and the antenna array is divided. The effect on radiation and reflection suppression is uncertain. Moreover, the vacant area of the antenna array is limited. The characteristics of EMS is influenced because of the restricted units. As a result, the performance of the antenna array is influenced.The radiation and the reflection suppression of the antenna array are difficult to control simultaneously.

To overcome the above challenges, the concept of EMS is introduced into antenna design. The antenna with rectangular radiation patch is utilized to form anisotropic EMS. The antenna has preternatural reflection characteristics and good radiation properties simultaneously. Two different antenna elements are used to form the antenna array [27,28]. Good radiation properties and reflection suppression are obtained simultaneously. This is a novel way to design antennas and overcome the challenges. In the above designs, two different antenna elements are utilized. However, in some applications only one kind of antenna element can be used, and multiple polarizations are required. Based on the excellent works, the anisotropic structures of antenna array are utilized [29]. As a result, the circular-polarized radiation and reflection suppression properties are obtained, respectively. However,the radiation area is reduced and the gain is influenced by using the split ring resonators. Moreover, the reflection is suppressed when the feeding of the antenna array is opencircuited. The radiation and reflection suppression of the antenna array cannot be realized simultaneously. To enhance the gain, a novel loading method and varactors are used [30].

In this paper, the rectangular radiation patch of the element antenna is utilized, which is beneficial to construct an anisotropic EMS. The electromagnetic surface antenna array (EMSAA) is formed by using single structured element antenna with 90° rotation and orthometric arrangement [7,29]. This orthometric arrangement of EMSAA is helpful to achieve reflection suppression and excellent radiation. For using rectangular radiation patch,the gain of EMSAA is maintained. Meanwhile, the linearand circular-polarized radiation properties are achieved.Moreover, the reflection is suppressed when the feeding of the antenna array is well matched. Thus, the reflection suppression and excellent radiation of EMSAA are achieved simultaneously.

2. Design of EMSAA element

According to the above analysis, the patch antenna with rectangular radiation patch is selected as basic elements to construct EMSAA. As Fig. 1 shows, the designed rectangular radiation patch is etched on the top surface of dielectric substrate. The metallic ground plane is installed on the bottom surface to guarantee absolute reflection. Polytef dielectric slab (F4B) is adopted as the substrate (ε=2.65, tanδ=0.002). The elements of EMSAA are fed with 50 Ω coaxial probes for impedance matching.The optimized design parameters areP=22.4,t=3.0,L=11.8,W=14.8,lf=3.7, units: mm. For easy of description, the element with coaxial probe skewingx-axis is denoted as E1 and the other one is denoted as E2. E2 is obtained by rotating 90° of E1. E1 and E2 own the same structures and parameters, so E1 and E2 are the same element.

Fig. 1 Schematic geometry of the EMSAA elements

Because the concept of EMS is introduced into antenna design, radiation and reflection properties of the elements are taken into consideration simultaneously. For radiation properties, the basic element is designed based on the empirical formula of patch antenna design. For reflection properties, the basic element is designed based on the phase cancellation principle. According to the array theory, the total scattering electric field can be given as

whereEr1,φr1andEr2,φr2are the magnitudes and phases of electric field reflected by E1 and E2, respectively. For the scattering electric fieldthe reflection suppression is produced when the phase difference satisfies the following condition [11,14]:

In this design, the phase difference condition of the asymmetric element can be represented as

whereφrxandφryare the phases of the electric field reflected by the elements forxandypolarized incidence, respectively. From the above analysis, the radiation and reflection properties of the elements could be achieved simultaneously.

For demonstrating the above design mechanism, the element antenna E1 is taken as an example. Full-wave numerical analysis is carried out in the Ansoft HFSS. The radiation boundary condition and lumped port excitation are used to achieve radiation properties. Meanwhile, the master-slave periodic boundary condition and floquet port excitation are utilized to obtain reflection properties.To acquire the desired radiation and reflection properties,all parameters of the element antenna are optimized.

The radiation properties of the element antenna E1 are shown in Fig. 2. The E1 resonates at 6.76 GHz and good impedance matching is achieved. The radiation patterns at 6.76 GHz are depicted in Fig. 2(b). The 2D radiation patterns show that main-beam patterns are all along normal direction and the maximum gain is 7.1 dBi. It can be concluded that the element antenna of EMSAA achieves good radiation properties.

The reflection characteristics of the element antenna E1 are shown in Fig. 3. As Fig. 3(a) plots, the reflection magnitudes are decreased forx-polarized incidence whilemaintained fory-polarized incidence. Thex-polarized waves are partially absorbed from 5.0 GHz to 8.0 GHz.The 0° reflection phases at 6.65 GHz (x-pol) and 5.2 GHz(y-pol) are achieved, respectively. Phase difference between 150° and 210° is obtained from 5.4 GHz to 6.7 GHz,shown in Fig. 3(b). From the above analysis, reflection suppression of EMSAA is expected based on phase cancellation and the absorption principle.

Fig. 3 Reflection properties of EMSAA elements

3. Analysis and performance of EMSAA

As for the asymmetric EMSAA elements, the reflection suppression will be influenced by the arrangement. The grated and orthometric arrangements are studied for obtaining better reflection suppression. The grated and orthometric arrangements shown in Fig. 4(a) and Fig. 4(c)are arranged with a single element while in Fig. 4(b) and Fig. 4(d) are ordered with the element tile. For easy of description, the four arrangements of EMSAA are denoted as EMSAA1, EMSAA2, EMSAA3 and EMSAA4, respectively.

Fig. 4 Different arrangements of EMSAA

The reflection properties of the four different EMSAAs are investigated and the results are shown in Fig. 5.For the grated arrangements, the reflection suppression of EMSAA2 is better than that of EMSAA1 forx-polarized incidence while the reflection suppression of EMSAA1 is better than that of EMSAA2 fory-polarized incidence.For the orthometric arrangements, the reflection suppression of EMSAA4 is better than that of EMSAA3 forxandy-polarized incidences. Considering bothx- andy-polarized incidences, it can be concluded that better reflection suppressions can be achieved by arranging EMSAA with the element tile. The reflection suppressions of EMSAA2 and EMSAA4 are almost the same. Thus, EMSAA2 and EMSAA4 are selected as the candidates. Referring to previous works [7,29], EMSAA4 possesses the ability to radiate circular polarization waves when the four element tiles rotate with a step 90° phase difference one by one along the anticlockwise. In this design, both radiation and scattering properties are taken into consideration. Thus, the orthometric arrangement of EMSAA4 is selected as the final arrangement of EMSAA. For easy description, each element of EMSAA is coded. As Fig. 4(d)shows, the coding order is from -xto +xand from -yto +y.

Fig. 5 Reflection properties of EMSAA

Because the EMSAA has a symmetrical structure, the reflection suppression properties of the EMSAA underxpolarized incidence are presented in Fig. 6. Because EMSAA can be treated as the EMS, a same sized metallic plane is taken as a comparison. For normal incidence, remarkable reflection suppression is achieved from 5.0 GHz to 8.0 GHz. The operation band of EMSAA is covered and in-band reflection suppression is obtained. The peak reflection suppression reaches 12.3 dB.

A 3D reflection patterns comparison of the EMSAA and the same size metallic plane is plotted in Fig. 7. For normal incidence, the strong backward scattering is redirected to four quadrants and the scattering power is weakened at 5.55 GHz, shown in Fig. 7(a) and Fig. 7(b).It indicates that reflection suppression at 5.55 GHz is achieved based on the phase cancellation principle and the absorption principle. As Fig. 7(c) and Fig. 7(d) present, planar pattern along normal direction is observed at 6.76 GHz, and the scattering power is also weakened. It infers that reflection suppression at 6.76 GHz is mainly obtained for the absorption of impedance matching. From the above analysis, it can be concluded that the EMSAA owns good reflection suppression properties.

Fig. 6 Reflection suppression of EMSAA for different incident angles

Fig. 7 3D reflection patterns

Since the elements E1 and E2 can be fed independently, the radiation properties are analyzed based on the feeding solution. Firstly, only the E1 elements are fed and the feeding phase is 0°. The radiation properties of EMSAA in this feeding solution are investigated and shown in Fig. 8. To describe reflection coefficients |S11| of EMSAA, the diagonal elements of EMSAA are selected(E11, E14, E15, E18). As Fig. 8(a) presents, the elements resonate close to 6.76 GHz and good impedance matching is achieved. The tiny difference is caused by the coupling of the EMSAA elements. The 2D radiation patterns at the resonant frequency are depicted in Fig. 8(b).The main-beam patterns are all along the normal direction and the gain reaches 14.4 dBi. The maximum crosspolarization level value is 24.0 dB lower than that of the main-polarization. Electric field distributions are observed on the 8λ6.76GHzsurface above the EMSAA. As Fig. 8(c) presents, the direction of electric field is along thex-axis direction. Thus, it can be concluded that the EMSAA possesses good radiation properties and radiatesx-polarized waves. From the above analysis, it infers that EMSAA also own the ability to radiatey-polarized waves when E2 elements are fed and the feeding phase is 0°.

Fig. 8 x-polarized radiation properties

Take a further step, the E1 and E2 elements are both fed. The feeding phases of E1 and E2 are 90° and 0°, respectively. The radiation properties of EMSAA in this feeding solution are investigated and shown in Fig. 9.

Fig. 9 Right-circular-polarized radiation properties

To describe the reflection coefficients |S22| of EMSAA,the diagonal elements of EMSAA are selected (E22, E23,E26, E27). As Fig. 9(a) depicts, the elements resonate close to 6.76 GHz and good impedance matching is achieved. The 2D radiation patterns of EMSAA at the resonant frequency are presented in Fig. 9(b). The mainbeam patterns are all along the normal direction and the maximum gain is 14.3 dBi. The maximum cross-polarization level value is 20.3 dB lower than that of the main-polarization. The axial ratio of the EMSAA is 0.24 dB in the operation band, shown in Fig. 9(c). Electric field distributions are observed on the 8λ6.76GHzsurface above the EMSAA at four different moments in a period. As Fig. 9(d)presents, the direction of the electric field rotates anticlockwise, that is, following the right-hand law. From the above analysis, it can be concluded that the EMSAA possesses good radiation properties and radiates right circular polarized waves. It also can be inferred that EMSAA radiates left circular polarized waves when the E1 and E2 are both fed and feeding phases of E1 and E2 are 0° and 90°, respectively. Moreover, the EMSAA also possesses the potential to radiate elliptical-polarized waves.

Take a further step, the radiation patterns comparison of the EMSAA element and the EMSAA are presented in Fig. 10.

Fig. 10 Radiation patterns comparison of EMSAA element and EMSAA

The gains of the linear-polarized EMSAA and circularpolarized EMSAA are almost the same. Compared to the EMSAA element, the gain of EMSAA is enhanced by 7.2 dB. It can be concluded that the EMSAA owns excellent radiation properties to radiate high gain and multipolarized waves.

4. Fabrication and measurement

To further validate the design method, the proposed EMSAA is fabricated by using the standard printed circuit board (PCB) technology. |S11| is measured by using a vector network analyzer (VNA) Agilent N5230C, and the radiation patterns and the axial ratio are measured in the anechoic chamber.

For the first case, the E1 elements of EMSAA are fed by the 1/8 power divider (RS8W2080-S of REBES) and the same size connecting lines with steady phase and low loss are utilized as phase shifter. Meanwhile, the E2 elements are terminated with matched load, plotted in Fig. 11(a). |S11| of diagonal E1 elements (E11, E14, E15,E18) is measured and shown in Fig. 11(b). The elements resonate close to 7.0 GHz and good impedance matching is achieved. The measured resonant frequency is 0.24 GHz higher than the simulated one, which may be caused by the fabrications. The 2D radiation patterns of EMSAA at the resonant frequency is depicted in Fig. 11(c).

Fig. 11 x-polarized measurement configuration and radiation properties of EMSAA

The main-beam patterns are all along the normal direction and the measured side lobe level values are almost 12.7 dB lower than those of the main lobe level. In addition, the measured results of cross-polarization are all less than -20.6 dB.

For the second case, a 1/2 power divider and two 1/8 power dividers are utilized to feed elements of EMSAA,depicted in Fig. 12(a). To distinguish the feeding phase,two kinds of connecting lines are used. The feeding phase of short connecting lines is 90° ahead of the feeding phase of long connecting lines. As Fig. 12(b) shows, diagonal elements of EMSAA are selected (E22, E23, E26,E27). The elements resonate close to 7.0 GHz and good impedance matching is achieved. The axial ratio of EMSAA is presented in Fig. 12(c), the values of the axial ratio are all less than 3 dB in the operation band while higher than the simulated results. The difference may be caused by bending of connecting lines. As Fig. 12(d)shows, the main-beam patterns are all along normal direction and the measured side lobe level values are 13.9 dB lower than those of the main lobe level. In addition, the measured cross-polarization level values are all less than-19.3 dB.

Fig. 12 Right-circular-polarized measurement configuration and radiation properties of EMSAA

As Fig. 13(a) shows, EMSAA is placed vertically on a foam platform to measure reflection suppression properties. Two horn antennas are initially set as horizontal polarization (xpolarization in simulation, referring to the coordinate axis). A piece of absorbing material is set between the antennas to reduce undesired coupling. The reflection suppression properties for normal and oblique incident waves are presented in Fig. 13(b). The measured results are consistent with the simulation results. The re-flection is suppressed from 5.0 GHz to 8.0 GHz with peak reduction of 14.0 dB, implying 46.2% relative bandwidth. The specular 6 dB reflection suppression is obtained from 5.65 GHz to 7.25 GHz for incident angle below 30°. From the above results, EMSAA possesses good reflection suppression performance. It can be concluded that EMSAA achieves reflection suppression and excellent radiation properties simultaneously.

Fig. 13 Reflection measurement configuration and results of EMSAA

Table 1 presents the properties comparison of this paper and the previous articles. For linear polarization, only 8 units of EMSAA work in this paper and [27], while 16 units work in [28] and [30]. For circular polarization,only 16 units of EMSAA work in this paper while 64 units work in [29]. It can be concluded that this work possesses the ability to radiate high gain linear and circular polarization waves. Also, the reflection suppression bandwidth is relatively wide. Same as [27], [28] and [30],the reflection suppression and excellent radiation achieve simultaneously in this paper.

Table 1 Properties comparison of this paper and previous articles

5. Conclusions

In this paper, a novel design of EMSAA with reflection suppression and excellent radiation performance is proposed. The element antenna possesses in-phase reflection characteristics and good radiation properties. The EMSAA is formed by using single structured element antenna with 90° rotation and orthometric arrangement.Simulation and experiment results show that linear- and circular-polarized radiation and wideband reflection suppression of EMSAA are achieved simultaneously. It is worth noting that the EMSAA owns the potential to radiate elliptical-polarized waves and this work offers a further strategy to solve the confliction between radiation and scattering of the antenna array.