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Abstract
In this paper, a shared aperture (SA) metasurface (MS) possesses ultra-wideband and wide-angle low-scattering performance is proposed and realized. Different from previous MS designs that mainly focus on one dimension, the SA MS design is divided into two dimensions, and the scattering performance is improved through effective integration of the two dimension design. Due to the introduction of the SA concept, a 10dB reflection magnitude reduction of MS is achieved almost from 4.8GHz to 16.4GHz (109.4%) for the incident angle below 30°. Simulated and measured results prove that our method offers an effective strategy to design high performance MS and promotes the MS for practical applications.
© 2017 Optical Society of America
1. Introduction
Metasurface (MS) is a kind of subwavelength artificial material which possesses the ability of manipulating magnitudes, phases, polarizations and propagation modes of electromagnetic (EM) waves [1–8]. These exciting and unusual features could be obtained through flexible designs and elaborate arrangements of MS elements. Thus, the MS has attracted focus research and extensive applications [9–19]. Recently, with the development of the stealth technology, achieving reflection suppression is highly desirable.
There are three main manners of the MS to achieve reflection suppression. The first one is EM invisible cloaks which make the incident waves transmit through the MS without reflection [20, 21]. The second one is radar absorber [22, 23] which transforms the EM waves energy into heat energy and reduces the reflection. The last one is reflective MS which redirects the incident EM waves to non-threatening space [24–26]. This method takes advantage of the reflection and provides more flexibility of designing low-scattering MS. For instance, some information science concepts could be utilized to design digital, coding, programmable and memory MS [27–30]. Moreover, optimization algorithm could also be applied to the MS design [26, 31]. These epoch-making reflective MS could obtain low-scattering performance in a wide frequency and spacial ranges. For the application requirement, a wider and lower reflection suppression frequency band is still urgently needed. However, it still remains a challenge for the MS to further broaden the bandwidth of reflection suppression, especially toward low frequency band and achieve 10dB magnitude reduction. In the recent report [25], two dual band MS elements are utilized to construct the MS and dual wideband rather than ultra-wideband 10dB magnitude reduction is obtained.
In order to overcome the above difficulties, shared aperture (SA) concept is introduced to design the MS. The SA concept is recently introduced in the field of phased array antennas for radar applications [32–34]. The advantage of the SA concept is that enabling the phased array antennas to operate at different frequency bands, polarizations, scanning directions in a common physical area. These capabilities are ascribed to sub-arrays of elementary radiators and then constitute the complete aperture of the phased array in vertical and horizontal dimensions. That is to say, a same sized physical area could obtain more interesting properties. Thus, the SA concept is a promising approach to improve the MS performance [35]. Unfortunately, up to now, there are few reports about utilizing SA concept to design the MS.
In this paper, different from previous MS design which mainly focuses on one dimension, the MS design is divided into two dimensions (vertical dimension and horizontal dimension) for the introduction of SA concept. In the vertical dimension, reflection suppression bandwidth could be broadened based on MS elements design. Air substrate is added to produce dual band MS elements and two different kinds of sub-tiles are formed. Therefore, the effective phase difference region (180° ± 30°) is broadened and a wider low-scattering bandwidth is expected. In the horizontal dimension, further decreasing reflection magnitude in ultra-wideband frequency ranges and wide-angle spatial ranges is the main goal. The MS consists of two kinds of sub-tiles. Each sub-tile possesses different operation band and reflection properties which are similar to the SA array antennas with different working band and radiation properties [33, 34]. The difference is that, based on different design goals, the SA array antennas radiate EM waves without interference while the sub-tiles reflect EM waves with interference. Optimization arrangement of the sub-tiles is utilized to decrease the reflection magnitude and obtain 10dB magnitude reduction. Meanwhile, the reflection suppression angle is also extended [36, 37]. As a result, ultra-wideband and wide-angle 10dB reflection magnitude reduction is achieved through effective integration of vertical and horizontal dimensions design.
2. Design of shared aperture metasurface
2.1 Design and analysis in the vertical dimension
According to the published literatures [38] and the authors previous works, square patch was selected as basic MS elements to construct the MS for its simple structure and wideband characteristics. As Figs. 1(a) and 1(b) present, single-layer reference elements are designed firstly. The designed square patches are etched on the top side of dielectric substrate, and metallic ground plane is installed on the bottom side to guarantee absolute reflection. Polytef slab (FR2) is adopted as the dielectric substrate (ε = 2.65, tanδ = 0.002). Then, an air substrate is added between dielectric substrate and ground plane. The square patches are painted on both sides of the dielectric substrate, shown in Figs. 1(c) and 1(d). In order to obtain desired characteristics, all parameters of the elements are optimized. For validating our design method, the reference elements are taken as a comparison. The element with larger scale structure is denoted as E1 and the other one is denoted as E2.
Fig. 1 Schematic geometry of (a) reference element E1, (b) reference element E2, (c) proposed element E1 and (d) proposed element E2. Brassy yellow indicates copper and blue indicates dielectric substrate. Design parameters of the elements are LP = 9.0mm, L1 = 7.34mm, L2 = 0.5mm, L3 = 4.7mm, L4 = 8.7mm, L5 = 1.9mm, L6 = 5.8 mm, t1 = 3.0mm, t2 = 3.0mm. (e) Reflection phases of the reference and proposed elements.
In order to investigate reflection characteristics of the four different elements, full-wave numerical analysis was carried out in the commercial full-wave simulation software Ansoft HFSS using a unitary cell with master-slave periodic boundary conditions and floquent port excitation. Reflection phase comparison of the four elements is presented in Fig. 1(e). The reference element E1 and E2 own single 0° reflection phase at 7.3GHz and 16.4GHz, respectively, while the proposed element E1 and E2 possess dual 0° reflection phase. Moreover, the lowest 0° reflection phase frequency shifts toward low frequency ranges. As Fig. 1(e) depicts, the proposed E1 possesses 0° reflection phase at 4.11GHz and 12.63GHz and the proposed E2 exhibits 0° reflection phase at 7.04GHz and 18.73GHz. Furthermore, the reflection phase bounding frequency of proposed E1 (E2) close to the 0° reflection phase frequency of proposed E2 (E1), which satisfies the condition to broaden the effective reflection phase difference region [16]. The reflection phase difference is also shown in Fig. 1(e). The phase difference of the proposed units stays in the range of 180° ± 30° almost from 4.5GHz to 17.5GHz, which is wider than that of the reference one. An ultra-wideband low-scattering performance is expected.
For revealing operation mechanism of proposed elements, current distributions of these elements at 0° reflection phase frequency were investigated. Due to the two proposed elements owning the same geometrical patterns, element E1 is taken as an example. The current distributions at 4.2GHz are presented in Fig. 2(a). It can be observed that the same current directions on top surface patch and middle surface patch are observed, and opposite current directions are found on ground plane. Strong current intensity (as arrow length shown) is emerged on middle surface patch and ground plane, indicating that a resonance is produced between them. Thus, the first 0° reflection phase frequency is mainly controlled by the middle surface patch. Figure 2(b) presents the current distributions at 12.3GHz. It shows that the same current direction is emerged on top surface patch and ground plane. Furthermore, the current intensity of the former is stronger than that of the latter. The reversed current direction and strong current intensity are observed on middle surface patch. Therefore, a resonance is produced between the top surface patch and middle surface patch, implying that the second 0° reflection phase frequency is commanded by them.
Fig. 2 Current distributions of the proposed element E1 at (a) 4.2GHz and (b) 12.3GHz. Reflection phases of proposed element E1 for (c) different L3 and (d) different L4.
In order to validate the above analysis, the reflection phase characteristics of the proposed E1 for different L3 and L4 are investigated. As Fig. 2(c) presents, with the increase of L3, the 0° reflection phase at high frequency shifts from 13.4GHz to 11.2GHz, while the 0° reflection phase at low frequency maintains well. The investigation of L4 is shown in Fig. 2(d). It can be seen that both the two 0° reflection phase frequencies skew downward. The variation range at low frequency is larger than that at high frequency. The parameters analysis results agree well with the current analysis results. Therefore, for obtaining the desired reflection phases, the middle surface patch could be adjusted firstly and then restructure the top surface patch.
2.2 Design and analysis in the horizontal dimension
As for two MS elements, the famous chessboard arrangement was widely used to obtain backward low-scattering performance. Based on reflection phase cancellation principle, 10dB reflection magnitude reduction will be achieved when the phase difference satisfies the condition [16, 17]
Generally, 180° ± 30° is set as a criterion. Nevertheless, as Fig. 1(e) presents, the reflection phases exceed phase difference region from 8.2GHz to 10.8GHz and the maximum value of the phase difference reaches 225.1° at 9.5GHz. According to the above theoretical conclusions, it is difficult to obtain 10dB magnitude reduction. Therefore, the functionality of horizontal dimension is to further decrease reflection magnitude and maintain the 10dB magnitude reduction.
Inspired by Ref [27, 31, 39], optimization arrangement methods are utilized to design the MS and smaller reflection magnitude is obtained. Thus, the arrangement of the MS in this paper is implemented using an optimization algorithm. For a normal incident x- or y-polarized plane wave, the total scattering electric field of the MS consisting of square patches could be expressed as [31]
where Amn, αmn are the illuminating magnitude and phase, Гmn, ϕmn are the reflection magnitude and phase, fmn(θ, φ) is the scattering pattern, θmn, φmn are the elevation and azimuth angles of source relative to unit cell, dx and dy represent the periodicity in x and y directions, respectively.In order to preserve the phase characteristics of elements, element tile that consists of 5 × 5 elements is treated as the unit cell. Therefore, dx and dy represent the element tile periodicity in x and y directions. The reflection will be greatly suppressed as the incident waves redirecting to numerous directions. It is a promising method to further decrease the reflection magnitude. In this paper, Genetic algorithm (GA) is utilized to optimize reflection phase arrangement (namely the MS arrangement) at 9.5GHz (the maximal phase difference frequency). In order to save the calculation time, a 2D inverse discrete Fourier transform (IDFT) is introduced [4, 31] and then Eq. (2) is deduced as
The cost function for a diffusion arrangement metasurface can be expressed asThe corresponding scattering performance is simulated and evaluated by minimizing the cost function. Therefore, the element tiles are arranged based on the simulated results and diffusion scattering of the MS is expected. In order to obtain optimal results in the entire frequency ranges, multiple different arrangements of the MS are full-wave simulated in Ansys HFSS and the best one is selected, shown in Fig. 3(a).Fig. 3 (a) The schematic layout of the MS. Simulated reflection reduction for (b) x polarization and (c) y polarization. 3D scattering patterns comparison of the MS and same sized metallic plane at 9.5GHz for (d) 0° and (e) 30° incident waves.
For demonstrating the effectiveness of the above optimized arrangement, scattering performance of the MS is investigated for x- and y-polarized incidence, shown in Figs. 3(b) and 3(c). For the normal incidence, it can be seen that remarkable reflection suppression is achieved from 3.0GHz to 20.0GHz, with a peak magnitude reduction of 30.5dB. The 10dB magnitude reduction is obtained almost from 4.7GHz to 17.5GHz for x polarization and from 4.1GHz to 17.1GHz for y polarization, inferring 115.3% and 122.6% relative bandwidth, respectively. The relative bandwidth is calculated with the following formula
where the fH and fL are the high and low frequencies of 10dB reflection magnitude reduction band, respectively. The low-scattering bandwidth is broadened and shifted toward low frequency ranges. Moreover, the reflection magnitude is decreased and 10dB magnitude reduction bandwidth is greatly expanded. It also can be observed that the reduction values at those frequencies close to 9.5GHz are below 10dB, verifying the effectiveness of the optimized arrangement. For the oblique incident waves, the simulated reflection magnitude in specular direction is also illustrated in Figs. 3(b) and 3(c). Although the deterioration of magnitude reduction is observed close to 3.0GHz, the ultra-wideband reflection suppression properties maintain well. Moreover, the specular 10dB magnitude reduction is obtained almost from 5.1GHz to 16.8GHz (x polarization) and from 4.8GHz to 17.1GHz (y polarization) for incident angles below 30°, implying 106.9% and 112.3% relative bandwidth, respectively. Thus, specular reflection could be effectively suppressed and ultra-wideband and wide-angle low-scattering performance is achieved.In order to reveal the reason for the further decreasing of reflection magnitude, the 3D scattering patterns of proposed MS and same sized metallic plane at 9.5GHz for normal incidence are investigated and presented in Fig. 3(d). It can be observed that, under x or y polarization, the strong reflection is distributed more evenly in the entire space. Unlike the chessboard arrangement MS which redirects incident waves to diagonal directions, this MS redirects the normal incidence to numerous directions. Thus, the reflection magnitude is decreased more deeply. Taking a further step, for the 30° oblique incidence along the yoz plane with different polarizations, the comparison of 3D scattering patterns at 9.5GHz is shown in Fig. 3(e). Although the incident angle increases to 30°, the MS also possesses the ability of redirecting the intercepted waves to numerous directions. The specular reflection is effectively suppressed in wide angles.
3. Fabrication and measurement
To demonstrate the low-scattering performance mentioned above, a proposed metasurface sample was fabricated by using common printed circuit board technique, shown in Fig. 4(a), and measured in the anechoic chamber. Five Nylon spacers (four spacers in the corner area and one spacer in the central area) are utilized to support the dielectric substrate above the ground plane, and then the air substrate is created. In practice, the dielectric substrate could be just placed upon the metallic plane and the whole MS weight is reduced. Moreover, the existence of air substrate provides more flexibility in designs. For example, the distance between dielectric substrate and metallic plane could be randomly adjusted and the MS possesses some different interesting characteristics.
Fig. 4 (a) Fabricated shared aperture MS. (b) The basic measurement setup. The reflection reduction for (c) horizontal (x in simulation) polarization and (d) vertical (y in simulation) polarization for 0° and 30° incident waves.
The basic setup of measurement is shown in Fig. 4(b). The sample is placed vertically on a foam platform. Two identical broadband horn antennas are utilized as transmitting and receiving devices, respectively. Both of the antennas are initially set as horizontal polarization (x polarization in simulation, referring to the coordinate axes) and connected to Agilent vector network analyzer N5230C to evaluate the reflection behavior. A piece of absorbing material is set between the antennas to reduce undesired coupling. The height of the sample is kept the same with the antennas and the distance is far enough to satisfy the far field measurement requirement. Moreover, the time-domain gating is applied for filtering out any undesired reflection, only the reflections coming from the sample are taken into account. Due to the restrictions of test condition, the reflectance of proposed metasurface is not measured in the frequency ranges higher than 18.0 GHz.
The measured scattering performance is presented in Figs. 4(c) and 4(d). For comparison, the simulated results are also plotted. It can be seen that the measured results agree with the simulated ones. The small discrepancies are attributed to the fabrication and the assembly. In our measurements, the reflection is suppressed from 3.0GHz to 18.0GHz with a peak reduction of 36.2dB. Moreover, for the normal incidence, the 10dB magnitude reduction is obtained from 4.8GHz to 17.5GHz for horizontal (x in simulation) polarization and from 4.4GHz to 17.2GHz for vertical (y in simulation) polarization, implying 113.9% and 118.5% relative bandwidth, respectively. Meanwhile, for the 30° incident waves, the specular 10dB magnitude reduction is achieved from 4.8GHz to 16.4GHz for horizontal polarization and from 4.9GHz to 16.8GHz for vertical polarization, implying 109.4% and 109.7% relative bandwidth, respectively. From the above results, ultra-wideband and wide-angle low-scattering performance of the MS is verified.
4. Summary
In this paper, a novel design method of introducing shared aperture concept to metasurface design for achieving ultra-wideband and wide-angle low-scattering performance was validated both analytically and experimentally. The MS design was divided into vertical and horizontal dimensions. At first, in the vertical dimension, an air substrate was added and a wider effective phase difference region was obtained. Then, in horizontal dimension, optimization arrangement of the MS was implemented and the reflection magnitude was further decreased in frequency and special ranges. Experiment results show that, compared to the same sized metallic plane, the reflection suppression was achieved from 3.0GHz to 18.0GHz with a peak reduction value of 36.2 dB. Moreover, a 109.4% 10dB magnitude reduction bandwidth was inquired for the incident angles below 30°. It is worth noting that the MS could be composed by other geometric MS elements and could be applied to stealth technology. Additionally, due to the great superiority of improving MS scattering performance, this work will attract more interest on the AS concept for more potential designs and applications.
Funding
National Natural Science Foundation of China (61501494, 61671464, 61271100); Doctoral Foundation of Air Force Engineering University (KGD08091601, KGD08091502).
Acknowledgments
Yuejun Zheng, Xiangyu Cao and Jun Gao contribute equally to this work.
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