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Abstract
An improved dual-polarized multifunctional switchable absorber/reflector with both wideband absorbing and wideband reflecting characteristics is presented in this paper. The proposed structure consists of three parts including a top-layer active frequency selective surface (AFSS) structure, a bottom-layer metal sheet and an air spacer in between. The polarization stability is satisfied by deploying the super-element topology, which contains four similar unitary elements arranged in a 2 × 2 matrix form. The PIN diode is employed as a RF switch in the AFSS structure for the purpose of switching. The bias networks responsible for different polarizations are intentionally separated through via holes. Multifunctional properties with four different operating states can be attained by controlling horizontally- and vertically-loaded PIN diodes independently. In addition, the biggest advantage of the proposed structure lies in its wideband features at both absorbing and reflecting states for different polarizations and incident angles. Finally, a prototype of the design is fabricated and measured to verify the simulation, and a good agreement between the simulated and observational results can be achieved under normal incidence as well as oblique incidence.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Electromagnetic absorbers are extensively applied in a variety of applications such as radar cross section (RCS) reduction, electromagnetic interference/compatibility (EMI/EMC) and electromagnetic protection (EMP) [1]. The Salisbury screen, comprising purely resistive sheets, is a microwave absorber that contains one resistive sheet separated by a dielectric slab of λ/4 thickness in front of a ground plane [2]. The Jaumann absorber is an extension of the Salisbury screen by using two or more homogeneous resistive surfaces to increase the absorption bandwidth at the expense of higher thickness [3,4]. Unlike the two classic types of absorbers mentioned above, the circuit analog (CA) absorber is characterized by using not only resistive but also reactive components, providing an effective solution to the problems of narrow bandwidth and bulky thickness [5,6]. The CA absorber is accomplished by employing lossy periodic structures such as frequency selective surfaces (FSSs) loaded with lumped resistors. Although FSS-based absorbers show better absorbing property with lower profile, they usually offer no flexibility once built. On the contrary, absorbers based on active frequency selective surfaces (AFSSs) allow actively modulating the operating frequency and absorbing intensity by controlling external excitations [7–13].
In order to adapt to the complex EM environment, the switchable absorber/reflector, which can realize the target scintillation by reducing RCS at the absorbing state and enhancing reflectivity at the reflecting state, is highly demanded in stealth systems. In recent years, some switchable absorber/reflector designs have already been reported [14–20]. A single-polarized switchable absorber/reflector, which can switch the function between a perfect absorber and a reflector, has been presented in [14]. To deal with the problem of polarization stability, a single-band switchable absorber/reflector for two perpendicular polarizations has been designed by using orthogonally oriented structures [15]. However, most switchable absorber/reflector designs suffer from narrow switchable bandwidth, and only several wideband switchable absorber/reflector have been reported till now. In [16], a switchable absorber/reflector with wideband properties has been designed by employing nested multi-split-ring structures. Although the switchable bandwidth is broadened compared to former studies, the reflection properties are undesirable. A switchable absorber/reflector with both wideband absorbing and reflecting bandwidths has been given in [17], but its performance deteriorates at oblique incidence. Besides, the wideband structures mentioned above only respond to single polarization so that their practical employments are limited. In [18], a polarization-insensitive wideband switchable absorber/reflector has been synthesized by incorporating lumped resistors and an air spacer in a single-band switchable absorber/reflector based on square loops. Nonetheless, two main open issues remain to be improved: the reflection coefficients at ON state and the angular stability (wideband absorbing performance at OFF state only maintains stable for the TE mode when the incident angle is less than 15°). A wideband switchable absorber/reflector with polarization independence has been introduced in [19]. Although both the absorption and reflection bandwidths are satisfactory, the proposed design only functions well under small incident angles. The absorption bandwidth shrinks greatly and meanwhile noticeable ripples can be observed once the incident angle is larger than 15°. In [20], a dual-polarized wideband multifunctional switchable absorber/reflector with additional polarization selection function has been provided, but the presence of lumped resistors connected in series with the PIN diodes in the structure brings about ohmic losses at the reflecting state. As a consequence, the reflection bandwidth at the reflecting state is narrower than the absorbing bandwidth, leading to a limited switchable bandwidth.
In this paper, a multifunctional switchable absorber/reflector with dual-polarized and wideband features has been developed. The proposed structure comprises an AFSS layer and a metal plane, separated by an intermediate air spacer. To ensure the polarization stability, the AFSS structure consists of an assembly of periodically arranged super-elements. Four unitary elements are placed in a 2 × 2 matrix form to form the super-element, where any two adjacent unitary elements are 90° rotational symmetric and any centrosymmetric ones are identical. The switching function is realized by employing PIN diodes, and two neighboring unitary elements are connected by a lumped inductor to provide dc bias for the PIN diodes in a row. Besides, horizontal and vertical structures are geometrically separated so that the PIN diodes loaded in different directions can be controlled independently, which introduces more polarization information and finally integrates four working states together (e. g. dual-polarized absorption, dual-polarized reflection, TE absorption/TM reflection and TE reflection/TM absorption). Wideband absorbing capability at OFF state is easily achieved by utilizing lumped resistors and optimizing air spacer thickness. Meanwhile, it is worth mentioning that each PIN diode is connected in parallel with a lumped resistor, and the shunt resistor will be shorted when the diode is switched on. This arrangement of PIN diodes and resistors in the structure allows minimizing the ohmic dissipation at ON state, leading to an excellent reflective performance within the entire band.
A comprehensive comparison has been conducted in Table 1 to illustrate the advantages of the proposed design with respect to the wideband absorber/reflector designs previously reported. Some critical parameters, including the absorption bandwidth, the reflection bandwidth, the realization of the polarization selection function, the polarization stability, the angle stability and the thickness compared to wavelength, are highlighted in the table. Considering the biasing network of active components, the issues of polarization stability is always a challenging issue in designing switchable absorber/absorber structures. As it can be observed from Table 1, the designs in [16] and [17] are limited to only one polarization applications. Beside, the structures in [18] and [19] suffer from the problem of angle stability and their performances maintain well under a small incident angle of 15°. Although the multifunctional absorber/reflector presented in [20] possesses better angle and polarization stability, its reflection bandwidth is not that ideal compared to those in [17–19] which achieve reflection properties in the entire operating band. Compared to many other studies on the absorber/reflector previously reported, the proposed design in this paper possesses competitive features of multifunction integration, polarization insensitivity, angular stability, and in particular, wideband characteristics at both reflecting and absorbing states within the entire target frequency band.
Table 1. Comparison between the Proposed Structure and Other Designs
The rest of the paper is organized as follows. A detailed description of the proposed structure is presented in Section II, including the simulated results, the equivalent circuit model and the parametric analysis. Then, a prototype is fabricated and measured to validate the simulated results in Section III, and the measurement uncertainty is also provided evaluate the experimental ones. Finally, a conclusion is drawn in Section IV.
2. Dual-polarized wideband multifunctional switchable absorber/reflector
2.1 Design and performance
The proposed structure comprises periodically arranged super-elements, where each super-element contains a 2 × 2 matrix and two neighboring unitary elements are 90 degrees rotated. The super-element configuration allows obtaining the stable polarization performance of the wideband multifunctional active absorber/reflector.
The unitary element (denoted by red dash lines) and the super-element (denoted by dark yellow dot lines) of the proposed structure are illustrated in Fig. 1. The unitary element consists of three parts including the active frequency selective surface (AFSS) structure on the top layer, the metal ground on the bottom layer and the air spacer in between. The top-layer AFSS structure is designed based on the modified Jerusalem cross structure, and a PIN diode in parallel with a lumped resistance Rs is inserted between the cross structure and each end-loading dipole. Adjacent unitary elements are connected by a lumped inductor Lp, so that the metallic structures together with the inductors allows the dc current to pass through without the need of additional bias lines. The horizontal and vertical parts of the Jerusalem cross structure are geometrically separated through a pair of via holes, thus the PIN diodes at different directions can be controlled independently. The F4BM220 material with the relative permittivity (εr) of 2.2 and the loss tangent (tanδ) of 0.0007 is used as the dielectric layer of the AFSS structure. The thicknesses of the air spacer and the F4BM220 substrate are h1 and h2, respectively.
Fig. 1. Geometry of the proposed dual-polarized wideband multifunctional active absorber/reflector. (a) Perspective view of the super-element. (b) Side view of the super-element. (c) Upper (left) and lower (right) views of the AFSS layer in the unitary element. The geometrical parameters of the proposed structure are decided as: p = 18 mm, lt1 = 3.8 mm, wt1 = 3.8 mm, lb1 = 9.5 mm, wb1 = 3.6 mm, lb2 = 9.1 mm, wb2 = 1.5 mm, lb3 = 3.5 mm, wb3 = 1.5 mm, g1 = 0.3 mm, g2 = 0.7 mm, dpin = 0.35 mm, h1 = 8 mm, and h2 = 0.8 mm.
The full-wave electromagnetic simulation software HFSS is employed in the design procedure, where a pair of Floquet ports are assigned as the excitation and two pairs of Master-Slave boundaries are set to four sides of the super-element to mimic an infinite array with a single unit cell. The lumped RLC boundary is used to model the PIN diode (Skyworks SMP1345-079LF), the lumped resistor and the lumped inductance. According to the datasheet [21], the PIN diode can be equivalent to a 1.5 Ω forward resistance Ron at ON state and a 0.15 pF capacitance Coff at OFF state. Besides, a 0.7 nH package inductance Lp is considered at both states as the SC-79 packaging option is adopted in the processing stage. The values of the lumped resistance Rs (Panasonic ERJ3RBD1500V) and the lumped inductance Lp are 150 Ω and 10 nH, respectively.
Figure 2(a) shows the reflection properties of the proposed structure. It is evident that the proposed design works as a dual-polarized wideband reflector with the return loss smaller than 1.5 dB within the entire operating frequency band when all PIN diodes are switched on at “11” state. The good reflection effect can be attributed to the short circuit condition of lumped resistors when adequate dc bias is applied to PIN diodes. On the contrary, the structure behaves as a dual-polarized wideband absorber when all PIN diodes are switched off at “00” state. The reflection coefficients are less than -10 dB from 3.43 GHz to 7.35 GHz with a fractional bandwidth of 72.7% under normal incidence. The cross-polarized reflection coefficients at all working states are below -20 dB. Thus, a dual-polarized wideband switchable behavior between absorbing and reflecting states can be realized by simply switching the on/off state of all PIN diodes. As shown in Fig. 2(b), the responses for different polarizations at “10” and “01” states can be clearly distinguished since the PIN diodes loaded horizontally and vertically can be activated separately by controlling their biases independently. The proposed structure functions as a wideband reflector for TE polarization and it can be deemed as a wideband absorber for TM polarization at “10” state. At “01” state, the opposite is the case that a wideband TE-polarized absorber and TM-polarized reflector can be achieved. In sum, the multifunctional design presents very stable dual-polarized performance and independent controllable properties with richer polarization information.
Fig. 2. Simulated reflection coefficients of the dual-polarized wideband multifunctional active absorber/reflector for different polarizations under normal incidence. (a) “00” state and “11” state. (b) “10” state and “01” state.
As shown in Fig. 3, the proposed structure is also studied under oblique incidence and it shows stable angular characteristics for both TE and TM polarizations at all working states. For TE polarization, the reflection coefficients rises gradually, and the absorbing bandwidth reduces to 3.74 GHz (3.57 GHz - 7.31 GHz) at “00” state and 3.80 GHz (3.56 GHz - 7.36 GHz) at “01” state as the incident angle increases to 30°. Meanwhile, the structure maintains stable reflecting performance for all working states with the increasing of the incident angle. In the case of TM polarization, the absorbing bandwidth is narrowed to 3.35 GHz (4.07 GHz - 7.42 GHz) at “00” state and 3.37 GHz (4.06 GHz - 7.49 GHz) at “10” state at the incident angle of 30°. Reflective properties remain satisfactory with the reflection coefficient decreases a bit at 6.48 GHz at “11” state as the incident angle increases, and a small dip can be observed near 7.70 GHz at “01” state when the incidence angle is larger than 30°.
Fig. 3. Simulated reflection coefficients of the dual-polarized wideband multifunctional active absorber/reflector for different polarizations under oblique incidence. (a) “00” state. (b) “11” state. (c) “10” state. (d) “01” state.
2.2 Equivalent circuit model
The equivalent circuit method (ECM) is employed to explain the operating mechanism of the proposed structure, and the equivalent circuit model is presented in Fig. 4. Based on the parallel between the real structure and a lumped-LC-network counterpart, the approximate analysis is useful for acquiring physical insights into the working principle of FSSs. The ECM allows isolating the major contributions which determine the frequency behavior. Otherwise, the response of such a complex structure may be hard to understand. As shown in Fig. 4, when the incident plane waves impinge on the structure, the metallic cross can be equivalent to a series resonant circuit (Ls1, Cs1, Rs1), where Ls1 is the equivalent inductance of the structure along the electric field direction of the incident waves, Cs1 mainly represents the equivalent gap capacitance between adjacent elements and Rs1 stands for the loss due to the finite conductivity of the copper. Besides, two parallel circuits of the loaded resistor and the PIN diode are connected in series with the series resonant circuit to model the lumped components. In addition, another series resonant circuit (Ls2, Cs2, Rs2) in parallel with the original one is also needed as the electric field also couples with the loading ends. The equivalent capacitance Cp models the interlayer coupling capacitance. The F4BM220 dielectric layer and the air spacer can be equivalent to two short transmission line sections (Z1, h1) and (Z0, h2), respectively. The characteristics impedance of the substrate can be calculated by the formula ${Z_1} = {Z_0}/\sqrt {{\varepsilon _r}} $, where Z0 is the free-space characteristics impedance. The short transmission lines can be converted to the combination of a series inductance (LTi, i = 1, 2) and a parallel capacitance (CTi, i = 1, 2), and the initial values can be calculated by the Telegrapher’s equations [22].
Fig. 4. Equivalent circuit of the dual-polarized wideband multifunctional active absorber/reflector. The values of the lumped elements are: Ls1 = 9.12 nH, Cs1 = 0.24 pF, Rs1 = 25.00 Ω, Ls2 = 4.01 nH, Cs2 = 0.05 pF, Rs2 = 6.51 Ω, Cp = 0.054 pF, LT1 = 1.25 nH, CT1 = 7.41 fF, LT2 = 8.00 nH, CT2 = 0.023 pF, Rs = 150 Ω, Lp = 0.7 nH, Coff = 0.15 pF, Ron = 1.5 Ω.
In order to validate the equivalent circuit model, the circuit results of the proposed design under normal incidence at both “00” and “11” states are obtained and compared with the simulated ones in Fig. 4. The circuit parameters are included in the legend of Fig. 4, whose starting values are obtained by using the semi-analytical approach mentioned in [23] and then optimized by performing interactive optimization in the Advanced Design System (ADS). As illustrated in Fig. 5, the reflection performance of the structure for both absorbing and reflecting states can be properly described by adopting the equivalent circuit model.
Fig. 5. Compared reflection performance of the dual-polarized wideband multifunctional active absorber/reflector for absorbing and reflecting states.
2.3 Parametric analysis
In order to analyze the effect of several parameters on the structural performance, a general study on the parametric variation is conducted by using the simulation software. As presented in Fig. 6(a), when the thickness of the air spacer (h2) increases, the reflection coefficient decreases a bit at “11” state, and meanwhile, the reflectance also reduces with larger bandwidth at “00” state. Considering the compromise between the structural thickness and the reflectivity at both states, an 8 mm thick spacer layer is chosen to reach the expected goals that realize the reflectance below -15 dB within the operation band at “00” state under normal incidence. From Fig. 6(b) it can be observed that the lumped resistance (Rs) has a greater influence on the depth of the first resonance than the second one at the absorbing state and the greater the resistance the deeper the resonance depth. At the reflecting state, the return loss is reduced as the resistance value becomes lager. Thus, a resistance of 150 Ω is selected to keep the in-band reflection coefficient less than -15 dB at “00” state.
Fig. 6. Simulated coefficients of the dual-polarized wideband multifunctional active absorber/reflector for different parameters. (a) Thickness of the air spacer h1. (b) Lumped resistance Rs. (c) PIN diode capacitance Coff.
In Fig. 6(c), typical capacitance values (Coff) are drawn from the datasheets of several commercially available PIN diodes. The reflectance curve varies greatly at OFF state and it remains unchanged at ON state when the equivalent capacitance of the PIN diode changes. The capacitance value of 0.15 pF is finally chosen due to the good absorbing property obtained.
3. Experimental results
In order to validate the simulated results, a prototype of the proposed absorbing/reflecting structure has been fabricated. The prototype, characterized by an overall size of 356 mm × 356 mm, is shown in Fig. 7. The AFSS structure and the metal sheet are printed on the F4BM220 substrate and the FR4 substrate, respectively. The thickness of the air spacer between the top and bottom layers is maintained by assembling 7 sets of nylon nuts (M3), nylon bolts (M3*15) and plastic gaskets (Φ7*3*2) on four sides of the array. The Skyworks SMP1345 Series of surface mountable PIN diode is selected as the RF switch due to its very low capacitance (0.15 pF). The thick film chip resistor ERJ3RBD1500V from Panasonic is used as the lumped resistor [24] in parallel with the PIN diode. The lumped components are soldered on the lower layer of the AFSS structure, which helps to protect them from external damages.
Fig. 7. Prototype of the dual-polarized wideband multifunctional active absorber/reflector. (a) Upper layer of the AFSS structure. (b) Lower layer of the AFSS structure. (c) Side view of the proposed structure.
Two independent groups of dc bias lines are printed on the back side of the F4BM220 substrate to provide power supply for the PIN diodes loaded horizontally and vertically, respectively. Therefore, four different feed modes can be achieved to switch the proposed design between different working states. Moreover, four lumped inductors, (4.7 nH, 6.8 nH, 10 nH and 22 nH), whose self-resonant frequencies are respectively 7 GHz, 6 GHz, 5 GHz and 4 GHz, are connected successively to attain wideband isolation between the bias network and the alternating current on the structure.
The experiment is conducted in an anechoic chamber and the measurement setup is presented in Fig. 8, where a pair of transmitting and receiving antennas is connected by a vector network analyzer (VNA) Agilent N5245A. The fabricated sample is placed in a custom-built frame and surrounded by absorbing materials, which contributes to reduce the unwanted edge diffraction. A two-channel dc power supply is employed in the experiment to provide bias for the PIN diodes loaded in different directions. The function of time-domain gating has been adopted in the vector network analyzer and the intermediate frequency bandwidth (IFBW) has also been reset for the purpose of removing undesired signals during the measurement process.
The measured reflection responses of the prototype at four operating states under normal incidence are compared with the simulated ones as depicted in Fig. 9. At “00” state, when no bias is provided to the PIN diodes, the -10 dB reflection bandwidth is 4.06 GHz (3.62 GHz - 7.68 GHz) with a fractional bandwidth of 71.86% for both polarizations. Meanwhile, the design shows good reflection performance almost covering the entire corresponding frequency band at “11” state when a voltage of 28 V is supplied to the PIN diodes in a row (about 0.78 V on each PIN diode). The isolation between the absorbing and reflecting states larger than 10 dB ranges from 3.83 GHz to 7.66 GHz. Thus, a wideband switching function can be attained by simply flipping all PIN diodes between ON and OFF states. Different from the situation at “00” and “11” states, the performance of the proposed design for different polarizations can be differentiated at “10” and “01” states by separately controlling the on-off state of the PIN diodes loaded in different directions. At “10” state, the structure reflects TE waves while absorbing TM waves in a wide band; contrarily, it can be seen as a wideband absorber/reflector for TE/TM polarization at “10” state. The characteristics of the proposed design under oblique incidence are also given in Fig. 9, and structure maintains good performance at the incident angle of 30°. Although the reflectivity at the absorbing state is slightly larger than -10 dB and the return loss at the reflecting state also increases at large incident angle of 45°, obvious switching features still remain unchanged.
Fig. 9. Measured reflection coefficients of the dual-polarized wideband multifunctional active absorber/reflector for different polarizations under oblique incidence. (a) “00” state. (b) “11” state. (c) “10” state. (d) “01” state.
Furthermore, a measurement uncertainty analysis for the wideband absorbing characteristic is carried out to evaluate the goodness of the agreement between the measurement and the simulation. The tolerances of the plastic gaskets thickness h1 (±0.5 mm), the substrate thickness h2 (±0.04 mm), the dielectric permittivity εr (± 2%), the resistance Rs (± 0.5%), the capacitance Coff (0.15 pF ∼ 0.2 pF) and the resistance Ron (1.5 Ω ∼ 2 Ω) of the PIN diode are taken into consideration to determine the reasonable reflectance range. As shown in Fig. 10, the reflectivity of most frequencies falls within the possible range inferred from the given conditions above. In addition to these parameter tolerances drawn from accessible product specifications, the uncertainty budget can be further expanded by the errors introduced by other influence factors such as system errors of VNA, the jitter of high frequency cables and the manual welding of surface mount devices (e.g. PIN diodes, inductors and resistors). Taking into account of the measurement uncertainty, a good agreement between experimental and simulated results can be attained despite a small deviation exists and dual-polarized wideband characteristics with multiple combinations of absorbing and reflecting states can be clearly demonstrated.
4. Conclusion
A dual-polarized wideband multifunctional switchable absorber/reflector is presented in this paper. The biggest advantage of the design lies in its wideband characteristics for different polarizations and incident angles at both absorbing and reflecting states. A prototype has been fabricated and measured to validate the simulation under different circumstances, and the observational results agree well with the simulated ones. The measurement demonstrates that the design possesses a wide switchable bandwidth (3.83 GHz - 7.66 GHz) with the isolation between different states larger than 10 dB. Furthermore, compared to other studies previously reported, the proposed structure leverages the strength in terms of multifunction, polarization insensitivity, angle stability and wideband switchable properties, which makes it a potential candidate for future stealth systems.
Funding
Key Laboratory of Radar Imaging and Microwave Photonics (Nanjing University of Aeronautics and Astronautics), Ministry of Education (NJ20210002); National Natural Science Foundation of China (61871219).
Acknowledgments
Dr. Huangyan Li thanks the Ministry of Education for its Grant of Key Laboratory of Radar Imaging and Microwave Photonics for young scientists.
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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