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Abstract
This paper reports a multiband Yagi-Uda shaped metamaterial absorber (YUMA) operating in X- and Ku-band of the microwave regime with the added functionalities of miniaturization, polarization controllability and wide incidence angle stability. The proposed YUMA shows three distinctive near-unity absorption peaks corresponding to 10.64 GHz, 12.08 GHz, and 14.09 GHz frequencies. The YUMA was analyzed under different oblique incidence angles for transverse electric (TE)-mode and different rotation angles of the top metasurface. The results showed that the proposed YUMA possesses multifunctional characteristics such as polarization controllability, and wide incidence angle stability. The comparison of simulated and measured results further demonstrates that the proposed absorber can be a potential candidate in polarization detection systems and transmissive polarizers. The proposed YUMA operating in the X- and Ku-band can have potential uses in several other applications, such as air traffic control, weather monitoring, military radar, and satellite communication.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Electromagnetic metamaterials have been realizing prodigious reputation due to their unique and unusual characteristics, such as negative refraction, cloaking, super lensing, perfect absorption and many more [1–8]. These metamaterials are different from natural materials because their properties are not dependent on the composition of the atoms rather depend on the arrangement of meta-structure or unit cell. By taking the advantage of the metamaterial aforesaid exciting properties, numerous microwave and optical devices for future applications can be designed [9–11]. Perfect metamaterial absorbers are the key and essential components in the field of optics. PMAs attained great importance after the investigation of the first metamaterial absorber by Landy et al. [12]. After this, various PMAs are designed for microwave, millimeter-waves, terahertz, and visible regimes of the electromagnetic (EM) spectrum [13–20].
Perfect metamaterial absorbers (PMAs) operating in the microwave regimes have also been extensively used in many diverse applications such as air traffic control, weather and military radars, the stealth applications, and satellite applications [21–24] along with PMA infrared-stealth technology in the infrared regime [25]. However, PMAs operating in the visible regimes are used for sensing, imaging, and solar photovoltaic applications [26–28]. PMA are considered important for the mentioned applications as they offer considerable interference reduction and radar cross section (RCS) reduction [29]. In addition, absorbers are also important in reducing the reflection levels in satellite communication devices where there is restriction of moving reflection source away from the system [29,30]. Likewise, multiband and wideband metamaterial absorbers have great demand in the recent applications such as radar cross section reduction (RCSR) [31], stealth technology [32], and MIMO antennas isolation for interference reduction [33]. Multilayered structures [34], multi resonance [35], fractal geometry [36], and lumped elements inclusion [37] are the most commonly employed methods to achieve multiband and wideband characteristics. However, these techniques have complex fabrication process and large thickness that can be hard to fit in compact housing of the devices. Contrarily, a single unit cell is an efficient way to create multi resonance behavior because of its simple and small profile along with the ease of fabrication [38].
Metamaterial absorbers with the added functionalities of multiple bands, wide incidence angle insensitivity, and polarization controllability are the valuable parameters for many applications such as ultrasensitive sensing, stealth technology, and radar cross section. Numerous research works have been reported in [39–44] on multiband and wideband metamaterial absorbers for microwave regime of electromagnetic spectrum. However, the reported multiband and wideband absorbers are designed using multilayer topology and exploit complex resonating structures. Hence, design of a multiband absorber having planar topology with simple conducting structures offering cost-effective and easy fabrication process would be a novel contribution in the microwave regime for sensing, stealth, radar, and communication applications.
In this paper, an asymmetric Yagi-Uda-shaped multiband metamaterial absorber in planar configuration is investigated. The structural configuration of the YUMA is composed of three layers; Yagi-Uda-shape having central conducting bar with five vertically placed conducting arms on top side of the substrate, a middle layer of FR4 substrate with thickness 1.6 mm, and a bottom copper thin layer as a ground plane. The compact planar structure in this paper offered three near-unity absorption peaks at 10.64 GHz, 12.08 GHz, and 14.09 GHz. Along with multiple near-unity absorption peaks, the proposed YUMA also attained several features, namely compact thickness, polarization controllability, and wide incidence angle stability. The in-depth investigation and parametric analysis of the proposed novel structure as YUMA demonstrated that the arms of the Yagi-Uda-structure along with the mutual coupling between the adjacent arms are the main contributor to the multiband absorption mechanism. Further, the measurements of the proposed YUMA demonstrated its employability for X- and Ku-bands applications.
The remainder of the paper is organized as follows: Section II illustrates the design and analytical segment of the proposed metamaterial absorber. Also, simulation procedure is discussed in this section. The simulation results are briefly discussed in Section III. Whereas comparison of simulation and measured results followed by the fabrication and measurement setup is elaborated in Section IV. Finally, the conclusion of the paper is elucidated in Section V.
2. Unit cell design and simulation setup
Figure 1 demonstrates the schematic of the unit cell of the proposed YUMA. The proposed YUMA is comprised of three layers; a top Yagi-Uda-shaped metallic structure and bottom layer made of pure metallic copper (electric conductivity σ = 5.8 × 107 S/m) is separated by a dielectric material FR-4 (relative permittivity ${\varepsilon _r}$ $= 4.3$ with loss tangent, tanδ = 0.025). The optimized design and parametric values of the proposed YUMA are shown in Fig. 1 (a). The copper thickness of the conducting structures on top and bottom ground plane is 0.035 mm.
Fig. 1. Optimized dimension, geometrical configuration, and unit cell simulation setup of the proposed YUMA. (a) The unit cell of the YUMA, with parametric values as P =12 mm, L = 9 mm, L1 = 4 mm, L2 = 3.5 mm, L3 = 3 mm, L4 = 2.5 mm, L5 = 2 mm, W = 0.5 mm, W1 = 0.5 mm, hs = 1.6 mm, (b) Side view representation of the unit cell of proposed YUMA, and (c) Illustration of infinite periodic array configuration of the proposed design in CST for periodic unit cell boundary conditions in x and y direction, while open space boundary condition is used in the z-direction, θ and φ represent the incident and polarization angle.
In this three-layered configuration of YUMA, the conducting structures having Yagi-Uda shape on the top layer allows the impedance of incident EM waves matched with the free-space due to electric and/or magnetic resonance. The bottom ground plane acts as a perfect reflector and blocks the transmission and middle dielectric substrate trap the EM waves and enhances the absorption due to the dielectric losses. The proposed YUMA unit cell is simulated using frequency domain solver of CST Microwave Studio. An illustration of the simulation setup along with the depiction of boundary conditions is shown in Fig. 1 (b) and (c). The plane wave is excited at normal and oblique incidence angle on the top surface of the absorber, i.e., along z-direction, as depicted in Fig. 1 (b). The unit cell boundary conditions are employed along x- and y- direction, and open add space boundary condition is along z- direction, as illustrated in Fig. 1 (b) and (c). The perfect absorption is attained as the reflection and transmission is zero. The zero reflection is attained due to electric and/or magnetic resonance of the top metasurface – at resonance, all the excited electromagnetic waves get trapped in the substrate layer. The subwavelength-sized vertical arms in the Yagi-Uda shaped metasurface resonates at different frequency that allows the electromagnetic wave to penetrate, will discuss in the forthcoming discussion.
The absorptivity of a metamaterial can be calculated from the given equation [45].
Where A, R and Τ represents the absorption, reflection, and transmission, respectively. Equation (1) can also be defined in terms of S-parameters: The absorber is backed with perfect reflector; therefore, transmission is almost zero, $|{{S_{21}}} |$≈ 0. Hence, Eq. (2) can be further simplified as3. Results and discussion
In this section, the absorption through the proposed YUMA has been investigated under the different operating conditions. Figure 2 illustrates the absorption of YUMA at normal incidence transverse electric (TE) wave polarization. It can be observed from Fig. 2 that the proposed YUMA exhibits three distinctive near-unity absorption peaks at 10.64 GHz, 12.08 GHz, and 14.09 GHz with an absorption value of 99.50%, 99.30% and 98.0% respectively. These three absorption peaks correspond to the three distinct resonances at three different frequencies that resulted because of the impedance matching with the free space for impinging TE wave due to electric and magnetic resonances.
Fig. 2. Simulated EM characteristics of the proposed YUMA at normal incidence for TE-mode (a) reflection and (b) absorptivity
To further understand the resonance mechanism of the proposed Yagi-Uda structure, the surface currents of the Yagi-Uda corresponding to three absorption peaks attained at 10.64 GHz, 12.08 GHz, and 14.09 GHz are plotted and shown in Fig. 3. The surface current plots depict that current density is maximum on the larger arm of the structure whereas current density gradually reduces from larger arms to the smaller arms. It can further be observed from Fig. 3 that current flows in the opposite directions on the first two conducting arms of the structure which leads to the formation of a magnetic dipole. Hence, the absorption of 10.64 GHz is due to the magnetic resonance in the structure. Next, the surface current is observed for 12.08 GHz, it is noticed that current density remains concentrated in the middle arms of the proposed Yagi Uda structure. Similar to Fig. 3 (a), the surface current flows in the opposite direction of the consecutive arms that corresponds to the magnetic dipole. Moreover, for the higher peak i.e., 14.09 GHz, it is noticed that the current density becomes maximum at the smaller arms of the Yagi Uda structure, as illustrated in Fig. 3 (c). Therefore, it can be concluded in the light of the current plots that impedance matching in the proposed structure is attained due to the magnetic resonance for the incoming plane EM wave. In addition, the absorption peaks mechanism also supports the theory that a larger arm will have resonance at the lower frequency, whereas higher frequency resonance will result because of the smaller arm due to lower wavelength. The surface current plots and wavelength mechanism confirmed that the three absorption peaks are attained due to magnetic resonance between the adjacent arms of the Yagi Uda structure.
Fig. 3. Simulated surface current distribution of the proposed YUMA for normal incidence of TE polarized wave at (a) 10.64 GHz (b) 12.08 GHz (c) 14.09 GHz.
Further, to observe the angular stability of the proposed design for the incoming EM wave, the absorption features are also analyzed by rotating (along the optical axis) the top Yagi-Uda shaped unit cell, namely α = 0°, 90°, 135°, and 150°. Figure 4 (a) corresponds to the condition when α = 0°. For α = 0°, the proposed YUMA shows three near-unity absorption peaks at 10.64 GHz, 12.08 GHz, and 14.09 GHz for TE mode. However, for TM-mode, two absorption peaks with absorptivity ∼ 33% and 36% are attained at frequencies of 10.65 GHz and 13.3 GHz, respectively. This is due to change in the polarization state (electric field vector orientation), therefore, resonance is modified, and two absorption peaks with lower absorptivity are attained. Considering the case α = 90°, (Fig. 4 (b)), three near-unity absorption peaks at 10.64 GHz, 12.08 GHz, and 14.09 GHz are noticed for TM mode whereas, for TE mode two absorption peaks are observed with absorptivity ∼ 33% and 36% at 10.65 GHz and 13.3 GHz, respectively. Figure 4 (c) shows that three absorption peaks are noticed having absorptivity as ∼ 70%, 50% and 50% at 11.4 GHz, 13.2 GHz, and 15.5 GHz, respectively, for both the TE- and TM-mode when α = 135°. Similarly, Fig. 4 (d) corresponds to the condition when α = 150°. In this situation, three absorption peaks have been observed at 11.4 GHz, 13.2 GHz, and 15.5 GHz having absorptivity of approximately 83%, 77%, and 77%, respectively for TE-mode. The angular stability of the proposed YUMA showed that multiple absorption peaks with average absorptivity less than 35% can be achieved for the TM-mode of excitation. Nevertheless, the absorption peaks remained intact at same three frequencies when YUMA is rotated along optical axis from α = 0° to α = 150°. Additionally, absorption of the metasurface can be controlled by mechanically rotating the top metasurface.
Fig. 4. Simulated absorptivity of the proposed YUMA under different rotation of the top metasurface along optical axes (a) α = 0° (b) α = 90° (c) α =135° (d) α = 150°.
The parametric analysis on the geometrical parameters of the proposed YUMA has also been carried out to understand the resonance mechanism of the structure. The effect on the absorption peaks by changing the width of the vertical conducting arms i.e., W1 from 0.1 mm to 0.5 mm (with a step size of 0.1 mm) and spacing between the adjacent arm S is depicted in Fig. 5. Figure 5(a) illustrates that absorption peaks shifted to lower frequencies at three resonant points when element width W1 is increased. Hence, by suitably tailoring the element width, the desire results can be achieved in the proposed YUMA. Similarly, the absorption features have also been inspected by varying the width of central horizontal bar i.e., W from 0.25 mm to 1 mm (with a step size of 0.25 mm), as depicted in Fig. 5 (b). It is represented in Fig. 5 (b) that absorption peaks shifted to lower frequencies at three resonant points when central horizontal bar width W is increased. Figure 5(c) shows the effect of inter element spacing between the arms of the Yagi-Uda metasurface. It is noticed that the third absorption peak shift to the higher frequency value with the increase of the spacing between the adjacent arms. It can also be observed that the absorption of the second peak remains changed with the increased of S. Whereas, the first absorption peak shows a slight shift to the higher frequency with the increase of S. Therefore, it can be inferred that alterations in the parameters of the top metasurface show profound effect on the magnetic resonance which causes absorption.
Fig. 5. Simulated absorptivity of the proposed YUMA (a) for different values of W1 (b) for different values of W (c) for different values of S.
For the angular stability and to predict the robustness, the absorptivity of the YUMA is studied under obliquity of incidence angles from 0° to 60° for TE-mode. The EM wave can strike with the metamaterial structure with any arbitrary incident angle. So, we designed such a dynamic meta-structure which almost equally treats with an incoming EM wave of any incident angle. As depicted in Fig. 6, it can be observed that the absorptivity of the designed YUMA remains almost same from 0° to 60°, however, a minor shift in the absorption peak is noticed when incidence angle is increase beyond 40°. A prominent effect on the higher band i.e., 14.09 GHz is observed for the obliquity of the incident angle. This shift is attributed to the change of impedance for the third absorption band could be useful for the detection of the angular excitation of the wave in the Ku-band.
To get better physical understanding of the absorption of the YUMA, the amplitude and phase of S-parameter (S11 and S21) are computed for normal incident wave and from these S-parameters, the effective parameters (permittivity, permeability, refractive index, and impedance) of the designed YUMA can easily be retrieved [46].
Within this context, impedance is also an important parameter to evaluate the YUMA resonance. Mathematically, impedance is given by equation $z= \sqrt {\mathrm{\mu}/\varepsilon } $ . Notably, if the impedance of the proposed absorber is perfectly matched with the free-space impedance then reflection of the incidence wave is almost zero. Therefore, near-unity absorption is achieved at the indicated operating frequency points. Once S-parameters are obtained from the numerical simulation, other material parameters such as impedance, refractive index, effective permittivity, and permeability of the designed absorber are extracted using [46].
where, k0 and d represent the wavenumber and thickness of the absorber, respectively. In Eq. (4), ${{\boldsymbol e}^{{\boldsymbol {in}}{{\boldsymbol {k}}_{\boldsymbol{0}}}{\boldsymbol d}}} = {\boldsymbol X} \pm {\boldsymbol i}\sqrt {\boldsymbol{1} - {{\boldsymbol X}^{\boldsymbol{2}}}} $ where ${\boldsymbol X} = \boldsymbol{1/2}{{\boldsymbol S}_{\boldsymbol{21}}}({\boldsymbol{1} - {{\boldsymbol S}_{\boldsymbol{11}}}^{\boldsymbol 2}+ {{\boldsymbol S}_{\boldsymbol{21}}}^{\boldsymbol{2}}} )$. These mathematical expressions are used to calculate the remaining parameters (i.e., relative permittivity and permeability). Effective permittivity and permeability of the proposed design are then calculated as:Table 1 summarizes the spectral features of the previously reported multiband metamaterial absorber with the YUMA. For comparison, several factors including numbers of unit cells, materials used, size, configuration and number of operating bands are taken into account. All the reported metamaterial absorbers are based on single layer device configuration and possesses different types of number of unit cells depending on the resonating frequencies. Most of the presented designs were composed of multiple unit cells and achieved dual and triple operating bands. In contrast, our proposed YUMA contain a single unit cell and shows triple band absorption features and its lattice period is also smaller than the reported metamaterial absorbers. Furthermore, the proposed YUMA is easily fabricable with readily available fabrication techniques and scalable to other operating frequencies.
Table 1. Comparison table for the present work with the previously reported work.
The effective permittivity, effective permeability, refractive index, and normalized impedance from the S-parameters are retrieved using the theory and mathematical expressions explained above, as depicted in Fig. 7. Figure 7 (a) shows the normalized impedance of the YUMA. It is evident that the real part of the impedance is ≈1, while imaginary part is approaching to zero for all the three operating frequency points at 10.64 GHz, 12.08 GHz, and 14.09 GHz.
Fig. 7. Retrieved parameters of the proposed YUMA (a) Impedance (b) Refractive index (c) Permittivity, and (d) Permeability.
Figure 7 (c) shows the effective permittivity of the proposed metamaterial absorber, it is noticed that there exist three electric resonance peaks at the operating frequency points of 10.64 GHz, 12.08 GHz, and 14.09 GHz. The peaks in the effective permittivity are due to the presence of the resonance of the top Yagi-Uda metasurface. Also, three magnetic resonance peaks are also found near the proximity of these frequency points (Fig. 7(d)). This is due to the generation of the anti-parallel currents produced by the top metasurface and the bottom ground sheet. These anti-parallel current loops cause an artificial magnetic dipole moment, which strongly linked with the incident magnetic field. Thus, the proposed design realizes the excitation of the magnetic resonances simultaneously.
Moreover, the E-field for TE-mode (keeping normal incidence case) at three different operating frequencies 10.64 GHz, 12.08 GHz, and 14.09 GHz are plotted to interpret the physical mechanism of the absorption. Figure 8 illustrates the surface E-field distribution of the proposed YUMA at three operating points, namely, 10.64 GHz, 12.08 GHz, and 14.09 GHz. For 10.64 GHz (Fig. 8(a)), the maximum E-field intensity is accumulated on first three arms (from the left) of the YUMA. Thus, this portion of the unit cell is a main contributor in absorption for the first band. For 12.08 GHz and 14.09 GHz (to Figs. 8(b) - (c)), the maximum intensities are localized at middle- and lower-arms, respectively, indicating that they are absorbing more energy.
Fig. 8. Simulated E-field distribution of the proposed YUMA for absorption peaks corresponding to (a) 10.64 GHz (b) 12.08 GHz, and (c) 14.09 GHz.
4. Fabrication and measurements
A finite sheet having 17 ${\times} $ 17 cells of the proposed YUMA are fabricated using chemical etching technology on FR4 with ground plane on the back side. A photograph of the fabricated prototype is shown is Fig. 9 (a). To verify the simulation results, the fabricated prototype is experimentally verified using the experimental setup shown in Fig. 9(b). A wall of RF absorbers with dimension 0.6 λ at lower frequency of resonance is created to ensure minimum reflection and impingement of EM waves only on the prototype, as shown in n Fig. 9(b). Two pair of the broadband horn antennas manufactured by AINFO (Model # LB-20265-SF) (as transmitter (Tx) and receiver (Rx)) connected to a well calibrated Network analyzer are also used in the experimental validation of the results. A twofold process was adopted for the measurement. Initially, complete measurement setup is calibrated by placing a metallic sheet between the horn antennas and absorbers wall at a far-field distance. Then, metallic sheet is replaced with the fabricated prototype and magnitude of reflection and transmission coefficients are measured over the frequency range from 10 GHz to 16 GHz. Finally, the obtained results are post processed to calculate the absorptivity from the measured S-parameters. The fabricated prototype is placed at θ = 0°, 30°, and 60° for incoming TE polarized wave from the horn antenna for the angular stability analysis. A comparison of measured and simulated results for TE polarized at θ = 0°, θ = 30° and θ = 60° angles are depicted in Fig. 10 (a) – (c). The results show that measured absorptivity peaks are intact with the simulated results. However, minor disagreements between simulated and measured results are attributed to the free space measurement setup which may cause EM leakage and inaccuracy in the fabrication process. The measured results are also consistent with the simulation results. The comparison of angular stability analysis of the prototype concluded that proposed YUMA is angularly stable for TE polarized wave and the absorptivity behavior of the absorber is not changing significantly.
Fig. 9. (a) Photograph of the fabricated YUMA (b) a picture of the adopted measurement setup for the validation of simulated results.
Fig. 10. Angular stability analysis of the proposed YUMA for TE polarized wave at (a) θ = 0°, (b) 30° and (c) θ = 60°.
5. Conclusions
This paper presented design, validation, and analytical analysis of a novel multiband YUMA at X and Ku- bands. The modeling of the proposed design is theoretically explained in the light of current plots, E-field distributions, and extracted parameters such as permittivity, permeability, impedance, and refractive index. The angular stability analysis demonstrated the angular insensitivity of the proposed YUMA. In addition, the design also exhibited polarization controllable characteristics over different rotations of top Yagi-Uda-shaped asymmetric resonator. The comparison of the simulated and measured results finally demonstrated that the proposed metamaterial absorber could be a useful alternative in microwave devices, communication, and airborne radar applications.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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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