An Ultra-thin Compact Wideband Metamaterial Absorber

This article reports an ultra-thin, compact metamaterial absorber with wideband absorption at microwave frequencies. The proposed structure contains circular and rectangular split rings, and has thickness of 0.049λ0 with respect to the center frequency of the bandwidth. This structure provides wideband absorption of 3.84 GHz from 12.80 GHz to 16.64 GHz with 90% absorptivity and absorption peaks are observed at 13.2 GHz and 16.5 GHz. The full width half maximum of the structure is 5.48 GHz from 12.09 GHz to 17 57 GHz. The absorption mechanism and polarization behavior of the structure have to be studied. The proposed metamaterial absorber is sensitive to polarization but wideband absorption is attained only for specific normal and oblique angles of incidence, with reduced absorptivity. The structure has been fabricated, and the measured results match well with the simulation responses. The advantage of the proposed absorber is its wideband absorption with compact (8 mm × 8 mm size), ultra-thin (1 mm thick substrate) structure compared to that of the other existing microwave metamaterial absorbers.


Introduction
Metamaterial [1], an artificial structure with unique properties has attracted attention in applications such as antenna [2][3][4][5][6], cloaking [7][8][9], absorbers [10], [11], and so on.In recent years, metamaterial absorbers have been used as promising alternative for conventional absorbers owing to their perfect electromagnetic absorption, compact size, ultra-thin thickness, and low profile design.These advantages have led to different microwave metamaterial absorbers in single band [12], [13], dual band [14], [15], multi-band [16][17][18] and wideband [19] applications in all frequency regimes [20], [21].Though metamaterial absorber has various benefits, it suffers from narrow bandwidth.Metamaterial absorbers with multiple dielectric layers [22], [23] are used for enhancing the absorption bandwidth but huge thickness is the major constraint in many applications.Multi-resonant [24] structures were proposed as wideband absorbers.These absorbers are the combination of multiple resonant structures exhibiting enhanced absorption bandwidth.The major limitations of these absorbers are their large size and the difficulty in controlling the structure at each absorption frequency.Single layer metamaterial absorbers [25], [26] possess moderate structure thickness and have enhanced absorption bandwidth.These structures are low profile and can be easily optimized to control the resonant frequency.Interesting applications of metamaterial absorbers are: stealth technology, specific absorption rate (SAR) reduction and electromagnetic interference (EMI) suppression.This article proposes a compact, wideband metamaterial absorber with 0.049λ 0 thickness operating at microwave frequencies.The proposed structure with circular and rectangular split rings provides absorption of 3.84 GHz at 90% absorptivity and full width half maximum (FWHM) of 5.48 GHz.This asymmetric structure is polarization sensitive.The electromagnetic field and surface current distributions were analyzed for the absorption mechanism of the proposed metamaterial absorber.

Structure Design and Simulation Results
The design of the proposed wideband microwave metamaterial absorber is presented in Fig. 1 with electromagnetic field vector directions.The top layer of the unit cell consists of outer rectangular split ring and inner two circular split rings with dimensions a = 8 mm, b = 3 mm, c = 3.5 mm, d = 2.5 mm, e = 2.7mm, g 1 = 0.5 mm, g 2 = 1.5 mm, g 3 = 0.8 mm, g 4 = 0.7 mm, g 5 = 0.6 mm, g 6 = 0.45 mm, w 1 = 0.35 mm, w 2 = 0.4 mm, w 3 = 0.5 mm, w 4 = 0.5 mm, r 1 = 2.55 mm and r 2 = 1.9 mm.The top layer metallic patch is etched on FR4 dielectric substrate of 1 mm thickness and dielectric permittivity ε r = 4.4.The other side of dielectric substrate is complete copper ground (conductivity = 5.8 × 10 7 S/m, thickness = 0.035 mm).The designed structure is simulated using Ansys HFSS with periodic boundary conditions and Floquet port excitations.The proposed structure is chosen such that each part of the structure resonates at particular frequency.The inner most circular split ring and the outer rectangular ring resonate at 16.69 GHz and 13.29 GHz, respectively.Then, the middle ring was incorporated and the structure was optimized to provide wideband absorption.
The absorptivity A(ω) of the structure is given by (1) [27] where S 21 (ω) and S 11 (ω) are the ratios of the transmitted and reflected power to the incident power, respectively.The complete copper ground blocks the transmission of electromagnetic waves (S 21 (ω) = 0) and therefore the absorptivity can be increased by reducing the reflection from the absorber structure.The proposed structure provides wideband absorption of 3.84 GHz at 90% absorptivity with two absorption peaks at 13.2 GHz and 16.5 GHz as shown in Fig. 2(a) and has FWHM of 5.48 GHz.The patch design and thickness of the dielectric substrate are optimized to match the free space impedance with the input impedance of the structure.The normalized input impedance (Z(ω)) is given by (2) [27] , and is shown in Fig. 2(b).At the absorption peaks, the imaginary and real parts of the normalized input impedance become zero and unity, respectively.The unity real part indicates the perfect impedance matching of the structure which leads to the maximum absorption.The effective permittivity ε eff and effective permeability μ eff are calculated using electric susceptibility χ es and magnetic susceptibility χ ms as shown in (3), (4) ( 5) and ( 6) [27], where k 0 is the wavenumber of the free space and d is the distance travelled by the incident electromagnetic wave.The retrieved real and imaginary parts of the electromagnetic parameters ε eff and μ eff are shown in Figs.3(a) and 3(b) and presented in Tab. 1.The negative ε eff and μ eff confirm that the proposed structure is double negative metamaterial and behaves as absorber in microwave region.
The parametric analysis of the proposed structure and its impact on absorptivity are demonstrated in Fig. 4 and Fig. 5.The analyses were performed by varying one parameter while keeping the other parameters as constant under 0 o of normal and oblique angles of incidence.In Fig. 4, the lengths of the rectangular split ring are varied and their impact on absorptivity is studied.Figures 4(a where the absorptivity decreases to less than 85% and 80%, respectively.Also, the absorption bandwidth obtained is narrow.For variations in the lengths d and e, the absorption bandwidth is retained, as shown in Fig. 4(c) and Fig. 4(d).However, the absorptivity decreases below 85% excluding d = 2.3 mm and e = 2.5 mm.The other parameters like  g 6 = 0.5 mm, where the absorptivity is above 90%, and it has decreased absorption bandwidth.The absorptivity under variations of the inner ring gaps is presented in Fig. 5(e).It illustrates that absorptivity and bandwidth decreases, except for g 5 = 0.8 mm where wide bandwidth is retained.
The distributions of electric and magnetic fields at the resonating frequencies are presented in Fig. 6 and Fig. 7, respectively.The magnetic and electric fields are highly concentrated in the rectangular split ring, contributing to high absorption at 13.2 GHz.At 16.5 GHz, the electromagnetic fields are higher in the circular split rings.The maximum absorption is obtained within the structure by the overlapping of electromagnetic fields.Figure 8 and Figure 9 show the surface currents distributed in the both layers of the proposed metamaterial absorber.At low frequency, the surface currents are mostly distributed in some parts of the rectangular and circular rings.Hence, both the rings contribute to high absorption at 13.2 GHz.whereas; absorption at 16.5 GHz is mostly due to the surface currents in the inner circular ring and partially by rectangular ring excitation.The anti-parallel surface currents create circulating loops around the magnetic field incident on the top layer of the structure.At these resonating frequencies, the electric fields are coupled with the copper patch, which leads to high absorption.The polarization sensitivity of the proposed absorber is studied under oblique and normal angles of incidence for Transverse Electric (TE) mode.The electric field direction is fixed, and the wave vector and magnetic field directions are changed to analyze the absorptivities under oblique incidence.To test polarization behavior of the structure under normal incidence, the magnetic and electric field directions are changed while the wave propagating direction is made normal along z direction.Figure 10(a) shows that, under oblique angles of incidence at 10°, 15° and 60°, the absorptivity drops to 78.7%, 71.1%, and 77% with bandwidth of 4.12 GHz, 4.28 GHz and 4.21 GHz, respectively.At 25°, 50° and 85°, the absorptivity decreases to 85.6%, 82.8% and 80.4% with bandwidth of 3.86 GHz, 3.96 GHz and 4.22 GHz, respectively.The polarization under normal angles of incidence is tested and analyzed in Fig. 10(b).At 25°, 35°, 50°, 60°, 75° and 85°, the absorptivity is reduced to 88.6%, 68.2%, 89.2%, 70.2%, 83.6% and 82.5% with bandwidth of 3.7 GHz, 2.64 GHz, 3.57 GHz, 4.29 GHz, 3.77 GHz and 3.70 GHz, respectively.The simulation results show the proposed structure is polarization sensitive and has wide bandwidth with reduced absorptivity at various angles.

Experimental Results and Analysis
To perform experimental test, 40 × 40 array of the proposed unit cells with dimension 320 × 320 mm was fabricated using printed circuit board (PCB) technology.This fabricated structure was tested in anechoic chamber, and the experimental measurement set up is shown in Fig. 11(a).It contained two broadband horn antennas operating at 2-18 GHz which were connected to PNA Network Analyzer Agilent E8362B.These two antennas were used for transmitting the electromagnetic waves and receiving

Absorptivity [%]
[28] 0.5 7.8 2.5 < 0. the reflected power from the fabricated structure.The power measurements were calibrated and measured from the network analyzer.The fabricated structure with the enlarged view is presented in Fig. 11(b) and Fig. 11(c).In the anechoic chamber, a copper sheet of equal dimension as that of the fabricated structure was initially positioned and the reflected power was measured as the reference.Then, the fabricated structure was positioned at the same location of the copper sheet and the reflected power from the structure was measured.The difference between the reflected and the reference power gives the actual power reflections.
The simulated absorptivity was compared with the experimentally measured absorptivity in Fig. 12(a).The measured response showed wideband absorption of 3.92 GHz which matched closely with the simulated response.The deviation in the measured and simulated absorptivity was due to the fabrication tolerances.The polarization sensitivity of the fabricated structure was tested under normal incidences for TE mode.The absorptivities under normal angles of incidence shown in Fig. 12(b) were measured by rotating the fabricated structure along its axis, with the antennas in static position.The result shows that, the structure is polarization sensitive and matches well with the simulated absorptivities under normal angles of incidence.The proposed metamaterial absorber is compared with ultra-thin structures in Tab. 2. Though previous structures were either ultra-thin or compact or both, the absorption bandwidths are very narrow when compared to the proposed structure.Table 2 confirms the advantage of the proposed absorber as wideband absorption of 3.84 GHz with 1 mm thick substrate and compact (8 mm × 8 mm size) structure.

Conclusion
A wideband microwave metamaterial absorber with circular and rectangular split rings has been proposed.The structure is compact and ultra-thin (0.049λ 0 thick).It is optimized to provide wideband absorption with two peaks at 13.2 GHz and 16.5 GHz.The simulated results show 3.84 GHz wideband absorption with 90% absorptivity and FWHM of 5.48 GHz.The electric field, magnetic field, and surface current distributions were analyzed distinctly at two absorption peaks to understand the absorption mechanism.The polarization behavior of the structure was examined for normal and oblique angles of incidence.The responses showed that the structure was polarization sensitive and for particular angles of incidence, wideband absorption was retained.However, the percentage of absorptivity reduced.The proposed metamaterial absorber has been fabricated and the practical measurements are similar to the simulated results.This microwave absorber with wideband absorption, ultra-thin and compact structure can be suitable for stealth and radar applications.

Fig. 1 .
Fig. 1.Top layer of the proposed unit cell structure.
) and 4(b) show the responses by varying the lengths b and c

Figure 5 (Fig. 5 .
Figure 5(d) represents the absorptivities by varying the gap g 6 at the bottom of the outer ring.It indicates that both the absorptivity and bandwidth are reduced except for