Metamaterials (MTMs) are artificial electromagnetic structures composed of metals and dielectrics. The initial impetus driving MTM research was the realization of their effective negative permittivity, permeability, and refractive index ~.
Various MTM structures are naturally coupled to either the electric or magnetic components of light in frequency ranges from radio to near optical , . The major advantage of MTMs over natural materials is that the macroscopic parameters can be designed to have desired values. Much of the recent MTM research has been focused on the real part of
Conventional absorbers cannot be made sufficiently thin because of diffraction limitations. The reduction in the electrical thickness of absorbers is one of the challenging aspects of their design. The need is urgent for designs of innovative absorbers that can overcome these two disadvantages of conventional absorbers. Tao et al. replaced the copper strip with a complete backplane and achieved 96 % absorption at 1.6 THz with a 16-μmthick surface. Their device showed good broad angle performance in both TE and TM modes . In their design, the metallic backing plate is necessary to prevent power transmission, which may represent a problem for stealth applications. Bilotti et al. proposed an SRR-based absorber by arranging SRR arrays behind a resistive sheet. This SRR-based absorber can be used in stealth technology, owing to the absence of metallic backing plates ~. However, a resistive sheet, which is used to match the impedance of the free space, is necessary in this type of design.
This study uses an ELC resonator structure with a resistor to control the electric coupling of the MTM and absorption. The magnetic coupling was generated by combining the center strip of the ELC and a thin strip on the bottom side of the substrate. The proposed absorber can be used for 2.45 GHz band applications such as radio-Frequency Identification (RFID), wireless local area network (WLAN), and industrial, scientific and medical (ISM) systems.
As an effective medium, an MTM can be characterized by a complex electric permittivity
respectively, , , , , .
Fig. 1 shows the unit cell geometry of the proposed absorber, which consists of an ELC resonator, a strip line, a lumped resistor, and an FR4 substrate (
The periodic metamaterial cell is analyzed by placing perfect electric conductors (PECs) on the top and bottom planes, and perfect magnetic conductors (PMCs) are placed on the front and back planes, as shown in Fig. 2(a). The magnetic field is perpendicular to the x-y plane and the electric field is parallel to the y-z plane. Therefore, the propagation direction is along the y-axis. The de-embed technique is used to obtain the
Figs. 3(a) and 3(b) show the simulated absorption characteristics of the proposed absorber for various ELC resonator widths (
Fig. 4 shows the simulated absorption characteristics of the proposed absorber for various strip lengths (
The absorption can be controlled by changing the resistance of the resistor. As shown in Fig. 5(a), the absorption increases with an increase in the resistance value (
Fig. 6 shows the simulated transmission, reflection, and absorption of the proposed absorber. An absorption of approximately 97 % is obtained at 2.45 GHz, and the half-max bandwidth is approximately 0.16 GHz. The transmission and reflectance are obtained as 0.05 and 0.01, respectively, at 2.45 GHz.
The operating mechanism of the proposed absorber structure at 2.45 GHz is verified by analyzing the electric field, magnetic field, and current distributions, which are shown in Fig. 7. The electric and magnetic resonances clearly are strongly generated by the two SRR elements and the two strips. Both the transmission and reflectivity are minimized because of the impedance matching and large loss in the absorber, and the incident energy will be converted to heat.
Figs. 8(a) and 8(b) show the retrieved effective permittivity and permeability of the proposed absorber. Fig. 8(c) shows the impedance
Fig. 9 shows photographs of the fabricated unit cell and five-unit cell absorber. Styrofoam was used in the absorber to elevate the absorber into the air.
Two horn antennas, placed 3 m apart, were used to measure the characteristics of the designed absorber, as shown in Fig. 10. Measurements were performed with an Agilent 8719ES Network Analyzer.
Fig. 11 shows the S-parameters of the two antennas with and without the absorber. Without the absorber,
This paper proposes an absorber using an MTM structure operating at 2.45 GHz, and a prototype was successfully implemented. The proposed absorber exhibited an absorption of approximately 97 % and a half-max bandwidth of approximately 0.16 GHz. The absorption performance can be controlled by adjusting the resistance of the resistor (R). The proposed absorber had a negative permeability and a negative permittivity at the operating frequency. The proposed metamaterial can be utilized to reduce the effect of body on a WBAN antenna.