No abstract

In this paper, a polarization-independent metamaterial absorber with enhanced bandwidth at two separate frequency bands is proposed over wide angle of incidence. The proposed structure consists of two layers of dielectric substrate. The unit cell is designed on the top surfaces of both the layers of the dielectric by parametric optimization in such a way that bandwidth-enhanced absorptions occur in C and X bands. The proposed structure is fabricated, and experimental results are in good agreement with the simulated responses. This bandwidthenhanced dual-band absorption nature is maintained for any angle of polarization under normal incidence, thus making the absorber polarization independent in nature. The structure also shows bandwidth-enhanced dual-band absorptions over wide angle of incidence up to 45°under TE polarization and 30°under TM polarization. Moreover, the proposed structure is ultra-thin, having total thickness of 3.2 mm, *k/14 and k/10 with respect to the center frequencies of two absorption bands.

This article presents a broadband circularly polarized (CP) rectifying antenna (rectenna) for microwave power transmission at 4.2-7.6 GHz, comprising a broadband CP antenna, and a rectifier circuit. The antenna consists of a standard Schiffman phase shifter and a pair of orthogonally positioned linearly polarized slot antennas with equal radiation strength. The rectifier circuit is composed of a microwave Si Schottky detector diode (HSMS-2862), a low-pass filter, a load resistor, and a ripple capacitor. Output dc voltage of 1.98 V over a 680 X load resistance and the maximum microwave-to-dc conversion efficiency of 81.6 % were measured when 34 dBm microwave power was transmitted at 5.6 GHz over a distance of 50 cm.

In this study, a novel multi‐band metamaterial absorber has been proposed which is ultra‐thin, compact, polarisation‐insensitive and wide‐angle absorptive. The proposed structure comprises two concentric metallic rings printed on a dielectric substrate which is backed by a metal ground plane. The proposed structure exhibits four distinct absorption peaks at 4.11, 7.91, 10.13 and 11.51 GHz with peak absorptivities of 98.81, 99.68, 99.98 and 99.34%, respectively, under normal incidence. The designed absorber is polarisation insensitive due to four‐fold symmetry, which has been confirmed by simulation and measurement studies. Moreover, the structure shows high absorption (over 90%) for oblique incident angles up to 45° for both transverse‐electric and transverse‐magnetic polarisations. The surface current distributions at the four absorption frequencies have been illustrated to explain the absorption mechanism of the structure. In addition, several parametric variations are performed to observe the effects of the geometrical dimensions on the absorption performance. The free space measurement method has been utilised to measure the responses of the fabricated structure, which are in good agreement with the simulated results. The proposed ultra‐thin (∼0.013λ0 thin corresponding to lowest absorption frequency) absorber is anticipated to be useful in various potential applications like stealth technology, electromagnetic interference, electromagnetic compatibility and wireless communication.

Three different metamaterial absorbers based on ultra-thin multi-layer structures, with different bandwidth characteristics, are presented. The proposed structure is composed of three vertically stacked metaldielectric layers backed by a metal ground. All the metallic patches are of crossed dipole shape and have different geometrical dimensions, which can be manipulated to design triple-band, 3 dB and 10 dB absorbers separately. The designed structures are polarisation-insensitive as well as wide-angle absorptive for both TE and TM polarisations. The 10 dB broadband structure, exhibiting an absorption bandwidth of 7.5% at 10 GHz, has been fabricated and the absorption performance has been verified with the simulated response. The proposed absorber has the advantages of ultra-thin thickness (λ 0 /50 corresponding to the centre frequency), compact size, simpler design, tunable absorption bandwidth and experimental validation, which makes it a promising candidate for many potential applications.Introduction: Electromagnetic (EM) wave absorbers are of critical importance due to their wide range of applications relating to such as radar cross-section reduction, stealth technology, the anechoic chamber, EM compatibility, EM interference and so forth. However, conventional microwave absorbers are mostly limited to a minimal thickness of one-quarter wavelength and have large surface mass densities, which restrict them from many practical applications [1]. With the recent advancement in the metamaterial approach, absorber structures can be made not only ultra-thin, but also near-unity absorption can be achieved over different frequency bands from the microwave to the terahertz range [2]. These metamaterial absorber structures usually consist of a periodic arrangement of metallic elements imprinted on very thin grounded dielectric substrates. These structures have the attractive properties of controlling the effective medium parameters so that the input impedance gets matched with the free space impedance, which results in minimal reflection from the structure. To date, several designs on metamaterial absorbers have been investigated exhibiting different characteristics, namely, single-band, multi-band, bandwidth-enhanced, polarisation-insensitivity, wide-angle absorption and so forth [3].In this Letter, an ultra-thin polarisation-insensitive and wide-angle metamaterial structure is presented that has variable absorption bandwidths. The proposed structure consists of three different cross-dipole-shaped metallic patches, each of which is printed on a very thin dielectric substrate and the overall structure is backed by a metal ground. By controlling the geometrical dimensions of these metallic patches, the proposed structure exhibits three different types of absorbers: a triple-band absorber, an absorber providing a 3 dB bandwidth of 20% and a broadband absorber having a 7.5% 10 dB absorption bandwidth. The designed absorbers satisfy polarisation-insensitive behaviours as well as provide high absorptions (above 80%) up to a...