A simple, fast and efficient method for designing wideband radar absorbers is proposed. The idea is to modify the circuit analog absorber method without perturbing the bandwidth. This is done by utilizing the asymptotic behavior of such an absorber at low frequency and replacing the band-stop resonating frequency selective surfaces with low-pass capacitive ones, which can be synthesized by square patches. It is shown that higher frequencies are not influenced by these modifications. A thin wideband capacitive circuit absorber (CCA) is presented with 28% reduction of thickness and 57% increase of bandwidth in comparison to the Salisbury screen. It is also explained why some optimized metamaterial designs fail to compete with the CCA method. For high permittivity layers, it is shown that the CCA is a better solution than the Jaumann absorber and improvements both in thickness and bandwidth are possible. A three layered ultra wideband (4-24 GHz) CCA is presented with total thickness of 15.1 mm. Finally, a design capable of handling oblique angles of incidence for both polarizations and fulfilling different mechanical, thermal and fabrication constraints is given. The absorption band covers the entire C, X and Ku radar bands (4-18 GHz), showing significant improvement compared to the published circuit analog absorbers. (Less)

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Capacitive Circuit Method for Fast and Efficient Design of Wideband Radar Absorbers

This letter focuses on the modification of a unit cell of a metamaterial absorber to decrease its operational frequency. By decreasing the operational frequency, we can increase capability of the absorber for practical applications and decrease sensitivity of the structure response to the curvature. The structure has been implemented by the array of the unit cell on the FR4 substrate in the front of a conductive plate. Both the simulated and experimental results indicate the structure provides 20% decrease in the minimum operation frequency with respect to the reference structure. Also, the operating frequency range of the structure with absorptivity more than 90% is 1.35–3.5 GHz, i.e., 88% fractional bandwidth. The structure has appropriate response under oblique incident angle from 0° to 40°.

A Metamaterial Absorber With a New Compact Unit Cell

Energy harvesting (EH) or scavenging is recognized as harvesting energy from ambient energy sources in the surrounding environment. This paper reports a literature review on radio frequency (RF) EH using different metasurface/metamaterial structures based on split-ring resonators (SRRs), electric inductive–capacitive (ELC) resonators, square-patch unit cells, square-ring unit cells, etc. The essential parameters in rectifying antenna (rectenna) design are included, such as receiving antenna efficiency, conversion efficiency, dimensions, supporting substrate properties, frequency band, and overall performance, etc. It is noted that rectenna design using conventional antennas such as microstrip antennas, monopole antennas, slot antennas, dielectric resonator antennas, etc. suffers from low power conversion efficiency with larger size. To overcome the above-mentioned constraints and enhance the conversion efficiency with smaller size, metasurface/metamaterial structures are used as EH collectors. An introduction to EH is discussed, followed by an overview of energy sources in the ambient environment. Several hypothetical and experimental studies on metasurface-based EH systems are summarized.

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A Comprehensive Review of Metasurface Structures Suitable for RF Energy Harvesting

Ultrathin dual‐layer triple‐band flexible microwave metamaterial absorber for energy harvesting applications

In this article, a dual-band microwave sensor using a complementary split-ring resonator (CSRR) is presented that determines the concentration of any liquid bi-mixture like water in ethanol and urea in whole milk. The proposed sensor is unique as it is designed, simulated and fabricated using a single-metamaterial cell structure to operate at dual frequency, i.e. 2.45 GHz and 5.8 GHz using an industrial, scientific and medical (ISM) band. The sensor is fabricated on FR4 substrate using a typical photolithography technique, and simulated results are in agreement with the measured results. Investigation of the sensing mechanism for impurity concentration (i.e. water in ethanol or urea in milk) is performed by placing the sample mixture in the pipette placed across from the sensor. As microwave sensors respond to the change in the dielectric constant of the nearby materials, when the liquid mixture concentration varies, there is shifting in the resonant frequency at which the sensor is designed. The proposed sensor is unique due to dual-band resonance, reusability, compactness (12 mm × 20 mm), low cost, noninvasiveness, nondestructiveness, and a user-friendly approach.

Dual-Band Microwave Sensor for Investigation of Liquid Impurity Concentration Using a Metamaterial Complementary Split-Ring Resonator

A compact ultra‐thin ultra‐wideband microwave metamaterial absorber

A new design has been proposed for a single layer polarization-insensitive dual-band metamaterial absorber at C and X bands. The proposed structure consists of a periodic arrangement of a circular resonator embedded in a square resonator. A commercially available FR4 dielectric has been used as a substrate with metallic grounded bottom and imprints on the other side. This structure resonates at 5.5 GHz and 8.9 GHz with absorptivity of 99.8% and 99.97%, respectively. It exhibits polarization-insensitive behaviour for Transverse Electric and Transverse Magnetic polarization under oblique and normal angles of incidence. The field distributions have been studied for better understanding of the absorption mechanism. The fabricated structure has been tested, and the experimental results are similar to the simulated ones. This polarization-insensitive metamaterial absorber with its ease of design and nearly unity absorption can be used for radar applications.

Design of Polarization-Insensitive Dual Band Metamaterial Absorber

Phytochemicals, especially botanical insecticides are currently of interest because of their successful application in plant protection as biocontrol agents. Biological activity of leaf aqueous extract of ten selected medicinal plants were evaluated against the fourth-instar larvae of gram pod borer Helicoverpa armigera (Hubner), (Lepidoptera: Noctuidae). Antifeedant activity of leaf aqueous extracts of Abutilon indicum L., Achyranthus aspera L., Aerva lanata L., Albiziz amara (Roxb), Andrographis paniculata Ness., Cardiospermum halicacabum L., Cassia tora L., Catharanthus roseus L (G) Don., Datura metal L. and Tribulus terrestris L. were evaluated in this study. Preliminary screening after 24 h of exposure with leaf aqueous extracts of the selected plants at a concentration of 1,000 ppm exhibited significant larval mortality rate. The percentage mortality rate ranged considerably from 10.8 to 72.8. The mortality rate was observed in the decreasing order of A. paniculata > C. roseus > D. metal > A. amara > C. halicacabum > A. indicum > C. tora > T. terrestris > A. aspera >A. lanata against the larvae of H. armigera. The results imply that leaf aqueous extract of A. paniculata, C. roseus and D. metal can potentially be used as ecofriendly pest control agents against the larva of H. armigera.

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Antifeedant Activity of Leaf Aqueous Extracts of Selected Medicinal Plants on VI instar larva of Helicoverpa armigera (Hübner)

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.

/pdf/an-ultra-thin-compact-wideband-metamaterial-absorber-3ivovbkjea.pdf

An Ultra-thin Compact Wideband Metamaterial Absorber

In this paper, a nearly perfect metamaterial absorber is proposed with enhanced bandwidth and polarization insensitive. The proposed unit cell structure consists of outer split ring and inner asterisk shaped resonators printed on FR4 dielectric substrate. The designed metamaterial absorber gives broad bandwidth of 2.01 GHz and peak absorptivity of 99.98 and 99.94 % at 19.4 and 19.8 GHz, respectively. The simulation results prove the polarization insensitive behavior of the structure for oblique and normal incidence of the polarized waves at angles of 25° and 85°, respectively. The effective parameter is retrieved and the field distributions are studied. This metamaterial absorber is well suited for K-band applications like radar and satellite communications for uses in weather radar, imaging radar and air traffic control.

S. Ramya

Papers

A compact ultra‐thin ultra‐wideband microwave metamaterial absorber

Design of Polarization-Insensitive Dual Band Metamaterial Absorber

Antifeedant Activity of Leaf Aqueous Extracts of Selected Medicinal Plants on VI instar larva of Helicoverpa armigera (Hübner)

An Ultra-thin Compact Wideband Metamaterial Absorber

Bandwidth Enhanced Nearly Perfect Metamaterial Absorber for K-Band Applications