A Circuit-based Model for the Interpretation of Perfect Metamaterial Absorbers

A popular absorbing structure, often referred to as Perfect Metamaterial Absorber, comprising metallic periodic pattern over a thin low-loss grounded substrate is studied by resorting to an efficient transmission line model. This approach allows the derivation of simple and reliable closed formulas describing the absorption mechanism of the subwavelength structure. The analytic form of the real part of the input impedance is explicitly derived in order to explain why moderate losses of the substrate is sufficient to achieve matching with free space, that is, perfect absorption. The effect of the constituent parameters for tuning the working frequency and tailoring the absorption bandwidth is addressed. It is also shown that the choice of highly capacitive coupled elements allows obtaining the largest possible bandwidth whereas a highly frequency selective design is achieved with low capacitive elements like a cross array. Finally, the angular stability of the absorbing structure is investigated.Comment: Accepted for publication on IEEE Transactions on Antennas and Propagatio

Waveguide Leaky-Wave Antennas: Modal Configuration and Dispersion Properties

Radiating modes supported by waveguide-based leaky-wave antennas are analyzed in terms of field distribution and a propagation constant. An analytic approach for computing the propagation constant of transverse electric modes in the partially open waveguide and on the basis of the transverse resonance method is presented, making use of a dielectric cover and a partially reflective surface cover

An Inkjet Printed Chipless RFID Sensor for Wireless Humidity Monitoring

A novel chipless RFID humidity sensor based on a finite Artificial Impedance Surface (AIS) is presented. The unit cell of the AIS is composed of three concentric loops thus obtaining three deep and high Q nulls in the electromagnetic response of the tag. The wireless sensor is fabricated using low-cost inkjet printing technology on a thin sheet of commercial coated paper. The patterned surface is placed on a metal backed cardboard layer. The relative humidity information is encoded in the frequency shift of the resonance peaks. Varying the relative humidity level from 50% to 90%, the frequency shift has proven to be up to 270MHz. The position of the resonance peaks has been correlated to the relative humidity level of the environment on the basis of a high number of measurements performed in a climatic chamber, specifically designed for RF measurements of the sensor. A very low error probability of the proposed sensor is demonstrated when the device is used with a 10% RH humidity level discrimination

Theory, design and perspectives of electromagnetic wave absorbers

Electromagnetic absorbers for free space and cavity absorption are discussed. Classical configurations are described as well as more recent designs. The presented layouts are able to provide absorbing behaviors spanning from ultra-narrow band to ultra-wideband. A comparison among various solutions is presented both in terms of achievable bandwidth and in terms of minimum theoretical thickness. The problem of cavity resonances is also addressed. It is shown that resonances can be damped by using alternative solutions with respect to the classic use of lossy magnetic materials

Analysis and Design of Ultra Thin Electromagnetic Absorbers Comprising Resistively Loaded High Impedance Surfaces

High-Impedance Surfaces (HIS) comprising lossy Frequency Selective Surfaces (FSS) are employed to design thin electromagnetic absorbers. The structure, despite its typical resonant behavior, is able to perform a very wideband absorption in a reduced thickness. Losses in the frequency selective surface are introduced by printing the periodic pattern through resistive inks and hence avoiding the typical soldering of a large number of lumped resistors. The effect of the surface resistance of the FSS and dielectric substrate characteristics on the input impedance of the absorber is discussed by means of a circuital model. It is shown that the optimum value of surface resistance is affected both by substrate parameters (thickness and permittivity) and by FSS element shape. The equivalent circuit model is then used to introduce the working principles of the narrowband and the wideband absorbing structure and to derive the best-suited element for wideband absorption. Finally, the experimental validation of the presented structures is presented.Comment: 8 pages, 10 figure

Wideband Scattering Diffusion by using Diffraction of Periodic Surfaces and Optimized Unit Cell Geometries

A methodology to obtain wideband scattering diffusion based on periodic artificial surfaces is presented. The proposed surfaces provide scattering towards multiple propagation directions across an extremely wide frequency band. They comprise unit cells with an optimized geometry and arranged in a periodic lattice characterized by a repetition period larger than one wavelength which induces the excitation of multiple Floquet harmonics. The geometry of the elementary unit cell is optimized in order to minimize the reflection coefficient of the fundamental Floquet harmonic over a wide frequency band. The optimization of FSS geometry is performed through a genetic algorithm in conjunction with periodic Method of Moments. The design method is verified through full-wave simulations and measurements. The proposed solution guarantees very good performance in terms of bandwidth-thickness ratio and removes the need of a high-resolution printing process

Chipless RFID Tag Exploiting Multifrequency Delta-Phase Quantization Encoding

A novel encoding paradigm for chipless RFID tags based on phase quantization is presented. The most distinctive features of this approach are represented by the low requirement on bandwidth and by the encoding scheme. The former is achieved by using only a multi-frequency reading without resorting to ultra-wideband systems whereas the latter relies on linking the information to the quantized difference between the TE and TM phase response of the tag, . The encoding mechanism is described as well as the decoding procedure. The reliability of the illustrated approach is experimentally validated by measurements on fabricated prototypes

Efficient design of multiple-fed leaky wave/Fabry-Perot antennas

A simple transmission line approach for the rapid analysis of multiple fed Leaky Wave antennas based on Fabry-Perot cavities is presented. The method is based on the superposition of the travelling leaky waves excited by each of the applied source. The propagation constant of the leaky waves inside the Fabry-Perot cavity is computed the Transverse Resonance Technique (TRM). The procedure is completely analytic since the antenna does not require a superstrate composed by unconventional FSS elements. This antenna configuration allows the synthesis of very high gain antennas while preserving reasonable bandwidth

Multi-frequency polarizarition converter with enhanced angular robustness

This paper presents a multi-frequency polarization converter for linear polarized waves. The reflection-based polarization converter is realized with a periodic surface. The unit cell comprises loaded dipoles printed on a metallic backed Teflon substrate 1.52mm thick. The unit cell topology results to be robust to the variation of the azimuthal incident angle. For this reason, the proposed device is particularly suitable for applications in which the direction of arrival of electromagnetic w ave is unknown