On the Bandwidth of High-Impedance Frequency Selective Surfaces

In this letter, the bandwidth of high-impedance surfaces (HISs) is discussed by an equivalent circuit approach. Even if these surfaces have been employed for almost 10 years, it is sometimes unclear how to choose the shape of the frequency selective surface (FSS) on the top of the grounded slab in order to achieve the largest possible bandwidth. Here, we will show that the conventional approach describing the HIS as a parallel connection between the inductance given by the grounded dielectric substrate and the capacitance of the FSS may induce inaccurate results in the determination of the operating bandwidth of the structure. Indeed, in order to derive a more complete model and to provide a more accurate estimate of the operating bandwidth, it is also necessary to introduce the series inductance of the FSS.We will present the explicit expression for defining the bandwidth of a HIS, and we will show that the reduction of the FSS inductance results in the best choice for achieving wide operating bandwidth in correspondence with a given frequency.Comment: 4 pages, 6 figure

Inverse scattering procedures for the reconstruction of one-dimensional permittivity range profile

In the present work we have presented a reliable and efficient algorithm for the data inversion, which is based on a fully nonlinear data model in conjunction with an optimization technique. The reconstruction of the permittivity range profile has been tested both on synthetic and real data to validate the electromagnetic code as well as to assess the accuracy and efficiency of the reconstruction procedure. We have studied the resolution of the algorithm and its robustness to the noise, demonstrating the ability of our procedure to be able to recognize the presence of high discontinuities even independently from the discretization fixed by the user. As a part of the ongoing improvement of the presented method, we have addressed the implementation of a new optimization algorithm, namely the particle swarm optimization, which has been customized and enhanced for our purposes. Finally, a detailed description of a fast and efficient procedure to evaluate the green’s function for a multilayered medium has been given. This is the groundwork useful for the next step toward a more reliable and versatile forward solver to be implemented in the inversion procedure

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

Characteristic Modes Analysis of a Near-Field Polarization-Conversion Metasurface for the Design of a Wideband Circularly Polarized X-Band Antenna

A metasurface (MS) based on loop elements operating in the near field of a linearly-polarized microstrip antenna is employed to realize a circularly polarized radiated field. The properties of the loop unit cell are highlighted with the help of the Characteristic Mode Analysis that is employed for investigating the achievable linear to circular polarization conversion bandwidth and providing the guidelines for the design of the final antenna. A finite structure comprising 4×4 unit cells is tailored for achieving a circularly polarized far field within the whole X-band adopted for satellite communications (7.25 GHz-7.75 GHz, 7.9 GHz-8.4 GHz). A simple but effective single-port excitation scheme is adopted, and the overall performance are assessed by measurements on the fabricated prototype. The good agreement between simulated and measured results confirms the reliability of the proposed approach as well as the meaningful insight provided by Characteristic Modes Theory

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

Normalization-Free Chipless RFIDs by Using Dual-Polarized Interrogation

A reliable encoding/detection scheme for chipless radio frequency identification (RFID) tags, free from any normalization procedure, is presented. The key strategy of the present approach consists in storing the information in the difference between vertically and horizontally polarized reflection coefficients of a completely passive tag. The measured reflection coefficients are preemptively filtered in the time domain to remove most of harmful effects due to the antenna coupling and environment multipath, and finally they are subtracted to obtain the differential response. A couple of chipless tag configurations suitable for providing the desired spectral response are presented. The resonators consist of an artificial impedance surface comprising either concentric rectangular loop resonators or square loop resonators loaded with stubs. The presented approach is experimentally verified in a non-anechoic environment, and its robustness is proved. This calibration-free approach could pave the way to practical applicability of chipless RFID tags in realistic scenarios with unknown response

Wideband radar cross section reduction of slot antennas arrays

A comprehensive analysis aimed at reducing the radar cross section (RCS) of array antennas, preserving at the same time their radiating performance, is presented. A microstrip slot array is considered as a test case to illustrate the proposed strategy for radar cross section reduction (RCSR). It is shown that a remarkable reduction of the radar signature can be accomplished over a frequency band as wide as two octaves by employing an array of periodic resistive elements in front of the radiating apertures. The monostatic and bistatic RCS of the proposed structures are investigated both for normal and oblique incidence. Different arrangements and geometries of the periodic resistive pattern are thoroughly analyzed showing the benefits and the drawbacks in terms of antenna gain and level of the scattered fields. Furthermore, the use of metallic parasitic elements for enhancing the antenna gain is considered, and the scattering phenomena caused by their presence are addressed, taking into account the appearance of grating lobes. The antenna designs are also analyzed by resorting to a bidimensional color plot presenting the variation of the reradiated field both in frequency and spatial domain. The guidelines illustrated by the proposed examples can be easily applied to other antenna architectures

Efficient Excitation of Characteristic Modes for Radiation Pattern Control by Using a Novel Balanced Inductive Coupling Element

A design strategy exploiting the Characteristic Modes Analysis (CMA) is described for improving the radiation efficiency of a mounted-on-platform radiator. To this aim, a novel Balanced Inductive Exciter (BIE) is introduced to improve the modal excitation purity of some Characteristic Modes (CMs). In fact, even if the optimal position of the exciters on the hosting platform is determined by using the Characteristic Modes Theory (CMT), the excitation purity of each mode plays a fundamental role in the radiation efficiency of the radiating system. In particular, achieving a good level of excitation purity strongly reduces the reactive power (Preac) stored in the near field zone and hence maximizes the amount of the radiated power (Pr). To better highlight the benefits offered by the presented approach, a set of BIEs is applied on a platform to obtain a fully-reconfigurable radiation pattern. The evaluation of the Preac, Pr and Equivalent Isotropically Radiated Power (EIRP) provided by the BIE reveals the importance of a pure modal excitation. To assess the reliability of the proposed BIEs some prototypes have been manufactured and tested

Analysis of the Performance Enhancement of MIMO Systems Employing Circular Polarization

The advantages of adopting circular polarization in multiple-input-multiple-output (MIMO) systems are illustrated for both line-of-sight (LOS) and multipath propagation. More in detail, an analysis of the MIMO performance attainable by employing orthogonal circularly polarized (CP) radiators with respect to orthogonal linearly polarized (LP) ones, has been addressed. At first, an accurate analysis is presented aimed at the evaluation of the channel matrix by comprehensively including also the effects of the antenna in LOS condition. In particular, the channel matrix has been calculated as a function of the antenna parameters and orientation, demonstrating that CP radiators are capable of obtaining better average values of the matrix eigenvalues with respect to LP ones. The analysis is therefore completed by evaluating the characteristics of a CP MIMO system operating in indoor environment representing this latter a more challenging condition where multipath propagation occurs. In this latter case, some meaningful numerical experiments have been performed by using a reliable ray-tracing solver, followed by a measurements campaign conducted in a real environment for validation purposes. Measurements, which are in good agreement with simulations, confirm the benefits of adopting circular polarization in MIMO systems with respect to LP