rЈ ϭ ͱG r ϭ ͱ1.47 ϫ 0.95 ϭ 1.18 and an effective read range given by: r ϭ rЈ ⅐ r 0 ϭ 5.90 m This means that the insertion of a proper designed metamaterial slab allows an increase of the effective read range parameter of about 40%. CONCLUSIONSIn this article, new insights and solutions to problems related to small antennas for RFID and wireless sensor applications by metamaterials have been provided. We have reviewed design requirements for passive UHF RFID tag antennas; we outlined the design process, and proposed a performance protocol for tag analysis. We have also shown that a metamaterial slab can be used to increase of about 40% the read range of the tag antenna without using a ground plane. NUMERICAL AND EXPERIMENTAL INVESTIGATION OF BASIC PROPERTIES OF WIRE MEDIUM-BASED SHORTENED HORN ANTENNAS

Abstract-In this paper, we derive quasi-static equivalent-circuit models for the analysis and design of different types of artificial magnetic resonators-i.e., the multiple split-ring resonator, spiral resonator, and labyrinth resonator-which represent popular inclusions to synthesize artificial materials and metamaterials with anomalous values of the permeability in the microwave and millimeter-wave frequency ranges. The proposed models, derived in terms of equivalent circuits, represent an extension of the models presented in a recent publication. In particular, the extended models take into account the presence of a dielectric substrate hosting the metallic inclusions and the losses due to the finite conductivity of the conductors and the finite resistivity of the dielectrics. Exploiting these circuit models, it is possible to accurately predict not only the resonant frequency of the individual inclusions, but also their quality factor and the relative permeability of metamaterial samples made by given arrangements of such inclusions. Finally, the three models have been tested against full-wave simulations and measurements, showing a good accuracy. This result opens the door to a quick and accurate design of the artificial magnetic inclusions to fabricate real-life metamaterial samples with anomalous values of the permeability.

Metasurfaces consisting of electrically thin and densely packed planar arrays of subwavelength elements enable an unprecedented control of the impinging electromagnetic fields. Spatially modulated metasurfaces can efficiently tailor the spatial distribution of these fields with great flexibility. Similarly, time modulated metasurfaces can be successfully used to manipulate the frequency content and time variations of the impinging field. In this paper, we present time-modulated reflective metasurfaces that cause a frequency shift to the impinging radiation, thus realizing an artificial Doppler effect in a non-moving electrically thin structure. Starting from the theoretical analysis, we analytically derive the required time modulation of the surface admittance to achieve this effect, and present a realistic timevarying structure, based on a properly designed and dynamically tuned high-impedance surface. It is analytically and numerically demonstrated that the field emerging from the metasurface is up-/down-converted in frequency according to the modulation profile of the metasurface. The proposed metasurface concept, enabling a frequency modulation of the electromagnetic field "on-the-fly", may find application in telecommunication, radar, and sensing scenarios.

Abstract-The recent extension of the orbital angular momentum (OAM) concept from optical to microwave frequencies has led some researchers to explore how well established antenna techniques can be used to radiate a non-zero OAM electromagnetic field. In this frame, the aim of the present paper is to propose a new approach to generate a non-zero OAM field through a single patch antenna. Using the cavity model, we first analyze the radiated field by a standard circular patch and show that a circular polarized (CP) TM nm mode excited by using two coaxial cables generates an electromagnetic field with an OAM of order ±(n − 1). Then, in order to obtain a simpler structure with a single feed, we design an elliptical patch antenna working on the right-handed (RH) CP TM 21 mode. Using full-wave simulations and experiments on a fabricated prototype, we show that the proposed antenna effectively radiates an electromagnetic field with a first order OAM. Such results prove that properly designed patch antennas can be used as compact and low-cost generators of electromagnetic fields carrying OAM.

No abstract