• POLARIZATION CONVERTERS
• METAMATERIALS
• CHIPLESS RFID
• REFLECTARRAYS

The aim of this thesis is to demonstrate the design of sub wavelength metamaterial for different applications such as reflectarray, polarization converters and chipless RFID systems. Metamaterials are a class of artificial materials designed with the purpose of obtaining properties which do not exist in nature. The planar version of these artificial materials are realized with arrangements of periodic structures known as Frequency Selective Surfaces (FSS).
The structure of this work is shown in the thesis navigation panel in Fig. 1.
Chapter 1 is mainly dedicated to the analytical models for the analysis of metamaterials based on periodic surfaces. In particular, the criteria of application for each model depending on the periodicity of the metamaterial and on the operative wavelength are presented.
In Chapter 2, the design of a Multi Band Single Layer Reflectarray is presented. In this chapter, the attention is focused on the problem of mutual coupling within the unit cell of the reflectarray. In fact, in order to design a multiband reflectarray, multiple resonant elements are required. In the case of sub wavelength unit cells, the resonant elements are physically close to each other, thus being highly coupled. In this case, the design of the reflectarray may not be feasible as it is not possible to tailor the geometry of the resonant elements in order to obtain the desired phase response of the cell for all the working frequencies of the reflectarray. In order to reduce the mutual coupling, a typical approach consists of printing the elements which work at different frequencies on different layers. Unfortunately, in several applications a multilayer structure is not desirable because of the resulting complexity of the final structure. This entails an increase of the weight, bulkiness and manufacturing cost of the reflecarray. In Chapter 2 of this thesis, an iterative design procedure for highly coupled resonant elements of multi band reflectarray is presented. This procedure is then applied to a tri band reflectarray, which has been fabricated. The prototype and the measured results are documented.
In Chapter 3, subwavelength metamaterials have been employed for the design of polarization converters. In particular, the working principle of these devices that are based on reflecting metamaterial is shown. Their working principle has been addressed with the interference theory approach. Moreover, the design of an ultra wideband linear polarization converter based on a grounded periodic surface is given. In this paper, an ultra-wideband linear polarization converting metasurface is also presented. The polarizer is based on a periodic arrangement of metallic elements printed on a grounded dielectric substrate. The unit cell of the metasurface is discretized in a 16x16 pixel matrix obtained with a genetic algorithm. This polarization converter is able to work from 8.12 GHz to 25.16 GHz with a relative bandwidth (-1dB) of 102%. In addition, in order to further extend the polarization converter bandwidth, the unit cell has been modified with a topology refinement algorithm. Consequently, the relative bandwidth has been extended up to 117%.
Chapter 4 reports the application of periodic surfaces to obtain ID and sensing functionality at radiofrequency. Indeed, this chapter is dedicated to chipless RFID. It is an enabling technology developed with the aim of reducing the cost of the classic RFID system by removing the Integrated Circuit. In the past years, the interest around this new technology has increased and extensive studies are currently ongoing in order to exploit its potentiality. Chipless RFID tags can be distinguished in two main categories on the basis of their working mechanism: time domain and frequency domain. The latter class of chipless tag will be considered in this thesis. In particular, the attention is focussed on the design of frequency domain chipless tags with high impedance surfaces. In addition, the techniques employed to transform a simple chipless RFID tag into a sensor are presented. Furthermore, applying one of the presented approaches, a chipless RFID sensor for humidity monitoring is shown. The sensor is realized on a thin sheet of paper using inkjet printing technology and is applied on a grounded cardboard layer. The paper substrate with the coating layer acts as a chemical interactive material (CIM) because it is sensitive to humidity variations. The relative humidity information is encoded in the frequency shift of these resonance peaks, which has proven to be up to 270MHz.
In the last section of Chapter 4, the design of a crosspolarized multi frequency chipless RFID tag, which is robust with respect to the rotation angle, is shown. The tag, which works in linear polarization, is realized with a periodic surface. The measurements of the fabricated prototype are in agreement with the numerical simulations.