• Radio Frequency IDentification (RFID)
• Chipless RFID
• Metasurfaces
• Complex-Source Beam diffraction
• Ray tracing
• Asymptotic high-frequency methods
• Additive manufacturing
• 3-D printing

In recent years, radio frequency identification (RFID) technology has gained greater interest because of its wide availability to applications in object identification, sensing, goods tracking and logistics. One of the most relevant limits toward the diffusion of RFID technology in place of traditional optical barcodes is the cost of the tags, even if it has dramatically reduced in the last decade to a few euro cents. Although radio frequency identification of objects provides valuable advantages with respect to barcode, such as the non-line-of-sight and high data rate reading procedure, it is necessary to further reduce the cost of tags to justify big investments of companies to build up a new identification technology. A feasible way to taking into account the tags cost could be the realization of a chipless RF barcode by removing the integrated circuit of the RFID tag. At the same time, in order to keep production costs reasonable, it is necessary to employ low-cost and easily available materials (as for instance the substrates for printed tags). Some of the advantages due to the use of RF are still maintained (e.g. correct working even without an optical visibility, quick reading of tags), and this approach leads to other advantages such as the absence of a minimum threshold power for activating the chip and the possibility to adopt the tag also in harsh environments. At the same time, some drawbacks with respect to using conventional RFID tag occur, since chipless tags cannot be easily reprogrammable and require a wideband reading system. Chipless RFIDs can be divided into two main categories: those that store information in the time domain (TD) and those that perform this task in the frequency domain (FD). In particular FD, chipless tags, also referred to as spectral signature-based tags, encode data into the frequency spectrum using resonant structures. Indeed, they allow to associate a bit of information to the presence or absence of a resonant peak at a predetermined frequency in the spectrum exhibited by the scattered field. One of the most relevant limitations of chipless RFID technology is that tag detection involves a calibration procedure based on two or three independent measurements obtained in the same scenario (tag, background, and eventually ground plane). Although this procedure is feasible in a controlled environment, such as a laboratory, it cannot be easily performed in a real scenario.
In this context, the author designed and tested several chipless RFID systems based on a FD encoding scheme. The proposed solutions are presented and analyzed in Chapter 1, Chapter 2, and Chapter 3. In the first chapter, the design and the implementation of a full Chipless RFID system, comprising the wideband reader, the antenna elements and the chipless tags have been presented. The information embedded in the spectral signature of the tags can be successfully retrieved even if the reader is several tens of centimeters away from the tag, because the latter is in the far-field (FF) region with respect to the antenna. The tags operate in cross-polarization, which is an essential feature for isolating the backscattered signal from the strong co-polar RCS response, originated by the objects surrounding the tags. The depolarizing property of a tag allows employing it also when it is mounted on metallic structures. Moreover, the calibration procedure with background is not necessary since the measured signal level is sufficiently higher than antenna coupling. In Chapter 2, a near-field reading system based on a Chipless 3D-printed tag, fabricated by using a PLA filament and a 3D low-cost printer, has been proposed. The tag has been metallized on the top and bottom surface in order to obtain a resonant-type structure by confining the electromagnetic fields within it. In addition, thanks to the metallization, the tag can be easily mounted on metallic surfaces. The reading process is performed by inserting a probe (a standard SMA connector) inside a small hole in the bottom surface of the tag. The proposed reading scheme is particularly recommended for anti-counterfeiting applications, allowing to avoid unwanted leakage of information and to ensure a closed reading system. Moreover, this reading system does not require any calibration procedure. In Chapter 3, a novel system for detecting radio frequency chipless RFID tags based on a near-field reading through a standard waveguide has been proposed.
The reader consists of a simple open waveguide. A dedicated Ultra Wideband (UWB) reader design can be avoided. The described approach guarantees the advantage of a wideband and reliable reading that does not require any calibration or intense data processing. The reading scenario involves locating the tag inside an object to be identified, for example a letter or a cardboard box, by placing the waveguide close to the Chipless
Tag. Therefore, this system can be successfully employed for typical logistics applications. Furthermore, an extensive analysis has been conducted, both simulated and measured, on the robustness of the system to the misalignment between the tag and the waveguide.
The signature of a chipless tag can be described by adopting different physical quantities according to the type of the specific encoding scheme. The Radar Cross Section (RCS) is commonly used if the information is encoded only in the magnitude of the response, as it happens in many Chipless RFID systems. When a chipless tag is illuminated by a reader, it scatters the incident energy in all directions of the space. It is worth observing that the scattered field is strictly related to the tag geometrical and electrical parameters. On the other hand, the energy scattered in directions different to that leading back to the reader antenna are generating undesired returns from the environment, which may disrupt information acquisition. Given the fundamental relevance of scattering theories in the context of the design and analysis of Chipless tags as well as that of taking into account returns from the environment in the reading procedure, in Chapter 4 an effective ray technique for analyzing scattering problems, has been discussed in detail. The main topic addressed in this chapter concerns the extension of the Uniform Geometrical Theory of Diffraction (UTD) to analyze the scattering from objects illuminated by Complex-Source Beams. In particular, a wedge consisting of a PEC (perfect electric conductor) material has been explicitly considered. The analysis presented is of great interest since CSBs represent an exact solution to Maxwell's equations, and can be used to construct a complete and efficient basis for expanding complex field distributions in scattering and radiation problems. Beyond the canonical problem analyzed, the proposed ray technique is particularly suited to analyze both scattering and radiation problems expecially when the objects under investigation are electrically large.