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This letter presents a $4 \times 4$ Butler matrix realized using stereolithography 3-D printing technology and liquid metal filling. The matrix is in coaxial technology and relies on circular branch lines and curved $50~\Omega $ lines. Input and output ports are aligned and placed on the same plane, while branch lines are organized using all three dimensions to minimize the length of the connecting lines. A prototype, designed at an operating frequency of 12 GHz, is manufactured and tested, showing a footprint of only $1.7\times 1.3\,\,\lambda _{0}^{2}$ . A good agreement between simulated and measured transmission coefficients is observed regardless of the selected input port. These encouraging results open the door to the realization of compact low-cost and high-performing RF beam-steering networks in coaxial technology, based on curing-free additive manufacturing processes.

Radio-Frequency IDentification (RFID) devices and sensors are among the main innovations of the last years, with an enormous impact on the Internet of Things (IoT) physical communication layer as well as on logistics and robotics. The aim of the present paper is to review the main technologies available for RFID sensors, and to identify the corresponding state-of-the-art when these technologies are applied to realistic IoT scenarios. Firstly, the concepts of radio backscattering and harmonic backscattering are analyzed, highlighting the pros and cons of each approach. Then, state-of-the-art solutions are reported, and the performance of each of them are discussed, to provide an overview of the potential of RFID-based sensing in different scenarios.

This study illustrates the sensing and wound healing properties of silk fibroin in combination with peptide patterns, with an emphasis on the printability of multilayered grids, and envisions possible applications of these next-generation silk-based materials. Functionalized silk fibers covalently linked to an arginine–glycine–aspartic acid (RGD) peptide create a platform for preparing a biomaterial ink for 3D printing of grid-like piezoresistors with wound-healing and sensing properties. The culture medium obtained from 3D-printed silk fibroin enriched with RGD peptide improves cell adhesion, accelerating skin repair. Specifically, RGD peptide-modified silk fibroin demonstrated biocompatibility, enhanced cell adhesion, and higher wound closure rates at lower concentration than the neat peptide. It was also shown that the printing of peptide-modified silk fibroin produces a piezoresistive transducer that is the active component of a sensor based on a Schottky diode harmonic transponder encoding information about pressure. We discovered that such biomaterial ink printed in a multilayered grid can be used as a humidity sensor. Furthermore, humidity activates a transition between low and high conductivity states in this medium that is retained unless a negative voltage is applied, paving the way for utilization in non-volatile organic memory devices. Globally, these results pave the way for promising applications, such as monitoring parameters such as human wound care and being integrated in bio-implantable processors.

This paper presents a beamforming network used to synthesize steerable nulls. The antenna topology consists of four patch antennas placed on the lateral sides of a cube. From the analysis of the array factor and the radiation pattern of the single radiating elements, the theory to realize fully steerable nulls on the azimuth plane is presented. The array, targeted for an operating frequency of 3.6 GHz, is fed by a reconfigurable radio-frequency network. The network, which bases its reconfigurability on eight phase shifters, makes it possible to excite couples of adjacent output ports with arbitrary amplitudes and phases. The radiation pattern of the topology is measured for different excitations, confirming the theory. This approach opens the way to a new beamforming approach, that can be applied to interference removal and radio localization applications.

This contribution describes the feasibility of a wireless sensor that uses Micro-Electro-Mechanical Systems (MEMS) on cellulose materials. In particular, the focus is on a paper-aluminum (paper-Al) bimorph cantilever formed by an Al foil glued on top of a paper layer. It is well known that paper relaxes its fibers in the presence of humidity. This means that the length of the paper layer increases (respect to the Al layer) when humidity is absorbed, thus determining the cantilever deflection. Such a mechanism is used to control a capacitor formed by the cantilever itself (Al layer) and by a metal plate printed on a PCB. The capacitor is then applied to a planar notch filter whose resonant frequency is hence determined by the humidity. The preliminary experiments show that the developed circuit features a 2.45 GHz center frequency and a sensitivity of about 3.7 MHz ner each nercent of relative humidity variation.

This paper presents a novel displacement transducer, based on a 3-dimensional coupled-line directional coupler, suitable for wireless passive vibration sensing. The 3-D printed transducer is based on two quarter-wave microstrip lines, which are aligned one on top of the other so that part of the signal flowing in one line is coupled to the other. The four ports of the directional coupler are realized on the same layer by adopting two transmission-line transitions. The circuit is manufactured with stereolithography 3-D printing. Selective metallization is achieved by embossing the desired patterns on the 3-D printed dielectric and then applying silver nanoparticle ink to the protruding areas. Vibrations change the distance between the lines, thereby modulating the coupling coefficient of the directional coupler. This way, the sensed vibration can be encoded in the amplitude modulation (AM) of a backscattered RF carrier. This information can be transmitted in real time and recovered by the reader using a simple AM demodulator without the need for any additional electronic component on the tag, that, thus, results in an ultra-low-cost chipless sensor.

This paper introduces a novel additive-manufacturing technique to obtain high-resolution selective-ink-deposition on complex 3-D objects, packages, and modules for 5G applications. The technique consists of embossing the desired pattern directly on the 3-D printed dielectric surface and then applying ink with a suitable tool. This approach is tested in combination with stereolithography 3-D printing technology to obtain selectively metallized 3-D circuits. In particular, the “clear” resin from FormLab is utilized for the 3-D printed dielectric, while the metallization is performed with silver nanoparticle ink from Suntronic. As a preliminary study, test samples containing lines with different widths are manufactured, demonstrating a pitch down to $135~\mu \text {m}$ and satisfactory sheet resistance of $0.011~\Omega /\text {sq.}$ (the electromagnetic characterization of the dielectric resin is reported in the Appendix). Then, two broadband multiport RF structures are developed to show the versatility of the proposed technology. First, an ultrawideband 3-D crossover, operating in the range 100 MHz–5 GHz, is conceived to test the suitability of the proposed technology to perform selective metallization on curved semienclosed areas. Then, the technology is applied to a multiple-input–multiple-output (MIMO) antenna system, based on four proximity-fed annular slot antennas, arranged on the lateral sides of a cube and decoupled by introducing a cross-shaped structure in the interior of the cube. This circuit offers a broad range of metallization challenges, as it features embossed and engraved parts, high-resolution patterns (line widths down to 0.7 mm) and sharp edges. Each slot radiates unidirectionally with the same polarization and uses the cube and its internal cross-shaped structure as a resonant cavity. The antenna system is designed to operate in the band 3.4–3.8 GHz, which is one of the sub-6-GHz 5G bands in Europe, and it is thought for hotspot and access-point applications. The final antenna topology is composed of only two blocks, weighs 21.29 g, and occupies a volume of $44.4\times 45.8\times 45.8\,\,\text {mm}^{3}$ , featuring an envelope correlation coefficient (ECC) lower than 0.005 and a total active reflection coefficient (TARC) lower than −6 dB in all the bands of interests.

In this contribution, the authors perform the design and show the experimental results relative to a prototype of a combined wireless power transfer (WPT)–power line communications (PLC) system, in which the WPT channel is interfaced to a PLC environment to allow data transfer when the cabled connection is no longer available. The main rationale behind this idea stays in the fact that PLC communication is now a popular choice to enable communications, for instance, in smart grids and in home automation, while WPT devices start to be available in the market (i.e. for mobile phones) and soon they will be a reality also for higher power (i.e. vehicle battery charging). In particular, theoretical insights about the requirements of the system are given; a two coils system has been implemented and a measurement campaign, together with simulations, show that the system is of great potentiality and could be used in applications where both wireless power and data transfer are needed (such as vehicles battery charging), achieving maximum power transfer and good data rate in order to transmit high-speed signals.

This paper describes the design, realization, and application of a custom temperature sensor devoted to the monitoring of the temperature differential between the leaf and the air. This difference is strictly related to the plant water stress and can be used as an input information for an intelligent and flexible irrigation system. A wireless temperature sensor network can be thought as a decision support system used to start irrigation when effectively needed by the cultivation, thus saving water, pump fuel oil, and preventing plant illness caused by over-watering.

In this work, a preliminary investigation of the behavior of a parallel-plate capacitor-like structure immersed in the seawater is presented as the first step towards the development of a non-inductive contactless power transfer system under seawater. The reference structure consists of two square copper parallel plates with a side of 18 cm. Four media have been considered (air, de-ionized water, seawater and tap water); the distance between the plates is varied from 0.5 to 50 cm per each medium, and the transmission coefficient of the capacitor is recorded for each case. The measurements are performed in the frequency range 100–50 MHz. An equivalent circuit model has been carried out and some considerations about the power transfer mechanism in capacitor-like structures in the seawater have been drawn to pave the way towards more applicable structures.

This paper presents a battery-less CMOS integrated RF tag with sensing and localization capabilities. In order to allow tag addressing, communication, and localization by external readers, the IC performs a back-scattering modulation of a sequence of received UWB pulses according to specific spreading codes. For this purpose a UWB RF switch is designed and connected to the UWB antenna. The adoption of backscattering is integral to achieve ultra-low power wireless communication. The use of quasi-orthogonal spreading codes allows communication and localization with multiple tags at the same time. The IC includes a low-power sensing interface suitable for internal temperature sensing or external capacitive sensors. The power required for operation is scavenged from an UHF signal and converted by means of an internal RF rectifier and of a dynamically reconfigurable charge pump circuit. An on-chip micropower oscillator clocks a digital control circuit. The circuit is implemented in a $0.18 \mu \mathrm{m}$ CMOS process. The architecture of the IC and preliminary test results are disclosed.

This effort presents an analysis of a system intended for wireless crack sensing and based on zero-power harmonic transponders. The sensors, which are fabricated by using a paper substrate, are suitable for both structural health and supply-chain monitoring. In the text, different techniques to recover information from the proposed tags are firstly explored, with a particular focus on the potentialities and trade-offs of each approach. Then, the impact of the presence of multiple-nodes on the performance of the system is considered and, finally, some proposals for crack localization in presence of multiple tags are explored and experimentally evaluated.

This paper proposes a combined harvesting system to improve the efficiency and flexibility of autonomous wireless network nodes, supplied by means of wireless power transfer technique. In particular, a mixed system for electromagnetic (EM) and thermal energy harvesting (EH), conceived for passive nodes of wireless sensor networks and radio frequency (RF) identification tags, is described. The proposed system aims at increasing the effectiveness and the efficiency of the EH system by integrating an antenna and a rectifier with a thermo-electric generator (TEG) able to perform thermal EH. The energy provided by the thermal harvester is exploited twice: to increase the rectifier efficiency by providing a voltage usable to improve the bias condition of the rectifying diode, and to provide additional dc energy, harvested for free. Ultimately, a great efficiency improvement, especially at low incident RF power, has been observed. The design methodology and the EM performance of a quarter-wavelength patch antenna, integrated with the TEG are resumed. Then, a test campaign to evaluate the thermal EH performance has been carried out. Afterward, a rectifier with variable bias voltage, operating at the same frequency of the antenna, has been opportunely designed to exploit the harvested thermal energy to bias the diode. A measurement campaign has been then carried out to test the efficiency increment obtained and to validate the proposed solution.

A rectangular ultra-wide band (UWB) cavity-backed slot antenna manufactured by combining 3D printing and inkjet printing technology is introduced in this paper. The antenna substrate is made of polylactic acid (PLA) and is fabricated by using fusion deposition modeling (FDM). Two different infills are used for the slot area and for the area corresponding to the microstrip ground plane to reduce waste and dielectric loss. Thanks to 3D printing versatility, a cavity with slant sides, providing high gain, has been designed. This shape is aimed at increasing the antenna impedance bandwidth and directivity. The rectangular-like proximity-coupled feeding line of the antenna is fabricated by depositing silver nano-particle ink on a layer of SU8. The prototype has been tested and measurements confirmed the satisfactory antenna operation over the whole band of interest, from 3.1 to 10.6 GHz.

The accurate modelling of a system a coupled coils is of paramount importance in the design of wireless power system. In particular, the resistances of the coils and their mutual inductance coefficient determined the efficiency of the device. In this contribution the authors compare the parameters of a two coils system obtained by two numerical codes with the corresponding experimental data. One of the numerical codes is COMSOL, the other is a research code developed at DESTEC. Comparison was performed on a wide frequency range where the skin effect on the current density distribution inside the conductor assumes a relevant importance. The measured system is part of a prototype of a hybrid WPT — PLC system, and the measurements of the scattering parameters have been taken in the frequency range from 30kHz to 30MHz by the use of a VNA. The simulations performed show that the high frequency behaviour of coils made by “thick” conductors is a difficult task and different codes are characterized by different performances.

A novel zero-power wireless crack sensor based on the harmonic radar principle is presented. The tag, fabricated on a paper substrate by means of the copper adhesive tape technology, is targeted for a fundamental frequency f 0 =2.45 GHz (ISM band) and consists of a system of two nested annular slots, a frequency doubler and a stub behaving as a band-stop filter. In presence of a crack, the stub, placed at the input of the doubler, is torn off and an alarm is sent to the receiver. Such a system is suitable for scenarios which involve the detection of any crack increase in a massively distributed population of cracked wall sensors. A wireless experiment demonstrates an operating range of the sensor from 1 to 5 m for a transmitted power EIRP of 25 dBm.

In this effort, the authors present the first demonstration of a flexible, inkjet-printed mm-wave rectenna. The design considerations and processes of the individual antenna array and rectifier are described, demonstrating a realized gain of 5 dBi, and a single-lumped-component via-less rectifier with up to a 2.5V dc voltage for an input power of 18 dBm, at the operation frequency of 24 GHz. The printed system is finally demonstrated in a wireless operation test, in a bent condition, thereby setting the foundation for the emergence of mm-wave-powered smart skins and wearables for the Internet of Things (IoT).

Internet of Things (IoT) is becoming a driving paradigm for the Information and Communication Technology (ICT) evolution. Beyond the well known sensing and communication features that must equip objects to become smart and thus to insert them into IoT world, energy autonomy and the largest available compatibility with hosting objects must be pursued. Chipless sensor tags are one of the most promising solution to cope with all these features concurrently; in fact, they have in-herent sensing and communication capabilities, they are passive, thus autonomous, they are extremely simple to allow an easy integration with the object manufacturing processes, and they can be still easily implementable by using eco-friendly materials. The main limit of classic chipless architectures, however, is that they are usually static (information is stored once the tag is built) and in any case sensing and dynamic information can be recovered only in presence of a stationary environment. This paper presents a review of some “multi-bit” sensor tag architectures based on the harmonic radar principle that can overcome this limitations and thus be profitably used to support the massive deployment of IoT devices.

In this paper the experimental results of a compact low-power rectenna in paper substrate, designed to operate in the Wi-Fi band, are presented. The complete prototype, based on an annular slot antenna and a single-diode rectifier, features a weight of 1.5 grams and shows an RF-to-dc conversion efficiency in the design band of about 40 % for a -10 dBm available input power, of about 28 % at -15 dBm, and in the range [10, 22] % at -20 dBm, corresponding to an output DC voltage in the order of 320, 240 and 60 mV respectively. Additionally, the rectenna features an efficiency higher then 7 % in the whole band 1.8-2.7 dBm for a power density estimated around 3 μW/cm2.

Energy harvesting is well established as one of the prominent enabling technologies [along with radio-frequency identification (RFID), wireless power transfer, and green electronics] for the pervasive development of Internet of Things (IoT). This paper focuses on a particular, yet broad, class of systems that falls in the IoT category of large area electronics (LAE). This class is represented by “smart surfaces.” The paper, after an introductory overview about how smart surfaces are collocated in the IoT and LAE scenario, first deals with technologies and architectures involved, namely, materials, antennas, RFID systems, and chipless structures; then, some exemplifying solutions are illustrated to show the present development of these concurrent technologies in this area and to stimulate further solutions. Conclusions and future trends are then drawn.

A 24-GHz patch array antenna with integrated feeding network has been fabricated exploiting a multi-layer cellulose-based (i.e. paper) substrate. The adopted microstrip circuitry exploits a copper adhesive laminate that is shaped by a photo-lithographic process and transferred to the hosting substrate using a sacrificial layer. The multi-layer structure is obtained by stacking and gluing two layers of photo-paper with an interposed copper ground plane. The measurements show an input reflection coefficient of about −29 dB at the centre frequency, an operating bandwidth with S 11 ⩽ −20 dB of 540 MHz and a gain of 7.4 dBi. The estimated radiation efficiency is 35%. The proposed design shows the feasibility of low-cost antenna systems for green wireless internet technology and applications up to the boundary between microwaves and millimetre-waves.

This paper presents a feasibility study for a very high data rate receiver operating in the K/Ka-band suitable to future Moon exploration missions. The receiver specifications are outlined starting from the mission scenario and from a careful system analysis. The designed architecture uses a low noise front-end to down-convert the incoming K/Ka-band signal into a 3.7 GHz intermediate frequency (IF). For maximum flexibility, a software defined radio (SDR) is adopted for the I/Q demodulation and for the analog to digital conversion (ADC). The decoding operations and the data interface are carried out by a processor based on field programmable gate array (FPGA) circuits. To experimentally verify the above concepts, a preliminary front-end breadboard is implemented, operating between 27.5 and 30 GHz. The breadboard, which uses components off the shelf (COTS) and evaluation boards (EVBs), is characterized by a 46 dB gain, a 3.4 dB noise figure and a &minus, 37 dBm input-referred 1 dB compression point. Finally, a 40 Msym / s quadrature phase shift keying (QPSK) signal is demodulated by means of a commercially available SDR, demonstrating the above concept. The importance of these results is that they have been obtained exploiting a class of miniaturized and low cost microwave integrated circuits currently available on the market, opening the way to a dense communication infrastructure on cislunar space.

This letter presents the design of a Schottky diode frequency doubler suitable for harmonic RFID tags. A microwave frequency doubler is implemented in a cellulose-based (paper) substrate, i.e., an ultra-low cost, recyclable and biodegradable material. The circuit exploits a distributed microstrip structure that is fabricated using a copper adhesive laminate to have low conductor losses. The measurements show a conversion loss of 13.4 dB at the output frequency of 2.08 GHz. This is achieved with an available input power of -10 dBm only. Finally a harmonic RFID experiment proves a reading range of 50 cm, obtained by transmitting 0 dBm and receiving a second harmonic of -60 dBm, i.e., well above the sensitivity of a typical microwave receiver.

Internet of Things (IoT) is becoming a driving paradigm for the Information and Communication Technology (ICT) evolution. This paradigm is not a revolution, yet is a way to look at, use and evolve existing technologies and apply them to new solutions. Among these technologies, Wireless Power Transmission (WPT) and related ones can be certainly considered enabling for IoT. WPT in fact, on the one hand is the core of Radio Frequency IDentification (RFID) systems, that represents likely the most promising physical layer communication platform for IoT; on the other hand WPT itself can be a means to power supply “things” and make them independent from grid connection and batteries, especially when massive localized concentration of granular devices can be envisaged, such as, for instance, in the case of “smart surface” implementation, “smart gravel” and so on. Example applications will be showed in this contribution in a comprehensive and holistic way, emphasizing the consistency of the developed technological solutions and architectures with short term and long term constraints and requirements for electronic apparatuses conceived for smart objects. Namely: data acquisition, wireless connectivity (short term), energy autonomy and object compatibility (long term).

At the moment, the most popular information encoding technique is based on the generation of a particular signature in the spectrum of the backscattered signal. Chipless tags which rely on spectral signature encode data by using resonant structures working as band rejection filters, designed to resonate at defined frequencies. In this paper, a parametric study of different layouts of spiral resonator coupled to a transmission line and band rejection filters on flexible substrate, namely a 100 um thick PET substrate, operating in the 2–4 GHz band is firstly performed with the aim of designing a chipless tag working either as identifier or sensor. The design has been carried out by means of an electromagnetic CAD, and the performance of the proposed structures has been evaluated by the analysis of the respective S-parameters. Then, a proof of concept of a novel passive, wireless and chipless crack sensor has been introduced as practical example.

This paper summarizes the main technologies adopted for the design circuits and systems on cellulose substrate useful for RFID tags. This in agreement with the needing of new ”green” and low-cost technological solutions, required from the project GRETA (GREen TAgs), in order to reduce the impact of the spreading of electronic devices in the environment. After a brief introduction and the description of the these technologies, the mandatory electromagnetic characterization of paper substrates is proposed. Finally the examples of designed and manufactured circuits, tags and systems exploiting cellulose substrates are reported.

This paper presents the design of a novel compact dual-layer harmonic tag in paper substrate, based on a system of nested annular slot antennas. The passive tag is interrogated by a signal at f 0 = 1.2 GHz and the signal transmitted back to the reader is converted to 2f 0 = 2.4 GHz in order for the system to be immune to clutter returns. The frequency conversion is performed by a single Schottky diode frequency doubler, which shows a theoretical conversion loss of 13 dB at the output frequency of 2.4 GHz for an available input power of only −10 dBm. Additionally, a tapered annular slot antenna has been proposed so as to increase the operational bandwidth.

This paper summarizes the most important technologies, concurrently participating to build the technological platform needed for a realistic implementation of the Internet of Things (IoT) paradigm. At the present state of the evolution of IoT, these technologies are mostly: Radio Frequency IDentification (RFID), Green Electronics (GE), Wireless Power Transfer (WPT) and Energy Harvesting (EH). This contribution briefly explains the reason for that, and shows a collection of scientific contributions which can be seen as examples. The deep description of the proposed systems can be found in the relative referenced papers.

This paper presents a miniaturized Doppler radar that can be used as a motion sensor for low-cost Internet of things (IoT) applications. For the first time, a radar front-end and its antenna are integrated on a multilayer cellulose-based substrate, built-up by alternating paper, glue and metal layers. The circuit exploits a distributed microstrip structure that is realized using a copper adhesive laminate, so as to obtain a low-loss conductor. The radar operates at 24 GHz and transmits 5 mW of power. The antenna has a gain of 7.4 dBi and features a half power beam-width of 48 degrees. The sensor, that is just the size of a stamp, is able to detect the movement of a walking person up to 10 m in distance, while a minimum speed of 50 mm/s up to 3 m is clearly measured. Beyond this specific result, the present paper demonstrates that the attractive features of cellulose, including ultra-low cost and eco-friendliness (i.e., recyclability and biodegradability), can even be exploited for the realization of future high-frequency hardware. This opens opens the door to the implementation on cellulose of devices and systems which make up the “sensing layer” at the base of the IoT ecosystem.

This paper describes the design and the optimization of a quarter-wave patch antenna with a Thermo-Electric Generator (TEG). The antenna operates in the Industrial, Scientific and Medical (ISM) frequency band 2.4-2.5 GHz. The TEG, mounted on the top of the antenna, is used to perform thermal Energy Harvesting (EH) in order to supply wireless nodes. This antenna topology is chosen in order to provide a good thermal transfer between the TEG and the ground plane of the antenna. The antenna design consists of the optimization of its radiation properties in presence of the TEG electromagnetic model. Finally, a prototype of the antenna was fabricated and the TEG was mounted on its top. The measurements show a reflection coefficient lower -10 dB in the overall frequency band and a gain of 2.3 dB at 2.45 GHz.

This work describes the realization of a fully organic “chip-less” tag, based on the harmonic RFID architecture, operating at 7.5 and 15 MHz. The tag is fabricated on paper substrate and includes an organic pentacene-based diode, as a non-linear component, to generate harmonics. The communication between reader and tag is provided by coupled resonators operating at the fundamental and harmonic frequencies. A measure campaign of the complete reader-tag system has been performed and the results are here reported.

The feasibility study of a harmonic RFID chipless sensing tag is presented. The tag sensor is composed by an antenna at f 0 = 1.2 GHz, a Schottky diode (HSMS2850), a balanced impedances bridge with a sensing variable element, and a second antenna working at 2f 0 = 2.4 GHz. When interrogated with a signal at f 0 the tag responds with a signal at 2f 0 only when the bridge is unbalanced due to a variation of the sensible impedance value of at least 5 Ω. With this approach the system is energetically autonomous, being activated by the received signal and responds only when there is a change in the sensed quantity. No IC for carrier modulation is required to transfer the information.

Wireless power transmission (WPT) is an emerging technology that is gaining increased visibility in recent years. Efficient WPT circuits, systems and strategies can address a large group of applications spanning from batteryless systems, battery-free sensors, passive RF identification, near-field communications, and many others. WPT is a fundamental enabling technology of the Internet of Things concept, as well as machine-to-machine communications, since it minimizes the use of batteries and eliminates wired power connections. WPT technology brings together RF and dc circuit and system designers with different backgrounds on circuit design, novel materials and applications, and regulatory issues, forming a cross disciplinary team in order to achieve an efficient transmission of power over the air interface. This paper aims to present WPT technology in an integrated way, addressing state-of-the-art and challenges, and to discuss future R&D perspectives summarizing recent activities in Europe. The work of N. Borges Carvalho and A. J. S. Soares Boaventura was supported by the Portuguese Foundation for Science and Technology (FCT) under Project CREATION EXCL/EEI-TEL/0067/2012 and Doctoral Scholarship SFRH/BD/80615/2011. The work of H. Rogier was supported by BELSPO through the IAP Phase VII BESTCOM project and the Fund for Scientific Research-Flanders (FWO-V). The work of A. Georgiadis and A. Collado was supported by the European Union (EU) under Marie Curie FP7-PEOPLE-2009-IAPP 251557 and the Spanish Ministry of Economy and Competitiveness Project TEC 2012-39143. The work of J. A. García and M. N. Ruíz was supported by the Spanish Ministries MICINN and MINECO under FEDER co-funded Project TEC2011-29126-C03-01 and Project CSD2008-00068. The work of J. Kracek and M. Mazanek was supported in part by the Czech Ministry of Education Youth and Sports under Project OC09075–Novel Emerging Wireless Systems.

The lumped-element finite-difference time-domain method is used to analyze quasi-optical multipliers based on diode loaded slot antennas. The method is validated firstly for a passive microstrip-fed structure then for the diode loaded case in both small- and large-signal regimes. The diode model is separately validated using a series diode mounted on a microstrip line. Input return loss and radiation patterns show good agreement with measurements and the concept of effective conversion loss is introduced and results show reasonable agreement between measurement and simulation. A new diode arrangement is introduced where dual offset diodes are placed in the slot instead of the conventional central diode. The diode position can then act as an extra design parameter. The performance of the two structures has been compared; currently best performance is still obtained for the central-diode structure. Finally, a fully quasi-optical structure is simulated with plane-wave excitation. Central and dual-diode structures are again compared and the diode position and input plane-wave field strengths are optimized. Slot voltage distributions, radiation patterns, and effective quasi-optical conversion losses are presented.

The inclusion of lumped elements, both linear and nonlinear, into the finite-difference time-domain (FDTD) algorithm has been recently made possible by the introduction of the lumped element FDTD method. Such a method, however, cannot efficiently and accurately account for two-terminal networks made of several lumped elements, arbitrarily connected together. This limitation can be removed as proposed in this paper by describing the network in terms of its impedance in the Laplace domain and by using appropriate digital signal-processing methodologies to fit the resulting description to Yee's algorithm. The resulting difference equations allow an arbitrary two-terminal network to be inserted into one FDTD cell, preserving the full explicit nature of the conventional FDTD scheme and requiring a minimum number of additional storage variables. The new approach has been validated by comparison with the exact solution of a parallel-plate waveguide loaded with lumped networks in the transverse plane.

Continous advances in material technology, in the field of integrated optics and optoelectronics, allow the realization of devices with geometries more and more compact and complex. Because of this trend, there is a parallel need for accurate fully numerical CAD tools. Among new ones, the FDTD method, already widely and successfully used for the characterization of microwave and millimeter-wave devices, is emerging in optics community because of its accuracy and versatility. However, in spite of the tremendous increase in computing power, the applicability of the method is still limited by the typical dimensions of optical structures. To overcome these limitations a specialized version of the FDTD algorithm for the rigorous analysis of 3D optical and optoelectronic devices is proposed and validated. This new technique is then used to characterize the optical behaviour of a MQW waveguide electroabsorption modulator.

The increasing attention of ICT community towards Internet of Things (IoT) is leading to the need of optimizing the management of an amount of information that grows with the addition of more objects connected to the internet. In this paper we will focus on the challenges that this scenario is inspiring and will dare suggest an holistic vision to look for solutions respectful of the environment.

In this article a paper-based contactless frequency doubler for harmonic RFID applications is described, fabricated and measured. The doubler, realized on paper substrate, generates the harmonic signal only by means of a Schottky diode. The fundamental and harmonic frequencies are wirelessly transferred to and from the doubler by coupled aerial resonators. The system operates at 7.5 and 15 MHz and these frequencies are chosen, without lack of generality, to accomplish for the future realization of a fully organic frequency doubler exploiting paper printed coils and organic diode (pentacene based), the present frequency limit of which is nowadays around 15 MHz. This perspective realization constrained also the geometry of the planar resonators that, although the diode at this level of the development is a conventional lumped one, are suited to host pentacene depositions and organic diode electrodes.

The behavior of packaged single and coupled MMIC via-hole grounds has been investigated by using a graded mesh FDTD code running on the massive parallel computer DEC 12000 with 4 K processors. Theoretical simulations have been compared with experimental measurements showing excellent agreement. Moreover, since the package introduces resonances, we have also investigated several different possibilities to choke off these resonances. It is shown that the common practice of inserting a damping layer just below the lid is often not effective. In particular, the importance of placing damping layers also on the side walls is demonstrated. >

This work investigates the design of a paper-substrate dipole antenna and its magnetic coupling to a UHF RFID chip. The magnetic coupling is realized by means of a heterogeneous transformer, the primary winding of which is implemented on the same paper-substrate of the antenna. The secondary winding, instead, is directly fabricated on the Si RFID chip, thus does not require for galvanic contacts between chip and antenna. The transformer insertion loss can be reduced to the device Maximum Available Gain (MAG) if a proper impedance termination of the primary (Z S,opt ) and secondary (Z L,opt ) windings is adopted. For the considered heterogeneous transformer the MAG is quite low and around −0.6 dB. Following this idea, a bow-tie dipole antenna is designed to meet Z S,opt at the operating frequency of 868 MHz. The antenna size is reduced by exploiting a meander line. As a result, the designed dipole features an overall length of about 40 mm. To the authors knowledge this is the first bow-tie antenna the design of which has been optimized for the magnetic RFID coupling concept.

This article deals with a novel low-cost technology for the assembly of flexible substrate antennas and UHF RFID Si chips. Such a technology exploits a magnetic coupling mechanism, thus avoiding the need for galvanic contacts between the Si chip and the antenna itself. The magnetic coupling is established by a planar transformer, whose primary and secondary windings are implemented on flexible substrate and Si chip respectively. As a result the Si chip can be assembled on the antenna with a mere placing and gluing process. First the idea has been validated by theory. Electromagnetic simulations of a square heterogeneous transformer (1.0 mm side) show a minimum loss of 0.6 dB at 900 MHz, under simultaneous matching conditions. Then a symmetrical PCB transformer (1.5 mm side) has been fabricated and tested. Measurements indicates a minimum loss of 0.3 dB at 900 MHz, thus confirming the validity of the proposed solution.

This study shows that, at the present state of development, one-bit frequency doubling tags seems to be feasible exploiting green technologies. The optimal process will exploit a paper substrate where both antennas and pentacene organic diodes could be deposited by means of ink-jet printing. The proposed frequency doubling tag has a fully uniplanar layout (no vias needed), whereas the use of simple non-linear devices is adequate to open the market of organic microwave RFID systems.

RF self-identification and triggering system are suited for the detection of fast moving objects. We developed a system consisting of a fixed transmitter, located in the path side, and of a receiver unit mounted aboard the vehicle. A slotted waveguide antenna array is used in the TX side to accurately define the position of the moving object. The paper describes the characterization of the transmitting antenna that has been designed using numerical simulation based on a home-developed FDTD algorithm. The antenna has been realized and measured, so that a comparison between simulation and experimental data is also reported.