In the past few years Radio Frequency Identification (RFID) has grown to be one of the most popular technologies in the area of identification systems. Following a brief survey of RFID systems, this paper provides a technical review of work undertaken in the field of time-domain chipless RFID tags and sensors. This paper aims not only to address the chipless tags which use Time Domain Reflectometry (TDR) concept for data encoding but also for the use of Ultra-Wideband Impulse-Radar (UWB-IR) as a time-domain measurement technique. The penultimate section intends to focus on time-domain reading setups and finally, a brief comparison between this method and other chipless techniques is provided.

A novel wireless sensor network system with compatibility of microwave power transmission (MPT) using a Gallium Nitride (GaN) power amplifier has been fabricated and tested. The sensor node operates using electrical power supplied by the MPT system. Time- and frequency-division operations are proposed for the compatibility. Under the frequency-division operation, receiving signal strength indicator of −85 dBm and packet error rate of <1% were measured when the available DC power of a rectifier with 160 mW output power. Under 120-min measurement, sustainable power balance between radio-frequency–DC conversion and power consumption in stable operation of the sensor node was achieved.

This work proposes a chipless radio frequency identification approach based on the working principle of the harmonic radar. A frequency multiplication stage is performed by a non-linearity (i.e. a Shottky diode) on the tag in order for the tag answer to be insulated from the interrogation signal, thus avoiding the need for clutter cancellation techniques. Firstly, the performance of a simple one-bit harmonic tag relying on a low-power frequency doubler is analyzed and then a novel crack sensor, implemented by adding a disposable band-stop filter, is presented. Both solutions demonstrate tag-to-reader operational distances beyond 1 m. The characterizing blocks (namely the frequency doubler and the filter) are fabricated on cellulose substrates (i.e. regular photographic paper), thus being conformal to their implementation for applications in the new paradigm of Internet of Things.

We proposed and examined a microwave power transfer system for electric vehicles (EVs). In this system, electricity is transmitted from a transmitting antenna over an EV to a receiving antenna on the roof of the EV. We used a rectenna to convert the received microwave power to direct current power. The conversion efficiency of a rectenna array is affected by the input power level distribution, and we have to form a flat-topped beam pattern to increase the conversion efficiency. We conducted an experiment to form a flat-topped beam pattern by using a phased array antenna. In this experiment, the output power of each antenna element is uniform and cannot be controlled independently. Hence, we controlled only the output phases of each antenna element and formed a flat-topped beam pattern. The distance between the transmitting antenna and the receiving area is 6.45 m, and the receiving area corresponds to a space in which the azimuth and elevation are in the range of −5°–5°.

The electromagnetic (EM) imaging technique at mm-band 60 GHz is proposed for data encoding purpose in the chipless Radio Frequency Identification (RFID) systems. The fully printable chipless RFID tag comprises tiny conductive EM polarizers to create high cross-polar radar cross-section. Synthetic aperture radar approach is applied for formation of the tag's EM-image and revealing the tag's content. The achieved high data encoding capacity of 2 bits/cm2 in this technique based on a fully printable tag is very convincing for many applications. The system immunity to multipath interference, bending effect, and printing inaccuracy suggests huge potentials for low-cost item tagging. Tags are also readable through a tick paper envelop; hence secure identification is provided by the proposed technique.

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.

This paper presents the design procedure of two ultra-high-frequency radio frequency identification reader antennas used in searching tagged items. They consist of microstrip arrays with alternating orthogonal dipoles, which are fed in series by a pair of microstrip lines. The dipoles are designed properly to provide the required bandwidth. The inter-element distance is adjusted to the center frequency, where the elements provide in-phase excitation and create two orthogonal electric-field components that give beams with direction diversity. Simulated results show that the return loss bandwidth (RL > 13 dB) of the first antenna design covers the required frequency band of ETSI (865–868 MHz) standard. In addition, simulated and measured results of the second antenna design indicate that the return loss bandwidth covers both the frequency bands of european telecommunications standards institute (ETSI) and federal communications commission (FCC) (865–928 MHz) standards. Regarding the coverage volume in the vicinity of the antenna, it was deduced that both antennas can read tagged items in a semi-cylindrical volume that extends to a radius of more than 50 cm. Finally, a case study of reading tagged books in front of a library cabinet with six shelves has been presented.

This paper discusses the electromagnetic (EM) signature of Arabic alphabets that can be considered as standards particles to form chipless tags. Normalized Arial font is suited as example but the method can be applied for any other font. The letters are realized by metallic strips or better, by conductive ink. All the 28 letters have been simulated and their EM signatures for both field polarizations are extracted. It is demonstrated that combining vertical and horizontal responses allow the identification of letters without ambiguity. Moreover, the case of letter with punctuation (one to three points) is considered in more details. Indeed, we propose to modify very slightly these letters by connecting the points to the body of the letters. This connection is made by a unique straight and very thin strip. Under this modification these letters exhibit more exploitable signatures. Finally, a lookup table for identification of the 28 letters is carried out.

The design of magnetic couplers for inductive power transfer has probably become the major challenge for those who wish to enter this promising research field. The number of variables that determine physical dimensions of a coupler is typically too high to allow analytical (exact) solutions in practical time when realistic magnetic materials are to be included. Thus, this paper suggests and describes a series of algorithms based on the finite element method (FEM) able to convert basic inputs (target inductances, primary current, frequency, and mechanical restrictions) into a geometric solution that satisfies user-defined targets for uncompensated power, open-circuit voltage, and short-circuit current. Advantages of these algorithms when compared with other existing design methods are: simplicity in terms of structure at the same time that require minimum user intervention to complete a full design; do not rely in expensive finite element solvers; user does not require previous background in FEM formulation. Experimental results show that the proposed design method based on two-dimensional FEM has errors of <8% when compared with three-dimensional FEM and can perform iterations in seconds. It is expected that the proposed routines encourage and provide design insights for practitioners, enthusiasts, and non-specialized engineers.

Design steps are outlined for maximizing the RF-to-dc power conversion efficiency (PCE) of a rectenna. It turns out that at a frequency of 868 MHz, a high-ohmic loaded rectifier will lead to a highly sensitive and power conversion efficient rectenna. It is demonstrated that a rectenna thus designed, using a 50 Ω antenna and lumped element matching network gives a superior PCE compared with state of the art also for lower resistive loading. By omitting the matching network and directly, conjugate impedance matching the antenna to the rectifier, the PCE may be further increased and the rectenna size reduced as it is demonstrated with a rectenna prototype measuring only 0.028 squared wavelengths at 868 MHz and demonstrating a PCE of 55% for a −10 dBm RF input power level.

Magneto-inductive waves are a form of propagation which only exists in certain types of magnetic metamaterials formed from inductively coupled resonant circuits. We present an investigation of their potential as contactless power transfer devices capable of carrying power along a surface between suitably prepared terminals while simultaneously offering a broadband data channel. Input impedances and their matching conditions are explored with a view to offering a simple power system design. A device with 75% peak and 40% minimum efficiency is demonstrated and designs with potential for better than 70% mean and 90% peak are reported. The product of planar magnetic coupling and metamaterial cell Q factor is determined to be a key optimization parameter for high efficiency.

This paper presents a study of Schottky diode rectenna (rectifying antenna) for radiofrequency (RF) energy-harvesting systems. These rectennas are suitable for wireless sensors with the rechargeable battery technology especially at low-power densities. A rectifying circuit is proposed with single high responsivity Schottky diode for RF–DC conversion. A matching circuit is optimized to improve not only the power transfer between the antenna and the diode, but also to reject harmonic signals. The radiating part is a monopole antenna, with a large bandwidth in the frequency domain and an omni-directional radiation pattern in the azimuthal plane. We show that antenna frequency response takes part in the improvement of the efficiency. The rectifier is integrated with the antenna on a printed circuit board, leading to 30% of size reduction with the same performance. The aim is to reach the highest efficiency with a single tone signal and a compact rectenna. This rectenna was simulated using both Agilent ADS and Ansoft HFSS software. An output DC voltage of 210 mV was measured inside an anechoic chamber which received a single tone signal of 2 µW/cm2 power density. The highest efficiency of 34% was obtained at a power density of 1.3 µW/cm2.

Wireless charging is emerging as a viable technology in many industries, including consumer, medical, and sensor electronics. An investigation of design principles is conducted for a wireless charging platform that is designed to charge devices of different sizes and technologies, using only through vias. It is shown that at a 5 mm separation distance, a coupling coefficient can be achieved which varies from 0.12 to 0.37 when staggered hexagonal transmitter coils (approximately 5 cm across) are used with an unstaggered square receiver coil, which declines to 0.06–0.11 at 2 cm separation. Without design measures, the coupling coefficient will approach zero at certain positions. The quality factors of the coils can be improved by stacking the coils in parallel, enabling the use of only through-vias, while the inductance can be controlled horizontally by increasing the number of turns in the inductor.

Current robotic systems have achieved great sophistication in kinematic motion, control, and neural processing. One of the most challenging limitations imposed on modern robotics is the portable power source needed to sustain tether-free operation. Energy storage devices such as batteries and combustion engines may be heavy, require a great deal of space, and invariably have a finite energy capacity. Methods to control such devices may also impose limitations as most robotic systems rely on either tethered or radiative communication. The unavoidable repercussion of these limitations is the ultimate reduction of mobility and operation time to achieve specific tasks. To address these challenges, we apply our quasi-wireless powering methodology to show the operation of two robotic devices over a 1×1 m2 surface. Both power and control signals are transmitted simultaneously, producing seamless storage-free functionality over the entire area with a communication technique that is not line-of-sight or radiation dependent. We demonstrate an average power transfer efficiency of 93% using commercially available toy robots and discuss parameters relating to the power and communication performance.

This article presents a modeling and parametric investigation of printed circuit board (PCB) coils used in inductive power charging systems by using intensive full-wave electromagnetic simulations. Low frequencies applications (below 1 MHz) are targeted. The proposed modeling approach and design methodology are validated for wireless power transfer systems including transmitting (Tx) and receiving (Rx) coils. The impact of ferrite materials used for shielding and efficiency improvement is also analyzed. Optimized PCB coils allowing a theoretical efficiency of 88.7% at 100 kHz and 98.5% at 1 MHz confirms that PCB coils are appropriate for wireless power transfer at such frequencies.