Pattern-reconfigurable antennas with multiple switchable beams, especially with both boresight and endfire directions, are highly desired for wireless communications. In this letter, a novel pattern-reconfigurable antenna is proposed that provides an efficient solution. By reconfiguring parasitic striplines placed around a radiating dipole and reflecting metal pieces under the dipole using p-i-n diodes, the antenna main beam can be switched to five directions in the elevation plane, approximately from $-90^{\circ }$ (left endfire), $-45^{\circ }$ , 0 $^{\circ }$ (boresight), $+45^{\circ }$ , to $+90^{\circ }$ (right endfire). The proposed antenna operates at 2.45 GHz with dimensions of about 0.57 $\lambda$ $\times\, 0.45\lambda$ $\times\, 0.28\lambda$ . An antenna prototype is fabricated and measured. For all five directional beams, the measured $|S_{11}|$ values are below $-$ 13 dB, and the measured realized gains range from 5.2 to 6.5 dBi. They agree reasonably well with the simulated ones.

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Pattern-Reconfigurable Antenna With Five Switchable Beams in Elevation Plane

An overview of transmission/reflection-based methods for the electromagnetic characterisation of materials is presented. The paper initially describes the most popular approaches for the characterisation of bulk materials in terms of dielectric permittivity and magnetic permeability. Subsequently, the limitations and the methods aimed at removing the ambiguities deriving from the application of the classical Nicolson–Ross–Weir direct inversion are discussed. The second part of the paper is focused on the characterisation of partially conductive thin sheets in terms of surface impedance via waveguide setups. All the presented measurement techniques are applicable to conventional transmission reflection devices such as coaxial cables or waveguides.

Electromagnetic Characterisation of Materials by Using Transmission/Reflection (T/R) Devices

Radio Frequency Identification (RFID) sensors, integrating the features of Wireless Information and Power Transfer (WIPT), object identification and energy efficient sensing capabilities, have been considered a new paradigm of sensing and communication for the futuristic information systems. RFID sensor tags featuring contactless sensing, wireless information transfer, wireless powered, light weight, non-line-of-sight transmission, flexible and pasteable are a critical enabling technology for future Internet-of-Things (IoT) applications, such as manufacturing, logistics, healthcare, agriculture and food. They have attracted numerous research efforts due to their innovative potential in the various application fields. However, there has been a gap between the in-lab investigations and the practical IoT application scenarios, which has motivated this survey of this research to identify the promising enabling techniques and the underlying challenges. This study aims to provide an exhaustive review on the state-of-art RFID sensor technologies from the system implementation perspective by focusing on the fundamental RF energy harvesting theories, the recent technical progresses and commercial solutions, innovative applications and some RFID sensor based IoT solutions, identify the underlying technological challenges at the time being, and give the future research trends and promising application fields in the rich sensing applications of the forthcoming IoT era.

https://www.mdpi.com/1424-8220/19/18/4012/pdf

Radio Frequency Identification and Sensing Techniques and Their Applications-A Review of the State-of-the-Art.

Recently, there is an increasing demand for food safety and quality monitoring in the agrifood sector, needing active and smart packaging. Radio Frequency Identification (RFID) has the potential to become one of the most promising technology for the food industry since it can satisfy all the requirements of a sensor and identification in food safety monitoring, packages tracking, inventory control, early warning, and easy check-out. The purpose of this paper is to introduce the recent advancement of the chipless RFID humidity, temperature, gas, pH, and bio-sensors, as these types of sensors are required for evaluating the quality of the food in the food industry. Additionally, the paper explains the requirements of the food industry for quality monitoring of the food products, covers the existing challenges of chipless RFID sensor technology, and highlights the future direction of the research.

Potential Chipless RFID Sensors for Food Packaging Applications: A Review

Radio-frequency identification (RFID) sensors are one of the fundamental components of the internet of things that aims at connecting every physical object to the cloud for the exchange of information. In this framework, chipless RFIDs are a breakthrough technology because they remove the cost associated with the chip, being at the same time printable, passive, low-power and suitable for harsh environments. After the important results achieved with multibit chipless tags, there is a clear motivation and interest to extend the chipless sensing functionality to physical, chemical, structural and environmental parameters. These potentialities triggered a strong interest in the scientific and industrial community towards this type of application. Temperature and humidity sensors, as well as localization, proximity, and structural health prototypes, have already been demonstrated, and many other sensing applications are foreseen soon. In this review, both the different architectural approaches available for this technology and the requirements related to the materials employed for sensing are summarized. Then, the state-of-the-art of categories of sensors and their applications are reported and discussed. Finally, an analysis of the current limitations and possible solution strategies for this technology are given, together with an overview of expected future developments.

https://www.mdpi.com/1424-8220/20/7/2135/pdf

Chipless RFID Sensors for the Internet of Things: Challenges and Opportunities

A novel chipless RFID humidity sensor based on a finite artificial impedance surface (AIS) is presented. The unit cell of the AIS is composed of three concentric loops thus obtaining three deep and high-Q nulls in the electromagnetic response of the tag. The wireless sensor is fabricated using the low-cost inkjet printing technology on a thin sheet of commercial coated paper. The patterned surface is placed on a metal-backed cardboard layer. The relative humidity information is encoded in the frequency shift of the resonance peaks. Varying the relative humidity level from 50% to 90%, the frequency shift has proved to be up to 270 MHz. The position of the resonance peaks has been correlated to the relative humidity level of the environment on the basis of a high number of measurements performed in a climatic chamber, specifically designed for RF measurements of the sensor. A very low error probability of the proposed sensor is demonstrated when the device is used with a 10% relative humidity level discrimination.

An Inkjet Printed Chipless RFID Sensor for Wireless Humidity Monitoring

The advantages of adopting circular polarization in multiple-input-multiple-output (MIMO) systems are illustrated for both line-of-sight (LOS) and multipath propagation. More in detail, an analysis of the MIMO performance attainable by employing orthogonal circularly polarized (CP) radiators with respect to orthogonal linearly polarized (LP) ones, has been addressed. At first, an accurate analysis is presented aimed at the evaluation of the channel matrix by comprehensively including also the effects of the antenna in LOS condition. In particular, the channel matrix has been calculated as a function of the antenna parameters and orientation, demonstrating that CP radiators are capable of obtaining better average values of the matrix eigenvalues with respect to LP ones. The analysis is therefore completed by evaluating the characteristics of a CP MIMO system operating in indoor environment representing this latter a more challenging condition where multipath propagation occurs. In this latter case, some meaningful numerical experiments have been performed by using a reliable ray-tracing solver, followed by a measurements campaign conducted in a real environment for validation purposes. Measurements, which are in good agreement with simulations, confirm the benefits of adopting circular polarization in MIMO systems with respect to LP.

Analysis of the Performance Enhancement of MIMO Systems Employing Circular Polarization

A novel low-cost null-steering antenna operating at 2.45 GHz is designed by exploiting an asymmetric excitation of two characteristic modes. This compact antenna allows a null shift of the radiation pattern by changing the relative phase between the two capacitive exciters (CCEs) located above the conductive plane. As a proof of concept, a prototype of the radiator has been realized by resorting to a discrete phase shifter in order to scan the null. The measured radiation patterns prove that the proposed design strategy is reliable and applicable to other kinds of structures. The manufactured antenna is capable of steering the null within a beam of 64° centered at broadside direction ( $\theta =0^{\circ }$ ), with depth nulls greater than 18 dB.

Null-Steering Antenna Design Using Phase-Shifted Characteristic Modes

A review of technological solutions for RFID sensing and their current or envisioned applications is presented. The fundamentals of the wireless sensing technology are summarized in the first part of the work, and the benefits of adopting RFID sensors for replacing standard sensor-equipped Wi-Fi nodes are discussed. Emphasis is put on the absence of batteries and the lower cost of RFID sensors with respect to other sensor solutions available on the market. RFID sensors are critically compared by separating them into chipped and chipless configurations. Both categories are further analyzed with reference to their working mechanism (electronic, electromagnetic, and acoustic). RFID sensing through chip-equipped tags is now a mature technological solution, which is continuously increasing its presence on the market and in several applicative scenarios. On the other hand, chipless RFID sensing represents a relatively new concept, which could become a disruptive solution in the market, but further research in this field is necessary for customizing its employment in specific scenarios. The benefits and limitations of several tag configurations are shown and discussed. A summary of the most suitable applicative scenarios for RFID sensors are finally illustrated. Finally, a look at some sensing solutions available on the market are described and compared.

A Review of RFID Sensors, the New Frontier of Internet of Things.

A design strategy exploiting the characteristic mode (CM) analysis (CMA) is described for improving the radiation efficiency of a mounted-on-platform radiator. To this aim, a novel balanced inductive exciter (BIE) is introduced to improve the modal excitation purity of some CMs. In fact, even if the optimal position of the exciters on the hosting platform is determined by using the CM theory, the excitation purity of each mode plays a fundamental role in the radiation efficiency of the radiating system. In particular, achieving a good level of excitation purity strongly reduces the reactive power ( $P_{{{reac}}}$ ) stored in the near-field zone and hence maximizes the amount of the radiated power ( $P_{r}$ ). To better highlight the benefits offered by the presented approach, a set of BIEs is applied on a platform to obtain a fully reconfigurable radiation pattern. The evaluation of the $P_{{{reac}}}$ , $P_{r}$ , and equivalent isotropically radiated power provided by the BIE reveals the importance of a pure modal excitation. To assess the reliability of the proposed BIEs, some prototypes have been manufactured and tested.

Francesco Alessio Dicandia

Papers

An Inkjet Printed Chipless RFID Sensor for Wireless Humidity Monitoring

Analysis of the Performance Enhancement of MIMO Systems Employing Circular Polarization

Null-Steering Antenna Design Using Phase-Shifted Characteristic Modes

A Review of RFID Sensors, the New Frontier of Internet of Things.

Efficient Excitation of Characteristic Modes for Radiation Pattern Control by Using a Novel Balanced Inductive Coupling Element