In recent few years, the antenna and sensor communities have witnessed a considerable integration of radio frequency identification (RFID) tag antennas and sensors because of the impetus provided by internet of things (IoT) and cyber-physical systems (CPS). Such types of sensor can find potential applications in structural health monitoring (SHM) because of their passive, wireless, simple, compact size, and multimodal nature, particular in large scale infrastructures during their lifecycle. The big data from these ubiquitous sensors are expected to generate a big impact for intelligent monitoring. A remarkable number of scientific papers demonstrate the possibility that objects can be remotely tracked and intelligently monitored for their physical/chemical/mechanical properties and environment conditions. Most of the work focuses on antenna design, and significant information has been generated to demonstrate feasibilities. Further information is needed to gain deep understanding of the passive RFID antenna sensor systems in order to make them reliable and practical. Nevertheless, this information is scattered over much literature. This paper is to comprehensively summarize and clearly highlight the challenges and state-of-the-art methods of passive RFID antenna sensors and systems in terms of sensing and communication from system point of view. Future trends are also discussed. The future research and development in UK are suggested as well.

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A Review of Passive RFID Tag Antenna-Based Sensors and Systems for Structural Health Monitoring Applications

Antennas are a vital component of the wireless body sensor networks devices. A wearable antenna in this system can be used as a communication component or energy harvester. This paper presents a detailed review to recent advances fabrication methods for flexible antennas. Such antennas, for any applications in wireless body sensor networks, have specific considerations such as flexibility, conformability, robustness, and ease of integration, as opposed to conventional antennas. In recent years, intriguing approaches have demonstrated antennas embroidered on fabrics, encapsulated in polymer composites, printed using inkjets on flexible laminates and a 3-D printer and, more interestingly, by injecting liquid metal in microchannels. This article presents an operational perspective of such advanced approaches and beyond, while analyzing the strengths and limitations of each in the microwave as well as millimeter-wave regions. Navigating through recent developments in each area, mechanical and electrical constitutive parameters are reviewed, and finally, some open challenges are presented as well for future research directions.

https://www.mdpi.com/1424-8220/19/10/2312/pdf

Recent Advances in Fabrication Methods for Flexible Antennas in Wearable Devices: State of the Art

Ultrawideband (UWB) technology can provide short-range, high bandwidth communications at a very low energy consumption level, which is quite attractive for wireless body area networks (WBAN). In such a system, the presence of the human body brings huge challenges for both the design of the wearable antenna and the propagation model. First of all, the coupling between the wearable antenna and the human body should be considered even in the initial steps of the design, in order to be able to deal with both the possibly deteriorating performance of the antenna as a result of the body and the exposure risk for the body. Second, the propagation channel in WBAN is influenced by the continuous movement of the human body, due to the time-varying scattering of the electromagnetic waves. Lots of researchers have been active in this area and some significant progress has been achieved recently. This paper retrospects the latest results in the field of wearable ultrawideband antennas, propagation channels, and their respective application in WBAN systems.

Wearable Ultrawideband Technology—A Review of Ultrawideband Antennas, Propagation Channels, and Applications in Wireless Body Area Networks

SUMMARY Millimeter-wave (mm-wave) radio is attracting attention as one of the key enabling physical layer technologies for the fifthgeneration (5G) mobile access and backhaul. This paper aims at clarifying possible roles of mm-wave radio in the 5G development and performing a comprehensive literature survey on mm-wave radio channel modeling essential for the feasibility study. Emphasis in the literature survey is laid on grasping the typical behavior of mm-wave channels, identifying missing features in the presently available channel models for the design and evaluation of the mm-wave radio links within the 5G context, and exemplifying different channel modeling activities through analyses performed in the authors’ group. As a key technological element of the mm-wave radios, reduced complexity beamforming is also addressed. Design criteria of the beamforming are developed based on the spatial multipath characteristics of measured indoor mm-wave channels.

Channel Models and Beamforming at Millimeter-Wave Frequency Bands

Graphene RF and microwave passive components such as coplanar waveguide transmission lines, open/short-circuited resonators and wideband antenna on paper substrate were designed, screen printed and characterized in this work. The experimental results demonstrate that the screen printed graphene passive components can be used for RF signal transmitting, processing and radiating/receiving; revealing that graphene ink can be a low cost alternative to much more expensive metal nanoparticle inks, such as silver nanoparticle ink. The screen printed graphene is processed at low temperature so that it is compatible with heat-sensitive flexible materials like papers, PTFE (Polytetrafluoroethylene) and textiles. The screen printed graphene passive components reported here are of high conductivity, high flexibility, light weight and low cost, making them ideal candidate for low cost wearable electronics. This work makes it prospective to manufacture RF and microwave passive components in mass production by screen printing in much lower cost to any other known techniques.

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Graphene radio frequency and microwave passive components for low cost wearable electronics

Body-centric wireless communications represent a well-established field of research, with many studies and applications developed in a range of frequencies that extend from 400 MHz up to 10 GHz. However, many advantages can be found in operating such systems at millimeter-wave frequencies. For example, compact antennas suitable for body-centric applications can be obtained together with other benefits, such as higher data rates and reduced interference and "observability." Meanwhile, numerical modeling of antennas and propagation at millimeter-wave frequencies represents a major challenge in terms of efficiency and accuracy. The aim of this paper is to provide a review of recent progresses and outstanding challenges in the field of body-centric communication at frequencies of 60 GHz and 94 GHz.

Antennas and propagation for body centric wireless communications at millimeter wave frequencies: a review

A coplanar waveguide (CPW) stub resonator has been fabricated on a pulsed-laser deposited KTa(0.5)Nb(0.5)O(3) (KTN) thin film (600 nm-thick) onto a r-plane sapphire substrate. It was designed to operate at 10 GHz when the applied bias voltage is zero. We show experimentally that the resonance frequency is shifted by 44% under a 70 kV/cm DC applied electric field. In addition, the dielectric characteristics of the KTN film have been assessed through post-processed measurements of CPW 50-Omega transmission lines using the conformal mapping method.

Highly tunable microwave stub resonator on ferroelectric KTa0.5Nb0.5O3 thin film

Bi(1.5-x)Zn(0.92-y)Nb(1.5)O(6.92-δ) thin films have the potential to be implemented in microwave devices. This work aims to establish the effect of the substrate and of the grain size on the optical and dielectric properties. Bi(1.5-x)Zn(0.92-y)Nb(1.5)O(6.92-δ) thin films were grown at 700 °C via pulsed-laser deposition on R-plane sapphire and (100)(pc) LaAlO(3) substrates at various oxygen pressures (30, 50, and 70 Pa). The structure, morphology, dielectric and optical properties were investigated. Despite bismuth and zinc deficiencies, with respect to the Bi(1.5)Zn(0.92)Nb(1.5)O(6.92) stoichiometry, the films show the expected cubic pyrochlore structure with a (100) epitaxial-like growth. Different morphologies and related optical and dielectric properties were achieved, depending on the substrate and the oxygen pressure. In contrast to thin films grown on (100)(pc) LaAlO(3), the films deposited on R-plane sapphire are characterized by a graded refractive index along the layer thickness. The refractive index (n) at 630 nm and the relative permittivity (e(r)) measured at 10 GHz increase with the grain size: on sapphire, n varies from 2.29 to 2.39 and e(r) varies from 85 to 135, when the grain size increases from 37 nm to 77 nm. On the basis of this trend, visible ellipsometry can be used to probe the characteristics in the microwave range quickly, nondestructively, and at a low cost.

Lingyan Zhang

Papers

Antennas and propagation for body centric wireless communications at millimeter wave frequencies: a review

Highly tunable microwave stub resonator on ferroelectric KTa0.5Nb0.5O3 thin film

Structural, Optical, and Dielectric Properties of Bi1.5–xZn0.92–yNb1.5O6.92−δ Thin Films Grown by PLD on R-plane Sapphire and LaAlO3 Substrates