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A first-of-its-kind design methodology to realize thermally actuated origami-inspired multilayer Miura frequency selective surfaces (MLM-FSSs) using a polyester-based substrate is presented in this article that can change its frequency response according to the variation in its ambient temperature. In order to maintain the optimum interlayer distance between the two-layer Miura-FSS structures, a specialized low-loss origami-inspired spacer layer is introduced that preserves its desired overall frequency response during different folding configurations. It also features an unprecedented high mechanical strength, making it suitable for a wide range of terrestrial, outer-space, and biomedical applications. The frequency response and its relationship to kinematics of the proposed structure are also presented in detail, which gives a unique insight into the operation principles of such structures. The proposed structure features twice the bandwidth as compared to single-layer Miura-FSS, an excellent angle of incidence rejection, and is the first demonstration of a fully inkjet-printed MLM-FSS that is realized completely by a rigid heat-sensitive substrate as opposed to traditional cellulose-based Miura-FSSs that lack self-actuation mechanism as well as rigidity, which is a key attribute of most practical thick origami structures. [ABSTRACT FROM AUTHOR]

Novel balanced-to-unbalanced power divider (BUPD) is introduced for arbitrary power division ratios and for arbitrary real termination impedances. It consists of one 180° transmission-line section (TL), two different 90° TLs and one isolation circuit comprising two different resistances and two different 90° TLs. Due to the arbitrary termination impedances, the conventional ways for the even- and odd-mode excitation analyses are impossible, requiring a new design method. Under the assumption of the perfect isolation between two output ports, the BUPD can be divided into two, based on which the design formulas for the characteristic impedances of all the TLs can be successfully derived. Then, the scattering parameters are inversely derived, which is quite different from the conventional ways where the design formulas are derived based on the scattering parameters. For the verification of the suggested theory, one prototype for the power division ratio of 5 dB and for the termination impedances of 60, 40 and 50 Ω is tested. The measured frequency responses are in good agreement with the predicted ones. [ABSTRACT FROM AUTHOR]

A branch-line hybrid (BH) topology is suggested, consisting of four equal lengths of transmission-line sections (TLs) with two different characteristic impedances and four identical L-sections with each a TL and an open stub. The topology can provide three distinct properties; lower characteristic impedances of TLs, longer electrical paths for the scattering parameters, and additional transmission zeros, enabling high performance of harmonic suppressions from $2f_{0}$ , where $f_{0}$ is a design frequency, wideband and selective harmonic suppressions. A compact equivalent circuit of a TL is suggested for harmonic suppression BH which can be applied for all possible power-division ratios. As a proof-of-concept demonstration, two prototypes for the power-division ratios of 0 and 6 dB are tested and measured. The measured frequency responses are in a good agreement with the predicted ones, given fabrication errors. [ABSTRACT FROM AUTHOR]

Wideband and compact 90° dc blocks are presented for the impedance-transforming function and designed based on the transmission-zero movement method combined with the pole (root) split method, which is the reason for the wide bandwidth responses, even with being very compact. For a proof-of-concept demonstration, one prototype is tested, and the measured bandwidth with the 10-dB return loss is 0.74–4.05 GHz (331%) with a design frequency of 1 GHz. [ABSTRACT FROM AUTHOR]

Abstract: In this article, a planar antenna with quasi‐isotropic radiation pattern is proposed for applications in drone communication. The proposed antenna consists of top‐loaded monopole and slotted loop antennas. The top‐loaded monopole provides a monopole‐like radiation pattern with a planar profile. To compensate for nulls in the monopole radiation pattern, a slotted loop antenna is added. Therefore, when two radiation patterns are combined, a quasi‐isotropic pattern is achieved with a low profile thickness of 7.2 mm. The performance of the proposed antenna is verified by simulation and experimental results. The antenna's 10‐dB impedance bandwidth is in the range of 2–2.4 GHz. The quasi‐isotropic radiation pattern is verified from both simulation and measured results. [ABSTRACT FROM AUTHOR]

By combining radio-frequency identification (RFID) and paper-microfluidics technologies, a low-cost first-of-its-kind platform for comprehensive liquid sensing, i.e., the “smart test strip,” is presented, which enables portable wireless real-time liquid sensing with handhold devices (e.g., cell phones), and integration of various multifunctional electrical and chemical sensors, for numerous Lab-on-Chip applications, including manufacturing control, environmental monitoring, and point-of-care medical diagnostics. The fabrication of RFID tags and two types of microfluidics are accomplished by a single inkjet-printing process in a cost-effective environmental-friendly additive manufacturing approach, which makes possible the production of disposable, lightweight, and flexible sensing platforms. Taking advantage of the proposed smart test strips platforms, we demonstrate two proof-of-concept high-performance electrical sensors based on interdigitated electrode topologies: a resistivity-based sensor with a 1782 $\Omega$/( $\Omega$*m) sensitivity; nevertheless, the proposed permittivity-based sensor with a 15%/$\epsilon _r$ sensitivity, but the proposed integrated wireless platform, can facilitate the integration of even more chemical and electrical sensors. In addition, two on-strip antenna prototypes have been designed, optimized, and tested to work at 2.4 GHz and 13.56 MHz, respectively. Furthermore, the wireless interrogation of a complete proof-of-concept smart test strip is presented, which shows an excellent sensing resolution of 1.33 $\Omega$ over the range of 0–1371 $\Omega$. [ABSTRACT FROM PUBLISHER]

Two miniaturized wire antennas using a 3D space-filling Hilbert-curve (HC) fractal geometry were constructed on a Kodak photo paper using inkjet printing. The proposed antennas each had the 3D dimensions of an 8 cm3 box and operate at very high-frequency band frequencies of 238 and 147 MHz. We wrapped 4- and 2-mm-thick planar wires around a 3D box made of photo paper using second- and third-order HC space-filling fractal designs. The electrical volumes of the proposed second-order HC and third-order HC fractal antennas were 0.004 λ0 × 0.004 λ0 × 0.004 λ0 and 0.002 λ0 × 0.002 λ0 × 0.002 λ0, respectively. The operation of the proposed antennas was verified using simulation and measurement results. [ABSTRACT FROM AUTHOR]

Three different equivalent circuits (asymmetric, symmetric, and mixed) of the 90° coupled transmission-line sections are proposed for Marchand baluns, including the accurate modeling of the effect of the connecting segment. The asymmetric one consists of one set of coupled transmission-line sections and two identical uncoupled transmission-line sections. Due to the asymmetry and a four-port circuit configuration, it seems to be difficult to derive the design formulas for the asymmetric equivalent circuit, thus leading to their indirect derivation by adding two identical transmission-line sections with effectively “negative” electrical lengths to the original 90° coupled transmission-line sections. The symmetric one may be obtained by combining the even- and odd-mode equivalent circuits, while the mixed equivalent circuit is combining the asymmetric and the symmetric ones. As a proof-of-concept, a symmetric and an asymmetric equivalent circuits along with a Marchand balun with a connecting section were implemented at 2 GHz, demonstrating a very good agreement between the simulated and the experimental values. [ABSTRACT FROM PUBLISHER]

The increasing use of backscatter radio for pervasive Internet-of-Things systems as a low-power and low-cost communication scheme will result in dense deployments of tags that need to operate under bandwidth (BW) constraints. Typical backscatter tags and computational radio frequency (RF) identification systems are unnecessarily limited to modulating data with “ON–OFF” switching front-ends, which leads to extensive spectrum occupancy, due to the rectangular pulses. This paper sets the foundations to overcome these limitations and proposes designs of RF front-ends that are capable of varying the tag reflection coefficient in a continuous way over time. Arbitrary waveforms can be generated to perform pulse shaping on the backscattered signals, which require significantly reduced BW. The principles presented in this paper will enable sophisticated tags to perform complex modulation schemes under BW constraints, while maintaining a very low RF front-end complexity, using a single nonlinear component such as a p-i-n diode or field-effect transistor. Migrating the idea of pulse shaping from complex power-demanding active radios to such minimal RF front-ends will complete backscatter radio as a communication scheme for simple RF tags and sensors in large-scale deployments. Each tag becomes a miniature software-defined radio that implements diverse communication schemes with increased spectral efficiency. [ABSTRACT FROM PUBLISHER]

In this effort, the authors implement the first ultralong-range chipless sensing sticker, by providing more than an order of magnitude increase in reading range, compared with the state of the art. The theoretical advantages of the use of millimeter-wave frequencies for high-performance chipless radio-frequency identification (RFID) sensor implementations are first argued before both a new fully inkjet-printed flexible device, based on the Van-Atta reflectarray structure, as well as a new chipless RFID polarimetric interrogation, and time-frequency data-processing approach is then presented and implemented, for operation in the Ka-band. The array, fully inkjet printed on Kapton HN polyimide, was demonstrated as being robust to variations of interrogation angle (between ±70° from normal), as well as to bending. With its demonstrated range, in excess of 30 m, and its proven adequacy for dense multitag and multisensing implementations in indoor environments, the structure may set the foundation for the emergence of flexible printable low-cost sensing smart skins for the Internet of Things. [ABSTRACT FROM PUBLISHER]

A simple method for inverting negative resistances of nonphysical circuits generated from approximating rational functions through vector fitting is proposed. This technique is applied to three resonant antenna structures. The challenge of applying this method to these structures lies in the accurate modeling of the ways in which the different resonant modes are excited, some via coaxially feeding a rectangular patch along the diagonal, while others via capacitively coupling energy from the driven patch to a parasitic patch. The equivalent circuits of these designs produce scattering parameter results that are consistent with the fitted functions. This methodology of resistance invertibility is a great tool that can be used to model antenna designs with low order equivalent circuits, drastically reducing the design time. In addition, valuable information that could aid in the optimization of microstrip antennas can be quickly ascertained through these techniques. [ABSTRACT FROM AUTHOR]

The feasibility of using near-hermetic organic materials for microwave and millimeter-wave packaging is investigated. The effects of moisture on both passive and active components on/inside the nonhermetic materials are tested. Microstrip patch antennas on three low moisture absorption substrates (0.1% or less) are subjected to immersion tests until saturated and the corresponding measured frequency shifts are found to be less than 0.25% for two of the three materials tested. The recovery time from saturation to normal operation is found to occur from natural evaporation at room temperature in 10 min or less. One of the three materials, a promising organic laminate material called liquid crystal polymer (LCP), showed no measurable change in weight or in antenna resonance frequency between the ambient (air) state and that after extended submersion in water. In addition, a novel laminated LCP package for a 13-25 GHz monolithic microwave integrated circuit (MMIC) formed from seven thin layers of laser-machined LCP is subjected to immersion testing. The seal for the embedded chip cavity (formed by the LCP conforming around the feeding transmission lines) is found to be robust for protecting the embedded chip. Standard hermetic materials are suggested to be a potentially unnecessary requirement for reliable, long-lifetime, high-performance RF systems. [ABSTRACT FROM AUTHOR]

The effect of multilayer geometry on microstrip antennas is investigated for the design of antenna-integrated mechanical structure. Changes in the gain of antenna due to the geometry have been determined using a transmission line analogy. Design of high-gain antenna in bandwidth is proposed away from structural resonance. Experiments are done on microstrip antennas covered by superstrates in order to verify high-gain conditions theoretically derived herein. The off-resonant conditions that use practical materials of moderate thickness make it possible to design the antenna-integrated mechanical structure with the perfect integration of high mechanical and electrical performances. [ABSTRACT FROM AUTHOR]

A new printed microstrip Yagi array antenna is proposed that can achieve a high gain and low backside radiation for various applications up to the millimeter-wave frequency range. The high front-to-back (F/B) ratio (up to 15 dB) is attributed to the constructive interference that takes place between the individual printed Yagi arrays in the design. Through the spacing of the elements, the directivity (between 9-11.5 dBi) and the FIB ratio can be altered to suit the application of interest. The operational principles of this design are discussed to give insight on the radiation mechanism of the antenna. An initial design at around 32.5 GHz is presented to show the performance capabilities of this configuration. An impedance bandwidth of 8.3% can be achieved around this frequency. Then, a parametric analysis is conducted to estimate the significance of the design parameters that affect the antenna's performance. Finally, measured return loss and radiation pattern performance at 5.2 GHz is displayed to validate the principles and simulated results of the design. The measured impedance bandwidth of 10% is achieved. The FIB ratio is 15 dB which is larger than values previously published by 5-10 dB. Additionally, a gain of 10.7 dBi is observed. To the author's knowledge, this is the first microstrip Yagi array antenna presented that has a high gain and a high F/B ratio designed using simple fabrication techniques. [ABSTRACT FROM AUTHOR]

Electronics packaging evolution involves system, technology, and material considerations. In this paper, we present a novel three-dimensional (3-D) integration approach for system-on-package (SOP)-based solutions for wireless communication applications. This concept is proposed for the 3-D integration of RF and millimeter (mm) wave embedded functions in front-end modules by means of stacking substrates using liquid crystal polymer (LCP) multilayer and μBGA technologies. Characterization and modeling of high-Q RF inductors using LCP is described. A single-input-single-output (SISO) dual-band filter operating at ISM 2.4-2.5 GHz and UNII 5.15-5.85 GHz frequency bands, two dual-polarization 2×1 antenna arrays operating at 14 and 35 GHz, and a WLAN IEEE 802.11a-compliant compact module (volume of 75 × 35 × 0.2 mm³) have been fabricated on LCP substrate, showing the great potential of the SOP approach for 3-D-lntegrated RF and mm wave functions and modules. [ABSTRACT FROM AUTHOR]

The mentioned paper (IEEE Trans. Compon., Packag., Manuf. Technol., vol. 7, no. 6, pp. 944–955, Jun. 2017) treats rat-race couplers (ring hybrids) and branch-line hybrids, but three errors are found for the ring hybrids: missing important references, wrong design parameters, and wrong interpretation on the termination impedances, which will be clarified further. [ABSTRACT FROM AUTHOR]

The article focuses on the development of radio frequency identification (RFID), an emerging compact wireless technology for the identification of objects. It notes that RFID, which utilizes electromagnetic waves for transmitting and receiving information, is considered an eminent candidate for the realization of an ubiquitous ad-hoc wireless network. Furthermore, the benefits of RFID over the conventional methods of identification include the higher read range, faster data transfer, and relaxation of line-of-sight requirements. It also mentions that Harry Stockman had introduced the principles of RFID in 1940s as he described the convergence of radio transmission and radar technology. It also profiles several guest editors, such as Manos M. Tentzeris, Harald Witschnig, and Erich Leitgeb.

Internet of Things applications based on backscatter radio principles have appeared to address the limitations of high cost and high power consumption. While radio-frequency identification (RFID) sensor nodes are among the most commonly utilized state-of-the-art technologies, their range for passive implementations is typically short and well below 10 m being impractical for "rugged" applications where approaching the tag at such proximity, is not convenient or safe. In this work, we propose a long-range "zero interception" ambient backscatter (LoRAB) communication system relying on low power sensor (tag) deployments. Without employing a dedicated radio transmission, our technology enables the "zero interception" communication of the tags with portable receivers over hundreds of meters. This enables low-cost and low-power communications across a wide range of missions by using chirp spread spectrum (CSS) modulation on ambient FM signals. A laboratory prototype exploiting commercial components (laptops, DAQ, software-defined radios (SDR) platform) have demonstrated the potential by achieving 130 m tag-to-reader distance for a low bit rate of 88 bps with the modulator current consumption at around 103 nA. [ABSTRACT FROM AUTHOR]

This work presents a novel tile based approach to constructing, in a modular fashion, massively scalable MIMO and phased arrays for 5G/B5G millimeter-wave smart skins and large-area reconfigurable intelligent surfaces for Smart Cities and IoT applications. A proof-of-concept 29 GHz 32 elements phased array utilizing 2 × 2 "8-element subarray" tiles was fabricated and measured and demonstrates + / - 30beamsteering capability. The unique benefits of the proposed tile approach utilizes the fact that tiles of identical sizes can be manufactured in large quantities rather than have arrays of multiple sizes serve various user capacity coverage areas. It has to be stressed that the proof-of-concept flexible 2 × 2 tile array features no performance degradation when it is wrapped around a 3.5 cm radius curvature. This topology can be easily scaled up to massively large arrays by simply adding more tiles and extending the feeding network on the mounting tiling layer. The tiles are assembled onto a single flexible substrate which interconnects the RF, DC and digital traces, allowing for the easy realization of on-demand very large antenna arrays on virtually any practical conformal platform for frequencies up to sub-THz frequency range. [ABSTRACT FROM AUTHOR]

An optically transparent metamaterial absorber that can be obtained using inkjet printing technology is proposed. In order to make the metamaterial absorber optically transparent, an inkjet printer was used to fabricate a thin conductive loop pattern. The loop pattern had a width of 0.2 mm and was located on the top surface of the metamaterial absorber, and polyethylene terephthalate films were used for fabricating the substrate. An optically transparent conductive indium tin oxide film was introduced in the bottom ground plane. Therefore, the proposed metamaterial absorber was optically transparent. The metamaterial absorber was demonstrated by performing a full-wave electromagnetic simulation and measured in free space. In the simulation, the 90% absorption bandwidth ranged from 26.6 to 28.8 GHz, while the measured 90% absorption bandwidth was 26.8–28.2 GHz. Therefore, it is successfully demonstrated by electromagnetic simulation and measurement results. [ABSTRACT FROM AUTHOR]

This review encompasses additive manufacturing techniques for crafting 5G electronics, showcasing how these methods innovate device creation with novel examples. A wearable phased array device on commonplace 3D printed material is described, with integrated microfluidic cooling channels used for thermal regulation of integrated circuit bulk components. Mechanical and electrical tunability are exemplified in an origami-inspired phased array structure. A 3D printed IoT cube structure shows the flexibility in the number of geometries additively manufactured 5G devices can adhere to. Finally, integrating 3D optical lenses with 5G electronics is shown. [ABSTRACT FROM AUTHOR]

As the demand for low-cost, flexible, and power-efficient broadband wireless electronics increases, materials and integration techniques become critical and face more challenges, especially with the ever-growing interest for “cognitive intelligence” and wireless applications, married with RF identification (RFID) technologies. This demand is further enhanced by the need for inexpensive, reliable, and durable wireless RFID-enabled sensor nodes, driven by applications such as logistics, aero identification, anticounterfeiting, supply-chain monitoring, space, healthcare, and pharmaceutical, and is regarded as one of the most disruptive technologies to realize truly ubiquitous ad hoc networks. [ABSTRACT FROM PUBLISHER]

In the above paper two equivalent circuits of $L_{T}-$ and $\Pi $ -types for a 90° transmission-line section (TL) are suggested only for $N = 1$. However, the design formulas for $N= n$ , where $n\ge 1$ and integer, are already published for $L_{T}$ -types and for $\Pi $ -types [3, Fig. 6(c) with $\Theta _{a}= 0^{\circ }$ ], , and the advantages for $N \ge 2$ are verified for $L_{T}$ -types in and $\Pi $ -types , , as well. [ABSTRACT FROM AUTHOR]

This article proposes a low-loss and light 3D-printed substrate-integrated waveguide (SIW). Despite the use of lossy polylactic acid (PLA) material, insertion loss is reduced, and bandwidth is increased due to a honeycomb substrate similar to air. To demonstrate the proposed concept, we fabricated microstrip-fed SIWs with solid PLA and honeycomb substrates, and compared their performance numerically and experimentally. Average measured insertion loss from 3.4 to 5.5 GHz for the honeycomb SIW is 1.38 dB, whereas SIW with solid PLA is 3.15 dB. Light weight is an additional advantage of the proposed structure. [ABSTRACT FROM AUTHOR]

In this paper, the far-field energy harvesting system for self-sustainable wireless autonomous sensor application is presented. The proposed autonomous sensor system consists of a wireless power supplier (active antenna) and far-field energy harvesting technology-enabled autonomous battery-less sensors. The wireless power supplier converts solar power to electromagnetic power in order to transfer power to multiple autonomous sensors wirelessly. The autonomous sensors have far-field energy harvesters which convert transmitted RF power to voltage regulated DC power to power-on the sensor system. The hybrid printing technology was chosen to build the autonomous sensors and the wireless power suppliers. Two popular hybrid electronics technologies (direct nano-particle printing and indirect copper thin film printing techniques) are discussed in detail. [ABSTRACT FROM AUTHOR]

5G has been designed for blazing fast and low-latency communications. To do so, mm-wave frequencies were adopted and allowed unprecedently high radiated power densities by the FCC. Unknowingly, the architects of 5G have, thereby, created a wireless power grid capable of powering devices at ranges far exceeding the capabilities of any existing technologies. However, this potential could only be realized if a fundamental trade-off in wireless energy harvesting could be circumvented. Here, we propose a solution that breaks the usual paradigm, imprisoned in the trade-off between rectenna angular coverage and turn-on sensitivity. The concept relies on the implementation of a Rotman lens between the antennas and the rectifiers. The printed, flexible mm-wave lens allows robust and bending-resilient operation over more than 20 GHz of gain and angular bandwidths. Antenna sub-arrays, rectifiers and DC combiners are then added to the structure to demonstrate its combination of large angular coverage and turn-on sensitivity—in both planar and bent conditions—and a harvesting ability up to a distance of 2.83 m in its current configuration and exceeding 180 m using state-of-the-art rectifiers enabling the harvesting of several μW of DC power (around 6 μW at 180 m with 75 dBm EIRP). [ABSTRACT FROM AUTHOR]

With the current development of the 5G infrastructure, there presents a unique opportunity for the deployment of battery-less mmWave reflect-array-based sensors. These fully-passive devices benefit from having a larger detectability than alternative battery-less solutions to create self-monitoring megastructures. The presented 'smart' skin sensor uses a Van-Atta array design enabling ubiquitous local strain monitoring for the structural health monitoring of composite materials featuring wide interrogation angles. Proof-of-concept prototypes of these 'smart' skin millimeter-wave identification tags, that can be mounted on or embedded within common materials used in wind turbine blades, present a highly-detectable radar cross-section of − 33.75 dBsm and − 35.00 dBsm for mounted and embedded sensors respectively. Both sensors display a minimum resolution of 202 μ -strain even at 40 ∘ off-axis enabling interrogation of the fully-passive sensor at oblique angles of incidence. When interrogated from a proof-of-concept reader, the fully-passive, sticker-like mmID enables local strain monitoring of both carbon fiber and glass fiber composite materials. The sensors display a repeatable and recoverable response over 0–3000 μ -strain and a sensitivity of 7.55 kHz/ μ -strain and 7.92 kHz/ μ -strain for mounted and embedded sensors, respectively. Thus, the presented 5G-enabled battery-less sensor presents massive potential for the development of ubiquitous Digital Twinning of composite materials in future smart cities architectures. [ABSTRACT FROM AUTHOR]

In this article, the implementation of a machine learning (ML) strategy based on neural networks for the real-time range-adaptive automatic impedance matching of wireless power transfer (WPT) applications is discussed. This approach for the effective prediction of the optimal parameters of the tunable matching network and the selection of range-adaptive transmitter coils (Tx) is introduced in this article, aiming to achieve an effective automatic impedance matching over a wide range of relative distances. We propose a WPT system consisting of a tunable matching circuit and three Tx coils that have different radii and are simultaneously controlled by trained neural network models, returning an output set of matching capacitances as well as the optimal single transmitter among the three transmitters. In addition, a proof-of-concept prototype of the entire real-time range-adaptive automatic impedance-matching system is built and characterized. Finally, the proposed approach achieves a power transfer efficiency (PTE) of around 90% for ranges within 10–25 cm. [ABSTRACT FROM AUTHOR]

The article focuses on the need for fast and accurate numerical simulation techniques due to advances in wireless and microwave communication systems. A number of practical geometries have been left untreated because of their complexity and the inability of existing techniques to deal with requirements for large size and higher resolution. An accurate characterization of the passive and active structures is the first step need for the development of efficient numerical algorithms.

The tremendous increase in the number of components in typical electrical and communication modules requires low-cost, flexible and multifunctional sensing, energy harvesting, and communication modules that can readily reconfigure, depending on changes in their environment. Current subtractive manufacturingbased reconfigurable systems offer limited flexibility (limited finite number of discrete reconfiguration states) and have high fabrication cost and time requirements. Thus, this paper introduces an approach to solve the problem by combining additive manufacturing and origami principles to realize tunable electrical components that can be reconfigured over continuous-state ranges from folded (compact) to unfolded (large surface) configurations. Special "bridge-like" structures are introduced along the traces that increase their flexibility, thereby avoiding breakage during folding. These techniques allow creating truly flexible conductive traces that can maintain high conductivity even for large bending angles, further enhancing the states of reconfigurability. To demonstrate the idea, a Miura-Ori pattern is used to fabricate spatial filters--frequency-selective surfaces (FSSs) with dipole resonant elements placed along the fold lines. The electrical length of the dipole elements in these structures changes when the Miura-Ori is folded, which facilitates tunable frequency response for the proposed shape-reconfigurable FSS structure. Higherorder spatial filters are realized by creating multilayer Miura-FSS configurations, which further increase the overall bandwidth of the structure. Such multilayer Miura-FSS structures feature the unprecedented capability of on-the-fly reconfigurability to different specifications (multiple bands, broadband/narrowband bandwidth, wide angle of incidence rejection), requiring neither specialized substrates nor highly complex electronics, holding frames, or fabrication processes. [ABSTRACT FROM AUTHOR]

Abstract: In this article, we propose a low‐cost three‐dimensional (3D) printed metamaterial absorber. The proposed metamaterial consists of a periodic structure of circular truncated cones. The dielectric materials supporting the circular truncated cone are made from a poly(lactic acid) filament using the 3D printer. The conductor of the circular truncated cone is developed by spraying silver‐coated copper. The performance of the proposed absorber is numerically and experimentally investigated. The simulated and measured absorptivity are 91% and 95% at 10.8 and 10.81 GHz, respectively. As a result of the 3D printing and silver spray coating processes, the proposed absorber can be easily fabricated at a low cost. [ABSTRACT FROM AUTHOR]

This article outlines the design, processing, and implementation of inkjet and 3D printing technologies for the development of fully printed, highly integrated millimeter-wave (mm-wave) wireless packages. The materials, tools, and processes of each technology are outlined and justified for their respective purposes. Inkjet-printed 3D interconnects directly interfacing a packaging substrate with an integrated circuit (IC) die are presented using printed dielectric ramps and coplanar waveguide transmission lines exhibiting low loss (.6-.8 dB/mm at 40 GHz). Stereolithography 3D printing is presented for the encapsulation of IC dice, enabling the application-specific integration of on-package structures, including dielectric lenses and frequency selective surface-based wireless filters. Finally, inkjet and 3D printing technology are combined to present sloped mm-wave interconnects through an encapsulant, or through mold vias, achieving a slope of up to 65° and low loss (.5-.6 dB/mm at 60 GHz). The combination of these additive techniques is highlighted for the development of scalable, application-specific wireless packages. [ABSTRACT FROM AUTHOR]

Ambient RF energy harvesting is a potential energy source for low-power and battery-less wireless sensors, enabling a range of applications from monitoring to security as part of the Internet-of-Things (IoT) scenario. One of the main challenges of ambient RF energy harvesting is the requirement of operation over a multitude of frequency bands of low ambient power densities resulting in a very wide aggregate operating bandwidth. In this paper, design examples of novel ultra-wideband energy harvesters are demonstrated with octave and decade bandwidths in the UHF and low microwave spectrum. The RF-dc conversion efficiency is maximized by tailoring the dimensions of a nonuniform transmission line used to provide broadband impedance matching. The design challenges in terms of impedance matching based on the Bode-Fano theoretical limit, losses and miniaturization are highlighted. Two prototypes are presented and their performance is evaluated. The octave band rectifier showed a measured RF-dc conversion efficiency of more than 60% over a frequency band of 470 to 860 MHz at 10-dBm input power. The decade band rectifier fabricated on Kapton substrate using inkjet printing featured a higher than 33% efficiency over a frequency band from 250 MHz to 3 GHz at 10-dBm input power. [ABSTRACT FROM PUBLISHER]

Although wireless sensor networks (WSNs) have been an active field of research for many years, the modules incorporated by WSN nodes have been mainly manufactured utilizing conventional fabrication techniques that are mostly subtractive, requiring significant amounts of materials and increased chemical waste. The new era of the Internet of Things (IoT) will see the fabrication of numerous small form factor devices for wireless sensing for a plurality of applications, including security, health, and environmental monitoring. The large volume of these devices will require new directions in terms of manufacturing cost and energy efficiency, which will be achieved with redesigned, energy-aware modules. This paper presents the state of the art of printed passives, sensors, energy harvesting modules, actives, and communication front ends, and summarizes the challenges of implementing modules that feature low power consumptions without compromising the low fabrication cost. The plethora of the modules presented herein will facilitate the implementation of low cost, additively manufactured, energy-aware IoT nodes that can be fabricated in large volumes with green processes. [ABSTRACT FROM AUTHOR]

In this paper, a novel matching circuit design method utilizing a genetic algorithm (GA) and the measured S-parameters of randomly moved coil configurations is discussed. Through the detailed comparison of different matching circuit topologies, the superiority of active matching circuits is clearly demonstrated, and potentially there is 21.4% improvement in the wireless power transfer efficiency by using a four-cell active matching circuit, which can create 16 different impedance values. Also, the matching circuit design simulation can be further simplified by choosing a much smaller subset of representative impedance values for the utilized time-changing coil configuration through the employment of $k$ -means clustering and use only these values for the derivation of the optimal matching circuit. This heuristic approach could drastically reduce the time for the matching circuit design simulation, especially for matching circuit topologies with a larger number of cells. [ABSTRACT FROM PUBLISHER]

This paper thoroughly investigates material characterization, reliability evaluation, fabrication, and assembly processes of additively manufactured flexible packaging and reconfigurable on-package antenna arrays for next-generation 5G/mmWave wearable and conformal applications. The objective is to bridge the technology gap in current Flexible Hybrid Electronics (FHE) designs at mmWave frequencies and address the challenges of establishing future design standards for additively manufactured flexible packages and System-on-Package (SoP) integrated modules. Multiple 3D printed flexible materials have been characterized for their electrical and mechanical properties over the 5G/mmW frequency band (26–40 GHz), and the inkjet printed interconnects on 3D printed Polypropylene (PP) substrates demonstrated excellent electrical and mechanical performance during a 10,000-time cyclic bending test over typical wearable flexible radii down to 1 inch. A proof-of-concept flexible on-package phased array with an integrated microfluidic cooling channel on 3D printed substrates was fabricated and measured, demonstrating ± 37 ∘ beam steering capability with efficient cooling. The proposed reconfigurable design and low-temperature fabrication approach using additive manufacturing can be widely applied to next-generation highly-complex on-demand FHE, flexible multi-chip-module integration, and on-package phased-array modules for 5G/mmWave wearable and conformal smart skin, digital twin and massive MIMO applications. [ABSTRACT FROM AUTHOR]

The rapid development and deployment of 5G/mm-Wave technologies for communication, sensing and energy harvesting applications have been on the rise. Consequently, the need for low-cost, scalable, agile and compact RF modules has become more prominent than ever. This paper presents a review of recent efforts in utilizing additive manufacturing techniques such as inkjet printing to sustainably accelerate the massive deployment of 5G/mm-Wave. First, a novel flexible and massively scalable multiple-input, multiple-output (MIMO) tile-based phased array enabled by additively manufactured microstrip-to-microstrip transitions is presented. Next, a novel Rotman-Based harmonic mmID tag for Ultra-Long-Range localization is presented. Finally, low-power, low-cost mm-Wave backscattering modules for localization and orientation sensing are demonstrated. [ABSTRACT FROM AUTHOR]

A preface for the December 2008 issue of the "IEEE Transactions on Microwave Theory and Techniques" is presented.

ABSTRACT In this article, a novel hybrid printing technology that realizes a microstrip patch antenna is introduced. The proposed hybrid printing technology uses both two- and three-dimensional (2D and 3D) printing technologies. The antenna's dielectric materials are built using a 3D printer. The metallic pattern is printed using a commercial inkjet printer. Polylactic acid (PLA) is used for the filament of the 3D printer and silver nanoparticle ink is printed on paper using the inkjet printer. For a highly efficient antenna design with lossy PLA material, the mesh PLA is printed and its electromagnetic parameters are characterized. Finally, the printed patch antenna shows a return loss of 24 dB and a peak gain of 7 dBi at 1.9 GHz. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:2602-2606, 2016 [ABSTRACT FROM AUTHOR]

In this paper, an inkjet-printed electromagnet-based touchpad employing spiral resonators is proposed. The proposed touchpad is fabricated by a direct patterning method using an inkjet printer with a conductive silver nanoparticle ink. The conductive patterns are easily printed on a paper substrate and sintered for achieving good conductivity. The proposed touchpad is composed of two spiral resonators that resonate at 0.94 GHz ( f1 ) and 1.83 GHz ( f2 ), respectively. When the first resonator is touched, f1 decreases from 0.94 to 0.81 GHz because of electromagnetic (EM) coupling between the finger and the spiral resonator. Similarly, when the second resonator is touched, f2 decreases from 1.83 to 1.55 GHz. Owing to the EM coupling distance between the spiral resonator and the finger, the frequency changes although the finger does not reach beyond a height of 1.27 mm on the spiral resonator. The performance of the proposed touchpad is validated using simulation and measurement results. [2016-0035] [ABSTRACT FROM AUTHOR]

This paper demonstrates the first-of-its-kind additively manufactured microfluidics-based flexible RF sensor, combining microfluidics, inkjet-printing technology, and soft lithography, which could potentially enable the first “real-world” wearable “smart skin” applications. A low-cost, rapid, low-temperature, and zero-waste fabrication process is introduced, which can be used to realize complex microfluidic channel networks with virtually any type of sensing element embedded. For proof-of-concept purposes, a reusable and flexible microfluidics sensor was prototyped using this process, which only requires 0.6- \mu \text L fluid volume to produce a 44% frequency shift between an empty ( \epsilon r=1 ) and a water-filled channel ( \epsilon r=73 ), demonstrating a sensitivity that is higher than most previously reported microfluidics-based microwave sensors. Seven different fluids were used to measure the sensitivity of the prototype and an overall sensitivity of 24\%/ \log (\epsilon r) was observed. The “peel-and-replace” capability of the presented sensor not only facilitates the cleaning process for sensor reusability, but it also enables sensitivity tunability. For bent/conformed configurations, the sensor’s functionality is good even for a bending radius down to 7 mm, demonstrating its great flexibility. After bending multiple times, the sensor still exhibits a very good performance repeatability, which verifies its reusability feature. The introduced additively manufactured RF microfluidics-based sensor would be well suited for numerous wearable and conformal fluid sensing applications (e.g., bodily fluids analyzing and food monitoring), while it could also be utilized in a variety of microfluidics-reconfigurable microwave components. [ABSTRACT FROM AUTHOR]

This paper discusses the feasibility of a real-time active matching circuit (MC) for wireless power transfer applications, especially for biomedical systems. One prototype of low-cost real-time automatic MC, utilizing a variable circuit topology, including discrete passives and p-i-n diodes, has been implemented and the principle has been verified by measurements. One genetic algorithm was introduced to optimize the design over a wide range of impedances to match. As a result of preliminary operation verification tests, the proposed real-time MC system results in improving the transfer coefficient in the range of 10–16-cm coil separation distance a maximum of 3.2 dB automatically in about 64 ms. Similar performance improvement results were observed in additional tests under misaligned conditions, as well as for nonsymmetrical Tx–Rx coil configurations further verifying the potential applicability of the proposed system to practical biomedical devices. [ABSTRACT FROM PUBLISHER]

A complete design and additive fabrication process of flexible wearable radio-frequency (RF) energy harvesters for off-the-shelf 2 W two-way talk radios utilizing inkjet printing technology is discussed in this paper. As a result of numerous output dc power measurements of fabricated proof-of-concept prototypes, a maximum output power of 146.9 mW and 43.2 mW was achieved with an H-field and E-field harvester, respectively. Also, the effect of misalignment between receiver and hand-held radio on harvesting performance is discussed in detail. To verify their potential in real-world wearable autonomous RF modules, the operation of E- and H-field energy harvesters was verified by utilizing an LED and a microcontroller communication module under on-body and on-bottle conditions, and the effect of the energy harvesters on the performance of the harvested communication systems was inspected through received power measurements in an anechoic chamber. [ABSTRACT FROM AUTHOR]

ABSTRACT An optically controlled reconfigurable antenna with operability in both wide and narrow bands is investigated for cognitive radio systems. The proposed antenna consists of a U-shaped patch for the spectrum sensing over a wide band and two open annuli for communication in narrow sub-bands within the frequency range of 3.1-10.6 GHz. The integration of narrow and wide bands devices makes the whole antenna structure compact with a dimension of 40 × 38.5 mm2. With an inherent property of being electromagnetically transparency, four appropriately placed laser-controlled photoconductive silicon switches are adopted to achieve reconfigurable frequency characteristics in the four bands of 5.8-6.8, 6.7-7.3, 7.0-8.4, and 7.9-9.2 GHz with the reflection coefficient below -10 dB. The wide-narrowband antenna is fed by two coplanar waveguides with the isolation of S21 < −15 dB, effectively allowing the spectrum sensing and the communication to operate simultaneously. An antenna prototype has been fabricated, and close agreement is observed between the measured and simulated results. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:1016-1023, 2015 [ABSTRACT FROM AUTHOR]

This article presents a state-of-the-art 3-D-printed dielectric reflectarray antenna featuring one-shot deployability and wide-angle beam-scanning ability. The design is enabled by a series of a “kirigami”-inspired two-stage snapping-like element structure that can be retracted by 66% to save space. The prototype is fabricated with a stereolithography (SLA) 3-D printing method with the Formlabs Flexible 80A photopolymer. A bifocal phase distribution method is utilized to optimize the RF performance of the array, realized −30° to −10° and 10° to 30° beam-scanning ability. The outcome of this work demonstrated a high-performance dielectric reflectarray design with ease of fabrication for 5G and satellite communication applications. [ABSTRACT FROM AUTHOR]

This paper introduces the first-of-its-kind wireless passive sensing platform combining radio frequency identification (RFID), microfluidics, and inkjet printing technology that enables remote fluid analysis and requires as little as 3 \muL of fluid. The demonstrated variable microfluidic capacitors, resonators, and RFID tags are fabricated using a novel rapid, low-cost, and low-temperature additive inkjet process, making them disposable. However, even with their disposable nature, the RF microfluidic devices exhibit repeatability and long-term reusability for accurately detecting water, various alcohols, and % content of water/alcohol mixtures down to 1% water in ethanol. While the main discussion is on fluid sensing, the demonstrated components can also be used in fluid-tunable RF applications. [ABSTRACT FROM AUTHOR]

A broadband circularly polarized (CP) antenna is developed with an omnidirectional radiation pattern in the horizontal plane. Four broadband CP rectangular loop elements are employed for broadband omnidirectional CP radiation. The four rectangular loop elements are first printed on a flexible thin dielectric substrate and then rolled into a hollow cylinder. A conducting cylinder is introduced inside the hollow cylinder for achieving desired omnidirectional CP performance. A feeding network consisting of four broadband baluns and an impedance matching circuit is designed to feed the four rectangular loop elements. The omnidirectional CP antenna has a circular cross section with diameter of 0.38 \lambda0. Experimental results show that the omnidirectional CP antenna has bandwidths of 41% (1.65–2.5 GHz) for \ axial\ ratio < 3 dB and 45% (1.58–2.5 GHz) for \ return\ loss > 10 dB. The gain variation in the omnidirectional plane is less than 1 dB for the frequency range from 1.65 to 2.5 GHz. Good agreement is obtained between simulated and measured results. [ABSTRACT FROM PUBLISHER]

A novel miniaturized 3-D cubic antenna for use in a wireless sensor network (WSN) and RFIDs for environmental sensing is introduced. The antenna produces a truly omnidirectional pattern in both E-plane and H-plane, which allows for non-intermittent communication that is orientation independent. The frequency of operation lies in the UHF RFID band, 902 MHz-928 MHz (centered at 915 MHz). The ultra-compact cubic antenna has dimensions of 3 cm x 3 cm x 3 cm (27 cm3), which features a length dimension of λ/11. The cubic shape of the antenna allows for "smart" packaging, as sensor equipment may be easily integrated inside the cube's hollow (or Styrofoam-filled) interior. The prototype fabrication was performed on six (planar) sides on liquid crystal polymer (LCP) substrate, and then folded into the cubic structure. The geometry of the design is inspired by the RFID inductively coupled meander line structures, which are folded around the sides of the cube. Due to the large number of freedom degrees, this antenna concept may be easily reconfigured for many values of impedances and design parameters. Experimental data verify the simulation results. [ABSTRACT FROM AUTHOR]

This paper presents a novel approach to integrate high-performance millimeter wave filters on top of wafers. The proposed method eliminates the dielectric loss by elevating cavity-based filters into the air with the aid of the polymer-core conductor surface micromachining technology. The electrical fields of the cavity are thus entirely in air. A coplanar waveguide input and output interface is designed for easy integration with other planar electronics. Several 60.GHz (V-band) air-cavity filters with superior performances, including two two-pole filters and one four-pole transmission zero filter using a novel capacitive coupling scheme, are developed and characterized to demonstrate advantages of the proposed technology. The filters exhibit excellent performances. Insertion losses as low as 1.45 dB for a two-pole filter and 2.45 dB for a four-pole transmission-zero filter have been observed at 60 GHz. Design curves and parametric analyses are included to help readers better understand key factors in optimizations. The proposed technology is capable of integrating high-performance millimeter-wave cavity filters on top of wafers, while providing easy integration with other electronic components. [ABSTRACT FROM AUTHOR]