Search

In this letter, we present a novel circularly polarized (CP) origami antenna. We fold paper in the form of an origami tetrahedron to serve as the substrate of the antenna. The antenna comprises two triangular monopole elements that are perpendicular to each other. Circular polarization characteristics are achieved by exciting both elements with equal magnitudes and with a phase difference of 90°. In this letter, we explain the origami folding steps in detail. We also verify the proposed concept of the CP origami antenna by performing simulations and measurements using a fabricated prototype. The antenna exhibits a 10-dB impedance bandwidth of 70.2% (2.4–5 GHz), and a 3-dB axial-ratio bandwidth of 8% (3.415–3.7 GHz). The measured left-hand circular polarization gain of the antenna is in the range of 5.2–5.7 dBi for the 3-dB axial-ratio bandwidth.

In this letter, a novel high-gain tetrahedron origami antenna is introduced. The antenna comprises a triangular-shaped monopole, a reflector, and two parasitic strip directors on a paper substrate. The directors and the reflector are employed to increase the antenna gain. The step-by-step origami folding procedure is presented in detail. The proposed design of antenna is verified by both simulations and measurements with a fabricated prototype. The antenna exhibits a 10-dB impedance bandwidth of 66% (2-4 GHz) and a peak gain of 9.5 dBi at 2.6 GHz.

In this research, a slotted patch antenna sensor is designed for wireless strain and crack sensing. An off-the-shelf RFID (radiofrequency identification) chip is adopted in the antenna sensor design for signal modulation. The operation power of the RFID chip is captured from wireless reader interrogation signal, so the sensor operation is completely battery-free (passive) and wireless. For strain and crack sensing of a structure, the antenna sensor is bonded on the structure surface like a regular strain gage. Since the antenna resonance frequency is directly related with antenna dimension, which deforms when strain occurs on the structural surface, the deformation/strain can be correlated with antenna resonance frequency shift measured by an RFID reader. The slotted patch antenna sensor performance is first evaluated through mechanics-electromagnetics coupled simulation. Extensive experiments are then conducted to validate the antenna sensor performance, including tensile and compressive strain sensing, wireless interrogation range, and fatigue crack sensing.

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.

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.

Nowadays, the explosive growth of Internet-of-Things-related applications has required the design of low-cost and ultra-low-power wireless sensors; backscatter communication has been introduced as a cutting-edge technology that could address the above constraints. For sensing applications, the monitoring of plant water stress is of high importance in smart agriculture. Instead of the traditional ground soil moisture measurements, leaf sensing is an old technology, which is used for the detection of plant water stress. Considering the above topics, this paper aims to provide a contemporary and literature review on fundamentals, applications and research efforts/progress on backscatter systems for leaf sensing applications. It is described how backscatter technology could be exploited for "one sensor per plant" applications in future precision agriculture.

This paper proposes miniaturized, ultrasensitive, flexible planar metamaterials for dielectric constant measurement with a wide dynamic range of the sensing related frequency. The proposed sensors are designed using a thin-substrate microstrip line loaded with tapered complementary split ring resonator (CSRR). The sensors' configuration can be easily integrated with RFID tags for future Internet of Things (IoT) applications. The minimum transmission frequency shifts 56% as the dielectric constant of the material under test (MUT) changes from 1 to 10. Compared to similar state-of-the-art planar sensors, the proposed sensors are almost 50% more sensitive. This extraordinary sensitivity level is of great importance for the development of high-precision near-field based sensors. Sensors' performance were verified numerically and experimentally. To further demonstrate the proposed sensor's practicality, one of the proposed sensors was used to characterize 3D printed dielectric. It was also used as a crack detector for 3D printed samples designed with artificial cracks.

In this paper, a miniaturized millimeter wave Radio Frequency Identification (RFID) system for the spatial localization and Internet of Things (IoT) applications is presented. The spatial localization of the RFID tags is enabled by the use of a Frequency Modulated Continuous Wave (FMCW) Radar as the reader. The radar is used to resolve the modulated backscatter signal returned by the RFID tags when interrogated by a Continuous Wave from the reader. The spatial information (range, angle with respect to the reader antenna) is contained in the returned signal's peak frequency and phase. The returned baseband signal is processed digitally in order to extract the required information regarding the spatial location of the RFID tag.

In our days, the massive growth of Internet-of-Things related applications has required the design and fabrication of low-cost and low-power wireless sensors. The last decade, backscatter communication has been introduced as a cutting-edge technology that could address the above limitations. In high precision agricultural applications, the monitoring of plant-water-stress (PWS) is of high importance for smart sensing. Instead of the traditional ground soil moisture sensing, leaf sensing is an old technology, which is used for the detection of PWS. Considering the above topics, this work aims to mention the development of a novel sensor node/tag for environmental leaf sensing which uses backscattering communication over ambient frequency modulated signals (FM music signals). The work is based on our previous works [1], [2] and it is described how ambient backscatter technology could be exploited for the “one sensor/plant” concept in precision agriculture applications.

This paper reports the first demonstration of antenna-in-package and seamless antenna-to-receiver signal transitions on panel-scale processed ultra-thin glass substrates, for high-speed 5G communication standards in the 28 GHz band. To demonstrate the benefits of g 1 ass for 5 G communications, package-integrated antennas with feedlines were modeled and designed on ultra-thin glass substrates laminated with low-loss dielectric thin films for highest bandwidth and efficiency in the mm-wave bands. The measured results for a miniaturized Yagi-Uda antenna, transmission lines, and through-package vias showed superior dimensional stability and good agreement with the simulated values on 100 µm glass substrates. The results showed low interconnect signal losses with a microstrip line loss of 0.108 dB/mm, and a through-package via loss of 0.095 dB/TPV. The Yagi-Uda antenna fabricated on glass substrates showed a center frequency of 28.18 GHz with a fractional bandwidth of 21.1%. The antenna also presented a wide-angle main lobe at the target frequency range implying good coverage of signal transmitting and receiving. The performance of package-integrated antenna, feedlines, through-package vias, and transmission lines on glass substrates was benchmarked in comparison to other 5G substrate technologies such as organic laminate, ceramic-based substrates, or fan-out wafer level packaging.

In this paper, a novel 3D and inkjet printed pressure-sensing button-shaped resonator is proposed. The button is fabricated using fast, cost-effective, flexible and environment friendly stereolithography (SLA) 3D printing and inkjet printing techniques. The resonant frequency of the button can be tuned from 2.7 GHz, for idle status, to 1.6 GHz, for a hard press. This design can be used for applications such as gesture detection for Internet of Things (IoT) devices and wireless pressure monitoring for buildings and constructions.

The overview of recent advances in additive manufacturing techniques such as different metallization methods is introduced. A fully printed 5G reconfigurable antenna is proposed. The fabrication process using 3D and inkjet printer is characterized. The flexible 3D printed material is used as the base substrate and the inkjet printed silver traces are used as the conductive traces. The operational frequency range of the proposed antenna can be changed by applying pressure on the flexible to change the shape of the structure. Thus, reconfigurability can be achieved. The center of the operational frequency can be adjusted between 26 GHz to 28.9 GHz. The adjusting rate is 11 %.

A novel wireless and passive temperature sensor that utilizes 3D printed inverted-F antenna (IFA) microfluidic and liquid metal technologies for the temperature sensor is introduced in this paper. The thermal volume expansion of liquid metal to progressively increase the length of the arm of IFA which will effectively increase or decrease with respect to temperature. In this way, the sensed temperature value can be accurately quantified by the change in its resonant frequency. Simulation result shows the sensitivity of temperature is 2.54 MHz/°C.

This paper presents an ultra-wideband, glass package-integrated, equal-split power divider with footprint smaller than the unit free-space wavelength corresponding to the operating frequency of 28 GHz 5G band. The utilization of precision low-loss redistribution layers (RDL) on ultra-thin glass substrates with stable electrical properties at mm-wave frequencies enable the ultra-wideband power dividing networks to have a small footprint in x-y-z dimensions along with excellent performance. This approach aggregates the benefits of ceramic, low-loss polymers and silicon: electrical performance of the ceramic, processability of polymers and the dimensional stability of silicon, simulated in glass substrate to realize fine features for the power dividers. The power divider exhibits low added insertion loss, minimal phase shift between its output ports and has a height less than 150-µm.

A first-of-its-kind fully inkjet-printed tunable hybrid n-ripple Miura (n-RiM) frequency selective surface (FSS) is presented that combines the advantages of n-RiM FSS structures (i.e. ultra wideband frequency & bandwidth tunability) and traditional Miura-FSS (i.e. ability to form multilayer FSS structures without mechanical support). They also feature high angle of incidence rejection, making them suitable for wide range of terrestrial and outer-space wireless communication applications.

This work presents a high efficiency, co-planar and low-complexity RF-to-dc rectifier of one diode, which is able to be directly connected, i.e., without a matching network, to a coplanar antenna, e.g., to a dipole or loop antenna. The rectifier was designed to optimally operate at 868 MHz and for low power input. Specifically, it presents 54.82% RF-to-dc efficiency for 20 dBm power input, while for −14.4 dBm the system is able− to supply continuously, i.e., with any boost converter, a backscatter sensor node with power consumption of 20 µW.

In this effort, the authors demonstrate for the first time the implementation of a Rotman-Lens-based rectenna system for mm-wave harvesting, in the 28 GHz band. The structure merges unique RF and DC combination techniques, for the realization of rectennas that display the unlike combination of high gain and large beamwidth. The Rotman-lens-based rectenna includes components–antennas, rectfiers, Rotman lens, and power combining network–whose design is first presented and performance is experimentally characterized and verified. Then, the fabrication and testing of the device, and that of a reference design–comprising a DC-combined array of rectennas of equal aperture–is presented. The novel Rotman-based rectenna harvester shows a 21-fold increase in the harvested power compared to its reference counterpart, while its angular coverage remains identical. The structure, therefore, demonstrates the surprising combination of high-gain and large beamwidth, thereby potentially enabling the emergence of passive long-range mm-wave RFIDs.

A novel multilayer flexible packaging fabrication process using only additively manufacturing techniques including inkjet and 3 dimensional (3D) printing is proposed. The 3D printed ramp structures and inkjet printed transmission lines on top of that are suitable for mm-wave inter-layer connections because lower parasitics are induced to the system. Moreover, a system-on-package (SoP) design for backscattering radio-frequency identification (RFID) is proposed. It has to be stressed, that an RF energy harvester operated at 26 GHz which is embedded inside the packaging using additively manufacturing techniques is proposed for the first time. The output voltage of the harvested energy at a distance of 20 cm away from the source is 0.9 V with transmitted equivalent isotropically radiated power (EIRP) equal to 59 dBm. The harvested energy is large enough to power the TS3001 timer for backscattering and can support all energy requirements of the entire SoP design so that the SoP design is fully autonomous and no external board or components are required. The system size can be shrunk to package level and thus paving the way for a multitude of novel miniaturized autonomous modules for wearable, IoT, and 5G applications.

This work outlines the first-of-its-kind integration of 3D and inkjet printing additive manufacturing processes for the realization of tunable origami structures. The outcome of this process demonstrated a tremendous structural strength improvement over paper-based origami structures, also featuring very good potential for more complicated origami-inspired designs. A mm-wave 4D tunable Miura-shaped frequency selective surface (FSS) is demonstrated using this process. This FSS shows both frequency and bandwidth tunability, and has excellent angle of incidence rejection.

In this paper, a spherical helix module based on 3D printing and microfluidic technologies was fabricated, tested and applied to drug delivery applications. A liquid metal alloy is introduced to enhance the sensitivity of the complex and drug-/liquid-reconfigurable helix topology. Moreover, the influence of the dimensions of the hemisphere inscribed by the helical turns on the resonant frequency of the structure is researched. The microfluidic channels with 1 mm radius are shaped in the form of double semispherical helix using a Form-2 3D printer. To fulfill effectively the liquid metal (LM) injection, movement and recycle for reuse, a syringe pump is integrated to flexibly control the flow of the fluid. Besides, the proof-of-concept prototype achieves excellent radiation gain and impedance matching. The realized sensitivity is 0.96 mL/GHz and 0.33 mL/GHz corresponding with the frequency shift from 2.28 to 2.01 GHz and 2.01 to 1.65 GHz.

This work describes the implementation of a machine learning (ML) strategy based on the neural network for real-time range-adaptive automatic impedance matching of Wireless Power Transfer (WPT) applications. This approach for the effective prediction of the optimal parameters of the tunable matching network and classification range-adaptive transmitter coils (Tx) is introduced in this paper 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 3 Tx coils which have different radius controlled by trained neural network models. The feedforward neural network algorithm was trained using 220 data and classifier’s in pattern recognition accuracy were characterized. The proposed approach achieves a Power transfer efficiency (PTE) around 90% for ranges within 10 to 25cm, is reported.

A first-of-its-kind design methodology to realize origami-inspired multi-layer frequency selective surfaces (FSS) with a thermally actuated smart shape-shifting spacer layer is presented. The spacer not only features low dielectric losses but can also change its shape to maintain optimum inter-layer distances as the Miura-FSS structure is folded to preserve desired frequency response. It also introduces unprecedented high mechanical strength making it suitable for wide range of applications. The proposed structure features twice as broad bandwidth as compared to single layer Miura-FSS and an excellent angle of incidence rejection.

This paper presents a dual-band circularly polarized (CP) antenna with wide 3-dB axial ratio (AR) beamwidth for compass navigation satellite system (CNSS) applications. The antenna is designed to be a double-layer structure and fed through a single coaxial probe. Two patches stacked together with truncated corners and symmetry L-shaped stubs are used to achieve two operating frequency bands, i.e., B1 (1.561 GHz) and B2 (1.207 GHz), for CNSS. In particular, a circular ground plane with four curved slots is adopted to achieve a symmetrical radiation pattern and a wide 3-dB AR beamwidth. At 1.561 GHz, the peak gain of the proposed antenna is 6.4 dBi, and the 3-dB AR beamwidths of 214°and 200°in the xz- and yz-planes are obtained, respectively. At 1.207 GHz, the proposed antenna has a maximum gain of 6.0 dBi, and the 3-dB AR beamwidths of 221°and 202°in the xz- and yz-planes.

A first-of-its-kind fully inkjet-printed electronically tunable active flexible frequency selective surface (FSS) using varactors with integrated on-package solar cells is presented in this paper. Each varactor is biased by a dedicated on-package inkjet-printed solar cell using a low-temperature fabrication process. The solar cell changes its output voltage with variation in light intensities that eventually leads to a change in capacitance of the varactor and overall frequency response of the FSS. The proposed design eliminates the use of labor intensive biasing network, bulky power supply and micro-controllers to tune the FSS frequency response. These structures presents an autonomous, ultra low cost on-package RF shielding mechanism for next-generation of system-on-chip packaging applications that can be tuned on-demand at different frequency bands by simple variation of incident light intensities. Thus making them useful for a wide range of terrestrial, outer-space and EMI shielding applications.

A novel chipless RFID sensor tag is investigated for angular velocity and displacement measurement. The designed sensor tag consists of horizontal and vertical I-shaped bar resonators which are both symmetrically arranged. The whole of the longest resonators acts as a sensing element while the others are used as encoders. The angular variation can be detected by measuring the Radar Cross-Section (RCS) magnitude of the sensing element as the object, to which the tag is attached, rotates. Meanwhile, a depolarizing technique is applied, which makes the detection process more robust. In addition, due to the fact that the dual polarization (DP) can increase the encoding capacity doubly in the same frequency band, the proposed tag with a compact dimension of 22 × 30 × 0.8 mm3 can encode 18-bit data. As compared with angular displacement and velocity sensors based on inductive coupling modulation, the proposed sensor tag can make a significant increase of the sensing distance.

A passive chipless RFID tag working as an angle sensor is presented. The proposed tag consists of a circle patch loaded with multiple slot resonators. The angle which the designed tag rotates is identified via measuring the scattered electric field in vertical polarization direction of I-shaped slot resonator. This is more convenient comparing with dual polarization, avoiding the use of complex measurement network. The proposed tag operates at the band of 5.6–11.6 GHz and its encoding capacity is 8 bits within an area of 2.01 cm2. The identification errors are no more than 5° in the simulated results.

A compact dipole antenna, operating at 915-MHz Industrial, Scientific, and medical (ISM) band of 902–928 MHz, is investigated for wireless capsule endoscope (WCE) systems. The proposed antenna consists of planar and vertical parts. By loading two symmetric semi-circular patches which form capacitive coupling with ring patch and two high impedance lines in the planar part, the simulated impedance bandwidth for| S 11 | less than −10 dB reaches 15.9% (0.87–1.02 GHz). Simulation results indicate that the proposed antenna has a dipole-like radiation pattern with a peak gain of −20.5 dBi.

A wideband frequency tunable circularly polarized (CP) crossed dipole antenna with a wide beamwidth is presented. The proposed antenna consists of two crossed dipoles, four screws, and a modified cavity. By replacing the screws, the proposed antenna can work in different frequency bands. The simulated results show the tunable impedance bandwidth for $\text{VSWR}\leqslant 2$ is 57% from 1.19 GHz to 2.14 GHz and the 3-dB axial ratio (AR) band is covered from 1.27 GHz to 1.92 GHz (41%). The 3-dB AR beamwidth and half power beamwidth are more than 120o at the operating frequencies. Thus, the designed antenna is a promising candidate for modern wireless communication system.

Additive packaging has garnered an increasing amount of interest due to its promising industrial scalability, and its materials and topological patterning versatility. The effort described in this paper benchmarks the performance of such a packaging approach relative to its commercial counterpart for the integration of devices operating in the mm-wave 5G bands. The approach first demonstrates the fully-inkjet-printed integration of a tunnel diode—a component empowering a plethora of emerging applications in these frequency bands—before comparing it to its pre-packaged counterpart. In an effort to enable an accurate and quantifiable comparison, the measured properties of the two integrated diodes were de-embedded using a TRL extraction method before their lumped element models were determined and compared. The comparison demonstrates a general and often significant reduction in all the packaging parasitics, up to 53%, using the additive approach. This result adds upon the growing evidence supporting the superior electrical performance of additive packaging for the integration of mm-wave dies and strengthens the appeal for the development of ultra-compact and ultra-high-performance fully-additively-manufactured mm-wave Systems in Package (SiP).

This paper demonstrates package-integrated power dividers with footprint smaller than the free-space wavelength corresponding to the operating frequency of 28 GHz band for 5G Antenna-in-Package (AiP), by utilizing precision low-loss redistribution layers (RDL) on glass substrates for highly-integrated mixed-signal systems. Two configurations of power dividers with two-way and three-way equal power split are modeled, designed and fabricated on glass substrates with thin-film build-up layers. This approach combines the benefits of ceramic and low-loss polymers for electrical performance, and silicon-like dimensional stability of glass for precision panel-scale patterning. Multilayered RDL with sub-20 micron features are utilized to design innovative power divider topologies with benefits in terms of low added insertion loss (

High-bandwidth 5G and 6G communication systems will inevitably migrate to 3D package architectures with backside or embedded dies and antenna-integrated packages for ultra-low losses and smaller footprints. With the trend to such 3D millimeter-wave (mm-wave) packages, the losses from the assembly and through-vias tend to dominate the overall losses. Traditional wirebond and thick solder interconnections lead to large mm-wave interconnect losses that are not acceptable for emerging 5G and 6G communications. This paper focuses on the material syntheses and process development of nanocopper interconnections with ultra-low interconnect losses for chip-last or flip-chip assembly in packages. The first part of the paper introduces the material synthesis of an innovative copper paste with shorter sintering times and temperatures. Optimized conditions are obtained to attain a conductivity of 1.4x10^7 S/m. This is equivalent to 82% increase in conductivity compared to that of solder. The surface roughness is also measured through atomic-force microscopy. Results suggest that the copper paste features higher roughness than that of solders. The second part of this paper discusses the potential of novel nanocopper paste to replace solders as a package assembly material, focusing on the effect of the conductivity and surface roughness with regard to the insertion loss in interconnection bumps. Based on the improved material properties of nanocopper paste, the model shows a 53% reduction in the dB scale at 28 GHz, by employing nanocopper paste. Die shear test for copper paste is also performed to show a high potential to replace solders as a flip-chip assembly material in both printed-circuit-board and mm-wave packaging technologies.

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.

A triple-band single-layer rectenna for outdoor RF energy applications is introduced in this paper. The proposed rectenna operates in the frequency bands of LoRa, GSM-1800, and UMTS-2100 networks. To obtain a triple-band operation, a modified E-shaped patch antenna is used. The receiving module (antenna) of the rectenna system is optimized in terms of its reflection coefficient to match the RF-to-DC rectifier. The final geometry of the proposed antenna is derived by the application of the Moth Search Algorithm and a commercial electromagnetic solver. The impedance matching network of the proposed system is obtained based on a three-step process, including the minimization of the reflection coefficient versus frequency, as well as the minimization of the reflection coefficient variations and the maximization of the DC output voltage versus RF input power. The proposed RF-to-DC rectifier is designed based on the Greinacher topology. The designed rectenna is fabricated on a single layer of FR-4 substrate. Measured results show that our proposed rectenna can harvest RF energy from outdoor (ambient and dedicated) sources with an efficiency of greater than 52%.

In this paper, we present a novel wideband circularly polarized (CP) composite, called cavity-backed crossed dipole antenna for 2.45 GHz industrial, scientific, and medical (ISM) band wireless communication. To excite the CP radiation effectively, a curved-delay line providing an orthogonal phase difference among the cross-dipole elements is attached at corners of the sequentially rotated elements. By choosing a proper radius of the curved-delay line, a wide input impedance of the antenna can be realized. Unlike conventional cross-dipole antennas, the proposed cross-dipole antenna is designed with an open stub added to the radiating arms of the dipole so that both impedance and axial ratio bandwidths are enhanced. The antenna is center-fed by a 50-Ω coaxial cable and is placed above a cavity-backed reflector to obtain a directional CP radiation pattern. With the advantage of being center-fed, a symmetric CP radiation pattern can be achieved across the entire operating bandwidth. To further improve the directivity and the radiation pattern, a rectangular cavity-backed reflector is used. Simulated and measured results confirm that the proposed antenna has good CP characteristics. The proposed antenna obtains a broad 3-dB axial ratio bandwidth of 49% (1.20 GHz, 1.96–3.16 GHz) and an impedance bandwidth of 67.7% (1.66 GHz, 1.69–3.35 GHz) for reflection coefficient (S11) ≦ −10 dB. It also yields an average CP gain of 9.2 dBic across the operating bandwidth and a peak CP gain of 10 dBic. Copyright © 2016 John Wiley & Sons, Ltd.

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 ( $f_{1}$ ) and 1.83 GHz ( $f_{2}$ ), respectively. When the first resonator is touched, $f_{1}$ 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, $f_{2}$ 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]

This paper presents the design, simulation, and validation experiments of a passive (battery-free) wireless frequency doubling antenna sensor for strain and crack sensing. Since the length of a patch antenna governs the antenna’s resonance frequency, a patch antenna bonded to a structural surface can be used to measure mechanical strain or crack propagation by interrogating resonance frequency shift due to antenna length change. In comparison with previous approaches such as radio frequency identification, the frequency doubling scheme is proposed as a new signal modulation approach for the antenna sensor. The proposed approach can easily distinguish backscattered passive sensor signal (at the doubled frequency $2{f}$ ) from environmental electromagnetic reflections (at original reader interrogation frequency $f$ ). To accurately estimate the performance of the frequency doubling antenna sensor, a multi-physics coupled simulation framework is proposed to aid the sensor design while considering both the mechanical and electromagnetic behaviors. Two commercial software packages, COMSOL and Advanced Design System (ADS), are combined to leverage the features from each other. The simulated performance of the frequency doubling antenna sensor is further validated by experiments. The results show that the sensor is capable of detecting small strain changes and the growth of a small crack.

A ‘button-shaped’ three-dimensional (3D) radio-frequency identification (RFID) tag combining 3D printing and inkjet printing technologies in fabrication for the first time is proposed. The button shape facilitates the tag's integration with clothes for the purposes of identification and access control. The proposed tag features a compact size (radius 15 mm, height 7.5 mm) and a good performance with a measured maximum reading range of 2.1 m (4.0 W equivalent isotropically radiated power) in the RFID Federal Communications Commission band (902–928 MHz). Furthermore, the combination of the 3D printing and inkjet printing technologies reduces the fabrication time and cost.

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.

A broadband antenna inspired by a windmill sail is developed for wireless body area network (WBAN) applications. The antenna consists of a couple of modified dipoles with four crossed symmetrical S-shaped arms printed on a flexible substrate. A $5 \times 5$ unit structure of artificial magnetic conductor (AMC) is integrated to reduce the backward scattering wave toward the human body, and simultaneously ensures a low profile with a thickness of 5.74 mm and a small size with the area of $46 \times 46~\hbox{mm}^{2}$ . The measured results on the phantom reveal that the AMC-integrated antenna accomplishes an impedance bandwidth of 63.5% (5.7-11.0 GHz) with ${ S}_{11} , a peak gain of 8 dBi, and a front-to-back ratio (FBR) greater than 15 dB. Health safety factors, such as specific absorption rate and temperature, are considered. According to the calculation of link budget in different scenarios, the reliable communication can be guaranteed within 10 m in line-of-sight (LOS) environment. The good performances make the proposed AMC-integrated antenna potential for wireless communication between miniaturized wearable sensors and a localized base station around body.

A facile, environmentally-friendly, low-cost, and scalable deposition process has been developed and automated to apply polyelectrolyte multilayers (PEMs) on flexible Kapton HN substrates. Two weak polyelectrolytes, poly(acrylic acid) and polyethylenimine, were deposited in an alternating, layer-by-layer fashion under controlled pH and ionic strength. Compared to strong polyelectrolytes, weak electrolytes can control the properties of the PEMs more systmatically and simply. To our knowledge, this work on surface modification of Kapton is not only the first to use only weak polyelectrolytes, but is also the first to take advantage of the surface properties of calcium-bearing additive particles present in Kapton HN. The resulting surface-modified Kapton HN substrate allowed for the inkjet printing of water-based graphene oxide (GO) inks and organic solvent-based inks with good adhesion and with desired printability. While the deposition of a single PEM layer on a Kapton substrate significantly reduced the water contact angle and allowed for the inkjet-printing of GO inks, the deposition of additional PEM layers was required to maintain the adhesion during post-printing chemical treatments. As a conceptual demonstration of the general applicability of this PEM surface modification approach, a flexible, robust, single-layered gas sensor prototype was fully inkjet printed with both water- and ethanol-based inks and tested for its sensitivity to diethyl ethylphosphonate (DEEP), a simulant for G-series nerve agents. The electrical conductivity and morphology of the sensor were found to be insensitive to repeated bending around a 1 cm radius.

This letter presents two high-gain, multidirector Yagi-Uda antennas for use within the 24.5-GHz ISM band, realized through a multilayer, purely additive inkjet printing fabrication process on a flexible substrate. Multilayer material deposition is used to realize these 3-D antenna structures, including a fully printed 120- μm-thick dielectric substrate for microstrip-to-slotline feeding conversion. The antennas are fabricated, measured, and compared to simulated results showing good agreement and highlighting the reliable predictability of the printing process. An endfire realized gain of 8 dBi is achieved within the 24.5-GHz ISM band, presenting the highest-gain inkjet-printed antenna at this end of the millimeter-wave regime. The results of this work further demonstrate the feasibility of utilizing inkjet printing for low-cost, vertically integrated antenna structures for on-chip and on-package integration throughout the emerging field of high-frequency wireless electronics.

This letter presents one of the first examples of the exploitation of 3-D printing in the fabrication of microwave components and antennas. Additive manufacturing represents an enabling technology for a wide range of electronic devices, thanks to its inherent features of fast prototyping, the reasonable accuracy, fully 3-D topologies, and the low fabrication cost. A novel 3-D printable flexible filament, based on NinjaFlex, has been adopted for manufacturing the substrate of a 3-D printed patch antenna. NinjaFlex is a recently introduced material with extraordinary features in terms of mechanical strain, flexibility, and printability. Initially, the electrical properties of this material are investigated at 2.4 GHz using the ring resonator technique. The capability of selectively changing the dielectric constant by modifying the printed material density by fine-tuning printing infill percentage is verified experimentally. Subsequently, a square patch antenna is prototyped through 3-D printing and measured to validate the manufacturing technology. Finally, exploiting mechanical flexibility properties of NinjaFlex, the antenna is tested under different bending conditions.

This communication presents a new reconfigurable origami bifilar helical antenna. This antenna can change its operating frequencies by changing its height. Also, analytical equations for the design of such antennas are derived based on an equivalent model of a standard helical antenna. An origami bifilar helical antenna is designed and its performance is verified using simulations and measurements.

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.

This paper demonstrates the combination of additive manufacturing techniques for realizing complex 3D origami structures for high frequency applications. A 3D-printed compact package for enclosing radio frequency (RF) electronics is built, that features on-package antennas for RF signal reception (for harvesting or communication) at orthogonal orientations. Conventional 3D printing technologies often require significant amounts of time and supporting material to realize certain structures, such as hollow packages. In this work, instead of fabricating the package in its final 3D form, it is 3D-printed as a planar structure with “smart” shape-memory hinges that allow origami folding to a 3D shape after heating. This significantly reduces fabrication time and effectively eliminates the need for supporting material, thus minimizing the overall manufacturing cost. Metallization on the package is performed by directly inkjet printing conductive inks on top of the 3D-printed surface with a modified inkjet-printed process without the need for surface treatment or processing. Inkjet-printed on-package conductive features are successfully fabricated, that are combined with RF energy harvesting electronics to showcase the proof-of-concept of utilizing origami techniques to build fully 3D RF systems. The methodologies presented in this paper will be enabling the manufacturing of numerous real-time shape-changing 3D complex structures for electromagnetic applications.

In this paper, a novel dual-band, dual-polarized, miniaturized and low-profile base station antenna operating in the frequency bands of 820–960 and 1710–2170 MHz is designed. Elements are arranged such that high-frequency elements are embedded in low frequency elements to reduce volume. A baffle is used to reflect the transmitted power density in the forward direction and also improve isolation between elements. Therefore, surrounding isolation baffles and rectangular baffles are appended around high-frequency elements and low-frequency elements, respectively. The diameter of the proposed antenna cover is only 200 mm, which is smaller than the existing antenna diameter of 280 mm. Compared with the other commonly used antennas, the proposed antenna also has some advantages such as concealment and low profile using a tubular form of radome, which can easily integrate the proposed antenna with the surrounding environment. The measured results verify that the proposed antenna meets the stringent design requirements: voltage standing wave ratio (VSWR) is less than 1.3, the isolation is greater than 30 dB, and the pattern parameters also meet telecommunications industry standards.

This paper proposed a hybrid printed electromagnetic pressure sensor using a metamaterial absorber. We used a three-dimensional printed flexible resin for the transformable substrate with inkjet printed conductive patterns stacked on the printed substrate to realize a hybrid printed electromagnetic pressure sensor. A square ring pattern was implemented for the metamaterial absorber unit cell. Since absorption frequency varied with substrate thickness, the device could be used as an electromagnetic pressure sensor with mechanically transformable substrate. Performance was numerically and experimentally measured, and absorption frequency increased from 5.2 to 5.66 GHz by applying 0 and 20 N pressure, respectively; device sensitivity = 7.75 × 108 Hz/mm (0.2 × 108 Hz/N); and repeatability was retained up to 100 cycles.

This paper proposes an electromagnetic based planar pressure sensor using a substrate integrated waveguide (SIW). The proposed pressure sensor is inspired by a rectangular SIW cavity and is additively manufactured using 3D printed dielectric material with inkjet printed conductive pattern. Since the resonance frequency depends on the SIW centre in transverse electrical mode, a circular cylinder with four bridges is placed at the SIW centre for perturbation. We inserted meshed material at the SIW centre, to facilitate soft pressing, and simplify producing frequency shifts due to capacitive coupling perturbation from different pressure levels. The proposed concept was numerically and experimentally verified, reducing resonance frequency from 4.28 to 3.71 GHz by increasing the pressure from 0 to 2.4 kPa. Device sensitivity = 2.4 × 108 Hz/kPa, and stable frequency change was observed over 100 repeats.

Implementation of multiple functions on the metamaterials can greatly broaden their potential application. Origami-based metamaterials, whose buck properties can be manipulated by tuning the specific folding angle, have currently become a supplemental method for conventional reconfigurable metadevices. Approaching kirigami with transformable features and idea of a functional metastructure, this paper numerically and experimentally investigates a multifunctional electromagnetic response with mechanically programmable characteristic based on an auxetic kirigami pattern in gigahertz frequency regime. The metamaterial unit cell has 2-dimensional (2D) five meta pixels. Each 2D meta-pixel can be transformed to the 3-dimensional (3D) meta-pixel and it is coded by ternary number (-1, 0, +1) for different folding levels. The kirigami pattern provides five transformable tessellations corresponding to five distinct functions of electromagnetic reflector, band-pass filter and polarization-selectable absorbers. The performance of the proposed programmable multifunctional metamaterial is numerically and experimentally demonstrated. This study can motivate the realization of kirigami computing, code-driven multifunctional metastructures.

This article presents the design and demonstration of a high-bandwidth antenna-in-package (AiP) module focusing on low-loss interconnects and Yagi–Uda antenna performance fabricated on a 100- $\mu \text{m}$ low coefficient-of-thermal-expansion (CTE) glass for the 28-GHz band. It shows the modeling, design, and characterization of key technology building blocks along with the process development of advanced 3-D glass packages. The building blocks include impedance-matched antenna-to-die signal transitions, Yagi–Uda antenna, and 3-D active–passive integration with backside die assembly on 100- $\mu \text{m}$ glass substrates. The design and stack-up optimization of antenna-integrated millimeter-wave (mm-wave) modules is discussed. Process development to achieve high-density interconnects and precise dimensional control in multilayered thin glass-based packages is also described. The characterization results of the key technology building blocks show an insertion loss of 0.021 dB per through-package via (TPV), leading to the whole-chain loss of less than 1 dB and a return loss lower than 20 dB. The fabricated Yagi–Uda antenna features high repeatability of wide bandwidth due to the process control enabled by glass substrates. The antenna measurements show a bandwidth of 28.2%, which covers the entire 28-GHz fifth-generation (5G) frequency bands (n257, n258, and n261). The flip-chip assembled low-noise amplifier with 80- $\mu \text{m}$ solder balls shows a maximum gain of 20 dB as desired. The performance of the glass-based package integrated antennas is benchmarked to other 5G substrate technologies, such as organic laminates or co-fired ceramic-based substrates.