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The above paper claims that if each 90° transmission-line section of a conventional ring hybrid (CR) is replaced with an asymmetric T-type, the resulting circuit conserves the properties of CR in the frequencies of operation. The theorem is, however, a fundamental mistake, which will be verified further.

This article outlines a number of inkjet-/threedimensional (3-D)-printed prototypes of RF and millimeter-wave (mmW) components, interconnects, and systems. We pay special attention to the major challenges related to applying current multidimensional printing technologies to the fabrication of flexible multichip modules (MCMs) and high-performance mmW components.

A novel wearable and flexible energy autonomous on-body sensing network is proposed featuring full operability through energy harvesting from a hand-held 464.5-MHz UHF two-way talk radio. Three different functions are provided utilizing the hand-held two-way talk radio as the only energy source for our proposed system. There are two types of energy harvesters (EHs) proposed for the presented system. The first EH that is mounted on the sensing capable backscattering RFID tags harvests the 464.5-MHz signal energy to drive the tags; the second EH that can be worn on hands harvests the same 464.5-MHz signal to produce both the dc power and the carrier signal. The second EH is more efficient than conventional ambient RF energy harvesting architectures because for the first time, both the dc and the second harmonics generated by the rectifier are utilized to enable two additional functions. The generated second harmonic is used to interrogate backscattering RFID tags for on-body sensing, while the dc power is used to power an RF amplifier in order to enhance the second harmonic to effectively extend the sensing and communication range. For the proof-of-concept demonstration, the measured dc and the second harmonic, 929 MHz, output power from the proposed EH are 17.5 and 1.43 dBm, respectively, while a two-way talk radio is 9 cm away. The measured second harmonic output power is increased to 13 dBm utilizing the harvester-powered RF amplifier, and the reading range of the custom backscattering sensor tag is extended to more than 70 m. Also, the interrogation of multiple sensor tags and the wireless detection of ammonia gas utilizing an inkjet printed flexible ammonia sensor are demonstrated showing the wide range of applications of the proposed approach.

This paper 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 IC die are presented using printed dielectric ramps and coplanar waveguide (CPW) transmission lines exhibiting low loss (0.6–0.8 dB/mm at 40 GHz). Stereolithography (SLA) 3D printing is presented for the encapsulation of IC dies, enabling the application-specific integration of on-package structures, including dielectric lenses and frequency selective surface (FSS)-based wireless filters. Finally, inkjet and 3D printing technology are combined to present sloped mm-wave interconnects through an encapsulation, or through-mold vias (TMVs), achieving a slope up to 65° and low loss (0.5–0.6 dB/mm at 60 GHz). The combination of these additive techniques is highlighted for the development of scalable, application-specific wireless packages.

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.

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.

A quadruple-polarization reconfigurable feeding network for an unmanned aerial vehicle (UAV) RF sensing antenna operating at 915 MHz is presented. It consists of one single-pole four-throw (SP4T) switch, one 90° hybrid coupler, two 180° hybrid couplers, and two single-pole double-throw (SPDT) switches. By controlling the SP4T and SPDT switches, quadruple polarizations with two linear polarizations (vertical and horizontal) and two circular polarizations (right-hand and left-hand) were obtained. The proposed reconfigurable feeding network with quadruple polarization modes in the 902–928-MHz range has an insertion loss of approximately 2.5 dB including switch loss, and it achieves amplitude and phase variations of up to approximately 0.5 dB without divided power or switch loss, and 3°, respectively. Using the proposed feeding network, a circular low-profile UAV RF sensing antenna with quadruple polarizations was designed. Its electrical size, 10-dB impedance bandwidths, and a peak gain are approximately $0.57\lambda _{0} \times 0.57\lambda _{0} \times 0.08\lambda _{0}$ at 915 MHz, 16%, and 6 dBi(c), respectively.

This paper proposes a gain-enhanced metamaterial (MM) absorber-loaded monopole antenna that reduces both radar cross-section and back radiation. To demonstrate the proposed idea, we designed a wire monopole antenna and an MM absorber. The MM absorber comprised lumped elements of subwavelength unit cells and achieved 90% absorbance bandwidth from 2.42&ndash, 2.65 GHz. For low-profile configurations, the MM absorber was loaded parallel to and 10 mm from the monopole antenna, corresponding to 0.09 &lambda, 0 at 2.7 GHz. The monopole antenna resonated at 2.7 GHz with a 3.71 dBi peak gain and 2.65 GHz and 6.46 dBi peak gain, before and after loading the MM absorber, respectively. Therefore, including the MM absorber increased peak gain by 2.7 dB and reduced back radiation by 15 dB. The proposed antenna radar cross-section was reduced by 2 dB compared with a monopole antenna with an artificial magnetic conductor.

A dual-band flexible antenna on a 3-D printed support is proposed for wrist worn applications. The antenna is aimed to work at 860 MHz and 2.4 GHz. A spatial diversity technique is used to overcome the blocking of the radiation by the arm. Characteristic Mode Analysis is used as a first step of the design process, analyzing different structures.

Package-integrated and ultra-thin low-pass filter (LPF) and bandpass filter (BPF) with footprint smaller than $0.5\lambda _{0}\times 0.5\lambda _{0}\times 0.025\lambda _{0}$ at the operating frequencies of 28- and 39-GHz bands are presented for 5G and mm-wave small-cell application. Such package integration of 5G filters with ultrashort 3-D interconnects allows for low interconnect losses that are similar to that of on-chip filters, but low component insertion loss of off-chip discrete filters. These thin-film filters exhibit a cross-sectional height of $188.5~\mu \text{m}$ and can be utilized either as embedded components or integrated passive devices in module packages. Three topologies of LPF and BPF in total are modeled, designed, and fabricated on precision thin-film buildup layers on glass substrate as a core. Large-area panel-compatible semiadditive patterning (SAP) process is utilized to form high-precision topologies to aid the low-cost fabrication of ultraminiaturized filters. SAP also enables the realization of ultra-thin traces to precisely model high-impedance inductive lines compared to conventional subtractive etching and printing techniques. This results in filters with low insertion loss, low VSWR, and high selectivity. The simulated and measured results of filters show an excellent correlation.

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.

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&ndash, 28.2 GHz. Therefore, it is successfully demonstrated by electromagnetic simulation and measurement results.

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.

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.

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.

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 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.

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.

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.

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.

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.

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.

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 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.