additive manufacturing (AM) or 3-D printing is the process of rapidly manufacturing complex parts that are used in a wide range of applications encompassing nearly unlimited types of critical and noncritical components. When considering metal AM, one of the more prominent processes involves layer-by-layer melting of fine metal powder into the desired part geometry, using an electron or a laser beam. The latter is referred to as the laser powder bed fusion (LPBF). The quality of the final printed part is directly impacted by the properties of the feedstock powder. This includes but is not limited to the metal powder size distribution, surface condition (i.e., oxidation), new or recycled powder, powder distribution surface nonuniformities, and streaks. The ability to determine metal powder properties prior to melting provides significant manufacturing quality control capability. Millimeter-wave nondestructive evaluation (NDE) techniques, spanning a frequency range of 30-300 GHz, offer several advantageous features for this purpose. These methods are noncontact, provide a high degree of measurement sensitivity to the metal powder properties of interest, and can provide real-time information. In addition, the reflection properties of the powder are the result of complex electromagnetic interactions among the powder particles and the irradiating wave. This article provides the results of a comprehensive investigation into the millimeter-wave reflection properties of several different types of metal powder at 32-40 GHz. The results demonstrate the ability to distinguish among metal powder types as a function of size distribution, powder stratification, alloy composition, recycled versus new and compacted powder using an open-ended circular waveguide probe, operating in its TE01 mode.

Titanium (Ti64) alloys are extensively used for additive manufacturing (AM) of complex and critical parts in the aerospace industry. Titanium powder particle size, shape, and presence of satellites (in case of recycled powder) are important Ti64 feedstock powder quality indicators. In this investigation, millimeter-wave characterization of Ti64 alloy and graphite powders was performed by studying the reflection coefficient response of the powder samples using a circular waveguide probe (an open cavity) operating in axially symmetric mode (TE0m) at Ka-band (32–40 GHz) and Q-band (42–50 GHz) frequency ranges. In addition to Ti64 and graphite powders, graphite-contaminated Ti64 samples, containing different amounts of graphite powder, were prepared and measured. The primary objective of this work has been to evaluate the efficacy of millimeter-wave reflectometry, and the capability of this measurement technique, for distinguishing among different contaminated Ti64 powder samples with low levels of contamination (by weight). The contamination levels ranged from 0.0015% to 5% (graphite percentage of Ti64 weight). Measurements were carried out at different powder (cavity) depths to determine the optimum sample holder depth for the increased measurement sensitivity. It is reported that a 0.05% (by weight) level of carbon contamination in powder feedstock can lead to defects in the final printed part. Crucially, the results of this investigation showed that graphite contamination levels as low as 0.0045% (percentage by weight) can be robustly detected by this method.

The increased demand for automobile applications and connected-vehicles in the market has prompted much research into intelligent automotive antennas. Connected-vehicles are being used to change automotive technology and serve as a framework for a smart society. This paper presents a thirty-two-port hex‑polarized massive multiple-input-multiple-output (MIMO) antenna that operates at GPS, Wi-Fi, and ultra-wideband (UWB) frequencies. In the proposed MIMO antenna, thirty-two-unit cells are populated in 3D configuration, where eight-unit cells are arranged in the horizontal plane and twenty-four-unit cells are arranged in four different vertical planes. The unit cell, based on an isomorphic inverse approach, consists of a semi-circular radiator with integrated meandered slots and achieves a tri-band operation at 1.5 GHz, 2.4 GHz, and 3.1 to 11 GHz. The unit cell has a size of 20 mm × 20 mm and the developed MIMO architecture has dimensions of 85 mm × 85 mm. The peak gain of the proposed antenna is 3.85 dBi, and the efficiency is 89.5%. The MIMO antenna assembly exhibits a calculated Envelope Correlation Coefficient (ECC) using far-field, less than 0.1. The Total Active Reflection Coefficient (TARC) is found to be less than -10 dB and the difference of Mean Effective gain (MEG) is less than 1 dB. Also, Apparent Diversity Gain (ADG) and Effective Diversity Gain (EDG) by far-field method are found to be above 9 dB. Antenna assemblies like the one proposed, are filled in various layouts on 3D vehicular space to combat the diversity reception issues, catering to spatial and polarization needs. In order to substantially validate the designed MIMO antenna for vehicular applications, housing effects and on-car analysis have been studied.

A growing amount of attention has been placed on the effective use of radio frequency (RF) waves for energy harvesting integrations of RFID with wireless communication systems and Internet of Things (IoT) applications. This work investigates multi-band energy harvesting utilizing a monopole antenna, with a specific concentration on three different frequency bands. The monopole antenna provides adjustable performance in a variety of frequency ranges. The monopole antenna has remarkable properties for short-range communication and identification operations in the lower frequency band [840 - 960 MHz], devoted to NRFID (Near field radio frequency identification) deployment. The energy harvesting circuit uses the middle-frequency band [2.4 GHz], to transform the antenna's radio frequency waves into electrical energy. This energy-collecting feature is very promising for IoT nodes, sensors, and low-power devices. A wide range of wireless communication applications are supported by the antenna's ability to transmit and receive data across long distances, as evidenced by the higher frequency band [4 - 4.7 GHz] that is set aside for data communication. The radiation pattern and the antenna's gain at frequencies of 2.4 GHz and 4.3 GHz is 3.040 dB and 3.464 dB. To evaluate the antenna's performance in RFID applications, the power conversion efficiency is calculated. With the use of a rectifier, the designed antenna has produced a 50% efficiency.

The demand for vehicular antennas increases in tandem with the need for multiple features in automobiles. The development of optically transparent antenna (OTA) has made it possible to deploy antennas on delicate surfaces such as glass. Earlier studies on OTA demonstrated its viability using materials such as transparent conducting oxides (TCO) and conductive polymers. A tri-band OTA is proposed in this paper for vehicular applications. The antenna operates at 1.8 GHz, 2.4 GHz and 3.39–12 GHz bands, covering automotive/wireless applications such as GSM, Bluetooth, Wi-Fi, vehicular communication and electronic toll collection. The proposed OTA is developed on soda lime glass, and the material TCO is used for the radiator and the ground plane. The antenna prototype is tested on windshield and in an anechoic chamber, the gain and efficiency are found to be greater than 1 dBi and 80%, respectively. Furthermore, a machine learning technique for vehicle classification is proposed, which could help in electronic toll collection, automatic vehicle identifier, and parking management applications. The presented algorithm achieves 80% classification accuracy with a minimum window size.

This paper presents the analytical design and radar cross-section (RCS) reduction of linear, tapered slot antenna for C band applications. The antenna was designed on an FR-4 substrate. The complete characterization of the antenna was carried out in terms of reflection coefficient, radiation pattern, antenna gain and RCS. The antenna operates over C band microwave frequencies and the centre frequency of operation is 6.7 GHz. It has been observed from the simulation that different types of metallic reduction strategies viz. rectangular cut, trapezoidal cut, cutout stripes, and semicircular cut, implemented on the reference antenna were able to provide RCS reduction. It was found that the cutout stripes technique has a minimum frequency shift of 200 MHz from the centre frequency of the reference antenna, better RCS reduction of 12dB at 6.7 GHz and preserves the radiation pattern to a large extent.

In this work, a linear eight element array of electric field sensors is realized for non-destructive testing of dielectric composite widely used in aerospace, medical, and automotive fields. An electrically short dipole antenna array was printed on photo paper using silver conductive ink, and the antenna transmission line was printed using polymer-based resistive ink. A zero bias Schottky diode was employed as the RF detector, and the output voltage was fed to a micro-volt meter. Planar dielectric composite sample of 3 mm thickness was illuminated by an X band (8–12 GHz) spot focusing horn antenna. An FR-4 sample of size 300 mm × 300 mm with a defect diameter of 20 mm was tested with this setup. First, the response from a defect free calibration sample is taken. Then, the measurement is carried out on the defective sample. The sensor array was used to map the electric field strength in defect free and defective regions of the sample.

This article presents a compact precision free-space microwave measurement setup with a choice of three dielectric lenses to tailor the antenna focal plane characteristics for extracting complex dielectric permittivity of small samples. Custom designed spot-focusing horn antenna pairs were used to achieve a compact setup with antenna separation distance, 2f l : 4λ c-8λ c and focal spot size, fs : 1λ c-1.5λ c , where λ c is the wavelength at center frequency. Using the compact free-space setup, relative complex permittivity (j) was extracted over 8-12 GHz for low-and high-loss dielectrics with lateral dimensions, 3.3 λ c× 3.3λ c and 10 λ c × 10 λ c. For large materials under test (MUTs), i.e., 10 λ c × 10 λ c , measurement accuracy in dielectric constant, Δ% was <0.65% and ≤1.14% for low-and high-loss dielectrics, respectively. For smaller MUTs ( 3.3 λ c × 3.3 λ c ), Δ % was <0.89% and ≤2.29% for low-and high-loss MUTs, respectively. The error in loss tangent ( Δ) varied over 0.002-0.016 and 0.015-0.056 for large ( 10 λ c × 10 λ c ) and small MUTs ( 3.3 λ c × 3.3 λ c ), respectively. For large MUTs, biconvex lens pair with the smallest f-s ( 1 λ c ) and f-l ( 4 λ c ) among the three lenses yielded the best accuracy in dielectric constant due to tight field focusing at the focal plane. The plano-convex lens pair yielded the best accuracy in loss tangent for large MUTs due to slow variation in the phase of the local plane wave. By tailoring antenna focal plane characteristics, a compact free-space setup that is 6×-10× smaller than the classical setup for handling MUTs that are 1/5th of the size used in classical setup is demonstrated without compromising the measurement accuracy.

Patterned deposition of highly flexible transparent conducting materials is essential to realize stretchable optoelectronic devices. Silver nanowires (NWs) are suitable for these applications because they possess high electrical conductivity and good optical transparency. However, NWs have poor surface adhesion and large roughness. Embedding them in a conducting polymer, such as poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS), is one way to overcome these disadvantages without affecting the optoelectronic properties. However, this is normally a two-step deposition process and difficult to pattern directly. In this work, we have formulated a stable and printable nanocomposite ink consisting of Ag NWs and PEDOT:PSS. This ink can be directly used for patterned deposition in a single-step process. The printed film shows 86% transparency and 23 ω/sq sheet resistance, which is suitable for flexible transparent electrode applications. The printed film shows good adhesion and excellent stability to mechanical deformation, with less than 20% resistance variation after 10,000 bending cycles. The nanocomposite also exhibits improved thermal stability, planarity, reduced contact resistance, and good optical transparency when compared to pure Ag NWs. We demonstrate suitability of this nanocomposite using two applications -a printed transparent flexible antenna radiating at Wi-Fi frequencies and a printed transparent flexible heater suitable for antifogging applications. The nanocomposite properties make it suitable as a transparent electrode in flexible optoelectronic devices such as photovoltaics and light-emitting diodes.

A hybrid microwave NDE technique employing near field sensor for electric field measurement in plane wave regime is proposed in this paper. A custom made spot focusing horn antenna has been used to illuminate the sample with defect. A graphene based electric field sensor is employed to measure the electric field with less field perturbation. Thin dielectric composites of thickness less than quarter wavelength has been inspected using this method. Samples of size 300 mm × 300 mm with machined defect radius of 10 mm and 5 mm with 0.5 mm depth were measured using the proposed hybrid method. The hybrid technique has a higher spatial resolution and insignificant perturbation to the electric field compared to the conventional methods.

In this paper, the resonance behavior of layered dielectric media is exploited for microwave non-destructive testing (NDT) of thin composite. Ultra wide band excitation over 1-20 GHz coupled to the composite using open ended coaxial probe was used to identify sample resonance in the absence of the defect. The time gated reflection coefficient (S11) of the sample measured at resonance was used to record the spatial variation in S11 for an insert with low dielectric contrast embedded in the thin composite. The proposed technique was validated using 3.5 mm thick glass fiber epoxy composite of 6 layers for 10 mm x 10 mm x 0. 4 mm inclusion with low dielectric contrast (| A e r | < 2) in between the composite layers. The measurements indicate enhanced sensitivity for the low dielectric contrast at sample resonance which is lost when inspected off resonance in agreement with the simulations.

The focal plane characterization of spot focusing horn antennas used for microwave NDE is presented using three experimental methods. Reflections from a metal sphere were analyzed in time domain to quantify the focal spot and spot size in the first method. In the second method, a rectangular waveguide padded with absorber was used as the sensor. Finally, graphene based miniaturized electric (E)-field sensor printed on photo paper using inkjet printing was used to measure antenna focal plane characteristics. The focal plane measured using the metal ball and waveguide techniques was within 5% of the simulated value. Higher error was recorded in focal spot measurement (≥25%) for the first two methods. The focal plane and spot size measured by the miniaturized E-field sensor was within 10% of the simulated antenna characteristics. The results indicate that the flexible printed sensor has better measurement accuracy and simple setup for field measurement.

The feasibility of non-contact and non-destructive evaluation (NDE) of planar aerospace dielectric composites using microwave is examined in this paper. Free space microwave measurement set up includes spot focusing horns to gather the scattering parameters of the composites with the help of (Gated-thru reflect line) G-TRL calibration. The dielectric properties are studied from the measured scattering parameters to characterize the material's defective and non-defective region. Particulate reinforced composite with an air gap of 0.1 mm thickness and 10 mm width is used for validation of the technique.

This manuscript presents the design of a size reduced microstrip rat-race (RR) coupler with improved spurious passband suppression performance. The coupler operates at 0.95 GHz, corresponding to 5G applications (sub-1 GHz 5G applications). The combined effect of coupled microstrip lines, slant connected split ring structure and the ring shaped defects offer deep suppression of harmonic signals beyond the passband till 4 GHz. The proposed coupler also offers 47.2% size reduction compared to a conventional RR coupler model.

The focal plane characterization of spot focusing horn antennas used for microwave NDE is presented using three experimental methods. Reflections from a metal sphere were analyzed in time domain to quantify the focal spot and spot size in the first method. In the second method, a rectangular waveguide padded with absorber was used as the sensor. Finally, graphene based miniaturized electric (E)-field sensor printed on photo paper using inkjet printing was used to measure antenna focal plane characteristics. The focal plane measured using the metal ball and waveguide techniques was within 5% of the simulated value. Higher error was recorded in focal spot measurement (≥25%) for the first two methods. The focal plane and spot size measured by the miniaturized E-field sensor was within 10% of the simulated antenna characteristics. The results indicate that the flexible printed sensor has better measurement accuracy and simple setup for field measurement.

The feasibility of non-contact and non-destructive evaluation (NDE) of planar aerospace dielectric composites using microwave is examined in this paper. Free space microwave measurement set up includes spot focusing horns to gather the scattering parameters of the composites with the help of (Gated-thru reflect line) G-TRL calibration. The dielectric properties are studied from the measured scattering parameters to characterize the material's defective and non-defective region. Particulate reinforced composite with an air gap of 0.1 mm thickness and 10 mm width is used for validation of the technique.

A novel ultrahigh frequency (UHF) near-field radio-frequency identification (RFID) reader antenna based on the complementary split ring resonator (CSRR) elements is presented in this communication. The antenna consists of a power divider and two arms. The two arms are terminated with two 50-Ω terminations. First arm is forward arm, which is a microstrip transmission line. The second arm is backward arm, which is loaded with CSRR elements, instigating backward wave propagation. The oppositely directed currents are generated by this configuration to produce strong and uniform magnetic field over the antenna plane for UHF near-field RFID operations. The proposed antenna operates from 0.76 to 0.88 GHz, and a total impedance bandwidth of 120 MHz is obtained. A near-field read range of 100 mm and an interrogation area of 220 mm ×180 mm over the antenna plane at a height of 50 mm is reported for this antenna.

This paper presents the design, testing, and analysis of a clover structured monopole antenna for super wideband applications. The proposed antenna has a wide impedance bandwidth (-10 dB bandwidth) from 1.9 GHz to frequency over 30 GHz. The clover shaped antenna with a compact size of 50 mm × 45 mm is designed and fabricated on an FR4 substrate with a thickness of 1.6 mm. Parametric study has been performed by varying the parameters of the clover to obtain an optimum wide band characteristics. Furthermore, the prototype introduces a method of achieving super wide bandwidth by deploying fusion of elliptical patch geometries (clover shaped) with a semi elliptical ground plane, loaded with a V-cut at the ground. The proposed antenna has a 14 dB bandwidth from 5.9 to 13.1 GHz, which is suitable for ultra wideband (UWB) outdoor propagation. The prototype is experimentally validated for frequencies within and greater than UWB. Transfer function, impulse response, and group delay has been plotted in order to address the time domain characteristics of the proposed antenna with fidelity factor values. The possible applications cover wireless local area network, C-band, Ku-band, K-band operations, Worldwide Interoperability for Microwave Access, and Wireless USB.

A multiservice radio-frequency identification (RFID) antenna with wideband axial ratio (AR) response around 2.4 GHz is presented in this study. This antenna offers dual band operation. The prototype follows a configuration constituting a driven element (radiator) and a parasitic element. The first band offers a voltage standing wave ratio (VSWR) bandwidth (3:1) of 183 MHz (0.85-1.04 GHz). The VSWR bandwidth (3:1) for the second band is 2.77 GHz (2-4.77 GHz). It also offers a 3 dB AR bandwidth of 1.1 GHz (2.1-3.2 GHz). The 2:1 VSWR bandwidth of 0.9 GHz (2.1-3 GHz) and cross-polarisation in the second band makes this antenna suitable for 2.4 GHz RFID reader applications. The other services supported by the proposed antenna include global system for mobile communication (0.88- 0.96 GHz), long-term evolution (2.3-2.4 GHz), wireless fidelity (2.4-2.5 GHz), bluetooth low energy (2.4 GHz) and worldwide interoperability for microwave access (3.5 GHz). To extract the multiservice characteristics, it is necessary to integrate this antenna to a handheld computing platform. For assessing the performance in such a platform, the prototype is optimised and merged with a larger ground plane of size 170 mm× 130 mm and studied in the course of this study.

This letter presents a compact ultrahigh frequency multiservice radio frequency identification reader antenna for near-field and far-field operations. The far field is circularly polarized, and the strong near field is generated using inductive coupling. The dimension of the prototype is 83 × 83 × 1.6 mm3. The prototype is based on a rectangular slot antenna, fed by an L-shaped feed. An integrated near-field and far-field radiator along with asymmetric slot perturbation result in circular polarization. The strong magnetic field in the near-field is generated by a configuration of stubs in the radiator, which bring oppositely directed current over the antenna plane. The antenna exhibits an impedance bandwidth of 110 MHz. The axial-ratio bandwidth (ARBW) of 37 MHz is also reported. A near-field interrogation area of 220 × 180 mm2 is reported for this antenna.

This study presents the design of a flexible compact rat-race coupler operating at 0.72 GHz on a 1 mmthick paper substrate. The ultra-wide stop-band effect caused due to the combination of the modified stepped impedance stub and the interdigitated resonator slot, offers suppression up to the 14th harmonic. The slow-wave effect caused due to the resonator slot leads to 59.6% size reduction compared with the conventional coupler at the design frequency. The signal transmission through the coupler for different substrate deformation steps has been investigated. Stable responses are reported for the same, making the prototype suitable for flexible electronics applications.

This paper presents the design of a novel linear analog planar phase shifter deploying split-ring resonator (SRR) and complementary split-ring resonator (CSRR) structures. Based on the advantages of these structures, a triband phase shifter is designed for multiple target systems to operate at 0.85, 1.69, and 2.46 GHz bands finding applications for European RFIDs, Satellite Radio Broadcast System, Mobile Services, and ISM Band, respectively. The effect of SRR and CSRR coupling with host transmission line is also analyzed on the basis of Bloch mode theory and the modes are validated through Eigen mode analysis. The proposed phase shifter design shows a good agreement between simulated and measured results. A 90° ± 8° shift in phase is observed at lower and upper bands, and a 135° ± 8° shift is observed in center band with reasonable group delay components.

This study brings out the design, fabrication and performance evaluation of a compact, dual band antenna on paper substrate. The two bands of operations are 2.18 to 2.50 and 5.22 to 5.72 GHz. These bands are used by the wireless sensor networks (WSNs) and radio frequency identification systems (RFID). The antenna becomes useful for the WSN nodes and RFID systems at these frequencies. The overall size of the antenna is 40 mm × 20 mm × 0.6 mm, which makes it a compact one. Antenna was tested for the effect of bending by placing it over curvilinear surfaces. The bending was done in horizontal (X) and vertical (Y) axes. These examinations will help to understand the effect of wind loading and other packaging related bending on antenna performance. The main features of this antenna are stability in performance under bent conditions, simple configuration and low cost of fabrication. A thin polyethylene layer has been used to laminate the antenna for permanence. The effect of lamination has also been explained in the course of the study.

This letter introduces a new method of achieving broad VSWR bandwidth and axial ratio bandwidth (ARBW) deploying fractal geometry in a single feed compact annular slot antenna loaded with diagonal slots. Various studies have been carried out for different iteration order (IO) and iteration factor (IF) on antenna performance. A measured VSWR bandwidth (2:1) of 400 MHz (1.37 GHz to 1.77 GHz) and measured ARBW of 360 MHz (1.47 GHz to 1.83 GHz) is achieved. A measured peak gain of 6.6 dB is reported at 1.775 GHz. This antenna can be applied to handheld devices applications and WSN node positioning requiring GPS Data.

This letter presents the design of a dual-band wearable fractal-based monopole patch antenna integrated with an electromagnetic band-gap (EBG) structure. The prototype covers the GSM-1800 MHz and ISM-2.45 GHz bands. The EBG structure reduces the radiation into the human body over 15 dB. It also reduces the effect of frequency detuning due to the human body. The performance of the antenna under bending, crumpling, and on-body conditions has been studied and presented. Specific absorption rate (SAR) assessment has also been performed to validate the antenna for its usefulness in wearable applications.

In this letter, a compact frequency selective surface (FSS) composed of a modified swastika unit cell having the smallest dimension of 7$,times,$7 mm$2$ is proposed. The design is aimed at the rejection of 5-GHz WLAN band. The unit-cell geometry resembles the shape of crossed dipoles to achieve compactness. The proposed FSS provides 400 MHz bandwidth with 20 dB insertion loss. The proposed design holds a stable response for TE and TM modes of polarization as well as oblique incidence angles, thus ensuring polarization and angular independent operation. The simulated results are validated with measured results obtained from the fabricated FSS. © 2013 IEEE.

In this letter, two triband RFID antennas are proposed, one for the radio frequency identification (RFID) reader and the other for the RFID tag. The antenna is the crucial part in designing any RFID system. The reader antenna operates at 3.6, 5.8, and 8.2 GHz. The tag antenna operating frequencies are 3.9, 5.9, and 8.2 GHz. The applications of these frequencies are RFID for logistics management, traffic toll collection, and tagometry (telemetry using RFID), respectively. Fractal structures are employed as radiating elements for this multiband antenna. © 2002-2011 IEEE.

This paper describes a novel FSS which functions as band stop filter to shield the GSM 1800 MHz downlink band. The FSS is designed to operate with the resonant frequency of 1820 MHz which is the centre frequency for the GSM 1800 MHz downlink band. The novelty is attributed to its unique geometry and the circular apertures endowed with it. The proposed geometry provides shielding effectiveness of 20 dB alongside with 133 MHz bandwidth. The structure holds identical response for both TE and TM Modes of polarization. In addition, the geometry with its circular apertures, a hitherto unexplored feasibility serves the purpose of ventilation and heat dissipation. The simulated results are validated using experimental measurements.