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This article presents a fabrication method for a $C$ -band, low-power, frequency-modulated continuous wave (FMCW) radar front–end technology demonstrator that is fully embedded in a representative aerospace composite structure. This demonstrator uses a structural glass-fiber-reinforced polymer laminate as both the mechanical load-bearing element and the multilayer substrate for the integrated microwave circuitry. The system design and optimization have been described in detail. The electromagnetic simulation of the technology demonstrator was shown to be in excellent agreement with the measurement results. Finally, the FMCW radar front-end was tested as a complete system and shown to yield correct distance-to-target estimations. The demonstrated integration technique will lead to smart skins where electronic circuits and complete systems are embedded in load-bearing composite structures to yield truly multifunctional capabilities.

The penetration of microwave power inside a human head model is improved by employing a dielectric loaded rectangular waveguide as the transmission source. A multi-layer reflection model is investigated to evaluate the combined material characteristics of different lossy human head tissues at 2.45 GHz. A waveguide loaded with a calculated permittivity of 3.62 is shown to maximise the microwave power penetration at the desired frequency. A Quartz (SiO2) loaded rectangular waveguide fed by a microstrip antenna is designed to validate the power penetration improvement inside an inhomogeneous human head phantom. A measured 1.33 dB power penetration increment is observed for the dielectric loaded waveguide over a standard rectangular waveguide at 50 mm inside the head, with an 81.9% reduction in the size of the transmission source.

Presented in this letter is an improved version of the point scatterer vector tracking (PSVT) method for noise estimation in microwave coherent Doppler tomography (CDT). This new process of multipeak PSVT (MP-PSVT) significantly improves the quality of the tomogram given a noisy dataset. Phase shifts caused by positional displacements between the microwave source and center of rotation are compared against multiple isotropic scatterers placed within an image plane. Correlation between a subset of estimated shifts is then used to formulate a singular noise function. This noise function is used to deembed the original dataset thereby compensating for all phase errors. The example performed in this letter leads to a 0.91 correlation coefficient in the linear regression model containing the estimated noise and the applied noise. This new MP-PSVT approach significantly outperforms the original PSVT method.

This work presents a comparison between two metasurface antennas operating at 10 GHz utilizing either a conventional hexagonal grid (with a continuous distribution of gap dimensions) or a Voronoi partition (utilizing a constant gap dimension). The Voronoi partition was generated using the point-shifting method (proposed by Lee and Sievenpiper) and is (to the authors’ knowledge) the first demonstration of this approach in the design of a radiating metasurface antenna. The traditional hexagonal artificial impedance surface (H-AIS) and Voronoi artificial impedance surface (V-AIS) antennas are shown to exhibit near identical surface impedance profiles, insertion loss, radiation efficiency, and realized gain. However, the Voronoi partition can be adapted to follow an arbitrary conformal surface, whereas the hexagonal grid is restricted to a 2-D plane. To reinforce this advantage, a third metasurface antenna on an arbitrary conformal geometry is designed utilizing numerically generated “reference” and “objective” fields. Lastly, the fabrication of all three antennas utilized a unique process that is compatible with conventional aerospace glass fiber-reinforced composite manufacturing. Hence, this work represents a significant step forward in the deployment of truly conformal multifunctional composite structures.

This work presents a technique to detect forest fire using incoherent scattering of radio waves from the ionized electron cloud of combusting plant material. Superheated forest matter ionizes a certain amount of the Alkali salt in plants creating a natural plasma frequency. This work investigates this plasma using a calibrated transmission line to identify the electromagnetic plasma frequency of forest fire. A plasma frequency of approximately 500 MHz has been identified by burning 4 grams and 6 grams of dry Eucalypt leaves. The plasma frequency was found to be stable as long as the biomass density is constant.

This article demonstrates a new method of determining depth in wideband coherent Doppler tomography (CDT) using only a monostatic arrangement. The method was developed to eliminate the smearing effects present when imaging a target wrapped around a conformal surface. All that is required is the addition of isotropic targets placed around the subject under test (SUT). Using only a single data set, two images containing differing reference planes are generated. The new algorithm locates the position of these points and correlates the dispersion of the peak spectral power for each one. It has appropriately been named the dual-layer peak matching (DLPM) technique. The DLPM technique will be performed in both the measured and simulated environments to consolidate the robustness of the results generated. For both examples, positional error falls within the range resolution for the measurement setup. All steps are demonstrated from the data acquisition to the final result, allowing the user to predict the depth (from here on referred to as the “ $\boldsymbol {\Delta z}$ ” location) of multiple scatterers in space. Finally, a tomographic image will be generated that encompasses the DLPM technique to eliminate smearing in the measured example.

This letter utilizes the holographic principal to design two metasurface antennas with an equivalent electromagnetic performance. The first antenna is designed with a continuous metasurface impedance variation and serves as the baseline antenna (comparable to similar antennas in the literature). The second antenna is designed using two discrete metasurface impedance values realized with two distinct unit cell geometries. Both holographic antennas are shown to exhibit the equivalent electromagnetic performance. However, the latter design (requiring only two discrete impedance values) simplifies the holographic pattern manufacture, and therefore suggests a new avenue to realize “switchable” metasurface antennas capable of beam steering. The “continuous” and “discrete” antenna designs presented in this letter are fabricated using a unique manufacturing approach to realize a structurally integrated antenna in a representative aerospace glass fiber reinforced composite laminate.

Determining the high-frequency conductivity of a material is one of the critical steps in material characterization. In this paper, we introduce a new technique for determining the conductivity which is based on shielding effectiveness measurement in a rectangular waveguide. The proposed method has been validated using the simulation and measurement. The proposed method has shown a very good accuracy for determining the conductivity and has significant advantages over traditional techniques, such as simple setup/postprocessing, no need for vector measurement, high repeatability, and being capable of measuring high conductivities precisely. Besides these advantages, the proposed method has some limitation such as being suitable for good and/or thick conductors.

This article describes the design and fabrication of an active four-port microwave switching network embedded into a multilayer aerospace composite structure. The multifunctional design is based on three broadband Integrated Device Technology (IDT) F2972 single-pole-double-throw (SP2T) RF switches and also includes a control and power supply circuit. The composite structure has been developed using a preimpregnated structural glass (S-Glass) material (Hexcel HexPly 914E). The conductive tracks of the switch and its ground plane have been fabricated using a nonwoven carbon–nickel–copper (CNC) veil material and a semiflexible conducting silver ink. This article contains an analysis of the electrical properties of both materials at microwave frequencies. The equivalent electrical conductivities of materials were determined using a curve fitting method that was then used to verify the computational modeling of the switching network. The RF switching network was developed based on a hybrid manufacturing approach. The RF transmission lines and a component of the control circuitry were transferred onto uncured pre-preg material using a laser to ablate select regions of the CNC veil material. Postcuring, flexible silver ink was additively printed onto the surface to create the remaining control circuits. The RF performance of the design has been evaluated, and it indicates that the overall performance was consistent with the measured S-parameters of the same circuit fabricated on Rogers 5880 substrate.

Ultra-wideband coherent Doppler tomography (UWB-CDT) is a method whereby complex frequency data are evaluated and back-projected at uniform angles around a subject under test (SUT). In the presence of phase shifts brought upon by the unwanted motion in the test equipment, generating a focused image becomes complex due to the nature of the back-projection integration process. This article will demonstrate a vector-based correction algorithm, which can be implemented during the nondestructive evaluation (NDE) of a target to compensate for down-range phase errors (brought upon by the factors external to the ideal measurement setup). This new method will be known as point scatterer vector tracking (PSVT). A coherent image can be constructed by using the returned scattering information contained within a complex time-domain envelope. For the proposed method, multiple vectors based on the estimated peak locations are used to track, correct, and reconstruct the path of an ideal point scatter. The demonstration of the algorithm with measured tomographic data combined with a simulated additive noise component is presented, along with a step-by-step approach to reconstruct the desired data set. It will be shown that by using this proposed method, a coherent CDT image with an average correlation coefficient of 0.71 can be recovered from the data that have been corrupted by the unwanted phase shifts.

This paper presents the analysis of magnetic LC (MLC) resonators loaded with differential microstrip transmission lines. The analysis is based on an equivalent circuit model describing the electromagnetic behavior of the MLC resonators. It is shown that a pair of microstrip lines loaded with MLC resonators exhibit a dual-mode bandstop response in the common mode of operation, while being transparent to the differential signals. This characteristic can be used in the design of differential transmission lines with wideband common mode suppression. Circuit models together with Bloch theorem are applied for the systematic design, where the dispersion analysis helps to estimate the maximum achievable common-mode rejection bandwidth. A systematic design procedure is developed for designing differential transmission lines using this method. The theoretical analysis is validated by designing and measuring a balanced transmission line using three cascaded MLC-based cells. The measurements exhibit more than 40-dB common-mode signal suppression for a fractional bandwidth of 32% around 1.3 GHz, whereas the fraction bandwidth for 20-dB suppression level is 57.5%. The fabricated prototype has a compact size of $0.09\lambda _{g}\times 0.37\lambda _{g}$ , where $\lambda _{g}$ is the guided wavelength at 1.3 GHz. A detailed comparison between the designed MLC-based differential transmission line and the state-of-the-art designs shows a superior performance of the MLC-based transmission lines in terms of size, fabrication simplicity, and the common-mode rejection level.

A compact 3-D antenna for microwave medical diagnosis is presented. The antenna is matched with an inhomogeneous human head allowing higher signal penetration so as to obtain more meaningful data reflected from inside the brain. Utilizing a folded inverted F-like structure and a slot-loaded ground plane, the antenna is a compact $0.23\,\,\lambda \times 0.23 \,\,\lambda \times 0.04 \,\,\lambda $ in size with respect to the lowest operating frequency. The antenna is backed with a rectangular cavity to minimize sidelobe and backlobe radiation. The cavity and folded antenna configuration achieve a highly directional radiation pattern toward the head with a front-to-back ratio of more than 17 dB. A curved inhomogeneous human head phantom is fabricated to validate the antenna performance for medical diagnosis applications. An operational frequency band from 1 to 1.7 GHz is achieved with the antenna attached to the surface of the phantom. Less than 0.0147 W/kg SAR is achieved throughout the operating frequency range, which is lower than the state of the art for comparable antennas. The salient characteristics of the proposed antenna make it a suitable candidate for microwave diagnosis of the human head.

Vanadium dioxide (VO2) has been identified as a material capable of transitioning from an insulator to a metal close to room temperature. Previous works have demonstrated potential applications of this material in various microwave components or devices. However, the complex permittivity and permeability characterization of the VO2 over X-band has not been previously reported. Knowledge of these properties is essential for accurate modeling of the components and devices utilizing VO2. This paper characterizes a VO2 film deposited on a quartz substrate in a WR90 waveguide using a vector network analyzer. The S-parameters, conductivity, relative complex permittivity, and permeability of the VO2 film are determined across the transition temperature.

Biodegradable natural fibre composites are increasingly developed for lightweight structural applications in automobile and construction industries with significant environmental benefit. This work aims to explore the sound and vibration properties of natural fibre composites that inevitably suffer menace of vibrations during their operations as engineering structures. Natural fibres (bamboo, cotton, flax) and polylactic acid (PLA) fibres were blended, carded, laid up and finally needle-punched to nonwoven mats before being consolidated at elevated temperature under compression. Vibration damping and acoustic measurements were conducted on these natural fibre composites and their behaviour was compared to that of a commercial panel, made of polypropylene (PP) matrix reinforced with hemp and kenaf fibres. From both the coincidence frequency analysis and wave number amplitude analysis, the cotton/bamboo/PLA composite laminate showed the best acoustic performance (coincidence frequency of 2448 Hz) that is related to the fineness of cotton fibres, bending stiffness, natural frequency and density of the composite panel. The results suggest that the bamboo/cotton hybrid composites can be a viable candidate for engineering applications that require good bending properties, high sound absorption and vibration damping properties.

© 1963-2012 IEEE. The performance of a multifunctional composite high-impedance surface (HIS) has been evaluated using the coherent Doppler tomography (CDT) and finite-impulse response (FIR) filtering techniques. A combination of embroidery and advanced laser manufacturing processes were used to fabricate the conformable multifunctional glass fiber reenforced polymer HIS. The CDT method was utilized because it enabled the generation of a high-resolution tomographic map of the HIS reflectivity. Tomograms generated at high incidence angles (>80° from normal) were used to localize and FIR filter unwanted scattering associated with the ground plane edges and HIS transition regions. The resulting scattered fields from a defect (metallic block positioned in the center of the tomogram) were then used to gain a significantly distinctive insight into the HIS scattering properties. Furthermore, unlike traditional methods for characterizing HISs, the CDT and FIR methods presented herein are applicable to electrically large and conformal HISs.

Orbital Angular Momentum (OAM), as a property of Electromagnetic (EM) fields has recently been proposed for Radio and Microwave communications. This paper investigates a new class of OAM radiation patterns for Radio and Microwave applications, namely, Quasi-OAM radiation patterns, induced by a proposed Quasi-Circular Array Antenna (QCA). Simulations and Experiments show that Quasi-OAM waves can be induced and preserved in the far-field using the proposed QCA apertures and configurations, demonstrating non-integer dominant OAM modes corresponding to l = +1 and l = −1 with a directional quasi intensity and rotational 2π phase profiles. The proposed method in this work significantly reduces aperture size and cost by using Quasi-Circular Arrays of N Q = 5 and N Q = 6 elements in lieu of conventional OAM circular aperture arrays with N = 8 elements.

Polylactide (PLA) nanocomposites with graphene nanoplatelet (GNP) contents of 0–15 wt% were prepared and characterised in terms of rheology and electrical conductivity. As expected for rigid nanofillers, GNP incorporation significantly enhanced the viscoelastic properties of the nanocomposites. Furthermore, above a certain GNP concentration the liquid-like melt flow behaviour changed into a solid-like behaviour. However, this rheological percolation threshold was found to decrease from near 9 wt% at 180 °C to below 6 wt% at 220 °C, detected and measured by Van Gurp-Palmen plots and Winter-Chambon method, respectively. It was also found that the viscoelastic properties of some of the nanocomposites increased with increasing temperature, which is in contrast to the rheology of ideal polymeric melts. The Electrical percolation threshold of the PLA/GNP system, which is determined by a sudden increase in DC conductivity and is indicative of establishment of interconnected conductive structures of GNPs within the matrix, was observed to be between 6 and 9 wt% GNPs. Morphological studies also showed physical contact between the platelets at 9 wt% while at 6 wt% they were still separated by the PLA matrix. These observations challenge the attribution of rheological percolation threshold to the formation of a percolating network of only filler particles within the matrix. A percolating network consisting of both polymer chains and filler particles might be the reason for the temperature dependency of rheological percolation threshold in the PLA/GNP system.

The increasing global use of carbon fibre (CF) in different industries has resulted in production of significant amounts of waste in the forms of CF-reinforced composites and dry CF scrap. The latter group which is mainly in the form of short fibres is generated during manufacturing of CF-reinforced composites and comprises a significant portion of total CF waste. However, little attention has been paid to recycling these waste fibres. This research provides a comprehensive understanding on the processability of CF waste into nonwovens using conventional textile machineries of carding and needle-punching, for applications in areas of electromagnetic interference (EMI) shielding and sound absorption. The carding of CF was carried out in the presence of polyamide 6 (PA6) fibres to minimise damages to CF during the recycling process. Hybrid CF/PA6 nonwovens were produced and the effects of operating parameters such as the number of carding passages, the ratio of CF/PA6 fibres (20/80, 50/50, and 80/20), and the number of cross-lapped web layers during the needle-punching process on the obtained features were explored. The results demonstrated that excellent EMI shielding effectiveness was achieved with the highest performance being 85 dB from the CF/PA6 80/20 samples. Increasing the content of CF, the number of carding passages and the thickness of nonwovens all positively contributed to enhancing the EMI shielding performance. The sound absorption performance of nonwovens was found to depend on the CF/PA6 ratio and the thickness of the tested samples. However, the second carding process negatively impacted on the sound absorption performance by reducing the fibres mass in final products. In addition, the relations between the composition of samples and employed methodologies with production yield, areal density, electrical conductivity and actual CF content of the developed nonwovens were all discussed.

An omnidirectional circularly polarized (OCP) antenna operating at 28 GHz is reported and has been found to be a promising candidate for device-to-device (D2D) communications in the next generation (5G) wireless systems. The OCP radiation is realized by systematically integrating electric and magnetic dipole elements into a compact disc-shaped configuration (9.23 mm $^{3} =0.008~\lambda _{0}^{3}$ at 28 GHz) in such a manner that they are oriented in parallel and radiate with the proper phase difference. The entire antenna structure was printed on a single piece of dielectric substrate using standard PCB manufacturing technologies and, hence, is amenable to mass production. A prototype OCP antenna was fabricated on Rogers 5880 substrate and was tested. The measured results are in good agreement with their simulated values and confirm the reported design concepts. Good OCP radiation patterns were produced with a measured peak realized RHCP gain of 2.2 dBic. The measured OCP overlapped impedance and axial ratio bandwidth was 2.2 GHz, from 26.5 to 28.7 GHz, an 8 % fractional bandwidth, which completely covers the 27.5 to 28.35 GHz band proposed for 5G cellular systems.

A three-dimensional (3D) frequency selective surface (FSS) with a response that is independent of incident angle is presented in this study. The proposed FSS is a periodic array of 3D hollow-tapered resonators. The proposed 3D FSS has reduced sensitivity to the angle of incidence of the electromagnetic wave compared with other 2D and 3D FSSs as a result of tapered structure. The transverse electric (TE) and transverse magnetic (TM) incident angle independences are obtained by tapering the width of a cylinder with a square cross-section from the upper opening to the lower opening. Impressive frequency stability and transmission characteristics have been achieved up to 80° for both TE and TM incidence angles. The principles of operation along with guidelines for the design of the proposed FSS structures are presented in this study. A prototype of the proposed 3D-tapered FSS is also fabricated and tested using a free-space measurement system to verify performance.

This letter presents an extension of the free-space method for the dielectric characterization of Eucalypt litter fire using the measured transmission phase shift of a propagating signal over a frequency range of 5–40 GHz. This method is especially suitable for a broadband, routine, nonintrusive, and accurate evaluation of fire dielectric properties from the measured transmission coefficient ( ${S}_{21}$ ). A 72 cm $\times $ 47 cm rectangular brick burning area prepared with Eucalypt ground litter was equipped with four K-type thermocouples at different heights to measure flame temperatures, which ranged from 283–865 K. The measured phase shift of an electromagnetic signal transmitted through the Eucalypt fire varied from 12.2 to −166.7° at 5.35 and 39.74 GHz. From this phase shift, the calculated real part of the relative permittivity of the fire was between 0.877 and 1, with the imaginary part from 0.02 to 0.09. The relative permittivity of less than unity confirms the formation of plasma in the combustion zone of fire, and shows good agreement with previous research. However, in this letter, the combustion zone region is defined with significantly higher accuracy, and characteristics are evaluated over a much broader frequency range. The extracted parameters are important for understanding of the reaction of electromagnetic waves in wildfire environments and can be used to design radar systems for forest fire detection.

Elastomeric, conductive composite yarns have recently received attention around the opportunity for them to offer special protective fields. A straightforward approach for fabricating tri-component elastic-conductive composite yarns (t-ECCYs) containing stainless steel wire (SSW) was proposed previously. This work mainly focuses on the electromagnetic shielding effectiveness (EMSE) of weft-stretchable woven fabric containing t-ECCY over the X-band under different testing conditions, e.g., single step-by-step elongation, cyclic stretch and lamination events. Results showed that a woven cotton fabric with weft yarn of t-ECCY not only exhibited superior weft stretch-ability to a higher elongation (&gt, 40%) compared with a pure cotton control but also had an acceptable 15-cyclic stability with 80% shape recovery retention. The t-ECCY weft fabric was effective in shielding electromagnetic radiation, and its EMSE was also enhanced at elevated elongations during stretch at parallel polarization of EM waves. There was also no decay in EMSE before and after the t-ECCY fabric was subject to 15 stretch cycles at extension of 20%. In addition, a 90&deg, by 90&deg, cross lamination of t-ECCY fabric remarkably improved the EMSE compared to a 0&deg, /90&deg, one. The scalable fabrication strategy and excellent EMSE seen in t-ECCY-incorporated fabrics represent a significant step forward in protective fields.

Orbital Angular Momentum (OAM) is a physical property of EM fields, which allows for the superposition of an infinite number of modes to exist on the same frequency carrier. However, as the number of OAM mode transmitted increases, so does the aperture size. Previous studies have shown that a QuasiCircular Array Antenna (QCA), where the annular aperture is truncated, is capable of producing a first-order Quasi-OAM noninteger mode. This paper investigates the capability of QCA apertures, in producing higher-order Quasi-OAM modes. The experimental results in free-space demonstrated herein, show that a QCA of six elements, is able to produce a non-integer Quasi-OAM mode of I = +1. 5, falling to the nearest integer mode I = +2 in the far-field.

This paper discusses different approaches to modeling Carbon Fiber Reinforced Polymers (CFRP). Modeling CFRP at fiber level has been studied, and the feasibility of homogeneous volume and thin layer approximation has been investigated. Finally, the simulation result for a WR90 waveguide with the proposed approximation is presented along with measurements.

In this paper, an antenna operating near a human head is analysed in the ultra-wideband (UWB) frequency range. The UWB antenna operating over a frequency range from 3.1 GHz - 6.7 GHz is evaluated in terms of reflection coefficient, E-field and H-field penetration, and specific absorption rate (SAR) of the human tissue when the antenna is operating in front of a realistic human head model. The study shows that at higher UWB frequencies, the power propagation inside the human head decreases significantly.

This paper describes the design and fabrication of an active four-port microwave switching network embedded into a multi-layer aerospace composite structure. The multifunctional design is based on three broadband Integrated Device Technology (IDT) F2972 single-pole-double-throw (SP2T) RF switches and also includes a control and power supply circuit. The composite structure has been developed using a pre-impregnated structural glass material (Hexcel HexPly 914E). The conductive tracks of the switch and its ground plane have been fabricated using a non-woven carbon-nickel-copper veil material and a semi-flexible conducting silver ink. The manufacturing was also based on a hybrid manufacturing approach. The conductive veil was transferred onto the un-cured pre-preg material using a laser ablation method. Post-curing, the flexible silver ink was printed onto the surface to create the control circuits. The RF performance of the design has been evaluated and indicates the overall performance was consistent with measured S-parameters of the same circuit fabricated on Rogers 5880 substrate.

The effect of homogeneous and frequency dependent inhomogeneous human head models are investigated while characterizing an on-head matched antenna for Biomedical applications. The antenna is designed to operate at 0.9 GHz and utilizes a low cost FR4 substrate to obtain the compact proportions of 30 × 30 × 1.6 mm3 (0.09 λ 0 maximum dimension). The antenna is characterized in two scenarios: in contact with either a homogeneous or an inhomogeneous human head phantom. The antenna is optimized to operate at the same frequency at the near field of both homogeneous and inhomogeneous human head phantom. The effect of choosing a homogeneous head phantom over a realistic inhomogeneous and frequency dependent human head phantom for antenna characterization is shown in terms of electric field, magnetic field, radiation pattern at the near field and the power penetration at 50 mm inside the phantoms. It is shown that the field and power penetration characteristics of the antenna significantly vary depending on the human head phantom used, which is a crucial finding for antenna dependent medical diagnosis applications.

Graphene–polyamide-6 composite (GC) filament was 3D-printed via melt extrusion (ME). The influence of specimen thickness and internal geometric designs on electromagnetic interference shielding effectiveness (EMI SE) and dielectric properties in the X-band frequency range (8.2–12.4 GHz) was investigated. Increasing specimen thickness from 1 to 5 mm did not improve EMI SE due to impedance matching and the associated reductions in electromagnetic (EM) wave reflection. It was demonstrated that the introduction of suitable internal geometric assemblies avoided impedance matching and significantly improved EMI SE. A material model for simulating EM response of 3D-printed GC was developed and experimentally verified. It was found that different internal geometric designs each displayed unique EM responses. However, geometrical inaccuracies in printed specimens resulted in differences between experimental EM response and that predicted by simulations. These inaccuracies stem from the small size of the features relative to the printer resolution and the ME printing methodology. Therefore, the limitations of a printer when replicating complex geometries must be considered to effectively apply internal geometric designs for enhancing EMI SE of 3D-printed components.

Graphene nanoplatelets (GNPs) were dispersed in poly lactide (PLA) and poly(butylene adipate-co-terephthalate) (PBAT) via melt-mixing. Effect of GNP incorporation on electromagnetic properties and electromagnetic interference shielding effectiveness (SE) of PLA and PBAT was investigated and the two systems were systematically compared. Furthermore, applicability of Sihvola's mixing rule of complex electrical permittivity to these nanocomposites was studied. GNP addition significantly enhanced permittivity of both polymers. Dielectric constants of PLA and PBAT nanocomposites had comparable values. However, above 6 wt% GNPs, PLA nanocomposites showed significantly higher dielectric loss than PBAT nanocomposites, even though pure PLA had lower dielectric loss than pure PBAT. This was attributed to the dispersion state of GNPs in the two matrices, detected in morphological studies. SE of both polymers increased with GNP addition due to enhancement of their dielectric properties. The difference in dielectric loss of the two systems was revealed in their ability to attenuate the radiation by absorption. At 15 wt% GNPs, 1 mm-thick PLA/GNP nanocomposite had an effective absorbance of 70%. This value was only 43% for PBAT/GNP nanocomposite. Variations of polymers' permittivities with GNPs were successfully modelled by Sihvola's rule. While both systems returned close values for model's fitting parameters, it better fitted PBAT/GNP nanocomposites. Keywords: Electromagnetic interference shielding, Dielectric properties, Poly lactide, Poly (butylene adipate-co-terephthalate), Graphene nanoplatelet, Nanocomposite

The rapid development of the embroidery of conductive threads into textiles and associated biomedical sensors and on-body antennas has given rise to many healthcare and emergency response applications. In this paper, we investigate the embroidery of conductive threads directly into a grade of composites called “preimpregnated” (pre-preg) materials. These pre-preg materials differ from traditional textiles in that they contain a B-staged epoxy resin that must be baked in an autoclave at temperatures greater than 170 °C, and under pressures upward of 700 kPa to achieve their maximum strength. The experimental characterization of pre-preg-realized transmission line structures embroidered with different types of threads, stitches, and sewing techniques establishes the best fabrication approaches. Unlike embroidery of conductive threads into textiles, the high processing temperatures and pressures of composite materials result in conductivity and dispersion properties similar to those found in copper. We demonstrate how to incorporate passive and active electronic circuit elements directly into this structural material and its subsequent durability during the hardening process. An embroidered pre-preg electronic (pregtronic) ultrawideband amplifier is realized numerically and confirmed experimentally. Because pre-pregs have a relatively long shelf life when stored within a freezer, as well as the capability to be conformably molded to any surface, the study of pregtronics lends itself to the development of storable, conformal electronics for aerospace, automotive, and other structural applications.

This paper investigates the enhancement of convective heat transfer within a sub-millimetre diameter copper tube using Al2O3, Co3O4 and CuO microparticle suspensions. Experiments are conducted at different particle concentrations of 1.0, 2.0 and 5.0 wt% and at various flow rates ranging from 250 to 1000 µl/min. Both experimental measurements and numerical analyses are employed to obtain the convective heat transfer coefficient. The results indicate a significant enhancement in convective heat transfer coefficient due to the implementation of microparticle suspensions. For the case of Al2O3 microparticle suspension with 5.0 wt% concentration, a 20.3 % enhancement in convective heat transfer coefficient is obtained over deionised water. This is comparable to the case of Al2O3 nanofluid at the same concentration. Hence, there is a potential for the microparticle suspensions to be used for cooling of compact integrated systems.

Biodegradable poly lactide (PLA)/graphene nanoplatelet (GNP) nanocomposites with 0–15 wt% (0–9.1 vol%) of nanofiller were prepared. Scanning electron micrographs showed good dispersion of GNPs in PLA at low concentrations while above 6 wt%, GNPs were found to become physically in contact forming a 3D network within the matrix. X-ray diffraction showed that GNPs had a significant effect on the semi-crystalline structure of PLA which was also confirmed by differential scanning calorimetry measurements. Electromagnetic properties and electromagnetic interference shielding effectiveness of the nanocomposites were determined over C- and X-bands (5.85–12.4 GHz). It was observed that GNP embedding in PLA did not alter its magnetic permeability but enhanced its electrical permittivity and conductivity markedly leading to higher shielding effectiveness. The contributions of reflection and absorption to the overall shielding performance of the nanocomposites were determined and reflection was found to be the dominant shielding mechanism. However, the absorption potential of the samples increased by increasing GNP loading with the effective absorbance being more than 80% for a 1.5 mm-thick sample containing 15 wt% of GNPs compared to less than 0.6% for pristine PLA. Keywords: Poly lactide, Graphene nanoplatelet, Electromagnetic interference shielding, Nanocomposite

This letter presents a dumbbell-shaped defected ground resonator and its application in the design of differential filters. The operation principle of the dumbbell-shaped resonator (DSR) coupled to differential microstrip lines is studied through a circuit model analysis. The proposed circuit model is validated through the comparison with the electromagnetic simulation results. It is shown that the bandpass configuration of microstrip-line-coupled DSR can be used to design higher order bandpass filters. The design procedure is explained by developing a third-order filter prototype. The designed filter shows more than 57-dB common-mode rejection within the differential passband.

Carbon fiber reinforced polymer (CFRP) materials are anisotropic lossy conductors which restrict their use in high-performance lightweight and load-bearing waveguides. The attenuation of CFRP waveguides has been improved in this research by investigating and comparing three different manufacturing techniques. WR-90 waveguides have been manufactured from Cytec IM7/977-3 prepreg and their transmission performance was evaluated. Three different techniques were implemented to reduce waveguide losses including: copper foil lining, conductive paint, and a new integrated metalized carbon fiber veil (CFV) material. The new lightweight high conductivity CFV was integrated into the walls of a CFRP waveguide which reduced the per unit length attenuation to below 1 dB/m. This was an improvement of up to 5.5 dB over raw carbon fiber and similar to the performance of a copper foil-lined waveguide.

This paper presents a compact coplanar waveguide fed antenna for medical diagnosis of the human head. Early diagnosis of the sophisticated organs inside the human body is possible through microwave medical imaging, which has recently achieved its popularity due to its numerous advantages. The crucial functional element for microwave medical imaging is the antenna. The antenna presented in this paper is conceived and evaluated in contact with an inhomogeneous human head tissue medium, increasing the signal penetration by removing the antenna–air–tissue transition loss. A combination of a meandered line and circular slot are utilized to achieve wideband performance at a subwavelength size. A rectangular cavity is introduced at the back of the antenna to attain directional radiation towards the inhomogeneous human head phantom. The antenna achieves increased bandwidth covering the lower microwave frequency region for medical diagnosis of a human head while minimizing ill-directed radiated fields which interfere with imaging computations. The salient characteristics of compact size, high near-field directionality, lower microwave operating frequency range and low impact on the skin surface make the proposed antenna a suitable candidate for microwave imaging of the human head.

© 2018 European Microwave Association. Conformal load-bearing smart skins (CLSS) provide the unique ability to integrate complex electrical systems into aerospace composite materials. This paper provides a preliminary investigation into the patterning of Artificial Impedance Surfaces (AIS) on these materials as they are well suited to develop conformal meta-surfaces (MTS). In particular, a composite veil material has been patterned using laser ablation and has been transferred directly onto a resin pre-impregnated (pre-preg) structural glass material. The new patterning technique was found suitable for the creation of fine geometric features with a resolution of approximately mathbf{100} mumathbf{m}. The material has been characterized by a reflected magnitude and phase measurement.

A varactor-based tunable bandstop filter has been proposed in this article. The proposed filter is based on a dualmode circuit developed by introducing inductive and capacitive couplings into a notch filter. The frequency tunability is achieved by using varactor diodes instead of the lumped capacitors in the circuit. Next, the equivalent circuit model has been implemented in planar microstrip technology using thin inductive traces and varactor diodes. The fabricated filter prototype shows a continuous center frequency tuning range of 0.66 - 0.99 GHz with a compact size of 0.12lg*0:16lg, where lg is the guided wavelength at the middle frequency of the tuning range., 3 pages

An analysis of the effect moisture has on the effective relative complex permittivity of ash particles created within fires has been presented within this paper. The inclusion of moisture within this hydroscopic material has a significant effect on the effective complex permittivity and, hence, its scattering ability. The investigations developed a mixing law model for describing moisture ingress in a 1000 $\ ^{\circ}{\hbox{C}}$ burnt eucalypt ash sample. Using the measured trend found in the eucalypt samples, the effective complex permittivity with respect to the inclusion of moisture has been modeled with an empirical modification to the tri-phase unified mixing law.

The Defence Science and Technology Group (Aerospace Composite Technologies) are in the process of developing a portable tool for evaluating the quality of surface features at microwave frequencies. This tool generates a Doppler tomographic image of the scene by recording the scattered electromagnetic field over a circular aperture. Previous versions of this tool were limited to planar surfaces. Scattering features that fell above or below the image plane were poorly resolved as a direct result of the imaging algorithm. This paper documents the development of a technique for surface curvature correction in the tomographic imaging algorithm. The final technique has since been implemented in a field-deployable prototype.

This article presents a bandpass frequency selective surface (FSS) with a narrowband frequency response. The designed FSS is made of miniaturized elements unit cell. The operation principle of the FSS is explained by using an equivalent circuit model, where the passband bandwidth can be controlled by the values of the circuit elements corresponding to the geometrical dimensions of the unit cell. The compatibility of the presented structure in designing higher order narrowband filtering responses is verified by designing a second-order bandpass FSS with 8:5% fractional bandwidth with a center frequency of 2:7 GHz.

© 2017 European Microwave Association. This paper has investigated the implementation of a conformal amplifier embedded into a high performance aerospace composite quartz fiber re-enforced polymer (QFRP) matrix. A cyanate ester resin system provided by Lonza (PT-30) was used to encapsulate the quartz fibers (Astroquartz II) and the passive/active elements. This enabled a streamlined aerodynamic functional skin to be formed over the leading edge of an airfoil. Apart from the active amplifier component, a passive bias-tee and power LED were also integrated into the skin. The conformal amplifier performance was measured and found to have low-loss and acceptable impedance matching between 0.01 GHz and 1.0 GHz. The amplifier gain is above 10 dB up to 2.0 GHz. The successful manufacturing process for this conformal amplifier has demonstrated the ability to integrate novel microwave circuits into composite materials on a highly conformal surface.

© 2017 IEEE. Several passive and active meta-structures have been successfully integrated into load-bearing high performance aerospace structural composite materials. These include an electrically small, metamaterial-inspired Egyptian Axe Dipole (EAD) antenna; a high impedance ground plane (HIGP) to mitigate any cross talk between adjacent antennas; and passive and active circuits including wide bandwidth and conformal amplifiers, bias-Tees, and powered LEDs. Several different manufacturing techniques have been tested and the outcome is a well-defined manufacturing process. These structures enable streamlined aerodynamic functional smart skins.