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In this work we investigate dynamic behavior of the LC director including the defect nucleation and the LC director field on the shape of the electrode using the Q-tensor method in the Super IPS cell. In general, the Q-tensor method can model the defect dynamics in a liquid crystal director field in addition to electrical behavior of the LC director. [ABSTRACT FROM AUTHOR]

We model the nucleation and dynamical behavior of defects from an inhomogeneous surface configuration using fast Q-tensor method. A fast Q-tensor method, which can model the defect dynamics in a liquid crystal director field, is applied to model the defect nucleation and dynamics. From the numerical modeling, we confirmed that surface inhomogeneity which makes strong strain energy in the local liquid crystal director field could cause defects. Experimental result has compared with numerical modeling in order to verify the simulation of the defect nucleation. [ABSTRACT FROM AUTHOR]

The electrochemical insertion–deinsertion of lithium into CuO electrode was examined by Cu K-edge X-ray absorption near edge structure (XANES) during the first electrochemical cycle. The XANES spectra in the Li[sub x]CuO (x: lithium content) system reveal that the initial insertion of lithium leads to the reduction of the Cu[sup 2+] in the pristine CuO to form the reduced nanosized Cu metal. In the successive deinsertion of lithium, the reduced Cu particles changed partially to the phase of Cu[sub 2]O. [ABSTRACT FROM AUTHOR]

A spectral inverse technique, applying the moment-method procedure with a series expansion for the induced field in each enlarged cell, is suggested to reconstruct permittivity profiles of inhomogeneous dielectric objects. In particular, averaging the reconstructed permittivity distribution over each cell with a weighting function becomes an effective tool to regularize the error arising from a Gaussian random noise of the scattered field. [ABSTRACT FROM AUTHOR]

Viewing-angle controllable liquid crystal display (LCD) without using an additional panel or pixel division is proposed. In the device, hybrid aligned nematic liquid crystal using a liquid crystal with negative dielectric anisotropy is used. The device shows narrow viewing angle when it is driven only by fringe-electric field, however, it exhibits wide viewing angle if driven by vertical- as well as fringe-electric field. With the approach, the viewing angle of the LCD can be controlled from 120° to 20° in horizontal direction. [ABSTRACT FROM AUTHOR]

For the size reduction and sharp attenuation of the low-pass filter in lower RF frequency range, a semilumped elliptic-function low-pass filter was proposed using microstrip meander line, chip capacitor, and radial stubs. To estimate more accurate inductance of microstrip meander line, it was modeled in an inhomogeneous dielectric environment, and then the calculated inductance was compared with the conventional case. To provide an additional attenuation pole in stop-band, radial stubs were added to the proposed structure. The 3-order low-pass filter has a cutoff frequency of 1GHz and dimension of 14.6 × 16.3 mm2 on the substrate with a relative dielectric constant of 3.38. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 1777–1779, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22545 [ABSTRACT FROM AUTHOR]

Compact semilumped bandstop resonators are simply designed using parallel-coupled microstrip line and lumped capacitors. The lumped capacitors loaded in between coupled lines increase the odd mode electrical length, and the frequency of transmission zero can be controlled by the lumped capacitors. For the accurate estimation of the transmission zero frequency, the four-port impedance parameter is derived using the even/odd mode analysis in an inhomogeneous dielectric medium. Two types of the bandstop resonators are proposed using the coupled line with the length of λg/8 (@2 GHz), and the calculated and measured results of transmission zero frequency are compared in 1–3 GHz ranges. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 367–369, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22135 [ABSTRACT FROM AUTHOR]

The concentration distribution and electronic structure of N atoms doped in multiwalled banboo-like carbon nanotubes (CNTs) are examined by photon energy-dependent x-ray photoelectron spectroscopy and x-ray absorption near edge structure. The inner part of the nanotube wall has a higher N concentration and contains molecular N2 presumably intercalated between the graphite layers. These results are supported by the self-consistent charge-density-functional-based tight-binding calculation of double-walled CNTs, showing that the intercalation of N2 is energetically possible and the graphite-like N structure conformer becomes more stable when the inner wall is more heavily doped. [ABSTRACT FROM AUTHOR]

High-density gallium (Ga)-doped indium oxide (In2O3) nanowires whose Ga content [Ga/(In+Ga) atomic ratio] is 0%, 7%, and 45%, were synthesized by thermal evaporation. They have an average diameter of 50 nm and consist of nearly single-crystalline cubic In2O3 structure with the [010] growth direction. High-resolution x-ray diffraction analysis reveals that as the Ga content increases the position of In2O3 peaks shifts to the higher angle and a crystalline form (Ga,In)2O3 is produced. We estimated a 0.4% reduction of the lattice constant for 45% Ga-doped nanowires. [ABSTRACT FROM AUTHOR]

Abstract This paper introduces a novel design featuring a thirty-two port diversity antenna with an elliptical shape, fed by an asymmetric coplanar waveguide (CPW). The antenna incorporates uneven meander lines, tailored for super-wideband (SWB) applications. The structure of the unit cell is of an elliptical patch with an elliptical slot, and it is connected to a rectangular stub and asymmetric meander line. The size of the single element is 22 mm × 20 mm, and it operates from 3 to 40 GHz. The bandwidth dimension ratio of the unit cell is 3911, bandwidth ratio is 13.33:1, and fractional bandwidth is 172.09%. The single element is developed into a 3-D thirty-two port diverse antenna, composed of a horizontal plane and four planes perpendicular to it. The diverse antenna has a peak gain of 12.5 dBi and an efficiency of 94%. The computed envelope correlation coefficient (ECC), as determined using S-parameters and far-field measurements, is less than 0.1. The attained diversity gains (DGs) of the developed MIMO antenna are above 9.9 dB in terms of both S-parameter and far-field computations. The obtained thirty-two port diverse antenna channel capacity loss (CCL) is below 0.25 bits/s/Hz. The mean effective gain (MEG) of the constructed thirty-two port antenna is below 2. To validate the appropriateness of the developed thirty-two port antenna for wireless indoor environment, an enclosure made of acrylonitrile butadiene styrene (ABS) is crafted and fabricated, and the characteristics of the proposed diverse antenna are investigated.

Abstract In this paper, a compact low-profile dual-band wearable textile antenna is proposed for on-body and off-body communications. The presented antenna works efficiently in the 5G n79 frequency band (4.4 − 5 GHz) and the ISM band (5.725 − 5.875 GHz). The designed antenna has an ultra-wide impedance bandwidth of 2.01 GHz and peak realized gains of 10.5 dBi and 12 dBi at 4.5 GHz and 5.8 GHz, respectively. The antenna has a small footprint (π × 0.3λ 0 2), which is inspired by circular fractal geometry. The performance of the presented wearable antenna is evaluated at various body parts, including the arm, wrist, and chest. The link margin is evaluated in the on-body and off-body communication scenarios, i.e., communication with the implantable antenna and the outside-body antenna, which is 80 dB and 65 dB at 4.5 GHz and 5.8 GHz, respectively. The 1 gm/10 gm specific absorption rate values at 4.5 GHz and 5.8 GHz are 0.12/0.098 and 0.11/0.082, respectively, which are significantly lower than the standard values, making the proposed antenna suitable for modern wearable applications.

Abstract This paper proposes a circularly polarized ultra-thin flexible antenna with a flexible rectifier and power management unit (PMU) for smartwatch/wristband applications. The flexible antenna is compact (0.17λ 0 × 0.20λ 0 × 0.0004λ 0) and has a stepped ground plane. A parasitic element is used at the substrate bottom to reduce the specific absorption rate (SAR) and enhance the gain up to 3.2 dBi, at the resonating frequency of WLAN/Wi-Fi (2.45 GHz). The SAR of the proposed design is also analysed at the resonating frequency, and it satisfies the guidelines of the International Commission on Non-Ionizing Radiation Protection (ICNIRP) and IEEE C95.1–2019 human safety standards. An impedance matching circuit is used between the antenna and the RF energy harvester to improve conversion efficiency. Polarization mismatch is avoided with the help of circular polarization, achieved by tuning stubs of size 0.02λ 0 × 0.044λ 0. The integration of the antenna and rectenna results in a good conversion efficiency of 78.2% at − 5 dBm of input power with a load resistance of 2 KΩ. The availability of RF signals allows the user to charge the smartwatch/wristband by connecting the PMU circuit with the RF energy harvester.

A design of an ultra-wideband transition from a stripline (SL) to a conductor-backed coplanar waveguide (CBCPW) is presented. The proposed transition provides smooth field transformation between adjacent transmission lines. The transition in a back-to-back configuration is measured to have <1.5 dB insertion loss and >10 dB return loss up to 25 GHz. The proposed transition can be easily integrated with SL-based circuits for various applications. [ABSTRACT FROM AUTHOR]

This paper proposes a novel pixel circuit and driving scheme that adopts low-temperature polycrystalline silicon and oxide thin-film transistors (LTPO TFTs) for mobile devices using active-matrix organic light-emitting diode (AMOLED) displays. The proposed pixel circuit and driving scheme provide uniform luminance and render flicker invisible at variable refresh rates (VRRs) from 1 to 120 Hz. Using the proposed pixel circuit with extended compensation time (tCOMP) improves the luminance uniformity at a high-frame rate. The proposed driving scheme applies a voltage to the driving TFTs (D-TFTs) higher than the programmed data voltage during the skip frame. This reduces the flicker caused by the hysteresis of D-TFTs during low-frame rate driving. A 6.0-inch quad high-definition (QHD) LTPO-based AMOLED display was fabricated using the new pixel circuit and driving scheme. Experimental results of the proposed pixel circuit show that the standard deviation of luminance was reduced from 0.056 to 0.008 by extending tCOMP from 2 to 8 µs. The flicker level was −51 dB, so there was no visual artifact during 1 Hz driving. A flicker-free LTPO-based AMOLED display with low power consumption is possible; driving can proceed in 1–120 Hz range.

Triplet–triplet fusion (TTF) has been an efficiency-enhancing mechanism in fluorescent organic light-emitting diodes (OLEDs) caused by the collision of two triplet excitons. However, achieving a high TTF ratio in fluorescent OLEDs has been difficult despite device strategies to maximize the triplet exciton density within a narrow recombination zone near the electron blocking layer (EBL) due to charge imbalance and hole accumulation between the TTF type emitter and EBL. Based on a trap-detrap mechanism, we were able to realize an improved TTF ratio and reduce hole accumulation by adding a TTF-assisting material (TTF-AM) in the TTF emitter. The TTF-AM served as the hole transport channel, triggering hole trap and detrap while improving the hole transport character of the emitting layer. Through the process of hole detrapping, the improved hole transport properties balanced carriers and generated more triplet excitons in order to activate the TTF mechanism from low to high current density ranges. By adjusting the TTF-AM, the TTF ratio of the anthracene-based emitter was increased from 5.5/20.1% to 13.4/25.5% (low/high current density), thereby resulting in more than doubled external quantum efficiency at low current density.

A generalized design technique of ultra-wideband planar transitions using EM-based analytical models is presented in this paper. Among various planar transmission lines, each transmission line has unique advantages over other line types, and a microwave component in a system can be designed to perform much better if it is implemented on a certain type of transmission line. Therefore, low-loss and high-performance transitions between transmission lines, with easy integrability with the main circuit board, are needed. The proposed transition design technique uses the Schwarz-Christoffel transformation, which is one of conformal mapping methods, and can be applied to design any planar transition between a pair combination of planar transmission lines based on TEM or quasi-TEM waves. To optimally match the characteristic line impedance and smoothly transform the electromagnetic field distribution between the planar transmission lines, entire cross-sections through the planar transition should be analyzed with proper models. In this paper, the cross-sections of planar transitions are categorized into 4 cross-sectional models, where each cross-section is divided into multiple regions for the analysis to obtain line capacitance. Therefore, for the 4 cross-sectional models, 7 types of basis structures are identified to obtain their capacitances by applying conformal mapping. By adding capacitances of a combination of the 7 analysis types, the total line capacitance, thus the characteristic line impedance, of any cross-sectional model of the planar transition can be obtained. The characteristic line impedance of each cross-sectional model calculated with the proposed analytical formulas is compared with the 3D EM calculations, and it is found that the deviated values are mostly well under 6%. This proposed technique enables to design various planar transitions efficiently and quickly for the maximum performance without parameter tuning trials, by providing optimal impedance matching and smooth field transformation up to mm-wave frequencies.

In this paper, a compact dual circularly-polarized (CP) planar multiple-input-multiple-output (MIMO) antenna is presented for a sub-6 GHz frequency band. The antenna consists of four identical resonating elements, which are placed in a mirrored-image pattern to obtain polarization diversity. Element 2 is a mirror image of element 1, and elements 3 and 4 are mirror images of elements 1 and 2. Each antenna element comprises an elliptical resonator, a 50-Ω microstrip feed line, and a rectangular stub integrated with the feed to increase the surface current path of the antenna, shifting the resonating frequency to the lower side. Additionally, the rectangular stub is lengthened towards the right side (along the +x-axis direction in the antenna element 1), which balances the magnitude and 90° phase variance among the horizontal (Ex) and vertical (Ey) fields. The proposed MIMO antenna supports both types of circular polarization, where radiators 1 and 3 radiate right-hand CP (RHCP) rays and radiators 2 and 4 radiate left-hand CP (LHCP) rays. Developing a compact-size MIMO antenna is a challenging task, especially when the antenna elements share the same ground plane and are placed less than half a wavelength apart. The mutual coupling in the proposed antenna is reduced by increasing the spacing between the elements without the use of any extra decoupling structure. Optimal spacing is maintained to achieve compact geometry with less inter-element correlation. The radiators are closely placed with an edge-to-edge spacing of 0.08λ0, where λ0 is the free space wavelength at 3.6 GHz. A peak gain of 5 dBi, efficiency of 90%, an envelope correlation coefficient (ECC) of less than 0.1, and isolation of more than 18 dB are obtained between different ports of the prototype antenna. The overall size of the antenna element is 17 mm × 17 mm × 1.6 mm, and the MIMO antenna is 40 mm × 40 mm × 1.6 mm.

In this work, two configurations (Config.-A and Config.-B) of planar four-port dual circularly-polarized (CP) multiple-input-multiple-output (MIMO) antenna are proposed for sub-6 GHz (3.4-3.8 GHz) applications. The MIMO antennas are designed on the FR-4 substrate with size of $60\times 60\times 1.6$ mm3. Each radiating element (unit cell) of the MIMO antenna is comprised of a microstrip feed line and an open slot ground plane integrated with two rectangular arms for realizing circular polarization. An I-shaped metallic strip is used in the proposed four-port MIMO Config.-A and Config.-B antennas for obtaining common voltage level in the ground plane. Both MIMO antenna configurations exhibit good impedance matching in the band of interest. In the MIMO Config.-A antenna, all ports provide the same CP band, while in the Config.-B antenna the ports-1/-2 and -3/-4 radiate at different CP bands. The proposed MIMO Config.-A and Config.-B antennas support both types of polarization (left-hand circular polarization (LHCP) and right-hand circular polarization (RHCP)). With the Config.-B antenna, the maximum 3-dB axial-ratio (AR) beamwidths of 118.9° and 92.7° are obtained at 3.536 GHz in the xz- and yz-planes, respectively. Furthermore, the I-shaped strip helps in improving 3-dB AR beamwidth in the MIMO Config.-B antenna. Inter-element isolation greater than 19 dB and envelope correlation coefficient (ECC) less than 0.12 (far-field) are achieved between the ports of the fabricated MIMO antenna prototype.

A conventional differential line (DL), commonly used on typical digital circuit boards for transmitting high-speed digital data, has fundamental limitations on the maximum signal bandwidth (~10 GHz), mainly due to signal skew, multiple line coupling, and EM interference. Therefore, to support super-high-speed digital data transmission, especially for beyond 5G communications, a practical high-performance transmission structure for digital signals is required. Balanced lines (BLs) can transmit the differential signals with multiple advantages of ultra-wide bandwidth, common-mode rejection, reduced crosstalk, phase recovery, and skew reduction, which enable super-high-speed transmission. In order to utilize the BLs in the DL-based digital circuit, connecting structures between a DL and BLs are required, but the DL-to-BL transition structures dominate the operating bandwidth and signal properties. Therefore, in this paper, properties, and design methods for two ultra-wideband DL-to-BL transitions, i.e., DL-to-CPS (coplanar stripline) and DL-to-PSL (parallel stripline) transitions, are presented. Both implemented DL-to-CPS and DL-to-PSL transitions provide high-quality performance up to 40 GHz or higher, significantly enhancing the frequency bandwidth for the transmission of digital signals while providing compatibility with the DL-based PCBs. The fabricated DL-to-CPS transition performs well from DC to 40 GHz with an insertion loss of less than 0.86 dB and a return loss of more than 10 dB, and the fabricated DL-to-PSL transition also provides good performance from DC to 40 GHz, with an insertion loss of less than 1.34 dB and a return loss of more than 10 dB. Therefore, the proposed DL-to-BL transitions can be applied to achieve super-high-speed digital data transmission with over 40 GHz bandwidth, which is more than four times the bandwidth of the DL, supporting over 200 Gbps of digital data transmission on PCBs for the next generation of advanced communications.

A low radar cross-section (RCS) wideband circularly-polarized (CP) dipole antenna using a frequency selective surface (FSS) is presented in this paper. A planar dipole antenna consists of a pair of slotted patches fabricated on top and bottom sides of the substrate is designed. The slotted patches are connected through a circular ring, which acts as a delay line to achieve circular polarization. In the proposed configuration, a band-stop FSS is employed for the antenna reflector, which improves the radiation performance and reduces the RCS of the dipole antenna. The impedance bandwidth (S11 ≤-10 dB) of the proposed antenna is 0.95 to 3.1 GHz and 3-dB axial ratio bandwidth (ARBW) is 1.9 to 2.35 GHz. The proposed antenna shows the maximum and average RCS reduction of 20 dB and 6.9 dB, respectively, in the frequency range of 3.7 to 18 GHz. The simulated and measured results show that the maximum gain of the antenna increases by 5.9 dB by using FSS.

A design of a modular antenna/detector array for the electron cyclotron emission (ECE) imaging system at the Korea Superconducting Tokamak Advanced Research (KSTAR) is proposed. The modular antenna/detector array is based on a unit antenna/detector module, which consists of an elliptical mini-lens, a dual-dipole antenna, an antenna balun, a low-noise amplifier, and a metal frame. The proposed modular antenna/detector array resolves the problem in the conventional antenna/detector array where one faulty channel requires the entire array to be removed for the service. With the proposed modular array, each channel module can be easily and independently removed and replaced without interference to the rest of the array, thus minimizing the interrupted service time for maintenance. Moreover, the unit channel modules can be efficiently updated under a variety of the tokamak operation conditions. The antenna/detector modules are optimized to have improved performance, and are tested in a W-band test setup, and consistently provide the gain increase by 10~20 dB as compared with the conventional antenna/detector array. A set of the proposed modular antenna/detector array is currently installed and tested in the KSTAR ECE imaging system, and will consistently produce the improved ECE imaging to monitor MHD instability activities under various plasma operation conditions.

A broadband compact-sized planar four-port multiple-input–multiple-output (MIMO) antenna with polarization diversity is presented. The proposed dual circularly polarized (CP) MIMO antenna consists of four G-shaped monopole elements, two of which are left-hand CP and the other two are right-hand CP. A vertical line strip in the G-shaped radiating element acts in balancing the vertical and horizontal electric field components to obtain 90° phase difference between them for circular polarization. Also, an I-shaped strip is incorporated between the ground planes of the G-shaped antenna elements to obtain equal voltage level in the proposed MIMO configuration. The dual circular polarization mechanism of the proposed MIMO/diversity antenna is analysed from the vector current distributions. The impedance bandwidth (S11 ≤ –10 dB) of the MIMO antenna is 105.9% (4–13 GHz) and the 3 dB axial ratio bandwidth (ARBW) is 67.7% (4.2–8.5 GHz), which is suitable for C-band applications. The overall size of the MIMO antenna is 70 × 68 × 1.6 mm3, and the minimum isolation between the resonating elements is 18 dB. The envelope correlation coefficient is less than 0.25, and the peak gain within the resonating band is 6.4 dBi.

A design method of narrow bandpass filters (NBPFs) of 4–6% bandwidth with ultrawideband suppression of harmonic passbands, utilizing two cascaded step impedance resonators (SIRs) in a suspended stripline, is proposed in this paper. The proposed design utilized the characteristics of a suspended stripline, which provides a much higher characteristic impedance ratio as compared with that of the microstripline, enabling ultrawideband harmonic suppression. As an example of the NBPF, a filter with a passband center frequency f0 of 0.75 GHz and bandwidth of 5% was implemented and proved to suppress the harmonic passbands up to 13.5 f0. Since the proposed filter was implemented on the suspended stripline, the passband insertion loss was only −0.9 dB, which is low as compared with other previous designs. The proposed filter is a compact high-performance low-loss NBPF, which can be applicable to various wireless systems.

Two designs of frequency triplers, which use planar transitions as baluns and an antiparallel diode pair to achieve wide bandwidth, are presented. The ultrawideband transitions are utilized for input and output impedance matching of the frequency triplers. The design process and operation principles are described in this paper. The implemented frequency triplers exhibit flat response over wide frequency range of 3 to 9 GHz. The proposed Type A frequency tripler shows conversion loss of ~18 dB with above 30 dB even harmonics suppression. With Type B frequency tripler, conversion loss of ~17 dB and above 25 dB even harmonic suppression are achieved.