High-directivity and compact microstrip coupler for RF power applications.

A microstrip coupler was designed, fabricated, and tested to monitor the power of the 500 MHz kW-level amplifier modules currently installed in an 80-kW solid-state transmitter. The proposed coupler consists of multi-section coupled lines and employs a reverse-coupling scheme, achieving a very compact structure. The dual-side configuration with good termination enables the coupler to have high directivity and wide bandwidth without using any lump components for better temperature stability. Under a low-power test, the measured coupling is 37.69 dB with high directivity from 82 to 700 MHz. The coupling is almost independent of the power at 499.65 MHz, while the directivity slightly decreases to 33.85 dB at the input power of 1 kW. The maximal insertion loss of 0.08 dB was observed at 1-kW input power. The proposed directional coupler suits many high-power applications, especially in low-frequency regimes.

Percolation Effect on the Complex Permittivities of Polymer Blends.

This study focuses on the measurement and analysis of the complex permittivities of polymer blends using the field enhancement method (FEM). The blends, consisting of air-powder or solvent-solute mixtures, are placed in a Teflon holder and inserted into the FEM cavity to determine the complex permittivity. The resonant frequency and quality factor of the FEM cavity coupled with the samples provide information on the blends' dielectric constant and loss tangents. To extract the complex permittivities of three specific samples of DC-840, MCL-805, and MCL-Siloxane, we employ effective medium theories and the high-frequency structure simulator (HFSS) together with the measured data. The results reveal that when the volume fraction of the DC-840 solute in the xylene solvent surpasses a specific threshold, the dielectric constants and the loss tangents experience a notable increase. This phenomenon, known as percolation, strongly correlates with the viscosity of polymer blends. The observed percolation effect on the dielectric behavior is further elucidated using the generalized dielectric constant and the Debye model. By employing these models, the percolation effect and its impact on the dielectric properties of the blends can be explained.

Mechanism and tuning sensitivity of symmetry-protected resonances in high-contrast gratings.

We develop a theory of refractive index tuning for symmetry-protected optical bound states (SP-BICs) in high-contrast gratings (HCGs). A compact analytical formula for tuning sensitivity is derived and verified numerically. We also discover a new type of SP-BIC in HCGs that has an accidental nature with a spectral singularity, which is explained in terms of hybridization and strong coupling among the odd- and even-symmetric waveguide-array modes. Our work elucidates the physics of tuning SP-BICs in HCGs and significantly simplifies their design and optimization for dynamic applications in light modulation, tunable filtering, and sensing.

Characterizing the dielectric properties of carbon fiber at different processing stages.

The polyacrylonitrile (PAN) fibers go through a series of chemical reactions in various processing temperatures/stages and finally turn into the so-called carbon fibers. Oxidization is the first stage, and it takes the largest proportion of the entire processing time for the tremendous change from a chain texture to a ladder texture. The pre-carbonization of carbon fibers is then achieved using a furnace with a higher processing temperature (typically at 700-900 °C). During the reaction processes, the color of the fibers changes from white (PAN) to light black (oxidation), and eventually to black (pre-carbonization). Characterizing the complex permittivity helps us determine the carbonization status of the fibers. This work employed the enhanced-field method (EFM) and the contour mapping method to determine the fibers' dielectric properties for the first time. Results show that both the real and imaginary parts of permittivity increase as the processing temperature rises. The dielectric constants change from 2.82 (PAN) to 6.50 (pre-carbonization), and the loss tangents increase from 0.007 (lossless) to 0.089 (lossy). This study provides a simple and effective method for characterizing carbon fibers' processing status and can be applied to the measurement of other fibrous materials.

Measuring the Complex Permittivities of Plastics in Irregular Shapes.

This work presents the measurement of the complex permittivities of high density polyethylene (HDPE), linear low density polyethylene (LLDPE), low density polyethylene (LDPE), polypropylene (PP), Nylon, and thermoplastic vulcanizates (TPV) in irregular shapes at the microwave frequency. A Teflon sample holder was employed to pack irregularly shaped plastic materials with various volumetric percentages. The samples were put into a resonant cavity with an enhanced electric field in its center, which is known as the enhanced-field method (EFM). The resonant frequencies and the quality factors at different volumetric percentages were measured by a network analyzer and compared with simulated results using a full-wave simulator (high-frequency structure simulator (HFSS)). Three simulation models, layer, ring, and hybrid, are proposed and compared with the experimental results. It is found that the hybrid model (denoted as Z5R5) with five heights and five radii in the partition is the most suitable. The complex permittivities of six plastic materials were evaluated by the contour maps of the HFSS simulation using the hybrid model. The measured complex permittivities of the irregularly shaped polymers agree well with their counterparts in bulk form.

W-band circular TM11 mode converter for gyrotrons.

This work proposes a methodology to convert a rectangular TE10 mode to a circular TM11 mode using an H-plane power divider at W-band. The divider evenly splits the input signal into two parts with the same amplitude and phase. One of the waves then goes through a wider rectangular waveguide with a lower cutoff frequency. After propagating through a specific length, the two waves differ by 180°. The two out-of-phase waves can jointly synthesize the circular TM11 mode with high mode purity. This power divider is structurally simple and capable of high-power operation. The full-wave simulation shows that the metal's conductivity affects the transmission of two-mode converters joined back-to-back. The measured back-to-back transmission agrees with the simulation result except for minor quantitative differences. The measured 3-dB bandwidth is 2.8 GHz with a center frequency of 93.6 GHz, which warrants the success of the TM11 mode gyrotrons.

A Two-Step Microwave Annealing Process for PAN Pre-Oxidation through a TM-Mode Cavity.

A novel microwave annealing system and a specific processing condition are proposed for the pre-oxidation of carbon fiber. The microwave annealing system consists of a TM-mode resonant cavity and a silicon carbide (SiC) susceptor. The TM-mode cavity enhances the electric field at the center. The SiC susceptor absorbs part of the microwave energy and converts it to heat. The enhanced fields and the SiC susceptor provide both nonthermal and thermal treatments for fibrous materials with various dielectric properties. Furthermore, a two-step microwave annealing process is used to oxidize polyacrylonitrile (PAN) fiber. The scanning electron microscopy (SEM) images, differential scanning calorimetry (DSC), and X-ray diffraction (XRD) results support the theory that the microwave annealing can achieve a high aromatic index of 66.39% in just 13 min, 9 times faster than the traditional processing time. The results of the Raman spectra also illustrate that the sheath-core factor of the microwave-heated specimen is closer to one than that of the conventional furnace-heated type, which agree with the images of the cross-section area.

Dielectric Properties of BaTiO3-Epoxy Nanocomposites in the Microwave Regime.

We synthesized BaTiO3-epoxy nanocomposites (particle size < 100 nm) with volume fractions up to 25 vol. %, whose high-frequency complex permittivity was characterized from 8.2 to 12.5 GHz. The maximum dielectric constant approaches 9.499 with an acceptable loss tangent of 0.113. The dielectric loss gradually saturates when the particle concentration is higher than 15 vol. %. This special feature is an important key to realizing high-k and low-loss nanocomposites. By comparing the theoretical predictions and the experimental data, four applicable effective-medium models are suggested. The retrieved dielectric constant (loss tangent) of 100-nm BaTiO3 nanopowder is in the range of 50-90 (0.1-0.15) at 8.2-12.5 GHz, exhibiting weak frequency dispersion. Two multilayer microwave devices-total reflection and antireflection coatings-are designed based on the fabricated nanocomposites. Both devices show good performance and allow broadband operation.

Frequency-induced negative magnetic susceptibility in epoxy/magnetite nanocomposites.

The epoxy/magnetite nanocomposites express superparamagnetism under a static or low-frequency electromagnetic field. At the microwave frequency, said the X-band, the nanocomposites reveal an unexpected diamagnetism. To explain the intriguing phenomenon, we revisit the Debye relaxation law with the memory effect. The magnetization vector of the magnetite is unable to synchronize with the rapidly changing magnetic field, and it contributes to diamagnetism, a negative magnetic susceptibility for nanoparticles. The model just developed and the fitting result can not only be used to explain the experimental data in the X-band but also can be used to estimate the transition frequency between paramagnetism and diamagnetism.

Nonlinear and self-consistent single-mode formulation for TM-mode gyrotrons.

This work develops a nonlinear and self-consistent framework for the single-mode simulation of TM-mode gyrotrons. Unlike TE modes, a nonlinear TM wave equation is derived by considering the additional axial modulation on the electron beam due to the interaction with the axial electric field. Together with the electrons' equations of motion, particle tracing simulation is conducted to model TM-mode oscillation. For a uniform structure, the electron-beam efficiency of the TM_{11}-mode gyrotron at the W band can achieve 30% over broad parameter space. Its beam-current, beam-voltage, and pitch-factor tuning properties are investigated under different magnetic fields. By optimizing the interaction structure of the proposed gyrotron backward-wave oscillator (gyro-BWO), the maximum interaction efficiency is higher than 30% with a frequency tuning range of more than 6 GHz at the pitch factor of 1.5. The peak efficiency can remain high of 32% at low beam voltage (10 kV) and low magnetic field (32.8 kG), indicating additional operating conditions. These special features may facilitate the development of low-cost and compact gyrotron systems and show great potential in the applications for TM-mode gyro-BWOs.

Fast, Nondestructive, and Broadband Dielectric Characterization for Polymer Sheets.

We propose a compact nearfield scheme for fast and broadband dielectric characterization in the microwave region. An open-type circular probe operated in the high-purity TE01 mode was developed, showing a strongly confined fringing field at the open end. This fringing field directly probed the freestanding sheet sample, and the overall reflection was measured. Without sample-loading processes, both of the system assembling time and the risk of sample damage can be significantly reduced. In addition, the nearfield measurement substantially simplifies the calibration and the retrieval theory, facilitating the development of easy-to-integrate and easy-to-calibrate dielectric characterization technique. The dielectric properties of more than ten polymers were characterized from 30 GHz to 40 GHz. We believe that this work fulfills the requirement of the fast diagnostic in the industrial manufactures and also provides valuable high-frequency dielectric information for the designs of 5G devices.

Characterization of the lossy dielectric materials using contour mapping.

The imaginary part of the complex permittivity of a lossy dielectric material is large and couples with its real part. The resonant frequency of a cavity with the sample depends not only on the real part of the complex permittivity of the sample but also the imaginary part, resulting in serious ambiguity in determining the sample's complex permittivity. This work proposes a contour mapping method to determine the complex permittivity. The full-wave simulation gives us the contours of the resonant frequency and the quality factor, which are functions of the relative dielectric constant and the loss tangent. By mapping the measured resonant frequency and the measured quality factor, one can uniquely determine the complex permittivity of the sample. Five liquids were examined, including three low-loss materials for benchmarking and two lossy materials. The measured complex permittivities of the three low-loss materials agree very well with the other methods. As for the lossy materials, the measured relative dielectric constant and the loss tangent of alcohol are 6.786 and 0.895, respectively. Besides, the measured dielectric constant of glycerin is 6.811, and its loss tangent is 0.562. The proposed contour mapping technique can be employed to measure the complex permittivity of liquids and solids from lossless to lossy materials.

Crystallographic, vibrational modes and optical properties data of α-DIPAB crystal.

The Crystallographic data of the α-DIPAB sample was measured using powder X-ray diffraction (PXRD). The crystal structure was also optimized using density functional based method. The calculations were performed both including van der Waals (vdW) interactions and excluding them to quantify the effects of vdW interaction on the crystal formation. The vibrational modes of DIPAB crystal corresponding to the Bromine deficient samples are calculated and presented. These show the origin of drastic change in dielectric response in Br deficient samples as compared to the ideal stoichiometric DIPAB crystal (Alsaad et al. 2018) [4]. Optical properties of an idealα-DIPAB were calculated using GGA and HSE06 hybrid functional methods implemented in VASP package. We mainly calculated the real and imaginary parts of the frequency-dependent linear dielectric function, as well as the related quantities such as the absorption, reflectivity, energy-loss function, and refractive index of α-DIPAB in the energy window of (0-12) eV.

Bandwidth broadening for stripline circulator.

This work provides a detailed analysis and simulation to demonstrate how to broaden the operating bandwidth of a circulator. A double-Y junction circulator is designed, and the shape of the central stripline is optimized with the knowledge of a modified equation. The equation predicts two resonant conditions. The overlapping of the two resonant conditions jointly constitutes the broad bandwidth. The bias magnetic field is simulated and then used in full electromagnetic-wave simulation. The designed circulator was fabricated in the S-band for communication purpose. The measured results agree very well with simulation. The overall operation range is from 1643 to 2027 MHz with the insertion loss less than 0.35 dB, reflection, and isolation better than 20 dB. The mechanism will be discussed.

Characterizing the complex permittivity of high-κ dielectrics using enhanced field method.

This paper proposed a method to characterize the complex permittivities of samples based on the enhancement of the electric field strength. The enhanced field method significantly improves the measuring range and accuracy of the samples' electrical properties. Full-wave simulations reveal that the resonant frequency is closely related to the dielectric constant of the sample. In addition, the loss tangent can be determined from the measured quality factor and the just obtained dielectric constant. Materials with low dielectric constant and very low loss tangent are measured for benchmarking and the measured results agree well with previous understanding. Interestingly, materials with extremely high dielectric constants (ε(r) > 50), such as titanium dioxide, calcium titanate, and strontium titanate, differ greatly as expected.

A modified calibration method for complex permittivity measurement.

This work proposes a calibration method to determine the complex permittivity of materials based on the cavity-perturbation method. The method can characterize a sample with relative large in volume or high in dielectric constant, which significantly broadens the scope of the conventional perturbation method. The theory is validated with a full wave solver and an experiment was conducted. A sample of silicon carbide was heated using high-power microwave and characterized with low-power signal, all operating in the same cavity but staggered in time sequence. The complex permittivity of the silicon carbide varying with temperature was measured and discussed. The proposed approach heats and measures the sample strictly by microwave techniques, which fosters the study of microwave∕material interaction.