CPA-induced Hz to THz broadband absorber with switchable perfect absorption between radio-microwave and THz frequency spectrum

The coherent perfect absorption (CPA) occurring in the graphene sheet suspended in air can be utilized to develop an ultrathin, ultra-broadband absorber working in the frequency range from a few hertz (Hz) to terahertz (THz) with perfect absorption. A graphene sheet is studied to induce the CPA to cover radio, microwave and lower THz frequency ranges. A graphene resonator able to provide the surface plasmon resonance (SPR) is combined with the graphene sheet to provide CPA at either side of a thin dielectric layer forms metamaterial structure with the cavity and enhances the absorption bandwidth in the THz region by creating a resonance near quasi-CPA frequency. A dielectric silicon resonator is embedded in the structure, which creates dipolar resonances between the resonances obtained by the formed cavity between the graphene sheet and resonator. This enhances the absorption level in the THz region. The absorption bandwidth is further enhanced to 7 THz by including a graphene disc at the top of the silicon resonator. Thus, the multiple multi-order resonances occurring in the silicon dielectric and SPR of graphene resonators are merged with the phenomena of CPA occurring in the graphene sheets to extend the CPA bandwidth in the THz regime. The doping level of graphene or its tunable Fermi energy based on the applied DC electric field provides the tunability in the total obtained absorption bandwidth. The symmetric structure provides polarization-insensitive behavior with an allowed incident angle of more than 45° with more than 90% absorption.

Multilayer Bolometric Structures for Efficient Wideband Communication Signal Reception

It is known that the dielectric layer (resonator) located behind the conducting plate of the bolometer system can significantly increase its sensitivity near the resonance frequencies. In this paper, the possibility of receiving broadband electromagnetic signals in a multilayer bolometric meta-material made of alternating conducting (e.g., silicon semiconductor) and dielectric layers is demonstrated both experimentally and numerically. It is shown that such a multilayer structure acts as a lattice of resonators and can significantly increase the width of the frequency band of efficient electromagnetic energy absorption. The parameters of the dielectric and semiconductor layers determine the frequency bands. Numerical modeling of the effect has been carried out under the conditions of our experiment. The numerical results show acceptable qualitative agreement with the experimental data. This study develops the previously proposed technique of resonant absorption of electromagnetic signals in bolometric structures.

A Low-Profile Dielectric Resonator Filter with Wide Stopband for High Integration on PCB

A low-profile dielectric resonator (DR) filter is proposed to achieve the feature of high integration and wide stopband. The high integration is due to the structure of printed circuit board (PCB) substrate instead of metal cavity, which can be easily integrated with other planar circuits. Thus, the proposed design can improve the integration level and reduce installation errors. Moreover, the out-of-band harmonics can be well suppressed by the structure combined with introducing rectangular hollowing in the center of the dielectric block, coupling the feed and loading 1/4λ wavelength branch. For demonstration, it is fabricated and measured. The simulated and experimental results with good agreement are presented, the insertion loss is as low as 1.1 dB, the profile height is only 0.77λg, and the stopband reaches 2.61f0.

Dipolectric antenna for high-field MRI

PURPOSE

To introduce the dipolectric antenna: a novel RF coil design for high-field MRI using a combination of a dipole antenna with a loop-coupled dielectric resonator antenna.

METHODS

Simulations in human voxel model Duke involving 8-, 16-, and 38-channel dipolectric antenna arrays for brain MRI were conducted. An 8-channel dipolectric antenna for occipital lobe MRI at 7 T was designed and constructed. The array was built of four dielectric resonator antennas (dielectric constant = 1070) and four segmented dipole antennas. In vivo MRI experiments were conducted in one subject, and the SNR performance was benchmarked against a commercial 32-channel head coil.

RESULTS

A 38-channel dipolectric antenna array provided the highest whole-brain SNR (up to a 2.3-fold SNR gain in the center of the Duke's head vs. an 8-channel dipolectric antenna array). Dipolectric antenna arrays driven in dipole-only mode (with dielectric resonators used as receive-only) yielded the highest transmit performance. The constructed 8-channel dipolectric antenna array provided up to threefold higher in vivo peripheral SNR when compared with a 32-channel commercial head coil.

CONCLUSION

Dipolectric antenna can be considered a promising approach to enhance SNR in human brain MRI at 7 T. This strategy can be used to develop novel multi-channel arrays for different high-field MRI applications.

Novel Method to Achieve Temperature-Stable Microwave Dielectric Ceramics: A Case in the Fergusonite-Structured NdNbO4 System

Microwave dielectric ceramics with permittivity (ε_r) ∼ 20 play an important role in massive multiple-input multiple-output (MIMO) technology in 5G. Although fergusonite-structured materials with low dielectric loss are good candidates for 5G application, tuning the temperature coefficient of resonant frequency (TCF) remains a problem. In the present work, smaller V⁵⁺ ions (r_V = 0.355 Å, with coordination number (CN) = 4) were substituted for Nb⁵⁺ (r_Nb = 0.48 Å with CN = 4) in the Nd(Nb_1-xV_x)O₄ ceramics, which, according to in situ X-ray diffraction data, lowered the fergusonite-to-scheelite phase transition (T_F-S) to 400 °C for x = 0.2. The thermal expansion coefficient (α_L) of the high-temperature scheelite phase was +11 ppm/°C, whereas for the low-temperature fergusonite phase, it was + 14 < α_L < + 15 ppm/°C. The abrupt change in α_L, the associated negative temperature coefficient of permittivity (τ_ε), and the minimum value of ε_r at T_F-S resulted in a near-zero TCF ∼ (+7.8 ppm/°C) for Nd(Nb_0.8V_0.2)O₄ (ε_r ∼ 18.6 and Qf ∼ 70,100 GHz). A method to design near-zero TCF compositions based on modulation of τ_ε and α_L at T_F-S is thus demonstrated that may also be extended to other fergusonite systems.

Low-Cost Electronics for Automatic Classification and Permittivity Estimation of Glycerin Solutions Using a Dielectric Resonator Sensor and Machine Learning Techniques

Glycerin is a versatile organic molecule widely used in the pharmaceutical, food, and cosmetic industries, but it also has a central role in biodiesel refining. This research proposes a dielectric resonator (DR) sensor with a small cavity to classify glycerin solutions. A commercial VNA and a novel low-cost portable electronic reader were tested and compared to evaluate the sensor performance. Within a relative permittivity range of 1 to 78.3, measurements of air and nine distinct glycerin concentrations were taken. Both devices achieved excellent accuracy (98-100%) using Principal Component Analysis (PCA) and Support Vector Machine (SVM). In addition, permittivity estimation using Support Vector Regressor (SVR) achieved low RMSE values, around 0.6 for the VNA dataset and between 1.2 for the electronic reader. These findings prove that low-cost electronics can match the results of commercial instrumentation using machine learning techniques.

Proposal: Apparatus for Sensing the Effect of Surface Roughness on the Surface Resistance of Metals

The root mean square surface roughness Rq of metals is detrimental in several microwave applications. Rq characterization methods are thus largely used and of great interest. In this work, a new dielectric loaded resonator (DR) design is proposed to evaluate the surface resistance variations of samples with different Rq. The new design is thought to make the measurement accuracy, usually strongly affected by the measurement repeatability, suitable for this study. We analyze the measurement method's sensitivity and accuracy in order to assess the possibility of using this new DR design for highly accurate surface resistance measurements sensitive to Rq variations.

Four-Port Dual-Band Multiple-Input Multiple-Output Dielectric Resonator Antenna for Sub-6 GHz 5G Communication Applications

A four-port dielectric resonator (DR)-based multiple-input multiple-output (MIMO) antenna is presented for sub-6 GHz MIMO communication. The dielectric resonator was fed through aperture feeding to achieve dual-band resonance. The DRA has the operating modes of TE01δ and TE10δ at 3.3 GHz and 3.9 GHz, respectively. The engineered antenna has port isolation of higher than 20 dB at the target frequencies without the employment of an extra isolation mechanism. Full-wave high-frequency simulation software was employed for the simulation computation. The antenna has a peak gain of 5.8 dBi and 6.2 dBi, and an efficiency of 88.6% and 90.2% at 3.3 GHz and 3.9 GHz, respectively. The proposed resonator has good MIMO diversity parameters. The optimal envelope correlation coefficient (ECC) is 0.01, channel capacity loss (CCL) is 0.1 bits/sec/Hz, and the total active reflection coefficient (TARC) is −22.46. The DRA elements are aligned orthogonally with adequate displacement for achieving polarization diversity and spatial diversity. The antenna delivers its applications in Sub-6 GHz 5G and WiMAX communications.

Gordon Coupler with Inductive or Capacitive Iris for Small EPR Resonators for Aqueous Samples

The Gordon coupler was introduced for use in EPR experiments at liquid helium temperatures. It provides an evanescent wave incident on the iris of a microwave resonator. Match of power incident on the coupler to the resonator is obtained by variation of the amplitude of an evanescent wave that arises from displacement of a dielectric wedge in a tapered waveguide. Reduced microphonics from helium bubbling was reported. The Gordon coupler was subsequently extended from cavity resonators to loop-gap resonators, initially at helium temperatures but later for aqueous samples. Plastics with low dielectric constants, usually Teflon, were used. Here, we extend the Gordon coupler for application in X-band five-loop-four-gap resonators using fused quartz, sapphire, or rutile dielectrics, noting that the size of the coupler can then be commensurate with dimensions of dielectric loop-gap resonators as well as dielectric tube resonators. Finite element modeling of electromagnetic fields has been carried out, and use of a capacitive iris that interfaces with the Gordon coupler reduces pulling of the resonant frequency when matching the resonator.

Compact Four-Port Circularly Polarized MIMO X-Band DRA

A circularly polarized (CP) multi-input multioutput (MIMO) dielectric resonator (DR) antenna (DRA) with compact size and four ports is implemented. CP radiation was achieved using the deformed DR geometry excited with aperture coupled feeding. A CPDRA with a single and two ports is investigated. The defected ground structure (DGS) was incorporated into the antenna for improving the isolation between the ports. The DGS was incorporated in such a way that the required phase difference between the generated orthogonal degenerate modes is preserved. This concept could be utilized in implementing a compact four-port CP antenna. The MIMO antenna provides a 10 dB impedance bandwidth of 38% (8.5–12.5 GHz) and a 3 dB AR bandwidth of 9.32% (9.2–10.1 GHz). The gain of the implemented antenna was around 6 dBi in the band where CP radiation was achieved. The MIMO performance parameters were calculated, and their values remained within the acceptable limits. The implemented antenna could suitably be used in X-band applications.

Beyond Q: The Importance of the Resonance Amplitude for Photonic Sensors

Resonant photonic sensors are enjoying much attention based on the worldwide drive toward personalized healthcare diagnostics and the need to better monitor the environment. Recent developments exploiting novel concepts such as metasurfaces, bound states in the continuum, and topological sensing have added to the interest in this topic. The drive toward increasingly higher quality (Q)-factors, combined with the requirement for low costs, makes it critical to understand the impact of realistic limitations such as losses on photonic sensors. Traditionally, it is assumed that the reduction in the Q-factor sufficiently accounts for the presence of loss. Here, we highlight that this assumption is overly simplistic, and we show that losses have a stronger impact on the resonance amplitude than on the Q-factor. We note that the effect of the resonance amplitude has been largely ignored in the literature, and there is no physical model clearly describing the relationship between the limit of detection (LOD), Q-factor, and resonance amplitude. We have, therefore, developed a novel, ab initio analytical model, where we derive the complete figure of merit for resonant photonic sensors and determine their LOD. In addition to highlighting the importance of the optical losses and the resonance amplitude, we show that, counter-intuitively, optimization of the LOD is not achieved by maximization of the Q-factor but by counterbalancing the Q-factor and amplitude. We validate the model experimentally, put it into context, and show that it is essential for applying novel sensing concepts in realistic scenarios.

Microwave Measurements of Electromagnetic Properties of Materials

A review of measurement methods of the basic electromagnetic parameters of materials at microwave frequencies is presented. Materials under study include dielectrics, semiconductors, conductors, superconductors, and ferrites. Measurement methods of the complex permittivity, the complex permeability tensor, and the complex conductivity and related parameters, such as resistivity, the sheet resistance, and the ferromagnetic linewidth are considered. For dielectrics and ferrites, the knowledge of their complex permittivity and the complex permeability at microwave frequencies is of practical interest. Microwave measurements allow contactless measurements of their resistivity, conductivity, and sheet resistance. These days contactless conductivity measurements have become more and more important, due to the progress in materials technology and the development of new materials intended for the electronic industry such as graphene, GaN, and SiC. Some of these materials, such as GaN and SiC are not measurable with the four-point probe technique, even if they are conducting. Measurement fixtures that are described in this paper include sections of transmission lines, resonance cavities, and dielectric resonators.

Cylindrical Dielectric Resonator Antenna-Based Sensors for Liquid Chemical Detection

A compact, cylindrical dielectric resonator antenna (CDRA), using radio frequency signals to identify different liquids is proposed in this paper. The proposed CDRA sensor is excited by a rectangular slot through a 3-mm-wide microstrip line. The rectangular slot has been used to excite the CDRA for HEM11 mode at 5.25 GHz. Circuit model values (capacitance, inductance, resistance and transformer ratios) of the proposed CDRA are derived to show the true behaviour of the system. The proposed CDRA acts as a sensor due to the fact that different liquids have different dielectric permittivities and, hence, will be having different resonance frequencies. Two different types of CDRA sensors are designed and experimentally validated with four different liquids (Isopropyl, ethanol, methanol and water).

3D-Printed Low-Cost Dielectric-Resonator-Based Ultra-Broadband Microwave Absorber Using Carbon-Loaded Acrylonitrile Butadiene Styrene Polymer

In this study, an ultra-broadband dielectric-resonator-based absorber for microwave absorption is numerically and experimentally investigated. The designed absorber is made of the carbon-loaded Acrylonitrile Butadiene Styrene (ABS) polymer and fabricated using the 3D printing technology based on fused deposition modeling with a quite low cost. Profiting from the fundamental dielectric resonator (DR) mode, the higher order DR mode and the grating mode of the dielectric resonator, the absorber shows an absorptivity higher than 90% over the whole ultra-broad operating band from 3.9 to 12 GHz. The relative bandwidth can reach over 100% and cover the whole C-band (4–8 GHz) and X-band (8–12 GHz). Utilizing the numerical simulation, we have discussed the working principle of the absorber in detail. What is more, the absorption performance under different incident angles is also simulated, and the results indicate that the absorber exhibits a high absorptivity at a wide angle of incidence. The advantages of low cost, ultra-broad operating band and a wide-angle feature make the absorber promising in the areas of microwave measurement, stealth technology and energy harvesting.

Melanin Radicals in Paraffin-embedded Melanoma Investigated Using Surface-type Dielectric Resonator for X-band EPR

We investigated melanin radicals in paraffin-embedded malignant melanoma (MM) using a surface-type dielectric resonator for X-band electron paramagnetic resonance (EPR) and analyzed the radical species. The surface-type resonator's performance was examined using 5 - 10 μL of 0.1 mM 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL) aqueous solution in a 1.0-mm (i.d.) glass capillary as well as 1,1-diphenyl-2-picrylhydrazyl (DPPH) powder. The surface-type detection has approximately two times poorer S/N ratio than commercial insertion-type detection. A sample of the paraffin-embedded MM specimen was used for the radical detection. We obtained an EPR spectrum of melanin radicals in the paraffin-embedded melanoma sample (size ∼3 × 4 × 3 mm). A single line (∼0.64 mT peak-to-peak line-width) with a small shoulder was observed and was identified as a pheomelanin-related radical. The pheomelanin radical can be directly related to the MM. Thus, the present results were a good indication for noninvasive measurement, as well as for detailed analyses of melanin radicals in human MM.

Dielectrically-Loaded Cylindrical Resonator-Based Wireless Passive High-Temperature Sensor

The temperature sensor presented in this paper is based on a microwave dielectric resonator, which uses alumina ceramic as a substrate to survive in harsh environments. The resonant frequency of the resonator is determined by the relative permittivity of the alumina ceramic, which monotonically changes with temperature. A rectangular aperture etched on the surface of the resonator works as both an incentive and a coupling device. A broadband slot antenna fed by a coplanar waveguide is utilized as an interrogation antenna to wirelessly detect the sensor signal using a radio-frequency backscattering technique. Theoretical analysis, software simulation, and experiments verified the feasibility of this temperature-sensing system. The sensor was tested in a metal-enclosed environment, which severely interferes with the extraction of the sensor signal. Therefore, frequency-domain compensation was introduced to filter the background noise and improve the signal-to-noise ratio of the sensor signal. The extracted peak frequency was found to monotonically shift from 2.441 to 2.291 GHz when the temperature was varied from 27 to 800 °C, leading to an average absolute sensitivity of 0.19 MHz/°C.