Cutoff Frequencies Of Modes Research Articles

Modal-Based Design of a Wideband Quadruple-Ridged Flared Horn Antenna

In this paper, a systematic design technique for quadruple-ridged flared horn antennas is presented, to enhance the radiation properties through the profiling of the ridge taper. The technique relies on control of the cutoff frequencies of specific modes inside the horn, instead of brute-force optimization. This is used to design a prototype antenna as a feed for an offset Gregorian reflector system, such as considered for the Square Kilometer Array (SKA) radio telescope, to achieve an optimized aperture efficiency from 2 to 12 GHz. The antenna is employed with a quadraxial feeding network that allows the excitation of the fundamental $TE_{11}$ mode, while suppressing all other modes that causes phase errors in the aperture. Measured results confirm the validity of this approach, where good agreement is found with the simulated results.

Power flux distribution in chiroplasma‐filled perfect electromagnetic conductor circular waveguides

AbstractTheoretical analysis of the electromagnetic wave propagation in circular waveguides with a chiroplasma core coated with a perfect electromagnetic conductor (PEMC) is presented. The presented formulations and analysis are general for any perfect electric, perfect magnetic, or perfect electromagnetic conductor (PEMC) waveguides filled with any general anisotropic/isotropic metamaterial including plasma. The characteristic equation for the modes in this waveguide is obtained, and the behavior of the dispersion curves and the power flux are examined and evaluated numerically. The results demonstrate that the behavior of the power flux transported in the guide, in magnitude and orientation, is highly determined by the chirality parameter and the plasma and cyclotron frequencies. The mode cutoff frequencies are sensitive to the variations in the filling material and chirality parameters and likewise affected by the variations in the PEMC admittance parameter.

Effect of Chirality on Dispersion Characteristics of Uniaxial Anisotropic Chiral Elliptical Waveguide

AbstractIn this paper, the dispersion characteristic of elliptical waveguide consisting of uniaxial anisotropic chiral medium is investigated. The eigenvalue equation for the waveguide that is to be studied is deduced by solving the Mathieu and modified Mathieu functions. The cutoff frequencies for several lower order even modes have been calculated and their dispersion characteristics are correspondingly plotted. The effects of elliptical eccentricity and chirality on the mode cutoff frequencies and mode transmission are examined numerically.

Propagation through chiroplasma waveguide using perfect magnetic conductor boundary conditions

Analysis of electromagnetic wave propagations in a cylindrical waveguide filled with a chiroplasma core coated with a perfect magnetic conductor is presented. The presented analysis and formulations is generalized for any general isotropic or anisotropic material, including plasma and metamaterials. The characteristic equations are obtained and the behavior of its characteristic curves and the energy flux are examined and evaluated numerically. The obtained results show that the behavior of the energy flux transported in the guide, in magnitude and orientation, is highly affected by the chirality parameter and the plasma and cyclotron frequencies. The mode cut-off frequencies and transported energies are sensitive to the variations in the material parameters, particularly the variations in the chirality parameter.

Comparison on thermal transport properties of graphene and phosphorene nanoribbons.

We investigate ballistic thermal transport at low temperatures in graphene and phosphorene nanoribbons (PNRS) modulated with a double-cavity quantum structure. A comparative analysis for thermal transport in these two kinds of nanomaterials is made. The results show that the thermal conductance in PNRS is greater than that in graphene nanoribbons (GNRS). The ratio kG/kP (kG is the thermal conductivity in GNRS and kP is the thermal conductivity in PNRS) decreases with lower temperature or for narrower nanoribbons, and increases with higher temperature or for wider nanoribbons. The greater thermal conductance and thermal conductivity in PNRS originate from the lower cutoff frequencies of the acoustic modes.

An Ultra-Compact Common-Mode Bandstop Filter With Modified-T Circuits in Integrated Passive Device (IPD) Process

Based on multi-cell modified-T circuits, this paper proposes a novel circuit topology to realize a common-mode bandstop filter with differential-mode all-pass characteristics for high-speed digital differential circuits. The circuit behaviors for both differential and common modes are analyzed based on symmetrical network analysis techniques. Two transmission zeros at common mode can be synthesized by the derived formulas. The proposed design flow is used to realize a test sample in the integrated passive device process. Simulated and measured results are in good agreement. The common-mode suppression band $(\vert S_{{ cc}21}\vert is from 1.6 to 4.7 GHz, whose fractional bandwidth is nearly 100%. The cutoff frequency of the differential mode can maintain up to 10 GHz with a constant group delay. Moreover, eye diagrams under different signaling rates all perform well. Compared with the other literature, the proposed common-mode filter has a very wide common-mode stopband, the highest differential-mode cutoff frequency up to 10 GHz, and the most compact circuit size of $0.017~\lambda_{g}\times 0.016~\lambda_{g}$ .

Dielectric waveguides for ultrahigh field magnetic resonance imaging.

The design of RF coils for MRI transmit becomes increasingly challenging at high frequencies required for MRI at 7T and above. Our goal is to show a proof of principle of a new type of transmit coil for higher field strengths. We demonstrate an alternative transmit coil design based on dielectric waveguide principles which transfers energy via evanescent wave coupling. The operating principles and conditions are explored by simulations. The waveguide is applied for in vivo imaging at 7T. The waveguide can be an efficient transmit coil when four conditions are fulfilled: (1) the waveguide should be operated just above the cutoff frequency of the lowest order transverse electric mode, (2) the waveguide should not operate at a frequency where the wavelength fits an integer number of times in the waveguide length and standing wave patterns become very prominent, (3) for homogeneous excitation, the waveguide should be bent around the object, and (4) there should be an air gap between the waveguide and the object. By choosing the dielectric and the dimensions adequately, the dielectric waveguide couples the magnetic field efficiently into the body. The waveguide can be redesigned for higher frequencies by simple adaptations and may be a promising transmit alternative. Magn Reson Med 76:1314-1324, 2016. © 2015 Wiley Periodicals, Inc.

Two-Material Ridge Substrate Integrated Waveguide for Ultra-Wideband Applications

A novel ridge substrate integrated waveguide (RSIW) with extended single-mode bandwidth is presented in this paper. The proposed structure is based on a substrate integrated waveguide including two dielectric layers with different permittivity. The ridge is implemented by a row of partial height metal vias, connected at the bottom by a metal strip. An RSIW with cutoff frequency of the fundamental mode at 2.5 GHz and of the second mode at 12 GHz is proposed, thus achieving a bandwidth of almost 5:1. This structure is suitable for applications covering the entire ultra-wideband. The proposed RSIW has been fabricated and tested to verify experimentally the electromagnetic properties of the fundamental and second mode. Finally, the role of the metal strip connecting the bottom of the ridge vias is discussed, showing through numerical and experimental results that the strip plays a fundamental role in avoiding the bandgap in the frequency response of the RSIW.

Guided modes in chiroplasma-filled perfect electromagnetic conductor parallel-plate waveguides

Theoretical analysis of electromagnetic wave propagation in a perfect electromagnetic conductor (PEMC) parallel-plate waveguide filled with a chiroplasma material is presented in this article. The derived formulations are general and can analyze perfect electric, perfect magnetic, or PEMC waveguides filled with any general isotropic/anisotropic material including plasma and metamaterials. The characteristic equation for the modes in this waveguide is obtained, and the behavior of the dispersion curves and the energy flux are examined and evaluated numerically. The results demonstrate that the chirality parameter, the plasma frequency, and the cyclotron frequency influence the behavior of the energy flux transported in the guide, in magnitude and orientation. The bifurcated mode cutoff frequencies are sensitive to the variations in the filling material parameters and likewise effected by the variations in the PEMC walls admittance parameter.

High-Frequency Guided Wave Scattering by a Partly Through-Thickness Hole Based on 3D Theory**Supported by the National Natural Science Foundation of China under Grant Nos 11474195, 11274226 and 61171145.

We present a theoretical investigation of the scattering of high frequency S0 Lamb mode from a circular blind hole defect in a plate based on the 3D theory. The S0 wave is incident at the frequency above the A1 mode cut-off frequency, in which the popular approximate plate theories are inapplicable. Due to the non-symmetric blind hole defect, the scattered fields will contain higher order converted modes in addition to the fundamental S0 and A0 modes. The far-field scattering amplitudes of various propagating Lamb modes for different hole sizes are inspected. The results are compared with those of lower frequencies and some different phenomena are found. Two-dimensional Fourier transform (2DFT) results of transient scattered Lamb and SH wave signals agree well with the analytical dispersion curves, which check the validity of the solutions from another point of view.

Critical Depths for Higher Modes by Minimally-invasive Multimodal Surface Wave (MMSW) Method: Simulations and Field Test

A Minimally-invasive Multimodal Surface Wave (MMSW) geophysical testing method, which is a hybrid of surface and borehole seismic methods, was developed recently by the authors to measure more extensive multi-mode dispersion data and thus improve the accuracy of inversion profiles. The new MMSW method employs a borehole geophone at selected depths to record seismic waves from different source offsets on the soil surface. Presented in this paper is a procedure for estimating a range of optimum geophone depths to capture a given higher mode by the MMSW method. Stiffness matrix and finite-element-based numerical simulations of the MMSW method are performed to identify the relationships between critical geophone depths and apparent cutoff frequencies of higher modes. Specifically, it is shown for increasing velocity profiles that 1) at a given borehole sensor measurement depth, the apparent cutoff frequencies of higher modes increase with mode number, 2) at a given frequency, the critical geophone depth at which a higher mode will first become dominant increases with mode number, and 3) for a given higher mode, the apparent cutoff frequency decreases as measurement depth increases. A preliminary field test is conducted using a vertical geophone placed at five different depths while impacts are applied to the soil surface from 3.66 to 43.89 m from the borehole, with an impact spacing of 3.66 m. Dispersion images from the five geophone depths are superimposed to produce a dispersion image having three modes with improved clarity relative to the surface-only Multichannel Analysis of Surface Waves (MASW) method. A comparison of the experimental and theoretical apparent cutoff frequencies for higher modes is used to validate the theoretical prediction of critical depths by the stiffness matrix method. Matching of experimental and theoretical apparent cutoff frequencies could provide additional optimization constraints to reduce the uncertainty of final inversion profiles.

High-frequency lowest torsional wave mode ultrasonic inspection using a necked pipe waveguide unit

We propose an effective method to transmit only the non-dispersive lowest torsional wave mode at a high frequency range even above the cutoff frequency of the third torsional mode. Unlike existing methods that tune the wavelength or phase of the target wave mode, the proposed method is based on the thickness change and the cutoff phenomenon. A specially configured necked waveguide, consisting of three regions of which the middle region is thinner than the so-called cutoff thickness, is put in end-to-end contact with a test pipe to transmit only the first torsional wave mode to a test pipe. After explaining the underlying role of the proposed necked waveguide, we propose a technique to mainly transmit the lowest torsional wave mode at a frequency where higher modes can also propagate. Numerical simulations and damage detection experiments were carried out to show the effectiveness of the proposed method.

Modal analysis and dispersion curves of an elliptical W-type single mode fiber

In this paper, the propagation of electromagnetic waves in W-type elliptical dielectric optical fiber having various cladding layers is presented. The presented fiber has concentric core and cladding which have elliptical cross section and the refractive index of one of the inner cladding (the cladding between first clad and last clad) maximum. Using elliptic cylindrical coordinates, boundary conditions are derived and longitudinal field components for the even and odd modes are obtained. The characteristic equation for the fiber to be studied is determined by solving the Mathieu (q > 0) and the modified Mathieu functions (q < 0). In order to study the fundamental mode, the modal index m is put as m = 1. Finally, the cutoff frequencies for several lower order modes have been calculated and their dispersion characteristics are plotted. The effects of elliptical eccentricity e on the mode cutoff frequencies and mode transmissions are also addressed. The analysis shows that one can control the propagation property of optical fiber by increasing the number of inner claddings. These claddings provide additional degree of freedom to control the modes.

Enhancement of single mode operation in coaxial optical waveguide using DB boundary conditions

In this study, a competent numerical strategy to compute the dispersion of optical waveguides is presented and propagation of electromagnetic waves in a coaxial optical waveguide with DB boundary conditions is instigated. For this intend, cylindrical coordinates are here being used to derive the DB boundary conditions and to obtain field components for the modes. The propagation constant for the waveguide to be studied is determined by solving the Bessel and the modified Bessel functions. The cutoff frequencies for various lower order modes have been calculated and their dispersion characteristics are plotted correspondingly. The behavior of the coaxial optical waveguide under DB boundary conditions is shown to be significantly different from that of coaxial optical waveguide and conventional optical waveguide under traditional or tangential boundary conditions. Finally, the effect of waveguide dimensions on the mode cutoff frequencies and fabrication issues are also addressed.

Coaxial‐to‐circular waveguide transition with broadband mode‐free operation

A simple transition between a circular waveguide and a coaxial cable having mode-free operation over more than an octave bandwidth is presented. The transition consists of a conical coaxial probe, a rectangular waveguide section and a linearly tapered transformer from the rectangular to circular waveguide. The transition shows a reflection coefficient of < –20 dB, a transmission coefficient of >–0.57 dB and a higher-order mode level of < −25 dB over a frequency range 1.04–2.53 times the TE11 mode cutoff frequency.

Photonic band gaps of a two-dimensional square lattice composed by superconducting hollow rods

In this paper by means of the plane wave expansion method, we have calculated the photonic band structure of 2D photonic crystals consisting of high temperature superconducting hollow cylinders arranged in a square lattice. Band structures were obtained at low frequencies and assuming TM polarization of the incident wave, for different inner radii of the cylinders and for two different temperatures (5K and 15K), showing the tunability of photonic band gaps with respect to these parameters. Interesting features, such as the decreasing of cutoff frequency and separation of photonic modes were observed by increasing both the temperature and inner radius. Permittivity contrast and the difference between the inner and outer radius lead to the appearance of new band gaps when compared with the case of solid cylinders. These band gaps can be modulated by the width of the shell and temperature, which may be used for the development of novel optical devices.

Half-Mode Buried Corrugated Substrate Integrated Waveguide

A half-mode buried corrugated substrate-integrated waveguide is proposed. Its dominant mode propagation behavior is almost identical to that of the half-mode substrate-integrated waveguide but offers a floating top conductor thereby permitting active devices connected to its longitudinal open-edge to be biased without the need for extra biasing chokes. The corrugations comprise open-circuit stubs buried in the substrate that are connected to the top wall of the waveguide by vias. These stubs are quarter-wave in length at a frequency twice the dominant mode cut-off frequency. The corrugations and vias are used to obtain an electric wall. Burying stubs in the substrate results in a low stub input impedance over a wide bandwidth which allows a monomode bandwidth of 3:1 to be obtained. Experiments for waveguides with a TE0.5,0 mode cut-off frequency of 2.8 GHz demonstrate its effectiveness.

Ballistic thermal conductance by phonons through superlattice quantum-waveguides

Ballistic thermal conductances (BTCs) by phonons through superlattice quantum-waveguides are investigated by using the scattering-matrix method and the elastic continuum theory. A comparison for the cylindrical model (CM) and the rectangular model (RM) is addressed. We find that for these two models, the quantum thermal conductance can be observed even when the superlattices exist in quantum-waveguides. At low temperature, BTCs for the CM and the RM present almost the same behaviors regardless of the periodic length of superlattices. However, at higher temperature, BTCs for the RM are larger than those for the CM stemming from lower cutoff frequencies of high order modes for the RM. We also find that BTCs undergo a noticeable transformation from the monotonic decrease to constant with increasing the periodic number of superlattices. A brief analysis of these results is given.

Acceleration and Deflection of an Electron Inside the Circular Sectoral Plasma Waveguides

The deflection of an electron and its acceleration when it interacts with the fields of transverse magnetic mode in a plasma-filled circular-wedge waveguide corresponding to different values of sectoral angle is analytically investigated by finding the field components of the TMpr mode in this waveguide. Then the cutoff frequency of some special modes for the π/2 and π circular sectoral plasma waveguide are calculated. However, it is found that the acceleration gradient and deflection angle depend strongly on the parameters of the microwave, the dimensions of the waveguide, and sectoral angle of plasma waveguide, specially. The deflection angle and the acceleration gradient of the electron during its shift along the waveguide for the mentioned cases are analyzed by giving typical values to the waveguide dimensions, the frequency and intensity of the microwave radiation, and the obtained results are discussed graphically from the presented figures.

Quantum entanglement in plasmonic waveguides with near-zero mode indices

We investigate the quantum entanglement between two quantum dots (QDs) in a plasmonic waveguide with a near-zero mode index, considering the dependence of concurrence on interdot distance, QD waveguide frequency detuning, and coupling strength ratio. High concurrence is achieved for a wide range of interdot distances due to the near-zero mode index, which largely relaxes the strict requirement of interdot distance in conventional dielectric waveguides or metal nanowires. The proposed QD waveguide system with near-zero phase variation along the waveguide near the mode cutoff frequency shows very promising potential in quantum optics and quantum information processing.