Time-domain Radiation Research Articles

Pulse Radiation Characteristics Prediction Method of Vivaldi Antenna based on Dipole Array

This paper presents a theoretical method to estimate the pulse radiation characteristics of Vivaldi antennas. Based on the surface current distribution and the ultra-wideband radiation principle, Vivaldi antenna is equivalently modeled as a dipole array, and the pulse radiation characteristics of a single Vivaldi antenna are brought out utilizing the spatial superposition approach. Then, the influences and results of the Vivaldi antenna pulse characteristics prediction with different construction ways of the dipole array and element numbers are furthermore investigated. Next, a quadratic spatial superposition technique is employed to complete the theoretical prediction for time-domain radiation characteristics of Vivaldi antenna arrays. Experiments and simulations are conducted separately to verify the proposed method for both single Vivaldi antenna and array. The validated results demonstrate that the dipole array-based theoretical prediction method can effectively capture the pulse radiation characteristics for both individual Vivaldi antenna and array operating in different modes, thereby addressing challenges associated with estimating radiation characteristics in ultra-wideband pulse applications.

Coherent Subcycle Optical Shock from a Superluminal Plasma Wake.

We propose exploiting the superluminal plasma wake for coherent Cherenkov radiation by injecting a relativistic electron beam (REB) into a plasma with a slowly varying density up-ramp. Using three-dimensional particle-in-cell and far-field time-domain radiation simulations, we show that an isolated subcycle pulse is coherently emitted towards the Cherenkov angle by bubble-sheath electrons successively at the rear of the REB-induced superluminal plasma wake. A theoretical model based on a superluminal current dipole has been developed to interpret such coherent radiation, and agrees well with the simulation results. This radiation has ultrashort attosecond-scale duration and high intensity, and exhibits excellent directionality with ultralow angular divergence and stable carrier envelope phase. Its intensity increases with the square of the propagation length and its central frequency can be easily tuned over a wide range, from the far infrared to the ultraviolet.

Towards time-domain modelling of wheel/rail noise: Effect of the dynamic track model

Transient events in railway rolling noise, such as the characteristic impulsive noise at switches and crossings, can significantly contribute to the perceived annoyance, despite being difficult to detect in the standard frequency-domain methods to analyse rolling noise. Studying these transient effects and their perception requires predicting the noise in the time domain. While several time-domain approaches exist for predicting the dynamic interaction of wheel and rail, predicting the associated rolling noise with adequate accuracy is computationally costly. The lack of a model for transient noise and the need for studying its perception was recently identified. Aiming for a comprehensive time-domain radiation model that includes the wheel and track contributions to rolling noise, this work focuses on the track radiation. The modelling approach taken here is based on a 2.5D formulation for the acoustic radiation and moving Green’s functions in the air. The computational cost, which lies mainly in the 2.5D BE calculations, is addressed by pre-calculating acoustic transfer functions. These transfer functions can be combined with different dynamic track models. Different dynamic track models in turn affect radiated sound field in different ways. Here, the sound fields produced by six different track models are compared, including different support types and analytical and numerical rail models. Several descriptors of the sound field are analysed. In terms of the radiated sound power and radiation efficiency, modelling the rail as a simple beam leads to similar results as elaborate numerical models up to about 5 kHz. In terms of the track-side sound pressure, simple beam models can provide similar results only up to 2.5 kHz. Euler-Bernoulli (E-B) beams seem unfit for time-domain predictions of the radiated noise as they over-estimate the bending wave speed at high frequencies. The results also show that the standard track decay rate (TDR) and the decay of acoustic sound pressure along the track are comparable.

Numerical investigations on nonlinear motions of ships sailing in large-amplitude waves based on hybrid HOBEM

This paper presents a nonlinear numerical approach based on hybrid higher-order boundary element method (HOBEM) to investigate the wave-induced motion responses of ships with forward speed. The radiation and diffraction forces in time domain are determined by convolution of the impulsive response functions computed from frequency-domain hydrodynamic data of the hybrid method using a combination of Rankine source and translating-pulsating Green's function. In its numerical implementation, steady flow effects are included in the inner domain encompassing the ship, 9-node bi-quadratic curvilinear elements are employed to discretize the boundaries and analytical quadrature formulas are derived to calculate the influence coefficients related to the Green's function over the control surface so as to improve the accuracy, efficiency and stability. Meanwhile, the nonlinear incident wave force and hydrostatic restoring force are calculated over the instantaneous wetted surface of the ship hull during the motion process to capture the nonlinear properties. Based on the method, a numerical program is developed to predict and analyze the motion response of a modified Wigley hull and a non-wall-sided S175 containership advancing in waves with different amplitudes and heading angles. By comparing the present computed results with experimental data and other numerical solutions, it is found that the nonlinear calculations can produce more accurate motion responses in large-amplitude waves with strong nonlinearity. And the present nonlinear method based on hybrid HOBEM is of higher accuracy and better stability than traditional nonlinear methods based on three-dimensional pulsating source (3DP) and translating-pulsating source (3DTP), especially for S175 with large flare near the waterline.

A Cylindrical Antenna With a Conductively Loaded Slot for Broadband Imaging Radar Applications

A cylindrical antenna with a conductively loaded slot (CACLS) is designed for broadband imaging radar applications. Through design considerations, such as conductive loading, cylinder termination, and the insertion of an electromagnetic (EM) wave absorber into the cylinder, the proposed CACLS is shown to be able to radiate temporally short and nondispersive pulses. Based on the final design of the antenna, parametric studies pertaining to the time-domain radiation performances depending on the dimension of the cylinder are performed. The CACLS with the determined design parameters is then fabricated and its performances verified through antenna measurements and a radar experiment. In addition to its excellent pulse radiation and reception performance, the realized antenna has a compact and physically robust structure, which makes it potentially suitable for many broadband imaging radar applications.

Design optimization and sensitivity analysis on time-domain sound radiation of laminated curved shell structures

Design optimization and sensitivity analysis on time-domain sound radiation of laminated curved shell structures

KleinPAT

We propose a new modal sound synthesis method that rapidly estimates all acoustic transfer fields of a linear modal vibration model, and greatly reduces preprocessing costs. Instead of performing a separate frequency-domain Helmholtz radiation analysis for each mode, our method partitions vibration modes into chords using optimal mode conflation , then performs a single time-domain wave simulation for each chord. We then perform transfer deconflation on each chord's time-domain radiation field using a specialized QR solver, and thereby extract the frequency-domain transfer functions of each mode. The precomputed transfer functions are represented for fast far-field evaluation, e.g., using multipole expansions. In this paper, we propose to use a single scalar-valued Far-field Acoustic Transfer (FFAT) cube map. We describe a GPU-accelerated vector wavesolver that achieves high-throughput acoustic transfer computation at accuracy sufficient for sound synthesis. Our implementation, KleinPAT, can achieve hundred- to thousand-fold speedups compared to existing Helmholtz-based transfer solvers, thereby enabling large-scale generation of modal sound models for audio-visual applications.

Multiparticle time-domain analysis of coherent undulator radiation

The physics involving the emission of coherent undulator radiation from a bunch of correlated electrons wiggling inside an undulator has been very well established in the frequency domain. With the aim of describing the process of emission of coherent undulator radiation in time domain while incorporating the effect of hyperbolic field profile of the undulator and various electron beam parameters, we present a time-domain analysis which can serve as a complimentary formulation to the frequency-domain analysis of the coherent undulator radiation. The formulation requires revisiting the equation of motion of an electron moving inside a planar undulator. An electron beam with substantial spread along the direction of the magnetic field will undergo betatron oscillations due to the hyperbolic profile of the undulator fields. We show that the betatron motion can be conveniently expressed in terms of Mathieu functions. The new set of equations of motion have been used to study the effect of betatron motion on the emitted undulator radiation. The single-particle time-domain analysis is then easily extended to multiparticle systems by correctly accounting for the phase-difference between the individual electromagnetic waves emitted by different electrons occupying different phase-space in the bunch.

A time domain three-dimensional sono-elastic method for ships’ vibration and acoustic radiation analysis in water

The classical three-dimensional hydroelasticity of ships is extend to include the effect of fluid compressibility, which yields the three-dimensional sono-elasticity of ships. To enable the predictions of coupled transient or nonlinear vibrations and acoustic radiations of ship structures, a time domain three-dimensional sono-elastic analysis method of acoustic responses of a floating structure is presented. The frequency domain added mass and radiation damping coefficients of the ship are first calculated by a three-dimensional frequency domain analysis method, from which a retardation function is derived and converted into the generalized time domain radiation force through a convolution integral. On this basis the generalized time domain sono-elastic equations of motion of the ship hull in water are established for calculation of the steady-state or transient-state excitation induced coupled vibrations and acoustic radiations of the ship. The generalized hydrodynamic coefficients, structural vibrations and underwater acoustic radiations of an elastic spherical shell excited by a concentrated pulsating force are illustrated and compared with analytical solutions with good agreement. The numerical results of a rectangular floating body are also presented to discuss the numerical error resultant from truncation of the upper integration limit in the Fourier integral of the frequency domain added mass coefficients for the retardation function.

Transient linear TEM horn array

This study presents an ultrawideband transverse electromagnetic (TEM) horn array with enhanced frequency- and time-domain performances for high-power transient and short-pulse radiation applications. The proposed linear array is composed of five combined TEM horn and loop elements. The compact size, wideband performance, and low-dispersion characteristics of the utilised element enable good time and frequency operations over more than two-octave bandwidth. The performance of the designed array is characterised computationally and experimentally. The scanning capabilities are also demonstrated; particularly for the time-domain operation using fidelity factor and energy patterns. Moreover, conventional frequency-domain side lobes (SLL) reduction techniques are utilised to minimise the time-domain radiation at undesired directions. The proposed array has VSWR 8 dBi, good quality radiation patterns, and SLL<−10 dB over more than 4:1 bandwidth. Excellent time-domain performance demonstrated by high fidelity factor over the array's field of view is achieved. Time-domain scan up to 40° is also demonstrated.

Scattering effect of submarine hull on propeller non-cavitation noise

This paper investigates the non-cavitation noise caused by propeller running in the wake of submarine with the consideration of scattering effect caused by submarine׳s hull. The computation fluid dynamics (CFD) and acoustic analogy method are adopted to predict fluctuating pressure of propeller׳s blade and its underwater noise radiation in time domain, respectively. An effective iteration method which is derived in the time domain from the Helmholtz integral equation is used to solve multi-frequency waves scattering due to obstacles. Moreover, to minimize time interpolation caused numerical errors, the pressure and its derivative at the sound emission time is obtained by summation of Fourier series. It is noted that the time averaging algorithm is used to achieve a convergent result if the solution oscillated in the iteration process. Meanwhile, the developed iteration method is verified and applied to predict propeller noise scattered from submarine’s hull. In accordance with analysis results, it is summarized that (1) the scattering effect of hull on pressure distribution pattern especially at the frequency higher than blade passing frequency (BPF) is proved according to the contour maps of sound pressure distribution of submarine׳s hull and typical detecting planes. (2) The scattering effect of the hull on the total pressure is observable in noise frequency spectrum of field points, where the maximum increment is up to 3dB at BPF, 12.5dB at 2BPF and 20.2dB at 3BPF. (3) The pressure scattered from hull is negligible in near-field of propeller, since the scattering effect surrounding analyzed location of propeller on submarine’s stern is significantly different from the surface ship. This work shows the importance of submarine׳s scattering effect in evaluating the propeller non-cavitation noise.

Semi-analytical formulation for a Unified Time and Frequency Antenna Characterization

The definition of parameters that characterize the radiation of electric and magnetic fields for antennas in the time and frequency domain on an unified representation is proposed. The formulation uses a straightforward semi-analytical formulation that can be subsequently applied on the analysis of excited antennas for an arbitrary source with temporal behavior. The effective height is a parameter for antenna analysis defined for quantities in far field region and can be used as a transfer function of the antenna. This transfer function can be described through the antenna singularities which can be obtained by singularity expansion. The Singularity Expansion Method (SEM) is capable to model an electromagnetic quantity with the singularities extracted by the current densities of an arbitrary object. This work proposes that the singularities are extracted by the Matrix Pencil method applied on the current densities. The current densities are obtained numerically through the method of the Finite Differences in the Time Domain (FDTD) for wired log-periodic antenna and, after the singularities are obtained, the formulation of the semi-analytical effective height equation is written. To validate the presented method, a formulation of the time-domain radiation pattern is presented and a corresponding frequency-domain radiation pattern is also presented using Parseval’s theorem.

A SPICE model of an antenna for transmitting

A SPICE model of an antenna for transmitting is proposed. This model allows for the calculation of the frequency-domain radiation fields for a range of frequencies in which the model is valid, it also allows for the direct calculation of the time-domain (TD) radiation fields for an arbitrary TD excitation signal, the spectrum of which should be within the modeling range. The model is then verified by two examples, both of them demonstrate its validity. This model can be a part of a complete system-level model for electromagnetic compatibility simulation.

A hybrid FDTD/SEM model for physics-oriented analysis of time-domain radiation properties of antenna-based sensors

A semi-analytical methodology is presented for the accurate analysis of time-domain radiation characteristics of antenna sensors. A locally conformal finite-difference time-domain technique is adopted to derive a minimal pole/residue spherical harmonic expansion of the equivalent currents excited on a suitable Huygens surface enclosing the sensing device. In this way, by using the singularity expansion method, the time-domain gain and effective height of the structure can be evaluated analytically, in terms of the newly introduced incomplete spherical Bessel functions, as the superposition of non-uniform spherical wave contributions attenuating along with the time and space according to the complex poles accounting for the natural resonant processes occurring in the device. The accuracy of the developed technique is assessed by application to an ultrawideband bow-tie antenna-based sensor for millimeter-wave radar measurements.

Optical input impedance of nanostrip antennas

We conduct an investigation into optical nanoantennas in the form of a strip dipole made from aluminum. With the finite-difference time domain simulation both optical input impedance and radiation efficiency of nanostrip antennas are addressed. An equivalent circuit is presented as well for the nanostrip antennas at optical resonances. The optical input resistance can be adjusted by varying the geometric parameters of antenna strips. By changing both strip area and strip length simultaneously, optical input resistance can be adjusted for matching impedance with an external feeding or loading circuit. It is found that the optical radiation efficiency does not change significantly when the size of a nanostrip antenna varies moderately.

Spherical Fidelity Patterns of UWB Antennas

The time domain radiation properties of UWB antennas are analyzed, especially the angular dependent impulse distortion, with respect to the signal transmitted in the main beam direction, is investigated. The correlation properties of the radiated pulses are determined by the so called fidelity <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</i> . The fidelity <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</i> is together with the peak pulse amplitude <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</i> a powerful tool for the characterization of impulse radiating antennas. The results are applied to two different UWB antennas, a Vivaldi antenna and a Bow-tie antenna. The spatial regions with good correlation and high peak power are determined for both by measurements for verification. The results are vital for UWB impulse communications, and for UWB Radar as well.

3 차원 시간영역 근사비선형 2 차경계요소법에 의한 선체의 대진폭 운동 및 파랑하중 계산

A three-dimensional time-domain calculation method is of crucial importance in prediction of the motions and wave loads of a ship advancing in a severe irregular sea. The exact solution of the free surface wave-ship interaction problem is very complicated because of the essentially nonlinear boundary conditions. In this paper, an approximate body nonlinear approach based on the three-dimensional time-domain forward-speed free-surface Green function has been presented. The Froude-Krylov force and the hydrostatic restoring force are calculated over the instantaneous wetted surface of the ship while the forces due to the radiation and scattering potentials over the mean wetted surface. The time-domain radiation and scattering potentials have been obtained from a time invariant kernel of integral equations for the potentials which are discretized according to the second-order boundary element method (Hong and Hong 2008). The diffraction impulse-response functions of the Wigley seakeeping model advancing in transient head waves at various Froude numbers have been presented. A simulation of coupled heave-pitch motion of a long rectangular barge advancing in regular head waves of large amplitude has been carried out. Comparisons between the linear and the approximate body nonlinear numerical results of motions and wave loads of the barge at a nonzero Froude number have been made.

Time Domain Analysis of the Near-Field Radiation of Shaped Electrically Large Apertures

The near-field radiation of several electrically large apertures has been computed based on the aperture field convolution method, including a square aperture, a circular aperture, and a serrated edge aperture. The frequency range is 8.0-12.0 GHz. The aperture diameters are limited to 5.00 m and the corresponding electrical size is 133-200 wavelengths. The time domain spectrum has been obtained by chirp z-transform with a Hamming window and the mechanism for the generation of the aperture near-field radiation in time domain is analyzed. The calculation results demonstrate that the near-field radiation of the aperture can be approximately seen as the synthesis of a plane wave from the aperture, cylindrical waves from the edges, and spherical waves from the corners. By this method, the direction, position, and magnitude of incoming waves in the near-field region can be estimated, and the aperture design can be modified to meet the requirements of the near-field radiation. The near field of the three apertures mentioned above has been compared, and it is shown that the serrated edge aperture has more uniform direct wave and lower diffraction waves, making it a candidate for compact range (CR) aperture design.

Terahertz Radiation-Band Engineering Through Spatial Beam-Shaping

We propose a terahertz (THz)-band engineering technique based on generation of specifically designed time-domain radiation. The desired time-domain radiation is formed by combining the time-delayed outputs of an array of photoconductive antennas. We have employed this technique to generate tunable narrowband THz radiation. By using an array of identical photoconductive antennas with identical spacings, we have demonstrated a 12.5-mW output peak-power over 0.05-THz bandwidth and a tuning range of 0.65-1.4 THz. Considering the 2.5-THz bandwidth of each individual photoconductive antenna, 50 times higher power efficiency is achieved as a result of radiation-band shaping.

Analysis of acoustic radiation mode in time domain

The acoustic radiation mode of plane, whose radiating operator is constructed by Rayleigh integral, is investigated in the time domain and its physical meaning is given. The relationship between the acoustic radiation modes of time domain and frequency domain is discussed. It is verified that the acoustic radiation modes are the natural property of the radiator and they can be obtained by different methods. These time domain radiation modes, whose shapes are only dependent on the geometry size and shape of the radiator, can radiate sound power independently. Especially, the first time domain radiation mode accounts for most of the sound radiation. All these simplify the calculation and control of the structure-borne sound power. Based on these observations, the sound power radiated from the vibrating plate is estimated by the time domain radiation mode for verifying the proposed method. The influence factors on the estimating accuracy in different conditions are discussed.