Far-field Propagation Research Articles

Understanding the Ultrawideband Channel Characteristics Within a Computer Chassis

A measurement-based, statistical ultrawideband (UWB) channel model is presented for the unique environment within a computer chassis. The channel impulse response parameters are derived from measurement data. Because of the short-range communications, a mixture of near-field and far-field propagation is characterized by the use of a breakpoint path-loss model. Due to the near-field nature of this environment, an unusually large number of clusters and rapid cluster decay time is noted in the channel impulse response. Additionally, electromagnetic interference measured inside a computer chassis that is generated by the motherboard in a custom fully operational mode is presented, wherein it is observed that the CPU clock does not appear as a significant interference source.

Evaluation of ground vibration induced by high-speed trains on bridge structures

A wide variety of field-measured data are used to evaluate the characteristics of ground vibration as induced by Taiwan high-speed trains on bridge structures. The measurements for analysis consist of various foundation types, geological conditions, and train speeds. Both near-field vibration (25 m from track center) and far-field vibration propagation are evaluated. Based on the results, the deep foundation in alluvium soils and the shallow foundation in rocks show higher near-field vibration level, whereas the foundation in gravelly soils has smaller vibration. The vibration level decreases with increasing concrete volume of structure and decreasing train speed. For vibration propagation, shallow foundations present higher vibration attenuation coefficients than those of deep foundations. Furthermore, the vibration attenuation is frequency dependent, and the vibration attenuation coefficient at low frequency range is much smaller compared with the coefficients at the middle and high frequency ranges. Based on these analyses, specific influence factors for ground vibration assessment are given.

聚焦高斯光束在湍流大气中的远场传输及相位补偿效果

聚焦高斯光束在湍流大气中的远场传输及相位补偿效果

An Acoustic Impedance Model for Evaluating the Ground Effect of Static-Firing Tests on a Rocket Motor

The accurate evaluation of the ground reflection effect is of importance for acoustic measurement conducted during static-firing tests of rocket motors. The effectiveness of existing acoustic impedance models was examined by comparing experimental data collected from acoustically hard surfaces. Through these comparisons and the subsequent evaluation of the effect of meteorological conditions, it was confirmed that the existing impedance models are not suitable for the evaluation of long-distance propagation over a hard surface, which corresponds to the far-field conditions in the present static-firing tests of solid rocket motors. A new acoustic impedance model is proposed in this paper. Comparisons of the acoustic measurement data indicated that the new model is effective for both near- and far-field propagation of acoustic waves. The proposed model was applied to acoustic data collected during static-firing tests of solid rocket motors assuming distributed acoustic sources along the exhaust jet axis in order to remove the ground reflection effect.

聚焦高斯光束在湍流大气中的远场传输及相位补偿效果

聚焦高斯光束在湍流大气中的远场传输及相位补偿效果

聚焦高斯光束在湍流大气中的远场传输及相位补偿效果

聚焦高斯光束在湍流大气中的远场传输及相位补偿效果

Propagation of a twisted anisotropic Gaussian Schell-model beam beyond the paraxial approximation

With the help of a tensor method, an analytical nonparaxial propagation formula for a twisted anisotropic Gaussian Schell-model (GSM) beam in free space is derived based on the generalized Raleigh-Sommerfeld diffraction integral. The far-field nonparaxial propagation expression for a twisted anisotropic GSM beam is also derived. The paraxial approximation is dealt with as a special case of our general result. Our numerical results show that the nonparaxial propagation properties of a twisted anisotropic GSM beam are closely related to the initial beam parameters (i.e., twist factor, transverse spot width matrix, coherence width matrix and wavelength) and the propagation distance. Our formulae provide a convenient and powerful way for studying the paraxial and nonparaxial propagation of an isotropic or anisotropic GSM beam with or without twist phase in free space.

Vector far-field propagation of a high-power laser diode

Based on the scalar nonparaxial theory, using a simple mathematical expression, a vector model to describe the propagation of a high-power laser diode beam is proposed. Through the analysis of the properties of the TM-mode propagation, a relative error criterion of optical intensity is discussed, it is found that the error is only correlative with the coordinate perpendicular to the junction plane and the propagation distance.

Jet Noise: Acoustic Analogy informed by Large Eddy Simulation

A novel approach to the development of a hybrid prediction methodology for jet noise is described. Modeling details and numerical techniques are optimized for each of the three components of the model. Far-field propagation is modeled by solution of a system of adjoint linear Euler equations, capturing convective and refraction effects using a spatially developing jet mean flow provided by a Reynolds-averaged Navier―Stokes computational fluid dynamics solution. Sound generation is modeled following Goldstein's acoustic analogy, including a Gaussian function model for the two-point cross correlation of the fourth-order velocity fluctuations in the acoustic source. Parameters in this model describing turbulent length and time scales are assumed to be proportional to turbulence information also taken from the Reynolds-averaged Navier―Stokes computational fluid dynamics prediction. The constants of proportionality are, however, not determined empirically, but extracted by comparison with turbulence length and time scales obtained from a large eddy simulation prediction. The large eddy simulation results are shown to be in good agreement with experimental data for the fourth-order two-point cross-correlation functions. The large eddy simulation solution is then used to determine the amplitude parameter and also to examine which components of the cross correlation are largest, enabling inclusion of all identified dominant terms in the Gaussian source model. The acoustic source description in the present approach is therefore determined with no direct input from experimental data. This model is applied to the prediction of sound to the experimental configuration of the European Union JEAN project, and gives encouraging agreement with experimental data across a wide spectral range and for both sideline and peak noise angles. This paper also examines the accuracy of various commonly made simplifications, for example: a locally parallel mean flow approximation rather than consideration of the spatially evolving mean jet flow and scattering from the nozzle; the assumption of small radial variation in Green function over the turbulence correlation length; the application of the far-field approximation in the Green function; and the impact of isotropic assumptions made in previous acoustic source models.

Analysis of a diffractive microlens using the finite-difference time-domain method

The finite-difference time-domain (FDTD) method is used as rigorous electromagnetic analysis model to calculate the field for a diffractive microlens (DML). The FDTD is used for the entire solution rather than using a near- to far-field propagation method to obtain the far-field energy distribution; thus, all the results are vector based. We derived a formula to calculate the magnitude of electric field, which is time dependent and can be used to graphically show the light wave propagation and focusing process through a DML. Both the comparison and the integral methods are presented to obtain wave amplitude in full solution space, and the distribution of light energy behind a DML is illustrated based on the wave amplitude. The formula of diffractive efficiency of the DML is derived from a time-averaged Ponyting vector, which can indicate the propagation direction of light energy. Application of these formulations in the analysis of a DML example demonstrates the high accuracy and efficiency of our method.

Optical path difference of the supersonic mixing layer

The distorted wavefront due to the density fluctuation of the supersonic mixing layer is measured by the nano-based planar laser scattering technique, with the spatiotemporal resolutions as 10(-1) mm and 1 micros (time interval between two exposures), respectively. The optical path difference (OPD) is analyzed from the viewpoints of correlation and structure functions. Depending on the structure of the flow field, the type of the correlation function is Gaussian or exponential. The power index of the structure function is about 1.1, and the consistency over many cases is quite good. The far-field propagation of the Gaussian beam is simulated by Fourier transform, and the Strehl ratio is calculated with the rms of the OPD. The beam center and beam spread about the center in the far field are calculated, and are dominated by the statistics of the tilt of the distorted wavefront. The results indicate that aero-optical aberrations can severely limit the performance of airborne laser systems.

The degenerating diffraction far-field propagation properties of the conical double half-Gaussian hollow beams

The far-field propagation properties of conical double half-Gaussian hollow beams in the condition of Collins formula are studied. Because of the cone angle of this kind of hollow beams, the diffraction is compensated and the inner diameter is turning bigger by the rule of geometric optics as the propagation distance is increasing, whereas the degenerating diffraction phenomenon is turned out. The far-field intensity distribution of the conical double half-Gaussian hollow beams in the condition of in-Collins formula is researched, and the results show that the far-field propagation properties can be depicted by this model. In the experiment, this kind of hollow beams are obtained by means of the dual-reflecting splitting optical system, and the inner diameter of the hollow beams is tested. The results show good agreement with the propagation theory in the condition of in-Collins formula.

Far-field evanescent wave propagation using coupled subwavelength gratings for a MEMS sensor

A technique was developed to couple near-field evanescent waves into observable diffraction orders in the far-field region. This investigation was of two gratings that have a 1.0 microm grating period in glass and 1.1 microm in silicon and are individually subwavelength, but when coupled together yield an 11.0 microm effective grating period. This effective grating period is not subwavelength to a 1.550 microm infrared incident source and exhibits higher diffraction orders. Optimum evanescent wave coupling efficiency was simulated by varying the grating thickness and the grating separation between the subwavelength gratings. This proposed evanescent wave coupling concept is being investigated for use in a bulk silicon MEMS accelerometer.

Analytical and numerical study of P-wave attenuation in rock shelter layer

The protection of underground structures against dynamic loadings has long been a topic of great interest in defense engineering. This paper focuses on the far-field blast-wave propagation and attenuation in rock shelter layer with an inclusion or filled medium. First, the effect of single inclusion on wave attenuation is explored analytically assuming that the interfaces between rock and the inclusion are completely consolidated. The formulae for the strength of total transmitted stress- and velocity-wave after the inclusion are subsequently presented. At last, numerical studies on the attenuation of one-dimensional P-wave in rock embedded with artificial cavity, steel plate and natural joint are carried out. The attenuation rules for various scenarios are discussed in detail in this paper which can provide useful guidelines to the design of intelligent defense structures.

Modeling cumulative sound exposure around marine seismic surveys

This article presents a method for reducing the computation time required for estimating cumulative sound exposure levels. Sound propagation has to be computed from every source position to every desired receiver location; so if there are many source positions, then the problem can quickly become computationally expensive. The authors' solution to this problem is to extract all possible source-receiver pathways and to cluster these with a self-organizing neural net. Sound propagation is modeled only for the cluster centroids and extrapolated for the entire geographic region. The tool is illustrated for the example of a marine seismic survey over a tropical coral reef. Resident fish species were expected not to flee the reef, but to stay among the corals for the entire duration of the survey. In such cases, the modeling of cumulative sound exposure levels is sometimes requested as part of environmental impact assessments. The tool developed combines a seismic source model, a near-field sound propagation model, and a far-field sound propagation model. The neural network reduces the computation time by a factor of 55. The cost is an error in modeled received levels of less than -1+/-3 dB re 1 microPa(2) s.

Propagation characteristics of silver and tungsten subwavelength annular aperture generated sub-micron non-diffraction beams

We examined the optical properties such as propagation modes, focal length, side lobes, etc. of metallic subwavelength annular apertures (SAA) and used finite-difference time-domain (FDTD) simulation to compare our experimental findings. Using two different metals, silver and tungsten, we examined the different optical transmission properties of the two metallic SAA structures. The far-field propagation of the silver SAA structure was found to be a type of quasi-Bessel beam when compared with a quasi-Bessel beam generated by a perfect axicon. The propagation characteristics of these two beams were found to match qualitatively. The far-field transmitted light generated by the silver SAA structure was found to possess a 390 nm sub-micron focal spot with a 24 microm depth of focus, which was much smaller than the focal spot generated by a perfect axicon. We also found that a silver SAA structure can generate a sub-micron quasi- Bessel beam that has a much lower far-field side-lobe when compared to that of non-diffraction beams generated by a tungsten SAA structure.

Long-range propagation of annular beam for lidar application

Long-range propagation characteristics of an annular beam were examined from the viewpoint of lidar application. The annular beam permits to use a reflecting telescope for transmission and improves the transmission efficiency because it can pass through the telescope without the obstruction of the telescope’s secondary mirror. As the annular beam can enlarge up to the telescope diameter, it can be eye-safe and its divergence is also reduced. The annular beam generated by a couple of Axicon prisms; conical prisms, transforms its beam shape into the central peak narrow beam; nearly non-diffractive beam, through the far-field propagation. We evaluated how the central peak intensity and its width of the transformed beam change in long-range of 3–100 km. The difficulty of the optical arraignment and the requirement of the beam quality to obtain the enough performance of the beam shape transformation were also examined theoretically and experimentally for the lidar application. The stability of the transformed beam was also confirmed in the atmospheric fluctuation.

Validation of a Time Domain Formulation for Propeller Noise Prediction

The present paper is oriented at the definition of a series of validation test cases for the far-field subsonic formulation of Farassat in the time domain, solving the Lighthill analogy for aeroacoustics in the Ffowcs William-Hawkings formulation for far-field noise predictions. The focus will be on the definition, validation and verification of four test cases for far-field acoustic propagation. The test cases are: (i) a rotating rotor with a pre-defined body force, (ii) a translating and rotating small disk case with a moving observer, (iii) a conventional helicopter rotor case, (iv) and the consistency test known as the Isom thickness noise. The acoustic pressure signature, sound pressure level and directivity have been calculated for each test case. Graphical outputs and comparisons of results with those of analytical solutions are presented.

Communication modes in scalar diffraction

The communication modes are a useful concept in studies of optical resolution, wave propagation, and image synthesis. We present an overview of earlier results on the communication modes in scalar diffraction theory. Besides the general theory, the modes are reviewed for the far-field and Fresnel regimes, and new eigenequations are derived for wide-angle diffraction. We prove a conjugate relationship between the transmitting and receiving modes in a general symmetric system. We also suggest an approximate method for far-field and Fresnel domain propagation, in which propagation amounts to a rotation of each mode in the complex plane. The main focus is on the near-field communication modes, where we present numerical examples of the modes and coupling strengths for a near-field geometry with a sub-wavelength size receiving domain. These results provide insights, for example, into the understanding of near-field scanning probe techniques.

GSM Base Station Electromagnetic Radiation and Oxidative Stress in Rats

The ever increasing use of cellular phones and the increasing number of associated base stations are becoming a widespread source of nonionizing electromagnetic radiation. Some biological effects are likely to occur even at low-level EM fields. In this study, a gigahertz transverse electromagnetic (GTEM) cell was used as an exposure environment for plane wave conditions of far-field free space EM field propagation at the GSM base transceiver station (BTS) frequency of 945 MHz, and effects on oxidative stress in rats were investigated. When EM fields at a power density of 3.67 W/m2 (specific absorption rate = 11.3 mW/kg), which is well below current exposure limits, were applied, MDA (malondialdehyde) level was found to increase and GSH (reduced glutathione) concentration was found to decrease significantly (p < 0.0001). Additionally, there was a less significant (p = 0.0190) increase in SOD (superoxide dismutase) activity under EM exposure.