Irregular Waveguide Research Articles

Spatial structure of the coherence parameter of the sound field in an irregular arctic waveguide

Results of numerical simulation of the total and coherent sound fields and the coherence parameter for a multimode acoustic signal excited by a monochromatic sound source and propagating in an irregular arctic waveguide are presented. Expressions used as the basis for the algorithm of the sound field calculation by the method of coupled normal modes are given. Both regular and stochastic sound scattering by horizontal inhomogeneities of the bottom, water medium, and ice cover are taken into account. It is found that, in the course of sound propagation in an arctic waveguide, an anomalous variation of the energy coherence parameter of the sound field as a function of distance is observed. This variation manifests itself in the form of local peaks of the field coherence parameter. This fact should be taken into account in both the measurements of the ice cover characteristics by acoustic methods and the evaluation of the efficiency of the operation of receiving arrays.

Efficient field representation for polygonal region

An efficient trigonometric-series representation is proposed for the solution to the Helmholtz equation in the region bounded by a convex polygon. The development is obtained using the domain-product technique. The approach enables the use of the unit in the capacity of a flexible key-block in the theory of irregular waveguide structures.

Theory of excitation of irregular waveguides

A variational method is proposed to study the excitation of waveguides by arbitrarily time-dependent sources, which is suitable for irregular waveguides.

Algorithms for Solving Problems of Nonstationary Electromagnetic Wave Propagation in Weakly Irregular Waveguides

Algorithms for Solving Problems of Nonstationary Electromagnetic Wave Propagation in Weakly Irregular Waveguides

Method of plane cross-section in the theory of axisymmetrical structures, containing the pieces of irregular coaxial and circular waveguides

The posibility to use the version of plane cross-section method for analisys of the axisymmetrical oversized waveguide sructures with one and two-coupled cross-section is considered. The solution of the methodical problem about principal wave scattering at the joint of coaxial and circular waveguides is presented.

The effect of a transverse irregularity on the electromagnetic fields excited by VLF transmitters at ionospheric heights in the Earthionosphere waveguide near the terminator

Abstract The effect of a transverse irregularity on the eigenvalues of the radial operator of the problem for the Earth-ionosphere irregular spherical waveguide is estimated for the case that the propagation path is in the neighbourhood of the terminator. The method is based on the solution of the horizontal eikonal equation for the complex phase which depends on the angular coordinates. The corrections denned in the first approximation allow us to evaluate the effect of the transverse irregularity on the fields excited in the waveguide by VLF transmitters located in the low ionosphere and at satellite altitudes.

The matrix theory of electromagnetic wave excitation in irregular waveguide structures

We have solved the problem of excitation of electromagnetic waves in irregular waveguide structures of arbitrary type by a system of extrinsic currents. The method is based on the Lorentz lemma using multiwave scattering matrices. The main advantages of the method are presented.

Echosignal temporal structure from an object of complex form in an irregular waveguide

Principal aspects of the problem of sound scattering from an object of complex form (CSO) in a free field and in an irregular waveguide are investigated. The CSOs are prototypes of real artificial submarine objects [I. L. Oboznenko, Complex Shaped Object Detection and Classification (EG&G WASC, Washington, 1995)]. The CSO is represented by a sum of the interacting elements of local reflection (LREs). It is shown that the LREs CSO are partly coherent scattered. In this condition, the CSO is a multipatch object containing glistening points (part of the first Fresnel zone of each LREs). Temporal structure computations of the multipatch echosignals from the four CSO’s models of real constructions have been made within ray approximation in imitation of an irregular waveguide. It is shown that the real signal can vitally distort and exceed the length of the CSO’s echosignal, an irregular waveguide caused by the multipath propagation and the multipatch CSO. This difference is observed by a statistical shift of the CSO along the waveguide direction at a different depth. Some classification indications of the CSO were provoked by scattering in an irregular waveguide.

Sound propagation in an irregular two-dimensional waveguide

A method is presented for the numerical calculation of a scalar wave propagating along a two-dimensional rough-sided or irregular waveguide. In this situation the wave becomes multiply scattered, with simultaneous interaction at the two boundaries. The field can be expressed in terms of a pair of coupled integral equations; these are derived and solved in an approach based on the parabolic integral equation method, which assumes that all energy is carried in a forward direction. An extended formulation encompassing backscatter is also derived, and a method given for its treatment. This paper serves in part to explain the computational results presented in B. J. Uscinski, “High-frequency propagation in shallow water,” J. Acoust. Soc. Am. 98, 2702–2707 (1995).

Excitation of the earth-ionosphere irregular waveguide by sources located in an anisotropic ionosphere

Abstract The electromagnetic fields excited in the Earth-ionosphere waveguide by arbitrarily oriented VLF transmitters (electric and magnetic dipoles) located in the ionosphere are treated. The formulation allows for a spherical radially and longitudinally irregular waveguide as well as for an anisotropic ionosphere. The generalized reciprocity theorem is used for the solution of the problem. Numerical techniques have been developed. To illustrate the field features conditioned by the longitudinal irregularities, the diurnal variations of the field components excited at the Earth's surface by the sources located in the low ionosphere and at satellite altitudes have been calculated for some propagation paths.

Ionospheric fields excited by sources located in the Earth-ionosphere irregular waveguide

Abstract The ionospheric fields excited by arbitrarily oriented VLF transmitters (electric or magnetic dipoles) located in the Earth-ionosphere waveguide are treated. The formulation allows for an irregular (in two directions) waveguide as well as for an anisotropic ionosphere. The transverse sections method was used for the solution of the waveguide problems. Numerical techniques have been developed. The diurnal variations of the field components excited in the low ionosphere and at satellite altitudes by ground-based dipoles have been calculated for some paths.

Design of a 3-MW 140-GHz gyrotron with a coaxial cavity

A design for a 3-MW 140-GHz gyrotron based on the use of a coaxial cavity is given. The cavity mode is TE/sub 21,13/, chosen so that the ohmic heating on both the inner and outer conductors would be low enough for CW operation. The mode selection process, nonlinear, multimode and time-dependent modeling of the beam wave interaction, and gun design are discussed in detail. An inverted magnetron injection gun (MIG) is used to accommodate the inner conductor. The radiation is coupled out via a quasi-optical mode converter, consisting of an irregular cylindrical waveguide section followed by a step-cut launching aperture and a single near-parabolic mirror. The design of these components is also described.

The Kirchhoff–Helmholtz integral theorem and related identities for waves in an inhomogeneous moving fluid.

The Kirchhoff–Helmholtz integral theorem (KHT), which expresses the wave field in a volume inside (or outside) a given surface in terms of the field’s value on the surface, is well known and widely used in acoustics of motionless media. In this paper, an extension of the theorem to acoustic‐gravity waves in an arbitrary inhomogeneous moving fluid is obtained as a corollary of the reciprocity‐type relations constituting the recently established flow reversal theorem (FRT) [O. A. Godin, J. Acoust. Soc. Am. 97, 3396(A) (1995); 98, 2866(A) (1995)]. The KHT takes a concise form when stated in terms of acoustic pressure and oscillatory displacement of fluid particles. The KHT is applied to study uniqueness of solutions to acoustic boundary value problems in moving media and to establish unitarity and other general properties of the scattering matrix for surface and volume scattering as well as in an irregular (range‐dependent) waveguide with flow. Relation of the scattering matrix properties to the FRT and to wave‐action conservation is discussed. [Work supported by NSERC.]

Calculation of VLW fields in a waveguide with A 3-D local inhomogeneity

The problem of a point-source field in an irregular impedance waveguide is solved. The 3-D inhomogeneity of one of the walls of the waveguide is given by an area-inhomogeneous impedance. To obtain a solution within the framework of the method of integral equations, we develop a procedure for asymptotic transformation of the 2-D equation into an 1-D equation with allowance for the waves reflected from all the inhomogeneity boundaries. The obtained 1-D integral equation for points that belong to both the path line and boundary contour of the inhomogeneity is solved numerically using an original algorithm. The results of model calculations in a near-earth waveguide for the case of ionospheric perturbations that are large on the wavelength scale are given.

On the problem of coastal refraction in an impedance waveguide

The problem of wave propagation in an irregular plane impedance waveguide is considered. This problem is similar to the classical coastal refraction problem with arbitrary geometry of the coastline. The two-dimensional integral equation is converted asymptotically in the large parameter kr≫1 to a one-dimensional equation, which contains, besides the conventional terms, a curvilinear integral taken along the interface (coastline). A convenient formula is obtained for the case of a straight coastline. The results of numerical calculations are presented.

Simulation of electromagnetic fields and calculation of cutoff frequencies of waveguides with complex cross-sections

An algorithm and a program for the calculation of the natural frequencies and fields of irregular waveguides are presented. The algorithm permits calculation of the fields of waveguides with complex cross-sections (L-, H, and E-profiles) and waveguides partially filled with dielectrics having constant electromagnetic characteristics. The method involves replacement of the 2-D electrodynamic problem by a discrete mathematical model, which can be interpreted as a system of electrically coupled circuits. Analyzing the behavior of the currents in a discrete model by electrical engineering methods, we obtain an approximate solution of the reference electrodynamic problem. Numerical calculation results and errors are given.

On solution of the vector 3-D locally irregular waveguide problem

The vector 3-D problem of a point-source field in a plane waveguide with a large-scale local inhomogeneity on one of its walls is considered. The field components on the boundary surfaces comply with the Leontovich conditions, which are used as a basis for obtaining expressions for the derivatives of the field vectors normal to the boundaries; these expressions reflect the 3-D nature of the inhomogeneity. The problem is reduced to a system of 2-D integral equations allowing for overexcitation and depolarization of the field scattered by the irregularity. The system of 2-D integral equations is asymptotically transformed over the inhomogeneity region on the surface of the walls bounding the waveguide space into a system of linear integral equations, for which the integration contour is represented by the line between the source and observation point, as well as by the linear geometric contour of the irregularity.

Asymptotics of eigenwaves of a smooth irregular spherical anisotropic waveguide

Based on the Debye series modified with respect to the method of horizontal beams and vertical modes, the asymptotics of eigenwaves of a smooth irregular anisotropic spherical anisotropic waveguide channel is constructed.

Effect of large-scale boundary irregularity on the ELF waveguide field

The effect of boundary irregularity on the ELF field in a waveguide is studied by the full-wave/ray method using the focusing-factor equation. The displacement of the normal-wave trajectories into the complex region and the field along them are estimated, and the applicability conditions of a longitudinal approximation of field phase and amplitude in a two-dimensionally irregular waveguide are determined. The transverse scale of an inhomogeneity that changes the qualitative dependence of the field on distance is found.

Numerical modeling of long‐range low‐frequency propagation in irregular Arctic waveguide

An improved model of the Arctic ice cover and the mode coupling approach have been used for numerical calculations of low‐frequency sound propagation over the irregular trans‐Arctic paths proposed for acoustic monitoring of Arctic climate change. The ice cover was represented by a rough elastic layer with the roughness statistically determined by its spatial spectrum and correlation between the upper and lower ice surfaces. Horizontal change of the ice statistics along the supposed paths was approximated by a step function in accordance with existing data. The bottom relief and horizontal variation of the typical sound speed profile were approximated by segment‐linear functions. In the ice modeling particular attention has been given to the influence of the ice cracks on low‐frequency propagation loss. This can be regarded as a shear wave attenuation in the broken ice plate which differs strongly from shear wave absorption in normal ice and mainly depends on a mean size of the ice fields. Calculated acoustic propagation loss over the trans‐Arctic paths differ from the empirical to experimental data after a 1000‐km distance, with the calculated loss significantly less than the empirical prediction. This is caused by a relative increase of the deep‐water mode contribution to the total sound field at such long distances.