摘要:

Rheometry serves as a method for the determination of effective length vectors of short antennas by means of electrolytic tank measurements. This paper reports on the application of rheometry to the three linear monopoles mounted for the purposes of the Radio and Plasma Wave Science Experiment on the Cassini spacecraft, which will fly to planet Saturn. The voltage signals induced by incoming waves from the Saturnian radio emissions will be recorded for further evaluation. By direction‐finding techniques one will trace back from the collected data to the source regions of the received radio waves and determine the wave polarization. An accurate direction finding is only possible if the effective length vectors of the antennas, which are affected by the spacecraft body, are known to a certain degree of accuracy. It is investigated how rheometry enables the determination of the effective length vectors with the help of a scale model. After a detailed discussion of the fundamentals of rheometry, the application of rheometry to the Cassini scale model is described. The results of the measurements are graphically depicted and discussed with the requirements for direction finding taken into consideration. Finally, an overview of the inflight antenna calibration is given, which will be possible by utilizing the strong Jovian radio emissions during Cassini's Jupiter flyb

ISSN:0048-6604    

DOI:10.1029/96RS01972

年代:1996

数据来源: WILEY

摘要:

This paper describes a comparison of two meteor radar systems operated simultaneously from June 29 to July 15, 1994, at the Buckland Park field station near Adelaide, Australia (35°S, 138°E). Both meteor systems operate on a narrow‐beam VHF wind profiler. The first meteor system was developed by the atmospheric physics group at the University of Adelaide. The second meteor system was the University of Colorado's meteor echo detection and collection (MEDAC) system. The goal of the campaign was to determine how closely the two similar meteor systems performed with regards to the detection of meteor trail echoes and the estimation of the Doppler frequencies. Classification of the signals in the resulting data set showed that a number of the echoes, including a class of echoes that appear to be from meteors traveling straight down the beam, were from sources other than under dense meteor trails. When the nonunder dense echoes were operated on by the Doppler frequency estimators, widely varying estimates between the two systems were produced. Only when taking into careful consideration the details of the detection routines, the signal composition of the data set, and performance characteristics of the Doppler estimators was the comparison satisfact

ISSN:0048-6604    

DOI:10.1029/96RS02028

年代:1996

数据来源: WILEY

摘要:

A generalized approach to the study of frequency‐independent antennas is presented which relies on the recently developed theory of fractal geometry. It is demonstrated that this fractal geometric interpretation allows for the ability to characterize a much wider class of frequency‐independent antennas. This includes radiating structures which are self‐similar in the discrete sense, the smooth sense, and even the “rough” sense. The antenna configurations in this paper are all self‐similar and have been parameterized in terms of a common similarity factor of τ. Finally, it is shown how this new theory of self‐similar fractal radiators may be employed to develop a multiband linear array design methodology for which the directive gain is a log‐periodic funct

ISSN:0048-6604    

DOI:10.1029/96RS02126

年代:1996

数据来源: WILEY

摘要:

The electromagnetic radiation due to a pulsed current point source located above a nonperfectly conducting Earth is investigated. The Cagniard‐de Hoop technique with polar variables is used in order to derive closed‐form expressions for the electric field above or on the surface of a nonperfectly conducting Earth. Early‐ and late‐time approximations are derived which are tested with an exact method that is presented in this paper. The approximate method yields excellent results and requires far less computation time. Numerical results for the electric field above the Earth are presented for both the exact and the approximate

ISSN:0048-6604    

DOI:10.1029/96RS02191

年代:1996

数据来源: WILEY

摘要:

Electrostatic image theory developed earlier for a point charge in an isotropic half‐space in front of an anisotropic half‐space with transverse isotropy is extended to an anisotropic half‐space with a slight deviation from transverse isotropy. Two alternative image solutions are found: a combination of a point charge and a planar surface charge, or a combination of a point charge and a half‐infinite line charge of quadrifilar nature. Both images are shown theoretically to give the same potential in the isotropic half‐space, which is tested by finding the potential function at the interface. As an example, image expressions for a point charge above the interface of a uniaxial anisotropic medium with its axis tilted from the vertical are found, and the effect of the tilt angle is studied numerically. The image expressions can be applied, for example, in scattering problems involving an obstacle in the presence of an anisotropic half‐space, which allows one to make use of the simple Green function corresponding to the isot

ISSN:0048-6604    

DOI:10.1029/96RS01727

年代:1996

数据来源: WILEY

摘要:

The recently introduced operator expansion formalism has brought new analytical power and computational efficiency to the scalar theory of rough‐surface scattering. Extending this method to electromagnetic scattering from a perfectly conducting surface is the subject of this paper. This simplest of boundaries imposes a Dirichlet‐like condition on the electric field component locally tangent to the surface; Maxwell's equations determine the normal component through a Neumann‐like condition, which requires an inverse operator to the normal differentiator. The resulting formalism, while considerably more elaborate than the scalar version, is nevertheless orders of magnitude more efficient than existing matrix methods, so that problems of three‐dimensional scattering from surfaces of multiscale roughness can now be computed acc

ISSN:0048-6604    

DOI:10.1029/96RS02164

年代:1996

数据来源: WILEY

摘要:

Milder's operator expansion formalism for scattering from surfaces of moderate slope is extended to the case of electromagnetic scattering from an interface between media of arbitrary isotropic electromagnetic properties, i.e., dielectric, magnetic, or imperfectly conducting materials. A numerical implementation of the second‐order formulas is described. It computes scattering amplitudes for complicated two‐dimensional surfaces in a few minutes on a workstation. Computations of oblique scattering from sinusoidal gratings demonstrate that the scheme is very accurate for moderate values of Fresnel number, surface curvature, and grazing angle. Practical applications include radar scattering from the ocean and optical scattering from rough metals and insulat

ISSN:0048-6604    

DOI:10.1029/96RS02165

年代:1996

数据来源: WILEY

摘要:

The diffraction of an inhomogeneous plane wave by a wedge is investigated. An integral representation for the total field is obtained and then evaluated by a uniform asymptotic procedure. The solution is expressed in the form of the uniform geometrical theory of diffraction (UTD) so that it can be applied to calculate the scattering from more complex shapes. The shadow and reflection boundaries of the geometrical optics field are found to be displaced from their conventional locations. The extent of the transition regions is also described. The solution is then extended to account for dissipative losses in the medium surrounding the wedge. To demonstrate the accuracy of the UTD solution, numerical results are presented and compared with those calculated from an eigenfunction solution.

ISSN:0048-6604    

DOI:10.1029/96RS02192

年代:1996

数据来源: WILEY

摘要:

This paper deals with mixtures of nonisotropic media. The mixing formulas to describe the effective material parameters of the mixture are complicated expressions where the different parameter dyadics are coupled. Using six‐vector notation, the different mixing rules become more manageable. However, for certain effective theories the matrix equations have been tractable only through numerical approach. Here we present a way to solve the effective material parameters in closed form, using an approach where the eigenvectors of the material six matrix are exploited. As a numerical example, the effective parameters of a mixture are calculated where the inclusion material is a mirror image of the background mediu

ISSN:0048-6604    

DOI:10.1029/96RS02559

年代:1996

数据来源: WILEY

摘要:

In this paper, generalized impedance boundary conditions (GIBC) are derived at an interface between isotropic, homogeneous dielectric media. The advantage of this technique is that rather than requiring that the tangential components of theandH→fields be continuous over the interface, an equivalent set of boundary conditions involving the electric field and its space derivatives is applied, reducing the problem dimension and therefore the computational needs significantly. The equivalence between the two sets of boundary conditions is validated for the case of an infinite dielectric slab. Finally, an application of the concept of GIBC to the analysis of dielectric waveguiding structures for high‐frequency applications is descri

ISSN:0048-6604    

DOI:10.1029/95RS03599

年代:1996

数据来源: WILEY