摘要:

In this paper, the singularity expansion method (SEM) is used to describe the electrostatic charge distribution on an array of thin linear antennas placed in a uniform electric field. The SEM, which has primarily been used to analyze transient scattering problems, decomposes the electromagnetic interaction process into various quantities such as singularities and modes. Using the SEM, the step plane wave induced transient current on the array is expanded in terms of its singularities (poles) in the Laplace transform (complex frequency domain.) The continuity equation is applied to the induced current expression to obtain the transient charge. The electrostatic charge distribution on the array is found by using the final value theorem on the transient charge expression. It is well known that the SEM factorization of a single linear element reveals that a single pole exists in the fundamental resonance region (near ωL/c= π, whereLis the length of the scatterer). For a two‐element array, two poles are observed in the fundamental resonance region. This trend continues such that ann‐element array hasnpoles in the fundamental resonance region. Associated with each pole is a unique modal current and corresponding charge distribution. For example, one of the two fundamental resonance region poles of the two‐element array produces half‐wavelength sinusoidal current distributions whose directions are the same on one scatterer but opposite on the other. The remaining fundamental resonance region pole produces half‐wavelength sinusoidal current distributions whose directions are the same on both scatterers. Corresponding to each mode is a coupling coefficient which determines how much a particular mode couples into the response. A generalization of these results for ann‐eleinent array will be given. Furthermore, the electric polarizability is derived in terms of the SEM electric charge description. The value of this research lies in the elegance and strength of the SEM to factor a problem into various quantities which depend on different variables of the problem. By using the SEM to analyze then‐element planar array, a much deeper comprehension of the fundamental aspects of the electrostatic interaction proc

ISSN:0048-6604    

DOI:10.1029/96RS01929

年代:1996

数据来源: WILEY

摘要:

Radiation characteristics of a plano‐convex lens are studied using Maslov's method, stressing the evaluation of an electromagnetic field around a caustic region when a uniform plane wave is incident axially to the flat plate side. The field expressions which are valid near a focal region are derived in terms of finite integrals with a single variable. As a special case, an approximate expression for a weakly focusing system is derived and compared with a corresponding expression obtained using Kirchhoff‐Huygens principle. Agreement between them is fairly good. Numerical computation for field distribution around a focal region is conducted, and the three‐dimensional pictures for the results are presented to help understand the focusing behavior of the

ISSN:0048-6604    

DOI:10.1029/96RS01330

年代:1996

数据来源: WILEY

摘要:

A rigorous modal expansion analysis is presented for a monopole antenna fed through an infinite perfectly conducting ground plane by a coaxial transmission line. An infinite perfectly conducting plate is introduced to facilitate the use of the modal expansion method. The surface current and input impedance of the monopole antenna result from finding the expansion coefficients of the electromagnetic field expressions by enforcing boundary conditions at conducting surfaces and regional interfaces. Numerical results for the current distribution and input admittance of a parallel plate antenna and a monopole fed by a coaxial line are presented and compared with available data in the literature. Very good agreement is observed. Also examined is the effect of the junction between the feed line and the monopole on the antenna's admittance.

ISSN:0048-6604    

DOI:10.1029/96RS01733

年代:1996

数据来源: WILEY

摘要:

Introducing a dielectric material near a conical antenna affects the transmitted and received signals. In this paper, wide‐angle cones with dielectric loading are analyzed in both the transmitting and the receiving modes. A Gaussian pulse is used as the exciting signa

ISSN:0048-6604    

DOI:10.1029/96RS01814

年代:1996

数据来源: WILEY

摘要:

The application of active and adaptive optics allows the construction of large diameter lightweight optical telescopes for observations below the seeing limit of the atmosphere. Active wavefront correction in a Cassegrain/Gregorytype radio telescope can be made with a deformable main reflector or deformable subreflector. Here we suggest the possibility of correcting spatially large‐scale wavefront deformations with a small size corrector located near the focus of the telescope. Using representative examples of the Institut de Radioastronomie Millimetrique (IRAM, Spain) 30‐meter diameter, millimeter‐wavelength telescope, we calculate the improvement expected from the correction of (1) the systematic component in homology deformations, (2) the large‐scale residual errors of a reflector adjustment, and (3) the beam degradation experienced in observations with a wobbling subreflector. The improvement in surface/wavefront precision obtained from piston correction with a corrector of some 50 elements is of the order of 30 – 40%. We investigate in particular the systematic component of homology deformations, their representation by low‐order Zernike polynomials, and their elimination by near‐focus correction. We study in detail the homology deformations of the IRAM 3

ISSN:0048-6604    

DOI:10.1029/96RS01124

年代:1996

数据来源: WILEY

摘要:

An experimental study was conducted to investigate the angular memory effect of millimeter‐wave scattering from two‐dimensional conducting random rough surfaces. The surfaces under investigation were machine‐fabricated with known Gaussian roughness statistics, and the copolarized and cross‐polarized angular correlation functions (ACFs) of scattering amplitudes were measured. It was found that for the case of reference antenna positions located bistatically in a backward direction, the measured ACF exhibits broad response when single scattering dominates but two peaks when multiple scatteringdominates. These observations are in good agreement with the second‐order Kirchhoff approximation (KA2). Specifically, the observed broad and peak responses are analytically identified to be due to the first‐order and second‐order (ladder and cyclical) scattering components, respect

ISSN:0048-6604    

DOI:10.1029/96RS01312

年代:1996

数据来源: WILEY

摘要:

Frequency domain interferometry (FDI) experiments were carried out on two medium frequency spaced antenna radars located in New Zealand and the Antarctic. These experiments were the first spaced antenna FDI experiments on MF radar and allowed simultaneous measurements of wind, angular distribution, spatial interferometry, FDI range, and scattering depth to be made. It was found that when correcting the FDI scattering depths for the scale of the radar volumes, typically large on MF radars, the adjustment should include a measure of the aspect sensitivity of the scattering. FDI provides a means of clearly identifying thin layers, and the occurrence of such layers often appeared here to be related to fluctuations in the wind. A very interesting example was a thin layer event that seemed to be closely related to a reversal of the wind apparently caused by a change in phase of the semidiurnal tide; enhanced turbulence following the event suggested breaking of the wave. Multiple frequency steps were used and, with analysis by frequency correlation functions, revealed the occasional presence of multiple scattering features in the radar volume.

ISSN:0048-6604    

DOI:10.1029/96RS01468

年代:1996

数据来源: WILEY

摘要:

Measurements of fog characteristics are rare, and therefore models for predicting the attenuation caused by fog on the propagation of electromagnetic radiation are scarce. Using experimental transmissions through moderate radiation fogs on a 500‐m path, a method is developed to infer the physical characteristics of fog from concurrent attenuation measurements at millimeter, infrared, and visible wavelengths. It makes it possible to retrieve the fog drop size distribution, as well as its time evolution, during the life cycle of a fog event. It yields realistic values for fog features, such as liquid water content and average drop diameter, which are not easily measured. Furthermore, on an experimental basis, simple relationships between inferred fog characteristics and measured attenuation are proposed. It is found that while both millimetric and infrared attenuations are directly related to liquid water content, visible attenuation strongly depends on the size of the fog droplets. Overall, using these relationships together with statistical distributions of inferred fog characteristics enables predictions of fog attenuation at any wavelength from millimetric to visibl

ISSN:0048-6604    

DOI:10.1029/96RS01725

年代:1996

数据来源: WILEY

摘要:

Reliable HF communications along short‐, medium‐ and long‐range paths require propagation assessment. Such assessment could be facilitated with the monitoring of ionospheric characteristics by continuously available passive means, i.e., measurement of the total electron content (TEC) using satellite‐emitted signals without a need for burdening the electromagnetic spectrum. With ubiquitous Global Positioning System (GPS) providing instantaneous time delay, or equivalently, TEC, values when needed, an assessment of HF propagation conditions may be available on a near‐real‐time basis. Both TEC and the peak electron density of the ionosphere, which determines the ordinary upper frequency limit (ƒ0F2( for HF sky wave vertical propagation, vary strongly with solar and geomagnetic parameters. Their ratio, the equivalent slab thickness, may vary to a lesser degree and hence be modeled with greater accuracy. A slab thickness model combined with real‐time TEC measurement anywhere on the globe may possibly yield an improved HF parameter prediction algorithm. To test the efficacy of the hypothesis, one has to ascertain the correlation, as exhibited by the correlation coefficient, between the TEC daily variability about the monthly mean and the ƒ0F2variability. To determine such correlation, a study compared Faraday TEC data as well as GPS‐generated TEC data collected in Israel and with corresponding ƒ0F2values obtained from vertical sounder measurements near the appropriate subionospheric location in Cyprus. The analysis shows that for large percentages of the time, very good correlation exists between TEC and ƒ0F2short‐term variations. The correlation coefficient varies between 0.7 or better during winter and summer months to about 0.5–0.6 during equinox months. A study of the diurnal dependence of the correlation indicates that a better correlation exists during daytime than nighttime. There was no indication that the coefficient is dependent on geomagnetic activity or on protonospheric electron content during t

ISSN:0048-6604    

DOI:10.1029/96RS01965

年代:1996

数据来源: WILEY

摘要:

Results of investigating the spatial and temporal HF field structure near the skip distance are presented. The data were obtained on midlatitude radio paths less than 1000 km in length, using broadcasting stations as sources of the probe signals. The signals reflected by the ionosphere were received and processed with a multichannel coherent receiver and the largest phased array of the decameter radio telescope UTR‐2. The strong dependence of the probe signal parameters on the ionospheric plasma conditions has allowed formulating and solving a number of model inverse problems of ionospheric diagnostics. It proves possible to determine parameters of the parabolic ionospheric layer, and the effective electron collision frequency, to evaluate horizontal gradients of the electron density and some parameters of the medium‐scale‐size ionospheric irregularities. A number of tests made both within the method and by means of comparison of the restored parameters against the data of other diagnostic techniques demonstrate the high potential and accuracy of the approach described in the

ISSN:0048-6604    

DOI:10.1029/96RS01737

年代:1996

数据来源: WILEY