摘要:

A physical optics formulation of scattering by thin wire helices of finite and infinite extent is presented. The high frequency limit of the integral representation of the physical optics field is obtained by the saddle point technique and is shown to be identical to the field computed by the geometrical optics approximation. For thin wire helices of finite extent, the physical optics approximation to the current should be supplemented by current waves traveling along the wire that are excited by the discontinuities at the ends of the wire. This modified physical optics approach is found to be a useful approximation to compute scattering by nonresonant helices that have a larger pitch than approximately 5 wavelengths. A hybrid method composed of the modified physical optics technique and the method of moments is applied, for the degenerate case of a linear wire, in order to extend the region of validity to cover the resonant cases. The modified physical optics approach is typically 5–10 times faster than Galerkin's method, and most important, it provides physical insight into the scattering phenomeno

ISSN:0048-6604    

DOI:10.1029/RS018i005p00639

年代:1983

数据来源: WILEY

摘要:

The properties of an infinite flat plate antenna in a cold isotropic homogeneous plasma are investigated for the case in which the antenna is excited by means of a delta function voltage source across the infinitely long and narrow slit. Special attention was paid to the influence of an insulating vacuum layer between the antenna and the plasma. The far field radiation pattern and the power transport into the plasma and along the plasma‐vacuum boundary are analyze

ISSN:0048-6604    

DOI:10.1029/RS018i005p00651

年代:1983

数据来源: WILEY

摘要:

The horizontal circular loop and the coaxial array of loops above a material half space are studied as antennas for directive transmission into the half space. In a practical situation the loops might be located in air with the directive transmission into the earth. In determining the optimum geometry for the single loop and the array, the far‐zone field patterns and directivities of these antennas when placed over lossless dielectrics are considered first. The directive properties for the lossless dielectric are found to be indicative of those for the same antenna over a medium with low loss when proper account is taken of the exponential attenuation experienced in the lossy medium. Parametric studies are used to obtain the maximum directivities for these antennas. For the single loop of resonant size, the optimum height over the interface is determined, and for the two‐element array consisting of a driven loop of resonant size with a single parasite, the optimum size and spacing of the parasitic reflector are found. Measured electric field patterns and gains of model antennas above an interface between air and fresh water are in good agreement with the theoretical resu

ISSN:0048-6604    

DOI:10.1029/RS018i005p00664

年代:1983

数据来源: WILEY

摘要:

As a synthetic aperture radar scans different portions of a rough surface, the direction of the unit vector normal to the mean surface of the effective illuminated area (resolution cell) fluctuates. In this paper the modulation of the like and cross polarized scattering cross sections of the resolution cell are determined as the normal to it tilts in planes that are in and perpendicular to the fixed reference plane of incidence. By using the full wave approach, the scattering cross sections are expressed as a weighted sum of two cross sections. The first cross section is associated with scales of roughness within the effective illuminated area that are large compared to the radar wavelength, and the second cross section is associated with small‐scale spectral components within the resolution cell. Thus both specular point scattering and Bragg scattering are accounted for in a self‐consistent manner. The results are compared with earlier solutions based on first‐order Bragg scattering t

ISSN:0048-6604    

DOI:10.1029/RS018i005p00675

年代:1983

数据来源: WILEY

摘要:

A model with four layered media is developed for the calculation of the radio loss in forests. In this model two lossy dielectric layers placed over a conducting flat earth are used to represent the crown and the trunk of the trees. Dyadic Green's functions in their eigenfunction expansion forms are used to analyze this model, and the electromagnetic fields are obtained from these functions. The theoretical results of the radio loss for vertical and horizontal radiating dipoles and for typical forests agree with the available experimental data. For this comparison the parameters for the layer representing the crown of the trees were estimated.

ISSN:0048-6604    

DOI:10.1029/RS018i005p00691

年代:1983

数据来源: WILEY

摘要:

This paper concerns an estimation algorithm of rain rates for a dual‐frequency radar at rain‐attenuating bands. The estimations are made based on the concept of least squares. In the algorithm, the ratios of received echo powers at adjacent range bins at each frequency are used to eliminate some unknown and/or undetermined factors, such as radar calibration factors. The algorithm, which is applicable when a part of echoes at one frequency is missing due to rain attenuation, is also developed. In the estimation algorithm, the relationship between specific attenuation and rain rate (k‐Rrelationship), and that between effectiveZfactor and rain rate (Z‐Rrelationship), are assumed to be expressed by the usual power law form. Thek‐Rrelationships are fixed; however, theZ‐Rrelationships are expressed by the least squares regression form in terms of the measured quantities and the rain rates. The results of computer simulation experiments are described to demonstrate the validity of the presen

ISSN:0048-6604    

DOI:10.1029/RS018i005p00697

年代:1983

数据来源: WILEY

摘要:

A well‐known rigorous technique proposed by Lin for predicting rain attenuation statistics for terrestrial links has been modified in order to apply it to 5‐min point rainfall long‐term distributions and to include the case of earth‐space links. This same technique leads to the construction of an effective predictive model for the rain depolarization statistics. Comparison of the so‐obtained theoretical results with experimental data has been quite en

ISSN:0048-6604    

DOI:10.1029/RS018i005p00709

年代:1983

数据来源: WILEY

摘要:

This paper summarizes measurements of ice depolarization made using a unique facility consisting of a dual‐site (7.3‐km site spacing), dual‐circularly polarized, low‐elevation angle (10.7°) receiving system for the SIRIO 11.6‐GHz beacon. Significant depolarization (cross‐polar discrimination as low as 10 dB) in the absence of attenuation occurred relatively frequently. However, the impact of the ice on the cross‐polar discrimination statistics for a 6‐month time

ISSN:0048-6604    

DOI:10.1029/RS018i005p00720

年代:1983

数据来源: WILEY

摘要:

A ray‐tracing program based on the Haselgrove equations and the Appleton‐Hartree formula has been developed and used to synthesize sweep‐frequency ionograms. We have looked at ionograms taken in summer periods at high‐latitude ionosonde stations in the vicinity of the dayside auroral oval. Especially the traces on ionograms from oblique reflections from an electron density enhancement have been studied. The enhanced ionization is simulated either by a two‐dimensional Gaussian distribution or by a double exponential function similar to the bottom part of a Chapman function. The enhancements are aligned with the earth's magnetic field direction. Ray paths in the magnetic meridian plane are calculated and discussed in connection with computed ionograms and observed ionograms from Canada and Greenland. The ray‐tracing exercise was done in order to estimate characteristics of the electron density enhancement from observed ionograms. By varying the parameters in the models of the ionosphere to obtain agreement between observed and synthesized ionograms we have been able to analyze temporal and spatial progress of an electron density enhancement that could be a feature of the auroral oval like the field‐aligned current sheets in th

ISSN:0048-6604    

DOI:10.1029/RS018i005p00725

年代:1983

数据来源: WILEY

摘要:

Some experimental data concerning amplitude and phase of two waves, one on 16 kHz and the other on 60 kHz, emitted in Rugby and received in Turin, are first presented. The amplitudes are affected by almost sinusoidal undulations; the oscillations of the 60‐kHz amplitudes are synchronous with those of the 16‐kHz amplitudes; furthermore, when the 60‐kHz amplitude increases, the 16‐kHz amplitude decreases in a specular fashion. This trend is explained in terms of a cross modulation imposed by traveling gravity waves that progressively modulate the ionization profile in the nocturnalDregion of the ionosphere and hence affect the absorption of the VLF and LF waves. From this phenomenon, information about the lower tail of the nocturnal ionosphericDregion can be o

ISSN:0048-6604    

DOI:10.1029/RS018i005p00738

年代:1983

数据来源: WILEY