摘要:

It often happens that tropospheric ducts produced by elevated layers have a modal system in the microwave range consisting of many virtually lossless modes. These weakly attenuated modes have their energy localized about the layer and are insignificantly excited by sources far removed from the layer where the modal height gains are characteristically evanescent. Despite such an unfavorable geometry for direct excitation of these essentially loss‐free modes, they may be indirectly excited within the line of sight region by mode conversion processes associated with lateral inhomogeneity of the layer. In this paper an approximate theory is developed for the mode conversion produced within the line of sight region by large‐scale lateral inhomogeneity of an idealized, vertically homogeneous layer. The theory is applied to a 3‐GHz case study with the following results: (1) lateral inhomogeneity associated with layers of low elevation is more effective for converting to lossless modes than is lateral inhomogeneity associated with layers of high elevation; and (2) lateral inhomogeneity near the horizon is more effective than lateral inhomogeneity near the transmitter for converting to lossless

ISSN:0048-6604    

DOI:10.1029/RS017i002p00305

年代:1982

数据来源: WILEY

摘要:

A solution is obtained for the high‐frequency scattering by a pair of parallel wedges where the edge of one wedge is illuminated by the shadow boundary field of the other. A generalized spectral extension of the uniform GTD is used to determine the contribution from the doubly diffracted ray. Expressions for the diffracted field are given for plane, cylindrical, and spherical wave illuminations. Incidence oblique to the edges is also considered. These expressions reduce to a closed form at the shadow boundaries and outside the overlapping transition regions where the results coincide with those obtained by the uniform GTD augmented by slope diffraction. The solutions to the scalar problems are then used to derive a dyadic diffraction coefficient for the doubly diffracted field, in the ray‐fixed coordinate system. The general expressions presented here directly apply to the diffraction by half planes, strips, and joined wedges (thick edges) where the number of terms involved significantly redu

ISSN:0048-6604    

DOI:10.1029/RS017i002p00323

年代:1982

数据来源: WILEY

摘要:

A coupled mode analysis of a parallel plate wave guide with non‐uniform impedance boundaries is presented. Closed form solutions for the first order estimates of the modal amplitude coefficients are obtained assuming small variations. An analytical comparison with an exact mode‐matching solution in the case of a stepwise change of the lower wall impedance is given and verified numerically. As an extension of the method, a wave guide with perfectly conducting but rough walls is analyzed. The possibility of selective mode coupling is pointed out.Full article is available on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D. C. 20009. Document S81‐002; $1.00. Payment must accompany

ISSN:0048-6604    

DOI:10.1029/RS017i002p00337

年代:1982

数据来源: WILEY

摘要:

The so‐called moment equation for a successively forward scattered wave was derived under a definite condition. The condition shows that the applicability of the equation is much more extensive. When a selective summation technique is applied to the derivation of the moment equation, another condition besides the above is necessary for the existence of the equation. The extra condition is due to the selective summation of scattered waves and shows the validity of the technique for thenth moment of forward scattered waves. For example, if the waves propagate along thezaxis, a necessary condition for the technique to be valid for the first, second, and fourth moments isk2l2B(0, 0) ≪ 1 wherekis the wave number in free space andlandB(r,z) are the correlation length and the correlation function of fluctuation of the medium, respectively. For all the higher moments the technique is applicable only to the case ofB(r,z) =Br,(r)δ(z) where δ(z) is the Dirac delta fun

ISSN:0048-6604    

DOI:10.1029/RS017i002p00339

年代:1982

数据来源: WILEY

摘要:

In Doppler satellite positioning techniques the receiver is often equipped with a monopole antenna. The location of the phase center of this antenna is needed if high accuracy positioning is to be achieved. The phase center, however, changes with the direction of incidence of the received electromagnetic wave. In this paper the phase center is calculated for the idealized model of a monopole antenna with an infinitely large, perfectly conducting ground plane. The analysis is carried out by using the boundary integral equation method for determining the surface current density on the conducting parts of the configuration.

ISSN:0048-6604    

DOI:10.1029/RS017i002p00349

年代:1982

数据来源: WILEY

摘要:

The frequency responses of the receiving channels of a radio astronomy synthesis array and, in particular, the variations between them for different antennas are factors that limit the accuracy and sensitivity of the instrument. A response that approximates a rectangular passband as closely as possible is advantageous from considerations of maximizing sensitivity within a given frequency band and minimizing the smearing of detail in the outer regions of a map. Tolerances on slopes, ripples, and other bandpass distortions can be determined by examining their effects with regard to loss in sensitivity and the introduction of errors in the assignment of complex gain factors for individual antennas by using calibration observations. These calibration errors are generally larger in amplitude than in phase and their effect is usually more serious than the loss in sensitivity. Velocity dispersion in waveguide transmission systems and the accuracy in adjustment of compensating time delays also affect the phase responses of the signal channels, and examination of these effects also leads to limits on system design parameters. Implementation of the tolerances derived, including some details of the filter specifications, is discussed for the very large array radio telescope of the National Radio Astronomy Observatory. Appendices contain an algorithm for performing a least squares solution for antenna gains and an analysis of the variation of the signal‐to‐noise ratio over the channels of a spectral line correlator sys

ISSN:0048-6604    

DOI:10.1029/RS017i002p00357

年代:1982

数据来源: WILEY

摘要:

The effects of a necessarily limited transmission and reception bandwidth upon a pulsed coherent radar experiment can be partially overcome by the use of the deconvolution method described. Unlike ‘conventional’ techniques, an accurate knowledge of the radar system pulse response is not required to estimate the propagation delays of reflected pulses (echoes). However, a knowledge of the complex pulse response facilitates the simulation (Monte Carlo) of a ‘generalized’ echo structure model from which a deconvolution analysis can yield 95% confidence limit estimates for the calculated echo delays. Simulation results are presented showing dependence between the errors in delay estimates, the form of pulse response, and deconvolution control par

ISSN:0048-6604    

DOI:10.1029/RS017i002p00371

年代:1982

数据来源: WILEY

摘要:

The usual picture of the development of temperature and humidity boundary layers in a land air mass that moves offshore is shown to be very wrong under one type of Foehn condition in southern California, and it is probable that similar conditions can prevail in widespread areas around the globe, notably the Mediterranean Sea and the monsoonal regions of the Near East and Southeast Asia. A formalism is developed for analyzing the modification that seems to represent the observations satisfactorily, and graphical solutions for radio and optical ducting are given. It is shown that offshore modification can lead to elevated layers rather than to surface based layers, and the height of the layer base is theoretically predicted. Values of evaporation and heat flux into such an air mass are calculated, and the distance offshore at which dew point depression becomes zero is predicted. A method for measuring the downward heat flux in elevated inversion layers is described and results are given.

ISSN:0048-6604    

DOI:10.1029/RS017i002p00385

年代:1982

数据来源: WILEY

摘要:

The nighttime polarization fluctuations of linearly polarized 136 MHz satellite signals received at Ascension Island, located near the southern crest of the equatorial anomaly, have been shown to be the manifestation of depolarization effect due to the diffractive scattering by small‐scale (<200 m) density irregularities with power law spectra. The theory can explain its coexistence withLband scintillation. The absence of this phenomenon at equatorial locations off the anomaly crests is attributed to two factors: (1) the ambient plasma densities are relatively low, and (2) the propagation angles of satellite signals are more nearly perpendicular to the geomagnetic fiel

ISSN:0048-6604    

DOI:10.1029/RS017i002p00399

年代:1982

数据来源: WILEY

摘要:

In this paper we consider an antenna consisting of an infinite plane metallic plate divided into two half planes by an infinitely narrow slit across which an alternating electric field of frequency ω is applied. The antenna is immersed in a cold nonmagnetized plasma. Between the homogeneous plasma and the antenna surface an ion sheath is assumed to be present. The fields are calculated for a given density profile of the electrons by a small signal theory. The electromagnetic energy radiated from the slit is determined for frequencies below the plasma frequency. It is shown that the radiated energy is completely absorbed in the sheath. The radiation admittance is calculated as a function of frequency, the sheath thickness, and the plasma density

ISSN:0048-6604    

DOI:10.1029/RS017i002p00411

年代:1982

数据来源: WILEY