Showing papers in "IEEE Transactions on Antennas and Propagation in 1977"

Abstract: The advent of precision microwave radiometry has placed a stringent requirement on the accuracy with which the dielectric constant of sea water must be known. To this end, measurements of the dielectric constant have been conducted at S -band and L -band with a quoted uncertainty of tenths of a percent. These and earlier results are critically examined, and expressions are developed which will yield computations of brightness temperature having an error of no more than 0.3 K for an undisturbed sea at frequencies lower than X -band. At the higher microwave and millimeter wave frequencies, the accuracy is in question because of uncertainties in the relaxation time and the dielectric constant at infinite frequency.

Abstract: This paper is concerned with the theoretical model for short pulse scattering from a statistically random planar surface with particular application to current state of the art radar altimetry, A short review of the assumptions inherent in the convolational model is presented. Simplified expressions are obtained for both the impulse response and the average backscattered power for near normal incidence under the assumptions common to satellite radar altimetry systems. In particular, it is shown that the conventional two-dimensional surface integration can be reduced to a closed form solution. Two applications of these results are presented relative to radar altimetry, namely, radar antenna pointing angle determination and altitude bias correction for pointing angle and surface roughness effects. It is also shown that these results have direct application to the analysis of the two frequency system proposed by Weissman, and a possible combined long pulse altimeter and two frequency system is suggested.

Abstract: A theory of characteristic modes for material bodies is developed using equivalent surface currents. This is in contrast to the alternative approach using induced volume currents. The mode currents form a weighted orthogonal set over the material body surface, and the mode fields form an orthogonal set over the sphere at infinity. The characteristic modes of material bodies have most of the properties of those for perfectly conducting bodies. Formulas for the use of these modes in electromagnetic scattering problems are given. A procedure for computing the characteristic modes is developed, and applied to two-dimensional bodies. Illustrative examples of file computation of characteristic currents and scattering cross sections are given for cylinders of different material constants.

Abstract: The resonant frequency is obtained in analytical form for a planar, circular disc antenna which is etched on a printed-circuit board so that the low-profile antenna is separated from the ground plane only by a thin layer of dielectric material. The formula is found to have an error of less than 2.5 percent when compared with experimental data.

Abstract: The propagation of radio waves is examined for communication paths that may be partly within a forest and partly in regions outside the vegetation. Analytic results are found for simple canonic geometries in which the fields can be described in terms of ray-trajectories. By viewing a realistic forest environment as a combination of such canonic cases, it is possible to evaluate radio losses in complex situations by using a ray-tracking approach. The pertinent fields can then be expressed in terms of relatively simple analytical expressions, which hold well for frequencies between 2 and 200 MHz.

Abstract: A procedure is given for designing a two-layer stripline log-periodic dipole (LPD) antenna array. Measured input impedance and far-field characteristics are demonstrated for a 12-element array for 1-2 GHz operation that are in good agreement with design specifications.

Abstract: Microwave scattering signatures of the ocean have been measured over a range of surface wind speeds from 3 m/s to 23.6 m/s using the AAFE RADSCAT scatterometer in an aircraft. Normalized scattering coefficients are presented for vertical and horizontal polarizations as a function of incidence angle (nadir to 55\deg ) and radar azimuth angle ( 0\deg to 360\deg ) relative to surface wind direction. For a given radar polarization, incidence angle, and azimuth angle relative to the wind direction, these scattering data exhibit a power law dependence on surface wind speed. The relation of the scattering coefficient to azimuth angle obtained during aircraft circles (antenna conical scans) is anisotropic and suggests that microwave scatterometers can be used to infer both wind speed and direction. These results have been used for the design of the Seasat-A Satellite Scatterometer (SASS) to be flown in 1978 on this first NASA oceanographic satellite.

Abstract: Corner reflectors are inadvertently formed by many surfaces on ships and military ground vehicles and can be dominant contributors to the radar cross section (RCS). The monostatic return from such structures can be reduced by ensuring that flat surfaces never meet at right angles. A criterion is developed that gives the required angle as a function of the RCS reduction desired and the electrical size of the corner faces.

Abstract: Given the true or any approximate current on a reflector, the radiated far-field is determined from a rapidly convergent series representation of the radiation integral. The leading term is a well-shaped J_{1}(x)/x beam pointing in a desired direction. Higher order terms provide perturbations to the leading term. The coefficients of the series are independent of the observation angles. Hence, once they are computed, the field may be determined very rapidly at large numbers of points. Initially, a suitable small angle approximation is made that places the radiation integral in the form of a Fourier transform on a circular disk. The theory is then extended such that the results are valid in both the near and the wide angle regions. Application to a rotationally symmetric paraboloid is presented herein. Other applications include the offset and dual reflectors and near- to far-field integrations. A modified form of the series can also be used for Fresnel zone computations.

Abstract: With the advent of satellite communications systems at frequencies varying from \sim140 MHz to 1600 MHz as well as navigation and ranging systems in the 1200-1600 MHz portions of the spectrum, the effect of equatorial irregularities on fading signals has become of importance. Recent observations of the signal statistics of scintillations at frequencies ranging from 136 MHz to 6 GHz reveals a power law fall off of irregularity sizes. Power spectra are now available for a variety of conditions and for frequencies from VHF to microwaves. During periods of intense equatorial activity, at frequencies to 360 MHz, Rayleigh scattering is frequently experienced. The latitudinal extent of the scintillation irregularity region has been established with a half occurrence width during years of moderate solar flux of plus and minus 12\deg . A correlation of in-situ measurements of irregularities from satellites had revealed the great variations in longitudinal patterns during any season. It has also allowed the \Delta N obtained from in-situ measurements to be utilized to predict scintillation excursions. New facets of scintillation activity in the equatorial region recently reported include weak daytime scintillation and the patchy nature of irregularities (small irregularities embedded in large structures) particularly noted during periods of low solar flux. Future studies will assist in the delineation of the extent of the equatorial region at frequencies from 1.5 to 6 GHz, and in the UHF range. From the viewpoint of the communicator the morphology of scintillations at microwaves has still not been reported from any long term program of measurements.

Abstract: A number of models which would explain ocean wave imagery taken with a synthetic aperture imaging radar are analyzed analytically and numerically. Actual radar imagery is used to support some conclusions. The models considered correspond to three sources of radar backscatter cross section modulation:tilt modulation, roughness variation, and the wave orbital velocity. The effect of the temporal changes of the surface structure, parametric interactions, and the resulting distortions are discussed.

Abstract: The basic integral equations for the electrostatic and magnestostatic problems are reviewed. These equations are solved with a digital computer for four typical aperture shapes. The results of the calculations are compared with Cohn's experimental values. Some data are given concerning the corresponding acoustic problems.

Abstract: Several important statistical properties of the HF sea echo and its Doppler power spectrum, which are useful in optimizing the design of radar oceanographic experiments, are established. First-and second-order theories show that the echo signal (e.g., the voltage) should be Gaussian; this is confirmed with experimental surface-wave data i) by comparison of the normalized standard deviation of the power spectrum at a given frequency with its predicted value of unity, and ii) by cumulative distributiun plots of measured spectral amplitudes on Rayleigh probability charts. The normalized standard deviation of the dominant absolute peak amplitudes of the power spectrum (which wander slightly in frequency) are shown from experimental data to be \sim 0.7 for the first-order peaks and \sim 0.5 for the second-order peaks. The autocorrelation coefficient of the power spectra is derived from measured data and interpreted in terms of the spectral peak widths; from this information, the correlation time (or time between independent power spectrum samples) is shown to be \sim 25-50 s for radar frequencies above 7 MHz. All of these statistical quantities are observed to be independent of sea state, scattering cell size, and relatively independent of radar operating frequency. These quantities are then used to establish the statistical error (and confidence interval) for radar remote sensing of sea state, and it is shown, for example, that 14 power spectral samples result in a sample average whose rms error about the true mean is 1.0 dB.

Abstract: A coherent, CW, dual-frequency, X -band radar was used to study microwave sea return from the Chesapeake Bay. It is shown that the product of the backscattered fields depends strongly on long surface wave properties. In particular, a sharp line is found in the product power spectrum whose frequency is that of the water wave whose wavelength is in resonance with the spatial period of the beat frequency between the two transmitted signals and whose wave vector is parallel to the horizontal line of sight. Thus, gravity wave dispersion relations can be obtained with the system. Furthermore, the degree of modulation of short waves by long ones is given by the intensity of the line. A broad background corresponding to the convolution of the single-frequency Doppler spectra is also seen in the product power spectrum. These results are shown to be interpretable by composite surface scattering theory.

Abstract: The use of an edge condition in moment method solutions is investigated. Using as an example TM scattering by a strip, various numerical schemes for treating the edge behavior are examined for low frequencies where an analytical solution of the quasi-static integral equation is available for comparison. The same numerical techniques are then applied in the general dynamic case. It is found that failure to incorporate the correct edge behavior in the current expansion can resuit in erroneous currents and anomalous behavior of the solution near edges. The use of ordinary pulses or triangle expansion functions with appropriately singular pulses at the edges circumvents these difficulties. In the case of pulses, a simple correction factor is derived which can be used a posteriori to correct the calculated edge current.

Abstract: We introduce a new approach for combining the integral equation and high frequency asymptotic techniques, e.g., the geometrical theory of diffraction. The method takes advantage of the fact that the Fourier transform of the unknown surface current distribution is proportional to the scattered far-field. A number of asymptotic methods are currently available that provide good approximation to this farfield in a convenient analytic form which is useful for deriving an initial estimate of the Fourier transform of the current distribution. An iterative scheme is developed for systematically improving the initial form of the high frequency asymptotic solution by manipulating the integral equation in the Fourier transform domain. A salient feature of the method is that it provides a convenient validity check of the solution for the surface current distribution by verifying that the scattered field it radiates indeed satisfies the boundary conditions at the surface of the scatterer. Another important feature of the method is that it yields both the induced surface current density and the far-field. Diffraction by a strip (two-dimensional problem) and diffraction by a thin plate (three-dimensional problem) are presented as illustrative examples that demonstrate the usefulness of the approach for handling a variety of electromagnetic scattering problems in the resonance region and above.

Abstract: High-frequency asymptotic solutions of electromagnetic edge diffraction by two different theories are studied. One is the uniform asymptotic theory which is a refinement of Keller's geometrical theory of diffraction. The other is Ufimtsev's theory of the edge wave, representing an improvedment over the classical physical optics theory. These two theories are summarized, their features compared, and their relations discussed. When the observation point is away from shadow boundaries and caustics, both uniform asymptotic theory and Ufimtsev's theory, up to (and including) order k^{-1/2} , agree with Keller's theory. Near or on shadow boundaries, uniform asymptotic theory gives an explicit field solution, while Ufimtsev's result contains a physical optics integral. The evaluation of that integral is not a trivial task. In a two-dimensional test problem, it is shown that both theories do give, up to order k^{-1/2} , an identical field solution everywhere including the edge, shadow boundaries, and transition regions.

Abstract: A spectral domain interpretation of high frequency diffraction phenomena is discussed using the concept of a spectral diffraction coefficient, which resembles Keller's coefficients. The solution of the two-dimensional problems of diffraction of an arbitrary field (with no caustics) by a half-plane is investigated and results are given for any observation angle including, in particular, asymptotic determination of the field at the shadow boundaries. The high frequency scalar diffraction by apertures is formulated in a systematic manner and the formulation, which is valid for any observation angle, is compared with that of other techniques. Results are also given for the diffracted field at the caustics.

Abstract: The microwave spectrum of oxygen (O 2 -MS) was investigated with a pressure-scanning dispersion spectrometer between 53.5 and 63.6 GHz under simulated atmospheric conditions. First, the strength and width parameters of 21 lines ( K^{\pm}= 1 to 25)were determined from low pressure ( K^{pm} = 25 to 35). Then, O 2 -MS intensities (dispersion and attenuation by oxygen and air) were measured between 100 and 800 torr and compared with Rosenkranz's band shape model. A set of interference coefficients was established to produce good agreement between experiment and theory. Also, at 61 GHz the refractivities of 13 atmospheric gases were checked. The improved calculation scheme of atmospheric molecular EHF ( \sim40 to 140 GHz) characteristics is presented as a transfer function incorporating a set of 144 line parameters (36 each of position, strength, width, and interference) with frequency, pressure and temperature dependencies.

Abstract: The electromagnetic field coupling through small apertures illuminated by an arbitrary incident plane wave is discussed for general aperture shapes. A set of new integral equations in a form highly amenable to numerical solution techniques is derived. Based on the application of the Rayleigh series method, an analytical solution is obtained for the first few terms of the expansion of the aperture E - field of a circular aperture. Numerical results are also constructed for the aperture field and the diffracted field of small rectangular apertures and compared with those of the circular apertures.

Abstract: The efficiency of an antenna combined with a matching network constructed from elements with finite Q is determined by applying the principle of conservation of energy. The conditions which must be satisfied to obtain maximum efficiency are examined. An analysis shows that for many electrically small antennas the conditions for maximum efficiency can be met using a simple L section as the matching network. Examples are presented to show the importance of including the matching network in efficiency calculations for electrically small antennas.

Abstract: A helical beam antenna with uniform conductor size has a nominal impedance of 140 ohms. This may not be convenient for some applications. The purpose of this note is to call attention to the fact that the impedance can be adjusted to 50 ohms by increasing the conductor size close to the feed point at the ground plane. This lowers the characteristic impedance of the conductor-ground plane combination (acting as a transmission line) and transforms the helix impedance to a lower value over a substantial bandwidth

Abstract: Microwave thermal emission from a layer of cloud or rain consisting of spherical particles has been investigated. Scattering effects are studied in great detail with both numerical and analytical approaches. In the absence of ground emission, it is found that scattering induces brightening for optically thin layers and vice versa for optically thick layers. As a function of observation angle brightening occurs near nadir while darkening occurs at large angles in the case of small optical thickness. For large optical thickness, darkening occurs at all angles because of backscattering effects. When the layer of cloud or rain is above an air layer and an ocean surface at a higher temperature, it is found that the darkening effect at large optical thickness is much more pronounced. The darkening effect is also larger for vertical polarizations because the ocean emits more vertically polarized components. The effect of thermal emission and molecular absorption by atmospheric gases is also taken into account. Results obtained from analytical formulas under single scatteirng assumptions are compared and illustrated.

Abstract: A model problem is formulated to evaluate the limitations imposed by the assumptions in the Wheeler or cap method for measuring the radiating efficiency of antennas. The antenna in the model is a circular loop and the radiation shield is a spherical metal shell. Calculated values of the actual efficiency of the antenna and the efficiency that would be measured using the Wheeler method are compared to determine the accuracy of the method and set guidelines for its application.

Abstract: Classical electromagnetic potential formulations are, with the exceptions of a few special cases of one-dimensional stratification, restricted to use in uniform media. A recently developed potential formulation that provides a flexible basis for numerical computation of time-harmonic field problems involving continuously and discretely inhomogeneous axially symmetric media is the topic of this paper. The formulation manifests itself in both a differential equation system and, alternately, a variational criterion. Typical numerical applications include solutions of scattering by arbitrarily shaped material bodies of revolution and radiation from inhomogeneously loaded rotationally symmetric antenna structures. Current numerical investigations by the authors, using Mei's unimoment method in conjunction with both finite-difference and finite-element techniques, have shown the formulation to be highly feasible for computation of field problems having dimensions as large as several wavelengths.

Abstract: A new method for applying continuous aperture design results to discrete antenna arrays is described. Instead of sampling the continuous aperture distribution, one requires that the pattern zeroes found in the continuous case also occur in the starting pattern for the discrete case. If the resulting pattern does not meet the design goal, a perturbation procedure is applied to the discrete distribution in order to bring the final pattern within specification. The approach is illustrated by two examples: 1) a 20 dB Taylor pattern, modified so that the innermost three sidelobes on one side of the main beam are down 30 dB and 2) a pattern in which the innermost seven sidelobes on one side of the main beam are at -25 dB, whereas on the other side they cascade in 5 dB steps from -45 dB next to the main beam to -15 dB for the seventh sidelobe.

Abstract: It is shown that the sidelobe level of circular arrays with a constant excitation amplitude can be reduced by adjusting the excitation phases of array elements. The resulting radiation pattern has equal-ripple sidelobes similar to a Chebyshev pattern. Examples are given.

Abstract: A general theory for the electromagnetic fields of dipoles in stratified isotropic media is outlined. The stratified model consists of a stack of layers sandwiched between two semi-infinite media. Either an electric or a magnetic dipole can be placed at any position in the stack, or in the upper or lower half-space. Dipoles can be electric or magnetic and can be oriented horizontally or vertically. The fields in the layer containing the source are given in terms of reflection coefficients, impedance and admittance terms, and wavenumber ratios. Recursion relations are developed to propagate the Hankel-domain field coefficients to other layers or to the half spaces. This allows the observation point to be placed anywhere except at the source. Numerical checks show that the derived algebra is at least self-consistent.

Abstract: The scattering properties of TM or TE illuminated lossy dielectric cylinders of arbitrary cross section are analyzed by the surface integral equation techniques. The surface integral equations are formulated via Maxwell's equations, Green's theorem, and the boundary conditions. The unknown surface fields on the boundaries are then calculated by flat-pulse expansion and point matching. Once the surface fields are found, scattered field in the far-zone and radar cross section (RCS) are readily determined. RCS thus obtained for circular homogeneous dielectric cylinders and dielectric coated conducting cylinders are found to have excellent agreements with the exact eigenfunction expansion results. Extension to arbitrary cross-sectioned cylinders are also obtained for homogeneous lossy elliptical cylinders and wedge-semicircle cross-sectioned cylinders, with and without a conducting cylinder in its center. RCS dependences on frequency and conductivity as well as the matrix stability problem of this surface integral equation method are also examined.

Abstract: Proof of the feasibility of aft mounting low-profile antennas on reentry vehicles is presented. This is an application of state of the art antenna hardware. Aft mounting presents the best reentry environment to antenna hardware, and its practicality has been demonstrated. Microstrip antenna principles can be used to produce very rugged antennas. Their low profile reduces many interface, mechanical, and survival problems. The pattern coverage for these vehicles must be toward the fore direction. This type of pattern requires an interaction between the vehicle and the antenna when the antenna is effectively shadowed from that direction. This paper describes the results obtained by actual experimental hardware. Contoured elements were used, as were foreshortened elements. These elements demonstrated ability to be fit around other aft equipment. The versatility of these elements were further demonstrated by tailoring them for two different types of coverage to accommodate different vehicle operational requirements.