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At least 199 records · Page 11

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NASA NTRS1982

The radar cross section of dielectric disks

A solution is presented for the backscatter (nonstatic) radar cross section of dielectric disks of arbitrary shape, thickness and dielectric constant. The result is obtained by employing a Kirchhoff type approximation to obtain the fields inside the disk. The internal fields induce polarization and conduction currents from which the scattered fields and the radar cross section can be computed. The solution for the radar cross section obtained in this manner is shown to agree with known results in the special cases of normal incidence, thin disks and perfect conductivity. The solution can also be written as a product of the reflection coefficient of an identically oriented slab times the physical optics solution for the backscatter cross section of a perfectly conducting disk of the same shape. This result follows directly from the Kirchhoff type approximation without additional assumptions.

D M Le Vine↗

NASA NTRS1983

An experimental investigation of sound radiation from a duct with a circumferentially varying liner

The radiation of sound from an asymmetrically lined duct is experimentally studied for various hard-walled standing mode sources. Measurements were made of the directivity of the radiated field and amplitude reflection coefficients in the hard-walled source section. These measurements are compared with baseline hardwall and uniformly lined duct data. The dependence of these characteristics on mode number and angular location of the source is investigated. A comparison between previous theoretical calculations and the experimentally measured results is made and in general good agreement is obtained. For the several cases presented an asymmetry in the liner impedance distribution was found to produce related asymmetries in the radiated acoustic field.

Fuller, C. R.↗

NASA NTRS1983

Sound radiation from annular ducts/nozzles using modal decomposition of in-duct acoustic power

Using a refined acoustic impulse technique, an experimental program was carried out to determine the proper procedure for measuring the acoustic power for both incident and reflected sound fields for annular ducts with and without nozzles at various flow conditions. Several radial and azimuthal pressure measurements were made, and using these complex pressure data, the modal content of the pressure field was determined. A mathematical approach for the modal decomposition of annular duct acoustic power was developed. The acoustic power due to each individual mode and also their total sum were derived. The acoustic characteristics of annular duct-nozzle systems were studied at various flow conditions. The results derived from the experiments include the termination reflection coefficient, the normalized transmission coefficients, the power transfer functions and the power imbalance.

Salikuddin, M.↗

NASA NTRS1983

Spinning mode acoustic radiation from the flight inlet

A mathematical model was developed for spinning mode acoustic radiation from a thick wall duct without flow. This model is based on a series of experiments (with and without flow). A nearly pure azimuthal spinning mode was isolated and then reflection coefficients and far field pressure (amplitude and phase) were measured. In our model the governing boundary value problem for the Helmholtz equation is first converted into an integral equation for the unknown acoustic pressure over a disk, S1, near the mouth of the duct and over the exterior surface, S2, of the duct. Assuming a pure azimuthal mode excitation, the azimuthal dependence is integrated out which yields an integral equation over the generator C1 of S1 and the generator C2 of S2. The sound pressure on C1 was approximated by a truncated modal expansion of the interior acoustic pressure. Piecewise linear spline approximation on C2 was used.

Moss, W. F.↗

NASA NTRS1983

Scattering of electromagnetic waves from a half space of densely distributed dielectric scatterers

The scattering of a plane wave obliquely incident on a half space of densely distributed spherical dielectric scatterers is studied. The quasi-crystalline approximation is applied to truncate the hierarchy of multiple scattering equations, and the Percus-Yevick and the Verlet-Weis results are used to represent the pair distribution function. The coherent reflected wave is studied with these approximations. The incoherent scattered wave is calculated with the distorted Born approximation. In the low-frequency limit, closed-form expressions are obtained for the effective propagation constants, the coherent reflected wave, and the bistatic scattering coeficients. Results at higher frequencies are calculated numerically. The advantage of the present approach is that, in the low-frequency limit, it reproduces the effects of specular reflection, Fresnel reflection coefficient, Brewster angle, and Clausius-Mosotti relation. In addition to the classical results, the bistatic scattering coefficients are also calculated. The theory is also applied to match backscattering data from dry snow at microwave frequencies.

Tsang, L.↗

NASA NTRS1984

Propagation and radiation of sound from flanged circular ducts with circumferentially varying wall admittances. I Semi-infinite ducts. II - Finite ducts with sources

Sound propagation in infinite, semiinfinite, and finite circular ducts with circumferentially varying wall admittances is investigated analytically. The infinite case is considered, and an example demonstrates the effects of wall-admittance distribution on dispersion characteristics and mode shapes. An exact solution is obtained for the semiinfinite case, a circular duct with a flanged opening: sidelobe suppression and circumferential-mode energy scattering leading to radiated-field asymmetry are found. A finite duct system with specified hard-walled pressure sources is examined in detail, evaluating reflection coefficients, transmission losses, and radiated-field directivity. Graphs and diagrams are provided, and the implications of the results obtained for the design of aircraft-turbofan inlet liners are discussed.

Fuller, C. R.↗

NASA NTRS1984

Scan blindness in infinite phased arrays of printed dipoles

A comprehensive study of infinite phased arrays of printed dipole antennas is presented, with emphasis on the scan blindness phenomenon. A rigorous and efficient moment method procedure is used to calculate the array impedance versus scan angle. Data are presented for the input reflection coefficient for various element spacings and substrate parameters. A simple theory, based on coupling from Floquet modes to surface wave modes on the substrate, is shown to predict the occurrence of scan blindness. Measurements from a waveguide simulator of a blindness condition confirm the theory.

Pozar, D. M.↗

NASA NTRS1985

Venus Surface Composition from Radio/radar Measurements

Three different radio techniques were used to study the electrical properties of the surface of Venus. The reflectivity of the surface at near normal incidence was studied using radar from the Pioneer Venus Orbiter (PVO) spacecraft. These measurements have inferred the corresponding dielectric constants. Surface emissivities were calculated using measurements of thermal emission brightness temperature obtained both from spacecraft and from the ground. These measurements also have inferred dielectric constant. The degree of linear polarization associated with thermal energy emitted from the surface at various angles was examined. The dielectric constant was calculated. The radar data have provided the highest surface resolution of the three techniques, and were the first to show the unusually high values of Fresnel reflection coefficient (approaching 0.40 in extreme cases) that are associated with many of the elevated regions of Venus. The distribution of small scale surface roughness was estimated.

Pettengill, G. H.↗

NASA NTRS1985

Diffraction Analysis of Mesh Deployable Reflector Antennas

A formulation and many representative numerical results for mesh reflector antennas are presented. The reflection coefficient matrix for the prescribed mesh configuration was determined and the local coordinate system of the mesh cells at each point on the curved reflector surface was accentuated. A novel strip aperture model was used to formulate the transmission coefficient matrix for a variety of mesh cell configurations. Numerical data are tailored to the dimensions of a conceptually designed land mobile satellite system (LMSS) which employs a large mesh deployable offset parabolic antenna. Results are shown for an offset parabolic reflector with mesh surfaces similar to the mesh surface of tracking and data relay satellite system (TDRSS).

Rahmat-Samii, Y.↗

NASA NTRS1983

Acoustooptic pulse-echo transducer system

A pulse-echo transducer system which uses an ultrasonic generating element and an optical detection technique is described. The transmitting transducer consists of a concentric ring electrode pattern deposited on a circular, X-cut quartz substrate with a circular hole in the center. The rings are independently pulsed with a sequence high voltage signals phased in such a way that the ultrasonic waves generated by the separate rings superimpose to produce a composite field which is focused at a controllable distance below the surface of the specimen. The amplitude of the field reflected from this focus position is determined by the local reflection coefficient of the medium at the effective focal point. By processing the signals received for a range of ultrasonic transducer array focal lengths, the system can be used to locate and size anomalies within solids and liquids. Applications in both nondestructive evaluation and biomedical scanning are suggested.

Claus, R. O.↗

NASA NTRS1984

Dual-mode coaxial feed with low crosspolarisation

Design data for optimized TE(11)-TM(11) dual-mode coaxial prime focus feeds is presented. The optimized feeds have a crosspolarization below -30 dB, an input reflection coefficient below 0.1 and very little phase error in their radiation patterns, over a 5 percent bandwidth. Experimental patterns are also given that show that the theoretically derived design data can be relied upon. The optimized feeds are useful for paraboloidal reflectors with focal-length/diameter ratios in the range 0.3 to 0.5, and will give overall efficiencies as high as 70 percent or more with sidelobes below -30 dB.

Collin, R. E.↗

NASA NTRS1985

Dipole and slot elements and arrays on semi-infinite substrates

The printed dipole or slot antenna on a semi-infinite substrate and infinite phased arrays of these elements are investigated. The solution is based on the moment method in the Fourier transform domain. The generalized impedance or admittance matrix can be expressed in rapidly converging infinite-integral or infinite-summation forms, allowing the accurate determination of the current distributions. Using the present formulation, the input impedance, resonant length, and radiation pattern for the isolated antennas, and the reflection coefficient for infinite phased arrays, are calculated.

Kominami, M.↗

NASA NTRS1985

A study of the polarization of light scattered by vegetation

This study was undertaken in order to better understand the factors that govern the polarization of light scattered from vegetation and soils. The intensity and polarization of light scattered by clover and grass in vivo and soil were measured at a number of different angles of incidence and reflectance. Both individual leaves and natural patches of leaves were measured. The light transmitted through the leaves was found to be negatively polarized. The light scattered from the upper leaf surface was found to be positively polarized in a manner which could be accounted for qualitatively but not quantitatively by the Fresnel reflection coefficients modified by a shadowing function of the form cos sup2 (g/2), where g is the phase angle. Findings indicate that the polarization of light scattered by vegetation is a more complex process than previously thought, and that besides the surface-scattered component of light, the volume-scattered and multiply-scattered components also contribute significantly to the polarization.

Woessner, P. N.↗

NASA NTRS1985

On the electromagnetic scattering from infinite rectangular conducting grids

The study and development of two numerical techniques for the analysis of electromagnetic scattering from a rectangular wire mesh are described. Both techniques follow from one basic formulation and they are both solved in the spectral domain. These techniques were developed as a result of an investigation towards more efficient numerical computation for mesh scattering. These techniques are efficient for the following reasons: (a1) make use of the Fast Fourier Transform; (b2) they avoid any convolution problems by converting integrodifferential equations into algebraic equations; and (c3) they do not require inversions of any matrices. The first method, the SIT or Spectral Iteration Technique, is applied for regions where the spacing between wires is not less than two wavelengths. The second method, the SDCG or Spectral Domain Conjugate Gradient approach, can be used for any spacing between adjacent wires. A study of electromagnetic wave properties, such as reflection coefficient, induced currents and aperture fields, as functions of frequency, angle of incidence, polarization and thickness of wires is presented. Examples and comparisons or results with other methods are also included to support the validity of the new algorithms.

Christodoulou, C.↗

NASA NTRS1986

Large reflector antenna study

In some applications, the wires used to construct the grids are plated over with highly conducting materials such as gold or silver. In those cases, depending on the frequency of operation, the coating may not be thick enough to prevent currents from flowing in the substrate. The conjugate gradient method, in conjunction with the fast Fourier transform is employed to solve the problem of scattering from such rectangular grids. An internal impedance is utilized to account for the effects of the substrate conductivity on the induced current densities. Calculated values of the reflection coefficient and induced currents from different coating thicknesses, angles of incidence and polarizations are presented and discussed.

Christodoulou, C. G.↗

NASA NTRS1986

On the electromagnetic scattering from infinite rectangular grids with finite conductivity

A variety of methods can be used in constructing solutions to the problem of mesh scattering. However, each of these methods has certain drawbacks. The present paper is concerned with a new technique which is valid for all spacings. The new method involved, called the fast Fourier transform-conjugate gradient method (FFT-CGM), represents an iterative technique which employs the conjugate gradient method to improve upon each iterate, utilizing the fast Fourier transform. The FFT-CGM method provides a new accurate model which can be extended and applied to the more difficult problems of woven mesh surfaces. The formulation of the FFT-conjugate gradient method for aperture fields and current densities for a planar periodic structure is considered along with singular operators, the formulation of the FFT-CG method for thin wires with finite conductivity, and reflection coefficients.

Christodoulou, C. G.↗

NASA NTRS1985

Ultrasonic characterization of changes in viscoelastic properties of epoxy during cure

A technique for using the longitudinal velocity (LV) of an ultrasonic wave to monitor the extent of cross linking (CL) during the cure of thermosetting resins is described. The method was developed by monitoring the rate of change in LV during the cure of a bisphenol-A epoxy resin with an amine adduct. The experiment included variations in the temperature and stoichiometry in order to express the rate of change in terms of the reaction kinetics. The pulse-echo method was used with a single transducer operating at 20 MHz. Numerical models were defined to account for the acoustic response of a single layer, the attenuation and the reflection coefficient. A linear relationship was observed between the inverse of the temperature and the log of the rate of change in the velocity, supporting the theory that the velocity could be used to monitor the extent of the cross-linking reaction. An activation energy of 11.9 kcal/mole was calculated for the mixture being investigated.

Winfree, W. P.↗

NASA NTRS1986

Input impedance of a probe-fed circular microstrip antenna with thick substrate

A method of computing the input impedance for the probe fed circular microstrip antenna with thick dielectric substrate is presented. Utilizing the framework of the cavity model, the fields under the microstrip patch are expanded in a set of modes satisfying the boundary conditions on the eccentrically located probe, as well as on the cavity magnetic wall. A mode-matching technique is used to solve for the electric field at the junction between the cavity and the coaxial feed cable. The reflection coefficient of the transverse electromagnetic (TEM) mode incident in the coaxial cable is determined, from which the input impedance of the antenna is computed. Measured data are presented to verify the theoretical calculations. Results of the computation of various losses for the circular printed antenna as a function of substrate thickness are also included.

Davidovitz, M.↗