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NASA NTRS1993

Maximum and minimum return losses from a passive two-port network terminated with a mismatched load

This article presents an analytical method for determining the exact distance a load is required to be offset from a passive two-port network to obtain maximum or minimum return losses from the terminated two-port network. Equations are derived in terms of two-port network S-parameters and load reflection coefficient. The equations are useful for predicting worst-case performances of some types of networks that are terminated with offset short-circuit loads.

Otoshi, T. Y.↗

NASA NTRS1992

Polarization ray tracing in anisotropic optically active media

Procedures for performing polarization ray tracing through birefringent media are presented in a form compatible with the standard methods of geometric ray tracing. The birefringent materials treated include the following: anisotropic optically active materials such as quartz, non-optically active uniaxial materials such as calcite, and isotropic optically active materials such as mercury sulfide or organic liquids. Refraction and reflection algorithms are presented which compute both ray directions and wave directions. Methods for computing polarization modes, refractive indices, optical path lengths, and Fresnel transmission and reflection coefficients are also specified.

Mcclain, Stephen C.↗

NASA NTRS1992

Improved instrumentation for monitoring the diurnal and seasonal cycles in the dielectric properties of forest canopies

The design and implementation of a dielectric measurement system that facilitates the automated and continuous in situ monitoring of the dielectric properties of several canopy constituents is presented. This system utilizes the same coaxial line reflection coefficient measurement technique as the portable dielectric probe (PDP) while incorporating several features that facilitate the automated monitoring of canopy dielectric properties. The new system is capable of continuously monitoring the dielectric properties of the canopy constituents in a near-simultaneous fashion. The implementation of a data logger as a user interface has increased the number of measurements that the instrument is able to store in memory while significantly improving system reliability.

Guerra, Abel G.↗

NASA NTRS1992

Dielectric measurement using an open ended coaxial line with an extended center conductor

Permittivity may be determined by measuring the complex reflection coefficient of an open-ended coaxial line placed in contact with a test sample. This method works best for liquid samples. For the measurement of non-liquid materials problems can arise. A perfect preparation is needed to provide a good contact between the tip and the dielectric medium. At times, the dimension of the sensor cannot be freely chosen, as is the case for the measurement of the dielectric constant of the xylem layer of a living tree. The influence of the installation of the sensor on a tree's water status must be minimized by using a small sensor. In such cases the capacitance cannot be optimized. By extending the center conductor of the open-ended coaxial line, some of these problems can be avoided. This provides an additional tool to tune the capacitance of the sensor by adjusting the length of the extension. Therefore the measurement accuracy can be optimized. The sensor also becomes sensitive to a larger volume. A comparative study of a flush and extended tipped probes shows that the ability to measure the dielectric constant of trees has been notably increased due to the extension of the center conductor.

Wegmueller, Urs↗

NASA NTRS1993

Enhancement effects in polarimetric radar returns: Phase difference statistics

The probability density functions (pdfs) of the co- and cross-polarized phase differences are derived for backscatter from vegetation using the coherent and incoherent scattering theories. Unlike previous derivations, no assumptions or observations other than the applicability of the Central Limit Theorem (CLT), the low fractional volume of the medium, the reciprocity of the scatterers, and the azimuthal symmetry of the scatterer's orientation statistics are employed. Everything else follows logically via the mathematics. The difference between the coherent theory and the incoherent theory is referred to as the backscatter enhancement effect. The influence of this enhancement effect on the phase difference pdfs is examined and found to be important under combined conditions of scatterer anisotropy and appropriate reflection coefficient values.

Lang, R. H.↗

NASA NTRS1993

Radar response of vegetation: An overview

This document contains a number of viewgraphs on surface and vegetation backscattering. A classification of vegetation based on general scattering properties is presented. Radar scattering mechanisms are discussed, and backscattering and reflection coefficients for soil back scattering models are given. Radar response to vegetation is presented, with the objectives to discriminate and classify vegetation; to estimate biomass, leaf area index (LAI), and soil moisture; and to monitor changes, including deforestation and growth. Both theory and observation (laboratory, field, air SAR, and European Remote Sensing Satellite (ERS-1) observations) are used to present backscatter coefficients and other data for various vegetation types. ERS-1 results include class statistics, comparison with theory, and biomass response and seasonal variation (LAI) for deciduous and coniferous forests.

Ulaby, Fawwaz T.↗

NASA NTRS1993

The Brewster angle effect in SAR polarimetry

For the double bounce case, where the radar signal is reflected twice before returning to the radar antenna, some polarization effects may be observed related to the dielectric constant of the two surfaces causing the reflections. The most noticeable effect would be that the returned signal would be preferentially H polarized. In fact, it may be possible to discern the Brewster angle for both surfaces. The locations of the Brewster angle will depend on the dielectric constant and permittivity of each surface. If it is assumed that both reflections are in the same plane of incidence, and that both surfaces are smooth and flat, there is a straightforward relationship between the degree of linear polarization m and both the dielectric constants of the two reflecting surfaces and the angle of incidence of the illuminating wave: m carat = cos 2(arccot (square root of (R(sub v) / R(sub h)))) where R(sub v,h) are the V and H polarized Fresnel reflection coefficients for two surfaces perpendicular to each other. The degree of linear polarization may be calculated from AIRSAR compressed Stokes data and compared with the given equation. The degree of linear polarization may also be calculated using tree models and compared with AIRSAR data. With further work, it may be possible to use the degree of linear polarization to determine surface parameters of certain imaged areas.

Chapman, B.↗

NASA NTRS1994

An electromagnetic noncontacting sensor for thickness measurement in a dispersive medium

This paper describes a general purpose imaging technology developed by the U.S. Bureau of Mines (USBM) that, when fully implemented, will solve the general problem of 'seeing into the earth.' A first-generation radar coal thickness sensor, the RCTS-1, has been developed and field-tested in both underground and highwall mines. The noncontacting electromagnetic technique uses spatial modulation created by moving a simple sensor antenna in a direction along each axis to be measured while the complex reflection coefficient is measured at multiple frequencies over a two-to-one bandwidth. The antenna motion imparts spatial modulation to the data that enables signal processing to solve the problems of media, target, and antenna dispersion. Knowledge of the dielectric constant of the media is not necessary because the electrical properties of the media are determined automatically along with the distance to the target and thickness of each layer of the target. The sensor was developed as a navigation guidance sensor to accurately detect the coal/noncoal interface required for the USBM computer-assisted mining machine program. Other mining applications include the location of rock fractures, water-filled voids, and abandoned gas wells. These hazards can be detected in advance of the mining operation. This initiating technology is being expanded into a full three-dimensional (3-D) imaging system that will have applications in both the underground and surface environment.

Chufo, Robert L.↗

NASA NTRS1993

PSIDD: A Post-Scan Interactive Data Display System for Ultrasonic Scans

An ultrasonic data display system was developed at NASA Lewis Research Center that allows the user to interactively examine digitized waveforms and processed information associated with any specific scan location of an ultrasonic contact scan. This information is displayed on a video display monitor and includes acquired time-domain waveforms, frequency-domain magnitude and phase spectra, and ultrasonic properties (pulse velocity, phase velocity, reflection coefficient, attenuation coefficient, attenuation coefficient error) as a function of frequency for a material. This report describes the system features and illustrates the system's usefulness for nondestructive materials characterization.

Roth, Don J.↗

NASA NTRS1995

Linearly tapered slot antenna impedance characteristics

The paper presents for the first time an experimental technique to de-embed the input impedance of a LTSA from the measured reflection coefficient. The results show that the input impedance is dependent on the semi-flare angle and the length of the LTSA. The Re(Z(sub in)) is large when the electrical length of the LTSA is small and is on the order of few thousand ohms. However for an electrically large LTSA the Re(Z(sub in)) is in the range of 55 to 130 ohms. These results have potential applications in the design of broad band impedance matching networks for LTSA.

Simons, Rainee N.↗

NASA NTRS1995

The determination of the constitutive parameters of a medium with application to a reinforced concrete pad

This report describes the experimental and analytical efforts performed to determine the constitutive parameters of a reinforced concrete pad on which an aircraft (the NASA Boeing 757) was parked while its internal electromagnetic environment was measured. This concrete pad is part of the Large Electromagnetic System-Level Illuminator (LESLI) test facility at the Phillips Laboratory, Kirtland Air Force Base, New Mexico. The relative dielectric constant, conductivity, index of refraction, and reflection coefficient have been determined over the frequency range of 0 to 300 MHz and are presented.

Poggio, A. J.↗

NASA NTRS1994

Studies of microwave scattering and canopy architecture for boreal forests

Our primary objectives during the last year have been to develop a helicopter-borne radar system for measuring microwave backscatter from vegetation and to use this system to study the characteristics of backscatter from the boreal forest. Our research is aimed at refining the current microwave models and using these improvements for more accurate interpretation of SAR data. SAR data are very useful for monitoring the boreal forest region because of the microwave signal's ability to penetrate clouds and to see at night. Meeting these objectives involves several stages of development. The first stage is the design and implementation of a frequency-modulated continuous-wave (FM-CW) radar system with the capability of measuring backscatter at three frequencies and four polarizations at each frequency. These requirements necessitate a twelve-channel radar system. Using three frequencies is advantageous because it allows us to look at different parts of the canopy. For instance, the lower frequency signal penetrates deeper into the canopy and allows us to see the ground while the high frequency signal is scattered more by the leaves and needles and typically does not penetrate to the floor of the forest. We designed the radar starting with the antenna system. We then developed the intermediate frequency (IF) and radio frequency (RF) sections of the radar. Also, the need to collect data from twelve channels during each flight line presented a complex data acquisition problem that we solved by using a high-speed data acquisition board. After construction, the radar was tested at the lab. We performed extensive testing of the IF and RF systems of the radar during this time. Once we were satisfied with the operation of the radar it was shipped to Canada for use in the second intensive field campaign (IFC-2) from July 16 - August 8, 1994. During IFC-2, we collected backscatter data over the experimental sites in the southern study area (SSA). Additionally, we used a ground-based step-frequency radar to measure the reflection coefficient of the forest floor at the old jack pine (OJP) and young jack pine (YJP) sites. The ground-based radar data have been processed and a frew examples are included in this report. we are currently processing the helicopter-borne radar data.

Lockhart, G. Lance↗

NASA NTRS1994

Kinetic aspects of reconnection at the magnetopause

Observations presented here support the kinetic (or single particle) description of reconnection where ions interacting with the magnetopause conserve their pitch angles or change them by equal amounts as in adiabatic motion. These observations include ion reflection and transmission at the magnetopause and time of flight effects associated with the magnetopause layers, with an emphasis here on ion reflection. Velocities of the reflected distributions predicted from this kinetic description are in good agreement with observed velocities. However, predicted velocities for the transmitted distributions are often higher than observed ones. Reflected distributions are also heated at the magnetopause; however, this heating is less important than the large scale ion motion. Reflection coefficients at the magnetopause are high (averaging 30%), appear to be the same on either side of the magnetopause, and have little or no dependence on ion mass. Time of flight effects result from the finite extent of the reconnection layers and are best observed at the edges of the layers.

Fuselier, Stephen A.↗

NASA NTRS1994

A feasibility study for long-path multiple detection using a neural network

Least-squares inverse filters have found widespread use in the deconvolution of seismograms and the removal of multiples. The use of least-squares prediction filters with prediction distances greater than unity leads to the method of predictive deconvolution which can be used for the removal of long path multiples. The predictive technique allows one to control the length of the desired output wavelet by control of the predictive distance, and hence to specify the desired degree of resolution. Events which are periodic within given repetition ranges can be attenuated selectively. The method is thus effective in the suppression of rather complex reverberation patterns. A back propagation(BP) neural network is constructed to perform the detection of first arrivals of the multiples and therefore aid in the more accurate determination of the predictive distance of the multiples. The neural detector is applied to synthetic reflection coefficients and synthetic seismic traces. The processing results show that the neural detector is accurate and should lead to an automated fast method for determining predictive distances across vast amounts of data such as seismic field records. The neural network system used in this study was the NASA Software Technology Branch's NETS system.

Feuerbacher, G. A.↗

NASA NTRS1995

Optimal Signal Processing of Frequency-Stepped CW Radar Data

An optimal signal processing algorithm is derived for estimating the time delay and amplitude of each scatterer reflection using a frequency-stepped CW system. The channel is assumed to be composed of abrupt changes in the reflection coefficient profile. The optimization technique is intended to maximize the target range resolution achievable from any set of frequency-stepped CW radar measurements made in such an environment. The algorithm is composed of an iterative two-step procedure. First, the amplitudes of the echoes are optimized by solving an overdetermined least squares set of equations. Then, a nonlinear objective function is scanned in an organized fashion to find its global minimum. The result is a set of echo strengths and time delay estimates. Although this paper addresses the specific problem of resolving the time delay between the two echoes, the derivation is general in the number of echoes. Performance of the optimization approach is illustrated using measured data obtained from an HP-851O network analyzer. It is demonstrated that the optimization approach offers a significant resolution enhancement over the standard processing approach that employs an IFFT. Degradation in the performance of the algorithm due to suboptimal model order selection and the effects of additive white Gaussion noise are addressed.

Ybarra, Gary A.↗

NASA NTRS1996

Computing Radiation From Axisymmetric Waveguide Feed Horns

Hybrid finite-element method developed for computing radiation patterns and reflection coefficients of axisymmetric waveguide feed horns that include dielectric bodies like rods and radomes. Dielectric inhomogeneities introduced by such bodies and even anisotropies of some dielectric materials pose no special problem in method because requisite dielectric properties readily incorporated into finite elements used to model bodies.

Chinn, Gilbert C.↗

NASA NTRS1996

Reflection Spectrum of Two Level Atoms by an Evanescent Laser Wave

An exact solution and numerical calculation of the reflection of two level atoms by atomic mirror are presented. The curve of reflection coefficient against Rabi frequency calculated shows some new features, and the physical machanism underlying is analyzed.

Tan, Weihan↗

NASA NTRS1994

Waveguide-coupled resonator filters on AlN on silicon

In the effort to continually reduce the size and cost of wireless communications products the level of integration has improved dramatically in recent years. In order to reduce future generations of wireless systems to single chip form, there is a need for on-chip filtering capabilities. In this paper, the first report of an experimental waveguide-coupled resonator filter for cellular radio applications is presented. Measured results indicate a typical insertion loss of 26 dB at a center frequency of 132 MHz using a 2 um AlN film on (001) less than 110 greater than Si. In addition, a laser probe analysis has been conducted and a theoretical analysis of the first order reflection coefficients is presented.

Liaw, H. M.↗