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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Swept frequency technique for dispersion measurement of microstrip lines

Microstrip lines used in microwave integrated circuits are dispersive. Because a microstrip line is an open structure, the dispersion can not be derived with pure TEM, TE, or TM mode analysis. Dispersion analysis has commonly been done using a spectral domain approach, and dispersion measurement has been made with high Q microstrip ring resonators. Since the dispersion of a microstrip line is fully characterized by the frequency dependent phase velocity of the line, dispersion measurement of microstrip lines requires the measurement of the line wavelength as a function of frequency. In this paper, a swept frequency technique for dispersion measurement is described. The measurement was made using an automatic network analyzer with the microstrip line terminated in a short circuit. Experimental data for two microstrip lines on 10 and 30 mil Cuflon substrates were recorded over a frequency range of 2 to 20 GHz. Agreement with theoretical results computed by the spectral domain approach is good. Possible sources of error for the discrepancy are discussed.

Lee, R. Q.↗
NASA NTRS1987

Microwave dielectric behavior of vegetation material

The microwave dielectric behavior of vegetation was examined through the development of theoretical models involving dielectric dispersion by both bound and free water and supported by extensive dielectric measurements conducted over a wide range of conditions. The experimental data were acquired using an open-ended coaxial probe that was developed for sensing the dielectric constant of thin layers of materials, such as leaves, from measurements of the complex reflection coefficient using a network analyzer. The probe system was successfully used to record the spectral variation of the dielectric constant over a wide frequency range extending from 0.5 to 20.4 GHz at numerous temperatures between -40 to +40 C. The vegetation samples were measured over a wide range of moisture conditions. To model the dielectric spectrum of the bound water component of the water included in vegetation, dielectric measurements were made for several sucrose-water solutions as analogs for the situation in vegetation. The results were used in conjunction with the experimental data for leaves to determine some of the constant coefficients in the theoretical models. Two models, both of which provide good fit to the data, are proposed.

Elrayes, Mohamed A.↗
NASA NTRS1988

Material parameter measurements at high temperatures

Alternate fixtures of techniques for the measurement of the constitutive material parameters at elevated temperatures are presented. The technique utilizes scattered field data from material coated cylinders between parallel plates or material coated hemispheres over a finite size groundplane. The data acquisition is centered around the HP 8510B Network Analyzer. The parameters are then found from a numerical search algorithm using the Newton-Ralphson technique with the measured and calculated fields from these canonical scatters. Numerical and experimental results are shown.

Dominek, A.↗
NASA NTRS1988

Measurement of the properties of lossy materials inside a finite conducting cylinder

Broadband, swept frequency measurement techniques were investigated for the evaluation of the electrical performance of thin, high temperature material coatings. Reflections and transmission measurements using an HP8510B Network Analyzer were developed for an existing high temperature test rig at NASA Lewis Research Center. Reflection measurements will be the initial approach used due to fixture simplicity even though surface wave transmission measurements would be more sensitive. The minimum goal is to monitor the electrical change of the material's performance as a function of temperature. If possible, the materials constitutive parameters, epsilon and muon will be found.

Dominek, A.↗
NASA NTRS1989

Testing Fixture For Microwave Integrated Circuits

Testing fixture facilitates radio-frequency characterization of microwave and millimeter-wave integrated circuits. Includes base onto which two cosine-tapered ridge waveguide-to-microstrip transitions fastened. Length and profile of taper determined analytically to provide maximum bandwidth and minimum insertion loss. Each cosine taper provides transformation from high impedance of waveguide to characteristic impedance of microstrip. Used in conjunction with automatic network analyzer to provide user with deembedded scattering parameters of device under test. Operates from 26.5 to 40.0 GHz, but operation extends to much higher frequencies.

Romanofsky, Robert↗
NASA NTRS1988

Measurement of the properties of lossy materials inside a finite conducting cylinder

A computer code was developed to automatically perform swept frequency reflection and transmission measurements using a HP5510B Network Analyzer and computer. This software is used in conjunction with a modified high temperature test rig to obtain reflection measurements from a flat material sample. The software allows data processing to eliminate measurement errors and to obtain a reflection coefficient in the frequency or time domain. A description of the program is presented.

Caldecott, R.↗
NASA NTRS1988

Technique for measuring the dielectric constant of thin materials

A practical technique for measuring the dielectric constant of vegetation leaves and similarly thin materials is presented. A rectangular section of the leaf is placed in the tranverse plane in a rectangular waveguide and the magnitude and phase of the reflection coefficient are measured over the desired frequency band using a vector network analyzer. By treating the leaf as an infinitesimally thin resistive sheet, an explicit expression for its dielectric constant is obtained in terms of the reflection coefficient. Because of the thin-sheet approximation, however, this approach is valid only at frequencies below 1.5 GHz. To extend the technique to higher frequencies, higher order approximations are derived and their accuracies are compared to the exact dielectric-slab solution. For a material whose thickness is 0.5 mm or less, the proposed technique was found to provide accurate values of its dielectric constant up to frequencies of 12 GHz or higher. The technique was used to measure the 8 to 12 GHz dielectric spectrum for vegetation leaves, teflon and rock samples.

Sarabandi, K.↗
NASA NTRS1992

Semi-coupled flow and thermal analysis of the field joint during rapid pressurization of the redesigned solid rocket motor

A transient, semi-coupled, multi-dimensional thermal and flow analysis methodology was developed to predict the thermal/gas dynamic conditions in the field joint region of the Redesigned Solid Rocket Motor (RSRM). Transient temperature response, pressure history, and combustion gas flow rate (within the field joint region), were of principle interest, in the course of this study. The thermal environment in the field joint was modeled using SINDA, a finite difference based thermal network analyzer. The combustion gas flow boundary condition was generated using the FLAP code; this code performs a transient, lumped-parameter, control volume analysis to solve the mass, momentum, and energy conservation equations. The FLAP computer code was modified to account for erosion of the NBR insulation material, following ignition. An independent grid sensitivity study was conducted to determine an appropriate grid distribution near the wall. The predicted results, obtained using an optimum grid distribution and computer generated flow boundary condition, were compared with subscale test data.

Ghaffarian, Benny↗
NASA NTRS1992

Anomalous capacitance of quantum well double-barrier diodes

The S-parameters of several different quantum well double barrier diodes have been measured. A technique has been developed for measuring whisker contacted diodes with an HP 8510B automatic network analyzer. Special coaxial mounts using K-connectors were designed to enable measurements up to 20 GHz. The voltage-dependent conductance and capacitance were derived from the measured reflection coefficient of each device. The C/V characteristics were observed to exhibit an anomalous increase at voltages corresponding to the negative differential resistance region (NDR). These are the first reported S-parameter measurements in the negative differential resistance region of quantum well double barrier diodes. A theory is presented that explains, in part, the observed results.

Boric, Olga↗
NASA NTRS1992

L-band radar scattering from grass

A radar system based on a network analyzer has been developed to study the backscatter from vegetation. The radar is operated at L-band. Radar measurements of a grass field were made in 1991. The radar returns from the grass were measured at three incidence angles. Ground truth and canopy parameters such as blade and stem dimensions, moisture content of the grass and the soil, and blade and stem density, were measured. These parameters are used in a distorted Born approximation model to compute the backscatter coefficients from the grass layer. The model results are compared with the radar data.

Chauhan, N.↗
NASA NTRS1992

Electromagnetic scattering from microwave absorbers - Laboratory verification of the coupled wave theory

W-band measurements of the bistatic scattering function of some common microwave absorbing structures, including periodic wedge-type and pyramid-type iron-epoxy calibration loads and flat carbon-foam 'Echosorb' samples, were made using a network analyzer interface to a focused-lens scattering range. Swept frequency measurements over the 75-100 GHz band revealed specular and Bragg reflection characteristics in the measured data.

Gasiewski, A. J.↗
NASA NTRS1994

Upgraded Near-Field Antenna-Testing System

Upgraded system for near-field testing of large microwave antenna built around commercial automated network analyzer (HP8510) designed for measurement and characterization of microwave circuits and components. Contains highly capable microwave receiver also suitable for far-field and radar-cross-section measurements. Receiver operates in fast-data-acquisition mode at rate of one new data point every millisecond without averaging. Also operates with greater sensitivity by use of averaging feature at time penalty of 0.2 ms per average. Time-domain option added to analyzer enables it to perform time-domain reflectometry.

Kunath, Richard R.↗
NASA NTRS1993

High temperature electromagnetic characterization of thermal protection system tile materials

This study investigated the impact of elevated temperatures on the electromagnetic performance of the LI-2200 thermal protection system. A 15-kilowatt CO2 laser was used to heat an LI-2200 specimen to 3000 F while electromagnetic measurements were performed over the frequency range of l9 to 21 GHz. The electromagnetic measurement system consisted of two Dual-Lens Spot-Focusing (DLSF) antennas, a sample support structure, and an HP-8510B vector network analyzer. Calibration of the electromagnetic system was accomplished with a Transmission-Reflection-Line (TRL) procedure and was verified with measurements on a two-layer specimen of known properties. The results of testing indicated that the LI-2200 system's electromagnetic performance is slightly temperature dependent at temperatures up to 3000 F.

Heil, Garrett G.↗
NASA NTRS1994

Cryogenic probe station for use in automated microwave and noise figure measurements

A cryogenic measurement system capable of performing on-wafer RF testing of semiconductor devices and circuits has been developed. This 'CryoProbe Station' can wafer-probe devices and circuits at cryogenic temperatures, thus eliminating the need for wire bonds. The system operates under vacuum created by a sorption pump. It uses an open cycle cooling system that can be cooled with either liquid nitrogen or liquid helium. Presently, it can reach temperatures, as low as 80 K and 37 K for each of the coolants, respectively. The temperature can be raised using a heater and it is stabilized to within 0.2 K by use of a temperature controller. The CryoProbe Station features a 1 by 2 inch stage that can hold large circuits and calibration standards simultaneously. The system is used with a Hewlett Packard 8510C Automatic Network Analyzer (ANA) to obtain S-parameter data over the frequency range 0.045-26.5 GHz. S-parameter data on HEMT (high electron mobility transistors) devices has been obtained with this station. With the use of DEEMBED software from NIST, detailed transmission line studies have been performed. Although the CryoProbe Station is designed for frequencies up to 26.5 GHz, useful transmission line data has been obtained for frequencies as high as 40 GHz. The CryoProbe station has also been used with the ATN noise figure measurement system to perform automatic, temperature dependent noise figure measurements.

Taub, Susan R.↗
NASA NTRS1995

Broadband Uniplanar Microstrip to Slot-Line Transitions

New in line uniplanar microstrip-to-slotline transitions for MIC/MMIC and phased array slotline antenna applications are described. Such transactions are compact and suitable to be used in an open environment or inside a package or a multichip module. The transitions share the concept of using a balun which consists of two microstrip lines connected to a slotline through a pair of coupled microstrips. The transitions are studied theoretically using the finite difference time domain (FDTD) technique and measured experimentally using an HP8510C Network Analyzer. For a back-to-back configuration, an insertion loss of less than 1.3 dB per transition is achieved over a 40% 3-dB bandwidth with a minimum of 0.6 dB at the design frequency.

Dib, Nihad I.↗
NASA NTRS1995

Application of FEM to estimate complex permittivity of dielectric material at microwave frequency using waveguide measurements

A simple waveguide measurement technique is presented to determine the complex dielectric constant of a dielectric material. The dielectric sample is loaded in a shorted x-band rectangular waveguide. Using a network analyzer; the reflection coefficient of the shorted waveguide (loaded with sample) is measured. Using the Finite Element Method (FEM), the exact reflection coefficient of the shorted waveguide (loaded with sample) is determined as a function of the dielectric constant. Matching the measured value of the reflection coefficient with the reflection coefficient calculated using the FEM utilizing the Newton-Raphson Method, an estimate of the dielectric constant of a dielectric material is obtained. A comparison of estimated values of dielectric constant obtained from simple waveguide modal theory and the present approach is presented.

Deshpande, M. D.↗
NASA NTRS1995

Reflectometer distance measurement between parallel conductive plates

This report presents an analytic and experimental investigation of the measurement problem in which a reflectometer is used to determine the distance to a target that is a highly conductive surface parallel to the reflectometer antenna ground plane. These parallel surfaces constitute a waveguide (WG) which can contribute parasitic perturbations that seriously degrade the accuracy of the measurements. Two distinct parallel-plate-waveguide (PPWG) phenomena are described, and their effects on both frequency and time-domain reflectometers are considered. The time-domain processing approach was found to be superior to a representative frequency-domain phase-measurement approach because of less susceptibility to perturbations produced by edge reflections and immunity to phase capture. Experimental results are presented which show that a simple radiating system modification can suppress parallel-plate (PP) propagation. The addition of a thin layer of lossy mu-metal 'magnetic absorber' to the antenna ground plane allowed a measurement accuracy of 0.025 cm (0.01 in.) when a vector network analyzer (VNA) is used as a time-domain reflectometer.

Hearn, Chase P.↗
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.↗