Arthur Ondrejka

Aperture excitation of electrically large, lossy cavities

We present a theory based on power balance for aperture excitation of electrically large, lossy cavities. The theory yields expressions for shielding effectiveness, cavity Q, and cavity time constant. In shielding effectiveness calculations, the incident field can be either a single plane wave or a uniformly random field to model reverberation chamber or random field illumination. The Q theory includes wall loss, absorption by lossy objects within the cavity, aperture leakage, and power received by antennas within the cavity. Extensive measurements of shielding effectiveness, cavity Q, and cavity time constant were made on a rectangular cavity, and good agreement with theory was obtained for frequencies from 1 to 18 GHz. >

Fields radiated by electrostatic discharges

The problem of electrostatic discharge (ESD) radiated fields problem is examined, both theoretically and experimentally. Measurements indicate that the electric fields can be quite significant (>150 V/m at a distance of 1.5 m), particularly for relatively low-voltage sparks (<6 kV). A theoretical dipole model for the ESD spark is developed to compute the radiated fields. The agreement between theory and experiment is good. The model can be used to predict the fields for a wide range of possible discharge configurations.<<ETX>>

The Detection of Incipient Faults in Transmission Cables Using Time Domain Reflectometry Techniques: Technical Challenges

The location and repair of faults in underground transmission lines is a difficult and time-consuming operation. The Department of Energy has sponsored research in the development of instrumentation to detect and locate incipient fault sites. Some of these methods rely on reflectometry techniques in either the time or frequency domain. NBS has investigated the feasibility of using such methods in extruded polyethylene cables.

Figure of merit for low frequency anechoic chambers based on absorber reflection coefficients

Return loss as a function of frequency and angle of incidence is studied to determine the effectiveness of the absorbing material used in an anechoic chamber. This alone is not enough to determine a figure of merit for an anechoic chamber or to compare the figure of merit for one anechoic chamber to that of another. While the information gained from return loss calculations and measurements as a function of angle of incidence is valuable, an overall measure of anechoic chamber effectiveness is necessary in order to compare different designs. In this paper, a new chamber figure of merit which is based on the decay time of the chamber is introduced. This decay time is, in turn, based on the average power absorbed by the chamber walls. The resulting model is simple and does not require intensive numerical computation. Calculations of the figure of merit for anechoic chambers which contain different types of absorbing materials are shown, and calculated and measured values of decay time for a primary standards calibrations facility are compared.

Time-domain measurements of the electromagnetic backscatter of pyramidal absorbers and metallic plates

A wideband time-domain measurement system has been developed for the evaluation of the backscatter performance of dissipative macrostructures. Backscatter measurements have been performed in an ordinary room environment on metal plates as well as samples of pyramidal absorbing material. The backscattering performance of pyramidal absorbers is evaluated in the 50- to 1000-MHz frequency range with a varying incident field angle of incidence. In the case of rectangular metal plates, numerically generated results are compared with measured data in order to gauge the accuracy of the system. >

Electrical material properties from a free-space time-domain RF absorber reflectivity measurement system

The scattering information obtained from the measurements of selected test structures is used to extract the relative permittivities of the various dielectric layers. Tests have been successfully conducted on single and multiple-layer dielectric panels, from which good estimates of material properties have been obtained. Results have been obtained in tests performed at both normal and oblique incidence. In addition, an edge-effect removal algorithm that significantly improves the estimated dielectric constant for small panels has recently been developed.

Analyzing broadband, free-field, absorber measurements

The authors present and analyze a method for the free-field evaluation of a broadband absorber in a nonideal testing environment. Using broadband, short-impulse TEM horns, a frequency-rich spectrum (equivalent pulse length <0.5 ns) illuminates a sample of the material under test and the reflections are recorded. Unwanted reflections from the sample edges, room environment, antenna and other systematic events are mathematically removed by a combination of time gating, background subtraction and systematic deconvolution. The result is an estimate of the reflection characteristics of the center at the sample. The authors also present an uncertainty analysis of the measurement technique.

The Co-Conical Field Generation System-a 40 GHz antenna calibration cell

A prototype test cell for generating standard fields suitable for antenna calibration from DC to 40 GHz has been developed and tested. A 1 meter scale model of the Co-Conical Field Generation System (CFGS), an expanding, constant impedance coaxial transmission system has been constructed. By maintaining symmetry throughout the cell, a uniform and calculable wavefront is established. This symmetry also maintains the dominant TEM mode structure and more importantly, allows for the gradual and efficient termination of the incident energy. Simple termination schemes have generally provided better than 20 dB absorption of the injected energy with complex designs showing much more promise. The field structure within the test volume has been mapped by measuring the reflections by a small passive scatterer. The measurement results agree well with transmission line theory and show no significant signs of mode degradation as energy propagates down the cell. Various numerical techniques agree well with the measurement results and transmission line equations, allowing for further study of scattering effects within the cell.

Low-frequency RF absorber performance with in situ and moveable sample techniques

The purpose of this paper is to describe two free-space measurement systems, developed at NIST for the evaluation of RF absorbers, and to demonstrate capabilities using measurement examples. The first consists of a moveable measurement system that performs in situ measurements of the reflectivity of a selected section of absorber wall in a chamber. The second is a fixed measurement system, deployed in conjunction with a moveable sample of absorbing material to be tested. Normal incidence results are presented for the in situ measurement system, and both normal and oblique incidence scattering results are shown for the moveable sample system. Typical Type-A measurement uncertainties are also shown for both systems.

Time-domain free-space evaluations of urethane slabs with finite-difference time-domain computer simulations

This paper describes the extraction of electrical material properties of lossy, carbon-doped urethane slabs using a free-space time-domain reflectivity measurement system developed by the National Institute of Standards and Technology. In order to validate the measurement results. The measurement system was simulated using the finite-difference time-domain method. Reflectivity measurement results and theoretical predictions as a function of carbon-doping, frequency, and angle of incidence are presented. Excellent agreement is obtained between measured and simulated results. Finally, complex permittivity results are presented for two urethane slabs with different carbon-doping levels, and the dispersion is a strong function of the amount of carbon.