Myron L. Crawford

Generation of Standard EM Fields Using TEM Transmission Cells

A new technique developed at the National Bureau of Standards (NBS) for establishing standard, uniform, electromagnetic (EM) fields in a shielded environment is described. The technique employs transverse electromagnetic (TEM) transmission cells that operate as 50 ? impedance-matched systems. A uniform TEM field is established inside a cell at any frequency of interestbelow that for which higher order modes begin to propagate. Standard field strength levels from 10 ?V/m to 500 V/m can be established with uncertainties of less than 1.0 dB to 2.0 dB inside the NBS cells for frequencies from dc to 500 MHz. The cells are especially useful for calibrating EM radiation hazard meters, for emission and susceptibility testing of small to medium sized equipment, and for special low level calibration of very sensitive field strength meters.

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. >

Expanding the Bandwidth of TEM Cells for EMC Measurements

This paper discusses the development of a modified (absorber-loaded) transverse electromagnetic TEM cell with expanded bandwidth for use in accurately characterizing electromagnetic interference (EMI) fields within a shielded environment. The cell is analyzed experimentally, both before and after the modification, to determine its radio-frequency (RF) characteristics, both as an RF transmission line and as an electromagnetic (EM) field simulator or detector. Comparative measurements are given to show the performance of the modified versus the unmodified cell in parameters such as voltage standing-wave ratio (VSWR), insertion loss, test-field uniformity, and reverse-coupling characteristics. The results of these measurements indicate an approximate two-fold increase in the upper useful frequency of the modified cell. An example of using the cell to evaluate the radiated emissions from a common electronic module (microprocessor timing circuit) is given. Finally, the technique of absorber loading is extended to larger cells, specifically a 3- × 3- × 6-m cell.

Methodology for Standard Electromagnetic Field Measurements

Establishing standards for electromagnetic (EM) field measurements is a multifaceted endeavor which requires measurements made (1) in anechoic chambers, (2) at open-sites, and (3) within guided-wave structures, and the means to transfer these measurements from one situation to another. The underlying principles of these standard measurements and transfer standards fall into one of the two categories: (1) measurements and (2) theoretical modeling. In the former a parameter or a set of parameters is measured, while in the latter a parameter or set is calculated employing established physical and mathematical principles. In the following discussion, the three measurement topics and field transfer standards mentioned above will be discussed with the guided-wave structures being restricted to the TEM cell. Throughout the discussion the interplay between measured quantities and predicted (modeled) quantities will be seen. The frequencies considered here range from 10 kHz to 18 GHz (and upward) and are dependent upon the physical constraints imposed by our ability to implement an actual measurement, subject to the conditions imposed by rigorous electrodynamic theory in a given analytical model.

Aperture coupling to a coaxial air line: theory and experiment

Coupling through a circular aperture in the shield of a coaxial air line is studied theoretically and experimentally. Polarizability theory is used to compute the effective dipole moments that excite the coaxial line in the internal region. Measurements of shielding effectiveness were made in a reverberation chamber over wide frequency ranges. Agreement between theory and measurements is generally within +or-10 dB. Recommendations for improvements in the measurements and theory are made for achieving the closer agreement that would be desirable for an artifact standard for shielding effectiveness measurements. >

Generation of EM Susceptibility Test Fields Using a Large Absorber-Loaded TEM Cell

This paper discusses the development of an electromagnetic simulator for accurate generation of broad-band suspectibility test fields within a shielded environment. The simulator consists of a large, 3 m X 3 m X 6 m, rectangular transverse electromagnetic (TEM) transmission cell that is loaded with RF absorber to suppress multimoding at frequencies above the cells waveguide cutoff or resonant frequencies. The paper describes the measurement facility and technique, and the experimental verification of pertinent test paramenters such as system VSWR, insertion loss, and test field uniformity. The measurement system is anticipated to provide swept, automated susceptibility measurements of electronic equipment to CW, pulsed, and EMP fields within the frequency band, 10 kHz to 1 GHz.

Electromagnetic fields with arbitrary wave impedances generated inside a TEM cell

It is shown that arbitrary electromagnetic fields and wave impedances can be generated inside a transverse electromagnetic (TEM) cell for RF susceptibility testing. This is achieved by simply exciting one port and terminating the other port with appropriate loads. Thus, TEM cells are not limited to producing a planar field environment with the free-space impedance of 377 Omega , but can also be used to generate high-impedance or low-impedance fields for special testing needs. Experimental results for only three load impedances are described. In principle, many other loads including reactive terminations could be used to create a particularly desired field and wave impedance inside the TEM cell. >