M T. Ma

Techniques for measuring the electromagnetic shielding effectiveness of materials. II. Near-field source simulation

Shielding effectiveness relates to the ability of a material to reduce the transmission of propagating fields in order to electromagnetically isolate one region from another. Because the shielding capability of a complex material is difficult to predict, it often must be measured. A number of approaches to simulating far-field source are studied, including the use of coaxial transmission-line holders and a time-domain system. In each case, the system frequency range, test sample requirements, test field type, dynamic range, measurement time required, and analytical background are considered. Data taken on a common set of materials are presented. >

Simple Approximate Expressions for Higher Order Mode Cutoff and Resonant Frequencies in TEM Cells

Simple approximate expressions for determining the cutoff frequencies of the first few higher order modes and the associated resonant frequencies in transverse electromagnetic (TEM) cells are presented. Both symmetric (seven TE and two TM modes) and asymmetric (three TE modes) cells are discussed.

Emission Characteristics of Electrically Small Radiating Sources from Tests Inside a TEM Cell

An electrically small radiating source of arbitrary nature may be modeled by an equivalent-dipole system consisting of three orthogonal electric dipoles and three orthogonal magnetic dipoles, each excited with arbitrary amplitude and phase. A method of determining the individual dipole moments and the cross-components of such a dipole system, by tests inside a transverse electromagnetic (TEM) cell, is presented along with some experimental results.

Shielding-Effectiveness Measurements with a Dual TEM Cell

Small-aperture theory is used to investigate the dual transverse electromagnetic (TEM) cell. Analyzing coupling through an empty versus loaded aperture leads to a simple model of dual-TEM-cell material shielding-effectiveness (SE) measurements. Experimental data are compared to theory with good agreement in the case of an empty aperture. Some of the difficulties in analyzing a loaded aperture are discussed.

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>>

A Study of Techniques for Measuring the Electromagnetic Shielding Effectiveness of Materials

Abstract: Shielding effectiveness relates to a materials ability to reduce the transmission of propagating fields in order to electromagnetically isolate one region from another. Because a complex materials shielding capability is difficult to predict, it often must be measured. A number of measurement approaches are studied including the use of a shielded room, coaxial transmission line holders, time domain signals, the dual TEM cell, and an apertured TEM cell in a reverberation chamber. In each case, we consider the systems frequency range, test sample requirements, test field type, dynamic range, time required, analytical background, and present data taken on a common set of materials.

Shielding Effectiveness Measurements Using an Apertured TEM Cell in a Reverberation Chamber

II TEST CONFIGURATION M e a s u r e m e n t s of n e a r f i e l d s h i e l d i n g effectiveness are performed in a reverberation chamber using an apertured transverse electromagnetic cell as the receiver. This configuration allows one to investigate the electricand magnetic-field shielding properties of a material simultaneously. Coupling to the cell is modeled using small-aperture theory, and predicted results agree well with measured data.

Excitation of a TEM Cell by a Vertical Electric Hertzian Dipole

From abstract: The excitation of a transverse electromagnetic (TEM) cell by a vertical electric Hertzian dipole is analyzed where the gap between the septum and side wall is assumed to be small. Approximate expressions for the field distribution and characteristic impedance are derived. These expressions are numerically evaluated for some typical geometries, and good agreement with previously published results is shown. The formation also allows a vertical offset for the septum position, thus offering more flexibility of increasing the size of the test area to accommodate larger pieces of test equipment.

Small Aperture Analysis of the Dual TEM Cell and an Investigation of Test Object Scattering in a Single TEM Cell

Introduction: Small aperture theory is used to investigate the dual TEM cell. Analyzing coupling through an empty versus a loaded aperture leads to a model of dual TEM cell shielding effectiveness measurements. Small obstacle scattering yields results for both the field perturbation and the change in a cells transmission line characteristics due to the presence of a test object in a TEM cell. In each case, theoretical values are compared to experimental data.

Input Impedance of a Probe Antenna in a TEM Cell

The input impedance of a probe antenna exciting a transverse electromagnetic (TEM) cell is analyzed via a variational formulation. The resulting impedance is shown to consist of two distinct terms; an ordinary rectangular waveguide contribution and a gap perturbation. Numerically generated curves for both are given and suggest that a simple algebraic approximation for the input impedance should normally suffice. The resistive portion is found to be proportional to the square of the probe length, while the reactive portion is largely capacitive. These results should enhance the use of probes inserted in TEM cells either to excite or to measure fields.