Richard B. Schulz

Shielding theory and practice

Plane-wave shielding theory is developed and discussed for a number of important cases such as single, double, and laminated shields. For application to design, the basic expressions are modified and plotted with universal parameters for convenient use in performance calculations of both solid and perforated sheets. Performances of solid copper and iron shields have been calculated and are presented in both tabular and graphical form. For these and other materials, measurement results of various experiments are tabulated for a number of different material forms and for various incident-wave impedances. Some consideration is given to shielding discontinuities and trends in modern shielding enclosures. >

ELF and VLF Shielding Effectiveness of High-Permeability Materials

Research was conducted to determine the low-frequency shielding effectiveness of magnetic materials. Both analytical and experimental approaches were used. This work is unique in that it provides a technique for experimental separation of the various terms in the shielding expression. Expressions of shielding effectiveness of flat sheets for very low frequencies are derived, and the results of experiments with the following are given: 1) sheet materials, including AMPB-65,1 HyMu 80, conetic AA, mumetal, copper-plated AMPB-65, and galvannealed steel; 2) AMPB-65 sheets perforated with various size holes and various numbers of 0.125-inch diameter holes; 3) an overlap junction of two AMPB-65 sheets with a) various numbers of fastening screws and b) various depths of overlap; 4) overlap junctions of copper-plated AMPB-65 sheets; 5) AMPB-65 sheets clamped in Lindsay structure; and 6) honeycomb-core stainless steel sandwiches.

RF Shielding Design

An RF shielding design procedure based on a nonuniform transmission-line analogy of shielding is presented. The theory treats the propagation of an EM wave through the walls of a shielding enclosure as a path through the material itself in parallel with one or more paths through defects, seams, or other discontinuities in the shielding structure. Data necessary for the design of a shield are presented in the form of material factors, seam factors, size factors, and others. Factor dependency on frequency is discussed. Two typical design examples are given.

Low-Frequency Shielding Resonance

A low-frequency resonance in the 5- to 200-kHz range has been discovered in typical shielding enclosures. It is due to unequal phase shifts in wave transmission to a receiving point via two or more parallel paths: one through the shielding material itself and others through leakage paths such as seams. Theoretical and experimental results are shown to be in close agreement and indicate a new approach to the design of shielding enclosures.

APD Measurements of V-8 Ignition Emanations

Amplitude-probability-distribution (APD) data are presented for ignition emanations from V-8 engines of used motor vehicles. Outputs from both single and multiple engines running at 1500 rpm were measured with an omnidirectional antenna over the frequency range from approximately 20 MHz to 1 GHz. In each case, the antenna was oriented similarly with respect to the vehicle(s)-vertical, height 3 m, distance 10 m. Results are presented on Weibull-distribution sheets and compare favorably with those of Hsu et al. [1] using a different technique. On each sheet, APD distributions are given for various received bandwidths between 1 and 300 kHz. For the larger bandwidths, curves are straight lines with large negative slopes. At narrower bandwidths and for multiple vehicles, slopes are less steep. Major conclusions from the APD results include the following. 1) Although overlapping of successive pulses at a detector becomes more prominent at narrower bandwidths, measurements are still valid and representative for a given bandwidth. 2) Measurements indicate good repeatability. 3) Curve shapes are essentially independent of tuned frequency, but do depend upon bandwidth and number of vehicles. 4) Amplitudes are functions of tuned frequency and number of vehicles.

Design of Minimum Weight and Maximum Effectiveness of Very-Low-Frequency Shielding

Shielding equations developed in a research program were used to generate data for use in the design of an optimum shield for a given set of constraints (a prescribed shielding level, minimum weight shield, etc.). Analysis consisted chiefly of computer processing of the modified equations. To reduce computer time, only symmetrical laminated shielding material was considered. Maximum shielding attenuation with respect to overall shield weight was considered for two cases, using the transmission theory of uniform shielding: 1) the lightest shield for a specified level of attenuation, and 2) the shield providing maximum attenuation within specified constraints of shield weight.

Low-Frequency Electrical Characteristics of RF Shielding Materials

Values of initial magnetic permeability relative to free space have been obtained by means of novel techniques which permit measurements on material in the form of a flat sheet, a box, or any other configuration containing at least a six-inch square flat surface. As a by-product of the technique, electrical conductivity values may also be obtained.

Shielding Density: An Electromagnetic Shielding Design Concept for Weight Sensitive Applications

Laboratory data obtained in a shielding study led to the conclusion that the size or shape of a shielding enclosure does not significantly affect its shielding effectiveness at frequencies below cavity resonance. A figure-of-merit expression for weight-sensitive applications was devised and termed shielding density. It relates shielding effectiveness per unit weight per unit area. This concept is useful in providing the shield designer with answers to three specific questions concerning maximum shielding effectiveness, lightest shielding enclosure, and optimum shielding density.

A Rational Basis for Determining the EME Capability of a System

A logical procedure is presented for determining the electromagnetic compatability (EMC) of a system, based upon an analytical approach developed earlier. The procedure is illustrated using as a system an airplane with a manageable number of electrical-electronic subsystems. The result is a single number which can be used in a weapon system effectiveness equation and is generally useful not only to EMC engineers, but also to other electronic engineers and managers. Byproducts of the procedure are enhanced highlighting of critical parameters for design purposes and a means for economic evaluation of EMC efforts.

Interference Characteristics of Streamer Discharges

This paper discusses analytical and experimental results of a study on streamer formation, discharge waveforms, and RF noise levels. Measurements disclose typical pulse parameters as follows: rise time = 20 ns, discharge time = 80 ns, and pulsewidth < 600 ns (dependent upon test sample). A mathematical model is developed to represent electric field strength in the vicinity of the streamer with supporting measured values.