Shielding efficiencies for a number of blends containing a variety of conductive fillers, including intrinsically conductive polymers, have been measured in the near field with a dual-chamber box and in the far field with a transmission line fixture. Although all samples satisfied the classical good-conductor approximation, most of them exhibited a crossover from being electrically thin (thickness skin depth) over the frequency range of interest, 1 MHz to 3 GHz. The theoretical relations for both near-field and far-field shielding which are prevalent in the literature do not accurately describe this region of crossover. The authors have derived expressions which describe the behavior accurately over the entire range of interest. Far-field shielding efficiencies as high as 70 dB at 1 GHz were measured for purely organic composites consisting of an intrinsically conductive polymer, polyaniline, dispersed in a thermoplastic matrix. >

EMI shielding measurements of conductive polymer blends

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. >

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

The electromagnetic shielding behavior of bounded-junction wire-mesh screens is discussed in the frequency range where the individual meshes are electrically small The screen, whose meshes are assumed to be square, is described by a equivalent sheet impedance operator The plane-wave shielding effectiveness of a planar-mesh screen is derived The problem of coupling between antennas on either side of the planar mesh is addressed, and its applicability to the measurement of mesh properties is described The low-frequency electric and magnetic shielding afforded by mesh enclosures is considered Because of the reactive character of the mesh surface, the plane-wave shielding effectiveness decreases with increasing frequency The enclosure's magnetic-field shielding effectiveness increases with increasing frequency, but saturates at a maximum value that depends on the geometry of the enclosure, the mesh size, and the mesh wire radius The enclosure electrostatic-field shielding effectiveness depends only on the enclosure and mesh geometries >

Electromagnetic shielding behavior of wire-mesh screens

The magnetic shielding effectiveness for closed and open shield structures is studied at extremely low frequencies. Analytical solutions are used for simple geometries, while more complex structures are evaluated using a finite-element method. Both highly conductive and ferromagnetic materials are studied, and their different shielding behavior is shown. Ferromagnetic shields give good results for small and closed shields and they also give a large field attenuation at close range to the source for open shield geometries. Highly conductive materials, on the other hand, are found to be suitable for large shield sizes. The attenuation is, however, reduced in the close vicinity of the source. Comparisons of numerical results with analytical calculations and measurements confirmed the high accuracy of the finite-element model. >

Geometrical aspects of magnetic shielding at extremely low frequencies

The problem of the electromagnetic shielding effectiveness of a thin, plane metal sheet of infinite extent on the electromagnetic field generated by a circular loop field source with uniform current I is solved by application of the quasi-near and near field approximations to the exact integral expressions. The results are shown to be in close agreement with experimental data as well as numerical integration results. Also, they are similar to the results obtained from the plane wave shielding theory of Schelkunoff.

New Theoretical Expressions for Predicting Shielding Effectiveness for the Plane Shield Case

The problem of low-frequency shielding of a loop axially perpendicular to a plane shield of infinite extent is analyzed by 1) the thin shield work of S. Levy, 2) solution of the vector wave equation, and 3) application of the transmission theory of shielding of Schelkunoff. Experimental data are obtained and compared with results of parts 1) and 3) in the frequency range 100 Hz to 50 kHz. The first analytical technique is not general, and the limits of applicability of the results are discussed. In the second solution, which is general, expressions are derived for the total electric and magnetic fields on both sides of and within the shield. The resulting expression for shielding effectiveness is not solved because of its complexity. The results of the third theory are adapted to the problem. The shielding effectiveness expression S = R + A + B is computer evaluated for the six shields considered (1/16-inch and 1/8-inch thick aluminum, copper, and steel). Although some approximations are made, this analytical method is the most useful in predicting the insertion loss of the shield, since the theory includes those parameters neglected in the first analytical technique.

Low-Frequency Shielding of a Circular Loop Electromagnetic Field Source

This paper presents experimental data that show an over-whelming dependence of system electromagnetic compatibility (EMC) on system cable effects, rather than leakage through cracks, seams, and other box (LRU and WRA) openings. A laboratory experiment compares cable-shield RF leakage from a pigtail-terminated cable with that of one that has been terminated peripherally. Theoretical justification for these empirical observations is presented. That a shielded conductor carrying an RF current i induces its own return current -i on the inside surface of the shield is proved. By confining this return current to the inside of the shield, external emissions are attenuated by the intrinsic shielding effectiveness (which can be quite a large number) of the shield.

Peripheral Cable-Shield Termination: The System EMC Kernel

An empirical ELF and VLF study of 1/16- and 1/32-inch thick shield cans (stainless steel, aluminum, copper, Polyform, annealed Hipernom, and annealed and unannealed coldrolled steel, mumetal, and Co-Netic) was performed to investigate shielding effectiveness versus frequency characteristics with emphasis on the effects of machining (notches, slots, holes, and louvers), dropping, annealing, and welding. The erratic nature of many of the shielding functions is explained by the resonance theory of shielding, and the shielding characteristics of certain shield cans are compared. The paper also presents a brief introduction duction to the automated testing system used during this study to obtain the numerous shielding effectiveness versus frequency curves that were required.

J. Ronald Moser

Papers

Low-Frequency Shielding of a Circular Loop Electromagnetic Field Source

Peripheral Cable-Shield Termination: The System EMC Kernel

An Empirical Study of ELF and VLF Shield Cans