A Handbook on Electromagnetic Shielding Materials and Performance

Description

There exists substantial material in the literature on the subject of electromagnetic shielding. Chap. 4 presents many references. For either an individual who has only recently been introduced to shielding or to a design engineer, however, much of the literature appears to be either confusing or poorly organized for design use. Missing in the literature are a series of useful design graphs including all the princi- pal variables presented in a clear, understandable manner. Thus, this handbook on Shielding was conceived to fill these voids. This handbook does not cover the topics of where and when to shield, and where to ground a shield. These topics are covered in Vol. 3 of the EMC Handbook Series. Rather, this handbook explains shielding theory and performance and presents many design graphs of shielding effective~ ness vs frequency as a function of shield metal and its characteristics, and E and H-fields and plane waves. Regarding the impedance of the fields (E, H, or plane waves), tie literature and manufacturers' data are often very misleading. For ex- ample, since the wave and circuit impedance which produced the field .are interlocked and since a circuit impedance is not infinite, E-field shielding effectiveness data are generally optimistic (too high) rela- tive to actual performance. In a converse manner, H-field shielding effectiveness data are pessimistic (too low) since a magnetic source circuit impedance is not zero. This handbook clarifies and quantifies these points, Another example of possibly misleading information is the use of MIL-STD-285 to measure and report the shielding effectiveness of test items to E and H-fields. The reference test distance per MIL-STD-285 is one foot (0.305 meters). Thus, for installations located in the near field which are greater than one foot from an interfering source, actual E-field shielding performance will be less and H-field perform- ance will be greater than that reported by MIL-STD-285 measurements. The converse applies for application distances between sources and metal barriers which are less than one foot away as illustrated in this hand- book. The discussions and design data on shielding effectiveness in this handbook are not restricted to homogeneous metals. In fact no real life and useful shielded compartment, box, cabinet, or room is homogen- eous since usually many penetrations of a six-sided shield configuration are necessary. Techniques used to reinstate the integrity of a shielded enclosure are discussed in Vol, 3 of the EMC Handbook Series. Shielding materials and performance of non-homogeneous metals are discussed in this handbook on Shielding. Some examples are pseudo-homogeneous shields made from metal deposition and flame-spray processes. Shields made of small-aperture metals are also presented. Examples include screens, wire meshes, cable braids and metalized textiles, all of which are discussed herein together with design data. The appendices of this handbook are perhaps the most important of all material presented. They contain 42 pages of design shielding effectiveness graphs for several metals whose thicknesses range from 0.0001 mil (2.54 pm) to 1 inch (2.54 cm). For both near and far- field calculations and associated frequencies, the design graphs cover source-to-metal distances ranging from 10 cm to 10 km. Fre- quency coverage is from 10 Hz to 30 GHz. All data were run-off on the HP-65 programmable calculator. For those who have an HP-65, the program is presented so that they can develop and use their own magnetic card. There also exist many design graphs other than direct shielding effectiveness which the reader should find useful. The author of this handbook invites the user to communicate with him. He especially invites comments, questions, or requests for further elucidation.