with being the conductivity in Siemens (mho) per meter (see Table 2). The calculated values of af in Table 2 are greater than measured values by a typical factor of two; however, all metal surfaces are excellent reflectors for SPS radiation. Bare metal surfaces of significant area will be rare within the confinement of habitable boundaries. The chamber material, instead, which reflects, absorbs, and transmits the field can be anisotropic and, in addition, it can be non-stationary varying the field in space or time or both. Such media are difficult to deal with from the standpoint of solving the reflection problem. Already the general case of oblique incidence on a lossy homogeneous material is quite complicated. To gain some handle on the rather loosely defined term "habitable structure", we categorize it in Table 3 by means of the enclosure material, and look first at the microwave properties of lossy dielectric material. Table 3. Electromagnetic Enclosure Properties Case Enclosure Properties Coupli ng Real Structure Equivalent 1 lossy dielectric diffuse, aperture brick, frame, stone house 2 composite of lossy dielectric and metal aperture, diffuse trailer, house with metal sidings, industrial building 3 metal plus dielectric coating aperture vehicle interior, aircraft cabin Table 2. Reflective Properties of Metals (2 GHz) Metal Conduct!vityfi oM, (mho/m)10° Skin Depth 6, pm Reflective Loss ar, % Aluminum 38.2 1.82 0.0152 Brass (66-34) 25.6 2.2 0.0185 Chromium 38.5 1.8 0.0151 Copper 58.1 1.48 0.0124 Gold 41.0 1.76 0.0148 Platinum 9.52 3.65 0.0307 Silver 61.7 1.43 0.0120 Steel 22.7 2.5 0.0210 Tin 8.8 3.80 0.0319
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