Emerson, 1973; Donaldson et al., 1978; Emslie et al., 1975), most notable from the comprehensive treatment by VonHippel. For a HSP, we look after material that is a good reflector K > 3) and has a small loss tangent (D < 0.01). Low loss dielectrics (Table 6) are artificial materials normally not common in habitable structures but especially developed for microwave applications. The materials commonly used to construct habitable space are more or less lossy (see examples listed in Table 7). Water is a very lossy dielectric, and the water content of materials determines to a large extent their dielectric behavior such as attenuation a (27) and reflectivity p (29). Water plays a role on the outside of habitable structures where it acts as a weather-variable shield. Also, the high water content of biota turns them into significant absorbers and reflectors. Their dielectric effects are simulated by a 4 percent aqueous salt (NaCl) solution. Reported dielectric properties of water are in Table 8; water content-dependent properties of various building materials are given in Table 9 and Figure 6. Wooden structures will allow a certain amount of diffuse coupling (see 2.3), while window areas are good direct-coupling apertures (2.1) with partial reflections on both sides. Table 6. Low Loss Dielectrics (D < 0.01, f = 3 GHz) K D K D Fused Quartz 3.78 0.00006 Phosphate 4.9 to 0.0018 to Glass 5.2 0.0046 Styrofoam 1.03 0.00010 Borosilicate 4 to 0.0012 to Glass 8 0.0058 Teflon 2.1 0.00015 Corning 7070 Glass 3.9 0.0031 Polystyrene 2.55 0.00033 Lucite 2.58 0.0035 Butyl Rubber 2.35 0.0009 Plexiglas 2.60 0.0057 Porcelain 8.9 0.0011 Phenolic Board 1.19 0.0058 Polyglass D 3.23 0.0012
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