indicative of hot spots; however, in this case, the reference to an outer incident field (see equation 7) is missing. In principle, it is possible to locate the source of reflections from a completely known spatial interference pattern, but finding that pattern at every point from a theoretical evaluation is prevented by the complicated problem of defining the reflecting boundaries. 5.3. Fields in Mine Tunnel (Dielectric Waveguide) Cavity boundaries made of dielectric (K > 3), low-loss (D < 0.02) material can guide SPS microwave radiation in any number of modes when the inner dimensions are larger than X . All of these modes are lossy (typically between 0.02 and 0.5 dB/m at 2.5 GHz) since a dielectric wall is not a perfect reflector. Detailed electric field distributions have been measured, for example, in mine tunnels (e.g., Emslie et al., 1975). The lowest attenuation rates are those when, in geometric optics terms, the wavefront impinges upon a reflecting boundary with small grazing angles. Even modes in this category display low Q-factors in closed tunnel chambers. A dielectric waveguide might guide SPS radiation to a resonance structure that otherwise is not coupled to the outside. In summary of Sections 2 to 5, we can say that the analysis has fallen short of its purpose. Other than an awareness of the significance of certain interaction processes (coupling, reflection, loss) which are essential to a HSP, no firm estimates of upper bounds and variabilities in the quantities entering equation (16) were obtained. This is mainly due to the almost infinite number of possible geometric space configurations for empty habitable space. 6. MITIGATIVE MEASURES FOR MICROWAVE HOT SPOTS Two principal means exist to achieve certainty that there is no hot spot problem; namely, to avoid penetration into the interior and/or to secure a uniform internal field distribution. In other words, apertures of sizes in the range « (0.3 to 3)Xq and low-loss, permeable building material are to be avoided at the exposure site to minimize the cross section X- (10), and little stored field energy is accomplished by suppressing internal reflections [low Q-factors (11)]. To circumvent an overapplication of absorber material, one should perform in suspected spaces a three-dimensional field scan and make sure that the free field SWR is within limits (say about + 6 dB, see Table 5) by judiciously placing absorbers and rearranging specular reflectors.
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