Average field strengths inside structures are expected to be less than those outside, due to coupling losses. The structure could, however, alter the spatial distribution of energy to produce localized areas of increased field intensity. Reflections within structures are the most likely causes of locally-increased field strength. Metallic surfaces are the most common elements of habitable structures likely to produce significant field increases. In the absence of any absorbing material, a single metallic reflection could increase power density at some locations by as much as a factor of four above that incident on the surface. More complex geometries involving multiple reflections could, in principle, produce larger field increases. However, such large increases require special geometries and very little microwave absorption in the system. Neither of these criteria is likely to be satisfied by real habitable structures. In particular it should be noted that the presence of people, who are significant microwave absorbers and reflectors, modifies reflection patterns in a time-varying way as the people move and absorb energy, reducing the field increases possible due to reflections. Measurements have been reported giving the ratio of microwave field strength inside to that outside single family houses illuminated by 2.6 GHz microwaves from a satellite (Wells, 1977). Average fields inside were found to be about 25% of those outside, although interior fields varied substantially from point to point. For the case of highest ratio of average inside fields to outside fields, the probability of encountering a microwave field inside that was larger than that outside was only 15%. If, in specific cases, interior microwave field strengths were found to be a problem, they could easily be reduced by methods described in the present report.
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