In an array of apertures (e.g., windows along the fuselage of an aircraft), the coupling increases when the line of apertures is parallel to the incident magnetic field vector (Jaggard, 1977). 2.3. Diffuse Coupling Microwave energy can penetrate into a cavity by means of "diffusion" through the walls when the wall conductivity is finite (Lee and Bedrosian, 1979). A wall is considered electrically "thin" when the skin depth 6 is larger than the wall thickness d (see equation 21). A few measurements have been reported on the insertion loss of typical composite building materials and are listed in Table 1. Table 1. Measured Insertion Loss of Building Materials (Wells et al., 1975; private communication)* Diffuse coupling plays a major role with nonmetallic structures. The total exposure power SX (8) will be attenuated; however, the amount reaching the inner space could still be substantial and contribute to a hot spot problem should field concentrating devices exist internally. 3. INTERNAL REFLECTIONS Reflections from the enclosure walls significantly affect the field distribution inside. The local energy density will be greater than obtained in free space when, at a test point, the majority of reflected energy is of proper phase to add to the energy arriving directly. The phase of a reflected signal and hence the local power density both are extremely sensitive to: size and shape of the enclosure, location of the test point, presence and location of objects (e.g., furniture, etc.) and persons. Wells, P. I., D. A. Hill, A. G. Longley, R. G. FitzGerrell, L. L. Haidle, and D. V. Glen (1975), An experiment design for the measurement of building attenuation, OT Technical Memorandum 75-199.
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