of the walls. A great deal of work has been done on the coupling problem in context with EMP (electromagnetic pulse due to a nuclear explosion) effects (Butler et al., 1978, and 166 references therein). The treatment of aperture coupling distinguishes two cases depending upon the relation of the largest aperture dimension d with respect to the wavelength A sufficient approximation for apertures with dimensions larger than the wavelength is to equate the coupled energy P^ with the incident field energy SX and assume for the cross-section X the surface area A of the opening. This yields simply that Theoretical calculations of the coupling through circular, elliptical, and rectangular cross sections confirmed (17) (e.g., Koch and Kolbig, 1968). An example of q^ for a circular aperture is given in Figure 1 as a function of (2d/\) > 2 (d is the diameter). 2.2. Aperture Coupling When the largest dimension d of an aperture falls below Xq/2 = 6 cm, the analysis of the coupled field energy is more complicated (Butler et al., 1978; Jaggard, 1977). The coefficient q. can vary between zero and 1.7 (Figure 1); ’ 2 however, the total energy transmitted will be small (e.g., S = 0.23 W/m , 1. The electric field strength increases by as much as a factor of 5 at the edge of the aperture (Butler et al., 1978). An example is given in Figure 2. 2. The coupling reaches a maximum when d « 0.4 XQ. The increase to q^ % 1.7 (Figure 1) is explained by the fact that energy is collected from all angles over a hemisphere surrounding the aperture. 3. Maximum transmission through a circular aperture occurs for grazing incidence of a polarized plane wave; i.e., the magnetic field vector is aligned with the center axis (Jaggard, 1977).
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