The field distributions of the following cavity shapes have been calculated rigorously (Harvey, 1970): (a) rectangular box, which most closely resembles a typical room, (b) circular cylinder, (c) spheres, (d) elliptical cylinders, (e) conical, toroidal, and cigar shapes, (f) confocal spheroids and paraboloids. The field equations for the geometries (a) to (c) are given in Appendix B. Case 3 (Table 3) might resemble to some degree one of these geometries. Theoretically, the peak value of the stored field energy density in an aperture- excited cavity can be up to two orders of magnitude above the incident value (e.g., Safari-Naini et al., 1977). Losses in a practical configuration, however, make a HSP very unlikely. The effective Q-value is calculated from where the ratio between time-averaged field energy and (a) power dissipated in the walls by conduction loss (Q,,), and (b) power radiated out through the aperture (Q»), and (c) dielectric heating loss by material in the cavity (QM) defines the individual quality factors. Large apertures cause a low value for Q (roughly the ratio between enclosure and aperture area). The ratio is close r\ to one for vehicles. In aircrafts, the enclosure-to-aperture ratio is on the order of 10 to 100. 4.2. Open Resonator For Case 2 (Table 3), a resonant wave field can be sustained by caustic shapes of two bounding, reflective surfaces, while free space provides the missing confining boundary. The mode density for an open resonator is (Weinstein, 1969)
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