diffraction and aperture radiation, and (d) from the presence of furnishings and biota. 4. Superimpose all possible resonant modes to the composite field pattern. To complicate matters further, a possibility exists for latent resonances; i.e., resonances that are tuned in by a dielectric object moving within the enclosure. A person inside a resonant cavity will cause a perturbation of the resonance properties roughly proportional to the filling factor kp which is defined by one-half the ratio between body and cavity volumes. The effective Q-factor (32) and shift in resonance frequency are approximately The combination of loss and high dielectric constant (D « 0.5, K « 60) makes the chances of sustaining a resonance in a small room very remote. For one person, the ratio k-j will vary typically between 10"^ (e.g., cabin of a vehicle) and <10”^ (e.g., large hall). 5. MEASUREMENTS No systematic measurements of a HSP in habitable structures have been reported, and only a few data on internal signal strength have been gathered at microwave frequencies. In some cases, these results were compared with values taken simultaneously on the outside to define a building penetration loss for the purpose of assessing signal coverage inside buildings. In those instances, the mean value for was always positive (see equation 4). The general consensus from these efforts was that such measurements are complicated by reflection, diffraction, and scattering both inside and outside the enclosure; that multipath effects cause signal level variations of about + 20 dB (see Figure 8) when scanning across an arbitrary room axis; that these results are extremely sensitive to size and shape of the enclosure and the presence and location of furnishings and people; that normal incidence of incoming radiation produced the lowest penetration loss L ; and that metals and glass with wire mesh effectively shielded the field. ' 2 The internal field E^ (5) is best mapped with a miniature (dimensions £ 0.1x) isotropic probe and a scanning mechanism. By coupling the linear motion of the probe and the measured signal strength to a recorder, an interference pattern similar to SWR curves is obtained (Crawford, 1974). Amplitudes and periods of the recorded oscillations can be analyzed to locate the source of the interfering signal for mitigative measures.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==