An experimental and analytical study of the use of bounds to estimate the coupling to a monopole inside a cavity with an aperture

We report the results of a comparative experimental and analytical study of the coupling of electromagnetic (EM) fields to a monopole inside a cavity with an aperture. Coupling measurements have been made over a wide range of frequencies using a number of monopole and aperture sizes. Measurements were made using two orthogonal cavity orientations. In some experiments, an absorbing sphere was placed inside the cavity to act as an additional energy absorber. The experimental results have been compared with estimates based on the bounds approach developed by Lee and Yang (1992). Some new expressions for the aperture cross-sections have been derived. Comparison has shown that the bounding approach developed by Lee and Yang using a power conservation approach is in close agreement with experimental results over a wide range of aperture and monopole sizes provided the frequency is high enough that the interior fields can be considered uniform. Violations of the bound are observed at lower frequencies when only a few cavity modes are excited and the uniform field approximation breaks down. It is also shown that the effects of absorbing bodies can be included. Use of this approach should simplify electromagnetic hardening of electronic systems.