FDTD simulations of electromagnetic coupling to internal resonance modes of metallic and resistive casings of electronic equipment

This paper examines coupling to electromagnetic modes found within cylindrical (NATO cylinder) and rectangular volumes with metallic and/or resistive surfaces using finite difference time domain (FDTD) simulations. Stripline geometries are positioned within the volume to simulate internal circuitry. External fields from sources in the near and far fields are coupled to internal modes via apertures of various dimensions in the external surfaces. The results have implications for protection of circuitry from high-power, ultra wideband excitation. Several parameters are considered. These include: volume and dimensions of cylindrical and rectangular casings; casing conductivity; aperture placement and size; electromagnetic frequency; and positioning of the simulated circuit and loads placed on that circuit. Calculations to date have demonstrated that intense internal fields can be excited via the casing apertures. Maximum coupling appears near the frequencies of characteristic modes of the casing geometries. Peak coupled field magnitude at the location of the simulated circuit can be 30 dB above the directly radiated value. External excitation from a high power RF source could excite amplified local internal fields that result in nonlinear responses on internal circuitry. Thus, shielding protection offered by the casings may be limited. Frequencies of greatest amplification are shown to be functions of cavity and aperture geometry, and casing conductivity. Impact of intrinsic loss of casings is shown to be minimal, but additional treatment with RF absorbers can be an effective mitigation strategy.