Coupling of external high frequency electromagnetic waves to aircraft wiring-theoretical background

The theoretical background for a method used for the measurement of the coupling of external HF electromagnetic waves to aircraft wiring is presented. Expressions for the coupling of the external incident electromagnetic fields to the aircraft skin and its associated wiring are derived. Emphasis is placed on the coupling into a small aircraft in resonance conditions, which would be expected to prevail in the HF band, where the aircraft dimensions constitute a major portion of the wavelength. It is demonstrated that, in resonance, the shielding effectiveness of the aircraft fuselage is practically reduced to zero. That implies a maximum coupling of the external incident electromagnetic waves to the aircraft wiring, thus increasing the interference coupled to the input of susceptible onboard avionics. These calculations are confirmed in actual measurements, where it is found that the shielding effectiveness is reduced, in resonance, to less than 10 dB