Shielding effectiveness measurements for a large commercial aircraft

The electromagnetic energy density in internal cavities of commercial aircraft is of increasing interest as the use of electronic systems to perform critical flight functions increases. This paper describes three test techniques which were used on a commercial transport aircraft to determine the external to internal coupling of electromagnetic energy over the frequency range 0.1 to 1 GHz. Coupling or shielding effectiveness measurements were performed on a Boeing 707 aircraft. The first technique used the external ambient electromagnetic environment as the excitation source. In the second technique, the aircraft was illuminated with band limited white Gaussian noise on a swept frequency carrier. The third technique used time domain analysis of the cavity electromagnetic environment generated by repetitive, short pulse excitation of the aircraft. Both of the direct excitation techniques included vertical and horizontal polarizations from three aspect angles. The shielding effectiveness data for the aircraft cockpit derived from the three techniques are in general agreement within measurement uncertainty. The minimum shielding effectiveness values are about 15 dB over the frequency interval 0.1 to 1 GHz. Polarization effects and aspect angle variations of a few dB were measured. The ambient electromagnetic environment excitation suggests the possibility of narrow band structure in the shielding effectiveness.