On the Qualification of Airborne Equipment for Radiated Emissions

Abstract

This work deals with the qualification of airborne equipment with regard to unintentional radiated emissions, which can couple into the antennas of navigation and communication systems of aircraft. It focusses on the qualification of flight test instrumentation installed inside research aircraft. Extensive transfer path measurements form the basis of this work; emphasis is laid on the statistical analysis of measurement data. Qualification is tackled on two fronts. Firstly, a new EMC test procedure is developed to detect potentially interfering emissions from flight test instrumentation on board. It makes use of detection antennas to measure the electromagnetic environment inside the aircraft. The procedure, performed on board, tests the final configuration of the airborne equipment, is faster than similar spectral measurements on the terminals of all relevant aircraft systems and covers the entire frequency bands, whereas conventional tests in many cases have just assessed single isolated frequencies. The substantiation of the method employs the concept of subtracted transfer path firstly introduced in this work. Secondly, the reduction of the coupling to aircraft antennas is evaluated after covering the aircraft interior with electromagnetic absorbing films. Results show that wave scattering is important on the cabin propagation and on the coupling to aircraft antennas. It is also confirmed that energy leaks to the exterior of the fuselage through aircraft doors and windows. Moreover, results indicate that the coupling to aircraft antennas decreases only modestly with absorbing material added to the insulation of aircraft with metallic fuselage.