Understanding skin current distributions on an aircraft at HF using eigencurrent expansions

In order to EMV test whole aircraft at HF, impractically large TEM cells or reverberation chambers and RF amplifier powers are required if the accepted principles are applied. Novel methods are called for which minimise RF amplifier power requirements and utilise more than the generally accepted uniform field volume of the TEM cell, or apply currents directly to the skin using direct current injection. Numerical modelling provides a valuable tool to explore and optimise these novel methods of applying EMV testing to whole aircraft at HF. However, the volume of data presented to the designer by such programs can be overwhelming: method of moments numerical models of small aircraft at HF produce several thousand sub-sectional basis functions. Comparisons between alternative methods therefore require either arbitrary decimation of this data or some other approach. Furthermore, the raw moment method data provides little insight into the ways in which the techniques may be improved. In this paper, a method of using eigencurrent expansions is demonstrated on an all-metal Aermacchi MB326H aircraft. It is shown that at HF, only a handful of the strongest skin eigencurrents are required to provide meaningful understanding of the DCI or free-space-generated skin currents on the whole aircraft This method may be used to systematically optimise the design of the DCI technique, including selecting injection sites, amplitudes and phases.