Modal excitation of the EMPTAC using multiple antennae

Modal excitation of a large aircraft was demonstrated by attaching electrically short monopole antennas to the aircraft's extremities (nose, tail, and wing tips) and feeding these antennas in-phase or out-of-phase using appropriate transformers and/or power amplifiers. Surface magnetic fields were measured at various points on the aircraft. Relatively pure single mode excitation was demonstrated for the lowest frequency modes. The resonances were much broader than those predicted by theory and the mode structure seemed to disappear for frequencies greater than four times the lowest resonant frequencies. The resonant frequencies were significantly different from those predicted by theory. It is shown that theory frequently incorrectly predicts this frequency by as much as 40%.<
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