Practical limitations to a natural mode characterization of electromagnetic transient response measurements

Practical limitations arising from limited signal-to-noise ratios in actual transient electromagnetic measurements limits the number of natural modes which can be extracted from such measurements. This number is shown here to be three to six for a complex scatterer, in this case an aircraft, five for a "fat" cylinder and thin wire. This is for double exponential waveform excitation of the aircraft and step function excitation of the fat cylinder and thin wire. A few more modes can be resolved if the excitation is altered so as to provide more high frequency energy, as can be accomplished by changing the step excitation to a delta function excitation, or by signal conditioning, for example making a derivative measurement that emphasizes the high frequency response. Even so, far fewer modes can be resolved experimentally than can be predicted. Experimental electromagnetic transient response measurements should therefore be made with these practical limitations in mind.