Letter to the editor [Use of periodic functions for equipment design]

It is pointed out that there has long been unease about using a periodic function, such as a sinusoid, as an input to determine the response of a network or of a structure excited by an electromagnetic field. It has been argued that periodic functions have infinite energy and violate causality because there is no way that one cycle of a periodic function differs from any other. An argument that shows that the traditional method of using sinusoids or other periodic functions to design equipment is asymptotically correct is presented