Discussion Forum

Interesting question. The use of "frequency domain" to solve RF problems removes the time parameter from the problem, i.e., the differential equation becomes literally "time-independent." See the attached image. A time-domain exponentially decaying signal is not a "single-frequency" signal. And I can see that an attempt to specify a complex frequency fails. That said, a signal that decays spatially can certainly be modeled in the frequency domain. Perhaps one of the real Comsol experts here can offer a comment on whether there is any hope at all for doing what you want to do via using a frequency domain interface. I would suspect that aside from some kind of trick (such as employing various transform operations in post-processing, or adding some clever analytic feature to the model that would effectively allow an interpretation as if it were time domain), you will probably have to work with a time domain solver.

Robert is right, a decaying signal is not harmonic. On the other hand, your model has to be linear to be suitable for frequency domain. So any quantities that depend on the stimulus in the model will be following the stimulus in a linear manner. It might be good enough to add the decay in the post processing.

Cheers Edgar

Dear Robert and Edgar,

Many thanks for taking time to share your thoughts. I agree with what you said and I do work with a time domain solver now. I could not think of an easy way to implement the decay I want in the post prosessing.

Still it would be interesting to read a comment from someone close to the development of comsol as to whether it is at all possible, what I asked, at least in principle.