Is it possible to model a near-field source on a 2D circular aperture?

Hello,

I have previously used the aperture fields (*.efe and *.hfe files) of a rectangular horn antenna to define an equivalent near-field source (by modeling a rectangular aperture of same dimension as that of the horn antenna). The comparison of the simulated far-field patterns obtained from the real antenna and the equivalent source had excellent agreement.

Now, I want to do the same thing for a circular aperture. For that I've already extracted the aperture fields from a conical horn antenna. However, I am unable to model a near field source which is circular in shape.

Any suggestions are welcomed. Thank you in advance!