Discussion Forum

Hi Everyone,

Just some setup info quick: Version 4.2, RF Module

I've been trying to do some work with comsol far field...As a test or toy problem, I chose to investigate the diffraction of light through a circular aperture in a perfectly conducing thin screen.

The hole diameter is 5 wavelengths, and I hit it with a plane wave using both port excitation in one model, and then using a background plane wave in another model.

At these dimensions, the answer for the Fraunhoffer far field diffraction pattern is well known, and I pulled it from a text book to compare Comsol's answer against.

Basically, on a screen in the far field, you would see a bright central spot, surrounded by progressively lighter (in intensity) concentric rings--An Airy pattern.

Using both the full field and scattered field formulation, however, I cannot get agreement between Comsol and the analytic solution after transforming the fields exiting the aperture (I do the transform on a half-sphere on the exit side of the screen which is slightly larger in diameter than the hole is).

My check is to see if the location of local maxima and minima are at their appropriate angle, taking the origin as the aperture center. For a given hole diameter the local maxima and minima always show up at the same angle. Problem is, Comsol doesn't produce good agreement. Qualitatively, however, I must say using the scattered field formulation produces what ~looks~ like Airy rings...

I've used the approach laid out in both the "patch antenna" [Although the tutorial on FF is only contained in the 3.5a PDF, not 4.2 like I'm using] and the "scattering cross section of a boat"

I attached a .pdf with all the details of my setup and the results in case you're interested. The solution is too big to upload, and attaching a blank setup before running would be rough, because I find it necessary to use cluster computing to get it done within an hour....yep, pretty big, I know...But simple in principle ;)

I must still be missing something, any help is very very much appreciated!
Thanks again!