Hi, I'm having some simulation results that I can't explain. I don't know if it is due to artifacts in the simulation (e.g. boundary condition) or I didn't understand correctly. Can someone help?

The simulation is quite simple (in Fig. 1): there is a square metal plane, 1 cm by 1 cm in size (~one \\lambda at 24 GHz), with a thickness of 1 mm. It is excited by a plane wave, with propagation direction perpendicular to the metal plane. I'm trying to simulate the scattered EM wave from the metal. The boundary condition is open (add space) on all 6 sides.

However, in the simulated result (in Fig. 2), there is radiation in the backside of the metal plane, which is almost exactly symmetric to the front-side radiation. In my understanding, ideally there shouldn't be any radiation because EM wave can't penetrate all the way through the metal, so there is no EM field (or surface current) on the other side. Is my understanding incorrect or is the backside radiation due to artifacts in the simulation?

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