I am trying to validate the effect of a slot in a PEC shield using CST Studio Suite  . I have built two models:

Two PEC plates separated by a physical air gap in the middle.

A PEC plate with a slot, modeled as a zero-thickness aperture / CST slot.

Both models are illuminated by a HEMP plane wave. The surrounding boundary is set to open on x/y sides. I record the E-field along a line/plane behind the plate in the propagation direction.

My expectation is that the peak E-field behind the aperture should decrease with distance from the plate, or at least not increase indefinitely. However, I observe the opposite: the farther the probe is behind the plate, the larger the peak E-field becomes. The two models show opposite trends behind the plate. For the physical air-gap model, the peak E-field grows with distance (z = 45 mm < 60 mm < 90 mm). Conversely, for the slot model, the peak E-field decreases with distance. Additionally, the peak signal amplitudes differ drastically between the slot and physical models. This looks very strange and possibly unphysical to me.

what caused it?