If there were microwaves being transmitted through the waveguide in the image below out the horn, and I place a sheet of metal in front of the horn (perpendicular to the propagation of the waves), will the standing wave ratio inside the waveguide be 1?

I know a standing wave will be formed between the horn and the sheet of metal because the transmitted and reflected waves will combine, but I'm not sure if the reflected waves will re-enter the waveguide through the horn.

I say that I think the SWR will be 1 because the horn matches the impedance of the waveguide with the impedance of free-space. But this would mean measurements of the voltage of the wave inside the waveguide will be fixed and there would not be a standing wave inside.