How can I limit the active working angle on a microwave motion sensor?

I have a microwave motion sensor that can be triggered in almost every angle.

I want to limit its angle of view to only one side of the board but so far I had no success.

What I have done:

1. Designed my board so that the surface that module mounts on is a ground plate. It is still triggered by motion that happens behind it.

2. Put an extra copper plate almost three time size of the whole device behind it and connected its copper layer to the ground of the board.

The only thing that comes to my mind and I haven't tried is to reduce the distance between the module and the board that it is mounted on to few milliliters. (Now it is about 1cm.)