Microwave retroreflectors are a thing: https://en.wikipedia.org/wiki/Corner_reflector#In_radar https://en.wikipedia.org/wiki/Corner_reflector#In_radar Too large

Microwave retroreflectors are a thing: https://en.wikipedia.org/wiki/Corner_reflector#In_radar https://en.wikipedia.org/wiki/Corner_reflector#In_radar Too large for personal use, but one could certainly line a wall with them. Would increase the danger of using such a weapon, to have a significant portion of the signal reflected back at the operator. Apparently there's been some very recent research into planar (and presumably more portable) microwave retroreflectors as well: https://ieeexplore.ieee.org/document/9060553 https://ieeexplore.ieee.org/document/9060553