Depends on the definition of "simple", imo. The first thing that comes to mind is research into materials with good (tens of dB), wideband absorption in the mmW

Depends on the definition of "simple", imo. The first thing that comes to mind is research into materials with good (tens of dB), wideband absorption in the mmWave bands. It's an area of active research [1] [2] (just a couple articles from a quick google, so caveat emptor). [1] https://www.cambridge.org/core/journals/international-journal-of-microwave-and-wireless-technologies/article/abs/broadband-millimeterwave-absorbers-a-review/4A05581C33F5E27E5AAB4CD16EC0D3D2 https://www.cambridge.org/core/journals/international-journa... [2] https://pubs.rsc.org/en/content/articlelanding/2022/tc/d1tc05078h https://pubs.rsc.org/en/content/articlelanding/2022/tc/d1tc0...