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The question is rather an everyday question: Think on an electromagnetic wave (e.g. let's say 60 GHz) and I would like to know how to absorb it such that there will be no transmission (through a wall) and no reflectivity (at the same wall).

A metal plate attached to the wall will remove any transmission (why is that, btw)? But reflectivity should still be present, right?

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  • $\begingroup$ Since this is physics stackexchange - just use black body. The rest is engineering. $\endgroup$
    – void_ptr
    Commented Apr 25, 2019 at 19:44
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    $\begingroup$ No transmission and reflectivity is not possible for real materials. You can minimize it but not fully erase it. $\endgroup$
    – Maxim
    Commented Apr 25, 2019 at 20:18
  • $\begingroup$ @Maxim Thanks! What do you mean by transmission and reflectivity is not possible for real materials? What exactly is the term "real" here? $\endgroup$
    – Ben
    Commented Apr 26, 2019 at 13:15

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For radio waves, you can achieve near-zero transmission and reflection by engineering a wall to exhibit a close impedance match with that of free space, and to possess electrical resistivity. The impedance match minimizes reflections and the resistivity dissipates the incident energy. You can learn more about these techniques by searching on stealth technology and radar cross-sections.

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  • $\begingroup$ Thanks a lot! To be precise: A metal plate will remove the transmission (almost completely) but reflections will still be there? $\endgroup$
    – Ben
    Commented Apr 26, 2019 at 13:17
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    $\begingroup$ a solid metal plate will act as a mirror, and very effectively bounce the wave without absorbing its energy. $\endgroup$
    – niels nielsen
    Commented Apr 26, 2019 at 17:17
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You use an EM absorbent material (such as carbon impregnated foam). As mentioned, impedance matching of the material to free space (377 ohms) would be ideal, but practically the material is formed into cones so that a wall of it looks like an egg-carton like grid. Incoming radiation then sees a smooth transition from air to absorber. This is used in EM "anechoic" chambers. 60 GHz is millimeter wave, so you might start by searching "millimeter wave anechoic chamber absorber".

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