Here's some info on how the absorbing cones in these chambers are constructed: https://en.wikipedia.org/wiki/Radiation-absorbent_material https://en.wikipedia.

Here's some info on how the absorbing cones in these chambers are constructed: https://en.wikipedia.org/wiki/Radiation-absorbent_material https://en.wikipedia.org/wiki/Radiation-absorbent_material "To be sufficiently lossy, RAM can be neither a good electrical conductor nor a good electrical insulator as neither type actually absorbs any power. Typically pyramidal RAM will comprise a rubberized foam material impregnated with controlled mixtures of carbon and iron. The length from base to tip of the pyramid structure is chosen based on the lowest expected frequency and the amount of absorption required."