How does the paint on a F-35 absorb radar waves instead of reflect them?

Question

I was doing some research for a project I am doing at school about military aircraft and I read that the F-35's paint is able to absorb radar waves instead of deflect them.

How does paint absorb radar waves?

Answer 1

For a radar beam to be reflected by an object (thereby producing a return) requires that there be an impedance mismatch between it and free space. The characteristic impedance of free space through which radar pulses travel is about 330 ohms, which is way different from that of a metal airframe. This means that if a radar pulse strikes that airframe, almost all of the pulse will bounce off it and almost none of it will be absorbed into the airframe.

On the other hand, if that pulse strikes something with the same impedance as free space or close to it, almost none of it will be reflected and almost all of it will be absorbed by that object.

Now we note that if the matched-impedance object possesses a certain amount of electrical resistance, the electric currents induced in it by the incoming pulse will be dissipated by its resistance, preventing those induced currents from producing an outgoing electromagnetic wave i.e., a return.

So stealth paint works by absorbing the incoming pulse and then dissipating it.

This process is not perfect, and so stealth designers strive to avoid flat surfaces and sharp 90 degree corners in the profile of the plane, so any returns generated will be sent off in a direction that does not point straight back to the radar station.