How do re-entrant triangles work by "trapping" radar signals and were are they put on the aircraft?

Question

Please explain in high detail.

Answer 1

Thats classified information......

Answer 2

Below the F-117A's skin one would find a special radar-absorbent structure, known as re-entrant triangles. The geometry of this structure "traps" the electromagnetic energy and gradually dissipates it by bouncing it off internal faces. Such radar-absorbent structure in the US was first used on SR-71.

On the surface of the aircraft, the principle is simple:  the radar waves aren't "trapped", they are reflected away from the source (the sender) by never presenting a face directly at the radar source.  If the signal is deflected away, and none of the signal returns, then the object is "invisible" on radar.

An example:  imagine you are looking at a tetrahedron (a pyramid) from the top, looking down.  The pyramid is composed of 4 triangles, with the pointy end looking at you.  If you sent radar waves toward the pyramid, there is no flat surface to reflect the waves back to you - they deflect off the surfaces of the pyramid and away from you.  So the pyramid is not visible by radar.  

"Re-entrant" triangles are like a bunch of pyramids placed side-by-side.  The principle is the same, the radar waves don't have a surface facing the radar source, and so the waves aren't reflected (they attenuate themselves on each reflection).  This design (pyramids placed side by side) is terrible from the tubulence standpoint, so the triangles either need to be very small, or covered by a radar-transparent material to smooth out the surface for airflow purposes.

Re-entrant triangles/pyramids are also used in anechoic chambers to dampen radar or acoustic waves.