Unexpected results in microwave absorption – part 1: different absorption mechanisms for metal-backed film and for material

It is commonly accepted that two synergistic effects maximize the microwave absorption power of a film, increasing the absorption power of the material and increasing the amount of penetration of incident microwaves into the film. However, this reasoning is incorrect as it confuses material with film and is therefore not consistent with the real nature of the absorption mechanism for a film. By differentiating film from the material, it is shown that reflection loss RL is a parameter only relevant for film and established that the real mechanism involves balancing the intensities of the two beams reflected from the two interfaces of the film when the beams are out of phase by π. It is commonly believed incorrectly that maximum absorption from film occurs when the resultant beam which is reflected back to open space from the interface with the metal-back is weakest. But it is shown here that the opposite is true, the maximum absorption occurs when this reflected beam reaches its maximum. A definition for film is proposed in the context of microwave absorption.