The problems in the quarter-wavelength model and impedance matching theory in analysising microwave absorption material

Abstract

It is shown here that many concepts in current mainstream microwave absorption theory are used inappropriately. Reflection loss RL has been used to characterize microwave absorption from material instead of film and the results have been rationalized by impedance matching theory. The quarter-wavelength model states that the reflection of microwaves with wavelength l from a film is minimized if the thickness of the film is m l /4 where m is an odd integer. But we show here that the model is wrong because the phase effects from interfaces have been overlooked. RL is an innate property only for metal-backed film. Impedance matching theory is developed from transmission-line theory for scattering parameter s 11 but cannot be generalized to RL.