Radiofrequency waves in lossy media The complex propagation constant

However small, the energy losses generated by the propagation of radio waves give matter in its gaseous, liquid or solid phase the properties of an absorbing medium. In this context, and in the presence of periodic sustained oscillations, the wave attenuated by energy dissipation is characterized by the propagation constant assimilated to a complex number.

The subject is approached in two distinct parts. In the first paragraph, dedicated to the attenuation of plane waves, the progression of the analysis leads to the dissociation of three physical phenomena related to the notions of conduction currents, displacement currents and dipolar absorption.

It has been shown that for radio waves, conduction currents are significantly involved in the presence of media with high electrical conductivity, at least greater than 100S.m ^-1 . In such media, currents are induced in accordance with the principles of electrokinetics. For moderately conductive media, i.e. below 100S.m ^-1 , displacement currents are superimposed on conduction currents, the origin of which is directly related to the dielectric properties of the medium. The search for approximate formulas for the propagation constant therefore facilitates the study of plane wave attenuation in matter. Examples based on physical data from electrolytes highlight the major impact of wave frequency. In this article, waves are localized from the 50Hz industrial frequency to microwaves around ten GHz. In addition to heat dissipation due to conduction and displacement currents, there are other losses due to forced oscillations in the dipole moment of certain molecules. So-called dipole absorption is reflected in the concepts of complex electrical permittivity and equivalent electrical conductivity.

The second paragraph focuses on the microscopic aspect of the phenomena. Qualitative reasoning briefly recalls the processes of electronic conduction in metals, electrolytes and certain composite materials. The absorption of the water molecule is also discussed, with a few details illustrating the diversity and overlap of the physical mechanisms involved.

Reading the article does not require in-depth knowledge of electromagnetism, but we recommend consulting the articles [E 1 020] and