This paper presents results of a study of electrophysical characteristics of composite radio materials in the frequency range from 10 MHz to 18 GHz. The composite materials are based on epoxy resin with multiwall carbon nanotubes (MWCNT) of various diameters. Coatings containing MWCNT have higher reflection and absorption coefficients. Varying filler concentration in the composite materials changes characteristics of the protective coating from reflective to absorbing electromagnetic radiation.