Transmission Loss Analysis of Unidirectional Carbon Fiber-Reinforced Polymers for Millimeter Waves for Linear and Circular Polarization

Carbon fiber-reinforced polymers (CFRPs) shape numerous industrial- and consumer-oriented products that benefit from lightweight construction while maintaining high durability. Electromagnetic waves transmitted through such conductive CFRPs experience strong attenuation that largely depends on the internal structure of the composite material. Therefore, we investigated the absorption loss of different unidirectional CFRP specimens with various numbers of carbon fiber (CF) plies and thicknesses. The results are presented for linear polarization (LP) and circular polarization (CP), and different orientations of the specimens at 45 GHz. Compared with LP, the transmission characteristics of CP hardly depend on the anisotropic character of unidirectional fiber plies. We observe that the absorption loss is mostly dependent on the number of conductive CF plies rather than the fiber volume ratio (FVR) of the sample. We present a multilayer homogeneous model that assigns an effective conductivity to a CF ply. The model assumes an alternation between dielectric epoxy resin and conductive CF plies. Iteratively, the effective conductivity of the conductive layers, in which the modeling results best fit the measurement results, is determined. In practice, this calculated effective layer conductivity can be used to estimate the transmission loss or shielding effectiveness of products made of CFRPs. The solver time of simulations becomes manageable by applying the multilayer simplification. For the first time, to the best of authors’ knowledge, effective conductivity values of CFs and their composites are presented at such high frequencies.