Characterisation of conducting polymer-loaded composite materials at oblique incidence and their application in radar absorbers

In previous papers, Wong et al. (1992), Wright et al. (1994), we have reported on the fabrication and microwave characterisation at normal incidence of large-area conducting polymer composites and radar absorbing materials (RAM) based on both paper and textile fabrics loaded with polypyrrole (PPy). The PPy-loaded composites were fabricated via a chemical oxidation technique. The morphology of the latter is such that it behaves as a complex impedance surface at microwave frequencies. This impedance surface can be modelled by an equivalent circuit comprised of a transmission line incorporating parallel R and C elements. Relationships between the amount of PPy loading on the substrate and the sheet values of R and C have been derived and their behaviour explained in terms of the substrate morphology and percolation effects. Typical variations of R and C with PPy loading are shown for a paper-based composite material. In practice, RAM has to be effective over a range of angles of incidence and incident polarisations; hence, in this paper, we present new data on the oblique incidence microwave characteristics of PPy-loaded paper composites and their performance in Salisbury screen RAM structures