This paper describes measurements at centimetre wavelengths (8.0–11.0cm) on a system representing a semi-infinite array of infinitely thin, perfectly conducting, metallic plates. The purpose is to illustrate a general experimental method of determining the properties of such artificial media. The parallel-plate system was chosen since, in addition to exhibiting the interface phenomena peculiar to artificial media, it is the only one for which a rigorous solution is known. The measurements were made using a parallel-strip transmission-line1specially developed for the study of two-dimensional structures which are non-uniform transverse to the direction of propagation.