In this paper a comparison is made between the index‐of‐refraction values measured on five metal‐strip test samples and values computed from four solutions that have appeared in the literature. Three of the samples are wave‐guide equivalents, and two are free‐space arrays. The techniques of measurement for the two types of structures are described briefly. It has been found that all four solutions have certain ranges of validity, but that only one is reliable over the full practical range of the design parameters. This solution, derived previously by the writer, is based on a transmission‐line equivalent circuit in which proximity effects are taken into account.