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Efficient use of the radio spectrum demands frequency reuse in increasingly localised regions in the built environment. It may be possible to employ frequency selective screens (FSS) to provide electromagnetic separation of adjacent areas. To study the signal isolation capability of these structures, field distributions have been measured inside a small enclosure with one wall clad with FSS which had very broad reflection bands. The FSS arrays were densely packed linear dipoles, on rigid and on flexible film substrates. Measurements were made at 2.45, 5.0 and 8.1 GHz. The spatial isolation was very significant: the signal attenuation averaged over the enclosed volume was 20 to 30 dB, but the field amplitudes and insertion losses were position dependent. The variations were characterised by Rayleigh statistics, except for the highest field levels. There, the observed probability density functions (pdfs) often deviated from the Rayleigh form, possibly owing to signal leakage around the FSS edges, especially with thin film substrates, indicating problems in applying it satisfactorily to existing surfaces. In an existing building, it might therefore be acceptable to replace only part of a wall with frequency selective material, metallically screening the remainder. In new buildings, frequency selectivity would be built in, with electromagnetic performance addressed at the design stage.