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Conformal slot arrays have been widely used in mobile communication systems due to their distinctive features such as compactness, ease of fabrication, high power handling, frequency-scanning capability and wideband/wide-scanning performance. So far, the focus has been on waveguide-fed slot arrays. On the other hand, cavity-backed slot (CBS) arrays have also been considered mainly because of their low-profile, flush-mountable features. Nevertheless, the bandwidth of CBS antennas has been very limited except for the traveling-wave slot configurations. Here, our goal is to achieve bandwidth enhancement for cavity-backed planar slot arrays by introducing a novel substrate/feed design. When conformal slots are placed over cavity-backed, planar or curved substrates, their bandwidth and radiation characteristics are altered due to the presence of undesired modes (surface-wave or parallel-plate modes) within the cavity. Hence, it is essential to use a frequency-dependent substrate to improve antenna performance. This can be achieved by using a multilayer frequency selective surface as an artificial substrate with a tailored frequency response.