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Transmission/reflection-band centre ratios of about 1.3 can be attained with multiple layers of inductive or capacitive grids, or with single-layer arrays of more complex elements. The copolar losses and the linear crosspolar performance of a 4-layer inductive grid and a 2-layer array of concentric rings are compared to illustrate some of the problems that can arise experimentally.
Concentric rings can be used as array elements in frequency selective surfaces. The transmission curves are more complex than those for simple rings, but provide more closely spaced reflection and transmission bands. In an example discussed here, a band centre frequency ratio of 1/1.3 is given by an array of compound elements with two concentric rings.
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