In an effort to improve the bandwidth of the single layer reflectarray, this paper investigates the use of double concentric circular ring elements arranged in a range of sub-wavelength grids on a single layer of substrate. Compared to the traditional λ/2 grid arrangements, when the radiating elements are arranged in grids less than λ/2, the reflected phase is more uniform over a wider frequency bands when radiating elements' parameters are varied; albeit with a reduced reflected phase range. The double concentric circular ring elements used here also allow an additional degree-of-freedom to improve the bandwidth. A comprehensive investigation on reflectarrays' performance with various grid spacings is conducted and the trade-off between the reflectarray gain and bandwidth is also discussed. Based on the concentric ring element, four offset-fed 0.43 m×0.43 m reflectarrays centered at 10 GHz with various element periodicities, namely λ/2, λ/3, λ/4 and λ/5 grids, are designed and developed. The measured results show that among the four reflectarrays, the one with λ/4 grid spacing achieves the broadest 2-dB gain bandwidth of 33% with an aperture efficiency of 36.2%.
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