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In this paper, we have outlined a novel procedure for accurately designing and optimizing wideband, cavity-backed Archimedean spiral antennas. We have used a precise transmission-reflection based waveguide measurement technique to accrately characterize a set of commercially available absorbers. From the complex permittivity and permeability data thus obtained, we designed a multi-layer composite absorber effective from 2 to 18 GHz. In EM simulation software FEKO, we modeled a 2-18 GHz Archimedean spiral antenna with the multi-layer absorber inserted in the cavity. With the FEKO optimizer, we determined the optimal thickness of each layer such that the absorption properties are sufficiently retained, co-pol RHCP gain maximized and crosspol LHCP gain minimized over the entire bandwidth.