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The characteristics of an Archimedean spiral (ARSP) antenna backed by an extremely shallow cavity (0.07 wavelength at the lower operating design frequency of 3 GHz) are analysed. It is revealed that the presence of the cavity causes noticeable variation in the antenna characteristics at low frequencies. To restore the wideband characteristics inherent to the ARSP, a ring-shaped strip absorber (R-ABS) is placed under the spiral arms. It is found that the wideband antenna characteristics are successfully restored with the use of the R-ABS. Subsequently, the R-ABS is divided into two arc-shaped strip absorbers and the volume of each is reduced by decreasing the arc-angle ARC from 180deg (corresponding to an R-ABS) to 0deg (corresponding to a cavity without an absorber). It is shown that when the arc-angle is chosen to be greater than degARC=90=, the antenna characteristics obtained for =ARC=180= are reproduced. Theoretical results are confirmed using results from experimental work.
Date of Publication: October 2008