Cavity-backed Archimedean spiral antenna with strip absorber

Abstract

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 phisARC from 180° (corresponding to an R-ABS) to 0° (corresponding to a cavity without an absorber). It is shown that when the arc-angle is chosen to be greater than phisARC=90°, the antenna characteristics obtained for phisARC=180° are reproduced. Theoretical results are confirmed using results from experimental work.