Equiangular Spiral Antenna Backed by a Shallow Cavity With Absorbing Strips

When a conducting shallow cavity is placed behind an equiangular spiral to obtain a unidirectional beam, the inherent wideband characteristics of the spiral deteriorate. To restore the wideband characteristics, a ring-shaped absorbing strip (R-ABS) is placed under the spiral arms. Analysis of the equiangular spiral with the R-ABS is performed using the finite-difference time-domain method. It is found that the R-ABS successfully restores the wideband characteristics. Subsequently, the R-ABS is divided into two absorbing strips, each specified by an arc-angle . Analysis reveals that the radiation characteristics obtained using the R-ABS are reproduced when the arc-angle is greater than . Throughout this paper an extremely small cavity depth is selected for the analysis: 0.07 wavelength at the lower operating design frequency of 3 GHz.