Coplanar Series-Fed Spiral Antenna Arrays for Enlarged Axial Ratio Bandwidth

We study two array antennas to expand a 3 dB axial ratio bandwidth. Each array is located at a quarter wavelength above the ground plane and analyzed using the moment method. First, we use paired spiral elements fed by balanced parallel lines to avoid unwanted radiation from the feedline. It is found that the antenna shows an axial ratio bandwidth of 30%. Next, the elements are separated and fed by a single feedline to simplify the feed system. It is revealed that the antenna can radiate a circularly polarized wave under a feedline radiation of less than −16 dB. The frequency responses show that an axial ratio < 3 dB and VSWR < 2 are obtained in a bandwidth of 21%, where the gain is more than 13.3 dBi. The simulated results are verified with experimental ones.