Re-visitation on the input impedance of two-arm frequency-independent antennas in free space

The proposed analytical close-form solutions for the input impedance of planar two-arm Archimedean spiral antenna in free space is a function of metallization ratio and can fulfill the Babinet´s principle fundamentally. Therefore, the applications of proposed model to other planar frequency-independent (FI) antennas are examined and discussed in this paper. It is found that Archimedean and equiangular spiral can be truly FI antennas based on their geometric formulas, while log-periodic and sinuous antennas exhibit FI properties only at self-complementary structure. Measurements and numerical simulations of several FI antennas validate the statement.