Numerical analysis of 4-arm Archimedean spiral antenna

The radiation fields of a 4-arm Archimedean spiral antenna excited with three types of balanced mode, M1(1, j, −1, −j), M2(1, −1, 1, −1) and M3(1, −j, −1, j), are evaluated on the basis of the current distributions which are determined numerically. An axial beam and conical beams are presented with experimental results. It is demonstrated that an axial ratio of less than 3 dB can be obtained over a wide angle range of about ± 60°, when the spiral is excited with modes 1 and 2. It is also found that the axial ratio for mode 3 is liable to be deteriorated by the reflected current, particularly around the spiral axis.