Pattern Purity of Coiled-Arm Spiral Antennas

The consequences of the coil-type arm treatment on the far-field characteristics of spiral antennas excited in the broadside mode (also referred to as Mode 1) are discussed. Computational analysis of the coiled-arm spiral antenna with two and four arms is performed using the moment method, and the effects on modal content, 3-dB beamwidth, azimuthal gain uniformity, off-axis axial ratio, and co-polarized broadside gain are reported. Predictions of the numerical calculations are verified experimentally through far-field measurements. It is found that the coil-type arm treatment produces significant modal contamination of the far-field pattern when compared to a conventional smooth spiral (with no arm coiling). It is shown that these effects can be mitigated by employing an antenna design that uses four spiral arms. For instance, the measured cross-polarization discrimination of the four-armed spiral at 47 degrees from broadside is improved by up to 15 dB with respect to a comparable two-armed spiral.