A Wideband Circularly Polarized Conical Beam From a Two-Arm Spiral Antenna Excited in Phase

Analysis is performed for a two-arm round spiral structure excited in phase, where the arms near the antenna center are backed by a small round disc. This spiral antenna has a frequency range where the radiation is bidirectional, having patterns symmetric with respect to the antenna plane. Subsequently, the bidirectional radiation is transformed into a unidirectional conical beam by backing the spiral with a cavity, where the cavity height is chosen to be extremely small (0.077 wavelength at the lowest design frequency 3.3 GHz). It is found that, as the distance between the spiral plane and the small disc inside the cavity is decreased, variations in the axial ratio and VSWR become smaller, as desired. Other antenna characteristics over a wide frequency range of 3.3 GHz to 9.6 GHz, including the gain and radiation efficiency, are also discussed.