Spiral antenna radiating a conical beam of circular polarization

A two-arm spiral antenna excited in phase radiates a circularly polarized (CP) bidirectional beam. This bidirectional beam is transformed into a CP unidirectional beam using a conducting cavity as a reflector. The height of this cavity is chosen to be extremely small to realize a low-profile antenna: 0.077 wavelength at the lowest design frequency. The antenna characteristics, including the radiation pattern, VSWR, axial ratio, gain, and radiation efficiency when a ring-shaped absorbing strip is placed behind the outer spiral arms, are analyzed and discussed. It is found that a conical radiation pattern is obtained over a wide frequency range of 1: 2.9.