A Compact, Broadband Spiral Antenna With Unidirectional Circularly Polarized Radiation Patterns

A low-profile, compact spiral antenna with a broadband circularly polarized (CP) response is presented. The antenna is backed by a ground plane and has unidirectional radiation patterns over its entire frequency band of operation. This antenna is a multilayer structure composed of a center-fed modified Archimedean spiral that exploits a novel loading structure, a ring-shaped absorber, and a feed network, which includes a 180° power splitter. The loading structure possesses both inductive and capacitive characteristics, which increase the equivalent electrical length of the antenna while maintaining its maximum dimensions. The Archimedean spiral is integrated into the multilayer dielectric structure along with its differential feed network. An optimized ring-shaped absorber is used on the periphery of the antenna to reduce the ground effects on the antenna performance. The proposed antenna occupies a volume that is 89% smaller than that occupied by a conventional ground-plane-backed Archimedean spiral antenna. At its lowest frequency of operation, the antenna has electrical dimensions of 0.21λ
_min
× 0.21λ
_min
× 0.09λ
_min
, where λ
_min
is the free-space wavelength at the lowest frequency of operation (0.5 GHz). Over the frequency range from 0.5 to 1.4 GHz (2.8:1), the antenna has a VSWR of 2.4:1, and it has a CP radiation pattern with an axial ratio better than 1.2 dB. Within this frequency range, the antenna has minimum and maximum realized gain values of -5.0 dBiC and 3.1 dBiC, respectively.