A Compact Circularly Polarized Antenna for 5.8-GHz Intelligent Transportation System

A compact circularly polarized antenna suitable for applications of an intelligent transportation system (ITS) is presented. The antenna consists of a disc patch operating in modal degeneration, which is gap-coupled with multiple annular sectors. The use of parasitic resonators coupled to the degenerated modes of the patch enhances the realized gain and the polarization purity, at the cost of only a modest increase in the area. Despite the small size of 35 × 35 mm2 (0.46 λ
₀
²
at the central frequency of 5.8 GHz), the experimental validation of the best prototype confirms a hemispherical coverage with a half-beam power of 80°, a maximum left-hand gain of 7.16 dB, a cross-polarization discrimination of 21 dB, and a 10-dB return-loss bandwidth exceeding 900 MHz, a performance suitable for ultracompact tag reader realized in cost-effective single-layer technology.