Compact dual-band circularly polarised antenna with omnidirectional and unidirectional properties
A novel dual-band circularly polarised (CP) antenna is presented in this study. The proposed antenna is composed of a U-slot patch and four sequentially rotated shorted monopoles. Different from conventional omnidirectional CP antennas with shorted monopolar patches, a novel capacitively coupled technique is proposed to achieve good impedance matching, which makes it possible to generate unidirectional CP patterns simultaneously. This antenna can generate omnidirectional CP radiation patterns in the global positioning system L1 band (1575 ± 5 MHz) for navigation application and unidirectional CP radiation patterns in the 2.4-GHz industrial, scientific, and medical band (2.40-2.48 GHz) for municipal wireless networks. The dual-band performance can be tuned separately. Experimental results confirm the good performances of a compact antenna with the size of 0.26 λ
 0
 × 0.26 λ
 0
 × 0.026 λ
 0
. The obtained 10-dB impedance bandwidths can cover the two required bands. The 3-dB axial ratio (AR) bandwidths cover the lower band and the upper band from 2.425 to 2.46 GHz. In addition, a wide 3-dB AR beamwidth of 200° is obtained at 2.45 GHz. The simple structure and good performances make this antenna a good candidate for handheld devices and vehicular applications.