A Compact Omnidirectional Circularly Polarized Planar Antenna With Excellent Axial Ratios and Pattern Roundness

A compact omnidirectional circularly polarized (OCP) planar antenna based on the modified complementary dipole is proposed. First, a magnetic dipole (M-dipole) formed by a half mode substrate integrated waveguide (SIW) cavity and a closely placed electric dipole (E-dipole) are used to produce the horizontal and vertical polarizations, respectively. Thanks to the compact arrangement of M-dipole and E-dipole, the antenna height is reduced by 63.1%, compared with traditional magneto-electric dipole antenna. In order to adjust conveniently the phase difference and save the space simultaneously, the phase shift structure is hidden in the SIW cavity, thus OCP waves can be radiated. Then, in order to eliminate adverse effects of the asymmetric structure, the E-dipole is bent such that the axial ratios (ARs) and the pattern roundness in the azimuth plane are improved significantly. Finally, the proposed antenna at 2.45 GHz is fabricated and measured. At the resonant frequency, measured ARs and gain variation in the azimuth plane are less than 1.8 dB and 1.5 dB, respectively. Coincident trend between simulated and measured results makes the proposed antenna a good candidate for WLAN applications.