Wideband Conical-Beam Circularly Polarized Microstrip Antenna for Large Ground Plane

A wideband conical-beam circularly polarized (CP) microstrip antenna for an electrically large platform is investigated. The antenna structure simply consists of a center-fed patch and eight shorting vias each of which is surrounded by a modified elliptical-ring slot. The θ-polarization of the proposed antenna is provided by the shorting vias and the coaxial probe, while the φ-polarization is generated by the modified elliptical-ring slots. Moreover, the amplitudes of the two orthogonal polarizations can be controlled separately and are less sensitive to the size of the metal platform. By employing the even and odd modes of the elliptical-ring slot, the proposed antenna exhibits a wide impedance bandwidth. A prototype of the antenna is fabricated and mounted on three ground planes, which function as electrically large metal platforms, with the radii of λ₀, 2λ₀, and 10λ₀. The measured results show that the wideband CP antenna with these three ground planes have the 10-dB impedance bandwidths of 19.4%, 19.5%, and 19.9%, and the corresponding 3-dB axial ratio (AR) bandwidths of 25.3%, 25.6%, and 24.8%, respectively. A detailed analysis on the radiation characteristics is also carried out for small to extremely large ground plane extensions.