A low-profile omnidirectional planar antenna with vertical polarization employing two in-phase elements

A novel technique for designing a low-profile miniaturized omnidirectional planar antenna with vertical polarization is presented. The proposed antenna is designed using two short in-phase vertical elements. To achieve in-phase radiation the short vertical pins should be λ/2 away from each other. To minimize the λ/2 transmission line a T-type 180 degree phase shifter with a capacitive impedance inverter can be used. However, the drawback of using this phase shifter is the generation of an out-of-phase conduction current going vertically down through the capacitor. The radiated fields from the conduction current cancel out radiated fields from in-phase currents on the two vertical elements, leading to the suppression of vertically polarized radiation. In this study, the conduction current is eliminated by substituting the capacitor connected to the shorting via with an open stub. The geometry of the open stub is optimized to obtain appropriate capacitance values giving the antenna lateral dimensions (λ/8X λ/8) and height (λ/40). An important advantage of the proposed antenna is omnidirectional radiation pattern and high gain.