Low Profile Vertically Polarized Omnidirectional Wideband Antenna With Capacitively Coupled Parasitic Elements

This communication presents a low profile and electrically small wideband antenna with omnidirectional radiation pattern and vertical polarization. A novel design approach manipulating the topology of a low profile folded monopole antenna with capacitively coupled parasitic elements in the same plane is presented to achieve omnidirectional radiation pattern. The 10-dB return loss fractional bandwidth of 43% is achieved with the dimension of 0.2λ_LF ×0.2λ_LF ×0.06λ_LF where λ_LF is the wavelength at the lowest frequency of the operation. Unlike the convention wideband λ/4 monopole antennas utilizing inductively coupled parasitic elements, the λ/2 folded monopole antenna allows for positioning the capacitively coupled parasitic elements in the middle of the antenna where maximum electric stored energy is formed. This, together with reducing the lateral dimension of the folded monopole antenna, enables the cancellation of radiated fields from electric currents in the horizontal plane of the proposed antenna, which is essential to achieve vertically polarized omnidirectional radiation. The compact parasitic elements introduce additional resonances that significantly increase the antenna bandwidth. Effects of design parameters on two resonant frequencies and impedance matching to a 50 Ω feed are investigated using the equivalent circuit model of the parasitic element and full-wave electromagnetic (EM) simulations. Based on this analysis, a design procedure to optimize the antenna topology is established.