Ultra-wideband, miniaturized, low profile, omnidirectional antenna using a novel reactive loading approach

This paper presents a novel approach based on reactive loading of a resonant structure to achieve ultra-wideband operation for a low-profile omnidirectional antenna. The use of the proposed reactive elements generates new resonances in lower frequency band than the original antenna resonant frequency. The key feature of the reactive load design pertains to its frequency response so that while the input impedance of the reactive load is seen as `infinite´ at the original resonant frequency, it is seen as `capacitive´ at newly generated resonant frequency. Proper combination of these resonant frequencies and their harmonics leads to a very wide bandwidth operation. A circuit model for the reactive load is first proposed and the realization of its electromagnetic model is followed. To implement the proposed method to antenna design, a λ/2 resonant folded monopole antenna exhibiting wider bandwidth than a λ/4 resonant monopole antenna is chosen. It is discussed that 10dB fractional bandwidth (FBW) of the antennas with the proposed reactive loads can be 45% and omnidirectional radiation pattern with vertical polarization be obtained in the operating frequency band.