A novel design approach to wideband, dual-mode resonant monopole antenna with stable, enhanced backfire gain is advanced. The sectorial monopole evolves from a linear, 0.75-wavelength electric prototype monopole under wideband dual-mode resonant operation. As theoretically predicted by the two resonant modes TE_3/5,1 and TE_9/5,1 within a 150° radiator, the operation principle is revealed at first. As have been numerically demonstrated and experimentally validated at 2.4-GHz band, the designed antenna exhibits a wide impedance bandwidth over 90.1% (i.e., 2.06–5.44 GHz), in which the stable gain bandwidth in the backfire, −x−direction(θ=90^∘, φ=180^∘) with peak value of 3.2 dBi and fluctuation less than 3 dB is up to 45.3% (i.e., 3.74–5.44 GHz). It is concluded that the stable wideband backfire gain frequency response should be owing to the high-order resonant mode in the unique sectorial monopole antennas.