Electrically small antennas usually exhibit limited bandwidth and reduced efficiency. The aperture tuning technique offers a means to achieve broad frequency characteristics for such antennas. However, determining the optimal positioning of the tuning element can be challenging. Consequently, a novel wideband tuning method based on characteristic mode analysis is introduced. According to the analysis of modal significance and characteristic current distribution of the small loop antenna, a small slot is created at the location where the current of the primary resonant mode is strongest. Two varactors are then inserted in parallel within the slot and adjusted by using bias voltage. This method enables the originally narrowband resonant antenna to attain an ultra-wideband tuning capability. Experimental results confirm the feasibility and effectiveness of the method, revealing a relative tuning ultra-wideband bandwidth (VSWR < 3) is 53.6% (103MHz to 178.5MHz) and the gain is about -3.9to -1.8dBi within the bandwidth, Furthermore, the minimum electrical size is only 0.086λ.