The fundamental working principle of front-to-back ratio (FTBR) enhancement of the quadruple inverted-F antenna (QIFA) is clearly revealed for the first time and a unified design method is then proposed in this paper. The characteristic mode analysis (CMA) demonstrates that the QIFA actually operates in two orthogonal modes, and analytical derivation validates that each mode acts as a pair of parallel magneto-electric (ME) short dipoles for radiation. More specifically, the FTBR of the antenna is determined by the amplitude ratio and phase difference of the two pairs of equivalent ME dipoles. The equivalent model and analytical method can greatly facilitate the design. Firstly, by leading or lagging phase of either characteristic mode through excitation, the beam direction can be flexibly prescribed as forward and backward. Secondly, the bending direction and bending degree of radiators respectively determine the phase and amplitude of its equivalent magnetic dipole, by which the FTBR can be improved. A practical design is carried out with the proposed method, and the antenna achieves an electrically small size of 0.068λ0 × 0.068λ0 × 0.052λ0 and an FTBR of 12.3 dB.