In this article, a fundamental method based on mutual conductance elimination is proposed for the ultimate decoupling of closely spaced antenna pairs. First, the mutual coupling between radiators is characterized by lossy J/K inverters, and the analysis with circuit models shows that the traditional lumped loading method can only suppress the mutual susceptance. Particularly, the decoupling will not be ultimate unless the mutual conductance is further eliminated, which requires equal modal conductance of the odd and even modes of the antenna pair. After that, a method based on antenna-shape modification is proposed, and three types of antennas are designed with characteristic mode analysis (CMA). As for the single-band and wideband patch antenna pairs, by changing the patch shape into a parallelogram and adjusting the patch angle, the modal conductance of the odd and even modes are regulated to be equal, and the isolation is thus significantly improved when compared with the traditional ones to be loaded with lumped elements. As for the inverted-F antenna (IFA) pair, enhanced isolation is achieved by changing the bending configuration, coupling gap, and ground length of the antenna. Good agreement between simulated and measured results shows that all these antennas have achieved isolation over 20 dB.