In this article, a pair of full-wave microstrip resonators is loaded at the nonradiative edges of the patch to improve port isolation of two-port copolarized single-patch antennas (SPAs) for full-duplex applications. First, the decoupling mechanism and its resulting radiation characteristics are exhaustively analyzed, which indicates that more similar radiation patterns and lower crosspolarization can be achieved based on the directly suppressing port coupling. Then, a transversal-section-like topology composed of an additional interference path and the SPA is proposed to realize this purpose. To clearly illustrate the operation principle in practical design, the phase shifts of both paths, that is, the interference paths from Port 1 to Port 2 via the patch and the full-wave resonators, are exhaustively investigated. Attributing to the realization of a stable 180° out of phase between the two paths, the resultant signal is highly and stably attenuated by the interfering signal, and the desired port isolation is satisfactorily attained by manipulating its amplitude readily. The measured results show a dramatic enhancement in isolation between the two ports from 4 to 55 dB with good impedance matching at the two ports. Most importantly, benefiting from the idea of direct port isolation, the radiation performance is hardly deteriorated, with a maximum gain of 8 dBi, beam squint angle of only 10°, and low crosspolarization in the upper half-space.