This paper proposes an analytical approach to diagnose coupling matrix and extract circuit model for low loss filtering multimode patch antennas from S₁₁ measurement for the first time. In contrast to existing reported methods by introducing extra simulation ports or radiation measurement data, we find an analytical method to recover the two-port Sparameters from one-port antenna’s S₁₁. On the one hand, the proposed method can diagnose coupling matrix and extract circuit model quantitatively and directly. On the other hand, it is applicable for multimode antennas. To verify the proposed method, a practical dual-mode filtering patch antenna is given as an example. The experimental results evidently validate the correctness and effectiveness of the proposed method.