A conventional microstrip EH1-mode bidirectional leaky-wave antenna (LWA) usually works as a traveling wave antenna, and its typical maximum broadside gain is commonly obtained when the phase and leakage constants of the used EH1-mode are exactly equal to each other. In this article, it is, for the first time, revealed that this bidirectional LWA can alternatively operate under standing-wave states when its terminations may be open-circuited. Under some of these states, the open-terminated LWA attains higher efficiency due to the extremely small leakage constant. However, it is unfortunately illustrated that split beams are generated at these concerned states owing to the inherent opposite directions of equivalent magnetic current distributions along the two edges of the strip conductor. In order to transform the unwanted split beams into a single high-gain beam at the broadside, a novel standing-wave bidirectional LWA is presented under the operation of the hybrid EH1 and EH3 propagating modes. By analyzing the transversal electric fields of EH1- and EH3-modes, respectively, it is demonstrated that their leaky electric fields can be crucially made in an antiphase manner. Next, the EH3-mode electric field is employed to periodically replace the original EH1-mode counterpart, so as to reshape the electric fields along the two parallel radiating edges toward a desired in- phase pattern. In such a way, the appealing high-gain beam can be successfully achieved. As an example, such a hybrid-mode standing-wave bidirectional LWA is designed, simulated, and tested. The measured and simulated far-field radiation performances show excellent agreement, and they both powerfully indicate that our reported antenna has enhanced its radiation gain at the broadside compared to the conventional EH1-mode traveling-wave bidirectional LWA