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In the slot-coupled microstrip antenna with a triplate line feed, the parallel-plate mode is excited by the slot, and it propagates between the ground planes of the triplate line. In an isolated element, excessive power loss is caused by the parallel-plate mode because of the power leakage of the mode. Also, the radiation pattern may be perturbed by mutual coupling due to the mode in array applications. Thus, a suppression technique of the parallel-plate mode is required in this antenna. To overcome the problems involved with the mode, we have proposed a novel geometry with a shunt-stub, and we have confirmed the effective performance of the proposal. We deal with a two element array of a slot-coupled microstrip antenna with a triplate line feed. Numerical and experimental considerations on the mutual coupling due to the parallel-plate mode are presented to demonstrate the effect of the configuration with a shunt-stub. Next, we show the radiation properties of the antenna by using numerical results. The moment method in the spectral domain is used to analyse the antenna, consequently, the analysis accounts for the parallel-plate mode as well as the radiated waves.