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This study investigated an improved shear horizontal surface acoustic wave (SH-SAW) ammonia sensor based on L-glutamic acid hydrochloride. In order to satisfy Barkhausen criteria for oscillation and avoid mode hopping, it used coupling of modes (COM) theory to design a SH-SAW resonator. This SH-SAW sensor presented excellent reversibility, sensitivity, and repeatability when ammonia was in the ppb level. According to the signal-to-noise ratio of two, the estimated limit of detection (LOD) is 13.53 ppb.