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A novel compact stub-coupled microstrip square open-loop bandpass filter (BPF) is proposed. The fork-shaped open stub is internally inserted inside the square loops using open gaps. The BPF has strong electric coupling, because the open sides of the two loops are proximately placed face-to-face in parallel. The rectangular stubs attached to the internal wall of loops connect to magnetic couplings; as a result the mixed coupling generates strong electromagnetic (EM) coupling in the BPF with an adequate input reflection coefficient at a centre frequency of 11.94 GHz. The measured results of the designed BPF exhibited return loss and insertion loss of 24.8 dB and 1.7 dB, respectively.