In this paper, a compact branch-line coupler based on slow-wave artificial dielectric substrate (SW-ADS) is proposed. The internal configuration of the coupler consists of two groups of metallic via cylinders with different heights. The first group comprises full-height metallic vias connected to the ground plane of a microstrip line, while the second group is shorter vias with mushroom-shaped caps, which are connected to the feed line. This unique arrangement enhances the capacitive loading, and, in turn, the effective dielectric constant of the structure is increased resulting in a substantial reduction in wave propagation velocity. The proposed coupler is designed and simulated using Ansys High-Frequency Structure Simulator (HFSS). A prototype of the coupler is subsequently fabricated and experimentally validated. The measured results show that the coupler exhibits good return loss with low insertion loss at the operating frequency of 5 GHz. Furthermore, the proposed structure achieves up to 90% size reduction compared with that of a conventional branch-line coupler at the same frequency, with overall compact dimensions of 0.083 × 0.043

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