A substrate-integrated-waveguide (SIW) cavity multiple-input-multiple-output (MIMO) antenna using hybrid boundaries and anti-symmetric U-shaped slots is proposed. Unlike conventional SIW cavities completely shorted by metallic vias, the proposed two cavities possess opened edges. Since shorted and opened cavity edges can be regarded as electrically and magnetically conducting boundaries, respectively, hybrid resonating boundaries are achieved. Excited by coaxial ports, antenna elements can radiate cavity energy through the opened edges. Moreover, antenna isolation can be significantly enhanced by introducing a pair of anti-symmetric U-shaped slots on the top and bottom planes. This design has been validated by experiments. With the overall size of 0.44λ0 × 0.44λ0 × 0.04λ0, the fabricated MIMO antenna exhibits operating frequency of 3.51 GHz, high isolation of 20.18 dB, peak gain of 3.15 dBi, and low envelope correlation coefficient of 0.12, which has potential applications for wireless systems.
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