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A compact self-isolated multiple-input-multiple-output (MIMO) antenna system is proposed. It is based on a quarter-mode substrate integrated waveguide (SIW) cavity where two edges are shorted by metallic vias and the other edges are open. A narrow slot with a rotated angle of 45°is utilised to divide the single cavity into two equal eighth-mode sub-cavities. These sub-cavities are fed by two coaxial ports to construct the proposed MIMO antenna system. A self-isolated mechanism that cavity energy inputted by one port is radiated into free space rather than transmitted to the other port is first observed. High isolation between two antenna elements can be obtained without additional isolating or decoupling networks although they are contacted with each other. A fabricated prototype operating at 3.5 GHz achieves the measured port isolation of higher than 18.5 dB, envelope correlation coefficient (ECC) of less than 0.04, the peak gain of 4.6 dBi, and maximum efficiency of 79.2%. With advantages of compact size, high isolation, low ECC, and good radiation performance, the proposed self-isolated MIMO antenna system is a good candidate for wireless communication.

A compact substrate-integrated-waveguide (SIW) cavity multipleinput-multiple-output (MIMO) antenna with enhanced bandwidth and high isolation is proposed. It is based on a triangle half-mode SIW cavity with one open edge and two edges shorted by metallic vias. By etching three slots, the single cavity can be divided into four eighth-mode sub-cavities. Two large sub-cavities with the same size are excited by two coaxial ports to construct two antenna elements and a wide slot between them is utilised as the de-coupler. Therefore, high isolation can be obtained. On the other hand, cavity energy can be coupled from the excited sub-cavities to the unexcited sub-cavities due to two narrow T-shaped slots serving as the couplers. Therefore, enhanced bandwidth can be achieved by merging two coupled modes into the required operating band. A fabricated prototype centring at 3.51 GHz has been measured. With a compact size of 0.59λ 0 × 0.59λ 0 , enhanced bandwidth of 160 MHz, high isolation of 19.5 dB, the antenna gain of 4.9 dBi, and radiation efficiency of 72.8%, the proposed design has potential applications for fifth-generation communication.

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A bandwidth-enhanced four-antenna multiple-input-multiple-output (MIMO) system based on a substrate-integrated-waveguide (SIW) cavity is proposed. By etching two long rectangular slots on the top plane of the cavity, four quarter-mode sub-cavities with the same size are constructed and then individually fed by a coaxial port to form a four-antenna MIMO system. Suitable port isolation is achieved by optimising the length and width of these slots. To enhance bandwidth, a pair of back-to-back L-shaped slots is cut to split each subcavity into three smaller parts. Three resonant modes are generated in these parts and merged into the operating band. A sample with the overall size of 0.58λ 0 × 0.58λ 0 × 0.02λ 0 has been fabricated. Measured bandwidth is enhanced from 80 to 230 MHz and measured isolation is better than 15.0 dB. Owing to covering the band of 3.40-3.60 GHz, the proposed antenna system has potential applications for fifth-generation communication.