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.

A dual-band antenna with good stopband characteristics is proposed. Different from conventional antennas, two transverse slots are etched on a substrate integrated waveguide (SIW) cavity. Two half modes are excited to achieve a dual-band operation and two non-radiating modes, a TE 110 mode and an even TE 120 mode, are induced to generate radiation nulls. With the aid of the two radiation nulls, the stopband characteristics of the proposed design including non-radiating gain and band-edge selectivity are significantly improved. A fabricated prototype is measured. The measured centred frequency of two passbands is at 3.28 and 3.77 GHz, respectively. The maximum gain is about 5.2 dBi in the passbands while the minimum gain is only −18.9 dBi in the stopband. The measured efficiency is 86% in the lower passband and 75% in the upper passband but it sharply decreases to lower than 5% between 3.36-3.7 GHz.

A dual-layer substrate-integrated-waveguide (SIW) cavity filtering antenna with a controllable radiation band and two radiation nulls is proposed. It consists of a coaxial feeding, two SIW cavities, three metal planes and three slots. An H-shaped slot is etched on the top plane and two rectangle slots are cut on the middle plane. A radiating mode (i.e. TE 210 mode) is excited to achieve a radiation band while two non-radiating modes (i.e. TE 010 mode and TE 110 mode) are utilised to generate two radiation nulls. Good filtering performance including sharp sideband selectivity and high stopband suppression is achieved by these radiation nulls. Moreover, the radiation band can be independently controlled without affecting the radiation nulls. A fabricated prototype with the size of 0.58λ 0 × 0.58λ 0 × 0.07λ 0 is measured. The measured centre frequency of the radiation band is at 3.49 GHz with the gain of 5.39 dBi and efficiency of 79.25%. However, the gain and efficiency of a radiation null at 3.29 GHz are −27.56 dBi and 0.07% while those of the other radiation null at 3.86 GHz are −27.29 dBi and 0.04%.

A half‐mode (HM) substrate‐integrated‐waveguide (SIW) cavity antenna is designed for tri‐band multiple‐input‐multiple‐output (MIMO) applications. Three cavity edges are shorted by metal posts while one aperture is open to radiate cavity energy into free space. To meet MIMO applications, a T‐shaped slot is etched to split the single HM cavity into two equal SIW subcavities and then they are excited by coaxial feeding ports to realize two antenna elements. To obtain tri‐band operations, four additional rectangle slots are cut, which introduces six quarter modes with different resonating regions. With the cavity size of 0.20λ0 × 0.40λ0, a fabricated prototype exhibits three operating bands with center frequencies of 3.33, 4.09, and 4.72 GHz and peak gain of 4.51, 4.05, and 3.93 dBi. Moreover, the measured port isolation is higher than 18.26 dB and the measured envelope correlation coefficient is below 0.05 within the frequency of interest.