Two ground radiation antennas with polarisation diversity performance were proposed for MIMO WLAN applications in wearable devices. The antennas can simultaneously excite the vertical mode and horizontal mode of the ground plane and the phase difference between the two modes can be controlled by utilising an inductor‐loaded metal strip in the ground plane, generating circular polarisation. A 3:1 VSWR bandwidth of 140 MHz with high isolation (above 17 dB) and a 3 dB axial ratio bandwidth of 100 MHz was obtained. The opposite rotations generated by the two antennas resulted in good diversity performance, which was verified by the measured envelope correlation coefficient.

This study investigates a compact and straightforward self-decoupled $2\times 2$ multiple-input multiple-output (MIMO) antenna set and its applications for current and future 5G terminal devices. The proposed self-decoupled MIMO antennas include a popularly used loop antenna and a compact loop-type ground-radiation antenna without utilizing any supplementary decoupling structures or undergoing complex tuning process. It is revealed that the loop antenna and the ground-radiation antenna can be modeled as an electric-current element and a magnetic-current element, respectively. This orthogonality allows the self-decoupled characteristic of the proposed MIMO antennas even though the antenna elements are tightly arranged and collocated together. An $8\times 8$ MIMO antenna system is further demonstrated for 5G MIMO applications, where both simulation and measurement are conducted to validate the feasibility of the proposed technique. It is concluded that the proposed MIMO antenna system is a smart way to generate high isolation and low correlation characteristics while having advantages of low profile and easy fabrication so that it can be recognized as a promising candidate for 5G applications.

A compact and easy to fabricate decoupling method is proposed to yield high isolation for the multiple‐input and multiple‐output (MIMO) ground radiation antenna (GradiAnt) system. The proposed MIMO antenna system is comprised of two symmetrical, closely spaced, loop‐type (GradiAnts) with a ground‐coupled loop‐type isolator inserted between them. The isolator can be seen as a series resonant circuit which is connected with lumped components to control decoupling. In the proposed MIMO GradiAnt system, a coupling null is induced due to the ground‐coupled isolator, which effectively can be used for isolation enhancement between two GradiAnts. Within WLAN band, a minimum 14 dB isolation with a peak value of 42 dB at 2.42 GHz is achieved. In this manuscript, the decoupling principle and controlling mechanisms are first explained, then the antenna performances and tuning mechanisms are discussed in detail. The simulation and the measurement of the MIMO antenna, including the scattering parameters, efficiency, radiation patterns, peak gains and envelope correlation coefficients are conducted to verify the performance of the proposed MIMO system.

A metal‐rimmed multiple‐input multiple‐output (MIMO) antenna design operating below the 1 GHz frequency band is proposed that offers a wide bandwidth, high isolation, low correlation, and compact size. The proposed technique is based on metal‐rimmed mobile phones with four gaps, and the overall design occupies a small size in the ground plane. Orthogonal ground mode resonances are controlled to be equal to the operating frequencies, providing high antenna performance and good diversity performance. The proposed main antenna is achieved by utilizing the upper‐side rim to tune the ground mode resonance along the length of the ground plane to be equal to the operating frequency; moreover, a coupling slot couples strongly with the ground mode resonance to achieve a ‐6 dB bandwidth from 0.695 to 1 GHz (305 MHz). The orthogonal ground mode along the width of the ground plane is used for diversity antenna design. Similarly, the ground mode resonance is made equal to the operating frequency by controlling the left‐side metal rim such that a simulated ‐6 dB bandwidth from 0.850 to 0.920 GHz (70 MHz) is achieved. The simulated isolation is more than 11.5 dB in the operating band, and the measured envelope correlation coefficients is below 0.1.

Here we propose a ground radiation antenna (GradiAnt) with circular polarization (CP) by applying the ground-mode tuning (GMT) technique to the ground plane. We demonstrate that a GradiAnt at an arbitrary location of the ground plane can simultaneously excite two orthogonal modes of the same magnitude, and the phase difference between the two modes can be adjusted via GMT to achieve CP in mobile antennas. The proposed GradiAnt can be viewed as a combination of an electric-coupler (J-type) and magnetic-coupler (M-type) to the modes of the ground plane, and both modes can be excited to the same amplitude by tuning the coupling type. GMT, applied to the ground modes using inductor-connected metal strips, can generate a 90o phase difference between the two modes of the ground plane. The proposed antenna design generates a 6-dB axial ratio bandwidth of 140 MHz from 2.38 to 2.52 GHz in the +z-direction. Right-hand circular polarization and left-hand circular polarization in the +z-direction and -z-direction, respectively, are obtained. The tuning mechanism is presented, and measurements are taken to validate the proposed technique.

This paper investigates the design of a novel single-feed dual-element antenna that can be used to achieve circular polarization (CP) performance for mobile devices at arbitrary locations. The severe conditions needed to achieve CP remain as difficult issues for small mobile antennas; this is the case because the ground plane, rather than the antenna structure, contributes to the dominant radiation as the antenna volume shrinks. A novel concept of decomposing the antenna into two elements, which are independently coupled to two orthogonal ground modes for far-field radiation, is proposed through a theoretical analysis based on characteristic mode theory. The proposed dual-element antenna is comprised a loop-type element and monopole-type element, where one element is directly fed and the other resonates parasitically. In this way, the magnitudes and phase difference between two radiated field components can be controlled through antenna tuning without needing to manipulate the ground plane. Both simulations and measurements are conducted to verify the proposed antenna design with a widely used smartphone.