For modern mobile terminals, the shrinking antenna space on terminal frames and the increasing number of 5G antennas motivate engineers to seek new space for antennas. The terminal back cover can provide a large area to accommodate planar antennas, thus, mobile planar antennas are becoming a hotspot recently. This communication proposes a novel approach to develop a dual-band planar antenna with an extremely low profile, which is an urgent need for modern ultra-thin terminals. The first two modes (TM _0.5,0 and TM _0.5,1 ) of the half-mode patch are adopted to cover the two target bands and a parasitic half-mode patch excited through electric coupling is added to expand the impedance and efficiency bandwidths. The realized dual-band patch antenna operating at partial 5G band N78 (3.40-3.60 GHz) and N79 (4.80-4.97 GHz) is with a profile of only 0.5 mm, about 0.006 λ _L (λ _L refers to the lowest frequencies of the operation band). Measured results show that the -6 dB impedance bandwidths are 3.19-3.42 GHz (230 MHz, 6.9%) and 4.77-5.03 GHz (260 MHz, 5.3%) with average efficiencies of -4.2 dB (39.0%) and -3.7 dB (42.6%) at the lower and higher bands, respectively. As far as the authors know, the proposed antenna is with the lowest profile and highest bandwidth-to-volume ratio compared to other reported planar dual-band antennas.

Nowadays, the MIMO can achieve fifth generation (5G) ultra-high capacity, but it is a great challenge for the smartphone antenna to achieve good isolation, high efficiency, and other performance in limited space. The paper designed and completed an eight-port dual-polarized high-efficiency shared-radiator antenna working in 3.5 GHz (3.4–3.6 GHz) for 5G mobile devices. The two antenna elements are regarded as one building block and share one radiator, and the size of one radiator is 17.1 × 17.1 mm${}^2$ (0.02 $\lambda$ × 0.02 $\lambda$, where $\lambda$ presents the free-space wavelength at 3.5 GHz). The MIMO system consists of four radiators, and the edge-to-edge distance between the radiators on the short side is 31.9 mm (0.038 $\lambda$), and the total size of the MIMO antenna system is 150 × 80 × 1.6 mm${}^3$ (0.176 $\lambda$ × 0.094 $\lambda$ × 0.0019 $\lambda$). The antenna uses an orthogonal placement of feed lines to produce dual polarization in the MIMO system, resulting in high isolation without introducing other decoupling structures. In addition, the reason for the high efficiency of the antenna is explained by the common mode (CM) and differential model (DM). Finally, the simulated results are as follows: the isolation is 14 dB; the total efficiency (TE) is 75–85%; the envelope correlation coefficient (ECC) is lower than 0.065; and the gain is 6.5 dB. The prototype is fabricated and tested: the isolation is better than 17 dB, the range of the measured TE is 60–75%, and the ECC is lower than 0.045. In addition, the influence of the human body model on the antenna are also discussed. Overall, the proposed MIMO antenna has a shared radiator with high isolation and high TE, and is more suitable for the current stage of 5G MIMO antenna technology. More importantly, the planar structure block is very simple to build and easy to fabricate on the substrate.