In this paper, we presented an ultra-wideband multiple-input multiple-output (MIMO) antenna system with high element-isolation for the application in 5G metal-frame smartphones. We proposed T-shaped and C-shaped slots on the metal frame generating four resonances to enhance the bandwidth. What’s more, we introduce modified H-shaped slots between each antenna-element to improve the element-isolation of MIMO antenna system. As a result, the MIMO antenna system has a wide bandwidth of 58% ranging from 3.3 to 6 GHz and the element-isolation is over 18 dB crossing the effective frequency band. Thanks to the decent element-isolation, the envelope correlation coefficient (ECC) between each antenna-element is below 0.05 providing a reliable anti-interference for the MIMO antenna system. In addition, the measured radiation efficiencies of the MIMO antenna system are higher than 40%. At last, we analyze the effects caused by user’s hands and head to guarantee the robustness of the MIMO antenna system in practical applications.

A compact circularly-polarized patch antenna with wide axial-ratio beamwidth (ARBW) operating at dual bands compatible for GPS satellite navigation application is proposed in this paper. The antenna consists of two stacked patches, which account for GPS L1 band (1575 ± 2 MHz) and L2 band (1228 ± 2 MHz). To realize significant size miniaturization, four pillars are introduced and strongly coupled to the lower patch. To adjust the beamwidth response of the far-field components, the antenna is backed by a modified metallic cavity, which in turn widen the 3-dB axial ratio beamwidth (ARBW) at two operating bands effectively Finally, the proposed design with excellent performance is realized in a size of 0.368 λ0 (diameter) and 0.105 λ0 (height). Simulated and measured results show that the ARBWs at L1 and L2 bands are as wide as 173° and 133°, respectively. For practical application, its satisfactory GPS signal receiving capability is also verified by outdoor positioning test.

!In this paper, 4×4 / 8×8 / 10×10 multiple input multiple output (MIMO) antenna systems composed of co-located self-neutralized antenna pairs are proposed to operate at 4.8-5.0 GHz (part of N77 band) for the fifth-generation (5G) communication applications. The proposed antenna pairs consist of a loop antenna and a monopole antenna, co-locating together with a compact size of 6.7×15.5×3.08 mm3. Meanwhile, the co-located design can miniaturize 50% of the antenna size, which is quite significant for practical mobile applications with limited space environment. In addition, the adoption of the self-neutralization technique between the co-located loop antenna and the monopole antenna contributes to a satisfactory isolation with better than 25 / 20 / 16.5 dB for the 4×4 / 8×8 / 10×10 MIMO systems respectively. Analysis of the isolation improvement is also discussed in this study. Moreover, the antenna pairs are able to generate complementary radiation patterns, enabling the envelope correlation coefficients (ECC) between the antennas to be less than 0.1. Also, efficiencies of the MIMO antenna system achieve up to 60% as there is no external decoupling structure deteriorating the radiation performance of the proposed antenna pairs. Finally, a 4×4 MIMO antenna system prototype is fabricated and measured to verify its prospect for 5G MIMO applications.

A size-compact and monolithically integrated millimeter-wave 16-way high-power combiner for advanced radio-frequency (RF) electronics applications is developed featuring a wide operational bandwidth and high isolations between the combiner's adjacent ports. The proposed combiner is operating at X band and is composed of 15 3-dB directional coupler units that are properly configured to achieve the overall structural compactness. The combiner is fast prototyped as a single part by employing metallic 3-D printing technique, which successfully overcomes the difficulty in its monolithic implementation by using conventional subtractive manufacturing techniques. The experimental results demonstrate that the 16-way power combiner achieves an approximately 21% operational bandwidth (8.5−10.5 GHz) at X band with port isolations of over 20 dB, an amplitude imbalance of about ±1 dB, a phase deviation of around ±1°, and good return and insertion losses. The excellent RF performance of the combiner makes it a good candidate for applications in multi-port high-power electronics systems where highly efficient power transmission and conversion among different ports are required.

In this paper, a high-order mode (TE340-mode) empty substrate integrated cavity (ESIC) fed high-gain circularly polarized 2×2 patch antenna array is presented. By using ESIC TE340-mode cavity, compact size and low-loss feeding network is achieved. Each cavity-backed circular patch is etched by a pair of inclined slits to achieve circular polarization. To verify the concept, a 2×2 antenna array is fabricated and measured. The measured -10 dB impedance bandwidth of the array is 19.58% (from 36.85 to 44.85 GHz). The measured 3-dB axial ratio bandwidth is 5.74% (from 42 to 44.5 GHz). The measured total gain is 14.02 dBi. The proposed high-gain and low-cost circularly polarized antenna array can be a proposing candidate in the future millimeter-wave communications.

A low-profile dual-band directional antenna operating at both 2.4 and 5GHz for wireless local area networks (WLANs) for unmanned aerial vehicles (UAVs) applications is proposed in the paper. Two pairs of dipole antenna arrays with different electrical lengths are utilized to achieve dual-band directional radiation performance, which is desirable for a remote-control unit of the UAVs. Note that the directional radiation characteristic is obtained according to the double-dipole antennas driven by antiphase signals (W8JK), provided the distances between different dipole antennas are properly optimized. Note that the distance between the two longer dipoles, as well as the two shorter dipoles, can be calculated by equations. For each pair of dipoles with equal electrical lengths, an antiphase along the two dipoles can be obtained, and directional radiation characteristics can be achieved in the dual band. Coplanar strips (CPS) are employed as feeding networks, which lie on the same substrate layer as the antenna arrays, resulting in a low profile of merely 0.762mm. Also, the overall size of the antenna together with the feeding network is 58.42×32.58×0.762mm³. To verify the performance of the proposed antenna, a prototype is fabricated and measured. The measured gain is 5.2 and 7.0dBi, respectively, in the 2.4 and 5.8GHz bands. The measured reflection coefficients as well as radiation patterns are consistent with those of the simulated results. The proposed antenna scheme can be a good candidate for UAV applications with the advantages of a lower profile, dual-band characteristics, directional radiation characteristics, and a simple assembly. Since the remote-control unit operates at 2.4GHz and 5.8GHz to control the UAV unit, the low-profile dual-band directional radiation antenna has extensive application prospects when integrated into the space-limited unit.