Surrogate models are widely used in antenna design for optimization efficiency improvement. Currently, the targeted antennas often have a small number of design variables and specifications, and the surrogate model training time is short. However, modern antennas become increasingly complex, which needs much more design variables and specifications, making the training time become a new bottleneck, i.e., in some cases, even longer than electromagnetic (EM) simulation time. Therefore, a new method, called training cost reduced surrogate model-assisted hybrid differential evolution for complex antenna optimization (TR-SADEA), is presented in this article. The key innovations include: 1) a self-adaptive Gaussian process surrogate modeling method with a significantly reduced training time while mostly maintaining the antenna performance prediction accuracy and 2) a new hybrid surrogate model-assisted antenna optimization framework that reduces the training time and increases the convergence speed. An indoor base station antenna with 2G to 5G cellular bands (45 design variables and 12 specifications) and a 5G outdoor base station antenna (23 design variables and 18 specifications) are used to demonstrate TR-SADEA. Experimental results show that more than 90% of the training time and about 20% iterations (simulations and surrogate modeling) are reduced compared to a state-of-the-art method while obtaining high antenna performance.

In this paper, a novel reconfigurable open slot antenna has been proposed for LTE smartphone applications to cover a wide bandwidth of 698-960 and 1710-2690 MHz. The antenna is located at the bottom portion of the mobile phone and is integrated with metal rim, thereby occupying a small space and providing mechanical stability to the mobile phone. Varactor diode is used to cover the lower band frequencies, so as to achieve a good frequency coverage and antenna miniaturization. The operational principles of the antenna are studied and the final design is optimized, fabricated, and tested. It has achieved the desired impedance bandwidth and the total efficiency of minimum 50% in free space throughout the required bands. The antenna performance with mobile phone components and human hand is also been studied. Furthermore, the SAR in a human head is investigated and is found to be within allowable SAR limits. Finally a multiple-input multiple-output antenna configuration with high isolation is proposed; it has an identical reconfigurable open slot antenna integrated at the top edge of the mobile phone acting as the secondary antenna for 698-960 and 1710-2690 MHz. Thus the proposed antenna is an excellent candidate for LTE smartphones and mobile devices.

A salty water antenna with beam-steering characteristics is proposed. The antenna combines the idea of the electronically steerable parasitic array radiator (ESPAR) antenna and the water antenna to realize a 360° beam steering. The unique features of water antennas such as transparency, reconfigurability and liquidity are fully utilized. Both the computer simulations and measurements are conducted to validate the performance. The proposed design has a frequency range from 334 MHz to 488 MHz ( $\text{S}_11$ < −10 dB), high radiation efficiency (> 60%), 13 different working states and a maximum gain of 5.8 dB. Compared with existing pattern reconfigurable liquid antennas, this new antenna has a wider bandwidth, more working states and better invisible ability. It is therefore a promising candidate for a range of VHF or UHF applications, such as internet of things (IoT) and maritime applications.

It has been shown that the correlation between the radiation patterns of two antennas can be measured in a reverberation chamber (RC). In this paper, it is shown that the self-correlation coefficient of an antenna, defined as the correlation between the radiation pattern of an antenna under test (AUT) and a transformed version of itself, can also be measured in an RC. It is found that the 3-D radiation pattern of the AUT can be reconstructed from the measured self-correlation coefficient by using spherical wave decomposition. Moreover, the axial ratio of the AUT can also be measured efficiently in the RC, when the pattern is directional. Numerical simulations and measurements in the RC and in an anechoic chamber have been undertaken. Good agreement is obtained, which confirms the validity of the proposed method. Thus, this novel method can become a very useful, cost-effective, and efficient method for 3-D antenna radiation pattern measurement.

A self-isolated 10-element antenna array operating in the long-term evolution 42 (LTE42) frequency band is proposed for 5G massive MIMO smartphone applications. The proposed antenna elements are placed in a 2D array configuration; they are placed symmetrically along the two long edges of the mobile chassis. The proposed antenna structure is a shorted loop antenna resonating at half-wavelength mode, which is rarely deployed by researchers due to its large size compared to other quarter wavelength antenna structures. It is a printed, shorted, and compact loop antenna of a total footprint area of 6 × 6.5 mm ² (λ/14.3 ×λ/13.2, where λ is the free space wavelength at 3.5 GHz). A small capacitive coupling flag-shaped strip is used to excite the proposed loop antenna. The compactness is achieved using an inward meandering that forms an internal loop in the element. The position and the dimensions of this loop are used to tune the resonant frequency and matching level at 3.5 GHz. The results (theoretical, simulated, and measured) show that the 3.5 GHz band (3.4-3.6 GHz) is achieved with impedance matching better than -10 dB, and total efficiency higher than 65%. A 10 × 10 MIMO system is formed and it has an excellent MIMO and diversity performance in-terms of the envelope correlation coefficient (below 0.055), and apparently it has the highest channel capacity (about 54.3 bps/Hz) among other MIMO systems of the same order. Simulation results of the specific absorption rate (SAR) demonstrates that the proposed antenna solution satisfied SAR criterion. Thus, the proposed ten-element MIMO antenna represent an excellent candidate for sub-6 GHz 5G smartphone applications.