A compact single-layer textile MIMO antenna is designed for wearable applications. The theory of characteristic mode is used to guide the antenna design and analyze its performance. The MIMO antenna utilizes a small ground plane as the main radiator, which is capacitively loaded by two strips along two orthogonal edges. The whole system occupies a volume of 38.1 mm × 38.1 mm × 2 mm, with each antenna having a dipole-like radiation pattern of linear polarization. Good isolation of above 12 dB is achieved due to the quasi-orthogonal radiations generated by the two antennas, providing pattern and polarization diversities. The envelope correlation coefficient between the antennas is below 0.01. The proposed antenna, fabricated on a flexible felt with a permittivity of 1.2, has a wide bandwidth of 20%. Due to its broadband behavior, the antennas remain well matched at the target band when worn on the body and bended. The loss of the human tissue results in the drop of the antenna gains to 1.6 dBi and 1.2 dBi, respectively, for the two antennas. The proposed antenna is competitive for wearable applications, due to its compact size, single-layer structure, easy integration, robustness, and reasonable on-body antenna gain.

In this communication, a compact pattern reconfigurable dual-antenna array is designed at 2.65 GHz for multiple-input multiple-output (MIMO) terminal applications, such as laptops. Each antenna is composed of two layers, i.e., the monopole layer and the planar inverted-F antenna (PIFA) layer. By switching the p-i-n diodes on each layer, its radiation patterns can be varied between conical and boresight patterns. Two such identical antennas are placed side by side on a common ground plane for MIMO application, providing altogether four operating modes. The center-to-center separation between the antennas is 0.25 wavelength, to keep the overall array compact. To facilitate an isolation of above 17 dB over all the modes for this compact array, two decoupling techniques based on the decoupling slits and the shielding wall are effectively combined. The envelope correlation coefficients (ECCs) of the far-field patterns in the four modes are below 0.05 in free space, and below 0.1 for most cases in the indoor and outdoor scenarios. The only exception is the monopole-PIFA mode in the outdoor scenario, but the ECC is still below 0.3 for this case. The measured efficiencies of the antennas are between -1.7 (68%) and -0.7 dB (85%) for all the modes. Therefore, good diversity and MIMO performances are expected for the proposed design.

In this work, wideband dual-polarized dielectric resonator antenna (DRA) arrays are designed for 5G mobile handsets and integrated with the sub-6GHz antenna. The dielectric resonators, surrounded by concentric air holes and metal vias, are excited by two orthogonal slots and fed through microstrip lines, providing a wide bandwidth from 25.3 GHz to 29.8 GHz. A high isolation of above 40 dB is obtained between the ports of the orthogonally polarized antennas. Based on the dual-polarized DRA, two 14 arrays are built and integrated with the longer metal frames of the handset, without occupying extra space on the chassis. The scanning angle for the proposed array is from 47 to 121 for the gain of above 10 dBi, and the overall handset antennas provide a large coverage efficiency of 80 % for a threshold gain of 5 dBi. User effect has also been investigated, manifesting that the horizontally polarized antennas are less influenced by the hands in both the one-hand and the two-hand data modes. Furthermore, a sub-6GHz antenna is designed with a wide operating band from 0.7 GHz to 0.94 GHz simply by making use of the shorter frames and the dual-layered DRAs, with a small ground clearance of 2mm remained. The performances of the arrays are validated by measuring the representative antennas. The proposed antenna is a good candidate for future handsets due to its high integration with the metal frames and with the LTE antenna, its large bandwidth and large coverage in free space and different user scenarios.

Detection of breast cancer in its early stage is critical for reducing the death rate. In this paper, a compact and flexible breast tumor sensing system is designed using dual-polarized UWB antenna arrays. Each UWB unit is composed of two orthogonally placed circular monopoles, which are printed on flexible and low-profile Kapton polyimide. The antennas are fed by single-side coplanar waveguides for size reduction. A wide band from 3.9 GHz to 19 GHz is obtained, with an isolation of above 16 dB over the operating band. High fidelity factors are achieved at different solid angles, indicating little distortion of the signal and stable sensing ability. To obtain the image of the tumor, eight UWB units are employed surrounding the breast phantom. Using the reflected and scattered signals from the antennas, 3D images of the breast are reconstructed using delay-and-sum (DAS) algorithm. Attributed to the dual-polarized UWB antennas, accurate location and size of the tumors are retained with little clutter. Moreover, the proposed system can detect two tumors with an edge to edge distance of 15 mm. Experiments have been carried out on rotating platforms, with the tumor successfully detected in the measurement. The proposed tumor sensing system is suitable for wearable applications due to its compact size and flexibility.

In this letter, a machine learning assisted antenna optimization method is proposed based on the random forest (RF) algorithm with data augmentation (DA). Using only a small number of samples, the prediction and optimization accuracy of the RF algorithm is ensured with repeated data augmentation, which balances different types of samples during the training. With the proposed DA-RF method, the AR bandwidth of a circularly polarized omnidirectional base station antenna is optimized. By learning the relationship between the loop orientations and the AR bandwidth efficiently, the AR bandwidth is improved by 41% compared with the best one in the samples. The estimation accuracy of the proposed method outperforms other similar methods, with fewer iterations as well. The method is also successfully applied to multi-objective optimizations.

Simple and low cost tag antenna with long reading distance in various environments is in great demand with its wide applications. This paper describes the design and optimization procedure of a single-layer tag antenna, which covers relatively long reading range when mounted on both metallic surface and various dielectrics. The antenna is based on the loop structure, with a small ka of 0.66. Characteristic mode analysis is first performed to inspect how a typical loop structure radiates in free space and on PEC. Antenna structure is then designed based on the characteristics of the loop. Afterward, equivalent circuits of the tag antenna on dielectrics and on metal are extracted, in order to provide guidance for antenna optimization. The proposed tag antenna achieves the maximum reading distance of 6.1 m and 14.1 m, respectively, when mounted on metal and dielectrics with low permittivity. Measurements of the tag antenna were carried out in different environments, including outdoor scenario, indoor scenarios with low and high reflections. The measured results agree reasonably well with the predicted ones. The tag antenna is proved to be competitive in perspective of simple structure, low cost, long reading distance, and tolerance on various background materials.