The recent advancement in the wireless technology has led to the advent of wearable antennas. These antennas are utilized for Wireless Body Area Networks (WBANs) purposes such as health-care, military sportive activities and identification systems. Compared to conventional antennas, wearable antennas operate in an environment which is in highly near proximity to the curved human body. Therefore, the wearable/flexible antenna performance parameters, including reflection coefficient, bandwidth, directivity, gain, radiation characteristic, Specific Absorption Rate (SAR), and efficiency are anticipated to be influenced by the coupling and absorption by the human body tissues. In addition, an electromagnetic band-gap structure is introduced in the wearable/flexible antenna designs to enable and give a high degree of isolation from the human body which also decreases the SAR dramatically. This paper reviews the state-of-the-art wearable/flexible antennas integrated with the electromagnetic band-gap structure on flexible materials concentrating on single and dual-band designs. Besides, it also highlights the challenges and considerations for an appropriate wearable/flexible antenna.

A compact wearable symmetrical e-slots antenna operated at 2.4 GHz was proposed for Medical Body Area Network applications. The design was printed onto a highly flexible fabric material. The final design topology was achieved by the integration of symmetrical e-slots antenna with an Electromagnetic Band-Gap (EBG) and Defected Ground Structure (DGS). The use of EBG was to isolate the body and antenna from each other whereas the DGS widened the bandwidth. This combination forms a novel and compact structure that broadens bandwidth. This broadened bandwidth makes the structure robust to deformation and loading in the human body. The design achieved a measured impedance bandwidth of 32.08 %, a gain of 6.45 dBi, a Front to Back Ration (FBR) of 15.8 dB, an efficiency of 72.3% and a SAR reduction of more than 90%. Hence, the integration of symmetrical e-slots antenna with EBG and etched DGS is a promising candidate for body-worn devices.

This paper presents a thorough review of the techniques involved in the enhancement of the efficiency performance of the reflectarray antenna. The effect of the selection of a suitable patch element or a proper feeding mechanism on efficiency improvement is studied in detail. Reflectarray loss quantification is examined in relation to the design techniques involved in the efficiency improvement. A low loss patch element with a wide reflection phase range and a properly illuminated reflectarray aperture are supposed to offer high efficiency performance. Additionally, the placement, the orientation and the position of a patch element on the reflectarray surface can also affect its efficiency performance. Mathematical equations were developed to estimate the efficiencies of circular and square aperture reflectarray antennas by considering their feed footprints. Moreover, a step by step practical method of predicting and measuring the total efficiency of a reflectarray antenna is presented. The two selected apertures of the reflectarray consisting of the square patch element configuration are fabricated and measured at a frequency of 26 GHz. Their measured efficiencies have been estimated using the derived equations, and the results were compared and validated using the efficiencies obtained by the conventional gain-directivity relation.

This work provides design and characterization of millimeter wave reflectarray antenna based on unit cells as well as periodic reflectarray design at 26 GHz. Diverse unit cell design configurations have been investigated based on simulations and scattering parameter measurements for feasibility of designing an optimum performance 5G reflectarray antenna. It has been demonstrated that the rectangular patch element provided minimum reflection loss of 1.02 dB and maximum bandwidth of 560 MHz. However, the phase error for rectangular patch element was observed to be 80°, which is much higher as compared to 10° and 13° in the case of rectangular ring and circular ring elements respectively. Periodic reflectarray antennas were also designed with main beam focused at 40° in the azimuth plane achieved with tilted array instead of using an offset feed. Radiation pattern measurements were carried out for further characterization where a maximum gain of 26.7 dB was provided by reflectarray designed with circular ring elements with variable radius. On the other hand, rectangular patch element array provided higher 1 dB gain drop bandwidth of 13.6% as compared to circular ring element reflectarray, which demonstrated a bandwidth of 13.1%. However, side lobe levels were observed to be higher at −18.4° for rectangular patch element reflectarray as compared to −19.4° in the case of circular ring elements based reflectarray antenna.

The narrow bandwidth and low gain performances of a reflectarray are generally improved at the cost of high design complexity, which is not a good sign for high-frequency operation. A dual resonance asymmetric patch reflectarray antenna with a single layer is proposed in this work for 5G communication at 26 GHz. The asymmetric patch is developed from a square patch by tilting its one vertical side by a carefully optimized inclination angle. A progressive phase range of 650° is acquired by embedding a circular ring slot in the ground plane of the proposed element for gain improvement. A 332-element, center feed reflectarray is designed and tested, where its high cross polarization is suppressed by mirroring the orientation of asymmetric patches on its surface. The asymmetric patch reflectarray offers a 3 dB gain bandwidth of 3 GHz, which is 4.6% wider than the square patch reflectarray. A maximum measured gain of 24.4 dB has been achieved with an additional feature of dual linear polarization. Simple design with wide bandwidth and high-gain of asymmetric patch reflectarray make it suitable to be used in 5G communications at high frequencies.

A resistively loaded wideband slotted patch antenna with optimized performance on lower frequencies is proposed for ground-penetrating radar (GPR) applications. The proposed design is backed by an optimized reflector composed of a periodic array of square loop elements, which enhances the antennas gain and directivity. The antenna shows good radiation characteristics and ease of integration with the GPR systems. The proposed structure features a compact size and wide bandwidth covering from 0.6 to 4.6GHz. The peak gain of 7 dBi is achieved. The fabricated prototype of the antenna along with an integrated optimized reflective surface has overall dimensions of 18×22×5cm³. The measured results validate the antennas performance in both free space and sandy medium, which enlighten its use for GPR applications.