This paper presents an elegant yet straightforward design procedure for a compact rat-race coupler (RRC) with an extended harmonic suppression. The coupler’s conventional $\lambda $ /4 transmission lines (TLs) are replaced by a specialized TL that offers significant size reduction and harmonic elimination capabilities in the proposed approach. The design procedure is verified through the theoretical, circuit, and electromagnetic (EM) analyses, showing excellent agreement among different analyses and the measured results. The circuit and EM results show that the proposed TL replicates the same frequency behaviour of the conventional one at the design frequency of 1.8 GHz while enables harmonic suppression up to the $7^{\mathrm {th}}$ harmonic and a size reduction of 74%. According to the measured results, the RRC has a fractional bandwidth of 20%, with input insertion losses of around 0.2 dB and isolation level better than 35 dB. Furthermore, the total footprint of the proposed RRC is only 31.7 mm $\times15.9$ mm, corresponding to $0.28\,\,\lambda \times 0.14\,\,\lambda $ , where $\lambda $ is the guided wavelength at 1.8 GHz.

Optically transparent electronic devices have attracted enormous interests in recent years. The development of transparent electronics emerges a lot of new industrial applications in variety of fields, such as displays, glasses, solar panels, satellite communications, terrestrial communications, integrated circuits, and sensors where optical transparency is required for unobtrusive placement of electronic devices on the surface. Over the last couple of years, there have been notable advancements in the development of transparent wireless electronics due to the emerge of new materials and fabrication technologies. Among transparent electronic devices, transparent antennas attract tremendous interests due to their widespread applications in healthcare industry, security sector, defence, sports, smart city, Internet of Things (IoTs) and many more. Many of these applications require antennas that are concurrently transparent and flexible. A transparent and flexible antenna can be easily integrated with displays, windows, solar cells and optoelectronic modules, thus, reducing the space in the integrated circuits. However, still the development of flexible-transparent electronics is associated with some challenges which continue to impede the progress of this emerging field. Among them are the contradictory relationship between the electrical conductivity and optical transparency of the transparent conductors, costly and complex processing of the transparent materials and unavailability of the appropriate materials. In this paper, we discuss current advancements in the development of flexible transparent antennas, including potential applications, various enabling materials and manufacturing approaches, technical hurdles, as well as prospects.

In this paper, a detailed investigation of the realization of conformal wearable transparent antennas by integrating conductive mesh with polymer has been presented. The proposed realization method is much simpler and more cost-effective than the existing realization methods of transparent antennas, and the prototype fabricated from the selected composite materials is more flexible and robust in bending operations than other transparent antennas. In this paper, the mechanical, electrical, and optical characteristics of the proposed composite material have been investigated to analyze its suitability for transparent flexible antenna realization. For concept demonstration, a prototype of a dual-band antenna operating at 2.33-2.53 GHz and 4.7-5.6 GHz has been fabricated and tested. These frequencies cover both the instrument, scientific, and measurement (ISM) and the wireless local area network (WLAN) bands. Full ground plane is utilized in the antenna design for on-body operations. The suitability of the antenna for wearable applications has been investigated by measuring its performance under physical deformation and testing its performance on phantom. Next, the RF performance of the antenna has been improved by using two layers of conductor to form the radiating element. Although transparency is slightly compromised, the double-layer element improves the gain and efficiency of the antenna.

A method to realize low-cost, optically transparent, flexible and unidirectional antennas is presented in this paper. The key to the method is making a flexible transparent reflector made using a new method- injection of pure water into a flexible transparent cavity that is made out of Polydimethylsiloxane (PDMS) polymer and integrating the flexible reflector with a flexible dipole made of transparent conducting mesh. This method is simple and inexpensive. Hence, it is useful for large scale production of transparent, flexible, robust and low-cost antennas as well as RF/microwave components for wearable body-area networks and other such systems that require flexible and transparent components. To validate the method, an antenna operating in the 2.45 GHz band was designed, fabricated and tested under various conditions. Its measured gain is 3.2 dBi and efficiency is 51%. To the best of our knowledge, this is the first water-based transparent flexible antenna. Due to the shielding effect of the water-based reflector, it produces low SAR in the human body when it is worn, as expected from a unidirectional antenna. Further, it is investigated and confirmed that the use of pure water in the reflector makes the antenna significantly more efficient as opposed to salt water.

In this paper, we present experimental investigations on washing durability of polydimethylsiloxane (PDMS)-conductive fabric composite, to validate its applicability for realization of flexible wearable antennas that can withstand multiple washing cycles. For this purpose, we designed an ultra-high frequency (UHF) radio frequency identification (RFID) passive tag antenna and fabricated several prototypes using such materials combination. Understanding the challenge of having a robust integration of lumped electronic component (e.g., RFID IC) on a flexible antenna, a new way to improve the interconnection has also been investigated. The tag prototypes were subjected to recurrent machine-washing tests, and after each washing cycle, their performance was analyzed mainly in terms of read range. The results reveal that, with a proper treatment on the antenna-IC fixture interconnection, the tag antennas developed with PDMS-conductive fabric composite can maintain their performance very well, showing a minimum degradation in read range after 15 cycles of washing.

Optically transparent flexible tag antennas are highly desired for some applications, where unobtrusiveness and capability of mounting on curved surfaces are required. On the other hand, circular polarization minimizes signal degradation due to polarization mismatch between the reader and tag antennas, thus ensuring good communication quality and accuracy. In this paper, we present an optically transparent and flexible radio frequency identification (RFID) passive tag antenna operating in the ultra-high frequency (UHF) band and exhibits circular polarization. It is thin, lightweight and completely encapsulated inside transparent polymer which protects it against dust, water, heat and mechanical stress. The tag antenna is made by utilizing highly flexible and transparent conductive-mesh-polymer composite through a simple and low-cost manufacturing process. The antenna design is based on a planar square-ring. It achieves a measured maximum read range of about 8.3 m and approximately 47 MHz (885-932 MHz) 3-dB axial-ratio bandwidth.