The field of flexible antennas is witnessing an exponential growth due to the demand for wearable devices, Internet of Things (IoT) framework, point of care devices, personalized medicine platform, 5G technology, wireless sensor networks, and communication devices with a smaller form factor to name a few. The choice of non-rigid antennas is application specific and depends on the type of substrate, materials used, processing techniques, antenna performance, and the surrounding environment. There are numerous design innovations, new materials and material properties, intriguing fabrication methods, and niche applications. This review article focuses on the need for flexible antennas, materials, and processes used for fabricating the antennas, various material properties influencing antenna performance, and specific biomedical applications accompanied by the design considerations. After a comprehensive treatment of the above-mentioned topics, the article will focus on inherent challenges and future prospects of flexible antennas. Finally, an insight into the application of flexible antenna on future wireless solutions is discussed.

Flexible Antennas: A Review.

Ultra-wideband (UWB) technology can offer broad capacity, short-range communications at a relatively low level of energy usage, which is very desirable for wireless body area networks (WBANs). The involvement of the human body in such a device poses immense difficulties for both the architecture of the wearable antenna and the broadcast model. Initially, the bonding between the wearable antenna and the human body should also be acknowledged in the early stages of the design, so that both the potentially degrading output of the antenna as a consequence of the body and the possibility of exposure for the body may be handled. Next, the transmission path in WBAN is affected by the constant activity of the human body, leading to the time-varying dispersion of electromagnetic waves. Few researchers were interested in this field, and some substantial progress has recently been considered. On the other hand, this paper covered both wearable and Non-wearable UWB antenna designs and applications with respect to their substrate characteristics. Finally, this review prospectively exposes the upgraded developments of (ON-OFF) body antennas in the area of wearable and Non-wearable UWB and their implementations in the WBAN device and aims to evaluate the latest design features that inspire the performance of the antennas.

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ON-OFF Body Ultra-Wideband (UWB) Antenna for Wireless Body Area Networks (WBAN): A Review

In order to keep pace with the growing research and development on flexible and reconfigurable electronics, this research presents an inkjet printed ultra-wideband (UWB) flexible antenna on photo paper. The antenna pattern comprises a circular patch with the double stepped symmetric ground plane and is fed by a coplanar waveguide (CPW) technique on one-sided photo paper. The system is designed, simulated, fabricated, and tested experimentally. It operates over 3.2–30 GHz (161% fractional bandwidth, FBW) range with a return loss of − 10 dB or less and a voltage standing wave ratio (VSWR) < 2. The proposed monopole antenna is of dimensions 33.1 mm × 32.7 mm × 0.254 mm with an electrical size of 0.35 λ × 0.35 λ at 3.2 GHz frequency. This design exhibits nearly omnidirectional radiation pattern over the entire impedance bandwidth. An average gain of 4.87 and a radiation efficiency of 86.61% was observed. The miniature size, higher operating range, relatively constant radiation pattern along with higher acceptable peak gain of the antenna on a paper substrate lend itself to wearable and Internet of Things (IoT) applications.

A CPW-fed flexible UWB antenna for IoT applications

In this paper, flexible, low profile and robust planar monopole antennas are designed and analyzed for its performance over conformal surfaces, intended for Ultra-Wideband (UWB) and Body Centric Wireless Communications (BCWC). By the aid of alike shape (modified rectangular shaped) radiator and truncated ground plane, two compact microstrip fed antennas are designed and fabricated on flexible substrates – Polyamide and Teslin (environmentally friendly, flexible and green-electronics). The proposed polyamide antenna with a size of 50 mm × 40 mm × 0.8 mm, radiates over frequency range from 1.8 to 13.3 GHz and teslin antenna with a dimension of 50 mm × 40 mm × 0.712 mm, covers the frequency range from 1.4 to 13.4 GHz. To validate the designed prototypes, the antennas are fabricated and the impact performance are analyzed, while flexing the antenna from its nominal straight arrangement (bending through its length, width, different bending radius and angles). Further, time domain analysis is carried out in free space and in various bent scenarios with pulse excitation to validate the antennas for UWB applications. The transmission coefficient characteristics (S21) and group delay for various bending conditions are measured and the fidelity factor for all the scenarios are greater than 0.82, which make the antennas good candidate for UWB-BCWC.

On the bending and time domain analysis of compact wideband flexible monopole antennas

In this paper, a flexible CPW fed ultrawide band (UWB) antenna with mu-negative (MNG) metamaterial is designed, fabricated, and tested for wearable applications. Initially, a UWB antenna of size 50 mm × 43 mm is fabricated on two different substrates, viz. flexible FR4 and semi-flexible Rogers RT/duroid 5880. A metamaterial structure fabricated on flexible FR4 shows a magnetic resonance from 7.2 GHz to 9.2 GHz with maximum stop band attenuation (−49 dB) and high MNG value (−2121.6) at 7.87 GHz. Then a (3 × 3) array of designed MNG metasurface is used as ground plane with flexible UWB antenna, which improves its overall gain and radiation pattern. The performance of the flexible antenna with/without metamaterial at various distances from flat and cylindrical three-layered human phantom of skin, fat, and muscle is studied. Further, the bending characteristics at different angles and performance over thin metallic sheet is also evaluated. Additionally, the peak specific absorption rate value averaged over 1 g of tissue at three chosen frequencies from UWB range (3, 5, 10 GHz) with/without metamaterial using 0.3 and 0.1 W of input power is also analyzed. The simulated and measured results are in good agreement which confirms that the designed antenna is a good candidate for wearable applications.

Design and performance analysis of a conformal CPW fed wideband antenna with Mu-Negative metamaterial for wearable applications

Compact wideband flexible monopole antennas are designed and analyzed for its performance for Body Centric Wireless Communications (BCWC). Two antennas with identical radiators on different substrates are designed and fabricated on polyamide and teslin paper substrates, deploying a modified rectangle-shaped radiator. With the aid of modifications in the radiating plane and defecting the ground plane, the polyamide based antenna is designed to operate between 1.8 and 13.3 GHz, and teslin paper based antenna is designed to operate between 1.45 and 13.4 GHz to cover the wireless communication technology frequencies and ultra-wideband range for various wireless applications. The reflection coefficient characteristics of the fabricated antennas on free space and on various sites of the body are measured and match reasonably well with the simulated reflection coefficient characteristics. The specific absorption rate (SAR) analysis is also carried out by placing the antennas on tissue layered model.

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Palaniswamy Sandeep Kumar

Papers

Design and Analysis of Wideband Monopole Antennas for Flexible/Wearable Wireless Device Applications

Investigations of Specific Absorption Rate and Temperature Variations for an UWB Antenna for Wireless Applications