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.

A compact textile ultra-wideband (UWB) antenna with an electrical dimension of 0.24λo × 0.24λo × 0.009λo with microstrip line feed at lower edge and a frequency of operation of 2.96 GHz is proposed for UWB application. The analytical investigation using circuit theory concepts and the cavity model of the antenna is presented to validate the design. The main contribution of this paper is to propose a wearable antenna with wide impedance bandwidth of 118.68 % (2.96-11.6 GHz) applicable for UWB range of 3.1 to 10.6 GHz. The results present a maximum gain of 5.47 dBi at 7.3 GHz frequency. Moreover, this antenna exhibits Omni and quasi-Omni radiation patterns at various frequencies (4 GHz, 7 GHz and 10 GHz) for short-distance communication. The cutting notch and slot on the patch, and its effect on the antenna impedance to increase performance through current distribution is also presented. The time-domain characteristic of the proposed antenna is also discussed for the analysis of the pulse distortion phenomena. A constant group delay less than 1 ns is obtained over the entire operating impedance bandwidth (2.96-11.6 GHz) of the textile antenna in both situations, i.e., side by side and front to front. Linear phase consideration is also presented for both situations, as well as configurations of reception and transmission. An assessment of the effects of bending and humidity has been demonstrated by placing the antenna on the human body. The specific absorption rate (SAR) value was tested to show the radiation effect on the human body, and it was found that its impact on the human body SAR value is 1.68 W/kg, which indicates the safer limit to avoid radiation effects. Therefore, the proposed method is promising for telemedicine and mobile health systems.

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Wireless Body Area Networks: UWB Wearable Textile Antenna for Telemedicine and Mobile Health Systems

Wireless body area network (WBAN) applications have broad utility in monitoring patient health and transmitting the data wirelessly. WBAN can greatly benefit from wearable antennas. Wearable antennas provide comfort and continuity of the monitoring of the patient. Therefore, they must be comfortable, flexible, and operate without excessive degradation near the body. Most wearable antennas use a truncated ground, which increases specific absorption rate (SAR) undesirably. A full ground ultra-wideband (UWB) antenna is proposed and utilized here to attain a broad bandwidth while keeping SAR in the acceptable range based on both 1 g and 10 g standards. It is designed on a denim substrate with a dielectric constant of 1.4 and thickness of 0.7 mm alongside the ShieldIt conductive textile. The antenna is fed using a ground coplanar waveguide (GCPW) through a substrate-integrated waveguide (SIW) transition. This transition creates a perfect match while reducing SAR. In addition, the proposed antenna has a bandwidth (BW) of 7-28 GHz, maximum directive gain of 10.5 dBi and maximum radiation efficiency of 96%, with small dimensions of 60 × 50 × 0.7 mm3. The good antenna's performance while it is placed on the breast shows that it is a good candidate for both breast cancer imaging and WBAN.

https://www.mdpi.com/2072-666X/12/3/322/pdf

Full Ground Ultra-Wideband Wearable Textile Antenna for Breast Cancer and Wireless Body Area Network Applications

In this article, a compact dual-port single element MIMO antenna is presented for millimeter wave (mmWave) 5G wireless applications. The presented antenna design consists of an annular ring slot that is fed by a Substrate Integrated Waveguide (SIW) at the two opposite sides of the substrate. The antenna resonates at 28GHz and its 10dB impedance bandwidth is 400MHz. The isolation between the ports is reduced by introducing additional annular slot that acts as a cavity which results in an isolation better than 20dB at 28GHz. The measured gain of both ports are 6.9 dBi which is the highest gain per unit element at 28GHz in the literature. The measured diversity gain and envelope correlation coefficient are 9.98dB and 0.065, respectively, at the band of interest. The measured and simulated results are in good agreement.

Compact SIW Fed Dual-Port Single Element Annular Slot MIMO Antenna for 5G mmWave Applications

This paper presents a multiple-input multiple-output (MIMO) antenna system for millimeter-wave 5G wireless communication services. The proposed MIMO configuration is composed of four antenna elements, where each antenna possesses an HP-shaped configuration that features simple configuration and excellent performance. The proposed MIMO design can operate at a very wideband of 36.83–40.0 GHz (measured). Furthermore, the proposed MIMO antenna attains a peak gain of 6.5 dB with a maximum element-isolation of −45 dB. Apart from this, the MIMO performance metrics such as envelope correlation coefficient (ECC), diversity gain, and channel capacity (CCL) are analyzed, which demonstrate good characteristics across the operating band. The proposed antenna radiates efficiently with a radiation efficiency of above 80% at the desired frequency band which makes it a potential contender for the upcoming communication applications. The proposed design simulations were performed in the computer simulation technology (CST) software, and measured results reveal good agreement with the simulated one.

Compact Quad-Element High-Isolation Wideband MIMO Antenna for mm-Wave Applications

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A Novel Dual-Band 28/38 GHz AFSL MIMO Antenna for 5G Smartphone Applications

A novel dual-layer ultra-wideband lotus-wearable antenna is presented in this paper for integration on astronaut’s flight jacket to monitor the vital signs of astronauts. The proposed antenna is designed and fabricated on a leather material as a substrate to operate over a frequency band (2– 12 GHz). The dielectric constant εr = 1.79 and loss tangent tan δ = 0.042 of the leather material are measured by using two different methods. The proposed antenna has three-strip lines in the 2nd layer for performance enhancement. The stretching effect of the proposed antenna on its impedance characteristics is studied. Furthermore, SAR calculations are performed in on-body environments to ensure that it operates properly in the nearness of the human body. Finally, the proposed design is simulated by CST simulator version 2016, fabricated using folded copper and measured by Agilent 8719ES VNA. The measured results agree well with the simulated ones.

https://www.jpier.org/pierc/pierc82/12.18010911.pdf

SAR Calculations of Novel Textile Dual-Layer UWB Lotus Antenna for Astronauts Spacesuit

In this paper, a novel multiband wearable fractal antenna suitable for GPS, WiMax and WiFi (Bluetooth) applications is presented. This antenna is designed to operate at four resonance frequencies are 1.57, 2.7, 3.4 and 5.3 GHz. The proposed wearable antenna may be attached to human body, so the specific absorption ratio (SAR) must be calculated. Therefore, another design to reduce SAR value with a spiral metamaterial meandered in the ground plane is introduced. In addition, a wearable fractal antenna system integrated on a life jacket is also presented.

SAR Calculations of Novel Wearable Fractal Antenna on Metamaterial Cell for Search and Rescue Applications

In this paper, two different methods are used for investigation the RF characteristics of three types of textile materials. Goch, Jeans and Leather substrates are studied. A microstrip ring resonator method and DAC (Dielectric Assessment Kit) method are used. Bluetooth antennas are designed and fabricated using these substrates. The results are compared for the two methods. Finally, all antennas are fabricated using folded cupper and measured by Agilent 8719ES VNA.

Investigation and comparsion of wearable bluetooth antennas on different textile substrates

In this paper a novel ultra-wideband double layer lotus wearable antenna is proposed for integration onto a flight jacket of the astronaut to monitor the vital signals of astronauts continually. This antenna designed and fabricated on leather material as substrate to operate over frequency band (2–12 GHz). The dielectric constant e r = 1.79, and loss tangent tanδ = 0.042 of the leather substrate are measured by using two different methods. This antenna is integrated with three strip lines for performance enhancement. Also, the SAR calculations for this antenna are performed on-body environments to ensure that operate properly in the nearness of the human body.

A. A. Shaalan

Papers

A Novel Dual-Band 28/38 GHz AFSL MIMO Antenna for 5G Smartphone Applications

SAR Calculations of Novel Textile Dual-Layer UWB Lotus Antenna for Astronauts Spacesuit

SAR Calculations of Novel Wearable Fractal Antenna on Metamaterial Cell for Search and Rescue Applications

Investigation and comparsion of wearable bluetooth antennas on different textile substrates

A novel UWB double layer lotus wearable antenna for integration on astronauts spacesuit