Revolutionizing The development of Wearable Antennas
Mona El Abbasi
Electrical and Computer Engineering
Electrical and Computer Engineering
American University of Beirut
American University of Beirut
Abstract— This review paper focuses on recently published wearable antenna technologies. The body-worn antennas are essentially any antenna being part of human body clothing. They are specifically used for high flexibility and efficiency communication purposes. Therefore, this study reviews the main technological advances and contributions in the field of wearable antennas for wireless body sensor network (WBSN) applications. This can include the detection of the body when exercise, the monitoring functions such as health care services and for general network connection.
Index Terms— Wearabla antenna; WBAN; technological ad-vances; review paper
Body area network is dedicated to cover the whole human body information both the personal body parts and the area around the body. These Body Sensor Networks (BSNs) orig-inate from Personal Area Network (PAN) thought, and they are wireless area networks with knobs normally placed on the human body or near it. Improvements in communication and electronic technologies have permitted the advancement of small sized and smart devices that can be situated on the human body or inserted inside it. When implanting most bio-medical sensor antennas within the body, they should be safe enough with no harm to the human body that’s why they should have omni-directional radiation pattern with free side lobes. WBANs proved their efficiency in supporting many applications, that includes urgent military services (as in cases of video and image transmission of soldier during a battle, survivability of firefighters and all police support) to improve their awareness when performing such applications (Winterhalter et al., 2005). Recently, wearable monitoring systems facilitates disease prediction by reducing the stays at health care centers thus improving the quality of hospital-ization (Kiourti and Nikita, 2012). Regardless the application area, an important aspect of wearable communications is the preservation of antenna performances (Wang et al., 2013). Smaller size, lightweight, least affected by human body, almost maintenance-free, ideally comfortable to the body of antenna and durability are required to fulfill the above needs. This was investigated by considering the on-body performance (jS21j path gain) . This review paper is divided into sections. Section II focuses on wearable antenna challenges. Then section III gives an insight on the most important previously wearable antenna networks throughout years based on their dedicated applications. To end up the paper with an open conclusion for more expanded future work.
AI. WEARABLE ANTENNA CHALLENGES
A. Human Body Near Field Effect
The antenna designed should be insensitive to near field effects to prevent any harm to human body, because the human body is a perfect absorber for electromagnetic waves (EMWs). Hence, it must be isolated as much as possible by using a ground plane . This will keep the antenna efficiency high and increase battery life time. Otherwise, the antenna performance is degraded and its efficiency may drop significantly from about -3 dB (50%) to -13 dB (5%).
B. Confined surface Area
Wearable sensors must be flexible and confined size rigid antennas. Such limited sensor volume sizes makes the de-sign of wearable devices more difficult and seamlessly into clothing.
BI. PREVIOUSLY WEARABLE ANTENNA STUDIES
The aim of this paper is to present the most recent wearable antenna studies and the main contribution in each. Rishani, Al-Husseini, El-Hajj & Kabalan  basically suggested the determination of permittivity of such textile wearable materials. This was based on Electromagnetic Band Gap (EBG) structures. They discuss the methods used to estimate the electrical characterization of textiles; the relative permittivity and loss tangent. In addition to, they provide latest wearable antenna designs, their applications, the RF energy harvesting and the analysis of rectifying circuits, with focus on their sensitivity and conversion efficiency. to match the average size of an adult
A. Wearable Antennas for VHF and UHF frequency appli-cations
As well said before, VHF/UHF wearable antennas also ensure omnidirectional radiation pattern with dominant ver-tical polarization that has no reply on the human body0 s positions. Wide band wearable antennas in the VHF/UHF band present large physical dimensions comparable with average adult size, but they are electrically small antennas at the lower frequencies . In 2008, Kota Furuya and the other authors proposed two wearable antenna designs for Digital Television in the frequency range 470 to 770 MHz. One of the designs constitutes T-shape monopole antenna with L shape parasitic element while the other is of T-shape monopole antenna with arrow geometries and dimensions as shown in the below figure . The vest antenna presents
[a] [image: image1.jpg] [b] [image: image2.jpg]
Fig. 1. Dimensions and Geometries of T-shaped monopole antenna vs. arrow shaped monopole antenna 
large physical dimensions: 40 cm wide, 20 cm thick, and 70 cm height which is comparable to human size. It is tested to give approximate high gain 5% to 10% dB gain . In 2009 Peter and Nilavalan, theoretically stated the performance deterioration of flexible UWB antennas. However, these an-tennas are designed using AgHT-8 which consists of ground and the radiator with the use of polymer film substrate . AgHT-8 antenna material has shown better performance with
A wideband low-profile printed E-shaped PIFA antenna 
which can work at six different communication bands:Single Channel Ground and Airborne Radio System (SINCGARS band at a frequency range of 30 MHz - 88 MHz), United States Air & Marine band (116 MHz - 174 MHz), ultra high frequency band (UHF) (225 MHz - 450 MHz), Public Safety UHF band (450 MHz-512 MHz) and Soldier Radio Waveform (1-2 GHz). This review paper provided novel opportunities to work on wearable antennas at VHF/UHF bands with an increased efficiency and more accurate systems when exposed to human body .
B. Wearable Antennas for WLAN Applications
The wearable antenna can be also suitable for wireless local area network applications. Sankaralingam & Gupta fabric studied the characteristics of versatile textile antennas. The cotton fabric textile antenna has been chosen as sub-strate and tested on microstrip circular disk. In this manner, Sankaralingam, S. and B. Gupta also exposed the design of textile circular wearable disk microstrip antenna working in the field of Bluetooth ISM band (2400 MHz -2485 MHz) using two separate parts a copper conducting part and a flectron electro textile one (smart cloth). When determining
A copper film antenna & AgHT-8 film antenna 
bending demonstrating its acceptable usage in wearable ap-plications. Due to this, researchers have raised a deep survey on wearable off body radio antenna links for VHF/UHF applications. Nepa & Rogier in 2015 presented the main challenges and future trends within this technology. They presented the different designs within UHF/VHF band to increase the antenna efficiency. One way is to keep an empty distance between the antenna and human body surface. As previously stated VHF band applications exhibit large space antenna dimensions and large gain. So, the authors designed a flexible fractal antenna printed on a coat with the helix folded over one users arm. This design is of quite large dimensions around 30 cm x 30 cm and operates at 136 MHz for land mobile radio link applications. The substrate of this antenna is a mixture of copper tape and conductive textile material. Recently, in 2017 a multiple frequency band body wearable antenna has been proposed by researchers
Mushroom-like EBG Surface 
the right dielectric loss of textile materials as suggested by Rishani, Al-Husseini, El-Hajj & Kabalan, high gain and high efficiency is obtained.The researchers in , proposed circular polarized wearable textile antenna as mobile appli-cations needs. The authors in  paper has demonstrated a wearable textile antenna with electromagnetic band gap structure which covers dual band WLAN frequencies (2.4 GHz & 5.2 GHz). The EBG substrate wearable antenna of
dielectric 1.7 is designed on a jeans material. The EBG are arranged in circular pattern by duplicating 6 same el-ements, surrounding the patch antenna. Because, the use of wearable antennas have backward radiation which can be danger on human body. The benefit behind the use of this structure is that it is safe on human body by reducing first the backward antenna radiation in both E-plane and H-plane by at least 10 dB and second by increasing the gain at the corresponding frequencies. In 2015, I. Sfar and L. Osman spent their study on the analysis and fabrication of wearable textile antenna working on 2.4 GHz and 5 GHz based on coplanar waveguide (CPW) configuration. Coplanar waveguide (CPW) 50 configuration is to feed the antenna as shown in the below figure. The used antenna
Dual-band textile antenna geometry .
material is jute fabric. This double G form antenna is of 3.8 mm thickness and a 0.2 mm gap distance that separates the strip from the coplanar waveguide ground plane. The dual band wearable antenna can create dual resonant modes depending on strip0 s electrical length this will lead to two different current paths . But this study didn’t focus on the antenna bent and its effect on its performance. That’s why Sankaralingam & B Gupta, included the bent effect on four flexible rectangular shaped microstrip wearable patch anten-nas for WBAN application and fabricated the patch as shwon below. These patches were employing different varieties of
compared to circular shaped wearable antenna design are as follows: larger physical area that means higher bandwidth, centered shorting pin excites higher order modes whereas in the case of rectangular patch there is a free gap distance in between. Moreover, rectangular patch implementation and analysis requires less electromagnetic simulation and is of easy design. It was shown in  that among these four antennas, the polyester antenna showed the best performance between the analyzed and fabricated measured resonant frequency results. The bandwidth of these four antennas is found to be 85 MHz for bluetooth applications. As stated before, the need of omnidirectional radiation pattern to meet the requirements of smart clothes and mobile devices. The human body movements, in wearable systems, cause the textile material based antennas to be bent all the time. Because of this in , the authors propose to use the daily wear polyster material and they examined bending effect on different radii cylindrical PVC pipes curved surfaces. It is shown that the antenna resonant length changes due to bending, this will cause more resonant frequency deviations. Generally, as antenna is bent more, as resonant frequency deviations are more with no effect on antenna performance .
C. Wearable Antennas for Energy Harvesting Applications
In  authors proposed wearable body systems used for energy storage. The energy storage system is composed of logarithmic body wear antenna followed by half and full wave rectifier electronic circuits. This energy storage system is used in far-field wearable antennas with wide frequency bandwidth. It collects all the radiations coming out from technological devices such as TV, computers, cellular and radio base stations etc.). Denim material is used to manufacture the antenna as shown in the below figure. It
Fabricated rectangular patch antenna geometry .
cotton and polyester materials for in contact wireless body applications. Rectangular shaped wearable antenna design
The proposed wearable antenna designed .
is shown that the output DC voltage is less than any other power source as in the case of batteries. But, at the same time this output dc voltage can be used to charge many power sources such as batteries, capacitors, mobile phones and any other portable device. This antenna operates at three different bands: 2.4 GHz, 5 GHz and 8 GHz. It also provides high gain about 5 dB. Researches spot the light on fully-wearable tri-band rectenna for energy harvesting
working at 200 to 900 MHz (the GSM band) and at 2.4 GHz (the WI-Fi band). MD material is being used and /20 UHF patch antenna is designed.The below figure illustrates the main incident RF sources of wearable antennas, the rectifying circuits of each and its effect on on-body wireless communication. The rectenna substrate fabrics used are con-
Fig. 8. (a) Multiple incident RF sources of wearable rectenna. (b) Har-vesting antenna Electromagnetic (EM) field sources. (c) Tri-band wearable rectenna circuit .
ductive and non-conductive. These new substrate materials show better performance than any other previously used materials. The paper in  presents a full description on Ultra-Wideband wearable active energy harvesting systems in frequencies ranging from 0.4 GHz to 8 GHz. This ultra-wideband antenna reveals excellent performance in terms of low VSWR, high efficiency about 90%, high gain around 25 dBi in the GSM band. However, this high gain dropped to half of its value when the same ultra-wide band antenna operates in Wi-Fi band. As presented in the above figure an
A wideband fractal active wearable notch antenna .
active notch antenna dimensions are 74.5 mm x 57.1 mm. The impedance matching circuit is connected to the amplifier network then to the rectifying circuit. It is found that the gain flatness of the energy harvesting system may be improved by using a low noise amplifier connected to the rectifier circuit that can reveal better gain flatness. Furthermore, Lee, H., & Roh, J. (2018) introduced energy harvesting techniques using wearable antennas for ultra wide-band on-body wireless sensors to harvest the free radiations from the different conventional power devices. The collected free electromagnetic radiations can recharge any of the portable smart electronics, extending their lifetime and saving more energy. The work in , presents the performance of the
fabricated antenna in three different places: in vaccum, on human0 s chest and on the human0 s arm. It is found that the most stable location is on body0 s back, because at this location the body orientation changes less compared to any remaining part such as the arm.
Inspite of the summarized studies provided, a full overview on wearable antenna studies remains hard to complete. From here, it is concluded that there are several wearable antenna design considerations. Of the most important considerations are: the election of material, critical parameters of antenna performance, conductive ground plane dimensions, material conductivity, the use of EBG structure, human body effect on antenna performance and specific absorption rate (SAR) acceptable levels. For future work, there will be a new design on wearable textile antenna and will tested directly on different parts of human body in real case environment. The upcoming review paper will concentrate also on new designs of wearable antenna for energy harvesting and collecting electromagnetic radiations from different radiant electronics.
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