a review of wearable/body worn antennas for body-centric ... · the characteristic of wearable...

International Journal Of Advancement In Engineering Technology, Management and Applied Science

(IJAETMAS)

ISSN: 2349-3224 || www.ijaetmas.com || Volume 04 - Issue 10 || October-2017 || PP. 31-44

www.ijaetmas.com Page 31

A Review of Wearable/Body Worn Antennas for Body-Centric

Wireless Communication (BWC)

Ajeet Thakur1, Garima Saini

2

¹Department of ECE, NITTTR Chandigarh, India

² Department of ECE, NITTTR Chandigarh, India

Abstract—A body worn antenna is meant to be part of human outfits and/or accessories such as belt, helmet,

smartwatch etc. These antennas are used for body-centric wireless communication purpose, which includes health

caring, tracking, navigation, public safety etc. This paper portrays review on wearable and body worn antennas

designed at different frequency bands for various applications. This literature review also tends to reveal the various

considerations in designing of wearable and body worn antennas from different textile as well as other material such

as FR 4 etc. and illustrates the effects of human body on antenna and vice versa.

Keywords—Integrating antennas; belt antennas; metal frame antennas; user’s wrist effects.

I. INTRODUCTION

With the development in wireless technology in the recent decade, wireless communication is becoming integral part in every aspect of human life and body centric wireless communication (BWC) is becoming inevitable for future [1]. Body-centric communication has its application within the sphere of personal area network (PAN) and body area network (BAN). Body-centric communication comprises in-body, off-body and on-body communication. On-body communication is communication between devices mounted on human body wirelessly, while off-body communication is RF link among body worn nodes and mobile devices or base units situated in surrounding environment; whereas in-body communication is wireless communication between on-body nodes and devices implanted inside human body [2-5]. Nowadays, portable electronic devices such as mobile phones, smart watches etc. have become integral part of human life. In future, a person is likely to carry range of sensors and devices which communicate among each other and outside world constantly [6], [7]. Wearable antennas and implantable devices are key technology to attain such goals [8].

Textile antennas using fabric textile material as substrate can be easily integrated inside clothing. Low dielectric constant is one of the essential properties of fabric textile material which improves impedance bandwidth of antenna and reduces surface wave losses. Therefore, body worn antennas can be considered part of human clothing for communication wirelessly. The characteristic of wearable antennas are based on criterion such as inexpensive, light weight, Low maintenance, robustness and no set up requirement etc. [9-11].

Wearable antenna made up of textile substrate generally developed with microstrip configuration because it is conformal and can be integrated inside clothing or accessories like belt, button, helmet etc. Also, it is not possible to keep the wearable antenna made up of textile flat every time as it bends frequently due to body movement etc. SAR level must be within acceptable limit when antenna is placed within, on or in the vicinity to body [12-14].

This paper has been organized as follow: Section II discussed various criteria for selection of fabric and section III includes safety concerns in designing of the antenna. Section IV gives an insight over the various antennas work in microwave frequency band and the interaction between antenna and human body. Finally, section V concludes the paper.

II. CRITERION FOR SELECTION OF FABRIC FOR WEARABLE ANTENNAS

In this section, various selection criteria of textile fabric for wearable antenna over the past decade are presented [15-22].

A. Permittivity

The permittivity ɛ is a complex quantity and can be expressed as

ɛ = ɛ0ɛr = ɛ0 ɛr′ − jɛr

′′ (1)

whereɛ0 = 8.854 × 10−12F/m, is the permittivity of vacuum.


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Generally, dielectric property depends on temperature, frequency and surface roughness and homogeneity, moisture content and purity of material too. The real part ɛr

′ of permittivity is called dielectric constant but it is worth noting here that it is not constant in frequency.

In designing of patch antenna, the dielectric constant of the substrate is 2.2 ≤ ɛr′ ≤ 12, but for textile fabric,

it is less than 2. The decrease in value of dielectric constant decreases the surface wave losses, therefore, increases spatial waves and hence impedance bandwidth of antenna also increases.

B. Loss tangent

It is also called dissipation factor and is defined as the amount of incident power converted into heat. It is ratio of imaginary part ɛr

′′ to real part ɛr′ of permittivity i.e.

tan δ =ɛr′′

ɛr′ (2)

Higher value of loss tangent results in more dielectric loss, in turn, causes reduction in radiation efficiency.

C. Thickness of Dielectric Material

Efficiency and bandwidth of patch antenna generally decided by dielectric constant of substrate and its thickness. The thickness ranges from 0.003λ ≤ h ≤ 0.005λ, where λ is wavelenth.For fixed value of relative permittivity, thickness is used to increase the value of bandwidth; but thickness cannot optimize the efficiency, so it is tradeoff between bandwidth and efficiency of antenna in relation of thickness. As it is clear that

BW~1

Q (3)

where Q is quality factor affected by radiation losses Qrad , ohmic losses Qc , surface wave losses Qsw and dielectric losses Qd and is given by

1

Q=

1

Qrad+

1

Qc+

1

Qd+

1

Qsw (4)

For thin substrate, Qrad is inversely proportional to substrate height, therefore, increase in height reduces Q, hence result in increase in impedance BW. Also thick substrate result in large size patch and thin substrate results in small size patch.

D. Surface Resistivity of Fabric

The electrical behavior of fabric can be evaluated by surface resistance and hence can be characterized by surface resistivity as fabric materials are planner in nature. These fabrics require low electric resistance to minimize electrical losses and so increase efficiency of antenna. However, surface resistance should be uniform over antenna area, but there may be some heterogeneity present due to fabric, hence results in discontinuity in electric current. Now if discontinuity obstructs the flow of current, it results in increase in fabric resistance.

E. Moisture content of Textile Fabric

Regain of fabric textile is ratio of masses of absorbed water in the specimen to mass of the dry specimen. Therefore water got trapped in fabric textile which results in change in EM properties of fabric, and hence increases dielectric constant and dielectric losses. Table 1 depicts various textile materials with dielectric constant.

TABLE1. DIFFERENT FABRIC TEXTILE MATERIAL AND THEIR DIELECTRIC CONSTANT

Textile Fabric Material Dielectric Constant

Wash cotton 1.51

Poly cotton 1.44

Curtain cotton 1.57

Polycot 1.56

Jean 1.67

Bed sheet 1.46


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www.ijaetmas.com 3

III. SAFETY CONSIDERATIONS

Some of the measures that must take into consideration during wearable antenna design are discussed as under [23-32]:

A. Specific Absorption Rate (SAR)

It is limited for safety purpose of antenna wearer. SAR value can be calculated using following formulae:

SAR =ςΕ

ρ (7)

whereς is electrical conductivity; E is r.m.s value of electric field and ρ is sample density.

Two standards viz. IEEE C95.1-2005 and IEEE C95.1-1999 are referenced here. According to IEEE 95.1-1999 SAR (specific absorption rate) averaged over 1g of tissue in cubic shape is restricted to less than 1.6 W/kg. Also according to IEEE C95.1-2005 standards, specific absorption rate average cover 10g of tissue in shape of cube (10g average) SAR restricts to less than 2 w/Kg.

B. Specific Absorption (SA)

SA per pulse (limitation for pulsed transmission if any), can be obtained as follow:

SA = SAR × Tp (8)

whereTp is pulse duration. SA was studied to compare the compliance with international safety regulation.

C. Effective isotropic radiated power (ERP)

EIRP should be restricted to safety regulator limits and it can be calculated from |S11| as

S11 =Pr

Pt (9)

wherePtis input power at transmitting end and Pr is received power by receiving antenna.

IV. BODY WORN ANTENNAS FOR MICROWAVE FREQUENCIES

A. Planar Inverted F Antennas

For many years, Planner inverted F- antennas (PIFA) have been used in mobile handsets, but recently

is used within the garments also.

In [1, 2] PekkaSalonen et al. presented a planar wire antenna which works as PIFA for wearable

applications in 2.4 GHz band. A flexible substrate of thickness 0.236 mm having dielectric constant and loss

tangent of 3.29 and 0.0004 respectively was used in the designing. By incorporating a second arm in PIFA

structure, dual band operation was obtained in Universal Mobile Telecom System (UMTS).The

configuration of the above mentioned antennas is shown in Fig. 1.

Fig. 1 Geometry of Wideband PIFA [1]


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B. Rectangular Microstrip Patch Antennas

In 2001, the first commercial small size fabric antenna design was developed by PekkaSalonen [1] by

using rectangular microstrip patch antenna, where fleece fabric and knitted copper fabric was used as

substrate and resonating element of the antenna respectively. This antenna worked in WLAN and with a

measured gain of 6.82 dBi at its resonance.

In the next year, PekkaSalonen in [2] described the use of various synthetic fabrics for the designing of

circularly polarized cooper based GPS antennas for wearble applications. In the experimental work, author

used five fabrics viz. Upholestery, fleece, Vellux,Cordura and Synthetic felt.The antenna made up of cordura

gave better result than others because it is able to maintain its mechanical dimensions even when stretched.

In 2005, YuehuiOuyang et al. designed electro-textile and copper based rectangular printed patch antennas

for body wearable application in WLAN band. Different types of cotton fabrics such as wash cotton, jeans

cotton, polyester combined cotton, polyester fabric and curtain cotton had been used as substrate in these

designs. The Flectron, Zelt and Shielditwas used as radiating/patch material. Fig. 2 illustrates the wearable

antenna structures that were wrapped across cylindrical pipes for experimental purpose. The return loss plot

of the antennas in bent condition with different radii is portrayed in Fig.3. The deviation of impedance

bandwidth and resonant frequency were observed and is shown in Table 2.

Fig. 2 Antennas in bent conditions with different radii [2]

Fig. 3 Effect of bending on return loss [2]

TABLE2. EFFECTS OF BENDING ON CHARACTERISTICS OF ANTENNA [2]

Sr. No. Bending

Radius(mm)

Measured

Impedance

Bandwidth

(MHz)

Measured

Resonant

Frequency

(GHz)

Measured

Gain (dBi)

1 Flat 113 2.43 9.62

2 50.8 111 2.55 8.19

3 63.5 115 2.535 9.32

4 76.2 114 2.525 9.16

5 88.9 117 2.52 9.50

It is clear from the table that due to bending, resonant frequency reduced due to increase in resonant length.


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C. Rectangular Slot antennas

In 2004, PekkaSalonen et al. used U slot design for dual band operation at GSM 1900 MHz and

WLAN 2450 MHz bands [3]. Copper plate was used for the radiating part while 3.5 mm fleece was used as

substrate material. The antenna is shown in Fig. 4. The upper and lower resonant frequencies were

determined by L and 𝐿ℎ dimension.

Fig. 4 Geometry of U-slot antenna [3]

D. Other antennas

Rectangular ring antennas, aperture coupledpatch antennas, coplanar circular patch antennas were

develpoedwwith the passage of time as per the demand of application. B.Sanz-Izquierdo et al. in [4]

explored a button shape wearable antenna. This antenna provided certain benefits in comparison to

microstrip based body worn antenna which often placed directly onto the clothing. Fig. 5 shows the initially

designed button antenna with coaxial feed. This antenna covers the 5.25 GHz Hiper LAN and 2.5 GHz

Bluetooth band.

Antenna Parameters Values

Disc diameter, Dd 16

Top disc diameter, Td 16

Base diameter, Bd 10

Cylinder outer diameter, D0 7

Centered via diameter, Vd 1.6

Tack button height, Th 7.9

Gap between disc and button, G 3

Fig. 5 Coaxial feed dual band antenna and its dimension [4]


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Fig. 6illustrates belt antenna which can be used as strap of a backpack and/or small wrist band and its return

loss plot respectively.

Fig. 6 Front view: Left, Back View: Right of belt antenna [4]

In 2007, B.Sanz-Izquierdo et al.in [5] designed a jacket mounted antenna for WLAN application and J.

J. Wang et al. in [6] presented a broadband vest and helmet antenna. In 2008, B.Sanz-Izquierdo et al.in [7]

also designed dual band belt antenna for body wearable application. In 2009, Carla Hertleer et al. [8]

introduced intelligent textile system which is highly suitable for garment integration and increases protection

level to the wearer. However, most of the textile materials are very thin (0.5mm); making it difficult for

engineers to design an antenna that operates in 2.4-2.483 GHz ISM band. In this paper author described

design and performance of textile fabric planar antenna employed on flexible foam for fire fighter outfits.

The antenna provides circular polarization with bandwidth greater than 180MHz even when antenna was

bent or compressed. In order to attain an antenna with greater bandwidth in ISM band, a patch antenna in

protective flexible pad foam is designed. Fig. 7 shows the truncated corner patch antenna and its return loss

plot.

Fig. 7 Truncated corner patch antenna on flexible foam [8]

In the year 2010, 2013 and 2015, B. Gupta et al. in [9] SweetyPurohit et al. in [12] and AnkitaPriya et

al. in [15] respectively reviewed various wearble textile patch antennas and their design consideration. Also

C. Liu et al. in [16] reviewed implantable antennas and safety concerns in the designing such as SAR, SA

etc. Sankaralingam et al. in [10] discussed transmission and refection methods for determination of fabric

material’s dielectric constant which were discuused in section II. J. A. Ray et al. in [11] presented various

PIFA designs used in the portable devices such as mobile phone etc.

Seungmin Woo et al. in [13] presented an Ultra Wide Band (UWB) diversity antenna as shown in Fig.

8for wireless body area networks (WBANs) for wrist watch applications. Here two radiators placed

symmetrically above top corner of ground plane and substrate of FR-4 material with relative permittivity 4.4


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is used in the designing. A stub is used between ends of radiators to achieve Ultra Wide Band (UWB) and

compact size. The antenna operates at frequency range of 2.9- 5.1 GHz and has a size of

40mmx40mmx5mm. Furthermore, the antenna was simulated on human wrist equivalent flat phantom and

antenna relational pattern was towards of body.

Fig. 8 Loop antenna on metal ring for mobile phones [15]

In this design, antenna contains a 1.0mm FR-4 substrate on which an impedance transformer is printed and

the exterior metal ring of a mobile. The designed antenna is fed from end of impedance transformer, and

then connected with exterior metal ring in order to excite a half-wavelength loop resonance. This paper

clearly shows that by adjusting the width and length of impedance transformer, good impedance bandwidth

can achieved.

Fu-Ren et al. in [14] integrated loop antenna design on the exterior metal ring on mobile. The antenna

is designed to work on GSM 900/1800 band. The proposed antenna is printed on 1 mm thick FR4 substrate

and the metal ring of mobile device. The Same concept was used by Saou-Wen Su et al. in [17] to design of

a loop antenna on to metal frame of smart watch Bluetooth devices. Fig. 9 depicts dimension and

corresponding return loss of the proposed antenna.

Fig. 9 Loop Antenna for smartwatch application [17]


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Here the metal frame of size 5x40x50mm is used with thickness of 0.3mm which act as front metal frame of

watch. The antenna resonates at 2.4GHz and 0.4mm thick FR-4 substrate is used in designing. The gap of

2mm is maintained between metal frame and system ground to attain good input matching and to avoid

grounding the antenna. The antenna is fed by 50Ω coaxial cable and radiation efficiency of 70% and peak

gain larger than 3dBi is attain by using the structure. Finally, antenna is tested on one-layer model of human

wrist as given Fig. 10to find out the impact of human hand on antenna performance.

Fig. 10 User’s wrist model [17]

In 2014, Ting- Yu Ku et al. [18] proposed a technique which is based on suspended micro strip line

method. The technique is used for characterization of finite size HIS structure. Additionally, an antenna

design along with finite size HIS also proposed for smart watch application. The HIS structure has the size

only 38x38 mm2, four frequency band in 2.4-2.48 GHz. By using finite size high impedance surface

structure antenna gain, front to back ratio and antenna radiation efficiency is increased by good extent. After

designing of finite size HIS, meandered monopole antenna which is fed by driven strip and fabricated on FR

4 substrate of 0.8mm thickness is placed on HIS structure. The antenna size is reduced to 5×15 mm2.

In 2015, Tamid Rashid et al. in [19] described design of textile coplanar monopole antenna with

electromagnetic band gap (EBG) for space. Here the antenna as shown in Fig. 11is designed for 5.8GHz

band and simulation results shows that gain is improved by 3.55dB with introduction of EBG and EBG

reduced radiation in undesired directions.

Fig. 11 Top view of antenna and one cell of EBG [19]

Although by introducing EBG layer, BW decreased but antenna gain improved by large extent.

In 2016, J.C. Wang et al. in [20] carried out the survey a wearable textile antennas used for wireless

body area network (WBAN) applications which are made up of textile materials with dual band and UWB.

The survey shows that every antenna including textile patch antenna and antenna with metamaterial

structures has their own advantage and drawbacks hence it is imperative to design optimized body WAN

antenna, making tradeoff between antenna size and its performance and complexity. Li-JieXu et al. in [21]

reported a metal frame antenna which is differentially fed as shown in Fig. 12. The proposed antenna has

wide frequency bandwidth i.e. from 2.27 GHz -6.14 GHz, so covering 2.45 GHz ISM and 5.8


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Fig. 12 Sketch of antenna and bone model [21]

GHz ISM (Industrial, Scientific and medical) band. Besides, it is also covering unlicensed Ultra Wide Band

(UWB) of 3.5-4.5 GHz which makes the antenna performance much robust when applied to close proximity

of human beings where frequency detuning effect occurs. The substrate used in designing here is Roggers

4350B having εr = 3.66, tan δ = 0.004 and consisting size of 38.6 mm× 18.6 mm which is enclosed by

wrist watch metal frame of thickness 0.3 mm. Furthermore, metal frame is used for purpose of antenna

radiation as well as outer rim of wrist watch. Inside the metal frame, two rectangular patches which are

separated by 1 mm distance are used. One edge of the patchis connected to the metal frame that acts as feed

part and other edge connected to metal frame acts as shorting pin. The antenna is placed 3 mm away from

human wrist so as to avoid direct contact with human tissue. Since the antenna covers unlicensed ultra wide

band of 3.5-4.5 GHz, so is quite useful in future communication where lower frequency can be used for

communication with implanted antennas and higher frequency with external base station antenna.

Additionally, the proposed antenna can be used as wearable repeater antenna for long distance

communication between external base station antenna and implanted antennas because of radiations pattern

normal to human body.

In 2016, F-eui Hong [22] et al. investigated the conservative exposure assessment of the flat phantom

because the regulation and the standard for compliance with the specific absorption rate (SAR) calculation

stipulates the use of flat phantom. This means that the calculated SAR in flat phantom is envisioned to

represent greater than the SAR in real exposure circumstances. Here, the SAR is inspected numerically by

comparing the SAR in the anatomical human body and flat phantom which analogous to a smartwatch

model in which planar inverted F antenna (PIFA) is used. The results clearly show that the flat phantom does

not harvest a conservative exposure approximation for smartwatch model. Yen-Sheng Chen in [23]

presented a structure of novel antenna made up over a miniature high impedance surface (HIS) for

smartwatch device applications. To overcome the effect of human body on the parameters of antenna and

further to build a very low-profile antenna structure, HISs are principally appropriate choice to wearable

applications. The smartwatch antenna should be highly directive, low specified absorption rate (SAR),

robust and low-profile in order to reduce impact of human body on antenna parameters, hence HISs are

predominantly a good choice to fulfill with these design objectives. Nevertheless, a HIS is generally

electrically too large that it cannot fit into the design part of smartwatch device applications. Besides, the

characterization of HISs is obtained by noting the reflection part of a unit cell, but this method befits

unsuitable for miniaturized and finite-size HISs. A new design method presented here is based on utilizing

fractional factorial designs (FFD) and the performances of the antenna are considerably enriched even

though the HIS size is only 0.3λ0 × 0.3λ0. As a result, the size of the proposed antenna is just 38 × 38 × 3 mm3 at 2.4 GHz which is smallest HIS structure till date. The directivity of the proposed antenna is 6.3

dBi and the maximum averaged SAR value is 0.29 W/kg just for input power of 100 mW; likewise, the

antenna radiation efficiency and antenna impedance matching are robust contrary to the loading effect of

user’s wrist tissues. Additionally, the proposed antenna moderates the built-up cost along with ease of

integration and fabrication; hence the proposed antenna is one of the strong contestant for smartwatch device

applications.

Sen Yan et al. in [24] designed a wearable UWB antenna with the analysis that, the antenna fabrications

except the testing connector is entirely done with textile materials. Besides keeping the low profile antenna

implements a multilayered structure and attains a worthy matching over the entire UWB band. As a ground

plane fully placed under the patch, so antenna’s performance doesn’t gets affected by the shielding of human

body when worn on it. Also the study under various kinds of deformations produced strong performance and


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great on-body reliability.Bin hu et al. in [25] discussed comparison of three types of textile based flexible

antennas which work at 2.45 GHz and is used in body area network (BAN). Here, the radiating element i.e.

patch and the ground plane is made up of using two types of conducting materials viz. Shieldex with the

thickness of 0.13 mm and copper foil tape (CFT) with the thickness of 005 mm. The total dimension of

proposed prototype antenna is decreased from 96 × 47 mm2 to 70 × 25 mm2 when compared with the

antenna without shorting pins and the measured value of return loss i.e. S11 is also decreased from -14.59 to -

33.30 dB when analyzed in free space.

Dougles H. Werner et al. in [26] presented two types of newly developed body wearable antennas. This

paper here demonstrates the transformational designs for conformal and lightweight body worn antennas

which are extremely efficient whereas simultaneously have a small size and highly compressed footprint and

a very low profile in the MBAN band. These prototypes are enabled by deploying a planar monopole

antenna over the top of an extremely truncated meta surface, that yields a very high front-to-back ratio in the

antenna radiation pattern for its size and excellently isolates the design from the loading effects due to the

user’s body. Additionally, the findings of this paper concentrate on compact, wide and narrow operational

bandwidths, planar and circularly polarized (CP) wearable outfits. The paper also infers the design strategy

of the antenna simulations and measurements as well as evaluations of on-body performance of these body

worn antennas. Besides, by introducing several interdigital capacitor loading schemes onto the meta surface,

the bandwidth widening techniques are exemplified and the selective functionality of integrated band of

radiating modules is validated using experimental measurements.

In 2017, Sang ilKwak et al. in [27] designed PIFA for SAR reduction using meta material in body

wearable application. In this paper, author proposed PIFA with artificial magnetic conductor (AMC) for

reduction in SAR. Antenna is designed for wideband code division multiple access band. AMC are type of

meta material which can act like perfect magnetic conductor, hence control antenna radiation pattern.

Therefore, antenna using AMC structure is able to prevent harmful electromagnetic fields which are emitted

towards human body. Results shows tremendous reduction of 43.3 % in SAR value at resonant/center

frequency.Hence using meta material SAR can be limited within tolerable limit and can protect human body

from electromagnetic waves. AMC is a metamaterial that contains dielectric board, unit cell and ground if

necessary. An AMC manipulates the EM wave propagation and also can control antenna’s radiation

properties. Generally, an AMC structure consists of high EM surface impedence capable of suppressing

surface currents and act like perfect magnetic conductor (PMC) in specified frequency range. Furthermore,

author described that an AMC without via contolstha antenna radiation pattern and reduces electromagnetic

waves in direction of human body. AMC structure enhance gain of antenna, prevents undesirable EM waves,

so maintaining performance of antenna by the application of AMC with ground plane. Further using slotted

AMC structure, SAR can be further reduced for body worn application. AMC structure is made up of

Taconic CER-10 (εr = 10.2 and loss tangent δ = 0.05) consisting thickness of 11.57 mm. The total size of

AMC is 54mm × 24mm with 3 × 7 unit cell array. Thereafter PIFA consisting a radiator, shorting pin

connected to ground plane and a feeding line. The proposed PIFA is fabricated using 0.017 mm of copper

substrate and dielectric costant of 3.5 (tectonic RF 35 A). Due to reduction in SAR value, designed PIFA

using AMC structure is good choice for body wearable application such as smart watch application, tablet

etc. In the same year Carlos Andreu et al. in [28] presented an antenna which is good candidate for the

upcoming in-body applications. In order to accomplish reliable measurement of UWB, implantable antennas

should operate in the propagation medium properly and to evaluate the performance of UWB channel, the

in-body antenna matching should be certain. Also, an omnidirectional antenna radiation pattern should be

achieved so as to communicate with the array sensor located in near the human body. Moreover, this letter is

dedicated to the study of the UWB in-body channel while an antenna miniaturization procedure is used

which maintain antenna’s omnidirectional radiation pattern operation bandwidth as well. To achieve the

aforesaid goals, an UWB monopole antenna consisting of circular patch is miniaturized directly and

optimized by considering the user’s muscle tissue. In order to evaluate the effect of antenna miniaturization,

the outcomes of the channel propagation measurement acquired using the miniaturized antenna is compared

with the results attained from a larger UWB monopole.

Heejae Lee et al. in [29] proposed an integrated all-textile antenna for radio frequency identification (RFID)

and global positioning system (GPS) in military beret application. The proposed antenna characteristics were

not affected by adding human head phantom. The proposed antenna with S11 = −10 DB on the head

phantom fully covers the (902 – 928 MHz) and GPS L1 band 1.563 − 1.587GHz and 915 MHz ISM

band. Therefore, the antenna is a good option for WBAN applications.

In 2017, Dingliang Wen et al. [30]presented a smartwatch antenna based on novel high impedance

surface (HIS) as shown in Fig. 13which in fit to the all-metal smartwatch applications.


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Fig. 13 Top view of HIS: Left, 3-D view of Smartwatch antenna: Right [30]

In this paper here a non-planar high impedance surface (HIS) is proposed rather than utilizing a traditional

planar one. The proposed antenna is principally comprised of a Planar Inverted F- antenna and a HIS. The

PIFA structure is placed over the HIS surface at a distance of 0.6mm and is fabricated on the upper surface

of FR4 substrate having a thickness of 0.8mm. The all-metal casing of the smartwatch works as ground of

the HIS structure, the FR4 substrate having 1.6mm thick is in the middle and a very thin copper layer is

printed on the substrate’s inner surface. Furthermore, the proposed HIS is based on cavity-shape and it uses

the metal frame of the smartwatch as its ground and in the presence of this cavity-shaped HIS structure, the

electromagnetic waves radiated towards the user’s wrist is reduced considerably as a result of which, a low

SAR can be realized for the proposed antenna. The user’s wrist phantom in the presence of HIS

Fig. 14 Comparison of reference and proposed antenna [30].

has a very small effect on the performance of proposed antenna and a very low SAR can also be observed.

The proposed antenna can acquire a gain of > 1.3 𝑑𝐵𝑖 and an antenna efficiency of > 40% covering the

frequency band of 2.4 to 2.484 GHz even if it is placed over a human wrist phantom (Fig. 14). Therefore it

can work excellently for Bluetooth or WIFI system.

Di Wu et al. in [31] proposed a cavity-backed annular slot antenna (Fig. 15) resonating at 2.4 GHz for

the use in smartwatches applications at WiFi band. The cavity is of cylindrical shape and is made up of

metallic material with a total volume of π × 212 × 10 mm3. The annular slot is cut along top surface edge

of the cavity (like smartwatch screen). The perimeter of the annular slot is nearby 1λ, which makes making

the proposed design small enough to be used in smartwatch applications. The transverse mode, current

distribution and resonant modes of the cavity-backed slot antenna are examined using simulation and the

outcomes are used to design a prototype smartwatch antenna. The proposed antenna for smartwatch is

evaluated in free space condition, on user’s hand phantom by the used of simulations and measurements. It is

apparent from the measured results that the antenna offers an efficiency of 57-66% on user’s hand model.

Furthermore, to evaluate the antenna performance in a more parctical environment, the electronic

components used within the smartwatch is modeled as a metallic block. For health risk issues pertains to

exposure of electromagnetic (EM) radiation, the Specific Absorption Rate (SAR) for the smartwatch in next-

to-mouth and the wrist-worn situations are also simulated as illustrated in Fig. 16. KawshikShikder et al. in

[32]presented a new wearable textile antenna. Ultra Wide Band (UWB) is presents in this paper for body

area networks. Along with a partial ground plane this antenna also consists a hexagonal radiating. A Dacron

fabric having permittivity 3 is used to make the substrate for the antenna. Optimization of geometry is done

to achieve Ultra wide bandwidth by: a) introducing novel slot pattern on the antenna’s radiating patch is

introduced with a slot pattern which is novel and b) a square indentation notch is introduced in the fractional

ground plane of the antenna. The icon for “wireless antenna” is this new slot. The projected antenna

substrate has the dimensions as 40×34×1.7 mm3 with bandwidth as 16.56 GHz which starts from 2.6 GHz


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and goes up to19.16 GHz and has less than -10 dB return loss. Also the gain varies from 2 dB up to 6.35 dB

with mean overall efficiencygreater than 83%.

Fig. 15 Cavity backed annular slot antenna [31]

Fig. 16 Measurement of SAR value [31]

To guarantee obedience with the IEEE C95.1-1999 standard for safety, 17.39 mW is set as the maximum

power that can be fed as input for the projected antenna. CST Microwave studio, a package for Commercial

electromagnetic simulation presents the details of the design and results of simulation for the proposed

antenna.

V. CONCLUSION

Despite of our best efforts, review of such a huge ream here is restricted to remain incomplete. Though

from this review, it is worth understood that there are several issues while designing body worn antennas or

wearable antennas for various applications. These design issue include: 1) antenna performance which

depends on material properties, 2) selection of substrate material, 3) performance enhancement using HIS

and EBG structure, 4) effect of human body on the antenna performance and vice versa, 5) SAR to be

maintain within acceptable limit.With the advancement in technology, new types of PCB like FR-4, Rogger

etc. shows better results when used in human accessories like smartwatch, helmet etc. as compare to textile

antennas which are highly affected by bending of user’s body. There is still enormous research needed in

this area to optimize the performance level. This field of body worn antennas may combine material science,

electronics circuit design and miniaturization technquesetc. for optimization in future.

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