Asia Pacific Symposium of Applied Electromagnetics and Mechanics (APSAEM2010) Kuala Lumpur, Malaysia, 28-30th July 2010

Design and Analysis of Conductive Textile PIFA

H.M.R. NURUL*1, F.MALEK, P.J. SOH, N.B.M. HASHIM, G.A.E VANDENBOSCH2.

A small, l ight weight planar inverted-F antenna (PIFA) fabricated using conductive textiles is presented in this paper. Conductive textile is made from conductive polymer or metal threads together with normal fabric thread. The antenna presented in this paper has a total dimension of 20 mm x 20 mm and has been optimized to be operational for Wireless Body Area Network (WBAN) with 2.45 GHz operating frequency. The antenna achieved the desired operat-ing frequency with 765 MHz bandwidth. Analysis of the different bending on the antenna has been carried out to in-vestigate the performance of the antenna when integrated on a human user.

Keywords: Conductive textile, conductivity, planar inverted-F antenna (PIFA).

1. Introduction

The antenna used for body area network, i.e a wear-

able a ntenna, enables the an tenna to be wo rn o r i nte-grated into clothing. Wearable antenna requires flexible material instead of conventional rigid substrates such as FR-4 o r R oger’s b oards. The s uitable materials f or wearable a ntennas a re c onductive t extiles. These c on-ductive t extiles ( or a lso k nown a s e lectrotextiles) a re typically made f rom c onductive m etals or polymer threads with normal fabric threads or conductive threads [1]. T hese c onductive t extiles f eel l ike n ormal f abrics and can be sewn into everyday clothings.

The a vailability o f n ovel e lectrotextiles o pens n ew possibilities f or w ireless c ommunication. Furthermore, the introduction of new wireless protocols for body area networks and personal area networks (BANs and PANs) operating i n t he 2.45 G Hz band, ba sed o n t he Z igbee (IEEE 80 2.15.4), Bluetooth [ 2] ( IEEE 8 02.15), W iFi (IEEE 8 02.11b/g), Wireless US B, a nd WiMAX ( IEEE 802.16a) pr otocols, poses some real challenges to antenna design b ased on t extile materials. T he em er-gence of i ntelligent t extile s ystems r equires t he devel-opment o f l ow-cost, flexible a nd l ightweight a ntennas that a re e asily i ntegrated i nto c lothing. Quite r ecently, the application o f new conductive t extile materials h as led t o t he de velopment of various wearable an tenna prototypes [3–6]. In addition, the development of wearable antennas i s a lso made p ossible by us ing new fabric m aterials w hich were previously u sed f or el ec-tromagnetic shielding.

Wearable antennas can be useful for many applica-

tions s uch a s f or t he paramedics, f ire fighters a nd military. B esides t hat, w earable a ntennas can al so b e

Wearable antennas can be useful for many applications such a s for t he p aramedics, fi re fighters a nd m ilitary. Besides t hat, wearable an tennas can also b e used f or monitoring t he m ovement of prisoners, children, a nd athletes. Hertleer e t. a l. has i nvestigated t he use o f antenna integrated into the fire fighters c lothings [7-8]. In this p aper, w e p resent a w earable p lanar inverted-F antenna manufactured using conductive textile operating in the 2.38-2.48 GHz band. As shown in Figure 1, with the exception of the connector, the antenna is manufac-tured e ntirely f rom t extile m aterials. The pr oposed antenna is a planar inverted-F antenna, designed entirely using c onductive t extile. T he t otal d imensions of t he antenna, including the ground plane is only 20 mm x 20 mm. Due to it compactness, the antenna can be hidden, under the shirt collar, for example. 2. Antenna Design

The first step in the design of a textile antenna is c hoosing a n a ppropriate m aterial f or t he a ntenna substrate and the conducting parts. Conductive textiles, also known a s e lectro-textiles, are c onductive f abrics constructed b y interlacing conductive m etal/polymer threads with normal fabrics. These fabrics are wearable, durable a nd f lexible, t hus m aking t hem s uitable t o be integrated into clothing [9].

The various types of conductive textiles avail-able today are such as: woven and non-woven conduc-tive t extiles, a nd t he c onductive m etalized t extiles. In [10], s everal requirements o f c onductive t extile f or antennas have been listed to guarantee its performance, as follows:

i. Low electrical resistance ii. Flexible iii. Light weight iv. Tear resistance

Table 1 tabulates t he parameters f or Z elt Co nductive Fabric, P ure C opper Polyester Taffeta F abric (PCPTF) and S hieldit Super, w hile their photographs are shown in F igure 2. E ach t extile h as i ts o wn a dvantages. An-

_______________________ Correspondence: H.M.R N urul, School of Computer and C ommunication E ngineering, U niversiti M alaysia Perlis. (UniMAP) email: nh_husna@yahoo.com.my *1 Universiti M alaysia Perlis (UniMAP) *2 ESAT-

TELEMIC, Katholieke Universiteit Leuven

488

other type of conductive fabric, Veilshield, is t ranspar-ent, while Shieldit Super has adhesive backing and can be e asily a ttached t o t he s ubstrate by i roning. Pure Copper Polyster Taffeta Fabric is a smooth fabric made using pure copper. I ts light weight and f lexible charac-teristics makes it easy to be cut and sewn like ordinary fabric. The Zelt conductive is composed of high quality nylon t affeta f abric, which i s t hen plated with c opper and t in. This f abric material is also durable, t ear r esis-tance and can withstand forming.

Fig. 1: The wearable planar inverted-F antenna pro-

totype

Table 1: Parameters of Conductive Textiles Conductive

Textile Thickness

(mm) Surface

Resistance (Ω)

Conductivity (S/m)

Zelt Conduc-tive Fabric

0.0635 0.05 1.75 x 105

Pure Copper Taffeta Fabric

0.08 0.05 2.5 x 105

Shieldit Super

0.15 1 6.67 x 105

Fig. 2: Conductive textiles

.The f abrication process f or the t extile antenna is pe r-formed b y c utting t he c onductive t extile a ccording t o the de signed di mensions. I n c onventional, non-conformal antenna design using FR-4 board, for exam-ple, the soldering technique is used to connect the SMA connector to the antenna. However, conductive t extile

material i s n ot r obust e nough t o withstand t he heat of the s oldering i ron. In t his ca se, a co nductive ad hesive with excellent electrical conductivity and high bonding strength is used as the interface for the fabric-connector interconnection. As s hown i n Figure 3, t he c onductive adhesive that consists of two components - pure s ilver, and its adhesive.

Fig. 3: Conductive adhessive

3. PIFA Patch Dimension Calculation

Figure 4 shows the geometry of the proposed anten-na. It consists of a ground pl ane a nd a main pa tch connected by a s horting plate . The c onnector us ed f or this de signed i s a pr obe feed c onnector. T he pr oposed design i s c arried o ut us ing C ST M icrowave Studio Software (MWS).

Fig. 4: Geometry of PIFA In order to model the conductive textile in the CST

MWS, the conductivity of the textile material has to be determined to define the textiles as custom materials. It is defined by its thickness and surface resistance, using the following equation [11]:

(1)

Where Rs = surface resistance of the material, t = thickness of the material and σ = conductivity of the material

The conductive textile used for the proposed antenna

is Pure Copper Polyster Taffeta Fabric (PCPTF), having 0.05Ω/square surface resistivity and 2.5 × 105 S/m

Main patch

Foam

Ground plane

Shorting plate

Probe feed

Height (h)

LG

W

L2 L1

tRs×=

1σ

489

conductivity. T he width, L1, a nd th e le ngth L2 are t he dimensions of the radiating patch, while the width, WG and t he l ength, LG, are the di mensions o f the gr ound plane. The radiating patch has dimensions of L1 × L2 and the ground plane has dimensions of LG× WG. The height of the antenna, h, is filled with an air substrate (εr = 1).

This height, h, is the height between the main patch and the ground plane, and is set as 5 mm. The shorting plate, with 5 mm w idth i s placed a t t he edge of t he radiating patch and the ground plane. The shorting plate is u sed to c onnect th e patch a nd the ground p lane. A planar i nverted-F a ntenna i s g enerally a quarter-wave (λ/4) resonant structure achieved by short-circuiting the radiating patch t o t he a ntenna’s g round plane u sing a shorting plate. The resonant frequency can b calculated using the closed form equation as [12].

(2)

Where c = velocity of l ight (3 x 108 m/sec) and f =

2.45 GHz. L1 and L2 = width and length of the conduct-ing element. Assume x is the total length. Where x = L1 + L2.

(3)

(4)

Therefore the length of the main patch is: x = L1 + L2 = 30.6 mm

L1 ≈ L2 ≈ 15.3

The o perating f requency of a m icrostrip patch a n-tenna i s i nversely p roportional t o i ts p hysical d imen-sions. The derived va lues of L1 = L2 = 15.3 m m a re used a s t he i nitial va lues. D uring t he optimization process using the CST, the patch dimension of 20 mm x 20 mm is obtained and sufficient in achieving resonance in the desired operating frequency. 4. Feeding Technique

The antenna i s fed using a probe feed, which is lo-

cated at a point on the patch where the input impedance is 50Ω.

The antenna will be integrated under the round part of collar s hirt. Due to di fferent size o f c ollar s hirt, analysis of d 00ifferent antenna bending has been done.

The feed location is selected based on the least S11 value obtained in the s imulation. Figure 4 shows the f eeding port location on the main pa tch, which i s fed from the bottom of the ground plane. In Figure 5, the probe feed is p laced at a cer tain distance f rom t he ed ge. T his distance is referred to as Rin. Table 2 shows the compu-tation of the calculation.

Fig.5: The feed point location

Table 2: Calculation of feed point location at 2.45GHz Frequency 2.45 GHz, 0.122 =oλ

1G

( )

( )m.

mm

ko

L

371

380658.124121

)122.0(12020

24121

)(120

23

21

=

−=

−=

−

λ

Where ; L1 (length of t he m ain pa tch) = 2 0 m m and k = 1.38065 x 10-23.

inZ

729.9237.1

1

1

1

=

=

=

mm

G

inR

mm31.8

2092.729

50cos

cos

1

1

=

=

=

−

−

π

π

mmx

lxZZ

in

o

Where Z0 = 50 and π = 3.142.

Optimized value using CST MWS

Rin = 8 mm

( )214 LLcf+

=

fcx

4=

( )GHzx45.24

103 8

=

mm6.30x =

xcf

4=

Rin

490

5. Results and Analysis

5.1 Simulated and measured results

The w earable an tenna p rototype is measured u sing the A gilent P NA E83628 Network Analyzer. The simulated and measured results are illustrated in Figure 6. T he result s hows that t he o btained simulated band-width is 765 MHz, and becomes narrow with 520 MHz when measured. A s light f requency upwards s hift has also been observed. The measurement also showed that the resonance obtained is between 1.97 and 2.46 GHz, and between 2.01 and 2.87 GH z when s imulated. At 2.45 GHz, the S11 obtained during simulation i s -22.93 dB, and -10.5 dB when measured. This is due to imper-fection during the fabrication process i.e. the fabricated height might not be exactly similar to the desired height.

Fig. 6: The simulated and measured S11 results 5.2 Analysis with different bending of antenna

The antenna is analyzed with 300, 600, 900 and

1200of bending. Based on the results shown in Figure 8, changes are observed i n t erms of S 11 and ba ndwidth. Even with both parameters changing, the antenna is still functioning w ithin t he desired 2. 45 G Hz frequency range. This is being kept consistent due to the location of the p robe f eed, w hich maintained i ts im pedance. Figure 7 shows example f the antenna bending.

Fig.7: Example of antenna bending

Fig. 8: The simulated S11result of the antenna with different bending.

5.3 Surface Current

Figure 9 shows the surface current flow at 2.45 GHz. Based o n t he fgure, t he s urface cu rrent i s l argely concentrated at t he e dge w here the s horting pl ate is located. Current f low i nduced from t he p robe feed is distributed t o t he m ain p atch a nd t he ground p lane through the s horting pl ate. Thus, t he surface current i s large along the edges where the shorting plate is located. The total lengths of the current flow, starting from a to b, which is 30 mm, indicate the excitation of a quarter wavelength r esonant mode a t 2. 45 GHz. E quations ( 5) to (6) show the steps to calculate the current length. For a quarter wavelengths, the wavelength of the frequency is divided by 4, as shown in equation (6) below.

Fig. 9: Surface Current at 2.45GHz

(5)

Where c = velocity of light (3 x 108 m/sec) and f = 2.45 GHz.

fc

≈λ

491

(6)

5.4 Radiation pattern

The el ectromagnetic wave i s formed by the combi-nation of e lectric f ield (E-field) and magnetic f ield (H-field). T he E-plane i ndicates t he el ectric f ield t hat radiates on t he vertical p lane w hile t he H-plane indicates t he magnetic f ield t hat r adiates o n t he horizontal plane. Figure 10 shows the polar plot for the radiation pattern at 2.45 GHz for E-plane and H-plane. Based o n t he r esults, the r adiation i s n early omnidirectional, where the E-plane is nearly shaped like an ‘8’, while the H-plane is nearly circular. This is due to the location of the probe feed not being in the middle of the m ain p atch. In o rder t o o btain a perfect omnidirectional radiation pattern, the connector has to be l ocated i n t he m iddle of t he patch, t o e nsure symmetry. One s uch example is f ound in [ 13], where the de signed antenna results i n a n omni di rectional radiation p attern, by de signing the an tenna symmetrically.

Antenna ga in i s a measure d escribing the pe rform-ance of an antenna, considering the di rectivity a nd the directional capabilities of the antenna [14]. The obtained gain of t he proposed a ntenna is 2. 684 dB wh ile th e obtained r adiation e fficiency of t he a ntenna i s 9 8%. The radiation efficiency, on the other hand, is the ability of a n a ntenna t o r adiate t he accep ted p ower i nto f ree space [15], while its total efficiency is the product of the radiation e fficiency an d t he r eflection e fficiency. A c-ceptable value for total antenna efficiency is within the range of 6 0% t o 90%. T he s imulated t otal e fficiency obtained for this antenna is 97%. Table 3 tabulates the simulated and measured results of the antenna gain and efficiency.

Table 3: Simulated and measured gain and efficiency

Simulated Measured

Gain (dB) 2.809dB 1.6018 dB Efficiency (%) 97.9% 52%

Fig. 10: Simulated and measured (a) E-plane and (b)

H-plane

6. Conclusion

A planar inverted-F antenna designed using conduc-tive t extiles operating in 2.45 GHz f requency has been presented. T he an tenna h as b een fabricated, w ith i ts measured S11 and bandwidth compared to its s imulated performance. The s imulated result s hows t hat t he antenna works well at 2.45 GHz, with S11<-20 dB, while S11<-10 dB is obtained in the experiments. The radiation pattern of the proposed antenna produced a nearly omni-directional r adiation p attern. The performance o f t he antenna h as also been investigated by va rying the bending of the antenna. The surface current distribution has been analyzed to investigate the current path for the 2.45 GHz frequency. Although this result is satisfactory, the structure can be further optimized in the future, by the addition of multi-band operation.

Acknowledgment

The authors would like to acknowledge the Univer-siti M alaysia P erlis ( UniMAP) S hort T erm Re search Grant Scheme (Grant No: 9001-00141) for the financial support.

4

mm 0.122 4

=λ

mm 0.122 =

mm 30 =

10(a)

10(b)

492

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