Robust Patch-Antenna for Wearable WLAN Applications

Rizwan Masood*, Syed Ali Mohsin**

* National Engineering and Scientific Commission, Islamabad, Pakistan, ** The University of Faisalabad, Faisalabad, Pakistan

Abstract-The design of a rectangular patch antenna with a

flexible polyimide substrate for rugged wearable

applications is described. During normal operation, the

designed antenna radiates at 2.4 GHz in the ISM band. Due

to mechanical stresses, the antenna structure is subjected to

twists and turns. For rugged use, a requirement is that the

twist and turns should not influence the far-field radiation

pattern and gain to an intolerable extent. The impact of

twist and turn angles on the antenna parameters is found in

this paper. The degree to which the substrate can be bent

without affecting normal antenna operation is determined.

A flexural endurance polymer like polyimide is found to be

suitable as a substrate for wearable antennas.

Index terms: Flexible Substrate, Polyimide, Patch Antenna,

Twist, Taper

I. INTRODUCTION

Patch antennas are popular as wearable antennas due to their thin structure that is flexible to some extent [1-2]. Wearable antennas are especially subjected to harsh and rough use that will bend, twist, and turn the antenna structure. Although these are transient changes, and the antenna structure reverts back to its normal shape when the stresses and strains are removed, yet normal electromagnetic operation is an absolute requirement during this state [3]. The variation in antenna parameters from their normal values due to antenna flexure has been studied by a number of authors [3-4]. The fact that the geometry of the antenna changes to some extent and we still expect normal operation conforming to the straight design is a stringent requirement; the subject has still not reached a mature state. The present paper further explores these unresolved issues.

The design required a flexible high-endurance substrate and our choice was polyimide with a dielectric constant of 3.5 and a Young's Modulus of 2.5 kN/mm2• Its Poisson ratio is 0.4 and coefficient of thermal expansion is 5.5xlO-5 IK. The high tensile strength and excellent thermal expansion properties make polyimide a substrate of choice in a wearable antenna that will be put to rugged use at a high temperature, for example, in fire rescue operations and other such rugged environments.

II. DISCUSSION AND RESULTS

A radiating patch is modeled by two methods; the transmission-line model, and the cavity model [5]. The effective dielectric constant remains constant at low frequencies (called static value) and approx imates to the

978-1-4673-0292-0/12/$31.00 ©20 121EEE

dielectric constant at high frequencies. The static value of the effective dielectric constant is given by [5] as [ ]-1/2

=Gr+1 Gr-1 1 12� GrejJ + + 2 2 w (1)

where w is the width of the patch, h is the substrate height

and Gr is the relative permittivity of substrate. The width

w of radiator is given by [5]

w- 1 J

2 (2) -2.f,� ,uoGo Gr + 1

and the length is given by [5]

1= 1 2M 2.f, � G rejJ ,uoG 0

(3)

where ,1L is the length extension because of the fringing

effects, ,uo is the permeability of free space and Go is the

permittivity of free space. And f,. is the resonance frequency of the patch element. These design relations

were applied to the polyimide substrate with Gr = 3.5,

loss tangent of 0.003 and height of 794 /lm. The values obtained were w = 43 (mm) and 1= 32.4 (mm)

The scattering parameters were calculated by the finite­difference time-domain (FDTD) method [6]. The software simulation package used was CST Microwave Studio [7]. The antenna and the FDTD computational domain are shown in Fig. 1. The ground plane of the antenna is taken as an outer boundary of the FDTD domain, since the antenna does not radiate on that side.

Polyimide Substrate

Domain wal ls wnere

absorbing boundary

conditions are enforced

Fig. 1. Computational domain for patch element on a polyimide substrate with absorbing boundary conditions

The length was parameterized by executing a parameters sweep for an exact resonance at 2.4GHz as shown in Fig. 2.

-s

-10

-fJ..wtr '\WV ( ',!�! I

1ien¢\:32 ..noth.,32-2 1ength·]2.� io:ngth'"'lH

-15

-20

-25 ,

m �� i "

freQUmCY/GHz

Fig. 2. Parameterization of length for exact resonance at 2.4GHz

Fig. 3 gives a plot of the return loss for the patch versus frequency which shows that the patch element resonates at 2.4 GHz and thus lies within the ISM band specified in WLAN IEEE 802.11 (b and g), [8].

51,1

-I' -I' -16 -20 j---""---------; ------------+-----------+-----------+ ------------ ,-------------- ;- ------------ ;------------+-------------; -------------j -12 1---;---;..---i--_--<_-'-_-i-_____ -;---1

, u u u u u u v U M Freca.oencv/Gll:

Fig. 3. Return loss of patch versus frequency showing resonance

ILA FAR-FIELD PATTERNS OF THE STRAIGHT PATCH ANTENNA

Fig. 4 shows the far-field radiation pattern for the straight patch antenna with the ground plane modeled as a perfect electric conductor. It is evident from the Fig. 4 that

there is no back lobe in the elevation direction (() > 90°) and that there is radiation only in the bore-sight direction. The patch in this case has a gain of 6.3 dB (magnitude)

and a bore-sight along () = 0° , that is, there is no beam squint. The values of gain and 3dB beamwidth are shown in the inset

Phi= 90 30

150

Gain Abs (Phi=90)

°

180

30 Phi=270

Frequency = 2_4 150

Theta I Degree vs_ dB

Main lobe magnitude = 6_3 dB

Main lobe direction = o_0 deg_

Angu.,r width (3 dB) = 132_3 deg_

(a)

.

Gan Abs (Theta=90)

o 30 330

150 210 180

Phi I Degree vs_ dB

(b)

(c)

Frequency = 2.4

Main klbe magntude = 2_7 dB

Man klbe du-ect,m = 270_0 deg_

Angu.,r wKith (3 dB) = 82_9 deg_

dB 6.34

4.42

2.88

1.34

0

-9.18

-17 .3

-25.5

X -33.7

Fig. 4 Far-field radiation patterns of the patch (a) Along elevation (b) Along azimuth (c) 3-D radiation pattern

ILB IMPACT OF SUBSTRATE TWIST

In order to study the impact of twist on the patch for wearable applications, the FDTD computational domain is extended to include the space next to the ground plane side. This is necessary since the after the twist (i.e., a bent substrate) the radiation on the ground plane side is no longer zero. The domain and the placement of the antenna are shown in Fig. 5.

Twisting torque to be

applied to the flexible

substrate

Fig. 5. Computational domain for patch antenna to study impact of twisting. The twist is applied in the direction indicated by the black arrows.

The far-field pattern of the patch with a twist of 1 ° is shown in Fig. 6.

Directivty Abs (Phi=90)

o

150 150 180

Theta I Degree vs. dBi

Directivty Abs (Theta=9 0)

o

180

Phi I Degree vs. dBi

(a)

(c)

Frequency = 2.4

Main lobe magnitude = 7.1 dBi

Main lobe dR-ection = 5.0 deg.

Angular width (3 dB) = 86.9 deg.

Side lobe level = -B.8 dB

Frequency = 2.4

Main lobe magnitude = -0.4 dBi

Main lobe direction = 304.0 deg.

Angular wkJth (3 dB) = 139.4 deg.

dBi 7.08

4.93

3.22

1.5

o

-8.98

-17

-24.9

-32.9

Fig. 6 Far-field radiation patterns of the patch antenna with an applied twist of 1° (a) Along elevation (b) Along azimuth (c) 3-D radiation pattern

It is evident from Fig. 6 that because of the twist in the antenna, there is a slight beam squint, i.e., the patch maximum lies along =5°. The 3-dB beamwidth for the patch is 86.9° (along elevation) and the side lobe level is -8.8 dB which being less than 3 dB can be accepted as tolerable.

The patch is then applied a twist of 2°. The resulting far-field pattern of the patch is shown in Fig. 7.

Directivty Abs (Phi=90)

o

150 150 180

Theta I Degree vs. dBI

(a)

Directivty Abs (Theta=90)

o

180

Phi I Degree vs. dBi

(c)

Frequency = 2.4

Main klbe magntude = 7.1 dBi

Main klbe d'ect'm = 5.0 deg.

Angular wKith (3 dB) = 86.5 deg.

SKie klbe ",vel = -8.8 dB

Frequency = 2.4

Main klbe magnitude = -{).4 dBi

Main klbe direction = 304.0 deg.

Angular width (3 dB) = 140.1 deg.

dBi 7. 09

4.94

3.22

1.5

0

-8.98

-17

-24.9 X -32.9

Fig. 7 Far-field radiation patterns of the patch antenna with an applied twist of 2° (a) Along elevation (b) Along azimuth (c) 3-D radiation pattern

Comparing Fig. 6 and Fig. 7, it becomes evident that the far-field radiation pattern of the patch remains within acceptable bounds with such a twist (2°). The patch gain and sidelobe level are still almost the same but there is only a slight decrease in 3dB beam width from 86.9° to 86.50

The twist is now increased to 5° and 6° and the corresponding results are shown in Figs. 8 and 9 respectively.

Directivity Abs (Theta=90)

o

180

Phi / Degree vs. dBi

(a) Directwity Abs (Phi=90)

o

180

Theta / Degree vs. dBi

(b)

(c)

Frequency = 2.4

Main lobe magnl:ude = -0.4 dBi

Main lobe direction = 304.0 deg.

Anguo,r width (3 dB) = 139.4 deg.

Frequency = 2.4

Main lobe magnl:ude = 7.1 dBi

Main lobe direction = 5.0 deg.

Anguo,r width (3 dB) = 86.8 deg.

Side lobe level = -8.8 dB

dBi 7.08

4.94

3.22

1.5

0

-8.98

-17

-24.9

x -32.9

Fig. 8 Far-field radiation patterns of the patch antenna with a twist of SO (a) Along elevation (b) Along azimuth (c) 3-D radiation pattern

Beyond 6°, the patch resonance is disturbed and starts to resonate beyond the ISM band. For verification, a sweep is performed for return loss versus frequency for different twist angles and the results are shown in Fig. 10.

The preceding analysis of a patch antenna serves to illustrate the effects that rugged environments can have on antenna performance. Other flexible substrate antennas, [9-12], can be adapted for use in harsh and rough environments and can be examined in the manner done in the present paper. It should be possible to design compensating radiation mechanisms when the antenna structure undergoes extreme bending or twisting.

o

180

Phi / Degree vs. dBi

(a) Directwity Abs (Phi =90)

o

150 150 180

Theta / Degree vs. dBi

(b)

(c)

Frequency = 2.4

Main lobe magnitude = -0.4 dBi

Main lobe d"ection = 304.0 deg.

Anguo,r width (3 dB) = 139.2 deg.

Frequency = 2.4

Man lobe magnitude = 7.1 dBi

Man lobe d"ect'Jn = 4.0 deg.

Anguo,r width (3 dB) = 86.8 deg.

Side lobe level = -8.8 dB

dBi 7. 09

4.94

3.22

1.5

0

-8.98

-17

-24.9

-32.9 x

Fig. 9 Far-field radiation patterns of the patch antenna with a twist of 6° (a) Along elevation (b) Along azimuth (c) 3-D radiation pattern

��f7�f7�J;���ff����������. L ..... 5 ············ ,·············· 1

......... n..u.1

2.1 l.l 2.l 2.4 2.S fTl!QU!flCV/Glz

Fig. 10 Return Loss of patch versus frequency with a sweep of twist angles (through parameterization)

III. CONCLUSIONS

It has been shown that in the presence of reasonable mechanical stresses and strains, the patch antenna, though twisted, gives near-normal operation. For the patch antenna taken as an example here, twist angle values up to 6° are acceptable since the twisted antenna still radiates in the ISM band with an acceptable far-field pattern. The substrate with the ground plane can be attached to a base of semi-rigid rubber material providing sufficient rigidity which will keep twisting or bending etc. within acceptable limits. The presence of this semi-rigid material will not affect the far-field pattern since the patch will have a ground plane adjacent to the slightly rigid base. With these considerations, the antenna can be used in on-fabric WLAN or bluetooth applications.

REFERENCES

[I] P. Salonen, Y. Rahmat-Samii, H. Hurme, M. Kivikoski, "Dual­band wearable textile antenna" Antennas and Propagation Society International Symposium, 2004. IEEE , vol. I , pp. 463- 466, 20-25 June 2004.

[2] Sankaralingam, S.; Gupta, B.; "A Circular Disk Microstrip WLAN Antenna for Wearable Applications," Annual IEEE India Conference (INDICON), 2009.

[3] A. Tronquo, H. Rogier, C. Hertleer, L. Van Langenhove, "Robust planar textile antenna for wireless body LANs operating in 2.45 GHz ISM band," Electronics Letters, Vol. 42, No. 3, February 2006.

[4] I. Locher, M. Klemm, T. Kirstein, G. Troster, "Design and characterization of purely textile patch antennas," IEEE Trans. on Adv. Packaging, Vol. 29, No. 4, November 2006.

[5] C. A. Balanis, Antenna Theory, Analysis and Design, New York, Wiley, 1997.

[6] A. Taflove and S. Hagness, Computational Electrodynamics: The Finite Difference Time Domain Method. Boston, MA: Artech House, 2nd edn, 2000.

[7] CST Microwave Studio; http://www.cst.coml [8] http://www.ieee802.org/ll/ [9] D'Souza, R., Gupta, R.K, "Printed Dual Band WLAN Antenna,"

IEEE International Conference on Electro/information Technology, 2006.

[10] Tae-Hyun Kim; Dong-Chul Park, "Compact dual-band antenna with double L-slits for WLAN operations," IEEE Antennas and Wireless Propagation Letters.

[II] Chi Yuk Chiu, Chi Hou Chan, Kwai Man Luk, "Small dual-band antenna with folded-patch technique," IEEE Antennas and Wireless Propagation Letters.

[12] Guo, y.x., Luk, K.M.; Lee, K.F.; Chow, Y.L., " Double U-slot rectangular patch antenna," Electronics Letters.