BOW-TIE MICROSTRIP ANTENNA

DESIGN

M.K.A. Rahim, M.Z.A. Abdul Aziz, C.S.Goh

Wireless Communication Centre, Faculty of Electrical Engineering, Universiti TeknologiMalaysia

813 10 UTM Skudai, Johor Malaysiaemail:mkamal@fke.utm-my, matjoingyahoo.com.csgoh@yahoo.com

ABSTRACT

In this paper, the bow-tie microstrip antennas have been designedwith two different angles of 400 and 80°. An investigaton on theeffect of the angle to the return loss and radiation patterns hadbeen carried out. The impedance matching network with theniicrostrip transmission line feeding was used in this study.Simulation and measurement results for the return loss andradiation patterns were presented

II. DESIGN CONSIDERATIONANTENNA

OF BOW TIE

Figure 1 shows the dimension of a bow-tie micirostripantenna, where a is the side length of the bow-tie strip, 0 isthe angle of equilateral triangular, L1, L2. W1, and W2 are thedimensions of the matching network. Resonant frequencycorresponding to the various modes described by [7]:

1. INTRODUCTION

In recent years, microstrip antennas have been widely used inboth theoretical research and engineering applications due totheir light weight and thin profile configurations, low cost offabrication, reliability, conformal structure and ease offabrication.The bow-tie microstrip antennas have been designed forwireless LAN application, where the operating frequency is at2.4 GHz. The bow-tie patch actually is the combination ofimaginary image of two triangular patches which arefabricated in a single substrate. Figure 1 shows the bow-tiestrip of a bow-tie microstrip antenna. Bow-tie microstripantennas have become attractive candidates in the present daycommunication scenario due to their compact naturecompared to rectangular patches. The ever increasing demandfor compact wireless communication equipment explicitlynecessitates research in compact antenna options and whichsparked interests of many researchers world wide in the fieldof bow-tie microstrip antennas.However in the literature, only very few attempts have beenmade towards the analysis of this type antenna. The bow-tiepatch microstrip antenna as a compact one, and suggested anempirical formula for the resonant frequency of this newgeodesy. The past research work on bow-tie antenna can beseen in [1-6]

fr _ Ckmn247

2c in2 ±mn+n2fr J3a -£r

(1)

(2)

where

fr is the resonance frequency

kmn is the resonating modes

m and n are nombor ofmodes

c is the velocity of light in free space.

a is the side length of the bow tie strip

The above expression is valid when the triangular resonator issurrounded by a perfect magnetic wall. The effect of a nonperfect magnetic wall on the resonant frequency can beincluded in an empirical fashion for easy calculation.

1-4244-0000-7/05/$20.00 02005 IEEE. 17

W4W2

Figure 1 Dimension ofBow Tie Microstrip Antenna

Resonant frequency for mn mode:

fmn in2 +mn+n2

Wavelength in free space:c

A =-0

f

Wavelength ofthe antenna:

Ag= rCeff

A number of suggestions have been made with regard to howto modify [81 to yield an accurate expression for a triangularmicrostrip patch antenna that is not enclosed by a perfectmagnetic wall. Most of the suggestions are about replacingthe side length a by an effective value aeff and leaving thesubstrate dielectric constant unchanged. The other set ofsuggestions proposes replacing both a and cr with theireffective values. An expression for aeffhas been arrived at bycurve fitting the experinmental and theoretical results for theresonant frequency for TMIO mode. It is given by

I = dig

Where d is the valueSmith chart

4

(10)

of "wavelength towards load" in the

(1 1)

Resonant frequency dominant mode is:

2c0-_2fj-

W geAd e2A _2(3)

W_-2B-1-ll1(2B-)W 1>4B-p)+0 39 0 11

dL 2Er cr

for -> 2d

Effective value of side length:

haeff=a+

Er(5)

Effective dielectric constant:

Er + 6r)-1eff 2 + (6)

4 1+

a

Accuracy of this empirical expression is claimed to be withinI % when compared with the value obtained from themoment method analysis. Knowing flo from above, theresonant frequency for higher order modes is calculated from

(1), that is

Z E I+ cr -1I 0.11Where A = ° r + £ (0.23+ )

60 2 Er + 1 Er

377,r2Z0g

I1I. SIMULATION AND EXPERIMENTAL RESULT

(A) Return Loss Result

The input return loss is shown in figure 2 and 3 with differentvalue of angle (0). Figure 2 shows the simulation result forangle 0 = 40° with a return loss of-23.6dB at the operatingfrequency of2.4 GHz, while the measurement retum loss is -

18

(7)

(8)

(9)

Side length:

2c2frTh

wfor -< 2

d(12)

(4) (13)

27.71dB at 2.48GHz.The BW from the measurement andsimulation result is nearly 3%.

Figure 3 shows -19.91 dB return loss from simulation atoperating frequency of 2.42GHz, while the measurementreturn loss is -28.92dB at 2.48GHz. The angle is at 0 = 80° .In considering that all the simulation and measurement returnloss for both antenna having the return of lower than -20dB,therefore the incoming signal is strong enough to be receivedat the receiver.The shifting of the frequency is due to the substrate of FR4which has dielectric constant between 4.0 and 4.7. In thisdesign the dielectric constant is 4.7. The shifting of thefrequency is also from the fabrication process of the hardware

Figure 2 Simulation and measurement return loss ofthe antenna with 0 = 40°

(b) H Plane

Figure 3 Simulation and measurement return loss ofthe antenna with 0= 80°

(B) Radiation pattern characteristic

Figure 4 and 5 show the radiation pattem for two differentangle for E and H Plane. The cross-polar isolation radiationpattern for 0 = 400 is nearly 20 dB for E Plane while for theH Plane has a very low cross polarization isolation. Thebeamwidth is 70 0 to 800. The result for 0 = 80° is slightlydifferent in term of the radiation pattem. It has a narrowbandwidth compared with aan angle 0 = 400. The cross polarisolation for both plane has a higher value than when 0 = 400 .The plotting in red colour is a co polarization while in bluecolour is a cross polarization.

(a) E Plane

Figure 4400

Radiation patterns of the antenna with 0 =

(a) E Plane

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V. CONCLUSION

The BW from the measurement and simulation result is nearly3% with a return loss more than 20 dB. The angle ofthe bow-tie microstrip antennas does not affect its return loss. As longas the design of the matching network is correct, the desiredreturn loss can be obtained. The radiation pattern for highervalue of angle (0) give a radiation pattem more directionalwith a HPBW of400 to 500 for E plane and 200 to 300 for Hplane.

REFERENCES

(b) H Plane

Figure 5 Radiation patterns of the antenna with 0= 800

IV. DISCUSSION

The bow-tie patch antenna is a combination of imaginaryimage of two triangular patches which are fabricated in asingle substrate. Therefore, the design of bow-tie microstripantenna is based on the design of triangular microstripantenna. Those formula used in the design of triangularmicrostrip antenna to calculate the side length, effective valueof side length and effective value of dielectric constant, arealso applicable in the design of bow-tie microstrip antenna.For the design of bow-tie antenna, the side length iscalculated and fix at a certain operating frequency. While thelength is depends on the angle of the equilateral triangular,where the angle, 0 is not less than 00 and not bigger than1800. Therefore, the bow-tie microstrip antenna can be designat any degree of 0.There is an impedance matching network with the microstriptransmission line feeding which are fabricated simultaneouslywith the antenna structure. The impedance matching networkis the most important part in the design of a microstripantenna in order to minimize the return loss. The acceptablereturn loss for a microstrip antenna is - OdB or below. It isbetter if the microstrip antenna having the return loss of lowerthan -20dB.By comparing the simulation results and the measurementresults, we can notice that the return loss of the antennas withsame angle is different. It is may be caused by these factors,they are: inaccurate in preparing the layout, impurities of theequipment used for fabrication and the quality ofthe substrateand connector.The bow-tie microstrip antennas which have been fabricatedare with the angle of 40° and 80°.The simulation results ofthese designed antennas having the return loss of -23.6dB and-19.9dB respectively. While for the fabricated antennas withthe angle of 400 and 80°, the return losses are -27.7IdB and -28.91dB respectively.

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