Bandwidth Enhancement of Microstrip AntennasUsing Shifted Parasitically Coupled Planar

MultiresonatorsK.Chattopadhyayl, S.K. Parui', S.Das2

, S.R. Bhadra Chaudhuri'

'Department ofElectronics & Communication Engineering,MCKV Institute ofEngineering, Liluah, Howrah, India

E-mail: chatterjUcrishnendu@rediffinail.com2Department ofElectronics & Tele-Communication Engineering,

Bengal Engineering & Science University, Shibpur, Howrah, IndiaE-mail: arkapv@yahoo .com.santanumdas@yahoo.com.proCsrbc@yahoo.com

Abstract - In this paper, a wideband planar multi resonatorantenna with parasitic coupling is proposed and studied.Impedance and radiation characteristics of this antenna arepresented and discussed. From the results, it has been observedthat, the Impedance bandwidth, defined by 10dB return loss, canreach an operating bandwidth of 251 MHz with center operatingfrequency of 3023 MHz, which is about four times that ofconventional reference patch antenna. The gain of studiedantenna is also observed with peak gain of about 8.33dBi.

coupled with radiating edges of fed patch symmetrically andalso coupled through some amount of anti-clockwise rotationfor both the parasitic patches . The result shows gradualimprovement of impedance bandwidth with respect toreference patch. In this study, the simulation has been carriedout by MoM based IE3D simulator software. The simulationresults confirm wideband characteristics of antenna. The detailresults are presented and discussed.

Fig.! Muitiresonators gap coupled structurewith suitabledimensions.

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II. ANTENNA DESIGN AND OBSERVATIONS

Microstrip antenna with length of L=3cm and width of W=4cm over a substrate of dielectric constant 2.55 and thicknessof 0.159cm has been considered as reference antenna . Thereference antenna exhibits 65 MHz (2.17%) impedancebandwidth with a centre frequency of 2.989 GHz having areturn loss of -20.46 dB. A planar multiresonator gap coupledstructure has been realized over the substrate with the sameparameter as reference antenna, for feed location X=1.1em,towards the direction of bandwidth enhancement as shown inFig.I . The fed patch is having the same dimensions ofreference patch. The other two patches having length ofLI=2.9cm and width of W=4cm are parasitically coupled withthe fed patch with a gap of S= 0.1cm.

Index Terms-Planar Multi-resonator antenna, wide bandantenna, parasitically coupled antenna, Method of Moment.

I. INTRODUCTION

THe major disadvantage of microstrip antenna is it'sinherently narrow impedance bandwidth . Intensive

research is going on to improve the bandwidth of microstripantenna [1]. The bandwidth of microstrip antenna increaseswith the increase of substrate thickness or with the decrease indielectric constant of substrate [2]. However there is a practicallimit on increasing the thickness and if it is increased beyond0.11.., surface wave propagation takes place, resulting indegradation of antenna performance . The use of electronicallythick substrate has also limited success, because of increasedprobe inductance and probe compensation techniques have tobe employed to obtain impedance matching [3,4].

The following work describes the planar multiresonatortechnique using parasitic patches for broadband operation. Inthis case, only a single patch is fed, and the other patches areparasitically coupled. The coupling between multipleresonators has been realized using small gap between thepatches. When a patch is placed close to the fed patch, it getsexcited through the electromagnetic coupling between twopatches. If the resonance frequencies of fed and coupledpatches are close to each other, then broadband operation isobtained. The overall VSWR will be the super position ofresponses of the two resonators , resulting in wide band width[5,6]. The study has been carried out on two parasitic patches

978-1-4244-4819-7/09/$25.00 ©2009 IEEE

d8[S(1,1)]

Now another technique of bandwidth enhancement with thesame gap between fed and parasitic patches for S=O.lcm hasbeen realized with anticlockwise shifting of both the parasiticpatches as shown in Fig.6. The simulation result has beenoptimized towards largest bandwidth for shifting of 8.25mm inanti clockwise direction. The impedance bandwidth has beenimproved to 205.4 MHz (6.79%) with an adjustment of feedlocation of X=1.2cm for proper impedance matching. Thereturn loss characteristic is shown in Fig.7. The shifting ofparasitically coupled patches in anti-clockwise direction, in anoptimized amount, results in optimum reduction of couplingbetween fed and parasitic patches to result in widebandoperation.

The shift in position of parasitic patches also results in

YSWR=2 circle. The return loss characteristics also show themultiple resonances, with lower frequency of resonance for fedpatch and higher frequency of resonance for parasiticallycoupled patch, with smaller length.

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Fig.4 Return loss characteristics of Gap-coupled structure forS=0.15cm.

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Fig.2 Return loss characteristics of Gap-coupled structurefor S=O.lcm.

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To obtain wideband operation rather than dual frequencyoperation, the gap has been optimized for S=0.15 em and itsreturn loss characteristics is shown in Fig.4. It shows awideband operation with a bandwidth of 195.5 MHz (6.463%).The corresponding impedance plot as shown in Fig.5 alsoconfirms the wideband operation as the entire loop is within

The return loss characteristics as shown in Fig.2 reveals that,there is a dual-frequency operation with two resonancefrequencies of 2.946 GHz and 3.112 GHz with a return loss of-29.53dB and -12.06dB respectively. The impedance plot asshown in Fig.3 depicting a large loop, which enters theYSWR=2 circle at 2.883GHz, leaves the YSWR=2 circle at3.03GHz and again enters at 3.092GHz and leaves at 3.14 GHz,is responsible for dual frequency operation. The large loop onimpedance plot is due to the strong coupling between the fedand parasitic patches as shown in impedance plot ofFig.3 .

Radiation pattern of the structure as shown in Fig.6 has beenstudied for the frequency with lowest return loss point at 2.973GHz and at two band-edge frequencies of2.923 GHz and 3.124GHz. E-plane pattern for 2.973 GHz operation is as shown inFig.9. Fig.l0 shows the radiation pattern for 3.124 GHzoperation in resonant direction i.e. for lI>=O°. At 2.973 GHz apeak gain of 8.33 dBi at lI>=Oo (co-polar direction) has beenobserved. For the lower and upper band-edge frequencies of2.923 GHz and 3.124 GHz this peak gain has been found to beof 8.05 dBi and 7.13 dBi respectively.

0.0

The interesting point to note for upper band-edge frequencyradiation pattern is the appearance of side lobes because athigher frequency the parasitic patches are resonating and theyexperience a phase delay with respect to the fed patch. Thebeam will try to shift in the +0 and -e directions . The result isthe appearance of side lobes in E-plane pattern . The half powerbeam width of this pattern is obviously small. The peak gain ofthis multiresonators gap coupled structure is 8.33dBi, which isabout 1.7dBi greater than the reference patch antenna . Thestudied antenna exhibits lower level of cross polarization

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clockwise rotation of parasitic patches .

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Fig.7 Return loss characteristics of Gap-coupled structurewith anti-clockwise shifted parasitic elements .

impedance mismatch at feed point. The impedance plot movestowards left on the smith chart . To achieve proper impedancematching, feed location has been optimized for X= 1.2cm fromthe center of fed patch, to maintain the impedance plot withinVSWR=2 circle. The impedance plot in Fig.8 also confirmsthe wideband operation.

(E, vs e for <1>=90°) with a value of about -20.6dBiapproximately.

III. CONCLUSION

In this paper, detail simulation results have been presented for acoaxial fed, gap coupled planar multiresonators structure.Simulation results exhibit gradual improvement in impedancebandwidth from 65 MHz to 251 MHz (about four times) withvery minor variation of resonance frequency from 2.989 GHzfor reference patch to 3.023 GHz for anticlockwise shiftedparasitically coupled elements. The maximum gain of theantenna has also been improved from 6.6 dBi for referencepatch to 8.33 dBi for multiresonators gap coupled antenna atthe expense of increased size.

ACKNOWLEDGMENT

This work is funded by AICTE, New Delhi.

REFERENCES

[I] Kumar, G. and K.P. Ray, Broad band Microstrip Antennas, Norwood,MA:Artech House,2003.

[2] W. Chen and K.F. Lee, "Input Impedance of Coaxially Fed RectangularMicrostrip Antenna on Electrically Thick Substrate", Microwave Opt.Technology Lett., No6, pp. 387-390,1993.

[3] R.Q. Lee, K.F. Lee and 1.Bobinchak, "Characteristics ofa two-LayerElectromagnetically Coupled Rectangular Patch Antenna", Electron. Lett,No23, pp.1070-1072, 1987.

[4] P.S. Hall, "Probe Compensation in Thick Microstrip Patches", Electron.Lett., No23, pp.606-607, 1987.

[5] Kumar G. and K.C. Gupta, "Broadband Microstrip Antennas UsingAdditional Resonators Gap Coupled to the Radiating Edges", IEEETrans. Antennas Propagation, Vol.AP-32, pp.1375-1379, Dec1984.

[6] Anandan C.K., P.K. Mohanan and K.C. Nair, "Broadband Gap Coupled

Microstrip Antenna", IEEE Trans. Antennas Propagation, Vol.AP-38,pp.1581-1586,1990.