A Printed 2.4 GHz/5.8 GHz Dual-band MonopoleAntenna for WLAN and RFID Applications with a

Protruding Stub in the Ground PlaneJyoti R. Panda and Rakhesh S. Kshetrimayum

Department of Electronics and Communication EngineeringIndian Institute of Technology

Guwahati-781039,IndiaEmail: j.panda, krs@iitg.ernet.in

Abstract—Design of a simple microstrip fed folded stripmonopole antenna with a protruding stub in the ground planefor the application in the WLAN and RFID is presented. Theantenna has two resonant paths, one in the radiating element(folded strip) and other in the protruding stub of the groundplane, supports two resonances at 2.4 GHz and 5.81 GHz, whichare the center frequencies of the WLAN and RFID. Effectivelyconsistent radiation pattern and large percentage bandwidth hasbeen observed. The percentage bandwidth at 2.4 GHz (2.06 GHzto 2.82 GHz) is 31.14 and the percentage bandwidth at 5.81 GHz(5.57 GHz to 6.08 GHz) is 8.75.The proposed antenna is simpleand compact in size providing broadband impedance matching,consistent radiation pattern and appropriate gain characteristicsin the RFID frequency range.

Index Terms—Folded strip, monopole antenna, microstrip lineprotruding stub, RFID and WLAN.

I. INTRODUCTION

Compact printed monopole antennas are indispensable forthe application in WLAN, UWB and RFID applications.Along with the compact size, the antenna should be lowcost, light weight, less fragile, low profile and finally, thefabrication methodology should be simple. Many compactprinted monopole antennas were fabricated for the wirelessapplications. Notable structures among them are CPW-feddual frequency monopole antenna [1], dual band CPW-fedstrip-sleeve monopole antenna [2], CPW fed L-shaped slotplanar monopole antenna for triple band operation [3], internalplanar monopole antenna for mobile phones [4], dual-bandplanar branched monopole antenna [5], etc. Similarly compactprinted antennas for RFID application at 5800 MHz arethere in the literature such as CPW-fed folded slot [6], T-shaped folded slot monopole antenna [7], F-shaped CPW-fedmonopole antenna [8], CPW-fed dual folded strip [9], semicircular CPW-fed folded slot antenna [10], CPW-fed U-shapedfolded slot monopole antenna [11], etc. The above-mentionedantennas might be compact in nature. But there is no suchprinted monopole antenna structure in the literature till now,which can be used simultaneously in the WLAN as well asRFID environment.

In this paper, a new printed microstrip fed folded stripmonopole antenna with a protruding stub in the ground planefor the application in the WLAN and RFID is presented. Theantenna has two resonant paths, one in the radiating element(folded strip) and other in the protruding stub of the groundplane, supports two resonances at 2.4 GHz and 5.81 GHz,which are the center frequencies of the WLAN and RFID.The antenna is constructed by a non-conductor backed foldedstrip with a microstrip feedline. The dual-band performancecan be easily obtained for this type of antenna by fine-tuningthe lengths of the two resonant paths in the folded strip andthe protruding stub in the ground plane.

II. ANTENNA DESIGN

The dual-band monopole antenna with a microstrip fedfolded strip and a protruding stub in the ground plane isprinted on the FR4 substrate of relative permittivity 4.4 and thethickness 1.6 mm. as shown in the Fig.1. A 50-Ohm microstripline is used for the excitation. The folded strip width and theprotruding stub width of the proposed dual-band monopoleantenna is 3 mm, same as that of the width of the microstripline. The remaining design parameters are given in the Fig.1.

The proposed antenna has two resonant paths, one in thefolded strip (𝐿𝛼) of the radiating element and other (𝐿𝛽) in theprotruding stub of the ground plane. The length of the resonantpath in the folded strip is 𝐿𝛼 =29.8 mm, which is 0.23𝜆1

at the first resonant frequency of 2.4 GHz (𝑓1=2.4 GHz).Similarly length of the second resonant path in the protrudingstub of the ground plane is 𝐿𝛽=12 mm, which is 0.23𝜆2 atsecond the resonance frequency of 5.8 GHz (𝑓2=5.8 GHz).By properly varying the lengths 𝐿𝛼 and 𝐿𝛽 we can fix theantenna resonance at 2.4 GHz and 5.8 GHz respectively. Theoverall adjustments of the geometrical parameters are done forthe improvement of impedance bandwidth in the 2.4 GHz and5.8 GHz bands.

III. RESULTS AND DISCUSSION

Fig.2 shows the photograph of the fabricated prototype ofthe proposed folded-strip monopole antenna with a protrudingstub in the ground plane for 2.4 GHz and 5.81 GHz for

Fig. 1. Geometry of the proposed antenna with M= 8 mm, N=12 mm andP=4.8 mm.

WLAN and RFID applications. Fig.3 shows the comparisonof the simulated reflection coefficient (∣𝑆11∣) (dB) and themeasured reflection coefficient (∣𝑆11∣) (dB) of the folded-strip monopole antenna with a protruding stub in the groundplane. The reflection coefficient measurement was done byusing the Rohde and Schwarz ZVA24 vector network analyzer.From the graph it is quite clear that there is reasonablygood agreement between the measured and the simulatedreflection coefficients (∣𝑆11∣) (dB). The experimental lowerand upper cutoff frequencies are quite clearly matching withthe simulated lower and upper cutoff frequencies and confirmgood fabrication quality of the fabricated prototype. With themeasurement, the first resonance occurs at 2.4 GHz havingthe reflection coefficient value of -40.48 dB with percentagebandwidth of 32.99 and the second resonance occurs at 5.81GHz having the reflection coefficient value of -20.21 dB withpercentage bandwidth of 10.11. Hence, from the experimentalresults, it is clear that the fabricated prototype can be usedfor the dual band WLAN and RFID applications around 2.45GHz and 5.8 GHz.

Fig.4 shows comparison of the simulated VSWR and themeasured VSWR of the proposed folded-strip monopole an-tenna with a protruding stub in the ground plane for WLANand RFID applications. From the graph it is quite clearthat there is good agreement between the measured and thesimulated VSWR.

Fig. 5 and Fig. 6 show the simulated antenna impedance vs.frequency and the gain (dBi) vs. frequency for the folded-stripmonopole antenna with a protruding stub in the ground plane.

In Fig. 5, the simulated variation of the antenna inputimpedance versus frequency of the folded-strip monopoleantenna with a protruding stub in the ground plane can beseen. At the resonance frequency of 2.4 GHz and5.8 GHz , theaverage value of the resistance (real part) is approximately 50-

Fig. 2. Fabricated folded strip monopole antenna with a protruding stubprototype (a) Top view, (b) Bottom view.

Fig. 3. Comparison of the simulated and the measured reflection coefficients(∣𝑆11∣) (dB) of the proposed dual-band monopole antenna for WLAN andRFID applications.

Fig. 4. Comparison of the simulated VSWR and the measured VSWR of theproposed dual-band monopole antenna for WLAN and RFID applications.

Fig. 5. Simulated antenna impedance (Ohm) vs. frequency of the proposedantenna.

Fig. 6. Simulated gain (dBi) vs. frequency of the proposed antenna.

Ohms and the average value of the reactance (imaginary part)is 0-Ohms signifies the adequate impedance matching occursat the two desired resonant frequencies. The Fig.6 depictsthe variation of peak gain in dBi of the proposed monopoleantenna with the frequency. The peak gain increases smoothlywith the frequency. The gain at 2.4 GHz is 2.27 dBi and thegain at 5.8 GHz is 2.99 dBi. The proposed antenna providessufficient and appropriate gain required for the operation inthe WLAN and RFID (2.4 GHz and 5.8 GHz) band.

Fig.7 shows the variation of the distance M between thefolded strip of the radiating element and the protruding stubin the ground plane when the other parameters such as N (=12mm) and P (=4.8 mm) remain constant. From the graph it isclearly visible that when M increases from 4 mm to 12 mm,the first resonant frequency (𝑓1) moves towards left, whichmeans that the first resonant frequency (𝑓1) decreases with theincrease of the distance M. But on the other hand, the second

Fig. 7. Simulated reflection coefficient (∣𝑆11∣) (dB) graphs varying M whenN=12 mm and P=4.8 mm.

resonant frequency (𝑓2) is remaining static at 5.8 GHz, but theperformance degrades at M=10 mm and 12 mm.

Fig.8 shows the variation of the length N of the protrudingstub in the ground plane when the other parameters such as M(=8 mm) and P (=4.8 mm) remain constant. From the graph itis clearly visible that when N increases from 8 mm to 16 mm,the first resonant frequency (𝑓1) moves towards left, whichmeans that the first resonant frequency (𝑓1) decreases with theincrease of the distance M. But on the other hand, the secondresonant frequency (𝑓2) is remaining static at 5.8 GHz.

Fig.9 shows the variation of the length P of the radiatingelement (means also the total length of the folded strip (𝐿1))when the other parameters such as M (=8 mm) and N (=12mm) remain constant. From the graph it is clearly visiblethat when P increases from 0.8 mm to 8.8 mm, the firstresonant frequency (𝑓1) moves towards left, which means thatthe first resonance frequency (𝑓1) decreases with the increaseof the distance M. But on the other hand, the second resonantfrequency (𝑓2) is also decreases with the increase of P, whichmeans the second resonant frequency (𝑓2) movers towards leftbut the performance degrades at P=8.8 mm.

The E-plane (xz-plane) and H-plane (yz-plane) radiationpatterns from the IE3D simulation [12] of the folded stripmonopole antenna with a protruding stub at 2.4 and 5.8 GHzare shown in the Fig. 10 and Fig. 11 respectively. The H-plane radiation pattern is purely omni-directional at all thesimulated frequencies. In the E-plane, the radiation patternis like a small dipole leading to a bi-directional radiationpattern. The E-plane radiation pattern is directional along 900

and 2700 respectively. In the E-plane, the radiation patternsremain roughly a dumbbell shape like a small dipole leadingto bidirectional patterns. Hence, this proposed folded stripmonopole antenna with a protruding stub demonstrates aconsistent radiation pattern in the desired band of frequencies.

Fig. 12 and Fig. 13 show the simulated surface currentdistribution of the proposed antenna at 2.4 GHz and 5.8 GHz

Fig. 8. Simulated reflection coefficient (∣𝑆11∣) (dB) graphs varying N whenM=8 mm and P=4.8 mm.

Fig. 9. Simulated reflection coefficient (∣𝑆11∣) (dB) graphs varying P whenM=8 mm and N=12 mm.

Fig. 10. Simulated E-plane (xz-plane)(Co-pol) radiation patterns at (a) 2.4GHz and (b) 5.8 GHz.

respectively. At 2.4 GHz, the current distribution on the foldedstrip (𝐿𝛼) is opposing in nature at the top of the strip and hencethe current cancellation occurs. That’s why the concentration

Fig. 11. Simulated H-plane (yz-plane)(Co-pol) radiation patterns at (a) 2.4GHz and (b) 5.8 GHz.

Fig. 12. Simulated surface current distribution of the proposed antenna at2.4 GHz.

Fig. 13. Simulated surface current distribution of the proposed antenna at5.8 GHz.

of current at the bottom of the folded strip (𝐿𝛼) is morecompared at the top of the folded strip. At the same time,

the current is moving upward on the protruding stub on theground plane (𝐿𝛽), which is shown in the Fig. 12. Similarly at5.8 GHz, again the current distribution on the folded strip (𝐿𝛼)is opposing in nature at the bottom of the strip and hence thecurrent cancellation occurs. That’s why the concentration ofcurrent at the top of the folded strip (𝐿𝛼) is more compared atthe bottom of the folded strip. At the same time, the currentis moving downward on the protruding stub on the groundplane (𝐿𝛽), which is shown in the Fig. 13. As known, theelectromagnetic coupling effect controlled the two excitedresonant frequencies at 2.4 GHz and 5.8 GHz between thefolded strips (𝐿𝛼) and the protruding stub (𝐿𝛽) in the groundplane.

IV. CONCLUSION

A simple microstrip fed folded strip monopole antenna witha protruding stub in the ground plane for the application in theWLAN and RFID is designed. Satisfactory dual-band opera-tion for WLAN and RFID application can be easily achievedby the proposed antenna, which provides two resonant pathsof different lengths for the excitation of the two resonant fre-quencies. The proposed antenna is simple and compact in sizeproviding broadband impedance matching, consistent radiationpattern and appropriate gain characteristics in the WLAN andRFID frequency range. Hence the proposed antenna may be asuitable candidate for the dual-band operation in WLAN andRFID applications.

ACKNOWLEDGMENT

The authors would like to thank Mr. Utpal Kumar Sarma,technical superintendent (High Frequency Laboratory) andMr. Dayananda Goswami, research fellow (DSP and Com-munication Laboratory) of Department of Electronics andCommunication Engineering, Indian Institute of Technology,Guwahati for their assistance in fabrication and measurementof the proposed antenna.

REFERENCES

[1] H. -D. Chen and H. -T. Chen, “A CPW-fed dual-frequency monopoleantenna,” IEEE Trans. Antennas Propagat.,vol. 52, no.4, pp. 978-982,Apr. 2004.

[2] C. -H. Cheng, W. -J. Lv, Y. Chen and H. -B. Zhu, “A dual-bandstrip-sleeve monopole antenna fed by CPW,” Microwave Opt. Technol.Lett.,vol. 42, no.1, pp. 70-73, Jul. 2004.

[3] W. -C. Liu, and C. -C. Huang, “A CPW-fed L-shaped slot planarmonopole antenna for triple-band operations,” Microwave Opt. Technol.Lett.,vol. 44, no.6, pp. 510-512, Mar, 2005.

[4] C. -A. Shen, and K. -H. Lin, “A broadband internal planar monopoleantenna for mobile phone,” Microwave Opt. Technol. Lett.,vol. 48,no.4,pp.768-769, Apr. 2006.

[5] M. N. Suma, R. K. Raj, M. Joseph, P. C. Bybi and P. Mohanan , “Acompact dual band planar branched monopole antenna for DCS/2.4-GHzWLAN applications,” IEEE Microw. Wireless Comp. Lett.,vol. 16,no.5,pp.275-277, May 2006.

[6] S. -Y. Chen and P. Hsu, “CPW-fed folded-slot antenna for 5.8 GHz RFIDtags,” Electron.Lett.,vol. 40, no.24, pp. 1516-1517, Jul. 2004.

[7] W. -C. Liu and P. -K. Hu, “Broadband CPW-fed folded-slot monopoleantenna for RFID application,” Electron. Lett.,vol. 41, no.17, pp.937-939,Aug. 2005.

[8] W. -C. Liu and C. -M. Wu, “CPW-fed shorted F-shaped monopole antennafor 5.8-GHz RFID applications,” Microwave Opt. Technol. Lett.,vol. 48,no.3, pp.573-575, Mar. 2006.

[9] W. -C. Liu and P. -C. Kao, “Compact CPW-fed dual folded-stripmonopole antenna for 5.8-GHz RFID application,” Microwave Opt.Technol. Lett., vol. 48 ,no.8, pp.1614-1615, Aug. 2006.

[10] D. Ma and W. -X. Zhang, “Broadband CPW-fed RFID antenna at 5.8GHz ,” Electron. Lett., vol. 42 ,no.22, pp.1258-1259, Oct. 2006.

[11] W. -C. Liu, “A coplanar waveguide-fed folded-slot monopole antennafor 5.8 GHz radio frequency identification application ,” Microwave Opt.Technol. Lett.,vol. 49, no.1, pp.71-74, Jan. 2007.

[12] IE3D version 10.2, Zeland Corp,Freemont, CA, USA.