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Multiple fractal-shaped slots-basedUWB antenna with triple-band notchfunctionalityAbhik Goraia, Anirban Karmakarb, Manimala Palc & RowdraGhataka

a Microwave and Antenna Research Laboratory, Electronics andCommunication Engineering Department, National Institute ofTechnology Durgapur, MG Avenue, Durgapur 713 209, Indiab Department of Electronics and Communication Engineering,Netaji Subhas Engineering College, Kolkata, Indiac Department of ECE, NFET, NSHM Knowledge Campus Durgapur,Durgapur 713212, IndiaPublished online: 23 Oct 2013.

To cite this article: Abhik Gorai, Anirban Karmakar, Manimala Pal & Rowdra Ghatak , Journal ofElectromagnetic Waves and Applications (2013): Multiple fractal-shaped slots-based UWB antennawith triple-band notch functionality, Journal of Electromagnetic Waves and Applications, DOI:10.1080/09205071.2013.852486

To link to this article: http://dx.doi.org/10.1080/09205071.2013.852486

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Multiple fractal-shaped slots-based UWB antenna with triple-bandnotch functionality

Abhik Goraia, Anirban Karmakarb, Manimala Palc and Rowdra Ghataka*

aMicrowave and Antenna Research Laboratory, Electronics and Communication EngineeringDepartment, National Institute of Technology Durgapur, MG Avenue, Durgapur 713 209, India;

bDepartment of Electronics and Communication Engineering, Netaji Subhas EngineeringCollege, Kolkata, India; cDepartment of ECE, NFET, NSHM Knowledge Campus Durgapur,

Durgapur 713212, India

(Received 3 July 2013; accepted 3 October 2013)

A planar elliptical shape band-notched UWB antenna with multiple fractal-shapedslots and a Sierpinski fractal curve-shaped ring resonator at the back of the substrateis introduced in this paper. The proposed antenna exhibits a triple-band notchcharacteristic. Koch fractal slot etched from the radiator is responsible for creatingnotched band centred at 5.5 GHz for wireless local area network rejection. TheMinkowski fractal slots in the ground planes create rejection characteristics at8.1 GHz to avoid interference with X-band uplink satellite communication systems.The RFID rejection band centred at 6.8 GHz is achieved by using a Sierpinskifractal curve-shaped ring resonator at the back of the substrate. Sierpinski fractal slotof third iteration on the radiator contributes to an impedance bandwidth of2.8–12 GHz with VSWR < 2 by improving matching at lower frequencies except atthree notched bands. The antenna gain varies from 1.5 to 4 dBi over the band.Stable radiation patterns are obtained throughout its operating frequency. Theantenna has a compact size of 41 mm × 45 mm.

Keywords: fractal antenna; UWB antenna; fractal resonator and band notch antenna

1. Introduction

In 2002, the Federal Communications Commission [1] released the regulation for UWBtechnology allocating the frequency spectrum from 3.1 to 10.6 GHz for UWB.[2–4]Since then, UWB communication systems have received great attention in the wirelessworld due to their merits such as high data rate, small emission power and low cost forshort range access and remote sensing applications. However, within the allocatedspectrum for UWB, there are some other narrow band communication systems, such aswireless local area network (WLAN) in the 5.15–5.825 GHz band, RFID communica-tion system centred at 6.8 GHz and satellite communication system operating in7.9–8.4 GHz range. Hence to avoid potential interference, additional filter circuitry isrequired but resulting in increase in total dimension of the antenna, making it incompat-ible with modern wireless systems. To overcome this disadvantage, various UWBantennas with band notch characteristics achieved by cutting slots either in the radiatoror in the ground plane such as a U-shaped slot,[5,6] an arc-shaped slot,[7] v-shapedslot,[8] Hilbert shaped slot [9,10] and, using partial annular slot,[11] using modified

*Corresponding author. Email addresses: rowdraghatak@yahoo.com; rowdra.ghatak@ece.nitdgp.ac.in

© 2013 Taylor & Francis

Journal of Electromagnetic Waves and Applications, 2013http://dx.doi.org/10.1080/09205071.2013.852486

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fractal shape slot,[12] using semi-hexagonal slot [13] or by introducing parasiticstrips.[14]

In this paper, a novel CPW-fed elliptical-shaped band-notched UWB antenna isproposed where the radiator consists of a Sierpinski fractal curve-shaped [15] slot. Thenotch band at 5.5 GHz is achieved by etching Koch fractal slot [15] of second iterationon the radiator patch, the notch band at 8.1 GHz is achieved by two Minkowski fractal[15] slot of second iteration realized on either side of the feed line on ground planeand the notch band centred at 6.8 GHz is achieved by using a Sierpinski fractal curve-shaped ring resonator at the backside of the substrate. The proposed antenna exhibits abandwidth ranging from 2.8 to 12 GHz with three-notched bands centred at 5.5, 6.8and 8.1 GHz. The three-notched bands can avoid interference with WLAN, RFID andsatellite communication systems. The paper is arranged as follows. Section 2 describesthe details of the design and parametric study which is followed by result anddiscussion and conclusion in Sections 3 and 4, respectively.

2. Antenna design and parametric study

The geometry of the proposed triple band-notched UWB antenna is shown in Figure 1.The basic radiator patch consists of an elliptical patch. The proposed antenna is printedon Taconic TLY-5 substrate with the thickness of 0.8 mm and the dielectric constant of2.2. The height of the initial elliptical monopole is chosen to be 18 mm which is λg/4at the lowest frequency of 3.1 GHz of the operating band. By etching a Sierpinski frac-tal curve slot of third iteration, the lowest frequency of the operating band shifts to 2.8from 3 GHz and also resonance characteristics at lower frequencies is improved whichis clear from the comparative study of S11(dB) as depicted in Figure 2. This is due tothe increment of effective electrical path due to fractal shape. Koch fractal slot of sec-ond iteration is etched out from the radiator to create a WLAN rejection band centredat 5.5 GHz. While the notch band centred at 8.1 GHz is obtained by etching out twosymmetrical Minkowski fractal slots of second iteration from the ground planes.

Figure 1. (a) Geometry of the proposed antenna. (b) Geometry of Koch fractal slot. (c) Geometryof Minkowski fractal slot.

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Finally, the notch band centred at 6.8 GHz is achieved by using a second iteratedSierpinski fractal curve-shaped ring resonator at the backside of the substrate as shownin Figure 1(a). The various iterations of Koch fractal curve is shown in Figure 3(a),while Figure 3(b) explains the formation of Minkowski fractal curve for first threeiterations and finally the algorithm for generating Sierpinski curve is in Figure 3(c).[15]The antenna was analysed using commercial CST Microwave Studio™.[16]

The effective length of the Koch fractal slot etched out from the radiator is chosenusing (1) and (2) and then optimized using CST Microwave StudioTM. The width andlocation of the slot is also adjusted to exactly tune the rejection band. f1 in (2) standsfor centre frequency of WLAN system which is 5.5 GHz. The Koch fractal slot in radi-ator of the antenna consists of 16 identical segments each of length L1 and thicknessTS1. Parametric studies involving length L1 and thickness TS1 is depicted in Figure 4(a)and (b), respectively .The parametric study shows that as the length of the segment L1increases, the notched band is shifted towards lower frequency without affecting theother notched bands. On the contrary, when the slot width TS1 increases, the notchedband shifts towards higher frequency. The parametric study shows that the optimumsegment length L1 and width TS1 for notch frequency of 5.5 GHz is 1.45 and 0.25 mm,respectively.

Figure 2. Comparison of S11 (dB) with and without second iterated Sierpinski fractal slot.

Figure 3. (a) Koch fractal curves for various iteration. (b) Minkowski fractal curves for first,second and third iteration. (c) Sierpinski fractal curves for first three iterations.

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L K ¼ 16� L1 (1)

L K � c

2� f1 � ffiffiffiffiffiffiffi

eeffp (2)

Minkowski fractal slots in the ground plane follows the similar theory as Koch fractalslot as explained earlier. The effective length of the Minkowski fractal slot is extractedusing (3) and (4) followed by optimization. The centre frequency 8.1 GHz is depictedby f2 in (4). The width and position of the slots are adjusted and optimized to tune therejection band at 8.1 GHz. The Minkowski fractal slot consist of 27 identical segmentseach of length L2 and thickness Ts2. Parametric studies involving various length L2 andthickness Ts2 are depicted, respectively, in Figure 5(a) and (b). The parametric studiesshow that notched band shifts towards lower frequencies as length L2 is increased whilethe notched band shifts towards higher frequencies as thickness Ts2 is increased, keepingother notched band intact. The parametric study shows that the optimum segment lengthL2 and width Ts2 for notch frequency of 8.1 GHz is 0.66 and 0.25 mm, respectively.

L M ¼ 27� L2 (3)

L M � c

2� f2 � ffiffiffiffiffiffiffi

eeffp (4)

Figure 4. Effect on VSWR due to Koch fractal shaped slot for (a) Different values of L1. (b)Different values of Ts1.

Figure 5. Effect on VSWR due to Minkowski fractal shaped slot for (a) Different values of L2(b) Different values of Ts2.

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The Sierpinski fractal curve ring resonator at the back of the substrate is made of 20identical segments of length L3 and thickness Ts3. The total length of the fractal slittaken can be explained using (5) and (6), where f3 in (6) is the centre frequency of theRFID rejection band at 6.8 GHz. The parametric studies involving various values of L3and Ts3 as depicted in Figure 6 shows that notch frequency decreases as L3 increasesand notch frequency increases as Ts3 increases. Finally, the optimum value of L3 andTs3 is chosen to be 2 and 0.25 mm, respectively.

L R ¼ 20� L3 (5)

L R � c

f3 � ffiffiffiffiffiffiffi

eeffp (6)

The gap between the radiation patch and the ground plane is Gp. An optimized valueof Gp is obtained as 0.75 mm. The optimized CPW centre strip width of CPW feed is1.524 mm. The final values of all the antenna parameters are listed in Table 1.

3. Results and discussion

The antenna layout is fabricated as per the dimensions given in Table 1. Figure 7(a)shows the fabricated prototype. The measured VSWR and simulated VSWR results arein close agreement as depicted in Figure 7(b). The radiator patch consist of third

Figure 6. Effect on VSWR due to Sierpinski curve shaped fractal slot for (a) Different valuesof L3. (b) Different values of Ts3.

Table 1. Parameters of the proposed antenna.

Antenna parameter Value (mm) Sierpinski ring resonator parameters Value (mm)

Ls 41 h 0.75Ws 51 L3 2Wg 21.3 Ts3 0.25Wo 1.524 Slot parameters Value (mm)Lg 20.75 L1 1.45Gp 0.75 TS1 0.25a 1 L2 0.66Rx 13.5 Ts2 0.25Ry 9 – –

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iterated Sierpinski fractal slot which results in better matching at lower frequencies. Inthis work, notching is done using Koch fractal slot and minkowski fractal slot for 5.5and 8.1 GHz rejection band, respectively, the algorithm of which is simpler as com-pared to [13]. The uniqueness of the antenna lies in achieving band notch characteris-tics at 8.1 GHz by using closed-loop fractal resonator at the back of the substrate asshown in Figure 2(a). Resonator at the back of the substrate does not affect the currentdistribution at the radiator or ground planes unlike other fractal slots. Figure 7(b) showsthe simulated and measured VSWR of the antenna. The measurement is performedusing Rhode and Schwarz ZVA40VNA. It is seen that the simulated and measuredVSWR closely resembles. Slight disagreement is caused due to fabrication toleranceand connector losses. The antenna shows wide impedance bandwidth extending from2.8 to 12 GHz. The current distribution shown in Figure 8 explains the notching charac-teristics. Current distribution shows the formation of standing waves at notch frequen-cies. It is observed that current density is maximum around Koch fractal slot at5.5 GHz as shown in Figure 8(a), whereas at 8.1 GHz, current is concentrated aroundthe Minkowski fractal slots as depicted in Figure 8(b). Finally at 6.8 GHz, current isconcentrated more around the ring resonator at the back of the substrate as depicted inFigure 8(c).

For UWB applications, the antenna is required to have omnidirectional radiationpattern. The measured cross-polar and copolar radiation pattern at frequencies 3.1, 7.5and 10 GHz are depicted in Figure 9. It is observed that at lower frequencies, the

Figure 7. (a) Photograpgh of fabricated prototype. (b) Comparison of simulated and measuredVSWR of the proposed antenna.

Figure 8. Current distribution on the antenna at (a) 5.5 GHz (b) 6.8 GHz (c) 8.1 GHz.

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E-plane patterns are like conventional monopole, while at higher frequencies, someripples are observed in the pattern, which is due to higher-order modes. Hence, theantenna exhibits satisfactory omnidirectional radiation characteristics and copolar andcross-polar levels have nearly 20 dB isolation throughout its operating band. Themeasured gain of the antenna is shown in Figure 10. It shows that the gain is almostflat except at rejection bands where gain decreases abruptly. The gain varies from 1.5to 4 dBi at the passband. Moreover, the distortion less transmission nature of theantenna is confirmed by studying the measured group delay characteristics as shown in

Figure 9. Measured co-polar and cross-polar patterns of the proposed antenna at (a) 3.1 GHz(b) 7.5 GHz (c) 10 GHz.

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Figure 11. It is observed that group delay excursion is within 1 ns except at notchfrequencies. This ensures satisfactory time domain characteristics and distortion lesstransmission.

4. Conclusion

This paper presents a novel fractal triple band rejection monopole ultrawidebandantenna. By incorporating Sierpinski curved fractal in the radiator, impedance matchingat lower frequencies is improved. The impedance bandwidth ranges from 2.8 to12 GHz. By using Koch fractal slots in the main radiator, the antenna shows good sup-pression ability at the 5.5 GHz WLAN. Minkowski fractal slots in the ground planescontribute to rejection band at 8.1 GHz satellite communication band and Sierpinskifractal ring resonator at the back of the substrate accounts to rejection band at 6.8 GHzRFID band. The slots follow fractal geometry due to which the area occupied is mini-mized and the surface current on the antenna remains unperturbed. The antenna gainvaries from 1.5 to 4 dBi over the band with dips at the rejection frequencies. The groupdelay excursion remains within 1 ns over the UWB region except at the rejection bands,hence the antenna remains distortion less. Also, the radiation characteristics suit UWBcommunication criterion of omnidirectional pattern.

Figure 10. Measured gain of the proposed antenna.

Figure 11. Measured group delay of the proposed antenna.

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Acknowledgement

Rowdra Ghatak is grateful to Department of Science and Technology, Govt. of India for support-ing this research under Young Scientist scheme vide sanction No. SR/FTP/ETA-0033/2010.

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[4] Azenui NC, Yang HYD. A printed crescent patch antenna for ultrawideband applications.IEEE Antennas Wirel. Propag. Lett. 2007;6:113–116.

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[9] Karmakar A, Ghatak R, Banerjee U, Poddar DR. An UWB antenna using modified hilbert curveslot for dual band notch characteristics. J. Electromagn. Waves Appl. 2013;27:1620–1631.

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