Hexagonal Boundary Fractal Antenna with WLAN Band Rejection Sreerag M Department of Electronics and Communication NSS College of Engineering, Palakkad, Kerala-678008, India. E-mail: sreeragm09@gmail.com Sudha T Department of Electronics and Communication NSS College of Engineering, Palakkad, Kerala-678008, India. E-mail: sudhat@nssce.ac.in Abstract In this work, a compact hexagonal boundary fractal antenna for Ultra Wide Band (UWB) applications with WLAN band-notch characteristics is proposed. Here a Coplanar Wave Guide (CPW) fed antenna with combinational slots is used. The proposed antenna covers a bandwidth of 3.5 to 12.1 GHz with band rejection in the range 5.1 to 6.0 GHz. Interference between UWB and WLAN band is reduced by band rejection characteristics. The antenna is simulated, fabricated and tested. Rectangular slots etched on the radiating element results in band rejection. Keywords: Fractal, Radiation Pattern, Band Rejection, WLAN. Introduction Ultra Wide Band (UWB) systems operates in a frequency range of 3.1 to 10.6 GHz with a power emission level of -41.3 dbm/mhz as per FCC standardisation. UWB has been considered as a promising wireless technology because of its merits like high data rate, robustness to fading, immunity against electromagnetic interference etc[1]. UWB band is divided into lower (3.1 to 5.15 GHz) band and upper (5.875-10.6 GHz) bands. UWB system have potentially low complexity and low cost, due to baseband nature of signal transmission. Applications of UWB antennas include navigation, biomedical systems, mobile satellite communications, radars, EM measurement and wireless sensor networks [2], [3]. UWB antenna that can suppress communication bands existing within UWB spectra is considered as an important challenge in antenna design. Several methods are employed to attain band rejection characteristics. Insertion of shaped slots, using parasitic element, split ring resonators, implementing fractal geometry are the few methods used to notch any unwanted frequencies [4]. A notch centered at 6.55 GHz is achieved by including split ring resonators and metallic shunt strips on the CPW feed line [5]. Design of dual compact split ring resonator on circular monopole resulting in notches centered at 5.33 and 7.9 GHz is discussed in [6]. By inserting a slot in the patch dual-band operation with a lower band from 1.68-2.06 GHz for GSM and upper band from 3.27 11 GHz for Ultra Wide Band is obtained [7]. Dual band notches of 3.4 to 3.6 GHz and 5.1 to 5.9 GHz is obtained by etching two identical square complementary split ring resonators in the radiation patch [8]. Embedding spur lines on the ground plane results in band-reject characteristic in 5.15 to 5.825 GHz frequency band of WLAN service [9]. A compact octagonal shaped UWB antenna employing koch fractal geometry results in WLAN band rejection. Here a C shaped slot etched on fractal monopole is responsible for band rejection [10]. Folded stepped impedance resonators on planar dipole antenna gives a dual band operation at 2.4 and 5.8 GHz [11]. A fractal antenna can operate at multiple frequencies simultaneously. Fractal antennas are based on the concept of a fractal, which is a recursively generated geometry that has fractional dimensions. In this work hexagonal boundary fractal antenna for WLAN band rejection is proposed. Band rejection in the WLAN range of 5.1 to 6.0 GHz is achieved by etching slots on the radiating element. The proposed system covers entire UWB band width and desired radiation patterns are obtained. This paper is organized as follows. Section 2 describes antenna design and in section 3 measurement results are discussed. Finally section 4 draws the conclusion. Antenna is designed and investigated using high frequency structure software (HFSS). Antenna Design The geometrical configuration of proposed antenna is shown in Figure 1. An FR4 substrate with relative permittivity of 4.4, a loss tangent of 0.02 and a thickness of h=1.59 mm is used for the proposed antenna. The substrate is having a cross section of 40x38 mm 2. The proposed antenna uses a CPW fed hexagonal boundary sierpinski carpet fractal structure. The 50 Ω CPW fed structure consists of the CPW transmission signal strip line with a signal strip width 2.6 mm. A partial ground plane is used for this structure. The length of the ground plane Lg and the width of the ground plane Wg are important design parameters, where Lg= 18.14 mm and Wg=17 mm. Figure 1: Antenna structure 326
Initiator for this hexagonal boundary carpet fractal is a hexagonal shaped antenna of 18 mm size. In first iteration, a central hexagon of 6mm size, which is one third size of the initiator is etched out. Next iteration is obtained by etching out additional four hexagons of one third size of center hexagon. Fabricated antenna structure is shown in Figure.2. Results and Discussion Antenna is investigated using high frequency structure software (HFSS) version 15 based on finite element method (FEM). Antenna is fabricated and measured by ZVB 20 vector network analyzer. The measured and simulated results are discussed below. A. Return Loss Simulated return loss curve of proposed antenna is given in Figure. 4. Simulated result shows that, the proposed antenna provides a sharp band notch of 5.1 to 5.9 GHz with impedance bandwidth ranging from 3 GHz to 11 GHz. Return loss curve indicates good impedance matching. From these curves, it is clear that both simulation and measurement results are comparable. The measured result shows slight deviation from the simulated results. This can be accounted for the connector and associated losses which are not considered in simulation. Figure 2: Fabricated structure A combinational slot of circles and rectangles are etched out from fractal structure to achieve band rejection in the region assigned for WLAN. Slot for band rejection is shown in Figure. 3. Length and width of rectangular slot is Ls and Ws respectively and radius of circular slot is represented as r. Ls is a combination of L1, L2, L3 and L4. Initially total length of slot Ls is taken as 29.4 mm and band rejection in the range 4.5 GHz to 4.9 GHz is achieved. As length of slot Ls is increased from 29.4 mm to 31.8 mm, band notched region shift towards higher frequency side. The final optimised design parameters are Ls =31.4 mm, Ws= 0.2 mm and r = 0.4 mm. Figure 4: Return Loss of proposed antenna Figure 3: Slot for band rejection Measured return loss curve of proposed antenna is given in Figure 5. Measurement result shows that, the proposed antenna provides a band notch in the range 5.1 to 6.0 GHz. Impedance bandwidth range from 3.5 to 12.1 GHz and the bandwidth covers entire UWB band. Return loss up to -24.25 db is obtained at 4.2 GHz which shows better impedance matching. Better return loss values are obtained at other resonant frequencies also. Measurement results are compared with simulation results. 327
Figure 7: Radiation Pattern at 6.4 GHz (Measurement) Figure 5: Return Loss of proposed antenna (Measurement) B. Radiation Pattern Radiation patterns at E plane and H plane are analysed at 4 different frequencies. For the proposed fractal antenna, Radiation pattern for E plane gives 8 shaped patterns at 4.2 GHz. Figure 8: Radiation Pattern at 7.1 GHz (Measurement) Figure 6: Radiation Pattern at 4.2 GHz (Measurement) Nearly omnidirectional radiation patterns are obtained for H plane at frequencies 4.2 GHz, 6.4 GHz, 7.1 GHz and 11.2 GHz. Figure 6 and Figure 7 shows the radiation pattern at 4.2 GHz and 6.4 GHz. Patterns are closer to dipole shape at other frequencies also. There is slight distortions in the E plane characteristics due to band rejection function. Radiation pattern at 7.1 GHz is given in Figure 8 and pattern at 11.2 GHz is given in Figure 9. 328
Conclusion A compact CPW fed Hexagonal boundary fractal antenna with band notch characteristics has been proposed in this paper. The designed antenna covers entire UWB bandwidth of 3.5 to 12.1 GHz. Proposed antennas is fabricated and measurement results reveal that simulation results are comparable with measurement results. Interference to WLAN band is reduced by etching out a combinational slot on the radiator. Radiation patterns obtained over the required bandwidth are nearly omnidirectional. Acknowledgment The authors wish to thank AICTE for the grant sanctioned under RPS for purchasing HFSS (No.20/AICTE/RIFD/RPS (POLICY-1) 59/2013-14, Entuple Technologies, Bangalore, India for fabricating the antenna and Centre for Research in Electromagnetics and Antennas CUSAT, Kerala, India for testing. References Figure 9: Radiation Pattern at 11.2 GHz (Measurement) C. Gain Gain of proposed antenna is relatively flat over entire UWB bandwidth. As expected there is significant reduction of gain in the WLAN band. The proposed antenna can perform well in the UWB band with a rejection in the 5.1 to 6.0 GHz. Gain plot of proposed antenna is given in Figure 10. Figure 10: Gain [1] Hojjatollah Fallahi and Zahra Atlasbaf, Study of a Class of UWB CPW-Fed Monopole Antenna With Fractal Elements, IEEE Antennas And Wireless Propagation Letters, VOL. 12,pp.1484-1487,2013. [2] Ultra Wide Band Antennas Design and Application, Daniel Valderas, Juan Ignacio Sancho, David Puente, Cong Ling, Imperial College press, London,pg no:167-168. [3] Tapas Mondal, Susamay Samanta, Rowdra Ghatak, and Sekhar R. Bhadra Chaudhuri, A Novel Tri-Band Hexagonal Microstrip Patch Antenna Using Modified Sierpinski Fractal for Vehicular Communication, Progress In Electromagnetics Research C, Vol. 57, 25 34, 2015. [4] Mohammad Jahanbakht, Abbas Ali Lotfi Neyestanak, A Survey on Recent Approaches in the Design of Band Notching UWB Antennas, Journal of Electromagnetic Analysis and Applications, pp. 77-84,2012. [5] Jawad Y. Siddiqui, Chinmoy Saha and Yahia M. M. Antar, A Novel Ultrawideband (UWB) Printed Antenna With a Dual Complementary Characteristic, IEEE Antennas And Wireless Propagation Letters, VOL. 14, pp.974-977, 2015. [6] Jawad Y. Siddiqui, Chinmoy Saha and Yahia M. M. Antar, Compact Dual-SRR-Loaded UWB Monopole Antenna With Dual Frequency and Wideband Notch Characteristics, IEEE Antennas AND WIRELESS PROPAGATION LETTERS, VOL. 14,pp.100-103, 2015. [7] Saira Joseph, Binu Paul, Shanta Mridula, and Pezholil Mohanan, CPW-Fed UWB Compact Antenna for Multiband Applications, Progress In Electromagnetics Research C, Vol. 56, 29 38, 2015. 329
[8] H.-Y. Lai, Z.-Y. Lei, Y.-J. Xie, G.-L. Ning, and K. Yang,UWB Antenna With Dual Band Rejection For WLAN/WIMAX Bands Using CSRRs, Progress In Electromagnetics Research Letters, Vol. 26, pp. 69-78, 2011. [9] H. J. Lee, Y. H. Jang, and J. H. Choi, Design of an UWB Antenna with Band-rejection Characteristic, Progress In Electromagnetics Research Symposium, Prague, Czech Republic,pp.155-157 August 27-30,2007. [10] Shrivishal Tripathi, Akhilesh Mohan, Sandeep Yadav, A Compact Koch Fractal UWB MIMO Antenna with WLAN Band- Rejection, IEEE Antennas And Wireless Propagation Letters, VOL. 99, pp. 1-4, 2015. [11] U. Deepak, T. K. Roshna, C. M. Nijas, K. Vasudevan, and P. Mohanan, A Dual Band SIR Coupled Dipole Antenna for 2.4/5.2/5.8 GHz Applications,IEEE Transactions On Antennas And Propagation, VOL. 63, NO. 4, pp.1514-1520, 2015. 330