Reduced Size Harmonic Suppressed Fractal Dipole Antenna with ...

Proceedings of 2010 IEEE Asia-Pacific Conference on Applied Electromagnetics (APACE 2010)

Reduced Size Harmonic Suppressed Fractal Dipole Antenna with Integrated Reconfigurability S. A. Hamzah*1, M. Esa#2, N. N. N. Abd Malik#3 #

Telematics Research Group, Faculty of Electrical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia 1 [email protected] 2 [email protected] 3 [email protected]

Abstract — Reduced size frequency reconfigurable fractal dipole antenna with harmonic suppression capability is presented in this paper. The proposed antenna consists of a narrow band radiating element, stub-filters and taper balun, producing harmonic suppression characteristics. In order to investigate the antenna performance, an extensive simulation (by CST and HFSS) and measurement works have been carried out. This technique is suitable for creating reconfigurable antenna with harmonic suppression or for single band antennas. Index Terms — Frequency reconfigurable antenna, harmonic suppression antenna, stub-filter, fractal dipole antenna.

I. INTRODUCTION Future wireless systems such as cognitive radio are placing demands on antenna designs. Recently, cognitive radio system has been introduced as one of the new features for radio communication system such as mobile handset unit [1]. This can be used to allow the user to share the spectrum frequency, allocated from other sources e.g. unused TV spectrums. Various techniques have been used to create reconfigurable antennas for cognitive radio applications [2][7]. The use of reconfigurable antenna by means of PIN diodes, MEMS switches or external circuit are effective methods proposed in the literature to change the operating frequency of the antenna. Two techniques reported on the creation of two antennas with wideband and narrow band features of omni-directional and directional patterns for sensing and frequency agile purposes [2]-[3]. Three monopole antennas having L-shaped configurations have been designed with the interband frequency being changed using a PIN diode. A DC voltage is used to control the reactance of a varactor diode for frequency tuning purposes that covers UHB band, mobile radio and wireless LAN [4]. In another reported work, a single patch antenna is integrated with parasitic elements by using RF switches to create a tunable feature from 550 MHz to 1500 MHz [5]. Recent studies of reconfigurable antennas include the use of vivaldi antenna [6] and log periodic dipole antenna [7]. They can operate in wideband and narrowband modes, respectively. The presence of harmonics is undesirable in many applications. Fractals can be incorporated, but this can cause significant undesired harmonic problems that are associated with higher order modes of the antenna.

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II. DESIGN STRUCTURE A reduced size of Koch fractal meander dipole antenna shown in Figure 1 has been designed and analysed [8], [9]. It has tunable capability of a reconfigurable operation within an observed range of 400 MHz to 3.5 GHz. The structure is constructed from the integration of Koch fractal dipole antenna, stubs and wideband taper balun. In this paper, further investigation on the performances are presented and discussed as given in Section III. Section IV concludes the paper.

Figure 1 Geometry of a proposed reconfigurable fractal antenna.

III. NUMERICAL SIMULATIONS The designed antenna has been successfully simulated. The simulated return loss responses obtained are depicted in Figure 2. The antenna has been implemented. Ideal switches have been inserted at the middle of the Koch curve section, for frequency configuration purposes. To suppress the higher order mode effectively, the stubs lengths have been varied at equivalents of quarter wavelength, λ/4. Besides that, the stub width, terminal length and the stubs location are optimized. Its prototype measured performances are shown in Figure 3. It can be observed that the antenna is operating well at band 1 of 673 MHz. The antenna return loss responses have been measured by using vector network analyzer. Figures 3 and 4 compare the simulation and measurement results.

Proceedings of 2010 IEEE Asia-Pacific Conference on Applied Electromagnetics (APACE 2010) Besides the factor of efficiency, the increasing appearances of higher order modes indicate that the selection method to reduce the size of the antenna is very important. In the research, the fractal antenna is a simple antenna design, but it is able to effectively suppress the undesired harmonic as well as good impedance matched.

Figure 4: Measured return losses of the fractal dipole antenna.

Figure 2. Simulated return losses of the fractal dipole antenna, N=2 at three conditions: (blue) without stub, (red) with stub1 and (black) with stubs 1, 2. In this study, f equals 710MHz (HFSS) while 1st HMcst=1.855GHz, 1stHMhfss=1.986GHz, 2nd HMcst=2.814GHz, 2nd HMhfss= 3.09GHz, 2nd HMcstnew =2.6181GHz, 2nd HMhfssnew=2.888GHz and suppressed return losses of 1stHM and 2nd HM equals -2.8 dB and 2 dB, respectively.

Figure 3: Measured return losses of the fractal dipole antenna without stubs, operating at 680 MHz, 1st HM = 1916 MHz and 2nd HM = 3008 MHz. There are two stubs, operating at 673 MHz and successfully suppressed the 1st HM and 2nd HM. In this study, antenna without stubs works at 680MHz, 1st HM=1916MHz and 2nd HM=3008MHz while the antenna with two stubs works at f = 673MHz and successfully suppressed the 1st HM and 2nd HM to be-2 and -3 dB, respectively.


The tunable return losses with higher order modes are presented in Figure 5. As shown in Figure 5, the operating band can be divided into three groups. The first group covers band 1 to band 6 having first higher order mode, 1st HM and second higher order mode, 2nd HM. The second group consists of bands 7 to 10 having single higher order mode, 1st HM while group three with no higher order mode. The bands in group three is colored to indicate that they have single frequency response which means they do no need a stub. Besides that, this figure obviously highlights the main issues in this work which is to avoid the higher order modes to interfere with the upper frequency bands (e.g. fr13 to fr15).Operating band is referred to as fr, 1st HM is used for order mode and 2nd HM for the 2nd higher order mode, respectively. The completed resonant frequencies and its equivalent higher order modes are given in Figure 5. To eliminate these higher order modes, one or two open circuit stubs are used depending on the λ/4 wavelength. The results obtained are shown in Figure 6. Tables II and III present a comprehensive description of the antenna performance for each band. As far as HFSS simulation results are concerned, the antenna can be tuned from 732MHz, 756MHz, 787MHz to 2971MHz, at one time. CST simulation results gave 710MHz, 738MHz, 763MHz to 3120MHz while measurement results show frequencies of 673MHz, 712MHz, 760MHz to 2460MHz. The length of stub 1 is from minimum of 19 mm to maximum of 42 mm, while stub 2 is from minimum of 16 mm to maximum of 30 mm to eliminate higher order modes. Meanwhile, Table II tabulated simulated and measured results for return losses and voltage standing wave ratios for band 1 to 15. The results show the stubs have strongly reduced antenna’s return loss as well as VSWR. The simulated and measured operating bandwidths are 2420MHz,

Proceedings of 2010 IEEE Asia-Pacific Conference on Applied Electromagnetics (APACE 2010) 2239MHz and 1789MHz, respectively as tabulated in Table III.

87.7°, 92.5°, 90° and 50.9°, respectively. The qualitative results by means of E- field patterns have computed with the highest bands of 13 to 15 are slightly different. This might be due to the tapered balun size that is relative to its operating wavelength. The figures have not been plotted due to limited pages. Finally, as seen from Table IV, the simulated gain gives higher than 2dB for bands 10 and above while bands 1 to 9 are below this value. The antenna radiations efficiencies are from 79% to 88%.

IV. CONCLUSION AND FUTURE WORK A fractal dipole antenna of reduced size has been successfully simulated and measured. It was found to operate well within the range of 400 MHz to 3500 MHz. The antenna exhibits reconfigurability feature. This suits the future cognitive radio system. Figure 5. Simulated tunable antenna return losses with higher order modes using HFSS. In this study the value of resonance frequencies as well as its higher order modes equals: fr1 = 689MHz, 1st HM= 1.855GHz and 2nd HM= 2.814 GHz, respectively

ACKNOWLEDGEMENT The work is supported by Universiti Teknologi Malaysia. The authors are grateful to Universiti Tun Hussein Onn Malaysia for supporting PhD studies of S. A. Hamzah.

REFERENCES

Figure 6. Simulated tunable antenna return losses with suppressed higher order modes using HFSS.

The proposed reconfigurable antenna works well in the cognitive radio (CR) standard frequency regulation which is governed by FCC in the TV, cellular radio and ISM bands (e.g. 400 MHz to 3.5GHz) with return loss below -10 dB and VSWR below 2.0. The antenna simulation bandwidth for each band, percentage bandwidth, half-power beamwidth on E-pattern, radiation efficiencies and gain are presented in Table IV. The antenna has bandwidths of 54MHz, 58MHz, 61MHz to 670MHz while its percentage is equals 7.6%, 7.9%, 8% and 21.5%, respectively. The antenna exhibits near omnidirectional radiation pattern which can be referred to its EHPBW. Bands 1, 2, 3, and 15 have wide beamwidths of


[1] http://www.nict.go.jp [2] Y. Hur, J. Park, W. Woo, K. Lim, C. H. Lee, H. S. Kim and J. Laskar, “A wideband analog multi-resolution spectrum sensing (MRSS) technique for cognitive radio (CR) systems”, proceeding of ISCAS2006, pp. 4090-4093. [3] F. Ghanem, P.S. Hall and J.R. Kelly,“Two port frequency reconfigurable antenna for cognitive radios”, Electronic letters, vol. 45, no.11, May 2009. [4] K. Ligusa, and H. Harada,“Antenna composition and technology for cognitive wireless communication”, pp. 843854, Springer 2009. [5] J. A. Zammit and A. Muscat, “Design and reconfiguration of low profile reconfigurable antenna for a cognitive radio system”, Proc. of IEEE conf. on WICT2008, 2008. [6] M. R. Hamid, P. Gardner, P. S. Hall, “Reconfigurable Vivaldi Antenna”, microwave and optical technology letters, vol. 52, No. 4, April 2010, pp. 785-786. [7] A. Mirkamali, and P. S. Hall, “Wideband frequency reconfiguration of a printed log periodic dipole array”, microwave and optical technology letters, vol. 52, No. 4, April 2010, pp 861-864. [8] S. A. Hamzah, M. Esa and N. N. N. A. Malik, “Reduced size microwave fractal meander dipole antenna with reconfigurable feature”, Proc. of ISAP 2010, pp. 1-4. [9] S. A. Hamzah, M. Esa and N. N. N. A. Malik, “Reduced size harmonic suppressed fractal dipole antenna with integrated reconfigurable feature”, accepted for presentation at SITIS 2010.

Proceedings of 2010 IEEE Asia-Pacific Conference on Applied Electromagnetics (APACE 2010)

Band Band 1 Band 2 Band 3 Band 4 Band 5 Band 6 Band 7 Band 8 Band 9 Band 10 Band 11 Band 12 Band 13 Band 14 Band 15

Table II Theoretical Predictions versus Measurement for Tunable Fractal Dipole Antenna Return Loss VSWR CST HFSS Measured CST 710MHz -26 732MHz -26 673 MHz -21 1.10 738MHz -32 756MHz -26 712 MHz -24 1.05 763MHz -43 787MHz -31 712 MHz -26 1.02 815MHz -36 837MHz -38 760 MHz -26 1.03 864MHz -30 889MHz -42 792 MHz -22 1.06 922MHz -23 956MHz -19 798 MHz -22 1.16 972MHz -32 1012MHz -18 858 MHz -19 1.05 1066MHz -22 1113MHz -17 858 MHz -17 1.07 1167MHz -21 1211MHz -17 930 MHz -20 1.17 1308MHz -28 1330MHz -19 1146 -24 1.31 MHz 1400MHz -9 1470MHz -10 1306 -42 2.03 MHz 1646MHz -12 1656MHz -14 1446 -9 1.68 MHz 1938MHz -15 1900MHz -19 1674 -9 1.43 MHz 2441MHz -16 2485MHz -20 1892 -12 1.12 MHz 3120MHz -17 2971MHz -19 2462 -20 1.32 MHz

Table III Channel Bandwidth: Simulated and Measured Results Items CST HFSS Measurement Channel bandwidth (MHz) 2410 2239 1789


HFSS 1.10 1.11 1.06 1.03 1.02 1.27 1.30 1.34 1.33 1.24

Measured 1.21 1.33 1.11 1.08 1.13 1.13 1.25 1.32 1.25 1.13

1.88

1.02

1.53

2.16

1.26

2.16

1.24

1.66

1.26

1.19