R F 2008 IEEE INTERNATIONAL RF AND MICROWAVE CONFERENCE PROCEEDINGS
December 22-4, 2008, Kuala Lumpur, MALAYSIA
M 08
Fractal Koch Dipole Antenna For Digital TV Application Mohd Nazri A. Karim, Mohamad Kamal A. Rahim, Thelaha Masri, Osman Ayop Radio Communication Engineering Department (RaCED), Faculty of Electrical Engineering, Universiti Teknologi Malaysia 81310 Skudai, Johor, Malaysia.
[email protected],
[email protected],
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Abstract- This paper describes the design of Fractal Koch Dipole array (FKDA) using log periodic technique for a sample 3 elements and design to operate at UHF band frequencies. The planar Fractal Koch has been investigated in a planar log periodic dipole array in order to miniaturize its size using fractal technique. The simulation process was carried out using 3D simulator software CST 2008. The antenna structure was fabricated using Fire Retardant4 (FR4) board, using wet etching techniques. The antenna is designed to operate at UHF Band (470 MHz- 806 MHz). The design procedures are elaborated in this paper. Both simulated and measured return loss S11 result exhibit similar characteristic which have same frequency range with a return loss magnitude less than -10dB. The comparisons in term of gain and radiation pattern at a single frequency for simulated and measured result are discussed in this paper. Keywords: Digital TV, Log Periodic, Fractal Antenna, Return loss, Radiation Pattern.
1. Introduction Digital TV (DTV) has already reached the implementing stage after a decade intense in research and development. The DTV can offer high data rate transmission, provide interactive service and operate at lower power. The DTV system is design to operate in UHF band (470MHz-806MHz). So it is very important to design broadband low cost and high performance antenna for digital television receiver. However it’s difficult to realize a built in antenna for UHF band because the wavelength of UHF is much longer than the devices. To overcome this problem, the technique to reduce antenna size is required so that the antennas can be embedded in the devices. So, it’s expected that the requirements of small and broadband antenna for this system will increase [1] [2]. In this paper, Fractal Koch dipoles are introduced as a technique to reduced antenna sized. The design considerations of the Fractal Koch antenna are elaborated in this paper. The effects by changing the degree of flare angle are observed. The results for
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simulated and measured return loss and radiation pattern are compared and have been analyzed.
2. Design Consideration of Fractal Koch Dipole Antenna This antenna can be used for any application that used this frequency range such as Digital TV, Digital Video Broadcasting (DVB) and many more. A straight dipole array antenna and fractal Koch dipole array antenna have been designed and compared in term of size reduction and their performance such as return loss and radiation pattern are discussed. The antenna was simulated using 3D simulator Computer Simulation Technology CST 2008 and was fabricated using printed circuit board FR4 board which has dielectric constant of 4.7 and tangent loss about 0.019. Figure 1 shows the layout of straight dipole and fractal Koch dipole antennas. Both antennas were designed using log periodic technique. This technique most commonly used for VHF and becomes popular for UHF band. It’s capable to obtain constant gain and input impedance over a wideband frequency. The design procedures for log periodic antenna are elaborate more details in [3]. As can be seen in figure 1, the parameter that are involved such as wn represent width of the dipole, dn represent distance between each element, h represent total height of the structure and w for total width of the antenna. In this project, the scaling factor that was used is 0.8. The dimension for straight dipole is about 222 mm length and for fractalized dipole about 190mm. So, the size reduction of the antenna between straight dipole and fractalized dipole can be calculated as follow:
Lsd − Lfd x100% = 15% Lsd For this structure, the reduction in size is around 15 percent due to the degree of flare angle. For this structure, the degree of flare angle is 45 degree. The reduction in sized is expected can be reduced more when the degree of flare angle and the number of iteration increased.
element as can be seen in figure 2. Otherwise the coupling is not strong enough that can cause the antenna can’t radiate effectively.
a) Straight dipole
Figure 2: Fabricated antenna
3. Result and Discussion I.
Figure 1: Layout from simulation
The scaling factor
τ=
τ
in this case used as [3]:
Ln rn = = 0.8 L n + 1 rn + 1
The antenna was fed using 50 ohm SMA connector. It’s connected to a central transmission line with a phase reversal between the dipole. This is required so that radiation is in backfire direction (towards smaller element). If the phase reversal is not used, radiation will occur in end-fire direction (towards larger element). This will cause scalloping in amplitude pattern and leads to erratic impedance behavior. This is referring to as end effect [3]. In simulation, the SMA connector needs to be designed precisely to obtain 50 ohm impedance to match with the line of the dipole. The excitation of the antenna can be fed using coaxial feeding technique or direct feeding technique. Figure 2 shows the prototype of fabricated fractal Koch antenna using wet etching technique. This antenna has been iterated with first iteration. It can be shrinking when more than one iteration applies to it but it needs to consider the coupling effect between the patch. In many cases, coupling effect is a big issue especially at the higher frequency. There are many papers discussed more details about coupling effect such as in [6], [7], [8]. The total size of the antenna structure is approximately 190 x 80 x 1.6 mm. The configuration of this structure need to be in criss-cross arrangement to get a better coupling between each
Figure 3 shows the simulated and measured return loss of the proposed designed. The return loss was measured using Vector Impedance Analyzer VIA Echo. From the graph, it can be seen that result for both simulated and measured exhibit similar characteristic which have same frequency range covered from 570 MHz until 810 MHz with a return loss magnitude less than -10dB. The bandwidth of the antenna is around 35 percent. Nevertheless, there is some shifting for the resonant at the frequency 700 MHz up to 800MHz but its still can be considered well enough for radiation.
Simulated and measured return loss
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Figure 3: Simulated and measured return loss
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II.
E Co & E Cross
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The radiation patterns for both simulated and measured are shown in figure 4. The measurement for the radiation pattern was done in anechoic chamber at 600 MHz. Figure 4a shows the simulated radiation pattern at 600 MHz. The radiation pattern radiated towards the smaller elements that so called end fire radiation. The dark pattern shows the strong radiation effect compared to light pattern as indicated in simulation software. Besides, the parameter that can be observed from this simulator such as efficiency, gain and many more, it depends on the setting that has been used. The radiation efficiency that was obtained is around 88 percent as depicted in figure 4 and the gain of the fractal Koch antenna is around 4 dBi. From the measurement, the radiation patterns for E-Plane and H-Plane have a similar pattern to the dipole antenna. The polarization of the antenna is in linear polarization which is for E plane, the cross polarization for E Co and E Cross roughly about 8 dB as well as for H plane. This antenna is omnidirectional pattern as can be seen at the polar plot. This is considering a very good radiation at the particular frequency. All measured radiation pattern and the data such as E-Plane co polar, E-Plane cross polar, H-plane co polar and H-plane cross polar has been taken and plot in polar plot as shown in figure 5.
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E & H Plane Figure 5: Measured radiation pattern
4. Conclusion A fractal Koch dipole antenna with three elements has been designed, simulated, fabricated and tested successfully. The antenna with 45 degree flare angle has been fabricated using FR4 board. The comparison in term of size reduction, performance in term of
Figure 4: Simulated radiation pattern
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return loss, radiation pattern and gain has been measured and tested. A bandwidth up to 35 percent is archived by using three elements fractal Koch dipole. Measured result shows that the return loss has been achieved to cover the UHF band application from 570 MHz up to 810 MHz that can be used for DTV application even though not cover all the frequency range of DTV application (470 MHz-806 MHz).
Acknowledgments The authors thanks to the Ministry of Higher Education for supporting the research work, Research Management Centre (RMC) and Department of Radio Engineering (RACED), Universiti Teknologi Malaysia for the support of paper presentation.
References [1] W.Y Chi, K.L Wong, “Wideband Printed Dipole Antenna for DTV Signal Reception”, TENCON 2007, Oct 2007. [2] D.H Choi, Y.T Im, Y.J Cho, S.O Park, “A Tunable Antenna For DVB-H Applications”, IEEE Antenna and Wireless Propagation Letters, Vol 6, 2007, pg 515-517. [3] C.A Balanis, “Antenna Theory: Analysis and Design”, John Wiley, Third edition 2005. [4] C.K Campbell, M.S Suthers, H.Kneve, “Design of Stripline Log Periodic Dipole Antenna”, IEEE Transaction on Antenna and Propagation, Vol. AP-25, Sept 1977. [5] D.H Choi, H.S Yun, S.O Park, “Internal Antenna With Modified Monopole for DVB-H applications”, Electronic Letter, Dec 2006, Vol 42. [6] R.R.Ramirez, and F. D. Flaviis, “A Mutual Coupling Study of Linear and Circular Polarized Microstrip Antennas for Diversity Wireless Systems”, IEEE Transaction On Antennas and Propagation, Vol 51, No. 2, Feb 2003. [7] M.Rammal, D.Eclercy, A. Reineix, B. Jecko, “Study of mutual coupling effect on radiated patterns of antenna arrays”, IEE Proc.-Microwave. Antennas and Propag., Vol. 144, No. 5, October 1997. [8] M. Gimersky and J. Bomemam, “Investigation of Mutual Cooupling Effect’s on the Radiation Pattern of Rectangular Patch Antennas”, Antennas and Propagation Society International Symposium, 1992. AP-S. 1992, Volume , Issue , 18-25 Jul 1992 Page(s):1194 - 1197 vol.2.
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