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Autonomous use of Fractal Structure in Low Cost, Multiband and Compact Navigational Antenna Rajeev Kumar Kanth, Pasi Liljeberg, Hannu Tenhunen Turku Centre for Computer Science (TUCS), Department of Information Technology, University of Turku, Joukahaisenkatu 3-5B, 20520 Turku Finland {rajeev.kanth, pasi.liljeberg, hannu.tenhunen}@utu.fi
Abstract- Different fractal structures and their relevance in navigational antennas have been studied. Based on multiband characteristics and pe rformance o f si erpinski gask et fr actal structures, a dual band, l ow profile antenna is dev ised in thi s paper. Cla ssical fra ctal st ructures h ave be en g enerated w ith extensive use of MATL AB, dime nsions o f the par asitic lay ers are d etermined v ia sev eral optimizations in Ma thCAD and finally design analysis is carried out using Ansoft Designer. The performance of this antenna is theoretically measured in terms of its return loss, gain radiation pattern and axial ratio. The multi layered physical antenna has been fabricated using glass epoxy substrate material contributing acceptable bandwidth in both ba nds. T he measured performance of t he fabricated antenna has be en analyzed and ev aluated with t he theoretical outcomes. Key Words- Koch Curve, Sierpinski Structure, dual band, Return Loss, Gain radiation pattern, Microstrip Antenna, Impedance match, Stacked Patches I. INTRODUCTION A recent trends show that there is a need for a multiband antenna to diminish the space usage and cost instead of using one an tenna fo r each specific s ystem. Modern telecommunication system r equires antennas having multiband characteristics, w ider b andwidths and smaller dimensions than conventionally possible. This has i nitiated antenna research in several directions, one of wh ich is using fractal structure in an tenna elements. In r ecent years several fractal geometries h ave been introduced for antenna applications with varying degree of success in improving the antenna characteristics such as r educing size of t he antenna, capable of resonating at multiple frequencies.
In this paper attempt has been made to explore multiband aspect o f f ractal antenna o perative at multiple frequency bands esp ecially on L an d S ban d w ith moderate an tenna gain o f -4dBi up to ±5 00. To accomplish t he e ntire w ork, MATLAB has been used to generate special classical fractal structures, M athCAD 2000 has been utilized to de termine the size of the parasitic layers of fractal structures, and finally the d esign of th e fr actal an tenna th at r esonates at 1.176 GHz and 2 .487 GHz simultaneously has been carried out with Ansoft Designer. This work is a continuous series of work that has been presented in papers [1], [2] and [3]. In recent years [4] - [11] several sta cked patches an d fractal g eometries ha ve been
1 1
Waqar Ahmad, 2Subarna Shakya, 1Li Rong Zheng
School of Information and Communication Technologies, Royal Institute of Technology (KTH), Sweden 2 Institute of Engineering, Tribhuvan University, Nepal
[email protected],
[email protected],
[email protected]
introduced for antenna a pplications with varying degrees of success in improving antenna characteristics. Som e of th ese geometries have b een particularly useful in optimizing t he dimensions of antennas operative at multiband environment. II. GENERATION OF FRACTAL STRUCTURES This section of wor k introduces th e method of gen erating classical fractals such as Koch Curve, Sierpinski Carpet and Sierpinski Gasket with extensive use MATLAB code and its versatile e ngineering an d gra phical fea tures.These fr actal structures h ave be en generated on basis of n umber of iteration cycles. As number of iterations increases, size of the structural details become tiny compared to resolution of the screen. Fig. 1, Fig. 2 and Fig. 3 respectively show the structures o f Koch Curve, Sierpinski Carpet and Sierpinski Gasket with four iterations. These structures have been created with the knowledge of their self si milar characteristics an d frequency ratio between two adjacent bands. The Sierpinski gasket structure is chosen among al l oth er classical str uctures du e t o its resemblance with equilateral triangle microstrip antenna (E TMSA). The method of generating this structure is to de velop the equilateral tr iangle with th e specified length an d an other inverted equilateral tr iangle is mad e by the joining th e mid points of the sides of the former triangle and being subtracted from the earlier one.
Fig. 1 Koch Curve
Fig. 2 Sierpinski Carpet
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Fig. 3 Sierpinski Gasket
136 TABLE I
SPECIFICATION OF FRACTAL ANTENNA
S. No. 1.
Parameters Frequency band L band S band Gain Axial ratio Polarization Return loss 3dB Beam width
2. 3. 4. 5. 6.
Value 1.16-1.19 GHz 2.46-2.50GHz -4dBi up to 50 3dB LHCP Min. -10dB 50
ANTENNA DESIGN The sp ecifications o f t he dual ban d fr actal antenna ar e shown in Table I. The parameters in the table are as per communication link betw een the tr ansmitter and r eceiver. The design of dual band sierpinski fractal antenna consists of three dif ferent s teps: dime nsion dete rmination, planer EM design and setting the probe position. The dimensions of the parasitic layers a re de termined using equa tions (1), ( 2) and (3). [12]-[14] To achieve the design specification, the glass epoxy and rogers RO3003 ha ve been used a s s ubstrates wh ose r elative dielectric constants ( r) are 4.4 a nd 3 .0 r espectively. In equation (1), t he resonant frequency (fmn) is expressed in terms of effe ctive si de l ength ( Se), mode of triangular patch antenna ( m=1, n=0), spee d o f light (c) a nd t he effective dielectric constant ( e). Sim ilarly the effect ive si de l ength (Se) is estimated with equation (2) where effective dielectric constant ( e) ca n be obt ained i n E quation (3) ut ilizing parameters lik e relative di electric con stant ( r), height (thickness) of th e gr ound p lane t o pa tch (h) an d s ide of equilateral triangle (S). III.
2c m 2
f mn
mn n 2
3S e Se
1 2
B.
Planer EM Design The la yers o f th e fra ctal antenna an d th eir width are illustrated in T able II. Th e planer EM desi gn s pecifies h ow several l ayers ar e st acked to ea ch other. Th e l etters‘t’, L.E. and U.E. r espectively stands for thickness, lower elevation and the upper elevation. C.
(1)
e
4h
S
Fig. 5 Lower Parasitic Patch Fig. 6 Upper Parasitic Patch A. Dimension Determination The dim ensions of la yers of th e fra ctal ant enna ar e determined usi ng e quations ( 1) - ( 3). S everal optimizations have been carried out to o btain the sp ecified characteristics of the fractal antenna. The dual frequency behavior is obtained t hrough the perturbation of th e c lassical S ierpinski via a r eduction of t he fr actal i teration and a m odification of the sca le pr operties. The inner t riangular slot co ntrols t he frequency ratio and maximum coupling is achieved by proper dimensioning of the lower patch. F ig. 4 illustrates b ottom layer while Fig. 5 and Fig. 6 show the dimensions of the middle layer and the upper pa rasitic layer patches respectively.
(2)
Probe Position The pr obe position is set to co-ordinate (0, 33) to get the desired s pecification. P ort I is the probe position shown in Fig. 4. During several optimization cycles it is found that this probe p osition gives th e su itable ch aracteristics i mpedance and matching with the input impedance of the antenna.
e
1
r e
2
1
r
2
20h 1 S
TABLE II
1 2
(3)
S.N.
Layers
LAYERS OF THE FRACTAL ANTENNA
Type
Material
1. Parapatch2 Signal
Copper
2. Substrate5 Dielectric Glass Epoxy 3. Substrate4 Dielectric 4. Parapatch1 Signal
t L.E. U.E. (mm) (mm) (mm) 0 16.62 16.62 0.8 11.32 16.62
Air(foam)
4
11.32 15.82
Copper
0
11.32 11.32
5. Substrate3 Dielectric Glass Epoxy
0.8 10.52 11.32
6. Substrate2 Dielectric
10
Air(foam)
1.52 10.52
7.
Active Signal Copper 0 1.52 1.52 Patch 8. Substrate1 Dielectric RogersRO3003 1.52 0 1.52 Fig. 4 Active Patch Layout of Fractal Antenna
9.
Ground M. Signal
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Copper
0
0
0
137 IV.
SIMULATED AND MEASURED RESULTS
Fig. 7 presents return los s di agram a nalyzed in A nsoft designer. It sh ows th at th e antenna i s sim ultaneously resonating at 1.76GHz and 2.85GHz. Moreover, it provides a superior b andwidth o f 40 MH z ra nging f rom 1.74 GHz t o 1.78 GHz and 210MH z ranging from 2.81GHz t o 3.02GHz respectively in L and S band. This v alue o f bandwidth is obtained after several iterations by optimizing the parameters in the d esign. The m easured Retu rn Loss d iagram i s sh own in Fig. 8. On e can notice th at th e devel oped antenna is resonating at 1.73GHZ and 2.94GHz generating a significant bandwidth in both the ban ds. This r esult has been measured with vector n etwork ana lyzer. I mpedance p lot an d gain radiation patterns are shown in Figs. 9- 10, a nd Figs. 11-12 respectively.
Fig. 9 Analyzed Impedance plot
Fig. 11 Gain in L Band
h
V.
Fig. 7 Simulated Return Loss of Dual band fractal antenna
V
Fig. 10 Measured Impedance plot
Fig. 12 Gain in S Band
RESULTS ANALYSIS
In the previous papers [1]-[3], simulations have been carried o ut for dua l band fractal antenna but this paper analyzes t he p erformance of the sim ulated and f abricated antenna. The simulated bandwidths in L band and S band are found a s 4 0 M Hz and 21 0 MHz r espectively. It fu lfils the characteristics of t he an tenna as per its s pecification. The bandwidth o f f abricated antenna is measured w ith v ector network analyzer. Fig. 8 illustrates appreciable bandwidth in both ba nds. Fig. 9 and F ig. 1 0 respectively show the simulated impedance plot analyzed with Ansoft Designer and measured i mpedance p lot w ith vector network an alyzer. These plots c learly show that there is formation o f a loop near unity. This implies impedance matches.
RESULTS ANALYSIS
Active Patch
Lower Parasitic Patch
Upper Parasitic Patch Fig. 8 Measured Return Loss of Dual band fractal antenna
Fig. 13 Layers of Fabricated fractal Antenna
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138 VI.
CONCLUSION AND FUTURE RECOMMENDATION
In this pape r des ign of the du al band sierpinski base d fractal antenna has been carried out. According to simulation result in terms of return loss and VSWR, the physical antenna was fabricated and its characteristics were measured with V ector Network A nalyzer (VNA). Both t he analyzed and measured results were compared and found to be sim ilar with certain shifting of t he fr equencies. Th e pr oposed antenna can be utilized in satellite navigational instruments. The ga in ra diation pa ttern was o bserved th eoretically fulfilling th e requirement of the antenna and measurement of its gain in anechoic chamber is future work. REFERENCES
[1]
Fig. 14 Developed Sierpinski dual band fractal antenna From gain radiation pattern shown in Fig. 11 and Fig. 12, it can be st ated th at th e gains along th e bore s ight are respectively 7.59dBi in L b and and 2.17dBi in S b and. The radiation pattern is dr awn for three different a ngles of P hi i.e. 00, 450 and 900, which gives the pattern appropriate to the specification. For dual ba nd, F ig. 1 3 i s sh owing th ree l ayers of fr actal antenna. Th ese la yers ar e a ctive pa tch, l ower parasitic patch and u pper p arasitic patch. Activ e patch is f abricated o n Rogers Ro3003 dielectric material where as lower and upper parasitic patches are fabricated on glass epoxy substrate. The measured r esonant frequencies of t he fr actal an tenna ar e 1.736GHz and 2.95GHz in L and S band respectively. It has been noticed that th ere i s a gap of an alyzed and m easured frequency. The s hifting of fr equency t o th e ri ght from 1.176GHz and 2.487GHz is due to the following reasons. Stacking of the layers is not ideal. The effective di electric constant mus t decr ease due to in sertion of some g aps amon g these th ree la yers of glass epoxy. Substrate 2 a nd sub strate 4 a re made up of white foam. Th eir ideal th icknesses are 10mm an d 4mm respectively. As their thicknesses are not uniform, it causes change in dielectric constant. Similarly the return loss of the antenna is measured with VNA in presence of air gap in between the sheets of the glass epoxy, which is also shifted due to the change in the effective dielectric material of the antenna. Fig. 14 is the actual dual band sierpinski fractal a ntenna developed f ulfilling the required specification. T he figure shows its active patch side where as the other side of the antenna has a coaxial probe, soldered with the patch through 3mm via. As glass epoxy substrate material is used, th e cost of th e antenna i s low. The stacked pat ches con struct the structure of antenna compact.
Rajeev Kumar Kanth, Alok K umar Singhal, Pasi Liljeberg, Hannu Tenhunen, “ D esign of Mu ltiband Fractal Antenna for Mobile and Handheld Terminals” C onference proceedings of Asian Himalayan First International Conference on Internet (AH-ICI, 2009) [2] Rajeev Kumar Kanth, Alok K umar Singhal, Pasi Liljeberg, Hannu Tenhunen, “ Analysis, D esign and Development Novel, Low P rofile Microstrip Antenna fo r S atellite Navigation” Conference proceedings of NORCHIP 2009 [3] Rajeev Kumar Kanth, Waqar Ahmad, Yasar Amin, Pasi Liljeberg, LiRong Zheng, Hannu Tenhunen, “ Analysis, Design and Development Novel, L ow P rofile 2. 487 G Hz Microstrip An tenna” Conf erence proceedings of 14th Int ernational Symposium on Antenna T echnology and A pplied Electromagnetics and t he American Electromagnetics Conference (ANTEM/AMEREM) 2010 [4] Yong-Xin Guo, Michael Yan Wah Chia, a nd Zh i Ning C hen, , “Miniature B uilt-In Mu ltiband A ntennas for Mobile H andsets” IEEE Transactions on Antennas and Propagation, Vol. 52, No. 8, August 2004 [5] Peter L indberg and Erik Ö jefors, “A Bandwidth Enhancement Technique f or Mobile H andset Antennas Using Wave tr aps” IEEE Transactions on Antennas and Propagation, Vol. 54, No. 8, August 2006 [6] Ar nieri, Lu igi B occia, Giandomenico Amendola, and Giuseppe Di Massa,” A C ompact H igh Ga in Antenna for Small Satellite Applications”, IEEE Transactions On Antennas And Propagation, Vol. 55, No. 2, February 2007 [7] Lei Bian, Yong-Xin Guo, L. C. Ong, and Xiang-Quan Shi, “Wideband Circularly-Polarized Patch Antenna” IEEE Transactions On Antennas And Propagation, Vol. 54, No. 9, September 2006 [8] Shing-Lung Steven Yang and Kwai-Man Luk,, “Design of Wide-Band L-Probe Patch Antenna for pattern Reconfiguration or D iversity Application” IEEE Transactions On Antennas and Propagation, Vol. 54, No. 2, February 2006 [9]
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[10] Josaphat Tetuko Sri Sumantyo, Ko ichi Ito and Masaharu Takahashi, “Dual Band circularly Polarized Equilateral Triangle Patch antenna for Mobile Satellite Communications”, IEEE Transactions on Antennas and Propagation, Vol. 53, No. 11, November 2005 [11] Qinjiang Rao, Tayeb A. Denidni, and Ronald H. Johnston, “A N ew aperture Coupled Microstrip Slot Antenna”, IEEE Transactions on Antennas and Propagation, Vol. 53, No. 9, September 2005 [12] J.D. Kr aus, Ronald J Mahefka a nd A hmad S K han “Antennas for a ll Applications” Third Edition ISBN-0-07-060185-2, PP 11-67 [13] Doulas H. W erner, Ra j Mitra “Frontier in El ectromagnetics” IE EE Press Series on Microwave Technology and RF, PP 48-170 [14] Girish Kumar, K.P.Ray, "Broadband Microstrip Antenna” Artech House 2003, ISBN 1-58053-244-6 Page NO. 80, 81,330
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