IMPROVEMENT OF FAR FIELD RADIATION PATTERN OF LINEAR ARRAY ANTENNA USING GENETIC ALGORITHM

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1 ISSN: (ONLINE) ICTACT JOURNAL OF COMMUNICATION TECHNOLOGY, MARCH 10, VOLUME: 01, ISSUE: 01 DOI: /ijct IMPROVEMENT OF FAR FIELD RADIATION PATTERN OF LINEAR ARRAY ANTENNA USING GENETIC ALGORITHM Sudipta Das 1, Soen Bhattacharjee 2, Durbadal Mandal 3 Departent of Electronics and Counication, National Institute of Technology, Durgapur, India Eail: sudipta.sit59@gail.co 1, soen.aec@gail.co 2, durbadal.bittu@gail.co 3 Abstract In this paper, the bea pattern of Linear Array Antennas with isotropic eleents is exained. The design goal is to reduce the sidelobe level with a iniu beawidth increase for the far field radiation pattern of the array by varying its electrical as well as its physical configuration. In this paper the cases of a uniforly excited and uniforly spaced array, uniforly excited and non-uniforly spaced array, and a non-uniforly excited and uniforly spaced array are exained for both syetric as well as asyetric array, and a coparison is done between the. Real Coded Genetic Algorith (RGA) is used to find the optial locations as well as the optial excitations for the proble as per the cases considered. Keywords: Syetric Linear Array Antenna, Asyetric Linear Array Antenna, Sidelobe Reduction, First Null Beawidth, Real Coded Genetic Algorith. 1. INTRODUCTION A lot of research works have been carried out for optiizing the radiation pattern of Linear Array Antenna for past few decades [1-13]. Array Antenna is fored by assebly of radiating eleents in an electrical or geoetrical configuration. In ost cases the eleents are identical. Total field of the Array Antenna is found by vector addition of the fields radiated by each individual eleent. There are five controls in an Array Antenna that can be used to shape the pattern properly, they are, the geoetrical configuration (linear, circular, rectangular, spherical etc.) of the overall array, relative displaceent between eleents, excitation aplitude of the individual eleents, excitation phase of the individual eleents, and, relative pattern of the individual eleents [1-2]. In any counication applications it is required to design a highly directional antenna. Array Antennas have a high gain and directivity copared to an individual radiating eleent. A Linear Array Antenna has all its eleents placed along a straight line, with a unifor relative spacing between eleents [3]. The goal in Array Antenna geoetry synthesis is to deterine the physical layout of the array that produces the radiation pattern that is closest to the desired pattern [7]. In this paper the design goal is to suppress the axiu relative sidelobe level (SLL) for a Linear Array Antenna of isotropic eleents, as well as to restrict the increent of First Null beawidth (BWFN). [4-6]. A radiation pattern with lower axiu sidelobe, thinner ain bea and ore and deeper nulls is preferred. This is done by designing the relative spacing between the eleents, with a non-unifor excitation over the array aperture. In this paper, RGA is used to get the desired pattern of the array [2]. Now in the rest of the paper, in Section 2, general design equations, both for syetric and asyetric array is discussed. In this Section, all the different cases are also discussed. In Section 3, briefly the RGA is introduced, the siulated results are discussed in Section 4 and 5, and the conclusion is drawn in section DESIGN EQUATIONS Radiation pattern of an array of antenna is strongly dependent on its geoetrical as well as its electrical architecture. All the eleents constructing a linear array ust be placed on a straight line. Geoetrical architecture of such an array eans how the eleents are placed along the line. Array eleents ay be uniforly placed throughout the array aperture, or syetrically placed with respect to the centre of the array, or asyetrically placed throughout the line. Electrical architecture gives the pattern by which the eleents are excited. They ay be excited with sae aplitude and sae progressive phase throughout the array aperture, or they ay be syetrically excited with respect to the centre of the array, or they all ay be entirely asyetrically excited. Thus, the configurations that can be considered are, A. Uniforly placed and uniforly excited array, B. Uniforly placed but non uniforly excited array, C. Non uniforly placed and uniforly excited array and D. Non uniforly placed and non uniforly excited array In this paper, the perforance of the Array Antenna is optiized for only first three conditions. If the eleents have syetry with respect to its centre for geoetrical and electrical configuration, it is used to analyze this case as syetric array and consider the reference point as the idpoint of the array, else the array is referred to as asyetric array and the reference point in this case is the end point of the array, or, the 1 st or last eleent of the array. Figure 1 depicts a design of an asyetric linear array and Figure 2 depicts a design of a syetric linear array. Array factor of a broadside linear array of M isotropic eleents placed along z-axis is AF = M { I 1 jkd 1 cosθ} = 1 exp( (1) 23

2 SUDIPTA DAS et al.: IMPROVEMENT OF FAR FIELD RADIATION PATTERN OF LINEAR ARRAY ANTENNA USING GENETIC ALGORITHM Fig.1. Geoetry of M eleent asyetric Linear Array Antenna placed along z-axis Where, I =Excitation aplitude of th eleent k = 2π / λ where, λ is the operating wavelength d =is the distance of th eleent fro the reference point θ Sybolize the zenith angle fro the positive z axis to the orthogonal projection of the observation point P. This is the ost general case, and, this equation can be used to analyze an asyetric array. Here the reference point is one end of the array. For analyzing a syetric broadside Linear Array Antenna of 2N isotropic eleents along z-axis the following equation can be used AF s Where, N { I n kd n cosθ } = 2 cos( ) (2) n= 1 I n =Excitation aplitude of n th eleent fro its id-point d n =is the distance of n th eleent fro the id-point of the array In our case the cost function, or the fitness function, called isfitness (MF) is defined as follows, MF = AF( θ sl + BWFN, I AF( θ initial ) + AF( θ, I ) sr 0 (3) BWFN, I current In this case θ0 is the desired direction of scanning, and the ain bea should be located here and here, θ [ 0, π], θ sl is the angle of axiu sidelobe for the lower band( AF θ, I ) ) ) ( sl and θ sr is the` angle of axiu sidelobe for the upper band( AF θ, I ) ). Thus the first ter in the right hand side ( sr of (3) gives the axiu sidelobe level with respect to the ain bea. By the second ter in (3), the beawidth increent is restricted. BWFN are the beawidth BWFN initial and current between the First Nulls for the initial condition and the current iteration. MF is lower for the Array Antenna which has lower SLL and lower BWFN as copared to the initial array (radiation pattern not optiized). Miniization of MF eans axiu reductions of SLL both in lower and upper bands. The evolutionary optiization techniques eployed for optiizing the current excitation weights and the inter-eleent spacing, resulting in the iniization of MF and hence reduction of SLL. 3. REAL CODED GENETIC ALGORITHM (RGA) GA is ainly a probabilistic search technique, based on the principles of natural selection and evolution [2]. At each generation it aintains a population of individuals where each individual is a coded for of a possible solution of the proble at hand and called chroosoe. Chroosoes are constructed with genes of rando values between (0, 1). Each chroosoe is evaluated by a function known as fitness function, which is usually the cost function or the objective function (called or MF) of the corresponding optiization proble. Steps of RGA as ipleented for optiization [13] of spacing between the eleents and current excitations are: Fig.2. Geoetry of 2N eleent syetric Linear Array Antenna placed along z-axis Initialization of real chroosoe strings of n p population, each consisting of a set of excitations. Size of the set depends on the nuber of excitation eleents in a particular array design. 24

3 ICTACT JOURNAL OF COMMUNICATION TECHNOLOGY, MARCH 10, ISSUE: 01 Decoding of strings and evaluation of MF of each string. Selection of elite strings in order of increasing MF values fro the iniu value. Copying of the elite strings over the non-selected strings. Crossover and utation to generate off-springs. Genetic cycle updating. The iteration stops when the axiu nuber of cycles is reached. The grand iniu MF and its corresponding chroosoe string or the desired solution are finally obtained. 4. NUMERICAL SIMULATION RESULTS This section gives the siulated results for various Linear Array Antenna designs obtained by RGA technique. Three Linear Array Antenna structures having 6, 12, and 18 eleents are assued, each aintaining a fixed spacing between the eleents. Paraeters such as axiu SLL and BWFN are studied for syetric as well as asyetric array. The paraeters for the RGA are set after any trial runs. It is found that the best results are obtained for an initial population of 1 chroosoes. Maxiu nuber of generations, N is liited to 0. For selection operation, the ethod of natural selection is chosen with a selection probability of 0.3. Crossover is randoly selected dual points. Crossover ratio is 0.8. Mutation probability is [13]. RGA technique generates a set of noralized array paraeters. I = 1 corresponds to unifor current excitation. Table 1. shows the axiu sidelobe level and the beawidth values for three sets of linear array designs, with the initial current distribution as I = 1, and unifor inter-eleent spacing as d = λ / 2. Tables 2-5 copare the radiation patterns for a syetric and an asyetric array for all the cases. Table 2. shows the radiation paraeters for all the sets of nuber of eleents (as considered in Table 1), for optiu unifor spacing d ( 0, λ) only. Table 3 shows the radiation paraeters for the optiu non-unifor spacing d ( 0, λ) with unifor excitation aplitude ( I = 1 ). Table 4. shows the respective radiation patterns for uniforly spaced ( d = λ / 2) arrays with optial non-unifor excitations. Table 5. shows the radiation patterns for all the arrays consisting eleents with optiu non-unifor excitation ( I ( 0, 1) ) & optiized unifor spacing ( d ( 0, λ) ). Table 1. Sidelobe Level & Main Beawidth for Different Sets of Linear Array with Unifor Excitation as 1 and Unifor Spacing as λ/2 No of Eleents Sl.No. No. of Eleents Initial ax SLL (db) Initial Beawidth ( ) Optiized Unifor Spacing Table 2. Optial Unifor Spacing Only Final ax SLL (db) Final BWFN( ) Syetric Asyetric Syetric Asyetric Syetric Asyetric Table 3. Optial Non Unifor Spacing Only No of Eleents Optiized Non-Unifor Spacing Final ax SLL (db) Final BWFN( ) Sy. Asy. Sy. Asy. Sy. Asy

4 SUDIPTA DAS et al.: IMPROVEMENT OF FAR FIELD RADIATION PATTERN OF LINEAR ARRAY ANTENNA USING GENETIC ALGORITHM Table 4. Optial Non Unifor Excitation Only (Unifor Spacing As λ/2) No. of Eleents Optiized Non-Unifor Excitation Final ax SLL (db) Final BWFN( ) Sy. Asy. Sy. Asy. Sy. Asy Table 5. Optial Non Unifor Excitation with Optial Unifor Spacing El Syetric Asyetric Final ax SLL (db) Final BWFN( ) Exc. Sp. Exc. Sp. Sy. Asy. Sy. Asy ANALYSIS OF RADIATION PATTERNS OF LINEAR ARRAY SETS Figure 3 depicts the radiation patterns for a uniforly excited linear array having 18 isotropic eleents with fixed inter-eleent spacing. The patterns are got directly fro the respective values fro Table 2. Finding optiized unifor spacing results in thinning of ain bea. Moreover, a lot extra nulls are inserted in the radiation pattern. For all the sets of eleents syetric array gives lower BWFN as copared with that of the asyetric array, except for the set of 18 eleents. The sidelobe for all the sets for both syetric and asyetric array is not altered except for the case of asyetric array with 18 eleents. While SLL is unaltered for the corresponding syetric array, that for the asyetric array is rather increased. Thus asyetric array has poorer perforance in this case. The result can be verified fro Table 2. It is seen that, with a negligible sacrifice in the SLL, 18 eleent asyetric linear array antennas gives lower BWFN as copared to the corresponding syetric linear array. SLL for all other sets are the sae as that of the initial pattern, both for syetric and asyetric array. Syetric array gives better result by providing lower BWFN except for the set of 18 eleents. Figure 4 depicts the radiation pattern for 18 eleent linear array for non-uniforly varied inter-eleent spacing. In this case, it can be seen that, for both syetric and asyetric array, BWFN is reduced, and SLL reduction is a bit better for syetric array, except for the set of 6 eleents. Again lower BWFN is provided with syetric array. Fro Figure 4, it can be seen fro the figure that unlike syetric array, for asyetric array the previously existing nulls are alost disappeared. But for the syetric Array, inserted nulls are quiet deep. Thus syetric array gives better result for the sets of larger nuber of eleents. The results can be verified by Table3. Figure 5 depicts the radiation pattern of 18 eleent linear array with non unifor excitation. BWFN is increased for both syetric as well as asyetric array fro that of the corresponding initial array. While syetric array gives lower SLL, the respective asyetric array provides lower BWFN (except for the set of 12 eleents). Moreover, while alost all the previously existing nulls are filled up for asyetric array, 26

5 ICTACT JOURNAL OF COMMUNICATION TECHNOLOGY, MARCH 10, ISSUE: 01 soe of the nulls are retained for syetric array. The results can be verified fro Table 4. Figure 6 depicts the radiation pattern for 18 eleent liner array with non unifor optiized excitation and unifor optiized inter-eleent spacing. While lower SLL is provided by syetric array, the corresponding asyetric array gives lower BWFN. It is clear fro the figure that SLL is noticeably reduced both the syetric and asyetric array. Fro the figure it can be seen soe extra nulls are inserted in the radiation pattern. While asyetric array suffers fro low null depth, corresponding syetric array has all such nulls quiet deep. Syetric array has an advantage over asyetric array by better SLL reduction perforance and deeper nulls in the radiation pattern, but it suffers fro the ain bea broadening, though negligibly sall. This result can be verified fro Table 5. Noralized Array Factor in db Radiation pattern of a ordinary 18 eleent linear broadside array -90 Optial Radiation pattern of a syetric Linear Array Optial Radiation pattern of an asyetric Linear Array Angle of arrival in degree Fig.3. Radiation pattern of the 18-elent Uniforly spaced Linear Array Antenna obtained using RGA Noralized Array Factor in db Radiation pattern of a ordinary 18 eleent linear broadside array -90 Optial Radiation pattern of a syetric Linear Array Optial Radiation pattern of an asyetric Linear Array Angle of arrival in degree Fig.4. Radiation pattern of the 18-eleent Non-uniforly spaced Linear Array Antenna obtained using RGA. 27

6 SUDIPTA DAS et al.: IMPROVEMENT OF FAR FIELD RADIATION PATTERN OF LINEAR ARRAY ANTENNA USING GENETIC ALGORITHM Noralized Array Factor in db Radiation pattern of a ordinary 18 eleent linear broadside array -90 Optial Radiation pattern of a syetric Linear Array Optial Radiation pattern of an asyetric Linear Array Angle of arrival in degree Fig.5. Radiation pattern of the 18-eleent Non-uniforly excited Linear Array Antenna obtained using RGA. Noralized Array Factor in db Radiation pattern of a ordinary 18 eleent linear broadside array -90 Optial Radiation pattern of a syetric Linear Array Radiation pattern of an asyetric Linear Array Angle of arrival in degree Fig.6. Radiation pattern of an 18 eleent Non Uniforly excited Linear Array Antenna with unifor spacing obtained using RGA The iniu MF values against nuber of iteration cycles are recorded to get the convergence profile of each set. Figures 7-8, 9-10 and portray the convergence profiles of iniu MF of linear array set having 18 eleents. Figure 7, 9 and 11 shows the convergence profiles for only non uniforly spaced, only non uniforly excited with unifor spacing d = λ / 2 and only non uniforly excited and uniforly spaced syetric array respectively. Figures 8, 10 and 12 shows the convergence profiles for an asyetric array for the respective cases. The prograing has been written in MATLAB language using MATLAB 7.5 on core (TM) 2 duo processor, 1.83 GHz with 2 GB RAM. 28

7 ICTACT JOURNAL OF COMMUNICATION TECHNOLOGY, MARCH 10, ISSUE: Fig.7. Convergence curve for RGA for non-uniforly spaced 18-eleent syetric Linear Array Antenna Fig.8. Convergence curve for RGA in case of non-uniforly spaced 18-eleent asyetric linear array Fig.9. Convergence curve for RGA for non-uniforly excited 18-eleent syetric Linear Array Antenna 29

8 SUDIPTA DAS et al.: IMPROVEMENT OF FAR FIELD RADIATION PATTERN OF LINEAR ARRAY ANTENNA USING GENETIC ALGORITHM Fig.10. Convergence curve for RGA for non-uniforly excited 18-eleent asyetric Linear Array Antenna Fig.11. Convergence curve for RGA for non uniforly excited 18 eleent asyetric Linear Array Antenna with unifor spacing Fig.12. Convergence curve for RGA for a non uniforly excited 18 eleent asyetric Linear Array Antenna with unifor spacing 6. CONCLUSION In this paper three Linear Array Antenna structures with variable spacings and excitations are considered. For the set of 18 eleent linear arrays SLL is reduced upto db for syetric and db for asyetric array, while the respective ain lobe beawidths are and against the initial SLL of db and initial beawidth of Fro the Tables and the corresponding figures it can be easily seen that, as the nuber of the eleents are increased, SLL reduction and BWFN aintainance perforances are iproved for both syetric and asyetric array. Siulated results show that a optial non uniforly excited and optial uniforly spaced Linear Array Antenna has a considerable sidelobe reduction with least first null beawidth increent. Moreover, extra nulls are inserted in the radiation pattern and this ultiately gives a design of an Array 30

9 ICTACT JOURNAL OF COMMUNICATION TECHNOLOGY, MARCH 10, ISSUE: 01 Antenna with lower interference fro undesired directions without significant sacrifice in directivity. Thus RGA is found to be proising evolutionary optiization technique for global optiization of any design proble. REFERENCES [1] C. A. Ballanis, Antenna theory analysis and design, 2nd edition, John Willey and Son's Inc., New York, [2] R. L. Haupt, and D. H. Werner, Genetic Algoriths in Electroagnetics, IEEE Press Wiley-Interscience, 07. [3] P. K. Murthy and A. Kuar, Synthesis of Linear Antenna Arrays, IEEE Trans. Antennas Propagat., Vol. AP-24, pp , Noveber [4] F. Hodjat and S. A. Hovanessian, "Non- uniforly spaced linear and planar array antennas for sidelobe reduction", IEEE Trans. on Antennas and Propagation, vol. AP-26, No. 2,pp 198-4, March [5] B. P. Ng, M. H. Er, and C. A. Kot, Linear array aperture synthesis with iniu sidelobe level and null control, Inst. Elect. Eng. Proc.- Microw. Antennas Propag., vol. 141, no. 3, Jun [6] K. -K. Yan and Y. Lu, Sidelobe Reduction in Array-Pattern Synthesis Using Genetic Algorith, IEEE Transactions On Antennas And Propagation, vol. 45(7), pp July [7] Majid M. Khodier and Christos G. Christodoulou, Linear Array Geoetry Synthesis With Miniu Sidelobe Level and Null Control Using Particle Swar Optiization, IEEE Trans. Antennas Propagat, Vol. 53, No.. 8, pp , August 05. [8] Peter J. Bevelacqua and Constantine A. Balanis, Miniu Sidelobe Levels for Linear Arrays, IEEE Trans. Antennas Propagat., Vol. 55, No. 12, Dececer 07, pp [9] Keen-Keong Yan and Yilong Lu, "Sidelobe Reduction in Array-Pattern Synthesis Using Genetic Algorith", IEEE Trans. Antennas Propagat. Vol. 45, No. 7, July 1997, pp [10] Francisco J. Ares-Pena, Juan A. Rodriguez-Gonzalez, Eilio Villanueva-Lopez and S. R. Rengarajan, "Genetic Algoriths in the Design and Optiization of Antenna Array Patterns", IEEE Trans. Antennas Propagat., Vol. 47, No. 3, March 1999, pp [11] Diogenes Marcano and Filinto Duran, "Synthesis of Antenna Arrays Using Genetic Algoriths", IEEE Antennas Propagat. Magazine, Vol. 42, No. 3, June 00, pp [12] B. Preetha Kuar and G. R. Branner, "Design of Unequally Spaced Arrays for Perforance Iproveent", IEEE Trans. Antennas Propagat., Vol. 47, No. 3, March 1999, pp [13] D. Mandal, S. P. Ghoshal, and A. K. Bhattacharjee, Deterination of the Optial Design of Three-Ring Concentric Circular Antenna Array Using Evolutionary Optiization Techniques, International Journal of Recent Trends in Engineering. vol. 2, No. 5, pp ,