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Performance Evaluation of Array Antennas International Journal of Modern Engineering Research (IJMER) www.ijmer.com Vol.1, Issue.2, pp-510-515 ISSN: 2249-6645 PERFORMANCE EVALUATION OF ARRAY ANTENNAS K.Ch.sri Kavya1 , Y.N.Sandhya Devi2 ,G.Sudheer Kumar2 , Narendra Neupane2 1(Associate Professor, Dept. of Electronics and Communication Engineering, KL University.) 2 PERFORMANCE(B.Tech Scholars, Dept. of EVALUATION Electronics and Communication OF Engineering, ARRAY KL University.) ANTENNAS ABSTRACT The purpose of the paper is to present a proper This paper will presents an optimization technique to reduce the side lobe level in the case normalization technique to obtain a low side lobe of linear array antennas. There several methods level and to avoid loss in main lobe directivity. are introduced for the side lobe reduction .But always the basic trade-off occur when II.METHODOLOGIES implementing amplitude weighting functions is that a trade between low side lobe levels and a loss A. Uniform Illumination method in main beam directivity always results . Here we Equal illumination at every element in an array made a comparison of three methods namely referred to as uniform illumination, results in uniform illumination method , Taylor line source directivity patterns with three distinct features. attenuation method and Taylor line source using Firstly, uniform illumination gives the highest redistribution method .The paper also presents aperture efficiency possible of 100% or 0 dB, for any different source distributions and their respective given aperture area. Secondly, the first side lobes for directivity patterns. a linear/rectangular aperture have peaks of approximately –13.1 dB relative to the main beam Keywords: Amplitude weighing, uniform peak; and the first side lobes for a circular aperture illumination, array antennas, normalization. have peaks of approximately –17.6 dB relative to the main beam peak Thirdly, uniform weighting results I.INTRODUCTION in a directivity pattern with the familiar sinc(x) or Array antennas offer a wide range of sin(x)/x where x=sin(θ) angular distribution, as opportunities in the variation of their directivity shown in Figure. patterns through amplitude and phase control. Through the use of individual amplitude and phase B. Directivity Pattern Calculations control, array antennas offer a wide range of The directivity pattern calculations given by directivity pattern shape implementations to the Hansen [2] and Raffoul and Hilburn [4] are become antenna designer .Synthesis of linear array antennas confusing even though the calculations are not has been extensively used in the last decades.[9]-[10] complex. .Common optimization goals in array synthesis are The below equation presents the calculation the side lobe suppression and null control to reduce for the voltage directivity pattern for a linear array of interference effects. High directivity antennas have N elements of isotropic radiators, where Δx is the defined main beams whose widths are inversely inter-element spacing, and an is the amplitude of proportional to their aperture extents. High directivity element n. Note that this equation is for the simplest antennas also have side lobes, which are often array case of uniform phase for that of a broadside undesirable as they may permit reception of energy fixed beam array. from undesired directions. The energy from the 2훱 푁 −푗 ( 푛훥 xsin 휃) undesired directions may contain interfering sources E(θ)= 푛=1 푎푛 푒 휆 (1) such as multipath or even deliberate jammers. The term uniform illumination is often used Use of these amplitude weighting functions to describe the array amplitude distribution when the have a well known effect on the peak of the main amplitude of all the elements is equal. If the voltage beam of the directivity pattern. The amplitude amplitudes all equal one Volt, the peak voltage tapering for side lobe reduction reduces the spatial 퐸푝푒푎푘 , for the ideal linear array of isotropic elements efficiency (or aperture efficiency) of the antenna. occurs when θ is zero and has a value given by Along with the reduction of peak directivity, Equation amplitude tapering also results in a broadening of the main beam. www.ijmer.com 510 | P a g e International Journal of Modern Engineering Research (IJMER) www.ijmer.com Vol.1, Issue.2, pp-510-515 ISSN: 2249-6645 퐸 푁 III. TAYLOR SYNTHESIS 푝푒푎푘 = 푛=1 푎푛 Since the 1940s, numerous researchers have contributed varying approaches for synthesizing If 푎푛 =1 amplitude distributions for the purpose of side lobe 푁 reduction. For this discussion, we will use the Taylor 퐸푝푒푎푘 = 푛=1 1 = 푁 (2) distributions as they are arguably more commonly used for array antenna pattern synthesis The Taylor UNIFORM ARRAY AMPLITUDE DISTRIBUTION yields an optimum compromise between beam width 2 and side lode level .The Technique introduced by 1.8 Taylor to pattern whose first few main lobes(closest to main lobe) are maintained at an equal level .The 1.6 remaining side lobe levels monotonically decreases 1.4 [5].The details of the analytical formulation are complex .They are presented in the literature [2][6]. 1.2 Taylor published his synthesis technique for 1 linear/rectangular [2] and circular [1] apertures in 0.8 1955 and 1960, respectively. This method also Element amplitude presents the same directivity pattern as that of Figure 0.6 in uniform case , except that a Taylor amplitude 0.4 weighting has been employed to reduce the near in side lobes . The 푛 parameter is used to define how 0.2 many near-in side lobes are held constant at the 0 desired amplitude level. For further detail on this -1.5 -1 -0.5 0 0.5 1 1.5 Element position (wavelength) parameter refer to Taylor [1-2]. The Normalized line source which yields the desired pattern is given by Figure 1.Plot of the uniform amplitude distribution for the eight element array 휆 푛 −1 푧′ I(z’)= 1 + 2 푆퐹 푝, 퐴, 푛 cos 2훱푝 (3) 푙 푝=1 푙 The coefficients SF (p,A,푛 ) represent samples for Taylor pattern and the can be obtained by Directivity pattern With Uniform Weighing 0 푆퐹 푝, 퐴, 푛 2 푛−1 2 -10 푛 − 1 ! 훱푝 1 − 푝 < 푛 = 푛 − 1 + 푝 ! 푛 − 1 − 푝 ! 푢푚 푚=1 -20 0 푝 ≥ 푛 The Taylor space factor is given by -30 2 푛 −1 푢 푛 =1 1− sin 푢 푢푛 Directivity pattern(db) -40 SF(u,A,푛 )= 2 (4) 푢 푛 −1 푢 1− 푛=1 푛훱 -50 푙 Where u=Πv=Π cos 휃 휆 -60 -100 -80 -60 -40 -20 0 20 40 60 80 100 푙 Angle in degrees 푢 =Π푣 =훱 cos 휃 푛 푛 휆 푛 Where 휃푛 represents the locatons of the nulls. From Figure 2.Directivity pattern of a linear array of eight the Figures 2 to 4, the directivity patterns of both are elements with uniform amplitude using MATLAB normalized to zero dB. As these patterns are not software normalized to a consistent peak, this is limited to no value in assessing the efficiency loss trade-off with side lobe reduction levels. www.ijmer.com 511 | P a g e International Journal of Modern Engineering Research (IJMER) www.ijmer.com Vol.1, Issue.2, pp-510-515 ISSN: 2249-6645 A. Attenuation Method ARRAY AMPLITUDE DISTRIBUTION USING TAYLOR DISTRIBUTION The attenuation method is analogous to 1 achieving the amplitude taper by increasingly resistively attenuating the field energy for radiators 0.95 toward the periphery of the array to achieve Voltages 0.9 less than 1. But, this method is the least efficient, and 0.85 the main beam gain loss is the greatest. Consider the example of a linear array of eight elements having 0.8 element patterns and an inter-element spacing of 0.75 0.695 λ. 0.7 The following series of plots were calculated 0.65 using Equation1, using software written in Element Amplitude(Voltage) MATLAB. Routines written to calculate the 0.6 amplitude weighting coefficients for array side lobe 0.55 reduction usually provide an for each element in 0.5 Voltage form. Not always, but often, the routines are -1.5 -1 -0.5 0 0.5 1 1.5 written to provide a maximum value of 1, The Taylor Element position (Wavelength) Figure3.Plot of the Taylor -20db 푛 =3 amplitude Voltages calculated for this amplitude illumination distribution. function are (from the outer elements to the center) 0.5828,0.7283,0.9147,1 from the figure 3.Where as in the case of uniform illumination every element have Directivity pattern of Array Using Taylor distribution the amplitude is equal to 1V. 0 This approach is further illustrated in Figures 5,6 -5 where the amplitude distributions and resulting -10 directivity patterns are presented for the range of Taylor weightings from –20 to –65 dB. The -15 attenuation method predicts very significant main beam pattern losses -20 -25 ATTENTUATION ARRAY AMPLITUDE DISTRIBUTION Directivity pattern(db) 1 -30 0.9 -35 0.8 -40 0.7 -80 -60 -40 -20 0 20 40 60 80 Angle in degrees 0.6 Figure4.The plot of the directivity pattern of the linear array using Taylor -20db 푛 =3amplitude for 0.5 near inside lobe reduction. 0.4 Elememt amplitude IV.NORMALIZATIONS 0.3 0.2 There are two physical methods for 0.1 generating amplitude distributions for array antennas. 0 Amplitude tapers can be created by either -1.5 -1 -0.5 0 0.5 1 1.5 redistributing the power among the elements or by Element position(wavelength) attenuating the power for the outer elements. With Figure 5.Plot of amplitude distributions for Taylor the attenuated method, power removed at the outer functions of -20db to -65db using the attenuation elements is attenuated in ohmic losses.
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