Microsoft PowerPoint - 제4장_Further Array Topics_4차수정

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1 목차 제 4 장. Further Array Topics 4-1. Scanning antennas. 4-. Case study Conformal Microstrip phased array for Aircraft test ; ⅰ) Microstrip array design ⅱ) Phase shifter design 4-3. Frequency scanning antenna Orthogonal beams and multiple beam former (MBF) Butler Matrix What is adaptive array? - 1 -

2 제 4 장. Further Array Topics Advanced topics to be dealt with here. - Scanning antennas. i) phased array antenna. ii) frequency scanning antenna. - Orthogonal beams and multiple beam formers. - What is adaptive array? 4-1. Scanning antennas i) Phased array antennas Large antenna systems are difficult to mechanically scan in a rapid fashion, and because of this, electronically scanned arrays have been developed. These arrays, which may have several thousand elements, are scanned by incorporating either ferrite or diode phase shifter in each feed line. - -

3 - Standard configuration for a linear array with a corporate or parallel amplitude distribution network, and a phase shifter for each array element. input port corporate (parallel) feed network. Key ingredients ; i) power divider circuit. ii) phase shifter to vary the array phase distribution, for example, to steer the beam electronically

4 4-. Case study-conformal Microstrip phased array for Aircraft Test with ATS-6. (ref; G.G. Sanford, Conformal microstrip phased array for aircraft tests with ATS-6. IEEE Trans. AP. vol. 6, no. 5, pp , Sept. 1978) ; One of the primary experiments planned for the ATS-6 satellite was an aircraft location and communication experiment. Design goal ; A single phased array mounted on one side of the aircraft satisfying the required specifications. antenna control unit Preamp & receiver antenna unit - 4 -

5 Requirements for the array; - operation from to MHz ; - VSWR <.1 referenced to 5Ω ; - fan-shaped beam, narrow in elevation plane and wide in azimuth plane; - 1 dbi right-hand circular gain; - right-hand circular polarization with 3 db maximum axial ratio; - sidelobes minimized in ocean multipath direction; - 3 bit digital phase shifter resolution ; and - aircraft operation at temperatures between +5 C and -5 C, altitude to 5 feet, humidity to 1 percent relative, and vibration levels characteristic of the mounting area. In order to make the array practical for commercial application, it must conform to four qualitative requirement - producible in quantity at low cost; - void of any significant effect on the aircraft structural or aerodynamic characteristics; - easy to install and replace on the aircraft ; and - inexpensive to operate and maintain

6 1 Selection of the radiation element. -conformal. Most suitable type. microstripantenna. 33 6dB 3 1dB 3 4dB 6 9dB ( 칸 : 5dB) 단일 patch 의 E-plane radiation pattern. (y-z plane) 마이크로스트립안테나구조와복사패턴의일례. z slots formed with respect to ground plane feed line y B slot A slot B feed point cooper ground plane dielectric radiating electric field element configuration A cooper ground plane Two slot model of the rectangular microstrip patch antenna chosen as a radiating element. x - 6 -

7 square patch resonant at design frequency l + 4 power divider l feed point The two orthogonal modes must be excited equally, in phase quadruture. (right handed circular polarization) cooper ground plane Dielectric substrate. Circularly polarized microstrip patch antenna

8 Array design - Suppression of grating lobe D (1 + sin θ ) 여기에서 D = separation = wavelength substrate (Teflon fiberglass) patch element θ 5 5 θ = max. beam steering angle maximum steering angle (from broadside) φ ground plane D antenna 8-element μ-strip patch array. max. beam 9.5 steering angle 9.5 주의 ) D ; 장에서는 9.5 ( 1+ cosφ ) 9.5 beam9 beam8 beam7 9.5 beam6 9.5 beam5 beam4 beam beam beam max. beam steering angle maximum beam steering angle θ = 5 이므로 D =.566 (1 + sin 5 ) D =.5 로선택. φ 로각도표시 horizon 1ⅹ8 linear array로선택하고 radiation pattern이 9-beam position beam이되도록설계.( 아래그림참조 ) - 8 -

9 H-plane pattern z y 11 (y-z) plane pattern 15 (x-z) plane pattern E-plane pattern x a. pattern of the element b. pattern of the linear array 1 5 overlap principle plane (E-and H-plane) pattern을 beam이되도록선택. antenna 15 3dB HPBW (half power beam width) beam6 beam5 beam4 beam3 beam center beam beam beam position Horizon [3dB HPBW] 그림에서와같이, beam width가15 인beam이 1 5 overlap 되어있다 Array antenna Horizon - 9 -

10 3 phase shifter design 1 - Constant phase progression with 9 steering increments and.5 spacing between elements. - With eight-element array, it is possible to select three bits for each element that will provide the desired phase progression. - The phase shifter for each element has three of the following bits : 3, 6, 1, and The arrangement of these bits and the resulting phase progression for each beam position are shown in the table below

11 Effective 3 phase resolution using 3-bit phase shifters BEAM POSITIONS Element Number Phase Bits Required (3 6 1) +18* (3 6 1) +18* * Fixed 18 phased shifter

12 - phase shifter circuit selection i) 3 bit ~ a loaded line phase shifter ~ 4 effective length of stub 4 Effective R. F. ground ( D. C. Bias point) open circuit The additional phase shift due to the capacitance of the back biased diodes is compensated by adjusting the stub lengths empirically. disadvantage) large I R loss for the diodes in the forward bias state of the pin diode

13 ii) 6, 1, and 18 bits ~ switched line phase shifters short to ground 4 short to ground - The phase shift difference between 4 D.C. bias point 4 L and L. 1 L 1 L L 1 - should be as short as possible for 6 and 1 bits. (Length for the 18 bit is less critical) L 1 D.C. bias point 4 Note) phase shifter circuit 1 Ohm characteristic impedance ⅰ) reduction of insertion loss ( 다이오드저항대선로특성임피던스비낮춤 ). ⅱ) power divider network design 용이. ⅲ) 선로폭을 reasonable하게유지할수있다는점

14 ; diode ; short to GND ; DC bias photograph of the completed array along side an unfinished antenna board showing the entire circuit. -gain 1dBi - polarization axial ratio<db for most beam positions and frequencies. -VSWR<1.8 power dividing circuit with desired fixed phase progression

15 4 Detailed view on the 8th element 8 DC GND OC DC GND OC Loaded line phase shifter for the 3 bit DC bias open circuit switched line phase shifter for the 6 bit switched line phase shifter for the 1 bit X ; DC bias. ; short to GND. note) 사용공간을줄이기위하여 open stub의뒷부분의 bending구조채택. - 6 bit와 1 bit 사이의 ground는공통으로구성. - pin diode 개로구성되어있는 bit는 3 phase shifter임

16 실제장착구조 ; antenna control unit Preamp & receiver antenna unit phased array location Horizon 41.5 정면도 확대된그림

17 Radiation pattern (electronically steerable in elevation) dB 3 7dB db dB 3 7dB db measured radiation patterns in elevation for two representative beam positions. Antenna test ; performance in terms of bit-error rate(ber), multipath rejection, ranging, and voice intelligibillity

18 4-3. frequency scanning antenna. antenna elements power 가 feed line 을진행해가면서 array element 에 coupling 되어원하는 amplitude distribution 을만들어준다. input port series distribution network FS(frequency scanning) antenna 주파수에따라 main beam 방향이변화되는 antenna. 이웃한소자간 r.f path length 는어떤특정주파수 에서 1 파장라고하면, 주파수 f 에서각 f 소자들은동상 (co-phased) 이되어 broadside beam을복사

19 d θ beam direction radiating element couplers 파장이이고주파수가 f ( f ) 인경우, feed input S sinuous feed terminating load 이웃한소자간의 phase difference φ 는 φ = π 1. General schematic diagram of a series fed frequency-scan antenna. 여기에서 s = 가되어 broadside beam 형성. f = f 이면 소자간간격이 d 이고위상차가 φ ( > ) 일때, 그림에서와같이 broadside 방향에서 θ 만큼편향 (deflected) 되었다고하면 Note) φ > 이려면 <, 즉 f > f 가되고그림에서 θ < 가된다. π 1 ( φ d) ( φ d) φ ( ) sin θ = = = = =. k π πd πd d

20 sinθ = 로부터 d 파장 ( 또는주파수 ) 이변함에따라 maximum beam direction이변화함을볼수있다. 이러한효과는활용분야에따라장단점이될수있다. 먼저 frequency scanning antenna로사용할경우, phase shifter도필요없고급전방식도직렬로서간단하며회로공간도대폭축소된다. Space for circuitry 반면에전파의복사방향이고정되어사용되는경우, 중심주파수 ( f ) 에서벗어나면요구되는복사방 향에서벗어난다. 요구되는복사방향으로부터반치각 (HPBW) 방향으로향하게되는경우, 신호출력레 벨도 3dB 떨어지므로, system bandwidth 에중대한제약이된다. - -

21 Δ f ex) 매우좁은빔폭이요구되는 array antenna가있다고하고, f = f + 에 beam이 broadside 방향에서 α 의각도만큼편향 (deflection) 된다고하면 ( ) sinα α = 1 d d f f f = 1 f = d f d f f = ( f 근처에서, f = f) f d Δf Δf = f f f d

22 만일 d = 이고 beamwidth가 라고하면, α = ± 1 일때, 신호출력은 3dB 감소된다. 이경우 Δ f 1 π α =± = f d 18 Δ f π 1.8%. f 18 =1 - 빔폭이매우좁고, 고이득 (high gain) 이요구되는경우, 매우큰 phased array가요구될수있다. 고이득의빔은 reflector antenna를사용하면용이하게얻을수있는데이렇게되면 mechanical scanning을사용해야한다. 이런경우, 비교적넓은범위의 scan angle을지닌소형 phased array와대형 reflector 안테나를결합하여 high gain을유지하면서제한된각도범위에서의 scanning(limited sector scanning) 이가능한 compromise arrangement를사용할수있다. - -

23 main-reflector aperture region system focal plane θ θ a i = M θ θ hba hbi = M f m f s sub-reflector θ hba θ a array feed Limited sector scanning Note) feeding array 만으로복사할때의 aperture 에비하여 imaging arrangement (sub-reflector + main reflector) 를사용하는경우의 effective aperture 가훨씬크다. 1 1 만일 M배로커진다면 beam width 도로줄고 scan angle도로줄게된다. M M - 3 -

24 4-4. Orthogonal beam and multiple beam formers(mbf) Radio Education and Research Center 아래그림은 multiple beam former(mbf) 와관련되어사용되는 linear array 를나타낸다. Beam 1 Beam 주어진주파수에대하여, 고정된방향으로형성된 beam들이동시에있기때문에각 beam port에 receiver가연결되어있으면 Beam forming network 동시에여러방향으로부터오는신호들을 수신할수있다. A B multiple beam former(mbf) - 4 -

25 MBF의구성 : - power divider/combiners at each array element. separate networks for each independent beam. - multiple beams이서로 orthogonal 하면 simultaneous MBF 구성가능. zero mutual coupling between beam ports. Beam 1 Beam ~ port A 에단위전압인가시 ; 전계 pattern EA ( θ ). Beam forming network ~ port B 에단위전압인가시 ; 전계 pattern EB ( θ ). A B - 5 -

26 - port A, B를동시에구동한경우의전계의 radiation pattern, E ( θ ) + E ( θ ); A, B port 간에 coupling 이없는경우, [ ( ) + ( )] [ ( ) + ( )] * E θ E θ E θ E θ dθ A B A B * * [ ] = EA( θ ) + EB( θ) EA( θ) + EB( θ) dθ * * * * A A s B B A B A B = E ( θ ) E ( θ) dθ + E ( θ) E ( θ) dθ + E ( θ) E ( θ) dθ + E ( θ) E ( θ) dθ. 여기에서 beam이서로 orthogonal 하기위한조건은, 즉이되려면 * B E ( θ) E ( θ) dθ =. A A B * { E ( ) ( ) } * A θ E B θ dθ { } * * * A B A B E ( θ) E ( θ) dθ + E ( θ) E ( θ) dθ = - 6 -

27 ex) aperture의길이가 L 이고 aperture distribution이 f ( x) 인경우, 전계의 radiation pattern ; L L E ( S sin θ ) f ( x )exp( jkxs ) dx = = beam 의 peak 가각각 g ( x) = f( x)exp( jkxsin α) A g ( x) = f( x)exp( jkxsin α). B S = sin α, s= sinα 인 pattern을주는두 aperture distribution은 ga ( x ) 와 gb ( x) 에의한두 beam이 orthogonal 하려면 E ( S) E ( S) ds. A B (A) 여기에서 L E ( S ) = f ( x )exp{ jkx ( S + sin α)} dx A L L E ( S ') = f ( y )exp{ jkx ( S sin α)} dy B L (B) (B) (A) 대입 ; - 7 -

28 L L * f ( x) exp{ jkx( S + sin α)} dx f ( y) exp{ jky( S sin α)} dy ds L L E ( s) E ( s) L L * L L A = f ( x) f ( y)exp{ jks( x y)} exp{ jk sin α( x + y) dxdy} ds B 적분변수 S 에대하여성립하는관계식인 exp( jsz) ds = δ ( z) 를이용하여먼저적분을행하면 exp{ jks( x y) ds = δ( kx ky) = δ{ k( x y)}. L/ L/ * L/ L/ f ( x) f ( y) exp{ jks( x y)} exp{ jk sin α( x + y)} dx dy ds L/ L/ * α L/ L/ = f ( x) f ( y) exp{ jk sin ( x + y)} exp{ jks( x y)} ds dx dy. δ{ kx ( y)} - 8 -

29 L L * L L = f ( x) f ( y)exp{ jk sin α( x y)} δ{ k( x y)} dy + dx * * f ( x)exp{ jksinα x) = f ( x)exp{ jkxsin α) L * = f( x) f ( x) exp( jkxsin α) dx= L regarded as the radiation pattern corresponding to an aperture distribution equal to the square of the modulus of the basic aperture distribution replacing S f ( x), with sinα, the beam spacing, If the spacing of the two beams corresponds to a zero of this radiation pattern, then the two beams are orthogonal

30 ex) The case of a uniform distribution [ f ( x) = 일정 = 1 ] over an aperture of length L. f ( x ) ( z) θ L L sin( π sl / ) ( π sl / ) SLL = 13dB f ( x ) = 1 uniform distribution x 이경우의 aperture source distribution에의한 basic radiation pattern ; sin( π SL / ). ( πsl / ) sin( π SL / ) 에서 first zero는그림에서와같이 ( π SL / ) S = sinθ =± 가되고 SLL은 S 가대략 ~ 의중간지점인 3 S = 에서 L L L L L s L 3 3π sin π sin L = = L 3 3π π 3π L 이므로, - 3 -

31 sin( π sl / ) ( π sl / ) 1 jkxsinα 만일 f ( x ) = 1에 e 에해당되는위상변화를준다면, 이때의 aperture source distribution은 jkx sinα = f ( xe ) f A, =1 jkxsinα 마찬가지로 e 에해당되는위상변화를준다면, jkx sinα f ( xe ) = f B. =1 L L L L 3π s SLL ( sidelobe level) db = log( ) 13[ db] 3π 이때, f 와 f 에대한 field radiation pattern은각각, A B

32 L jkxsinα jkxs L E ( S) = f( x) e e dx A L jkx( S+ sin α ) L = f ( x) e dx, =1 f A L jkxsinα jkxs L f B E ( S) = f( x) e e dx B L jkx( S sin α ) L = f ( x) e dx, =1 여기에서 two beams EA ( S ) 와 EB ( S) 가서로 orthogonal 하기위해서는 EA( S) EB ( S) ds = 를만족해야한다

33 서로독립적인두빔에대한가장단순한예 : EA () s EB () s s = 1 s = 1 s = sinθ 1 1 E ( S) E ( S) ds =. A B 지금다루고있는일반적인경우에대하여는 L * jkxsinα L EA( S) EB( S) ds = f( x) f ( x) e dx 으로부터 E ( S) E ( S) ds = A B 이되기위하여는 L * jkxsinα L f( x) f ( x) e dx= 이되어야하는데 f ( x ) = 1 인경우,

34 π L sin sin L α jkxsinα e dx = 로부터 L π L sinα sinα =± n 일때, 즉 sin α = ±, ±, 일때, L L L angular spacing EA( S) EB ( S) ds = 가됨을볼수있다. Radio Education and Research Center L 4dB corssover -13dB first sidelobe sin x Orthogonal beams x 옆의그림은 angular spacing S 가 S = ±, ±, 일때, L L EA( S) EB( S) ds 을만족하는서로 orthogonal한빔들을도시 하고있다. note) 한 beam 의 peak 는다른 orthogonal beam 의 zero 에해당. s Caution) 이는일반적으로성립하지는않는다. ex) f ( x) cos π x = L

35 Note) 임의의 aperture source distribution 에대하여 orthogonal beam set 가언제나존재하는것은 아니다. 아래그림의예와같이 triangular aperture source distribution 의경우, orthogonal beam 관계를줄수있는 beam 간의 angular spacing 이존재하지않는다. Illumination f ( x) Radiation pattern and sidelobe level due to f x ( ) Angle to first zero Radiation pattern for f ( x) Spacings and crossover level for orthogonal beams 6dB L ( No zeros ) Physical explanation : 위의예에서와같이, triangular aperture source distribution의경우, Field radiation pattern f x 의 sidelobe들이모두동상 (same phase) 이므로, 그러한 sidelobe를갖는두빔패턴의 cross product, 즉 E s E s ds는평균적으로 이될수없다. E s E s ds A( ) B( ) A( ) B( ) ( )

36 Multiple beam formers for linear arrays 3 well-established types ⅰ) Butler matrix Ref : J.Butler and R. Lowe, Beamforming matrix simplifies design of electronically scanned antennas, Electronics Design, 9, P.17, ⅱ) Maxon matrix. Ref : R. Hansen (Ed), Microwave scanning antennas, vol. III, Academic press, London, chapter ⅲ) Rotman lens. Ref : W. Rotman and R.F. Turner, Wide angle lens for line source application, IEEE Trans., AP-11, PP , 1963 본강의에서는대표적인예로서 Butler matrix에대하여살펴보기로한다

37 4-5. Butler matrix. Radio Education and Research Center N 개의 input ports 중에하나의 input port를급전 ( 구동 ) 하여, 그에상응하는, 복사각도가서로다른 N 개의 beam 을복사할수있도록구성한 feed system 을 beam-forming matrices 라하는데, 가장 대표적인예가 Butler matrix 이다. Beam-forming matrices = hybrid junction + fixed phased shifter Combination. ex) two-element array with beam-forming matrix. A 9 θ d = 4 φ 1 B Simple example of Butler beam-forming matrix Hybrid junction + 9 phase shifters Magic tee hybrid junction

38 - Properties of the hybrid junction Hybrid Magic T junction. θ ⅰ) ports (1 and 4, and 3) are uncoupled. ⅱ) port 1 ⅲ) port 4 port port 3 In-phase transmission port port 3 Out of phase transmission ( 18 차이 ) A 9 d = 18 4 φ 1 B 먼저 port 1을구동한경우, magic T의 port 와 3으로동상으로분배된후, 안테나소자 A로진행해가면서 9 의위상 변위를겪으므로, 안테나소자 A에서의복사파는안테나소자 B에서의복사파비하여위상이 9 늦다

39 따라서아래그림과같이왼쪽으로 45 기운방향으로복사된다. A 45 l = 4 B ; path-length delay l for maximum beam direction for port 1 beam. A 9 φ θ 그런데 port 4를구동하게되면 magic T의 port, 3으로 18 B 와 의위상차이가있어서, 즉 port 로결합된파의위상이 = port 3으로결합된파의위상보다 18 앞서는데, port 쪽 에 9 의위상변위기에의하여안테나소자 A의위상이 B에 18 4 비하여 9 앞서게되기때문에오른쪽으로 45 기운방향으로 1 복사가이루어진다. d Port 1 beam Port 4 beam 따라서 port 1과 4를각각구동한경우에형성되는 beam을도시하면다음과같다. Port 1과4는uncoupled 되어있기때문에두빔은독립이되어별도로동시에존재한다

40 지금까지다룬 -element array에포함된원리들은 array element의개수가 array 에적용될수있다. ex) Butler beam-forming matrix for a four-element array. ant out in N (N : integer) 인모든 Transmission phase relationships for a 9 phase-lag hybrid junction. ant 먼저 port 1 을구동한경우, 각 array element 의 relative aperture phase distribution 에대하여생각해보자. Ant. element 1 ; 45 9 = 45 Ant. element ; 9 9 = Ant. element 3 ; 45 = driven input Ant. element 4 ; 9 = port 1 구동 ;, 135, 7,

41 ant Ant. element 1 ; Ant. element ; = Ant. element 3 ; Ant. element 4 ; = port 구동 ;, 45, 9, 135. driven input ant Ant. element 1 ; Ant. element ; Ant. element 3 ; Ant. element 4 ; = = 45 9 port 3 구동 ;,45,9, driven input

42 ant Ant. element 1 ; 9 Ant. element ; Ant. element 3 ; Ant. element 4 ; 9 9 = = port 4 구동 ;, 135, 7, 45 driven input Relative aperture phase distribution. 3 port 1 ;, 135, 7, port ; port 3 ;, 45, 9, 135, 45, 9, port 4 ;, 135, 7, 45 four separate beams produced

43 4-6. What is adaptive array? - Adaptive array antenna systems are still currently the subject of intense interest and investigation/development for radar and communications applications. - The principal reason for the interest is their ability to automatically steer nulls onto undesired source of interference, thereby reducing output noise and enhancing the detection of desired signals. - These systems usually consist of an array of antenna elements and a real-time adaptive receiver-processor which has feedback control over the elements weights. 아래그림은 adaptive array의개념도를도시하고있다. 각 antenna element로부터의수신신호들을 Phase shifter와 attenuator를사용하여 weighting 과정을거친후 summation하게된다. B ' S u B B 3 1 B ( ) W = Aexp jϕ n S 1 A 1 ϕ 1 S A ϕ S N A N ϕ N Array Processor B 3 θ B 1 S N = SnW 1 n Optimization criterion Adaptive array and resulting pattern

44 Simple example : two-element array B( jamming θ ) θ B θ B Ant. Ant. a S S1 S Phase Attenuator shifter W = Aexp( jϕ ) Two-element adaptive array and jammer B. 여기에서 Ant.: omnidirectional antenna. S 1 S : signal received by the first antenna element. : signal received by the second antenna element. ( π ) S = S1 exp j a sin θb k 이신호 S 가, 위상이 ϕ j 이고감쇄에상응하는계수A 로서정의되는계수 W = Ae ϕ 에의하여 weighting 이된다면, S = S + WS B

45 Jammer로부터의 signal S B 를상쇄시키려면 ( ) ( ) SB = S1 + WS = S1 + Aexp jϕ S1exp j πa sinθb W S = S1 1+ Aexp( jϕ) exp( j( πa ) sinθb ) = 이로부터 Aˆ ( a ) = 1, ˆ ϕ = π π sinθ B jπ ( ˆ ) ( ) ( ) Wˆ = Aˆ exp jϕ = e exp j πa sinθb = exp j πa sinθb 이러한 weighting 과정에대하여, 임의각도 antenna pattern = radiation pattern) 는 ( ) ( ) = + ˆ ( ) S θ S1 1 W exp j πa sinθ θ 로서입사하는전파에대한수신신호 (receiving F ( θ ) θ ( ) 의함수로서의 received signal은다음과같은 characteristic function F θ 에비례, ( ) ˆ ( ) ( ) ( ) ( ( ) ) F θ = 1 + W exp j πa sinθ = 1 + exp j πa sinθb exp j πa sinθ ( πa )( θ θ ) = 1 exp j sin sin B 따라서 radiation pattern F ( θ ) 를구해보면아래그림과같이 Jammer 방향으로 zero가된다. Wˆ

46 B F e α π a j Note) ( θ ) = 1, α = ( sinθ sinθ ) j( α ) j( α ) j( α ) j α j( α ) j( α F = e e e = e 1 e e ) ( θ ) j 1 1 ( ) ( ) ( ) ( α ) ( 1) ( j) sin( α ) = e Optimized pattern. B sinθ B ( ) τ = sinθ π a F ( θ) = sin( α ) = sin ( sinθ sin θb )

47 Concluding remarks - Let s review what we have discussed in Chapter. 4 제 4 장. Further Array Topics 4-1. Scanning antennas. 4-. Case study Conformal Microstrip phased array for Aircraft test ; ⅰ) Microstrip array design ⅱ) Phase shifter design 4-3. Frequency scanning antenna Orthogonal beams and multiple beam former (MBF) Butler Matrix What is adaptive array?

THE JOURNAL OF KOREAN INSTITUTE OF ELECTROMAGNETIC ENGINEERING AND SCIENCE Sep.; 26(10),

THE JOURNAL OF KOREAN INSTITUTE OF ELECTROMAGNETIC ENGINEERING AND SCIENCE. 2015 Sep.; 26(10), 876 884. http://dx.doi.org/10.5515/kjkiees.2015.26.10.876 ISSN 1226-3133 (Print) ISSN 2288-226X (Online) Design