II. PHYSICAL OPTICS (PO) METHOD

Transcription

1 IN: IO 9001:2008 Certfed Internatonal ournal of ngneerng and Innovatve Technology (IIT) Volume 2, Iue 2, Augut 2012 A Novel Method for RC Reducton of a Complex haped Arcraft Ung Partal RAM Coatng A. Upendra Raju, yoth Balakrhnan Abtract - In th paper, a novel method of partal coatng to reduce the Radar Cro ecton (RC) of a complex haped perfectly electrcally conductng (PC) arcraft, of electrcally large ze, propoed. The phycal optc (PO) method ued to dentfy the hot pot of the PC arcraft. An emprcal procedure then propoed to coat partal contguou area wth Radar Aborbent Materal (RAM). The PO then modfed to compute the RC reducton of the partally coated arcraft. The level of coatng can be adjuted o a to obtan the requred reducton n RC. It hown that, wth an approprate choce of the partal coatng level, the RC can be reduced to the ame level a that of a fully coated arcraft, over a wde frequency range. The weght, volume and thu cot reducton through the propoed partal coatng technque are alo quantfed. Index Term - RC (Radar Cro ecton), PO (Phycal Optc), RAM (Radar Aborbng Materal), PC (Perfectly lectrcally Conductng), Partal Coatng. I. INTRODUCTION RC reducton ha been an mportant topc amongt reearcher becaue of t mportance n mltary applcaton for ncreang urvvablty by reducng the detectablty of bode. Technque uch a hapng, actve loadng, pave loadng, and RAM coatng are ued to reduce detectablty. Thee method are normally employed n tealth technology for RC reducton [1, 2], and among them RAM coatng the mot wdely ued method [1]. RC may be determned by expermental meaurement, but uch a procedure may not be practcal and applcable for all angle of obervaton and object dmenon [3]. Therefore, numercal electromagnetc technque are requred for the computaton of RC. The electrcal ze of a practcal arcraft large and hence the ue of low frequency technque uch a Method of Moment (MoM) and Fnte lement Method (FM) are nfeable. Depte a dramatc mprovement n the peed and torage capacty of computer n recent year, the practcal ze of the object that can be handled va the MoM approach tll of the order of about 25 or le. ven method uch a the Fat Multpole Method (FMM) are computatonally neffcent [4]. The hgh frequency technque, though computatonally mpler, offer the dered accuracy n RC predcton. They can alo be modfed to handle both metallc and coated urface n the ame complex haped body [5,6]. There are many hgh frequency RC predcton technque avalable n lterature [7]. The mplcty of hgh-frequency predcton method due to the aumpton that each part of the body eentally catter energy ndependent of all other part. Therefore, the feld nduced on a porton of the target are due only to the ncdent wave and not the energy cattered by other 52 part. Th make t relatvely mple to etmate the nduced feld and to ntegrate them over the body urface to obtan the far cattered feld and, therefore, the RC. Th paper deal wth the RC analy where t mplctly aumed that the radar frequency hgh enough uch that the correpondng wavelength mall compared to the phycal dmenon of the catterng body. electng an approprate method for RC computaton depend on the man objectve behnd the catterng calculaton, the computng equpment, and the geometry of the target, t electrcal length and t conductvty [4]. Hgh frequency aymptotc method are wdely ued to compute the RC of three-dmenonal complex target at hgh frequence, becaue of ther ablty to provde conderable phycal nght nto the dffracton mechanm and predct the RC at a reaonable computatonal cot. Phycal optc (PO) one uch hgh frequency method and utable to compute the feld cattered by complex object, nce t yeld bounded reult at any pont n pace, ncludng thoe pont where boundary layer overlap [8, 12]. PO ha been ued n th paper. II. PHYICAL OPTIC (PO) MTHOD The phycal optc (PO) method a hgh-frequency technque where the target replaced by nduced urface current that erve a the ource of cattered feld. The bac aumpton n PO are () the rad of curvature of the urface are large compared to wavelength, () current ext only n the area that drectly llumnated by the ncdent wave, and () current on the llumnated urface have the ame charactertc a thoe on an nfnte plane tangental to the urface at the pont of ncdence. PO eentally baed upon Huygen prncple, whch tate that each pont on the prmary wave front gve re to econdary wave front. Hence, a phercal wave front, a t propagate, gve re to econdary phercal wavelet. Th provde a mechanm for the bendng of wave and account for the dffracton phenomenon n the hadow regon. In PO, the target replaced by nduced current. The feld due to the radatng ource aumed to ext ndependent of the cattered. Thu, fnte feld ext n both the llumnated and hadow regon and obvouly mut alo be contnuou at the hadow boundary. The effect of the cattered manfet a a feld due to the ource nduced current on the catterer, known a the PO current.

2 IN: IO 9001:2008 Certfed Internatonal ournal of ngneerng and Innovatve Technology (IIT) Volume 2, Iue 2, Augut 2012 A. PO for A Conductng Body The PO current for a conductng body gven by, 2nˆ H 0, llumnated, hadow regon regon Where nˆ the unt normal vector of the urface potoned H outward and the ncdent magnetc feld vector? In the preence of a perfectly conductng urface, the total electromagnetc feld of a ource may be expreed a a uperpoton of the ncdent feld and cattered feld by the urface of the body. The cattered feld can be expreed n term of the radaton ntegral over actual current nduced on the urface of the cattered a [9], e r jkr ˆ ( ˆ j ) ( ), ne ˆ (1) jk( rˆ rˆ ). r (2) d. B. Po for a Radar Aborbng Materal Coated Body Radaton from a body coated wth a thn layer of aborbng materal ha been tuded by [6]. For an ncdent plane wave, the total cattered feld from a coated body gven a, Where, (3) and M M are the cattered feld due to the electrc ( ) and the magnetc current ( M ), repectvely. The expreon for, M, and M are gven detal n [6]. The RC then can be computed ung the well known formula, 2 2 lm 4 r, 2 (4) r Where and repectvely. are the ncdent and cattered electrc feld III. RADAR ABORBING MATRIAL (RAM) RAM coatng the mot wdely ued method for RC reducton [1]. Radar aborbng materal (RAM) reduce the energy reflected back to the radar by mean of aborpton. Radar energy aborbed through one or more of everal lo mechanm, whch may nvolve the delectrc or magnetc properte of the materal. Delectrc RAM ue carbon a the bac materal becaue of t poor conductvty, but thee 53 materal are not ealy appled to operatonal weapon platform and they are uually too bulky and fragle n operatonal envronment. Hence, magnetc aborber are ued wdely for operatonal ytem [7]. Alo, magnetc materal offer the advantage of compactne a they are typcally around one-tenth of the thckne of delectrc aborber and they can be effectve over a wde range of frequence. The lo mechanm n magnetc aborber prmarly due to a magnetc dpole moment. Compound of ron are the bac ngredent n thee aborber. Carbonyl ron, made of oxde of ron (ferrte) ha been ued extenvely a radar aborbng materal. Permttvty (ε) and permeablty (μ) are two properte that are cloely aocated wth the effectvene of thee aborber. They both vary conderably wth frequency n dfferent way for dfferent materal. For a coatng to be effectve, t hould have a thckne that cloe to a quarter wavelength at the frequency of nteret [14]. The ferrte layer of approxmately quarter wavelength thckne over a conductng urface act a an mpedance matchng layer a well a aborber. RC reducton ung a magnetc RAM of ntered Nckel Znc Ferrte [7] tuded n th paper. IV. CATTRR MODLING For llutratve purpoe an arcraft of length 4m, wng pan 7m and heght of 1m ued, to repreent a cale model of typcal fghter arcraft Me 163B-1. There ha been no ntenton to reproduce exactly the degn of any partcular fghter arcraft. The arcraft modeled ung 29 urface. The fuelage and noe of the arcraft modeled ung 8 curved urface and the wng and tal are modeled ung 21 flat urface. For PO applcaton purpoe, the urface of the arcraft dcrtzed nto trangular element wth 10 element per wavelength. The arcraft placed wth the noe along the X-ax and wng along the Y-ax, a hown n Fg. 1. The mulaton wa done at frequence of 1, 3, and 10 GHz, repreentng the L,, and X wave band repectvely. The larget dmenon of the arcraft,.e., the wng pan, 23, 70, and 233 at 1, 3, and 10 GHZ, repectvely. A plane electromagnetc wave, of unt ampltude and gven frequency, ncdent on the noe of the arcraft and the btatc-rc oberved n the azmuthal plane. The number of unknown, memory, CPU tme, and the charactertc of the RAM at the above ad frequence are lted n Table 1. The mulaton wa done on a Tyrone 8 proceor parallel machne wth 32 GB memory. FKO [10] wth phycal optc (PO) ued for the mulaton. V. RC RDUCTION UING PARTIAL RAM COATING Though magnetc RAM are advantageou compared to ther delectrc counterpart, the preence of ron make

3 IN: IO 9001:2008 Certfed Internatonal ournal of ngneerng and Innovatve Technology (IIT) Volume 2, Iue 2, Augut 2012 them heavy and alo requre klled peronal for coatng them unformly on the urface [7]. To make the catterrer not too heavy due to coatng, one hould reduce the amount of coatng materal to a mnmum, wthout compromng on the RC reducton. Th alo reduce the cot. From the catterng center approach of a target, t can be concluded that, t may not be neceary to coat a target fully. Intead, t enough to coat only the major catterng center called hot-pot [13]. In th paper, the hot-pot are frt dentfed and a novel method of only coatng the hot-pot on the urface of a complex haped arcraft wth RAM propoed, ntead of coatng the entre urface of the arcraft. A mlar method of RAM avng by coatng only the edge of a flat plate ha been dcued n [13]. There are alo recently, method of RC reducton ung meta materal [15-17] for partal concealment of object [18] are avalable n the lterature. In Fg. 2, the normalzed value of reducton n btatc RC hown wth varyng RAM coatng thckne, expreed n term of wavelength. It evdent from Fg. 2, that maxmum reducton n RC can be acheved when the coatng thckne of the order of /4. Alo t can be een that the RC reducton ubtantal for thckne value rangng from 0.18 to 0.4. A coatng of thckne 3.7mm n the range of 0.18 to 0.4 at all the three frequence of 1 GHz, 3 GHz and 10 GHz. Hence, a 3.7mm coatng thckne ued n the preent tudy. arcraft at 1 GHz. Analy of the RC how that the noe cone of the arcraft a major contrbutor to the backcatter cro ecton, havng an RC of -15 dbm 2 at 0 o. A the vewng angle ncreae, the broadde of the arcraft contrbute to the RC, and the RC ncreae. Two RC peak of dbm 2 at 31 o and 8.44 dbm 2 at 162 o are due to the cylndrcal ecton of the arcraft facng the radar. A mnmum RC of -17 dbm 2 at 42 o due to the obtue angled corner between the cylndrcal body and wng of the arcraft. The maxmum RC peak of 16 dbm 2 at 90 o due to the edge of the wng. There a decreae n RC to dbm 2 at 148 o, due to acute angled corner between the wng and cylnder. o the major contrbutor to the RC are the cylndrcal ecton of the arcraft and the edge of the wng. Only thee part are coated wth RAM. A maxmum reducton n RC of about 4 dbm 2 obtaned at 90 o wth partal coatng. nce the arcraft ymmetrc wth repect to the XZ-plane, the ame reult wll be obtaned for = 180 o to 360 o. Fg. 1. Plane Wave Incdence at the Noe of an Arcraft and RC Pattern In the - Plane. Fg. 2. Normalzed value of reducton n Btatc RC of arcraft wth RAM thckne (expreed n term of wavelength) at 1GHz. A. Monotatc RC of arcraft The monotatc RC of an arcraft anlyzed at 1GHz ung phycal optc (PO) method. The dcretzaton of the body of the arcraft reulted n 95,175 unknown, a hown n Table 1. Fg.3 how the monotatc RC of a PC and RAM coated 54 Fg. 3. Monotatc RC of arcraft at 1 GHz. B. Btatc RC of arcraft Analy of the btatc RC at 1GHz, gven n Fg. 4, how that for an ncdent plane wave, peak are oberved at 63, 137, and 180 and a mnmum at 0. For an ncdent M plane wave, the noe of the arcraft appear lke a pont ource, hence, mnmum RC of -15 dbm 2 oberved at 0. A the vewng angle ncreae, the broadde of the arcraft contrbute to the RC, and the RC ncreae. The peak RC of 9.3 dbm 2 at 63 due to the edge of wng and the ncreae n RC after 100 due to the cylndrcal ecton of the arcraft and the peak RC of -3.2 dbm 2 at 137 due to the corner between cylndrcal body and wng. Maxmum RC of dbm 2 oberved at 180 o due to the curved urface of the tal ecton and alo due to forward catterng. Hence, t may be concluded that the major contrbutor to the RC are the cylndrcal ecton of the arcraft, the corner between wng and cylnder and edge of the wng. The btatc

4 IN: IO 9001:2008 Certfed Internatonal ournal of ngneerng and Innovatve Technology (IIT) Volume 2, Iue 2, Augut 2012 RC of the arcraft at 3 GHz and 10 GHz are preented n Fg. 5 and 6, repectvely. The RAM coatng appled to the entre arcraft and the RC calculated at all three frequence. ntered Nckel Znc Ferrte [7] ued a RAM, wth the properte gven n Table1. The RC at 1 GHz, 3 GHz and 10 GHz are plotted n Fgure 4, 5 and 6, repectvely. It can be een from Fg. 4, that RC ha reduced by 3.23 dbm 2, dbm 2, 0.34 dbm 2, and 1.55 dbm 2 at 63, 80 o, 137, and 180, repectvely. In order to determne the urface for coatng, for effcent RC reducton, the average current denty of all the cell of each of 21 flat and 8 curved urface calculated. The urface wth an average current denty greater than -48 dba/m have been choen for the RAM coatng. Baed on th method, 5 urface out of 29 were choen for RAM coatng. The avng n urface area due to electve coatng ha been computed to be about 80% of the total arcraft area. The RC for th partally coated cae gven n Fgure 4, 5, and 6. It can be een that wth th partal coatng the RC reducton comparable to the fully coated cae. The urface current dtrbuton on the PC and RAM coated arcraft gven n Fg. 7. Fg. 5. Btatc RC of arcraft at 3 GHz. VI. CONCLUION In th paper, a novel method of partally coatng the arcraft to obtan reducton n RC comparable to that obtaned by a fully coated arcraft preented. The RC of a complex haped arcraft of ze 23, 70 and 233 at 1, 3, and 10GHz frequence repectvely determned. In order to effectvely reduce the RC wth mnmum amount of RAM coatng, a method to dentfy the hot-pot for RAM coatng ha been propoed. Phycal optc (PO) technque ha been ued frt to dentfy the hotpot on the arcraft and only thee hot-pot are coated wth RAM ntead of coatng the entre arcraft. To tudy the effectvene of the propoed technque, the monotatc and btatc RC of arcraft are analyzed. It ha been found that the partal (.e., hot-pot) RAM coatng reduce the RC, and th reducton n RC comparable to that of a fully coated body. Th gve a gnfcant avng of nearly 80% n weght and volume of RAM, and thu cot of the RAM coatng. Th reducton n volume of RAM coatng valuable wth repect to complexty of applyng RAM coatng. Fg. 6. Btatc RC of arcraft at 10 GHz. Fg. 4. Btatc RC of arcraft at 1 GHz. Fg. 7(A). Current Denty on A PC Arcraft & Fg.7 (B). Current Denty On A RAM Coated Arcraft. 55

5 Frequency GHz IN: IO 9001:2008 Certfed Internatonal ournal of ngneerng and Innovatve Technology (IIT) Volume 2, Iue 2, Augut 2012 Table 1: Memory and CPU Tme Requrement for PC and RAM Coated Arcraft Peak Total tme taken No. of Arcraft memory for mulaton unknown ued (MB) () PC 95, Fully RAM coated 190, electvely RAM coated 116, PC 857, Fully RAM coated 1,715, electvely RAM coated 1,051, PC 4,622, Fully RAM coated 9,244, electvely RAM coated 5,646, RFRNC [1] K.. Vnoy and R. M. ha, Radar Aborbng Materal from Theory to Degn and Characterzaton, Kluwer Academc Publher, Boton/London, [2] Mauro A. Alve, Rafael. Port and Mrabel C. Rezende, mulaton of the radar cro ecton of a tealth arcraft, BMO/I MTT- Internatonal Mcrowave & Optoelectronc Conference (IMOC 2007), pp , [3] H. Oraz and A. Abdolal, Ultra wde band RC optmzaton of multlayered cylndrcal tructure for arbtrarly polarzed ncdent plane wave, Progre In lectromagnetc Reearch, PIR 78, , [4] A. Upendra Raju, and yoth Balakrhnan, Performance analy of CM technque for predcton of RC of a complex haped body, Internatonal ournal of Appled ngneerng Reearch (IAR), IN , Volume 7, Number 4, pp , [5] Andrey. Andrenko and Makoto Ando, Analy of lne current wave dffracton from Metal backed delectrc trp by extended PO method, MMW'98 ympoum Proceedng. Kharkov, Ukrane, pp , eptember 15-17, [6] F Adana, I G Dego, O G Blanco, P Lozano and M F Catedra, Method baed on phycal optc for the computaton of radar cro ecton ncludng dffracton and double effect of metallc and aborbng bode modeled wth parametrc urface, I Tran. on Antenna and Propagaton, Vol. 52, No.12, pp , December [7].F Knott,.F. chaeffer and M.T. Tuley, Radar Cro ecton, 2 nd dton, ctech Publhng, Inc [8] Danel Bouche, Fredrc Molnet, and Raj Mttra, Aymptotc Method n lectromagnetc, prnger-verlag, Berln- Hedelberg-New York, 1997, Chapter 7. [9] N. A. Albayrak, RC Computaton wth PO/PTD for conductng and mpedance object modeled a large flat plate, M.c. The, Blkent Unverty, uly [10] 56 RAM properte [7] ε = j 9.0 µ =1.2 +j 12.0 ε = j 6.3 µ =0.9 +j 6.3 ε = j 6.3 µ =0.1 +j 0.32 [11] A.A.Abdelazz, Improvng the performance of an antenna array by ung radar aborbng cover, Progre In lectromagnetc Reearch Letter, Vol. 1, pp , [12] D. Klement,. Prener and V. ten, pecal problem n applyng the phycal optc method for backcatter computaton of complcated object, I Tran. on Antenna and Propagaton, Vol. AP-36, No.2, pp , February [13] A. K. Bhattacharyya, lectromagnetc catterng from a flat plate wth rm loadng and RAM avng, I Tran. on Antenna and Propagaton, Vol. 37, No.5, pp , February [14] Paul avlle, Revew of Radar Aborbng Materal, A Techncal Report, Defence R&D Canada, anuary [15] M.. de Co, Y. Alvarez, and F. La-Hera, A novel approach for RC reducton ung a combnaton of artfcal magnetc conductor, Progre In lectromagnetc Reearch, Vol. 107, pp , [16] L. Varga, Radar gnature Control Ung Metamateral, A Techncal Report, Defence R&D Canada, November [17] H. Oraz, A. Abdolal, and N. Vaegh, Applcaton of double zero metamateral a radar aborbng materal for the reducton of radar cro ecton, Progre In lectromagnetc Reearch, PIR 101, pp , [18] A M Lebedev, A I Fedorenko, ffcent partal concealment of convex conductve body, PIR ympoum, uzhou, Chna, ept , pp , AUTHOR PROFIL A. Upendra Raju receved h M.c. n lectronc cence from Bangalore Unverty n He currently a reearch tudent n the Department of lectronc cence, Bangalore Unverty, Bangalore, Inda. H current feld of nteret are n computatonal electromagnetc and radar cro ecton control. yoth Balakrhnan receved her M.c. n Phyc from Pune Unverty n 1976 and Ph.D n Phyc from Indan Inttute of cence, Bangalore n he currently wth the Department of lectronc cence Bangalore Unverty. Her nteret are n materal characterzaton, power electronc, devce phyc and mcrowave.