Ultra Wide Band TEM Horn and Reflector Antenna Designs for Down and Forward Looking Ground Penetrating Radars

Transcription

1 Ultra Wide Bad TEM Hor ad Reflector Atea Desigs for Dow ad Forward Lookig Groud Peetratig Radars Ahmet Serdar TURK, Ahmet Kea KESKIN, Mustafa Dagca SENTURK Abdullah MAGAT, Mehmet Burak OZAKIN, Serka AKSOY Yildiz Techical Uiversity Electroics ad Telecommuicatio Egieerig Departmet Davutpasa, Eseler, 34, Istabul, Turkey Abstract This paper presets ultra-wide bad (UWB) TEM hor ateas ad TEM hor-fed parabolic reflector desigs, which are suitable for dow-lookig ad forwardlookig vehicle-mouted impulse GPR systems, respectively. O this scope, partial dielectric loaded, Vivaldi form ad array cofiguratios of the TEM hor structure are ivestigated, desiged, simulated ad measured. Vivaldi shaped TEM hor-fed parabolic reflector atea prototype is proposed to reach hyper-wide bad impulse radiatio performaces from 3 MHz up to 5 GHz for multi-bad GPR operatio that ca provide both deep ad high resolutio imagig. The gai ad iput reflectio performaces are demostrated with measuremet results. Keywords- Ultra-wide bad atea, TEM hor, Parabolic reflector, Groud peetratig radar, Impulse radiatio. I. INTRODUCTION Ultra wide bad (UWB) ateas have bee usig icreasigly i may applicatios of high-speed wireless commuicatio, high resolutio oise radar, RF jammig ad EMC test systems. The operatioal frequecy bads of the impulse ad UWB radar systems ca be very broad startig from VHF bad up to millimeter waves. The UWB operatio provides some critical advatages, such as improved detectio, adaptive ragig ad target resolutio performaces. There are a few types of UWB ateas which operate at various frequecy bads, such as hor, spiral, bi-coical ad log-periodic arrays []. Groud-peetratig radar (GPR) is a well-kow leadig o-destructive testig ad through-wall imagig techology for the detectio, idetificatio ad imagig of subsurface structures ad buried objects []. The frequecy bad of the GPR sigal is the key factor for the detectio performace. Higher frequecies are eeded for better resolutio, evertheless lower frequecy bads are preferred to detect somethig buried too deep due to the dramatically icreased wave atteuatio i the soil with icreasig frequecy. Thus, UWB GPR systems are proposed to beefit from both low ad high frequecies. For impulse GPR, the impulse duratios ca vary from a few aosecods to hudred picosecods correspodig to a broad spectrum from MHz to GHz. It ca be exteded up 5 GHz for stepped-frequecy GPR systems, which may use microwave tomography methods for high resolutio imagig [3]. The target detectio ad idetificatio performace of the GPR depeds sigificatly o the proper desig of the UWB trasmitter ad receiver (T/R) ateas, which should have high gai, arrow beam, low side lobe ad iput reflectio levels over the wide operatioal frequecy bad []. O this scope, TEM ad ridged hors are oe of most favorites due to their high gai, arrow beam ad low iput reflectio characteristics over : badwidth ratio [3]. The TEM hor atea cosists of a pair of triagular or circular slice shaped coductors formig some V-dipole [4]. The partial dielectric loadig (PD) techiques ca be applied to improve the UWB gai performaces [5]. For istace, the PD-TEM hor was itroduced by Turk as a efficiet UWB impulse radiator over : frequecy bad [6-7]. Furthermore, its array combiatio was desiged to obtai exteded UWB characteristics up to 5: [8]. The double-ridged hor atea (DRHA) operates at very wide frequecy bad by usig ridges iside the hor that adjust the upper ad lower cut-off frequecies of the propagatig modes [9]. Although its atea gai is sufficietly high ad almost stable over the wide bad, dielectric aperture loadig methods ca be applied to improve the gai performace. Nevertheless, typical big hemispherical leses icrease the physical dimesios ad weight of the atea. Hece, the partial dielectric les loadig has also bee proposed for DRHA to improve the gai performace ad to elimiate radiatio patter deterioratio without sigificat chages o the atea sizes ad weight []. This paper starts with a geeric survey for UWB TEM hor atea types, which are suitable for multi-sesor adaptive, stepped-frequecy ad impulse mode UWB GPR systems. For example, the partial dielectric loaded TEM (PDTEM) hor, Vivaldi shaped TEM hor (PDVA) ad array combiatios have bee itroduced by Turk, as efficiet UWB impulse radiators operatig at 5 MHz - 5 GHz bads [6-]. A ovel versio of Vivaldi shaped TEM hor desig is proposed as feeder of the parabolic reflector (PR) to obtai hyper-wide bad atea characteristics from MHz up to 5 GHz. The hybrid PLTLM (partially loaded trasmissio lie atea method) ad ARM (aalytical regularizatio method) is used for proper pre-desig of PDVA-fed reflector atea. The desiged atea models were measured betwee. 5 GHz. The atea gai, VSWR ad radiatio patter performaces are preseted ad compared with /3/$3. 3 IEEE

2 stadard -8 GHz DRHA. It is show that PDVA ad PDVA-PR desigs are quite suitable for multi-bad GPR operatios, due to advatages of up to db ehaced gai performaces over : badwidth ratio. II. PLTLM PROCEDURE FOR TEM HORN ANALYSIS TEM hor is a kid of travellig wave atea. Hece, its structure ca be cosidered as combiatio of micro-strip trasmissio lie segmets, which are characterized by their local geometrical ad costitutioal structure parameters. The staircase modellig is used for the aalysis. 3D atea geometry is firstly divided ito N umber of elemetary cells, which are chose locally homogeeous ad sufficietly small i wavelegth. The, the structure is reduced to the equivalet D trasmissio lie with correspodig characteristic impedace defiitios. The iput impedace of each lie segmet ad its characteristic impedace are expressed i Eqs. () ad (), respectively [6]: Z Z Z jz ta l Z jz l Z i i i ta r 38 log 8 w d r ; =,,...,N () w ; for d where, β is propagatio costat, l is segmet legth, w is segmet width, d is segmet height, L is the total arm f r N r legth, c Z ad i is the equivalet atea lie output impedace. The iput ad local reflectio coefficiets of the th segmet lie are give by Eqs. (3) ad (4): Zi Z i Zi Z ;,,..., N (3) j ( l z) () z i e zl ; ; z L (4) Usig the Eq. (4), the discrete voltage ad curret distributio fuctios over the atea lie are determied by Eq. (5) as follows: ( ) jz V z V e ( z) j z I ( z) I e ( z) ; zl;,,..., N V ad I coefficiets are calculated iteratively usig the iitial values, such as the excitatio voltage at the atea lie feed ad the source impedace. The itegral equatio techiques are used to compute radiated field. This method is faster tha direct umerical techiques [7]. III. PARTIAL DIELECTRIC LOADED TEM HORN ANTENNA DESIGNS TEM hor structure cosists of a pair of triagular or circular slice shaped coductors formig some V-dipole structure ad characterized by L, d, ad parameters which correspod to the arm legth of atea, feed poit gap, coductor plate agle ad elevatio agle, respectively [4]. The covetioal TEM hor shows bad () (5) pass filter-like gai behavior due to the arm legth that limits the lower cut-off frequecy of the radiated pulse. Thus, dielectric fillig ad partial dielectric loadig techiques are employed to broade the operatioal bad up to twice [5-6]. Some TEM hor, PDTEM hor, PDVA ad PDTEM hor array desig cofiguratios are give i Table ad Fig., with the gai ad VSWR measuremet results show i Fig.. It is see that partial dielectric loadig approach used i PDVA- ca attai two times broadeed gai characteristics tha VA- with a VSWR level less tha. The gratig arm model PDVA structure i Fig. c, which is basically Vivaldi shaped wig versio of the PDTEM hor is a special desig proposal for metal detector adaptive operatio of GPR [7]. TABLE. UWB GPR atea desigs Fig Model Physical Descriptios =, =6, d=.5cm, TEM- L=cm, a r =, air-filled PDTEM- =, =6, d=.5cm, L=cm, r =3, dielectric loaded b VA- =, ( -6 ), d=.4cm, L=cm, r =, air-filled PDVA- =, ( -6 ), d=.4cm, L=cm, r =3, dielectric-loaded =, = 9, d =.5cm, c L =45cm, aperture: cm 5cm PDTEMA- =, = f(l)( - ), 45 d =.cm, L = 5cm, r =3.5, dielectric loaded d TEMRHA Aperture: width=4.4 cm, height=5.9 cm, depth=7.9 cm. TEM hor feeder: =, ( - ), d=.4 cm, L= 7 cm..b 3 PDVA-3 =, ( -6 ), d=.5cm, L=3 cm, r =., dielectric-loaded For vehicle mouted systems, the array combiatio of PDVA with TEM hor atea ca be proposed (see Fig. c) for hyper-wide bad GPR operatios to exted badwidth ratios up to 5: (5 MHz to GHz) [8]. The partial dielectric loadig method ca yield about 5 db gai icremet alog 4-4 GHz frequecy bad for stadard DRHA structure []. Moreover, the TEM hor fed ridged hor atea (TEMRHA) show i Fig. d is proposed to exted the lower cut off frequecy of the DRHA from 8 MHz to 4 MHz, which provides two times broadeed badwidth ratio []. The physical descriptios ad geometrical view of the Vivaldi shaped TEM hor (PDVA-3) desiged as UWB feeder for parabolic reflector is give i Table ad Fig. 3. The ear field illumiatio of this atea is calculated by PLTLM (see Sectio II). The, the ARM procedure described i Sectio IV is used for computatio of the UWB reflector radiatio.

3 dielectric feed gap r plate o resistive sheet Figure. Atea gai characteristics of TEM hor desigs feed plate Dielectric o resistive sheet IV. ARM FORMULATION FOR PARABOLIC REFLECTOR Scalar diffractio problem of a ifiitely log, smooth, logitudially homogeeous ad perfectly coductig cylidrical obstacle correspods to the Dirichlet boudary coditio for E-polarized icidet wave. Cosiderig that XOY plae cross sectio is deoted by the closed cotour S, the icidet ad scattered scalar wave fuctios (u i (p) ad u s (p)) must satisfy the Helmholtz equatio give i Eq. (6) ad the Dirichlet boudary coditio i Eq. (7) []. k u s ( p), p R \S (6) s( ) s( ) i u ( p ) u ( p ) u ( p ), p S (7) (c) where, S smooth cotour of the domai D i D space R s( that belogs to the smoothess class C,. u ) ( p) s( ad ) s u ( p) are limitig values of u ( p) i the ier ad the outer sides of S, respectively. The solutio of the BVP is writte i Eq. (8) usig Gree s formula ad the boudary coditio i Eq. (7). i ( ) i H ( k q p ) Z( p ) dl p u ( q ), q, ps 4 (8) S s( ) s( ) u ( p) u ( p) where, Z( p), p S ; is the uit outward with respect to S ormal of the poit p. The ukow fuctio Z(p) is costructed by solvig Eq. (8), ad usig parameterizatio of the S cotour specified by the fuctio ()=(x(),y()) that smoothly parameterizes the cotour S by the poits of -,. The itegral equatio represetatio of the first kid i Eq. (8a be equivaletly rewritte by meas of the () parameterizatio as follows: (d) Figure. TEM hor desig illustratios PDTEM hor geometry (side view) Vivaldi form TEM hor (side view) (c) PDTEM hor array (3D view, adapted from [8]) (d) 3D illustratio of TEM-RHA geometry (adapted from []) l si K(, ) ZD( ) d g( ), -, (9) with the ukow fuctio Z D () ad the give fuctio g(), where, i ' ' / Z ( ) l( ) Z( ( )), g( ) u ( ( )); -, () D l( ) [ x ( )] [ y ( )], x, y C Q ()

4 Here K(,) fuctio is rather smooth sectio of the Gree s fuctio i compariso with l si part that represets the mai sigularity of the Eq. (4). The fuctios i Eq. (9) are represeted by their Fourier series expasios with k s,m, z m, g m coefficiets. Subsequetly, oe ca obtai a ifiite system of the liear algebraic equatios of the secod kid []: ˆz ˆ k ˆ z gˆ, s,,.. () s s, m m s m where, ˆ k k ˆz z gˆ g / max(, ),,,,.. (3) ad s, is the Kroecker delta fuctio. Fially, the scattered field u s (q) for qr are obtaied by the itegral equatio represetatio of the Eq. (9) with ay required accuracy by the trucatio method [3]. s, m s m s, m s, m,,, s s The ARM procedure is derived for aalysis of scatterig from parabolic reflector atea. The -D cylidrical reflector structure illustrated i Fig. 3 is cosidered. The cross-sectio of the reflector is a parabolic arc of the outer focus b, ier focus a ad thickess c. It is modeled i ARM as a closed cotour L goes from poit A to poit D ad back to A which correspods to -,. The total cotour L which cosists of four parts is defied as: L L AB L BC L CD L DA ; Total cotour legth (4) L AB b ta( ) / ; Outer cotour legth (5) L CD a ta( ) / ; Ier cotour legth (6) L BC Rc, LDA Rc ; Edge cotour legths (7) where; b a c ad ba c (8) cos cos 3 ad the edge-rollig coefficiet R is. The parameterizatio of the cotour lie is separately implemeted from A to D ad back to A by meas of the variable l[a,d] as follows [4]: b cos x cos acos x cos y c si ( l L x c cos ( l L AB AB si y b cos si y a cos ; l A, B (9) ; l C, D () ( a b)cos ( R ) c cos cos ( a b)si ( R ) c si cos ; l B, C () x c cos ( l L y c si ( l L where l CD CD ( a b)cos ( R ) c cos cos ( a b)si ( R ) c si cos ; l D, A () L ;, L (, ), ( )l ( )( l L l (3) L AB ) ( L L BC CD BC ) (4) (5) The PDVA type TEM hor feeder is assumed to be located o the focus of the reflector. The ear field illumiatio of the PDVA-3 o the reflector is defied as the icidet wave at ARM algorithm. V. UWB REFLECTOR DESIGN AND PERFORMANCES The geometry of the desiged reflector atea with PDVA-3 feeder is give i Fig. 3, with the umerical ARM results show i Fig. 4. This structure ca also be adapted for offset feedig, which is more suitable i forward-lookig GPR applicatios due to low tilt agle. Figure 3. D cross-sectio geometry of parabolic reflector atea with PDVA type TEM hor feeder The measuremet results plotted i Fig. 5 ad Fig. 6 show that radiated impulse sigal has highly arrow beam widths ad db improved gai performaces ca be achieved over 5: badwidth, from 3 MHz up to 5 GHz.

5 Figure 4. ARM simulatio results of the ormalized radiatio patters Figure 7. Test ad measuremet setups 5 GHz impulse GPR, 5 GHz Stepped frequecy GPR Figure 5. Time domai trasformatios of 5 GHz impulse radiatio gai measuremets for differet observatio agles Depth rage Scaig directio (c) Figure 6.Atea gai measuremet results; (Bold) PDVA fed parabolic reflector, (Solid) -8 GHz double ridged hor atea, (Dashed) PDVA VI. TEST SETUP AND PERFORMANCE RESULTS The measuremet setups for both impulse ad steppedfrequecy GPR test systems, which use PDVA-3 T/R atea head, are exhibited i Fig. 7. The dow-lookig performace results are plotted i Fig. 8 for buried object ad through-wall detectio scearios. (d) Figure 8. B-sca measuremet data (backgroud removed) - Buried object detectio (metal, glass, pipe, water pipe) (c) Buried object detectio by SF-GPR (time domai trasformatio) (d) Through-wall imagig of 4 huma bodies by impulse GPR

6 The demostratio of forward-lookig GPR test sceario for 5 GHz UWB impulse radiatio by TEM-RHA atea ad the FDTD (Fiite Differece Time Domai) simulatio results for a metallic object buried at cm (black box) are give i Fig. 9. It is see that target backscatterig ca be distiguished better from surface reflectios; i the case of atea dow-lookig grazig agle is chose as the Brewster agle (~ 4 ). VII. CONCLUSION I this study, dielectric loaded, Vivaldi shaped ad array versios of TEM hor ateas were surveyed for UWB GPR systems. The partial dielectric loaded Vivaldi shaped TEM hor (PDVA) fed parabolic reflector was itroduced to achieve high atea gai ad arrow beam width characteristics over a hyper wide bad greater tha 5:. The hybrid approach of PLTLM ad ARM algorithms are used to make fast ad reliable pre-desig of the PDVA-PR atea structure. It is show that atea attais highly efficiet UWB radiatio performaces, which is cosidered suitable especially for multi-bad forward-lookig groudpeetratig radar operatios. ACKNOWLEDGMENT This work was supported by grat E of TUBITAK (The Scietific ad Techological Research Coucil of Turkey) research fud. (c) (d) Figure 9. Forward-lookig sceario ad FDTD simulatio results of UWB GPR system with TEMRHA (Lx = m, Lz = 6 cm) Illustratio of forward-lookig sceario with buried metal object Atea located at low grazig agle (~ 5 ) (c) Atea located at Brewster agle (~ 4 ) (d) Atea located at high grazig agle (~ 6 ) REFERENCES [] R. J. Fotaa, Recet system applicatios of short-pulse ultrawide bad techology, IEEE Tras. Microwave Theory Tech., Vol. 5, No. 9, Sept. 4. [] D.J. Daiels, Groud Peetratig Radar (IEE Radar, Soar, Navigatio ad Avioics Series),d ed., IEE Press, Lodo, 4. [3] A. S. Turk, A. K. Hocaoglu, A. A. Vertiy, Subsurface Sesig, Wiley-Itersciece, New Jersey,. [4] B. Scheers, M. Acheroy ad A.V. Vorst, Time-domai simulatio ad characterizatio of TEM hors usig a ormalized impulse respose, IEE Proc. Microwave Ateas Propagatio, vol. 47, o. 6, ,. [5] A. G. Yarovoy, A. D. Schuki, I. V. Kaplou, L. P. Ligthart, The dielectric wedge atea, IEEE Tras. o Ateas ad Propagatio, vol. 5, o., 46-47,. [6] A. S. Turk, D. A. Sahikaya, Partial Dielectric Loaded TEM Hor Desig for Ultra-Widebad Groud Peetratig Impulse Radar Systems, Ultra-Widebad Short-Pulse Electromagetics Series 7, chapter 34, Spriger, Berli, 7. [7] A. S. Turk, Ultra-widebad Vivaldi atea desig for multisesor adaptive groud-peetratig impulse radar, Microwave ad Optical Techology Letters, 48, (5), , 6. [8] A. S. Turk, H. Nazli, Hyper-wide bad TEM hor array desig for multi bad groud-peetratig impulse radar, Microwave ad Optical Techology Letters, 5 (), 76-8, 8. [9] C. Brus, P. Leuchtma, R. Vahldieck, Aalysis ad simulatio of a 8-GHz broadbad double-ridged hor atea, IEEE Tras. Electromagetic Compatibility, 45, pp. 55 6, 3. [] A. S. Turk., A. K. Keski, Partially dielectric-loaded ridged hor atea desig for ultrawide bad gai ad radiatio performace ehacemet, IEEE Ateas ad Wireless Propagatio Letters, vol., pp. 9-94,. [] A. S. Turk, A. K. Keski, Vivaldi shaped TEM hor fed ridged hor atea desig for UWB GPR systems, 6th Iteratioal Workshop o Advaced Groud Peetratig Radar (IWAGPR ), -4 Jue, Aache-Germay,. [] Yu. A. Tuchki, Wave scatterig by a ope cylidrical scree of arbitrary profile with Dirichlet boudary value coditio, Soviet Physics Doclady, vol. 3, 7-3, 985. [3] E. Karacuha, A. S. Turk, E-polarized scalar wave diffractio by perfectly coductive arbitrary shaped cylidrical obstacles with fiite thickess, It. J. Ifrared ad Millimeter Waves,, ,. [4] O. M. Yucedag., A. S. Turk., Parametric desig of ope eded waveguide array feeder with reflector atea for switchable cosecatsquared patter, Applied Computatioal Electromagetics Society (ACES) Joural, vol. 7, Issue 8, pp ,.