A Novel UWB Vivaldi Antenna Array for Radar Applications - Ijser

International Journal of Scientific & Engineering Research, Volume 7, Issue 5, May-2016 ISSN 2229-5518

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A Novel UWB Vivaldi Antenna Array for Radar Applications Osama M. Dardeer, Tamer G. Abouelnaga, Ashraf S. Mohra, Hadia M. El-Hennawy Abstract— This paper presents a novel four elements ultra wideband (UWB) antipodal Vivaldi antenna (AVA) array design for radar and microwave imaging applications. Initially, the radiation ares of the AVA is designed based on an elliptical curve shape. Next, the substrate end is shaped as a triangular shape and a negative index metamaterial (NIM) has been incorporated into the AVA aperture to act as a director. A gain enhancement of 2 dB is obtained for the proposed element compared to conventional one. The proposed antenna element is fabricated and measured. There is a good consistency between the simulated and measured return loss. In addition, four UWB Vivaldi antennas with substrate end shaping are placed along the z-axis in a linear antenna array configuration. The four NIM structures of the four antennas are placed together to form a sheet which is perpendicular to the four antenna substrates. A gain enhancement of about 6 dBi is obtained compared to the proposed single element through the whole UWB frequency range. The proposed antenna array bandwidth extends from 3 GHz to 16 GHz. The proposed antenna array is fabricated, measured, and good agreement is obtained between simulated and measured reflection coeffcients. Index Terms— Ultra-Wideband, Vivaldi Antenna, Linear Antenna Array, Radar Applications.

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1 INTRODUCTION

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H E Fed eral Com m u nication Com m ittee (FCC) has approved the frequ ency band betw een 3.1 to 10.6 GH z for u ltra w id eband (UWB) applications in 2002 [1]. There are great attentions from the acad em ic and ind u strial com m u nities to this UWB rad io system w hich is 7.5 GH z w id e. Designing a UWB antenna w ith stable end fire rad iation patterns and high gain rem ains a challenge still. This kind of antennas find s lots of ap plications like rad ar, rem ote sensing, m icrow ave im aging, and UWB com m u nication system s. Planar antennas are preferred becau se of the m erits of lightw eight, low pro file, and cost effective [2]. Tapered slot antenna (TSA) is a good cand id ate to achieve w id e im ped ance band w id th, stable rad iation pattern, and high gain characteristics [3]. H ow ever, the antipod al Vivald i antenna (AVA) provid es m ore com p act size and low er reflections from the feed ing stru ctu re than TSA [4]. It is w orthy to note that balanced antipod al Vivald i antenna (BAVA) is a com pact and versatile d esign that w as introd u ced by Langley et al. [5]. The u niqu e characteristics of the Vivald i antenna enabled it to be u sed in m any ap plications w here UWB and end fire rad iation p attern is requ ired like high range rad ar system s [6], d etection of on -bod y concealed w eapons [7], see throu gh w all ap plications [8], and im aging of tissu es for the d etection of cancerou s cells [9]. For these applications, the antenna shou ld achieve w id e im ped ance band w id th, narrow half pow er beam w id th, and high gain. Different antenn as in [10], [11], [12], [13], [14], [15], [16], [17], [18] are presented to fu lfill these requ irem ents. In this p aper, the shape of the rad iation flares is d esigned in the form of elliptical cu rves. The basic d esign has been explained analytically and the equ ations governing the elliptical cu rves has been presented . N ext, a negative ind ex m etam aterial (N IM) has been incorp orated into the AVA apertu re to act as a d irector. The proposed elem ent signicantly im proves the antenna rad iation pattern and gain. In ad d ition, this elem ent is u sed in a fou r elem ent UWB antenna array that operates from 3------16 GH z w ith m ore than 12 d Bi m axim u m gain. The paper is organized as follow s. Section 2 d escribes the configu ration

of the prop osed antenna elem ent. Section 3 is d evoted to the 4elem ent array. The p aper is conclu d ed in Section 4.

2 PROPOSED ANTENNA ELEMENT

2.1 Antipodal Vivaldi Antenna Design The antenna is d esigned on a low cost FR4 su bstrate w ith d ielectric constant εr =4.3, thickness h=1.5 m m , and loss tangent δ=0.025. The antenna geom etry is show n in Fig. 1. The proposed antenna inclu d es three parts: m icrostrip line feed section, transition section and rad iating section. Transition section is the m ost critical part in the antenna d esign since it is a grad ient stru ctu re and acts as a balu n. This section is form ed by intersecting tw o id entical half ellipses w ith the top and bottom m etals besid es an offset d istance from the beginning of the feed line. The shape of the rad iation are is d esigned in the form of elliptical cu rve. This taper line is a qu arter elliptical arc of d ifferent m ajor/ m inor axis ratio to form a sm ooth transition to red u ce reflection. As a consequ ence, this elliptical con figu ration provid es a good broad band characteristics and it is one of the optim u m cu rvatu res [19].

Figure 1: Antipodal Vivaldi antenna geometry.

Theoretically, Vivald i antenna has infinite band w id th [20] and

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generally the band w id th of the Vivald i antenna is proportional to its length and apertu re. Therefore, the size of the antenna increases w hen UWB perform ance is requ ired . The low er frequ ency lim it d epend s on the length of antenna and the effective d ielectric constant εeff. Low er frequ ency lim it is calcu lated from [21]: (1) f min  [c / 2( LH )(  eff )]

 eff  [( r  1) / 2]  [( r  1) / 2][1  (12h / LH )]0.5

(2)

w here h is the su bstrate height and LH is the antenna length. The stru ctu ral p aram eters are listed in Table 1, and have alread y been optim ized . Extensive param etric stu d ies are carried ou t u sing Com pu ter Sim u lation Technology Microw ave Stu d io (CST MWS) to stu d y the effect of d ifferent param eters on the antenna perform ance to select the proper d im ensions of the m ajor and m inor axes of the intersecting ellip ses. The d im ensions of the antenna are 60 × 45 × 1.5 m m 3. As show n in Fig. 1, the shape of the rad iation flare is d esigned in the form of elliptical cu rve. This taper line is a qu arter elliptical arc and the tw o cu rves are d enoted : Es1 and Es2. In the equ ations d escribing the tw o cu rves, u and v represent the m ajor and m inor axes respectively. Using the stru ctu ral param eters in Table 1, the tw o elliptical cu rves can be d escribed by the equ ations: Es1:

(a)

(b)

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y  [v 1  [ x  (offset  d / 2)]2 / u 2 ]  (2b  c  a) , (offset  d / 2  x  LH ) Es2:

(3)

y  [v 1  [ x  (offset  d / 2)]2 / u 2 ]  (a) , (offset  d / 2  x  LH )

(4)

TABLE 1: STRUCTURAL PARAMETERS OF THE PROPOSED AVA. B C D offset W

16 mm 4 mm 43.5097 mm 6.53 mm 15.0794 mm

LH L a u v

60 mm LH-offset-d (2b+c-w)/2 LH-(offset+d/2) b+c-a

Fig. 2a show s the effect of length d on the proposed antenna band w id th perform ance. An enhancem ent of 8 d B is obtained as d changes from 48.5 m m to 43.5 m m at 6 GH z. Also, an enhancem ent of 2 d B is obtained at 10 GH z. As a resu lt, d is chosen to be 43.5 m m . The offset length is chosen to be 6.53 m m in ord er to achieve w id e im ped ance band w id th w ith good retu rn loss as show n in Fig. 2b. When the antenna apertu re w id th w gets sm aller, the antenna has a better retu rn loss in low frequ ency, bu t from th e point of view of the antenna gain, it gets w orse as show n in Fig. 2c. Therefore, w hen the w idth w increases, the gain is enhanced and the resonance point is shifted to high frequ ency becau se the cu rrent path becom es shorter as illu strated in Fig. 3.

(c) Fig. 2 (a) Simulated return loss of antenna with different d, (b) different offset at d = 43.5 mm. (c), different w at d = 43.5 mm and offset=6.5 mm.

(a) (b) Fig. 3 (a) Current distribution at w=10mm, (b) w =15mm.

2.2 Substrate End Shaping N orm ally antip od al Vivald i antennas su ffer from low axial gains and tilted beam s. The techniqu e proposed in [18] has been applied here to correct both problem s. This techniqu e is based on the su bstrate end shaping. It is a sim ple geom etrical m ed ication that extend s the su bstrate beyond its length to an ap prop riate length. This extend ed length acts as a lens and can be consid ered as an integral part of the antenn a in ord er to be easy to fabricate. A triangu lar end shaping is consid ered since it prod u ced the highest im p rovem ent in gain throu ghou t the entire frequ ency band of operation. This triangle is equ ilateral and extend s for a d istance of half the antenna leng th w hich is 30 m m . There is an enhancem ent of 2 d B w hen su bstrate end shaping is ap plied , and this can be illu strated in Fig. 4.


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of extend ed length beyond three N IM cells is not d esirable for practical ap plications, in ad d ition to the sim plicity of fabrication.

Fig. 5 NIM unit cell.

Fig. 6 NIM configuration.

Fig. 4 Effect of the substrate end shaping on the realized gain.

2.3 Negative Index Metamaterial Structure Metam aterial is a m anm ad e com posite stru ctu re m ad e of d ielectrics m etals. It im plies that it is not a hom ogenou s m aterial, rather inhom ogeneou s artificial. Metallic or d ielectric stru ctu res in m etam aterials are typically m u ch sm aller than the w avelength. So, at a w avelength scale, one can assu m e an effectively hom ogenou s m aterial bu t w ith very d ifferent physical properties. So, m aterial cou ld be d ivid ed as, first a positive ε and μ m aterial, this kind of m aterials is called d ou ble positive m aterials or natu ral d ielectric m aterials. If an electrom agnetic w ave propagates in su ch a m aterial, you w ill see a prop agating w ave w ith som e loss or attenu ation. Second , a negative ε and a positive μ su ch as m etals, thu s m etals are also called epsilon negative m aterials. If electrom agnetic w ave prop agates in a m etal, it experiences hu ge attenu ation that is w hy w hen m etals are d iscu ssed , the skin d epth effect is d iscu ssed too. Actu ally the w aves w ill attenu ate a lot into the m etam aterials. In 1960s, m aterials having sim u ltaneou sly negative μ and negative ε are theoretically proposed and they prod u ce a negative refraction ind ex. This m aterials are called a left hand ed m aterial [22]. In 1996, negative epsilon m aterial cou ld be obtained by arranging the m etal w ires in a sim ple cu bic lattice. Bu t the problem still rem ained how a negative μ cou ld be obtained . In 2000 by sm ith, the qu estion w as answ ered w here a stru ctu re consisting of period ic array of split ring resonators and continu ou s w ires that prod u ced the negative perm eability and negative perm ittivity is proposed [23], [24]. Fig. 5 show s a N IM stru ctu re u nit cell. It is constru cted of tw o self com plem entary archim ed ean rectangu lar spiral shapes. The N IM u nit cell w ith d im ensions of 10×10 mm2 (A×B) is d esigned on an id entical su bstrate m aterial that is u sed for the AVA d esign. The N IM is excited w ith an electric field along the y-d irection and a m agnetic field along xd irection as illu strated in Fig. 6. The resonant frequ ency of N IM cell is controlled by the gap betw een spirals [14], [15]. The incorp oration of N IM in ad d ition to the su bstrate end shaping d id not signicantly affect the retu rn loss (S11) of the antenna. Different techniqu es are available for incorporating N IM into AVA. In this p aper, single layer of N IM is inserted perpend icu larly to the su bstrate i.e. betw een the tw o arm s of the antenna. The final stru ctu re of the N IM incorporated AVA w ith su bstrate end shaping is show n in Fig. 7. A fu rther increm ent

Fig. 7 The proposed NIM incorporated AVA with substrate end shaping.

2.4 Proposed Antenna Radiation Pattern

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The sim u lated and m easu red rad iation patterns of the antenna are show n in Figs. 8------10. From these figu res, it is fou nd that the proposed antenna exh ibits good u nid irectional rad iation patterns in the elevation and azim u th planes. Over the operating frequ ency band , the m ain lobes of the rad iation patterns are fixed in the end fire d irection (x-axis d irection).


(Elevation plane, θ=90o) (Azimuth plane, φ=0o) Fig. 8 Normalized radiation pattern of the antenna at 3.2 GHz.

(Elevation plane, θ=90o) (Azimuth plane, φ=0o) Fig. 9 Normalized radiation pattern of the antenna at 6 GHz.

(Elevation plane, θ=90o) (Azimuth plane, φ=0o) Fig. 10 Normalized radiation pattern of the antenna at 9.7 GHz.

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betw een elem ents. This spacing is not obviou s in u ltra w id eband antenna arrays to avoid the grating lobes in the w hole UWB frequ ency sp an since they operate at very large frequ ency band w id th [26].

2.5 Fabrication and Measurement of The Proposed Antenna Element The band w id th of the proposed antenna extend s from 2.8 to 16 GH z. The fabricated antenna is show n in Fig. 11. Fig. 12 presents the sim u lated and m easu red retu rn loss of proposed antenna. There is a good agreem ent betw een the tw o resu lts. The d ifference betw een sim u lated and m easu red S11 m ay com e from the connection loss betw een the printed circu it board and SMA connector, and the u nstability of the FR4 su bstrate param eters at 12 GH z.

Fig. 13 Array Configuration.

3.2 Element Spacing Many trials are carried ou t to choose properly the d istance betw een elem ents w hich is the m ost im portant param eter that affects m u tu al cou pling. Of cou rse, a large d istance resu lts in less m u tu al cou pling effects, and consequ ently a better isolation betw een array elem ents can be achieved , w hile the SLL w ill be d eteriorated . There is alw ays a trad e-off betw een half pow er beam w id th (H PBW) and SLL [27]. Table 2 lists d ifferent valu es of the elem ent spacing, H PBW and SLL in both the elevation and azim u th planes at 10 GH z.

Fig. 11 Photograph of the fabricated antenna.

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TABLE 2 HPBW AND SLL FOR DIFFERENT ELEMENT SPACING AT 10 GHZ. Element Spacing

Fig. 12 Return loss performance of the proposed antenna.

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22 mm 25 mm 30 mm

Elevation plane (XYplane) HPBW SLL 50.9o -5.2 dB 54.1o -4.1 dB 57.2o -4 dB

Azimuth plane (XZplane) HPBW SLL 19.1o -20 dB 17o -21.8 dB 14.1o -20.3 dB

PROPOSED 4-ELEMENT ARRAY

3.3 Realized Gain

3.1 Array Configuration In ord er to obtain better range resolu tion in a rad ar system , a w id e band w id th is requ ired . Also, for tw o ad jacent targets at the sam e range, they mu st be separated by at least one beam w id th to be d istingu ished as tw o objects. Bilgic and Yegin in [25] achieved 13.5o half pow er beam w id th bu t covers only the frequ ency band from 8.75 to 12.25 GH z. In this paper, fou r UWB Vivald i antennas w ith su bstrate end shaping are p laced along the z-axis in a linear array con figu ration as show n in Fig. 13. The Dolph-Tschebysche d istribu tion w ith a sid e lobe level (SLL) of -20 d B has been chosen for this d esign. Accord ingly, the fou r excitation coefficients are (0.576------1------1------0.576). The fou r N IM stru ctu res of the fou r antennas are assem bled together to form a large sheet w hich is perpend icu lar to the fou r su bstrates. This stru ctu re is proposed d u e to high gain featu res. For narrow band antenna arrays, the elem ent sp acing shou ld be less than one w avelength (λ) to avoid the grating lobes. Moreover, it shou ld be greater than half w avelength (λ/ 2) to m inim ize the fad ing correlation and m u tu al cou pling

Figu re 14 show s the antenna gain for single elem ent and array stru ctu res for the UWB frequ ency range. In the frequency band s betw een 3 GH z and 9 GH z, the array gain is abou t 5 d Bi m ore than the single elem ent gain and exceed s 6 d Bi beyond 9 GH z. If the elem ent spacing is increased from (λ/ 2) u ntil (λ), the array gain is increased . This spacing shou ld n't exceed one w avelength to avoid the grating lobes. The azim u th plane rad iation pattern com parison betw een a single elem ent and a 4elem ent antenna array is show n in Fig. 15.

Fig. 14 Array and single element antenna gain at 25 mm element spacing.



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d istribu tion. A prototype of the proposed single and fou r elem ents antenna array w ere fabricated and m easu red over the UWB frequ ency range. Good agreem ent w as obtained betw een m easu red and sim u lated reflection coefficients. The antenna array gain w as increased by abou t 6 d Bi com p ared to that w as obtained w ith the proposed single elem ent. The proposed antenna array is su itable for rad ar and m icrow ave im aging applications w here a d irectional UWB antenna is requ ired . Fig. 15 Azimuth radiation pattern at 10 GHz and 25 mm element spacing.

REFERENCES [1]

3.4 Fabrication and Measurement of The Proposed Four Element Antenna Array [2]

The fabricated fou r elem ent antenna array is show n in Fig. 16. Fig. 17 presents the sim u lated and m easu red retu rn loss (S11) of proposed array. The tw o resu lts are below -10 d B across the entire frequ ency band of operation.

[3]

E. J. Thomas, ‘‘First Report And Order, Revision Of Part 15 Of The Commission’s Rules Regarding Ultra-Wideband Transmission Systems Fcc, fcc02-48,’’ May 2002. [Online]. Available: http:/ / www.fcc.gov/ Bureaus/ EngineeringT echnologies=Orders=2002=fcc02048:pdf M. E. Bialkow ski, A. M. Abbosh, Y. Wang, D. Ireland, A. A. Bakar, and B. J. Moham m ed , ‘‘Microwave Im aging System s Em ploying Cylind rical, H em ispherical And Planar Arrays Of Ultraw id eband Antennas,’’ in Microwave Conference Proceedings (A PM C), 2011 Asia-Pacific, Dec 2011, pp. 191---194. L. Lew is, M. Fassett, and J. H unt, ‘‘A Broad band Stripline Array Elem ent,’’ in A ntennas and Propagation Society International Symposium, 1974, vol. 12, Jun 1974, pp. 335---337. J. Bourqui, M. A. Cam pbell, J. Sill, M. Shenoud a, and E. C. Fear, ‘‘Antenna Performance For Ultra-Wid eband Microwave Im aging,’’ in Radio and Wireless Symposium, 2009. RW S ’09. IEEE, Jan 2009, pp. 522--525. J. D. S. Langley, P. S. Hall, and P. N ew ham , ‘‘Novel Ultraw ideBandw id th Vivald i Antenna With Low Crosspolarisation ,’’ Electronics Letters, vol. 29, no. 23, pp. 2004---2005, N ov 1993. S. Maalik, ‘‘Antenna Design For UWB Radar Detection Application,’’ Master d issertation, Dept. of Comm unication Engineering, Chalm ers University of Technology, 2010. A. S. Atiah and N . J. Bow ring, ‘‘Design Of flat Gain Uwb Tapered Slot Antenna For On -Bod y Concealed Weapons Detections,’’ PIERS Online, vol. 7, no. 5, pp. 491---495, 2011. Y. Yang, Y. Wang, and A. E. Fathy, ‘‘Design Of Com pact Vivald i Antenna Arrays For Uwb See Through Wall Applications,’’ Progress In Electromagnetics Research, vol. 82, pp. 401---418, 2008. A. Lazaro, R. Villarino, and D. Girbau, ‘‘Design Of Tapered Slot Vivald i Antenna For Uw b Breast Cancer Detection ,’’ Microwave and Optical Technology Letters, vol. 53, no. 3, pp. 639---643, 2011. [Online]. Available: http:/ / dx.doi.org/ 10.1002/ m op.25770 R. Natarajan, J. V. George, M. Kanagasabai, and A. K. Shrivastav, ‘‘A Compact Antipodal Vivaldi Antenna For UWB Applications,’’ IEEE Antennas and Wireless Propagation Letters, vol. 14, pp. 1557---1560, 2015. L. Yang, H . Guo, X. Liu, H . Du, and G. Ji, ‘‘An Antipod al Vivald i Antenna For Ultra-Wid eband System ,’’ in (ICUW B), 2010 IEEE International Conference on Ultra-Wideband, vol. 1, Sept 2010, pp. 1---4. H . C. Ba, H . Shirai, and C. D. N goc, ‘‘Analysis And Design Of Antipodal Vivald i Antenna For UWB Applications,’’ in (ICCE), 2014 IEEE Fifth International Conference on Communications and Electronics, July 2014, pp. 391---394. Y. Wang, A. Bakar, and M. Bialkowski, ‘‘Reduced-Size Uwb Uniplanar Tapered Slot Antennas Without And With Corrugations,’’ Microwave and Optical Technology Letters, vol. 53, no. 4, pp. 830----836, 2011. [Online]. Available: http:/ / dx.doi.org/ 10.1002/ mop.25878 R. Singha and D. Vakula, ‘‘Corrugated Antipodal Vivald i Antenna Using Spiral Shape N egative Ind ex Metam aterial For Ultra Wid eband Application ,’’ in (SPA CES), 2015 International Conference on

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[5]

[6]

Fig. 16 Photograph of the fabricated antenna array.

[7]

[8]

[9]

[10] Fig. 17 Return loss of the antenna array. [11]

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CONCLUSION

[12]

This p aper presents a UWB AVA w ith rad iation flares in the form of elliptical cu rves. The antenna d im ensions are 60 × 45 × 1.5 mm3 w ith a triangu lar shape extension and a N IM perpend icu lar to the su bstrate. The proposed elem ent is capable of achieving an inpu t im ped ance band w id th from 2.8 GH z to 16 GH z experim entally. Then, fou r id entical antenna elem ents w ere em ployed to introd u ce a linear array. The excitation coe fficients w ere calcu lated accord ing to the Dolph -Tschebysche

[13]

[14]



[15]

[16]

[17]

[18]

[19]

[20] [21]

[22]

[23]

[24]

[25]

[26]

[27]

Signal Processing A nd Communication Engineering Systems, Jan 2015, pp. 179---183. A. M. D. Oliveira, M. B. Perotoni, S. T. Kofuji, and J. F. Justo, ‘‘A Palm Tree Antipodal Vivaldi Antenna With Exponential Slot Edge For Improved Radiation Pattern,’’ IEEE Antennas and Wireless Propagation Letters, vol. 14, pp. 1334----1337, 2015. L. Chen, Z. Lei, R. Yang, J. Fan, and X. Shi, ‘‘A Broadband Artificial Material For Gain Enhancement Of Antipodal Tapered Slot Antenna,’’ IEEE Transactions on Antennas and Propagation, vol. 63, no. 1, pp. 395----400, Jan 2015. I. T. Nassar and T. M. Weller, ‘‘A Novel Method For Improving Antipodal Vivaldi Antenna Performance,’’ IEEE Transactions on Antennas and Propagation, vol. 63, no. 7, pp. 3321----3324, July 2015. K. Kota and L. Shafai, ‘‘Gain And Radiation Pattern Enhancement Of Balanced Antipodal Vivaldi Antenna,’’ Electronics Letters, vol. 47, no. 5, pp. 303----304, March 2011. A. M. Abbosh and M. E. Bialkowski, ‘‘Design Of Ultrawideband Planar Monopole Antennas Of Circular And Elliptical Shape,’’ IEEE Transactions on Antennas and Propagation, vol. 56, no. 1, pp. 17----23, Jan 2008. P. J. Gibson, ‘‘The Vivaldi Aerial,’’ in Microwave Conference, 1979. 9th European, Sept 1979, pp. 101----105. S. Wang, X. D. Chen, and C. G. Parini, ‘‘Analysis Of Ultra Wideband Antipodal Vivaldi Antenna Design,’’ in Loughborough Antennas and Propagation Conference, LAPC, April 2007, pp. 129----132. V. G. Veselago, ‘‘The Electrodynamics Of Substances With Simultaneously Negative Values Of ε And μ,’’ SOV PHYS USPEKHI, vol. 10, no. 4, pp. 509---514, 1968. J. B. Pendry, A. J. Holden, W. J. Stewart, and I. Youngs, ‘‘Extremely Low Frequency Plasmons In Metallic Mesostructures,’’ Phys. Rev. Lett., vol. 76, pp. 4773----4776, Jun 1996. [Online]. Available: http:/ / link.aps.org/ doi/ 10.1103/ PhysRevLett.76.4773 D. R. Smith, W. J. Padilla, D. C. Vier, S. C. Nemat-Nasser, and S. Schultz, ‘‘Composite Medium With Simultaneously Negative Permeability And Permittivity,’’ Phys. Rev. Lett., vol. 84, pp. 4184----4187, May 2000. [Online]. Available: http:/ / link.aps.org/ doi/ 10.1103/ PhysRevLett.84.4184 M. M. Bilgic and K. Yegin, ‘‘Wideband Offset Slot-Coupled Patch Antenna Array For X/ Ku-Band Multimode Radars,’’ IEEE Antennas and Wireless Propagation Letters, vol. 13, pp. 157----160, 2014. H. M. Bernety, R. Gholami, B. Zakeri, and M. Rostamian, ‘‘Linear Antenna Array Design For UWB Radar,’’ in IEEE Radar Conference (RADAR), April 2013, pp. 1----4. B. Kasi and C. K. Chakrabarty, ‘‘Ultra-Wideband Antenna Array Design For Target Detection,’’ Progress In Electromagnetics Research C, vol. 25, pp. 67----79, 2012.

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