Printed Compact Lens Antenna for UHF Band Applications

Transcription

1 Prgress In Electrmagnetics Research C, Vl. 6,, 6 Printed Cmpact Lens Antenna fr UHF Band Applicatins Kyadan Krth Ajith * and Amitabha Bhattacharya Abstract A new micrwave lens antenna suitable fr ultra-high frequency (UHF: 3 MHz 3 GHz) band applicatins is prpsed. An imprved bw-tie antenna and a planar metamaterial lens design is presented. db imprvement in bresight gain and a directive radiatin pattern is achieved with the lens. The applicatin f the designed antenna is demnstrated in a grund penetrating radar (GPR) experiment. The size f the antenna is very cmpact cmpared t ther antennas fund in the literature used fr similar applicatins.. INTRODUCTION The crucial requirements fr a GPR antenna are: ultrawide bandwidth at lw frequency f peratin and high frnt-t-back rati, keeping mderately gd gain and radiatin efficiency t meet the pwer budget. Resistively laded bw-tie antenna is suitable fr GPR applicatins because f its ultra-wide bandwidth and its ability t radiate shrt pulses. Hwever, radiatin efficiency has been very lw fr typical resistively laded antennas due t lt f pwer dissipated in the resistrs. Wu et al. [] prpsed a cavity backed, half ellipse shaped antenna with lumped resistr lading t suppress any reflectins frm the end. Wang et al. described the design f bw-tie antennas with high radiatin efficiency []. Lestari et al. [3] prpsed bw-tie antenna with lumped resistrs lading. In their paper, time dmain pulse radiating ability f the antenna was analyzed. Frequency dmain antenna parameters f this antenna, such as the radiatin pattern, gain and frnt-t-back rati, were nt discussed by them. In [4] capacitive slts were cut n the arms f the bw-tie, and micrwave absrbers were put n each arm, t prvide a cmbined resistive and capacitive lading. Absrbers n the back side f the antenna als helped in suppressing back-lbe. The impedance bandwidth (VSWR :) f this antenna was frm. t. GHz. A stable radiatin pattern was btained up t abut 3 GHz. The input impedance was Ω making it difficult t feed directly using a caxial cable. Uduwawala et al. [] gave a detailed study f bw-tie antenna with resistr laded at the ends. Vee diple antennas [6, 7] were als cnsidered fr GPR applicatins. Lgarithmic spiral antenna [8] is anther candidate fr similar applicatins at UHF bandwidth. The advent f micrwave lens antennas dates back t 946 when Kck [9] intrduced metal lens antennas by prducing artificial dielectrics. Artificial lenses are preferred ver dielectric lenses at micrwave frequencies because the latter tends t be bulky and heavy. A planar lens is in many ways advantageus cmpared t a cnvex lens. Planar lens is easy t fabricate and is less bulky. Pzar prpsed a planar lens in []. Micrwave lenses have evlved using cncepts f gemetrical ptics. Mst f the lens designs in literature are cncerned with GHz frequencies. Lens antennas have a great ptential that bandwidth independent antennas can be made if we can design the lens with desirable prperties. Waveguides are the simplest examples f artificial dielectrics having refractive index less than unity since inside the waveguide, phase velcity is greater than the velcity in free space. Studies n single-layer Received 7 Nvember, Accepted 3 January 6, Scheduled 4 February 6 * Crrespnding authr: Kyadan Krth Ajith (ajithkkrth@gmail.cm). The authrs are with the Department f Electrnics & Electrical Cmmunicatin Engineering, Indian Institute f Technlgy Kharagpur, India.

2 Ajith and Bhattacharya planar lenses have been fund scarce in the literature. Lens designs at sub-ghz frequencies are even mre scarce. With the increasing ppularity f metamaterials, the planar lens antennas are becming ppular again. Manipulatin f electrmagnetic fields by metamaterial is extensively discussed in []. Zhu et al. [] presented a metamaterial superstrate fr cnversin frm linear t circular plarizatin. They als btained an imprvement in antenna gain. Imprvement f directivity f Vivaldi antenna has been presented in [3]. Qi et al. [4] succeeded in cntrlling the radiatin pattern ver a bandwidth frm t 8 GHz. This is a significant effrt because metamaterials are inherently narrwband structures. Planar flded diple antennas [] were als used with metamaterials. Erentk et al. [6, 7] have dne wrk with sub-ghz frequency metamaterials. In this paper, we describe a resistively laded bw-tie antenna and its imprvement in perfrmance characteristics with the applicatin f a planar lens. Sme f the results have been presented in a cnference paper [8]. The experimental results including GPR scan result fr detecting an bject buried under sil are described in this paper. The rest f the paper is rganized in three sectins. In Sectin, the design f the antenna is presented in detail. Sectin 3 describes measurement results and discussins n it while Sectin 4 cncludes the wrk.. ANTENNA DESIGN.. Imprved Bw-Tie Antenna Design Bw-tie antennas have been studied systematically by a number f researchers by using cnfrmal transfrmatin. DuHamel and Isbell [9] described the design f frequency independent antennas including bw-tie antenna and shwed that it can be transfrmed t a pair f cplanar lines by a simple cnfrmal mapping. Carrel [] derived expressins fr the characteristic impedance f infinite cplanar fin antenna. Mst f the bw-tie antenna designs have been ptimized fr Ω input impedance. Recent develpments n the analysis f printed cplanar strips based n cnfrmal mapping [, ] have added t ur understanding f bw-tie antenna. Ideally, a bw-tie antenna is infinite in extent and is specified nly by angles. But in practice, the antenna has t be finite and can be cnsidered as a bradband antenna with a stable radiatin characteristics ver the bandwidth. A bw-tie antenna with aflareangleθ and radius r is represented in the plar crdinates by, z = re jθ () This analytic functin can be cnfrmally transfrmed int w-plane by the functin, w =lnr + jθ () Figure shws the pictrial representatin f the bw-tie antenna in the z-plane described by Eq. (), and its cnfrmal transfrmatin in the w-plane, represented by Eq. () in Fig.. Fig. (c) represents the elliptical bw tie. Cnsider the elliptical arm as cmpsed f narrw strips f cncentric circles. Each circular strip transfrms t a rectangular strip in the w plane. By jining all these strips tgether we get the cnfrmal transfrmatin in Fig. (d), a gradually tapered cplanar transmissin line. The t infinity t infinity θ 6 4 t infinity 3 t infinity In r In r (c) (d) θ Figure. Cnfrmal transfrmatin. Infinite bw-tie. Cnfrmal transfrmatin f. (c) Elliptical bw-tie. (d) Cnfrmal transfrmatin f (c).

3 Prgress In Electrmagnetics Research C, Vl. 6, CPS-Z (Ohm) 4 3 ε =. ε = 4.4 ε = r r r CPS-Z (Ohm) 4 3 h/b =. h/b =. h/b = a/b a/b Figure. Characteristic impedance f cplanar stripline. Characteristic impedance variatin f cplanar stripline w.r.t. substrate permittivity. Characteristic impedance variatin f cplanar stripline w.r.t. its dimensins. Bwtie Planar lens Figure 3. Final prttype f bw-tie antenna. Phtgraph f the fabricated bw-tie antenna. The arm f the bw tie. variatin f characteristic impedance f the twin strip line is shwn in Fig. as btained frm [3]. The dimensins are shwn in the inset f the figure. It can be seen frm the figure that, as a/b rati decreases, the characteristic impedance als decreases. S in the case f elliptical bw-tie we initially have a lwer characteristic impedance. It is als knwn frm Carrel [] that the impedance f the bwtie antenna decreases as the flare angle increases. Fig. shws variatin f characteristic impedance when relative permittivity is, 4.4 and. An FR4 substrate with a relative permittivity 4.4 and thickness. mm has been chsen in this design. The feed pint impedance will depend n the curvature f the ellipses and can be ptimized t give a gradual transitin in the input impedance t match Ω s that it can be fed easily with a micrstrip t parallel strip line transitin. Cncentric slts are cut n the elliptical arm in rder t prvide a resistive lading. The designed antenna is an RC-laded bw tie inspired by Lestari et al. [4] with sme mdificatins. The antenna is fed with a Ω cax thrugh a micrstrip t parallel strip line transitin. An artificial dielectric lens is then placed in frnt f the antenna t fcus the radiatin. Figure 3 shws the fabricated bw tie with lens. Fig. 3 gives a clse lk at ne arm f the bw tie at the feed lcatin. A sheet f graphite f mm thickness is put ver each arm t btain resistive lading. The resistivity f graphite has been determined by using Hall effect measurement and is.33 4 Ωcm. Varius design parameters with reference t Fig. 3 are: Ls = mm, Ls =4.8mm, Ls =4.7mm, Ls 3 =4.6mm... and s n. G =.mm, G =.3mm, G =.4mm...,etc.

4 4 Ajith and Bhattacharya The slt width increases as we mve away frm the feed pint and hence the effective resistive lading increases twards the end. The verall size f the antenna is 3 3 cm... Metamaterial Lens Figure 4 depicts the fcusing actin using artificial materials. In the figure, medium represents air, and medium represents a material belnging t the particular quadrant. The quadrant materials are DPS (Duble Psitive) materials, and quadrant 3 represents the DNG (Duble Negative) materials. These tw types f materials can be utilised fr lens applicatin. Quadrants and 4 media d nt allw the wave t prpagate. A planar lens has been designed by intrducing metallic pattern inclusins n an FR4 epxy substrate as shwn in Fig.. The metallic inclusins serve t increase the refractive index f the Figure 4. Fcusing actin by materials in first and third quadrants. Figure. Metamaterial lens. Designed 3 array f unit cells. The unit cell frm [6].

5 Prgress In Electrmagnetics Research C, Vl. 6, 6 Table. Design values in mm. a e.774 h k. b 3 f. i l.938 c g j m. d.696 H = tan.8.6 E = tan Magnitude.4. S S... 3 Figure 6. Simulatin f the unit cell in CST Micrwave Studi. Simulatin setup. Magnitude f S vs. frequency. medium. But ne has t be careful t minimize the reflectins frm the lens surface. The basic unit cell is frm the paper by Erentk et al. [6] shwn in Fig.. Erentk et al. used their design with lumped element lading, and the material was a narrw-band vlumetric metamaterial. But in this wrk the ENG unit cell frm their design is ptimized fr lens design. It has been used withut the lumped elements and has been fund t have a very brad band respnse. The final designed values f unit cell dimensins are given in Table. The simulatin is carried ut with the time-dmain slver in CST micrwave studi with the bundary cnditins shwn in Fig. 6. Perfect electric and perfect magnetic bundary cnditins have been applied n the bundaries parallel t y-axis and x-axis, respectively. In z-directin, pen bundary cnditins have been applied. This simulates an array f unit cells, infinite in extent in x and y directins. The transmissin S parameter btained frm this simulatin is shwn in Fig. 6. The size f the unit cell at 3 MHz is nly λ/. At 3 GHz, the unit cell size is λ/. Between these frequencies, the lens acts like a resnant metamaterial and can exhibit a negative refractive index. The individual atms getting plarized under the influence f the incident electric field make the planar structure a Huygen s surface [4]. A sharp dip in S is bserved at.8 GHz. But this result is btained upn nrmal wave incidence. In the case f ur planar lens, being illuminated by the bw-tie antenna, the angle f incidence is nt always perpendicular. In a recent study by Ginis et al. [], it has been shwn analytically that very thin metasurface can exhibit a bradband behaviur irrespective f the resnant nature f the unit cell. This is evident frm ur experimental results presented in Sectin 3. The distance f the lens frm the bw-tie antenna has been determined by simulatin fr minimum reflectin frm the lens surface and maximum gain ver the bandwidth. The lens respnse t dminant plarizatin cmpnent f electric field is shwn in Fig. 7. The hrizntal axis represents distance alng the antenna, and the vertical axis represents distance away frm the antenna. The designed bw-tie antenna is placed alng the hrizntal axis. Figs. 7(c) and 7(d) shw the bw-tie antenna with the lens placed alng the x axis at 3 mm distance n the vertical axis. The phase f the E x cmpnent f electric field vectr emanating frm the bw tie is shwn. The bw-tie antenna has a spherical wavefrnt in the near field. The manipulatin f the wavefrnt emerging ut f the lens results in an increase in bresight directivity thrughut the band.

6 6 Ajith and Bhattacharya (c) (d) Figure 7. Phase manipulatin by the lens at different frequencies. Phase f E x at MHz, in the near field f bw-tie antenna: withut lens, with lens. Phase f E x at GHz, in the near field f bw-tie antenna: (c) withut lens, (d) with lens. 3. EXPERIMENTAL RESULTS AND DISCUSSIONS 3.. Return Lss and Input Impedance The return lss fr the cncerned frequency range is shwn in Fig. 8. The plt shws an ultra-wide impedance bandwidth extending frm 3 MHz t 3 GHz. It is seen that the presence f the metamaterial has nt affected the bandwidth. S the reflectins frm the lens surface is negligible. Return lss (db) Figure 8. Return lss vs. frequency measured simulated Realized Gain, Radiatin Pattern and Frnt-t-Back Rati Figure 9 shws the antenna gain pattern in E- and H-planes fr the entire frequencies f interest and cmpares it with the nrmal bw tie. Cmparing H-plane pattern f the bw tie in Fig. 9 with that f lens antenna in Fig. 9 and E-plane pattern f the bw tie in Fig. 9(c) with that f lens antenna in

7 Prgress In Electrmagnetics Research C, Vl. 6, Angle (degree) Angle (degree) Angle (degree) (c) Angle (degree) (d) Figure 9. Measured radiatin pattern in the UHF band. H-plane pattern f bw tie: withut lens and with lens. E-plane pattern f bw tie: (c) withut lens and (d) with lens db E-plane H-plane db E-plane H-plane db E-plane H-plane db db 8 (c) db 8 Figure. Radiatin pattern in E and H plane at selected frequencies. Fig. 9(d), the imprvement in bresight gain is clearly appreciable. The measured peak gain is. db at.4 GHz. The maximum gain btained withut lens is 6.4 db at 94 MHz. Figure 9 shws a clear imprvement in gain at deg (bresight) at all frequencies, and the gain is imprved by at least db at frequencies abve. GHz as evident frm Fig.. Radiatin patterns f the antenna at MHz,. GHz, and GHz are shwn in Fig.. At GHz, the antenna s majr beam splits, and bresight gain gradually decreases at higher frequencies. Nevertheless, it is useful fr applicatins as grund penetrating radar, because higher frequency signals d nt penetrate much int the grund. S, decrease in gain at thse frequencies is tlerable. Frnt-t-back rati is defined by the rati f gain in the bresight directin t that in the back side f the antenna. The nrmal bw-tie antenna has a frnt-t-back rati clse t db as evident frm the

8 8 Ajith and Bhattacharya Gain (db) with lens withut lens F/B rati (db) with lens withut lens Figure. Cmparisn f measured gain and frnt-t-back rati. Measured realized gain. Measured frnt-t-back rati. measurements. An imprvement f up t 6 db is btained fr the frnt-t-back rati by using the lens as seen in Fig Radiatin Efficiency Radiatin efficiency measurement is carried ut using the mdified Wheeler cap methd fr UWB antennas utlined in [6] by Schantz. Fr this, a hllw metallic sphere f cm diameter is used as shwn in Fig.. The radius r f the metallic shell is chsen such that r>λ/π at the lwest frequency f peratin. Fr the calculatin f radiatin efficiency, tw sets f measurements are needed: ne is reflectin cefficient f the antenna in free space, and the ther is reflectin cefficient f the antenna inside the metallic shell centered at the rigin. If S FS is the measured reflectin cefficient f the antenna in free space and S WC the measured reflectin cefficient f the antenna inside the shell, then the radiatin efficiency e r is calculated using the frmula [6]: e r = ( S FS )( S WC S FS ) (3) Figure shws the radiatin efficiency f the antenna. It can be seen that the efficiency f the designed bw-tie antenna with metamaterial lens is greater than 68.% frm 3 MHz t. GHz. with lens withut lens Radiatin Efficiency (%) Figure. Radiatin efficiency by using Schantz cap methd fr UWB antennas. Radiatin efficiency measurement setup. Radiatin efficiency vs frequency.

9 Prgress In Electrmagnetics Research C, Vl. 6, 6 9 Efficiency drps t 46.% at.8 GHz. Efficiency f the bw-tie antenna with lens is less than that withut lens in mst f the band because f the lsses within the metamaterial GPR Experiment An experimental GPR has been set up with the fabricated bw-tie antenna with lens and a hand-held vectr netwrk analyzer as shwn in Fig. 3. This GPR is used t detect an bject buried at a depth f cm in a test bed filled with sand. Fig. 3 shws the measured GPR B-scan image f the bject. The target is a metallic hllw rectangular bject f size 4 cm 3 shwnintheinsetfthe same figure. A brief descriptin f this measurement is given belw. This is a stepped frequency cntinuus wave (SFCW) type radar. The whle setup, shwn in the figure, is mbile, and the lwest rack cntains the antenna with lens just tuching the grund. The antenna is cnnected t prt f Vectr Netwrk Analyzer (Agilent FieldFx VNA-N996A). The VNA is interfaced with the laptp n the tp rack t acquire cmplex reflectin cefficient data. The data are acquired n regular interval as we mve the GPR ver the linear track n the surface where the target is buried. Each measurement instance is called an A-scan. These frequency dmain data are then inverse Furier transfrmed t extract the depth infrmatin. The aggregatin f all the A-scan, after applying sme data prcessing algrithms, gives the image f the target. This is called the B-scan image and shwn in Fig Cmparisn The designed antenna has a very cmpact size and very gd gain cmpared t the ther antennas fund in literature. The size and features f this antenna are cmpared with the ther antennas in the literature in Table. The wrk presented in this paper can cver the UHF bandwidth with the highest gain btained in the band while the size f the antenna is nly.3λ at the lwest frequency f peratin. Table. Cmparisn with ther GPR antennas fund in literature. Ref. Type Lwest perating frequency Highest perating frequency Max. gain in the perating bandwidth Frnt-t-back rati [4] RC laded bwtie MHz GHz Nt knwn Nt knwn cm [7] wire bwtie MHz GHz Nt knwn Nt knwn cm [3] Resistive laded MHz GHz Nt knwn Nt knwn 3 7cm bwtie [8] Cavity-backed lgarithmic 4 MHz 3.8 GHz 6.6 db db cm 3 spiral [6] Resistive laded Vee MHz 8GHz db Nt knwn cm diple [] Cavity backed Resistive laded half-ellipse MHz 7 MHz Nt knwn Nt knwn 3 8 cm bwtie This wrk bwtie withut lens 3 MHz 3GHz 6.4 db db 3 3 cm This wrk bwtie with lens 3 MHz 3GHz. db.39 db 3 3 cm Size

10 Ajith and Bhattacharya Figure 3. GPR experiment. Experimental GPR setup. GPR B-scan image f a target buried in sand. 4. CONCLUSION A cmpact printed lens antenna fr UHF band has been presented. Frm the measurements it has been verified that by the use f a lens, bth the gain and frnt-t-back rati have been imprved. An imprvement in gain up t db is btained cmpared t a bw-tie antenna withut lens. The imprvement in frnt-t-back rati is up t 6 db. Thus, mnidirectinal radiatin frm a bw-tie antenna can be made directive by this technique. The antenna is als cmpact in size cmpared t ther antennas fund in the literature, cnsidering its lwest frequency f peratin. The antenna is suitable fr UHF band, and its applicatin in grund penetrating radar is demnstrated. REFERENCES. Wu, B., Y. Ji, and G. Fang, Analysis f GPR UWB half-ellipse antennas with different heights f backed cavity abve grund, IEEE Antennas and Wireless Prpagatin Letters, Vl. 9, 3 33,. [Online]. Available: Wang, J., Y. Su, C. Huang, M. Lu, and Y. Li, Design f bw-tie antenna with high radiating efficiency fr impulse GPR, IEEE Internatinal Gescience and Remte Sensing Sympsium, 94 97, Jul.. [Online]. Available: arnumber= Lestari, A. A., E. Bharata, A. B. Suksmn, A. Kurniawan, A. G. Yarvy, and L. P. Ligthart, A mdified bw-tie antenna fr imprved pulse radiatin, IEEE Transactins n Antennas and Prpagatin, Vl. 8, N. 7, 84 9, Jul.. [Online]. Available: lpdcs/epic3/wrapper.htm?arnumber= Lestari, A., A. Yarvy, and L. Ligthart, RC-laded bw-tie antenna fr imprved pulse radiatin, IEEE Transactins n Antennas and Prpagatin, Vl., N., 63, Oct. 4. [Online]. Available: Uduwawala, D., M. Nrgren, P. Fuks, and A. Gunawardena, A deep parametric study f resistr-laded bw-tie antennas fr grund-penetrating radar applicatins using FDTD, IEEE Transactins n Gescience and Remte Sensing, Vl. 4, N. 4, 73 74, Apr. 4. [Online]. Available: 6. Kim, K. and W. Sctt, Design f a resistively laded Vee diple fr ultrawide-band grundpenetrating radar applicatins, IEEE Transactins n Antennas and Prpagatin, Vl. 3, N. 8,

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