L-Probe Fed Planar-Rectangular Microstrip Patch Antenna Mounted on a Cylindrical Ground Surface

Transcription

1 58 VO.5 NO. MARCH -Probe Fed Planar-Rectangular Microstri Patc Antenna Mounted on a Cylindrical Ground Surface Pradyot Kala * and Reena Pant * Dean, Researc & Develoment, Sree Ganati Institute of Tecnology, Gaziabad-INDIA Deartment of E&I Engineering, M J P Roilkand University, Bareilly-INDIA radyot_bly@yaoo.co.in Abstract: Tis aer resents a teoretical study of te different caracteristics of an -robe fed lanar rectangular microstri atc antenna mounted on a cylindrical ground structure. A transmission line model is used for te calculation of its inut imedance. Te radiation attern of te structure is obtained by aerture metod. Effect of ground curvature and different arameters of -robe (vertical lengt) are studied. It is found tat -robe rovides a wide-band imedance bandwidt wic is furter enanced by mounting te atc on a cylindrical ground lane. A curved grounding structure can increase te 3 db beamwidt of a lanar-rectangular atc. Teoretical results are comared wit existing exeriments results and a very good agreement is observed. Index Terms: Wideband antenna, Cylindrical Ground -robe feed, Rectangular atc, Microstri antenna. I. INTRODUCTION Te raid growt of wireless communications resulted in te roduction of a large variety of wireless devices. In wic broad imedance bandwidt is greatly desired. Toug te Microstri antenna as te benefit of ligt weigt, low fabrication cost besides its ability to integrate well wit microwave circuits but its inerently narrow imedance bandwidt is its major drawback wic needs focused remedial effort. Various tecniques to enance its imedance bandwidt, suc as alications of matcing networks, multile resonators, stacked structure, te reactive loading using U-saed slot, lossy materials, and te caacitive robe-fed structure ave been reorted in te oen literatures [-8]. Recently, some aers using an -saed robe wic allows single layered structure ave been exerimentally demonstrated. Tis -saed robe is an excellent feeding tecnique for a tick substrate ( ~. λ ) microstri antenna, were te ig inductive effect of te long robe can be suressed by te caacitive effect introduced by te bend of te robe itself [9-7]. -robe feeding tecnique was first resented by uk et al [9-] in 998. A foam layer wit a tickness of around % of te wavelengt was used as te suorting substrate. An imedance bandwidt of 35% and an average gain of 7.7dBi were acieved. Te radiation attern was stable across te assband. In anoter aer uk et al roosed a novel -robe roximity fed U-slot wideband single-layer single-atc antenna. Tey found tat using two broadening tecniques simultaneously te roosed antenna aear to be 9% wider tan tat using te U-slot alone and 5% wider tan tat using te - saed robe alone. Tey also roosed a dualband microstri antenna comosed of two atces and two -robes. Te antenna dislays wide-bandwidt caracteristics and can be imlemented as a dual-mode GSM/PCS base station antenna. In 995, i et al [7] observed tat te bandwidt can be furter increase wit te eigt of te atc. However, if te atc eigt is furter increased, te extra induced inductance (due to vertical ortion of -robe) will be too large to be cancelled efficiently by te caacitance (due to orizontal ortion of te robe) over a wide range of frequencies. As a result, imedance bandwidt decreases wen te atc eigt exceeds. λ. Tus tey roosed an -robe fed lanar atc antenna wic was mounted on a cylindrical or a conical surface, wic furter increase te effective eigt of te atc witout lengtening te vertical ortion of te robe, for good matcing over a wide bandwidt. Te investigation of te IJMOT ISRAMT

2 59 VO.5 NO. MARCH caracteristics of te roosed antenna was based on simulated and measured results. Tey found tat db bandwidt enanced by 5% tan tat of te atc mounted on a flat ground lane, and a 3 db beamwidt is gradually widened wit te reduction of te cylinder radius. Te work done by researcers till date are eiter exerimental or simulated and no teoretical model as been resented so far, terefore, te resent work roosed a teoretical analysis of an -robe fed lanar structure mounted on a cylindrical grounding structure. Te effects of air-ga tickness, lengt of vertical/orizontal section of -robe, and radius of ground lane on te return loss are comuted teoretically. Te effect of cylinder radius on radiation attern is also comuted teoretically and results are comared wit measured/simulated results available in existing literature literatures. II. THEORY A. Structure Te geometry of te roosed antenna is sown in Figures -. Different arameters of -robe fed antenna are as follows: Substrate Permittivity: ε r (air) -Stri Parameters: W r robe diameter, v vertical eigt of -robe, y distance of robe end oint from one of te radiating edge, orizontal lengt of -robe, and ga between atc and orizontal art of -robe, and 5 Ω (caracteristic imedance of coaxial cable). Patc Parameters: lengt of atc, W widt of atc, center eigt of antenna from ground. Ground Parameters: R Radius of ground cylinder. B. Equivalent Circuit of -Probe Figure 3 sows te R--C equivalent circuit of a lanar rectangular atc mounted on a curved cylindrical ground and fed by an -robe. It is observed tat, te vertical ortion of -stri is equivalent to a series combination of resistance R vs and inductance vs and can be written as [8] R vs vs π f µ σ W () W.3 ln nh () W were in meter But, te equivalent circuit of orizontal ortion of -stri as two arts. A series combination of distributive resistance R s and inductance s offered by an infinitesimally smal orizontal ortion of -stri ( ) as R π f µ σ s W s.3 ln W W nh (3) (4) and distributed caacitances C f-g and C f- over te orizontal section of -robe for a lengt of. Were C f-g caacitance between a infinitesimally small orizontal ortion of - stri ( ) and te ground ε rε W C f g (5) IJMOT ISRAMT

3 6 VO.5 NO. MARCH v SMA y o Rectangular Patc Air as Substrate Ground Plane (A) W and, C f- caacitance between infinitismally small orizontal ortion of -stri ( ) and te atc ε rε W C f (6) Tus te equivalent model of -stri feed can be roosed as series combination of equivalent circuit of its orizontal and vertical sections. C. Equivalent Circuit of Patc and Ground Te equivalent electrical circuit of te atc at te -robe-end oint will be arallel combination of R,, and C [8]. C C atc cos π y eff (7) C C C (8) atc g f (B) Figure : An -robe fed lanar-rectangular atc antenna (A) Front View, (B) Isometric view for lanar ground. W C ω (9) r r Q R () ω C were, C -g is a arallel late caacitance between rectangular atc and te ground R C W ε rε eff g () eff eff is te effective eigt of an equivalent arallel late caacitor wose caacitance is equal to resent structure and eff is te effective lengt of te atc wic includes te fringing lengt and ence includes te effect of fringing caacitance (C f ) [8]. D. Inut Imedance Figure : Isometric view of an -robe fed lanar-rectangular atc antenna mounted on a cylindrical ground surface Tus te equivalent circuit of -robe fed rectangular atc antenna can be obtained by combining te equivalent circuit of -robe wit IJMOT ISRAMT

4 6 VO.5 NO. MARCH C f- s R s Patc C f R vs y C -g C f vs v C f-g SMA Ground Plane Figure 3: R--C equivalent circuit of -robe fed atc Equivalent circuit of T.. equivalent of T.. equivalent of Equivalent circuit of vertical -robe section orizontal -robe section orizontal -robe section atc at y distance outside te atc inside te atc vs R vs R C -y y in Figure 4: T.. equivalent circuit of -robe fed rectangular atc antenna te arallel R--C equivalent circuit of rectangular atc, as sown in Figure 3. From te above equivalent circuit, we can directly comute inut imedance of te atc as in R vs jω vs tan tan ( α[ y ]) ( α [ ]) y () were Rs jωs (3) jω C f g ( Rs jωs ) j C f g γ ω (4) tan tan ( α y ) ( α ) y (5) IJMOT ISRAMT

5 6 VO.5 NO. MARCH R jω s s (6) jω ( C f g C f ) ( R jω ) j ( C C ) s s ω f g f γ (7) F U z ε e 4π r sin c ( ς ) jk r. e V jγ Re jkrsin sin φ α cos dα (4) atc R jπ f C jπ f (8) were [( R ) secα R] φ k ς sin. cos (5) Using te values of inut imedance, return loss and VSWR can be comuted using te following relations reflection coefficient in o (9) o VSWR s () in Re turnoss log db () E. Radiation Pattern in in α α γ kr sin sinφ cos cos sin (6) and U are te left most and rigt most inclination angle of te elements from te reference line wic contributes on radiation. Total far field radiation for two curved slots saced at distance aart may be written as [8] F (7) xt keff F yt Fy cos sin cosφ (8) Aerture model of an antenna is constructed by utting all te radiating aerture slots along te eriery of te antenna. Te slots are assumed to be aving a widt equal to te ga between te atc and te ground at a oint wic is due to te fringing field effect [9]. Te aerture excitation field corresonding to TM, mode (widts are radiating edges) is given by Y F () F U y x ε e 4π r sin c ( ζ ) jk r. e V jγ Re α sin dα jkrsin sin φ (3) α Reference line Figure 5: Ray Diagram α IJMOT ISRAMT

6 63 VO.5 NO. MARCH keff F zt Fz cos sin cosφ (9) Te serical comonents of electric field vector generated by curved and straigt slots can be obtained by rectangular comonents, using te following relationsis [8] F ( F cosφ F sinφ) cos F sin (3) x y F F x sin φ F cosφ (3) φ y Tus te E-lane (XY lane) and H-lane (Y lane) radiation attern can be exressed as [8] ( XY lane) F F Fφ (3) ( Y lane) π F F Fφ φ (33) π III. DESIGN PARAMETERS Table design arameters of an -robe fed rectangular atc antenna mounted on a cylindrical ground (same dimensions as in [7]) Parameters Value [mm] W v ε r. r.5 y 6. R 5,,, infinite (ground lane) IV. RESUTS AND DISCUSSION In tis aer te antenna as been analyzed for (i) a flat ground, and (ii) a ground wit cylindrical radius of 5 mm, wile keeing te z rest of te antenna arameters fixed. Figure 6 sows te return loss of te antenna for tese two structures. From tis figure, it can be seen tat te imedance bandwidt of te -robe fed atc antenna increases wen te radius of te cylindrical structure decreases. Te equivalent circuit of te antenna is given in Figure 4, consisting of two resonant circuits. One resonance is due to te atc (i.e., arallel R C resonant circuit), and te oter resonance is due to te series combination of te inductance ( ) and resistance (R ) offered by - robe and te caacitance (C ) wic is due to te energy stored between robe-ground and robe-atc. Te broadband oeration of te antenna is due to tis double resonance. It is observed tat teoretical bandwidt of flat ground is aroximately 36 % wic can be increase u to 46% for a ground radius wic is equal to twice of te atc dimension. Tis enancement is basically due to sift of resonant frequency towards iger frequency range. Figure 7 sows te TM mode (Y- lane) radiation attern of te structure. It is observed tat te radiation attern for lower radius ground structure is flatter wit resect to flat ground structure. Te beamwidt for flat ground structure is 68 wile it is 6 in te case of curved ground surface. Tis observation imlies tat gain of te structure decreases as radius of ground decreases. It is also observed tat te teoretical results are in good agreement wit exerimental results obtained by i et al [7]. From equivalent circuit of -robe antenna (Figures 3 & 4) it is observed tat total caacitance offered by te orizontal section of -robe (due to resence of atc and ground) gets affected by its vertical eigt. It is minimum were lengt of vertical section is alf of atc eigt from ground lane and tends to infinity wen orizontal section of -robe reaces near te ground lane/te atc, because at tat extreme orizontal section do not carry IJMOT ISRAMT

7 64 VO.5 NO. MARCH any ower. Tus te total ower carried by te vertical section will transfer eiter to te ground lane (loss of ower) or to te atc. From Figure 8 it is observed tat maximum imedance matcing between -robe and atc is obtained at v 6 mm. Te resonance frequency of te atc sifted towards iger frequency side, wic is due to te series induct- ance roduced by te vertical section of - robe. On te oter and te resonance frequency offered by te -robe sligtly sifted towards lower frequency side due to (i) te series inductance roduce by vertical section of -robe, and (ii) te increase in caacitance wit sifting of orizontal section of -robe from te centre, wic reduces te overall bandwidt of te antenna. -5 Flat Ground Teoretical Radius 5 mm Teoretical Flat Ground Measured Radius 5 mm Measured -5 v 8 mm 7 mm 6 mm 5 mm 4 mm Return oss [db] Frequency [GHz] Figure 6: Te variation of return loss wit frequency for flat ground and R5 mm Return oss [db] Frequency [GHz] Figure 8: Te variation of return loss wit frequency for different vertical section lengt of -robe. (for flat ground). Angle [degree] Flat Ground Teoretical Radius -7 5 mm Teoretical Flat Ground Measured Radius -8 5 mm Measured -9 Relative Power [db] Figure 7: Te Y- lane radiation attern for flat ground and R5 mm Return oss [db] 8 mm 7 mm 6 mm 5 mm 4 mm Frequency [GHz] Figure 9: Te variation of return loss wit frequency for different vertical section lengt of -robe. (for R 5 mm). IJMOT ISRAMT

8 65 VO.5 NO. MARCH Figure 9 sows te variation of return loss wit te lengt of orizontal section of -robe. It is observed tat mismatcing (return loss) increases as te lengt of orizontal section deviates from its otimized lengt ( 6 mm). V. CONCUSIONS A broadband -robe fed lanar-rectangular atc antenna mounted on a cylindrical ground surface is teoretically analyzed. Altoug tis antenna requires a sligtly ticker substrate, it rovides large imedance bandwidt tan te antenna wit te same geometric arameters on lanar ground. Due to symmetric osition of te -robe wit resect to atc and ground curvature, te antenna resents linear olarization. REFERENCES [] H. F. Pues and A. R. Van de Caelle, An imedance matcing tecnique for increasing te bandwidt of microstri antennas, IEEE Trans. Antennas Proagat., vol. 37, , 989. [] D. M. Pozar and B. Kaufman, Increasing te bandwidt of a microstri antenna by roximity couling, Electron. ett., vol. 3, , 987. [3] R. Q. ee, K. F. ee, and J. Bobincak, Caracteristics of a two-layer electromagnetically couled rectangular atc antenna, Electron. ett., vol. 3, PP. 7 7, 987. [4] S. D. Targonski, R. B. Waterouse, and D. M. Pozar, Design of wideband aerture-stacked atc antennas, IEEE Trans. Antennas Proagat., vol. 46, 45 5, 998. [5] S. Gao,. W. i, P. Gardner, and P. S. Hall, Dualolarized wideband microstri antenna, Electron. ett., vol. 37,. 6 7,. [6] T. Huyn and K. F. ee, Single-layer single-atc wideband microstri antenna, Electron. ett., vol. 3,. 3 3, 995. [7] K.. Wong and Y. F. in, Small broadband rectangular microstri antenna wit ci-resistor loading, Electron. ett., vol. 33, , 997. [8] M. A. Gonzalez de Aza, J. aata, and J. A. Encinar, Broadband cavity-backed and caacitively robe-fed microstri atc arrays, IEEE Trans. Antennas Proagat., vol. 48, ,. [9] K. M. uk, C. Mak, Y.. Cow, and K. F. ee, A novel broadband microstri atc antenna, Electronics etters, vol. 34, , 998. [] K. M. uk, Y. X. Guo, K. F. ee, and Y.. Cow, robe roximity fed U-slot atc antenna, Electron. ett., vol. 34, , 998. [] K. M. uk, C. H. ai, and K. F. ee, Wideband - robe-feed atc antenna wit dual-band oeration for GSM/PCS base, Electronics etters, vol. 35,. 3-4, 999. [] Y. X. Guo, K. M. uk, and K. F. ee, U-slot circular atc antennas wit -robe feeding, Electronics etters, vol. 35, , 999. [3] C. Mak, K. M. uk, K. F. ee, and Y.. Cow, Exerimental study of a microstri atc antenna wit an -saed robe, IEEE Trans. Antennas Proagat., vol. 48, ,. [4] X. Guo, C. Mak, K. M. uk, and K. F. ee, Analysis and design -robe roximity fed-atc antennas, IEEE Trans. Antennas Proagat., vol. 49, ,. [5] Steven S.. Yang and K.M. uk, Wideband foldedatc antennas fed by -saed robe, Microwave Ot. Tecno. ett., vol. 45, , 5. [6] P. Hazdra, M. Mazanek, and J. Čermak, Wideband rectangular microstri atc antenna using -robe feeding system, Radioengineering, vol. 6,. 37-4, 7. [7] P. i, K. M. au, and K. M. uk, A study of te wide-band -robe fed lanar atc antenna mounted on a cylindrical or conical surface, IEEE Trans. Antennas Proagat., vol. 53, , 5. [8] I. J. Bal and P. Bartia, Microstri Antenna. Dedam, MA: Artec House, 98. [9] Reena Pant, Pradyot Kala, S. S. Pattnaik, and R. C. Saraswat Analysis of a circularly olarized cylindrical-rectangular microstri atc antenna, International Journal of Microwave and Otical Tecnology, vol. 3, , 8. IJMOT ISRAMT