Helical Antenna Performance in Wideband Communications

Transcription

1 Helical Antenna Pefomance in Wideband Communications Maja Šekelja 1, Zoan Blažević, Maino Maslać 3 1,,3 Univesity of Split, Faculty of Electical Engineeing, Mechanical Engineeing and Naval Achitectue, Ruđea Boškovića bb, Split, Coatia 1 msekelja@fesb.h, zblaz@fesb.h, 3 mmaslac@fesb.h Abstact: In this pape, we employed the electomagnetic theoy to the design of a.4-ghz wideband helix antenna. The designed antenna was fist simulated acoss wide bandwidth using 4NEC softwae, and then two identical antennas wee made fom available mateials and finally measued using a vecto netwok analyze. The measued and simulated esults ae found to be in a quite ageement. Modeate discepancies among measued and simulated esonant fequencies noted ae contibuted chiefly to the use of a high-loss PVC pipe as the dums fo the coils and, to a smalle degee, to the loss in the coppe at micowave fequencies and to the influence of the wie insulation. 1. INTRODUCTION Vaious applications in communication systems pesent a geat task fo antenna designes who ae always seaching fo ways to achieve desiable antenna chaacteistics and impove existing antenna designs. Since the fist intoduction of helical antenna by Kaus in 1946 [1], many eseaches have lagely employed compehensive analytical analysis, numeical and empiical techniques in ode to pedict adiation chaacteistics and impove helical antenna popeties. Helical antennas found impotant applications in WLAN communications and satellite systems, pimaily due to thei wide bandwidth, consideing the fact that they belong to taveling-wave type of antennas, and cicula polaization because of thei loop-dipole geomety []. Although, numeous modifications to the basic geomety of the conventional helical antenna have been poposed and used to impove antenna popeties [, 3, 5], in this pape we will focus on the conventional cylindical helical antenna geomety in ode to examine basic electomagnetic popeties of a helix in two modes of adiation. With the pupose of exploing the electomagnetic model of a helix configuation, we daw attention to the poblems of a pactical helical antenna design. In this pape, helix antenna was analyzed both theoetically and expeimentally. Section descibes the fundamental concept of a helical antenna design with the analytical definition of helical antenna adiation modes. Section 3 points out the main paametes fo helical antenna design. Section 4 pesents the pactical antenna design, the numeical simulation and empiical esults, along with the compaisons with the othe liteatue concening optimum design antenna paametes. Finally, section 5 concludes the pape.. THEORETICAL BACKGROUND OF A HELICAL ANTENNA Geneally, thee ae two basic opeating modes of a helix, the tansmission line mode [, 9], often elated to the popagation of the electomagnetic wave though an infinite helix, and the adiation mode, concening the helix antenna design. We shall examine electomagnetic popeties of a helix in two adiation modes, nomal and axial mode. A simple and pactical helix configuation, consisted of a conducto bent in a fom of a helix aound the imagined cylinde with pefectly conducting gound plane, is shown in Fig. 1. a). Helical antenna geomety is descibed with the following paametes: diamete of helix D, spacing between tuns S, cicumfeence of helix C = πd, pitch angle α = tan - 1 (S/πD), numbe of tuns N, total length of the antenna L = NS, total length of the wie L n = NL, wie length of one tun L = (C + S ) 1/ and adius of wie conducto a []. When the pitch angle is < α < 9, a helix geomety is fomed, but when α = o α = 9, we obtain a loop antenna of N tuns and a staight wie, espectively. The natue of these two modes of adiation depend on antenna size and geomety configuation compaed to the wavelength λ. When the dimensions of the helix ae small compaed to the wavelength, the maximum adiated field is pependicula to helix axis. In the nomal mode helix antenna, we use the appoximation that the total adiated fa field is a sum of the E θ and E φ components of N shot dipoles with a length S, and N small loops with a diamete D, espectively []: and E ki Se = (1) jk jη sinθ θ ( /) jk k D I e E = η sinθ, () φ 4 whee I is constant cuent acoss a tun, k = π/λ is the popagation constant and is distance coodinate in spheical coodinate system (, θ, φ) fom the oigin of the helix geomety. Deivation of (1) and () is given in Appendix. In geneal, these electic field components ae out of phase so the total fa field of a nomal mode helical antenna is

2 usually elliptically polaized [3] and mostly inconvenient fo many applications due to its naow bandwidth of adiation. Axial mode of adiation is achieved when the cicumfeence of helix is appoximately one wavelength, obtaining the constuctive intefeence of waves fom opposite sides of tuns, hence ceating the maximum adiation along the axis. Helical antenna in this mode of adiation is ciculaly polaized taveling-wave wideband antenna. Using the appoximation that helix is an antenna aay of N equally spaced and unifomly excited elements [3], we obtain total fa field patten, E tot, by multiplying the antenna facto with the patten of a cicula loop element appoximated as cosθ [3]: E tot [( N ) ψ ] [ ψ ] π sin / = sin cosθ N sin /, (3) whee ψ and p ae defined fo odinay end-fie adiation []: L ψ = k cos S θ p, (4) and N + 1 AR =, (8) N HPBW = 5. (9) C S N λ λ The discepancy fom the pue cicula polaization, descibed with axial atio AR, depends on the numbe of tuns N and it appoaches to unity as the numbe of tuns inceases. Because of the tavelling-wave natue of the axial-mode helix, the input impedance, given in (7), is mainly esistive and insensitive to fequency vaiations ove a wide bandwidth of the antenna. Based on a geat numbe of expeimental esults, King and Wong developed moe accuate expessions fo these antenna paametes [5, 6]. Fo example, they established that in the equation (6), valid fo 1 < α < 15, 3/4 < C/λ < 4/3 and N > 3, numeical facto can be be much lowe than 15, usually between 4. and 7.7 [4], poviding a new expession fo the helical antenna gain: L / λ p = S / λ + 1. (5) N π D NS tan1.5 Ge = 8.3 λ λ tanα p p N. (1) 3. PARAMETERS FOR THE ANTENNA DESIGN Classical design data in [1] define optimum design paametes in a elatively naow ange pactically neglecting the influence of the wie s diamete on the antenna s chaacteistics. The conclusions in [4], bought on extensive computations and compaisons with numeous empiical and numeical esults in [1, 5, 6], established optimum paametes fo helix antenna design and evealed the influence of the wie s adius on helix antenna popeties. The optimization of a helix antenna design in [4] was accomplished by geat numbe of computations of vaious antenna paametes poviding staightfowad ules fo a simple helix antenna design. Empiical fomulas fo antenna gain G, input impedance R in ohms, half powe beamwidth HPBW in degees and axial atio AR, developed by Kaus [1], ae given by: C S G = 15N λ λ, (6) R = 14 C, (7) λ whee λ p is wavelength at peak gain [6]. 4. ANTENNA SIMULATION AND MEASUREMENT We designed an axial mode helix antenna fo the cental fequency of 43 MHz, as in [7], with the following paametes: λ = 1.34 cm, D = 4 mm, C = 13 mm, S = 33 mm, L = 4 cm, N = 1, a = 1 mm, α = 14, AR = 1.4, C/λ = 1.7 and L/C = 3. Expected gain was in the ange of (14 ± 1.5) dbi, accoding to [4]. Hence, the chosen pitch angle, the axial atio and the cicumfeence wee in ageement with equiements established in [4] fo an optimum wideband helical axial mode antenna design. As we can see fom Fig. 1. b), the insulated wie conducto was wound aound the cylindical PVC pipe and attached to the ound coppe eflecto with a diamete of 17 cm. Adjustment element was made of coppe sheet, accoding to [7]. Both modes of adiation wee simulated in 4NEC, based on Numeical Electomagnetic Code (NEC) and Method of Moments, using linea wie segments. In [8] some conclusions wee made about the impovements that can be achieved by using a Method of Moments compute code, called the Cuved code, by using the cuved basis and testing functions.

3 a) b) c) Figue 1 Helical antenna: a) standad helical geomety [3], b) pactical helical design and c) helical geomety simulation in 4NEC. The simulation with a sufficient numbe of linea wie segments and unde the limitations with espect to the detemined wavelength and the numbe of segments and thei length, showed satisfying esults fo this simplified simulations. These numeical simulations consideed only infinite pefectly conducting flat gound plane (theoetically) and pefectly conducting wie in a vacuum. Thin wie appoximation was the main limiting facto in numeical modelling and simulation of the helical antenna due to the geat numbe of segments needed in helix geomety appoximation. The helical antenna model, consisted of 49 segments (Fig. 1. c)), was simulated at thee diffeent fequencies in ode to emphasize the antenna pefomance at two diffeent adiation modes. Fig.. a), b) and c) show, fom left to ight, the simulation esults of fa field 3D adiation patten, the amplitude and phase cuent distibution along the wie, vetical and hoizontal fa field D patten, at thee diffeent fequencies. These simulation esults in Fig. confim that the helical antenna configuation achieves a high gain of 13.8 dbi in axial mode at the design fequency of 43 MHz, which is in ageement with the esults in [4], while nomal mode is achieved at lowe fequencies, fo example at 18 MHz. The fa field pattens at 4 MHz illustate that helix dimensions ae big compaed to the wavelength and the axial mode is stating to collapse into a nomal mode. Simulated SWR esults of a helical antenna in Fig. 3. a) and b) show vey good match (SWR nea o lesse) fom 15 MHz to 5 GHz, especially in the egion of 3 GHz. The expeimental SWR esults of both designed helical antennas ae geneally vey simila to each othe, diffeing only in a cetain phase shift. Also, we can notice a good match fom 13 MHz to 5 GHz, paticulaly aound 5 MHz almost as if the simulated and the expeimental esults diffe only in a few hunded megahetz fequency shift, which was actually expected. It is impotant to highlight that the fequency esolution in expeimental esults was moe than two times bette than in the simulation esults. Also, consideing the fact that the eal antenna design includes lossy PVC pipe inside helix instead of a vacuum, and the finite ound coppe eflecto instead of infinite pefectly gound plane, we can obseve a good ageement of expeimental and simulated esults concening the SWR and wideband measuements. The expeimental esults, shown in Fig. 3. c) and d), exhibit impulse esponses in wideband measuements, based on a vecto netwok analyze with tansmitting and eceiving helical antennas. Using these esponses to evaluate the antenna gains and the Fiis equation fo fee space loss (11) between two identical antennas sepaated by a distance d, we obtain a helical antenna gain G h of appoximately 1.5 dbi estimated fo a 3 8 MHz fequency band: a FS λ = Gh d (11) Fig. 3. d) shows the atio of appoximately 6 dbi between the E θ and E φ component of the adiation field at 4 MHz, achieving the antenna gain of appoximately.5 dbi estimated in a nomal mode of adiation. The encounteed poblems, egading lossy PVC mateial used in ou antenna design, included high conductivity of a PVC as well as the inceasing pemittivity (in compaison to vacuum) inside the helix.

4 a) f = 18 MHz λ = 1,67 m b) f = 43 MHz λ = 1,3 cm c) f = 4 MHz λ = 7,5 cm Figue Radiation pattens in the case of infinite flat gound plane fo vaious fequencies, fom left to ight: the fa field 3D adiation patten, the amplitude and phase cuent distibution along the wie, vetical and hoizontal fa field D patten.

5 SWR a) SWR The SWR of two designed and one simulated helical antenna helix1 1 helix1 helix 4NEC Fequency (MHz) The SWR of two designed and one simulated helical antenna helix1 1 helix1 helix 4NEC 5. CONCLUSION The adiation chaacteistics of helical antennas wee examined on the basis of the electomagnetic theoy and compaed to the given liteatue. The designed antenna was simulated using 4NEC softwae and measued with the vecto netwok analyze. Theoetical, expeimental and simulation esults wee in quite ageement. Cetain discepancies among measued and simulated esults wee contibuted to the common poblems in pactical design of the antenna, e.g. the use of a high loss PVC dum. The esults, illustated in this pape pesent the intoduction in ou helical antenna eseach. Vaious applications of a helical antenna in wideband communications and a geat numbe of modifications of its basic geomety poducing diffeent adiation popeties, pesent a geat challenge fo antenna designes and eseaches in development and pediction of new and impoved helical antenna designs. The futue wok will be focused on developing a suitable electomagnetic model of helical wie stuctues. Powe (db) Powe (db) b) Fequency (MHz) c) Wideband measuements fom 3-8 MHz, axial mode helix d = 1 m d = m Time delay (μs) Wideband measuments fom MHz, d = 1m nomal mode helix axial mode helix Time delay (μs) d) Figue 3 SWR helical antenna measuements a) fom 13 1 MHz and b) fom 1-5 GHz, and impulse esponses of adio channel with helical antennas c) and d). REFERENCES [1] J. D. Kaus: Antennas, McGaw Hill, nd ed.,1988. [] C. A. Balanis: Antenna Theoy Analysis and Design, John Wiley and Sons, Inc., 3 d ed., 5. [3] I. Ghoeishian: The Spio-Helical Antenna, M. S. thesis, Viginia Politechnic Institute, August 1999, etdig.pdf. [4] A. R. Djodjevic, A. G. Zajic, M. M. Ilic, G. L. Stube: Optimization of Helical antennas [Antenna Designe's Notebook], IEEE Antennas and Popagation Magazine, Vol. 48, No. 6, Decembe 6, p.p [5] H. E. King, J. L. Wong: Chaacteistics of 1 to 8 Wavelength Unifom Helical Antennas, IEEE Tansactions on Antennas and Popagation, Vol. AP-8, No., Mach 198, p.p [6] J. L. Wong, H. E. King: Empiical Helix Antenna Design, IEEE Intenational Symposium on Antennas and Popagation, May 198, p.p [7] R. den Besten: Helical/helix antenna cookbook ecipe fo.4 GHz and/o WiFi applications, [8] E. D. Caswell: Analysis of a Helix Antenna Using a Moment Method Appoach With Cuved Basis and Testing Functions, M. S. thesis, Viginia Politechnic Institute, Septembe1998, theses/available/etd / unesticted/etd.pdf. [9] K. L. Coum, J. F. Coum: RF Coils, Helical Resonatos and Voltage Magnification by Coheent Spatial Modes, Micowave Review, Septembe 1, p.p. 1-1.

6 [1] D. Poljak, V. Doić, S. Antonijević: Compute Aided Design of Wie Stuctues: Pat I, Univesity of Split, 7. APPENDIX Radiated fields of the basic antennas Thin staight dipole The cuent of an infinitesimal (l << λ) and vey thin linea dipole (a << λ), positioned at the oigin and oiented along the z- axis, is taken to be constant along the wie: I e ( x', y', z' ) = ezi, (1. a) and magnetic vecto potential can be calculated fom: jkr μ e A( xyz,, ) = ( x', y', z' ) dl' I e. (. a) R C Geneally, (x, y, z) ae the obsevation point coodinates and (x, y, z ) the coodinates of the souce, R is the distance fom any point of the souce to the obsevation point in fa field egion (R = ) and C is the path along the length of the souce. The length of wie l equals S accoding to appoximation in Sec. and the infinitesimal section of linea wie dl equals dz. Thus, the magnetic vecto potential can be calculated as in []: l + jk μil z l π μi A( xyz,, ) = ez e dz' = e e 4 jk. (3. a) When the tansfomation between the ectangula and spheical components is pefomed, we obtain []: μil jk A = e μil Aθ = e A = φ cosθ jk sinθ. (4. a) Using the continuity equation fo the magnetic vecto A and electic scala ϕ potential function [1]: A= jωμεϕ (5. a) and the elation fo the time hamonic electic field: E = ϕ jω A, (6. a) we can calculate the electic fa field component E θ fom (4. a): Small loop ki sin l θ jk Eθ = jη e. (7. a) If we assume a thin loop antenna with a vey small cicumfeence, positioned on the x-y plane, at z =, then the cuent distibution can be appoximated to be constant along the wie: Iφ = I. (8. a) The magnetic vecto potential is also calculated fom equation (. a) whee dl is the infinitesimal section of the loop antenna with the adius a = D/. Pefoming the tansfomation between ectangula and spheical components, we obtain: and dl ' = adφ ' (9. a) ( ) R a asinθ cos φ φ' = +. (1. a) Finally, the cuent I e in spheical coodinates is given by: Ie = e Iρ sinθcos ( φ φ' ) + Iφ sinθsin ( φ φ' ) + Iz cosθ + + eφ Iρ sin ( φ φ' ) + Iφ cos ( φ φ' ) +. + eθ Iρ cosθ cos ( φ φ' ) + Iφ cosθ sin ( φ φ' ) Iz sinθ (11. a) The magnetic vecto potential in fa field egion is given by: kμa I sin θ A jk ea φ φ = ej φ e (1. a) 4 and the electic fa field component E φ by: ( ka) I sinθ jk Eφ = η e. (13. a) 4