A Wide-Beam Broadcasting Antenna Using a Curved Dipole on Reflector Plane

Transcription

1 1th WSEAS Iteratioal Coferece o COMMUNICATIONS, Heraklio, Greece, July 3-5, 8 A Wide-Beam Broadcastig Atea Usig a Curved Dipole o Reflector Plae RANGSAN WONGSAN School of Telecommuicatio Egieerig, Istitute of Egieerig Suraaree Uiversity of Techology 111 Uiversity Aveue, Muag District, Nakho Ratchasima, 3 THAILAND Abstract: - This paper presets a wide-beam atea for broadcastig statio by usig a curved dipole atea, which its both eds are electrically shorted o the surface of a square coductig reflector. The Method of Momet (MoM is utilized to solve umerically the electrical currets that distribute alog the curved dipole, which are used to determie the impedace characteristics ad the radiatio patters of this atea. The beamwidth of azimuth patter of this atea with shorted-ed curved dipole o the reflector is aroud 1 degrees that is very suitable for TV broadcastig statio. Havig cofirmed the validity of this approach, the UHF-bad atea prototype is fabricated, tested experimetally ad shows good performace. Key-Words: - Curved dipole, broadcast atea 1 Itroductio Broadcastig system has bee extesively ad cotiuously used for distributig iformatio over wide rage of the service area. The atea applied for the broadcastig statio of the ultra high frequecy televisio (UHF TV requires either uidirectioal or omidirectioal beam with sufficiet gai, high power hadlig, ad badwidth of each chael is at least 6 MHz [1]. Moreover, the atea should possess physically strog structure, easy fabricatio ad cost effectiveess. Most maufacturers offer cofiguratios that ca be used to broadcast a wide variety of azimuth ad elevatio patters at either low or high power with their simple feedig system, low widload, ad ability to be side- or top-mouted. Covetioally, the popular atea utilized for this purpose is the UHF atea pael system [] is oe type of broadcastig ateas that cotais horizotally polarized wire/patch dipoles as a array arragemet. However, to achieve omidirectioal patter, the pael system must be placed aroud the tower at least four paels because of the beamwidth of each pael is ot much wide. I additio, the structure of covetioal pael system is probably complex ad required the fully feed system. From these aforemetioed literatures of the covetioal UHF TV broadcastig ateas, it is obvious that the atea which possesses simple structure ad simple feedig system is desirable. A atea made of curved-wire dipole, which the both eds are shorted o the metallic reflector plae, is a attractive oe sice the structure ad feed system is very simple. Moreover, the beamwidth of curvedwire dipole is larger tha straight dipole. Most of substatial works related to the shaped or curved dipole structures. Krisha et al. [3] preseted a V-shaped wire-loop with a butterfly-like structure. The elliptically polarized radiatio of S- shaped dipole atea was preseted by Elkamchouchi ad Nasr [4]. Besides these, Dubost [5] preseted the radiatio impedace ad badwidth of a short-circuited dipole parallel to a perfect reflector plae. As far as we kow, there is o iformatio about the curved dipole that the both eds are shorted o a metallic reflector plae, which is ecessary for the structure that requires the simple fabricatio, simple feeder, ad high power hadlig. Sice impedace, radiatio patter ad beamwidth are the importat characteristics that determie the efficiecy of the atea. Therefore, this paper focuses o these characteristics of a shorted-eds curved dipole. Theoretical backgroud of this atea will be determied by usig Method of Momets (MoM. The Pockligtos itegral equatios for total filed are first formulated ad are subsequetly solved by usig Method of Momets to determie the ukow curret desities. Fially, the iput impedace of the atea at the ceter feed, the radiatio patters ad beamwidth will be determied ad validated by measuremet. ISSN: ISBN:

2 1th WSEAS Iteratioal Coferece o COMMUNICATIONS, Heraklio, Greece, July 3-5, 8 Theory The structure of a shorted-ed curved dipole atea is composed of a curved-wire dipole, which its both eds are short-circuited o a metallic reflector plae as show i Fig.1. The curved-wire dipole of the legth L, the wire radius b ( b λ, ad the curved radius a is aliged alog φ - directio at which the feed ceter of this curved dipole is located at φ = π. The dimesio of a square reflector plae of the width W is oe wavelegth as show i Fig.. Pockligtos itegral equatio ad solved umerically usig the method of momets. The poit-matchig techique is the used to satisfy the itegral equatio at discrete poits o the axis of curved dipole rather tha attemptig to satisfy this equatio everywhere o its surface [6]-[7]. Thus, a system of liear algebraic equatios is geerated to form the matrix model ad the iversio of this matrix ca realize the curret distributio o the curved dipole, which is used to first determie the iput impedace of this proposed atea. I this work, the Pockligto s itegral equatio for the total field calculatio, which are geerated by a electric curret source radiatig i a ubouded free space [8], which ca be writte i the form of e + = + L jkr t I ( φ k d j4 E φ πωε φ R, (1 L φ Fig. 1 Geometry of the shorted-ed curved dipole o a reflector plae. t where E φ is the total radiated field due to the curret I ( φ. Resultig from a source field E φ, that could be caused by voltage applied at the atea feed poit or icidet plae wave i E φ. Here φ ad φ are the source locatio variable ad the observatio locatio variable, respectively, ad R is the distace from ay oe poit o the source to the observatio poit. For small diameter wires, the curret o elemet ca be approximated by a fiite series of odd-order eve modes [7]. Thus, the curret o the elemet ca be writte as a Fourier series expasio of the form: M ( φ ( φ I = I g, ( m m= 1 where ( g φ is etire domai basis fuctios [8], which is give by g ( ( m 1 πφ φ = cos, φ π, L (3 Fig. Aalysis model of the shorted-ed curved dipole o a reflector plae. The curret distributio of the shorted-ed curved dipole atea ca be obtaied by usig the where L is the correspodig legth of the elemet. After some rigorous derivatios ad usig etire domai fuctios with Pockligto s itegral equatio, the itegral equatio for each elemet of the proposed atea ca be writte as [8] ISSN: ISBN:

3 1th WSEAS Iteratioal Coferece o COMMUNICATIONS, Heraklio, Greece, July 3-5, 8 ( m 1 ( m 1 ( m 1 M 1 t m+ 1 π L Eφ = Im ( 1 G ρρφφ,,, / z, j4πωε m= 1 L + k G z L π L πφ cos dφ, L ( ρρφφ,,, /, φ (4 where I m represets the ukow curret coefficiet of mode m o elemet. Because of the proposed atea comprises two elemets such as drive ad reflector elemets. Therefore, = is the total umber of elemets. Gree s fuctios is give as ( ρρ φφ z φ G,,,, jkr jkr+ e e = + R R, (5 + where R ± represets the distace from each wire radius to the ceter of ay other wire which ca be show by ( ( ( = ±. (6 ± R a a z z b φ φ From (4, i accordig with referred to as poitmatchig method, i matrix form ca be expressed as electric fields plottig the azimuth patters ca be expressed i (1. jkr N μe jk( ρsiθ cosφ+ φsiθ siφ Eφ = jω siθ e 4π r = 1 M + si ( φ si ( φ L I m +. + m= 1 φ φ { (1 3 Numerical Results For illustratio, the shorted-ed curved dipole atea was desiged to operate at the ceter frequecy of 47 MHz. From the prelimiary study it was foud that, at this operatig frequecy, the radius of curvature a ad the wire radius b should be 1.15 cm ad.5 λ, respectively, therefore, the good matchig coditio ad the proper gai of this atea could be obtaied. Computatios for this proposed atea have bee made usig the formulas of previous sectio. For the curret itself, covergece was about 5.5 percet chage i goig from 3 to 5 segmets. I our case, 4 segmets for a half-wave legth of atea will give the accurate results as show i Fig.3. It is foud that the appropriate legth of this curved dipole should be half-wavelegth due to the curret magitude at the both eds is zero. where [ V ] [ Z m m][ Im] =, (7 ad ( m 1 m+ 1 π L ( 1 ρρ,, φφ,, L Z = G z m ( m 1 + k G z πφ Vm L π L ( m 1 cos dφ L ( ρ, ρ, φ, φ, φ (8 = 4πε. (9 The, the ukow coefficiets I m ca be obtaied from the matrix model i (7 by usig a umber of matrix iversio techiques. Fially, the total Fig. 3 Curret distributio of curved dipole. The legth of curved dipole is the importat parameter to characterize the impedace characteristics. The curved dipole legth is varied from.5λ to.75λ while other parameters are fixed. The umerical results of the iput resistace ad reactace of a shorted-ed curved dipole as a fuctio of the frequecy are illustrated i Figs. 4. It is obvious that the larger the legth, the larger the ISSN: ISBN:

4 1th WSEAS Iteratioal Coferece o COMMUNICATIONS, Heraklio, Greece, July 3-5, 8 resistace ad the reactace. For the resoat frequecy observed from the zero reactace, it ca be see that the resoat frequecy will be higher whe the dipole legth is shorteed. The resoat frequecy of 47 MHz is realized whe the dipole legth is aroud.5λ. Resistace (Ohms Ld =.5 wavelegth Ld =.375 wavelegth Ld =.5 wavelegth Ld =.65 wavelegth Ld =.75 wavelegth (a E-plae Frequecy : f (MHz (a Iput resistace 6 4 Ld =.5 wavelegth Ld =.375 wavelegth Ld =.5 wavelegth Ld =.65 wavelegth Ld =.75 wavelegth Reactace (ohms Frequecy : f (MHz (b H-plae Fig. 5 Radiatio patter of curved dipole (b Iput reactace Fig. 4 Impedace characteristic of curved dipole. Fially, the umerical results from (1 for the azimuth patters i E-plae ad H-plae are show i Fig. 5 (a ad (b, respectively. Note that the radiatio patters are approximately directioal ad similar to Yagi-Uda atea but it yields wider beamwidth. Note that the radiatio patters are approximately directioal ad similar to Yagi-Uda atea but the proposed atea ca yield wider beamwidths (approx. 1 degrees i both plaes. 4 Experimetal Results To verify the theoretical calculatio, the curved dipole, which the both eds are short-circuited o the reflector plae is fabricated ad the iput impedace is measured by usig a HP87C Network Aalyzer. The prototype of the fabricated atea is show i Fig. 6. To achieve impedace matchig, the λ 4 coaxial balu (1:1 is mouted at the ceter gap of curved dipole to balace iheretly ubalaced system. The measured results are superimposed as dashed lie i Fig. 7. It should be poited out that the good agreemet betwee theory ad experimet ca be observed that the iput ISSN: ISBN:

5 1th WSEAS Iteratioal Coferece o COMMUNICATIONS, Heraklio, Greece, July 3-5, 8 impedace is aroud 5 Ω at the operatig frequecy of 47 MHz. I Fig. 8 (a ad (b, the measured patters are compared with the radiatio patters of the shorteded curved dipole, which are obtaied by usig MoM, icludig the cross-polarizatio from measuremets that ca be radiated i each plae about -5dB ad - db, respectively. It is foud that the measured mai-beamwidths tred to the calculated results. However, overall agreemet is good. Fig 6. Photograph of the prototype of curved dipole Theory Experimet 35 Resistace (Ohms (a E-plae Frequecy : f (MHz (a Iput resistace 15 Theory Experimet 1 Reactace (ohms Frequecy : f (MHz (b Iput reactace Fig 7. Theoretical ad experimetal results of iput impedace of shorted-ed curved dipole (b H-plae Figure 8. Theoretical ad Measured results of radiatio patters of curved dipole icludig their cross-polarizatios ISSN: ISBN:

6 1th WSEAS Iteratioal Coferece o COMMUNICATIONS, Heraklio, Greece, July 3-5, 8 5 Coclusio Iput impedace is a importat characteristic that affects the atea efficiecy. It must be well ivestigated so that the matched atea ca be desiged. While the wider beamwidth i azimuth plae of atea is the major requiremet of broadcastig statio. This paper focuses o the iput impedace characteristics ad the beamwidth of a so-called shorted-ed curved dipole o a reflector plae. This atea is developed to be a UHF TV broadcastig atea due to its simple structure ad simple feedig system. I order to ivestigate iput impedace of this atea, the straightforward procedure is applied. Startig from formulatios of itegral equatios of the structure based o the Pockligtos itegral equatio. The etire domai basis fuctio is used i the Method of Momets to solve the electric curret desity o the curved dipole. The iput impedace ca be foud from this curret subsequetly. From the umerical results, the frequecy respose of iput impedace ca be ivestigated, which is very useful to fid the optimum matchig coditio by simply adjustig the legth of curved dipole. Next, the radiatio patters have bee calculated ad plotted to determie the beamwidth of atea. Fially, the measured results give good overall agreemet with the theoretical results. [7] G.A. Thiele, Calculatio of the curret distributio o a thi liear atea, IEEE Tras. Ateas ad Propagatio (Commuicatios, vol. AP-14, pp , September [8] C.A. Balais, Atea Theory-Aalysis ad Desig, USA, Joh Willey & Sos, Ic., 198. Refereces: [1] Iteratioal Telecommuicatio Uio, TRANSMITTING ANTENNA CHARAC- TERISTICS AT VHF AND UHF, Recommedatio ITU-RBS.1195, [] D.Casciola, G.L. Mieers, ad R.A. Surette, UHF Atea Choices, IEEE Tras. O Broadcastig, vol. 45, pp , March [3] S. Krisha, L.-W. Li, ad M.-S. Leog, A V- Shaped Structure for Improvig the Directioal Properties of the Loop Atea, IEEE Tras. Ateas Propag., vol.53. pp , Jue 5. [4] H. Elkamchouchi ad M. Abu Nasr, THE S- SHAPED DIPOLE ANTENNA, Proc. IEEE Microw. Ad Millimeter Wave Techology, 4, ICMMT 4 th Iteratioal Coferece, pp. 19-, August 4. [5] G. Dubost, Flat Radiatig Dipoles ad Applicatios to Arrays, RESEARCH STUDIES PRESS A DIVISION OF JOHN WILEY & SONS LTD. pp. 8-36, [6] R.F. Harrigto, Matrix methods for field problems, Proc. IEEE, vol. 55, pp , February ISSN: ISBN: