A New Design of Log-Periodic Dipole Array (LPDA) Antenna

Joural of Commuicatio Egieerig, Vol., No., Ja.-Jue 0 67 A New Desig of Log-Periodic Dipole Array (LPDA) Atea Javad Ghalibafa, Seyed Mohammad Hashemi, ad Seyed Hassa Sedighy Departmet of Electrical Egieerig, Uiversity of Shahrood, Shahrood, Ira, Departmet of Electrical Egieerig, Shahid Rajaee Teacher Traiig Uiversity, Tehra, Ira School of New Techologies, Ira Uiversity of Sciece ad Techology, Tehra, Correspodig author:sedighy@iust.ac.ir I. INTRODUCTION Abstract- This paper presets a ew approach for desig of the logperiodic dipole array atea (LPDA) based o usig of differet desig parameters i the LPDA elemets to cotrol the atea behavior. I the proposed procedure, the desig parameters ca cotrol the value of forward gai over the operatig frequecy rage, ad also adjust the gai flatess. Furthermore, this desig procedure ca decrease the LPDA dimesios i compariso to the covetioal desig. Based o the proposed desig method, several LPDA ateas are desiged ad simulated umerically by the method of momet. The simulatio results show that this LPDA desig procedure ca be a good cadidate for desig of optimized atea i differet frequecy bads. Idex Terms- Gai cotrol, gai flatess, log-periodic dipole array, miiaturized LPDA Atea desig is a challegig topic amog the commuicatio experts for widebad commuicatio systems []. The Log-periodic dipole array (LPDA) which ca be used i these systems is a widebad directioal atea ad cosists of parallel dipoles with differet legths []. Its cocept has made a huge impact o both commercial ad military applicatio, sice it operates over a broad bad of frequecies. The covetioal desig method of LPDA is proposed by Carrel [] ad modified by Butso ad Thompso []. I this method, all the dipoles are iside the same agular sector, ad the dipole legths ad distace betwee them ca be calculated if the two desig parameters, defied as scale ad space factors (τ, σ), are kow. The τ ad σ are foud from the costat directivity cotour curves give by Carrel [] ad corrected by Butso ad Thompso []. With this covetioal method, it will be easy to desig a LPDA. However, the size of the atea based o this method is usually so large i the VHF bad []. I additio, the covetioal method has ot ay cotrol o value of forward gai over the operatig frequecy rage so that i Mauscript received -Dec.-0 ad revised -Feb.-0, P-ISSN: -088 Accepted o 8- April-0 E-ISSN: -96

68 A New Desig of Log-Periodic Dipole Array... Fig.. The LPDA Atea cofiguratio (a) covetioal desig, (b) proposed desig. covetioal desig the gai is usually decreased by icreasig the frequecy [6].The abovemetioed issues have bee addressed by a multitude of works [7, 8]. I all of these works, a umerical optimizatio techique such as Geetic Algorithm (GA) or Particle Swarm Optimizatio (PSO) is used for improvemet of covetioal LPDA respose. However, these techiques caot show the direct effect of desig parameters o optimized results. Compared to the covetioal method, i this paper we propose a ew approach for desig of LPDA where the scale factor is ot a costat value ad chages i differet regios of the atea elemets. Usig the simulatio results, we show that the mai advatage of the proposed approach is the possibility to cotrol the forward gai o operatio frequecy rage by chagig the scalig factor i differet regio of atea elemets. Based o the applicatio, the gai cotrol ca be applied uder specific requiremets such as the miimum forward gai at the begiig frequecies, the miimum forward gai at the ed frequecies ad the gai flatess over the operatig frequecy rage. As aother advatage, the proposed desig method ca be used for miiaturizig of covetioal oe. The paper is orgaized as follows. Sectio II outlies the operatio priciple of the proposed desig method of LPDA. The effect of the desig parameters values o forward gai is also cosidered i this sectio by usig of some full-wave simulatios. Compared to the covetioal LPDA, the ability of miiaturizatio i the proposed LPDA is ivestigated i sectio III where the results verify that the classical LPDA ca be miiaturized by smoothig the atea gai i the badwidth.

Joural of Commuicatio Egieerig, Vol., No., Ja.-Jue 0 69 II. OPERATION PRINCIPLES Fig. (a) shows the covetioal LPDA cofiguratio where the atea boom is composed of two separated parallel which are shorted i the ed by a metal plate. The scale ad space factors i the covetioal desig are give as follows [-]: d R () L L L LN...... R R R R N () d d d L L L () () Where L is the legth of the th elemet, R is the distace of th elemet from the iput port ad d is the space betwee two adjacet elemets. I covetioal desig method, the first elemet legth L is determied by the lowest frequecy ad the the other elemet legths are specified by () where τ is a costat value. As it show i Carrel desig curves [], the maximum atea gai i its badwidth is icreased by choosig of higher τ which results i log atea boom legth i a specified badwidth. Clearly, if we chose a lower τ, the atea boom legth is decreased as well as the maximum gai. I this covetioal method, there is ot ay cotrol o the atea gai versus frequecy ad the atea gai is usually decreased by icreasig the frequecy. I the ew proposed LPDA, we use a o costat τ versus elemets to cotrol the atea gai i its frequecy bad. I more details, we kow that the loger elemets cotribute i the radiatio i lower frequecies while the shorter oes cotribute i the higher frequecies. Therefore, the atea gai ca be cotrolled with chagig τ i differet regio of atea elemets by o costat fuctios versus the elemet positios. If we wat to decrease the gai i a specified frequecy rage, we ca decrease τ i the correspodig elemets i the atea cofiguratio. This fact is depicted i Fig. (b). As it ca be see, two differet τ are selected i this ew proposed atea compared with the covetioal oe with sigle τ. I fact, the values of τ ad τ cotrol the frequecy bad of maximum gai.

70 A New Desig of Log-Periodic Dipole Array... Elemet Legth (cm) 0 0 0 0 0 Elemet Positio (cm) Fig.. Atea elemets legth versus elemets positio i three cases, classic, ad Table I. Studied LPDA cases Case Regio of High gai τ τ Start of Bad 0.9 0.9 Middle of Bad 0.89 0.9 Ed of Bad 0.9 0.9 To validate the proposed desig, we desig ad simulate three kid of LPDAs i frequecy rage of. GHz to. GHz (See Table ) with two differet τ values istead of a costat oe which is used i the classical desig. The first oe is a covetioal LPDA ( Case) which has high gai i the start of the bad. The secod oe with high gai i the middle of the bad is amed as case A, ad fially, the third case with the high gai i ed of the bad is case B. As tabulated i Table. I, the classic atea has oe costat τ value versus the atea elemets. I all cases the umber of atea elemets, N, is chose as 0 ad the spacig factor for d is determied by usig Carrel desig curves to achieve the optimal value for gai. The other spaces betwee each two adjacet elemets (d, d, ) are determied by () based o usig correspodig τ. Notice that, the τ values are chose by doig some simulatios to kow the atea behavior with these coefficiet variatios. The legth of atea elemets versus positio of the elemets is depicted i Fig. for these three cases. Notice that based o the desig formulas, the scalig factor is related to the iverse slope of the lies.

Joural of Commuicatio Egieerig, Vol., No., Ja.-Jue 0 7 0 Gai (dbi) 9 8 7 6.... (a) VSWR.... (b) Gai (dbi) 0 9 8 7 6.... (c).... (d) Fig.. Compariso of three desiged LPDA give i Table I (a) Gai versus frequecy by FEKO (b) VSWR versus Frequecy by FEKO (c) Gai versus frequecy by HFSS (d) VSWR versus Frequecy by HFSS. VSWR As show i Fig. i the classical structure, the scalig factor is a costat value ad the lie slope is ot chaged. For cases (A ad B) based o the ew proposed desig method, we use two differet values for τ ad it ca be see that the lie slope is chaged i the middle for A ad i the ed for the B. I case A, we icrease the τ i the middle elemets of the atea ad decrease the τ i the first elemets which results i high gai i the middle frequecy bad of the atea. But i case B, we decrease τ i start ad middle elemets while it is icreased i the last oes which causes gai ehacemet i the ed of atea badwidth. The commercial full wave simulatio software, FEKO is used to simulate the proposed ateas. The simulatio results of S ad gai i the whole atea badwidth for these three cases are show i Fig.. As it ca be see, by chagig the scale factor coefficiet, we ca set the gai behavior i the frequecy rage of operatio. Moreover the radiatio patters of the desiged atea are demostrated i Fig. at three differet frequecies, start of badwidth (.6 GHz), middle of badwidth ( GHz) ad ed of badwidth (. GHz). To validate the simualtio resutls, the

7 A New Desig of Log-Periodic Dipole Array... (a) (b) (c) Fig.. E-Plae (right colum) ad H-plae (left colum) radiatio patter of three desiged LPDA give i Table I at.6 GHz (black), GHz (blue) ad. GHz (red). (a) (b) (c) simualtios are doe by Asoft HFSS software also which are show good agreemets with the FEKO oes. III. COMPACT LPDA DESIGN By the ew proposed method i LPDA desig, we ca desig a compact LPDA. For this purpose, we

Joural of Commuicatio Egieerig, Vol., No., Ja.-Jue 0 7 Elemet Legth (cm) Compact 0 0 0 0 0 Elemet Positio (cm) Fig.. Atea elemet legths versus elemets positio. Gai (dbi) 0 9 8, FEKO 7, HFSS 6 Compact, FEKO Compact, HFSS.... (a) VSWR (b) Fig. 6. Compariso of the desiged compact LPDA ad covetioal oe (a) Gai versus frequecy (b) VSWR versus Frequecy., FEKO, HFSS Compact, FEKO Compact, HFSS.... Table II. Miiaturized ad Covetioal LPDAs Compariso Case Total Legth, cm τ τ 9.8 0.9 0.9 Compact. 0.9 0.9 should try to decrease the first elemet legth to achieve a more compact structure. Therefore, we chose this elemet legth as short as possible oly to satisfy the VSWR coditio of the atea desig (VSWR < ). The we use two differet values for τ as it tabulated i Table.. Notice that the total umber of elemets is 0 ad the atea works betwee. GHz to. GHz same as the previous cases discussed i sectio II. The atea elemet legths versus elemet positios are depicted i Fig. beside the classical oe.it is clear that the atea elemets legths ad positios follow two differet τ. The first elemet legth i the compacted LPDA has about 0% compactess rather tha the classical oe. Also, the proposed LPDA boom legth is.9 cm which show about 7% compactess compared with the classical LPDA boom legth. I other words, the proposed

7 A New Desig of Log-Periodic Dipole Array... Fig. 7. E-Plae (right columb) ad H-plae (left columb) radiatio patters of the desiged compacted LPDA give i Table II at.6 GHz (black), GHz (blue). GHz (red). desig procedure compacts the LPDA i two dimesio; first elemets ad boom legth. The simulatio results of VSWR ad gai reported i Fig. 6 show that the proposed atea has a good performace compared with the covetioal oe. These results show that the proposed method ca be used to compact the covetioal LPDA. Notice that the HFSS simulatio results show i Fig. 6 validate the FEKO oes, also. Moreover, this compact atea shows a smooth gai variatio versus frequecy. The radiatio patters of this compact LPDA are demostrated i Fig. 7 at three differet frequecies, start of badwidth (.6 GHz), middle of badwidth ( GHz) ad ed of badwidth (. GHz). IV. CONCLUSION The gai cotrol ad size reductio of LPDA was demostrated i this work based o optimizatio of scale desig factor. For these purposes, the scale factor was ot chose as a costat value ad was chaged i differet regios of atea elemets. Moreover, by taperig the scale factor, a smoothly varied gai is achievable. Three kids of the proposed LPDAs have bee desiged ad simulated with two differet scale values. The simulatio results verified the proposed method ability to gai cotrol of LPDA i the operatio badwidth as well as size reductio. REFERENCES [] Y. Zehforoosh, A. Siahcheshm, Ultra widebad Moopole Atea Excited by a Capacitive Couplig Feed with Double Bad Notch Fuctio, Joural of Commuicatio Egieerig, vol., o., 0. [] C.A. Balais, Atea Theory, Aalysis ad Desig, rd ed., New Jersey: Joh Wiley & Sos, 00, pp. 6-6. [] R.L. Carrel, Aalysis ad desig of the log-periodic dipole atea, Ph.D. dissertatio, Dept. Elect. Eg., Uiversity of Illiois, Uiversity Microfilms, Ic., A Arbor, MI, 96. [] P.C. Butso ad G.T. Thompso, A ote o the calculatio of the gai of log-periodic dipole ateas, IEEE Tras. Ateas Propagat., vol., o., pp. 0-06, 976.

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