Progress In Elecromagneics Research Leers, Vol. 21, 129 137, 2011 A NEW DUAL-POLARIZED HORN ANTENNA EXCITED BY A GAP-FED SQUARE PATCH S. Ononchimeg, G. Ogonbaaar, J.-H. Bang, and B.-C. Ahn Applied Elecromagneics Laboraory Deparmen of Radio and Communicaion Engineering Chungbuk Universiy, Cheongju 361-763, Korea E.-J. Cha Deparmen of Medicine Chungbuk Universiy, Cheongju 361-763, Korea Absrac A new dual-polarized horn anenna fed by a microsrip pach is proposed. The pach is excied in wo orhogonal polarizaions by small gaps beween he pach edge and he microsrip open end. A horn anenna operaing a 14.9 GHz is designed, fabricaed and esed. Measuremens show ha he horn has a reflecion coefficien of less han 10 db, and a por isolaion greaer han 30 db, over 14.6 15.2 GHz, and a gain of 12.34 dbi and 10-dB beamwidhs of 87 and 88 a 14.9 GHz. 1. INTRODUCTION Dual-polarized anennas are used in such diverse applicaions as radio asronomy, radar polarimery, frequency-reuse and polarizaiondiversiy communicaions, and anenna measuremens [1, 2]. Dualpolarized anennas can be realized by exciing wo linearly polarized waves whose elecric field vecors are a 90 degrees o each oher or by simulaneously uilizing righ- and lef-hand circularly polarized. Dual-polarized horn anennas have been radiionally realized by feeding he inpu square waveguide wih wo orhogonal coaxial probes [3]. In some applicaions, i is favorable o excie a horn wih a prined-circui srucure. In his case, a ransiion beween he coaxial cable and he prined-circui ransmission line can be eliminaed, making i easy o inegrae he horn wih prined circuis. Received 26 January 2011, Acceped 28 February 2011, Scheduled 8 March 2011 Corresponding auhor: Sodnomseren Ononchimeg (ononchimegs@yahoo.com).
130 Ononchimeg e al. There have been some research resuls on horn anennas excied by prined circui srucures such as he microsrip probe, dipole and pach anennas [4, 5]. Dual-polarized horn anennas fed by prined circui elemens have been sudied in [6 8]. In exising resuls [6 8], he por isolaion is no high due o a srong muual coupling beween feeding elemens. In his paper, we propose a new compac dual-polarized horn anenna wih high por isolaion. A square pach anenna is excied a wo orhogonal edges by coupling hrough a small gap beween he pach edge and he microsrip open end. The pach launches wo orhogonal and linear polarized waves in he square waveguide of he horn. The dual-polarized horn proposed in his paper has a narrow bandwidh due o he use of a microsrip pach in he feeding circui so ha i is suiable for narrow-band applicaions. The proposed anenna is analyzed and opimized using he widely-used elecromagneic simulaion sofware Microwave Sudio TM by CST. The designed anenna is fabricaed and is performance is measured and compared wih he simulaion. 2. ANTENNA DESIGN Figure 1 shows he srucure of he proposed horn anenna. The horn consiss of wo microsrip lines, a gap-fed dual-polarized pach, a square waveguide and a pyramidal horn. Open ends of wo microsrip lines excie orhogonal currens on he pach by he capaciive coupling hrough a small gap a he pach edge. The pach launches TE 10 and TE 01 fundamenal modes in he square waveguide. Walls of he square waveguide are elecrically conneced o he ground plane of he circui board via closely-spaced hrough holes. The horn and he circui board are joined ogeher wih a silver-epoxy conducive adhesive. Small screws can also be used for he assembly. The horn anenna is designed in hree seps. Firs, we design a pyramidal horn anenna wih specified characerisics. Nex, we design a dual gap-fed recangular pach. Finally, we combine he pach and he horn, and adjus relevan dimensions for good impedance maching. Design requiremens of he proposed anenna call for he cener frequency of 14.9 GHz wih impedance bandwidh of 200 MHz and 10-dB beamwidhs of 87 in boh E- and H-planes. The anenna is o be used as a feed in a dual-polarized reflecor anenna for erresrial poin-o-poin communicaions. Figure 2 shows a pyramidal horn anenna wih a square inpu waveguide. The widh and heigh of he square waveguide are chosen o be same as he broad wall widh (15.75 mm) of a sandard recangular
Progress In Elecromagneics Research Leers, Vol. 21, 2011 131 Horn Waveguide Subsrae ( ε r,an δ ) Slo Radiaing Pach Meal srip Via hole Por 1 Por 2 Ground plane Figure 1. Srucure of he proposed anenna. l g l h w h w g w h w g w g w h Waveguide x y z Horn z x y Figure 2. Srucure of a horn wih a square waveguide. waveguide WR-28. The iniial lengh of he square waveguide is chosen o be l g = λ 0 /2, which is a half wavelengh a 14.9 GHz. The square waveguide lengh has an effec on he reflecion coefficien, which is minimum when l g is around λ 0 /2. For he pyramidal secion we choose iniial dimensions such ha w h = 2w g and l h = λ 0 /2. The lengh and aperure size of he horn deermine he bandwidh. The larger hey are, he smaller he beamwidh will be. For a given beamwidh and he anenna size limiaion, one can deermine heir values by gradually increasing hem unil he desired beamwidh is obained. In
132 Ononchimeg e al. he simulaion, he square waveguide is excied wih he TE 10 mode using he wave por in Microwave Sudio TM sofware. Saring from iniial dimensions of he horn, we firs adjus he lengh of he square waveguide such ha a minimum reflecion coefficien occurs a 14.9 GHz. Nex we adjus he lengh and aperure size of he pyramidal secion so ha boh E- and H-plane 10-dB beamwidhs are approximaely 83 degrees, aking ino consideraion ha beamwidhs are increased when he horn is excied by a pach. Afer numerous sessions of parameer sweeping, we obained he following opimum dimensions: w g = 15.8 mm, l g = 9.0 mm, w h = 30.76 mm, l h = 13.0 mm, and = 2.0 mm. Figure 3 shows he reflecion coefficien and gain paerns of he designed horn. The horn has a reflecion coefficien less han 30 db over 14 16 GHz wih a minimum value of 52 db occurring a 14.9 GHz. A 14.9 GHz, he horn has a gain of 12.9 dbi and 10-dB beamwidhs of 81 and 83 in E- and H-planes, respecively. (a) (b) Figure 3. Simulaed performances of he horn alone. (a) Reflecion coefficien and (b) gain paerns. Nex, we design a dual-polarized pach anenna ha will excie he square waveguide. Fig. 4 shows he srucure and dimensional noaions of he pach. The pach is excied by a gap beween he microsrip open end and he pach edge. Wih gap coupling, good impedance maching and high isolaion beween pors are achieved. The deailed design of he pach is described in [9]. Following he procedures in [9], we obain he final opimized dimensions of he pach: w p = 5.35 mm, g = 0.20 mm, w m = 2.26 mm, w = 40.00 mm, ε r = 2.5 and h = 0.787 mm, and an δ = 0.001. The iniial pach size is deermined by he resonan condiion [9]. The coupling gap widh is adjused for he minimum reflecion coefficien. This process is repeaed unil he feed circui has he lowes possible reflecion a he
Progress In Elecromagneics Research Leers, Vol. 21, 2011 133 w w w p g w p Radiaing Pach g w m Por2 w m x z z y Por1 h x y Ground plane Subsrae ( ε r,an δ) Figure 4. Dual-polarized pach fed by gap coupling. Figure 5. pach. Simulaed reflecion coefficien and por isolaion of he cener frequency. Fig. 5 shows he performance of he designed pach. A 14.9 GHz, he pach has a reflecion coefficien of 26 db and a por isolaion of 29 db. The impedance bandwidh ( 10 db reflecion) is 4.3% (644 MHz) and he anenna gain is 6.7 dbi. Finally he pach is insalled inside he square waveguide in a manner shown in Fig. 1. A recangular meal srip is prined around he pach. Closely spaced via holes on he srip connec he waveguide wall o he subsrae s ground plane. Microsrip lines feeding he pach
134 Ononchimeg e al. ener he square waveguide hrough small slos in he waveguide wall as shown in Fig. 1. Afer assembly, he reflecion coefficien of he enire anenna srucure is compued. Only he lengh of he square waveguide, he widh and heigh of he slo are adjused o obain an opimum reflecion coefficien. The horn anenna of final design has a square waveguide lengh of 11.00 mm and slo widh and heigh of 6.00 mm and 2.00 mm, respecively. All oher dimensions of he anenna remain unchanged from hose obained in he design of each par. 3. ANTENNA FABRICATION AND MEASUREMENT The designed anenna is fabricaed and esed. The fabricaed anenna is shown in Fig. 6. Two coaxial-o-microsrip adapers are insalled on he subsrae o measure he anenna performance. Anenna gain, reflecion coefficien, por isolaion and radiaion paerns are measured in an anechoic chamber using he HP 8720C nework analyzer and far-field anenna es insrumens. Figure 7 shows a comparison of he measured and simulaed reflecion coefficiens, and he por isolaion of he fabricaed anenna. (a) (b) (c) Figure 6. Fabricaed anenna. (a) Horn, (b) pach and (c) assembled anenna.
Progress In Elecromagneics Research Leers, Vol. 21, 2011 135 Measured resuls agree well wih he simulaion. The impedance bandwidh ( 10 db reflecion) is 4.1% (600 MHz). The bandwidh of he dual-polarized horn is deermined by he microsrip pach of he feeding circui since he horn alone has a much larger bandwidh. The bandwidh of he feeding circui can be increased by placing anoher pach wih a slighly differen resonan frequency on op of he base pach as ofen done o increase he bandwidh of a microsrip pach [10]. The reflecion coefficien of he anenna is less han 10 db and he por isolaion greaer han 30 db over 14.6 15.2 GHz. The por isolaion of he assembled anenna is beer han ha of he pach alone. Figure 8 shows gain paerns a 14.9 GHz of he fabricaed anenna wih por 1 excied and por 2 conneced o a mached Figure 7. Reflecion and isolaion performances of he fabricaed. (a) (b) Figure 8. Gain paerns of he fabricaed anenna a 14.9 GHz on (a) E- and (b) H-planes.
136 Ononchimeg e al. load. The proposed anenna s maximum gain is 12.34 dbi, and is 10-dB beamwidhs are 87 and 88 in E- and H-planes, respecively. Measured gain paerns agree well wih he simulaion in he upper hemisphere, where he maximum error is abou 3 db. 4. CONCLUSION In his paper, we proposed a new compac dual-polarized horn anenna operaing a Ku-band. Good impedance maching, high por-isolaion, and compacness of feeding are achieved by exciing he horn wih a square pach fed by wo orhogonal gaps beween he microsrip open end and he pach edge. The proposed horn anenna can be easily inegraed wih prined circuis. The horn anenna and he pach are designed separaely and assembled ino one srucure. Only minor adjusmens of dimension are required in opimizing he assembled anenna srucure. The designed anenna is fabricaed and esed. Measuremens of he fabricaed anenna show ha he proposed anenna has a gain of 12.34 dbi, 10- db beamwidhs of 87 and 88 in E- and H-planes, respecively, a 14.9 GHz. The reflecion coefficien is less han 10 db and he por isolaion is greaer han 30 db over 14.6 15.2 GHz (600 MHz). The dual-polarized horn anenna proposed in his paper can be uilized in such applicaions as feeding a prime focus parabolic dualpolarized reflecor anenna and dual-polarized horn arrays where he horn needs o be easily inegraed wih prined circuis. ACKNOWLEDGMENT This research was suppored by he Basic Science Research Program hrough he Naional Research Foundaion of Korea (NRF) funded by he Minisry of Educaion, Science and Technology (2011-0001045). REFERENCES 1. Secmen, M., S. Demir, and A. Hizal, Dual-polarised T/R anenna sysem suiable for FMCW alimeer radar applicaions, Proc. IEE Microwave Anennas Propaga. Conf., Vol. 153, 407 412, 2006. 2. Lempianien, J. J. A. and J. K. Laiho-Seffens, The performance of polarizaion diversiy schemes a a base saion in small/micro cells a 1800 MHz, IEEE Trans. Vehicular Tech., Vol. 47, No. 3, 1087 1092, Aug. 1998.
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