Cost Effective Dual Band Short Backfire Antenna

Transcription

1 International Journal of Electrical & Computer Sciences IJECS-IJENS Vol:9 No:09 30 Cost Effective Dual Band Short Backfire Antenna M. Javid Asad and M. Zafrullah Abstract-- Short backfire antennas have been developed with single impedance bandwidth characteristics. In this paper, a new cross dipole excitation structure for short backfire antenna is presented that achieves dual impedance band width characteristics. The cross dipole excitation structure consists of H-shaped and straight cross dipoles. The cross dipole structure is mounted on the slotted coaxial line. The antenna is developed on elliptical ground plane with dielectric rim. A cost effective design is proposed to get metallic rim from the dielectric rim. The pertinent features of this short backfire antenna are (i) dual impedance band width (ii) reconfigurable circular polarization (iii) cost effective fabrication. It is demonstrated that this short backfire antenna can achieve voltage standing wave ratio (VSWR) bandwidths of 21% and 20.36% for 2:1 VSWR, axial ratio of 1.8 db with axial ratio ( 3dB) bandwidth of 1.94% and a gain of 12 dbi. The antenna structure is described and simulation and experimental results are presented. Index Term-- Dual impedance band, circularly polarized, short backfire antenna I. INTRODUCTION The demand for directional antennas with high gain and wideband radiation characteristics for different wireless applications has increased in recent years [1]-[3]. WiMax (world interoperability for microwave access) used for point to multipoint communication has become hot area [4]-[5]. The subscriber directional antennas communicate with the base stations connected to internet in WiMax wireless systems. Short backfire antenna (SBA) is one of the competitive candidates for such wireless applications due to its good characteristics. SBA possesses compact structure and good radiation characteristics like gain of dbi and side lobes of -20 db [6] making it s wide use in telemetry, tracking and mobile / maritime satellite communications [7]-[9]. The popular feeding structure for SBA is half wavelength dipole antenna. Dipole excited SBA has narrow impedance bandwidth. The natural impedance bandwidth of circularly polarized SBA is 3-5% for voltage standing wave ratio, VSWR < 1.5 [10]. Short backfire antennas with waveguide excitation structure possess improved impedance bandwidth M. Javid Asad is in Telecom Engineering Department, University of Engineering & Technology Taxila Pakistan.Telephone: , mjavid_pk@yahoo.com M. Zafrullah is professor in Electrical Engineering Department, University of Engineering & Technology Taxila Pakistan.Telephone: , drzafrullah@uettaxila.edu.pk characteristics but are bulky and complex [11]-[12]. SBA with unbalance fed slotted patch can have considerably improved radiation and impedance characteristics [13]-[15]. For satellite communications, circularly polarized antenna is desirable to avoid power loss due to Faraday polarization rotation. The cross aperture excitation structure is widely used for producing circular polarization in microstrip antennas [16]- [19]. Unbalance-fed cross aperture SBA has been developed for achieving circular polarization [20]. In this paper, a new excitation structure for short backfire antenna is proposed and developed with dual impedance bandwidth and circular polarization characteristics. The excitation structure consists of H-shaped and straight cross dipoles. This structure reduces blockage of radiations between sub-reflector and primary reflector by providing less physical area and more effective area as compared with cross aperture excitation structure. The tuning and feeding screws are used for impedance matching and improving impedance bandwidth. SBA is developed on elliptical primary reflector. The use of dielectric and Al tape metallic rims make the antenna cost effective. The antenna can find use in different wireless applications like mobile communication, telemetry, and feed of cut parabolic reflectors. Simulation and experimental results are presented and discussed. II. ANTENNA STRUCTURE The configuration of dual band circularly polarized short backfire antenna is shown in Fig. 1. The excitation structure consists of H-shaped and straight cross dipoles. The primary reflector of SBA is elliptical metallic plate with major axis d maj and minor axis d min. The circular sub-reflector of diameter d src is placed at a height h src from the primary reflector. Numerous simulations were carried out to design the antenna by using electromagnetic simulator. The geometrical parameters of SBA are listed in Table I. The cross dipole excitation structure is fed by a slotted coaxial line. d f2 and d f1 are diameters of outer and inner conductors of coaxial line respectively. The coaxial line consists of a slot with length, L SL and width, W SW in its outer conductor. The feed line is used to support the subreflector and so it does not require any extra support. III. RESULTS The short backfire antenna is excited by dipole structure consisting of straight and cross dipoles. A feeding screw shortcircuits the inner conductor with the outer conductor of slotted coaxial line. A tuning screw is located opposite to the primary reflector. The antenna was characterized using automated experimental setup consisting of agilent network analyzer 8753E and standard gain horn antennas. Dual impedance bandwidth is achieved by the combination of straight and H- shaped cross dipoles. The gap between straight cross dipole and H-shaped cross dipole is 5mm (0.0447λ 0 ). The big and the small arms of H-shaped cross dipole are 3.4mm (0.0304λ 0 ) and 4.4mm (0.0393λ 0 ) short as compared with the arms of straight cross dipole. Fig. 2 shows the simulated and measured results for VSWR of cross dipole excited SBA. Good

2 International Journal of Electrical & Computer Sciences IJECS-IJENS Vol:9 No:09 31 agreement is observed between simulated and measured VSWR. The antenna exhibits dual impedance bandwidths which are found to be of 21% and 20.36% for VSWR 2. Circular polarization is achieved by using unequal arms of straight and H-shaped cross dipoles. The H-shaped cross dipole consists of orthogonal H-shaped dipoles. Each arm of the dipoles consists of T-type part, of lengths S x or S y, inserted into the straight part, of lengths L xb or L yb respectively. The T- type part inserted into the straight part of H-shaped dipole is moved outwardly for adjusting the axial ratio and providing reconfigurable circular polarization. The axial ratio at broadside and the gain are plotted in Fig. 3. It is seen that axial ratio (AR) is 1.82 db with bandwidth for AR 3 db of 1.94% and the gain is 12dBi. Short backfire antenna is developed on elliptical primary reflector with dielectric rim. The elliptical geometry of the primary reflector produces different beam widths in two planes, xz and yz, because the radiations reflected from primary reflector are different in these planes making antenna useful for applications like feed for cut parabolic reflectors. The metallic rim, used to improve side lobe level, is usually welded with primary reflector and a separate radome is assembled with the rim making fabrication of antenna complex and costly. SBA is developed by using inexpensive dielectric rim. The metallic rim is fabricated by using Al tape on inside of the dielectric rim, shown in Fig.1(a), making fabrication of SBA simple and cost effective. The simulated and measured radiation patterns at 2.68 GHz) are shown in Fig. 4. The half power beam widths are 56º and 65º in Φ = 0º and Φ = 90º planes respectively. First side lobe level is less than -22 db. The effect of height of metallic rim on the performance of short backfire antenna is shown in Table II. When the height of metallic rim is increased from 0.223λ 0 to 0.268λ 0 then the radiations are concentrated more towards bore sight direction reducing beam widths in both planes. The side lobe level is also improved. IV. CONCLUSIONS A new excitation structure has been proposed and developed for short backfire antenna. The proposed excitation structure for SBA not only provides dual impedance bandwidth but also reduces blockage of radiations between sub-reflector and primary reflector. The outwardly movement of T-type part of H-shaped cross dipole provides reconfigurable circular polarization. The dielectric Al tape rims make the fabrication of the antenna cost effective. The effect of metallic rim on the performance of SBA is discussed. The developed SBA achieves voltage standing wave ratio (VSWR) bandwidths of 21% and 20.36% for 2:1 VSWR, axial ratio of 1.8 db with axial ratio ( 3dB) bandwidth of 1.94% and a gain of 12 dbi. Simulation and experimental results show good agreement. Being low cost, high gain and dual band directional antenna, developed SBA can find applications in different wireless systems, such as mobile communication, telemetry and cut parabolic reflectors. (a)

3 International Journal of Electrical & Computer Sciences IJECS-IJENS Vol:9 No:09 32 d src Rim Tuning screw z Sub-reflector Slotted line y x Cross dipole Slot h d h src h rm Primary reflector N-type (b) d maj Feeding screw y Rim Cross dipole d src d min z x S x L yb L ya d s1 d src d s1 S y d f1 d f2 L xb L xa L ty d s2 L tx (c) Fig. 1. Short backfire antenna (a) 3D view (b) Side view (c) Top view

4 Gain (db) Axial ratio (db) VSWR International Journal of Electrical & Computer Sciences IJECS-IJENS Vol:9 No: Measured VSWR Simulated VSWR Frequency (GHz) Fig. 2. Simulated and measured results for VSWR of SBA 5 4 Measured axial ratio Simulated axial ratio Frequency (GHz) (a) Measured gain Simulated gain Frequency (GHz) (b) Fig. 3. Simulated and measured results for SBA (a) axial ratio (b) gain.

5 Relative power (db) Relative power (db) International Journal of Electrical & Computer Sciences IJECS-IJENS Vol:9 No: Measured RHCP, o Simulated RHCP, o Measured LHCP, o Simulated LHCP, o (degree) (a) Measured RHCP, o Simulated RHCP, o Measured LHCP, o Simulated LHCP, o (degree) (b) Fig. 4. Simulated & measured radiation patterns at 2.68GHz (a) Φ = 0º (b) Φ = 90º

6 International Journal of Electrical & Computer Sciences IJECS-IJENS Vol:9 No:09 35 T ABLE I Geometrical parameters for SBA. (λ 0 = Free space wavelength at 2.68GHz) d maj Major diameter of primary reflector mm (1.965λ 0 ) d min Minor diameter of primary reflector mm (1.429λ 0 ) h rm Height of rim 74.0 mm (0.661λ 0 ) d src Diameter of sub-reflector 50.0 mm (0.447λ 0 ) h d Height of dipole excitation structure 28.0 mm (0.25λ 0 ) h src Height of sub-reflector 56.0 mm (0.5λ 0 ) L SL Length of slot in coaxial line 56.0 mm (0.5λ 0 ) L xa Length of arm of straight cross dipole in x direction 18.8 mm (0.168λ 0 ) L xb Length of arm of H-shaped cross dipole in x direction 14.4 mm (0.129λ 0 ) L tx Total arm length of straight cross dipole in x direction 47.6 mm (0.425λ 0 ) S y Length of T-type part of H-shaped cross dipole in y 8 mm (0.072λ 0 ) direction S x Length of T-type part of H-shaped cross dipole in x 8 mm (0.072λ 0 ) direction d s2 Diameter of arms of straight cross dipole 3.0 mm (0.0268λ 0 ) d s1 Diameter of arms of H-shaped cross dipole 2.0 mm (0.0179λ 0 ) d f1 Diameter of inner conductor of coaxial line 3.5 mm (0.0313λ 0 ) d f2 Diameter of outer conductor of coaxial line 7.0 mm (0.0625λ 0 ) W SW Width of slot in coaxial line 2 mm (0.0179λo) L ya Length of arm of straight cross dipole in y direction 21.7 mm (0.194λ 0 ) L yb Length of arm of H-shaped cross dipole in y direction 18.3 mm (0.164λ 0 ) L ty Total arm length of straight cross dipole in x direction 53.4 mm (0.477λ 0 ) Rim height (mm) T ABLE II Effect of height of metallic rim. (λo is free space wavelength at 2.68GHz). Φ = 0 Radiation Plane Φ = 90 Radiation Plane Beam width (degree) Side lobe (db) Rim height (mm) Beam width (degree) Side lobe (db) REFERENCES [1] P. F. Driessen, Gigabit/s indoor wireless systems with directional antennas, IEEE Trans. Commun., vol. 44, no. 8, pp , Aug [2] R. B. Waterhouse, D. Novak, A. Nirmalathas, and C. Lim, Broadband printed sectored coverage antennas for millimeter-wave wireless applications, IEEE Trans. Antennas Propag., vol. 50, no. 1, pp , Jan [3] S. Yang, S. H. Tan, and J. S. Fu, Short backfire antennas for wireless LAN applications at millimeter-waves, in Proc. IEEE AP-S Int. Symp., vol. 3, Jul. 2000, pp [4] S. J. Vaughan-Nichols, Achieving wireless broadband with MiMax, Computer, pp , Jun [5] S. M. Cherry, WiMax and Wi-Fi: Separated and unequal, IEEE Spectrum, p. 19, Mar [6] K. M. Chen, D. P. Nyquist, and J. L. Lin, Radiation fields of the short backfire antenna, IEEE Trans. Antennas Propag., vol. 16, no. 5, pp , Sep [7] Y. Yamada, T. Takan, and N. Ishida, Compact antenna equipment for maritime satellite communication systems, in Trans. IECE, vol. 62-B, 1979, pp [8] K. Fujimoto and J. R. James, Mobile Antenna Systems Handbook, 2nd ed. Norwood, MA: Artech House, 2000, pp [9] A. Kumar and H. D. Hristov, Microwave Cavity Antennas. Norwood, MA: Artech House, 1989, pp [10] S. Ohmori, S. Miura, K. Kameyama, and H. Yoshimura, An improvement in electrical characteristics of a short backfire antenna, IEEE Trans. Antennas Propag., vol. 31, no. 4, pp , Jul [11] A. A. Kishk and L. Shafai, Gain optimization of short -backfire antenna with different excitation types, in Proc. IEEE AP-S Int. Symp., vol. 24, Jun. 1986, pp [12] M. S. Leong, P. S.Kooi, Chandra, and T. S.Yeo, Theoretical and experimental investigations of two-dimensional waveguide-excited short backfire antenna structure, in Proc. Inst. Elect. Eng., vol. 136, June 1989, pp [13] Z.N. Chen and M.Y.W. Chia, A center-slot-fed suspended plate antenna, IEEE Trans. Antennas Propag., vol. 51, pp , Jun [14] S. Gao, L.W. Li, M.S. Leong, and T.S.Yeo, A broad-band dualpolarized microstrip patch antenna with aperture coupling, IEEE Trans. Antennas Propag., vol. 51, pp , Apr [15] R. L. Li, D. Thompson, M. M. Tentzeris, J. Laskar, and J. Papapolymerou, Development of a wide-band short backfire antenna excited by an unbalance-fed H-shaped slot, IEEE Trans. Antennas Propag., vol. 53, no. 2, pp , Feb [16] H. Iwasaki, A circularly polarized small-size microstrip antenna with a cross slot, IEEE Trans. Antennas Propag., vol. 44, no. 10, pp , Oct [17] C.-Y. Huang, J.-Y.Wu, and K.-L.Wong, Cross-slot-coupled microstrip antenna and dielectric resonator antenna for circular

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