Compact Coupled Sectorial Loops Antennas for Ultra-Wideband Applications
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Compact Coupled Sectorial loops Antennas for Ultra-Wideband Applications Adel Elsherbini 1, Kamal Sarabandi 1 1Radiation Laboratory, Electrical Engineering and Computer Science Department, University of Michigan, 1301 Beal Ave., Ann Arbor, MI 48109, USA. Email: [email protected] Abstract This paper presents the structure and design of a number of significant modifications to the UWB coupled sectorial loops antennas (CSLA) to achieve further size reduction and unidirectional radiation over a very wide band. The variations of the radiation pattern at high frequencies of the original CSLA were less than desirable. Also, for many applications, unidirectional radiation pattern over a wide band is desired. Initially, an approach for size reduction is presented where the two sectorial loops are brought from a coplanar arrangement into two planes making an angle less than 180 o. Then, in order to further improve the directivity and the front-to-back ratio, a Vivaldi in conjunction with a folded CSLA was used. Finally, to achieve a unidirectional UWB radiation pattern, a Vivaldi-CSLA backed by a corner reflector is designed. The proposed antennas are simulated and measured, and the measured results showed good agreement with the simulation results. 1. Introduction Ultra-wideband (UWB) systems are gaining increased popularity in the recent years, since they have the potential of very high data rate for communication applications and very good resolution for Radar applications. Among the challenges in UWB systems design is antenna design. Several UWB antennas with very good time domain characteristics have been proposed recently such as UWB dipoles and monopoles, TEM horns and Vivaldi antennas. Among the newly proposed antennas is the coupled sectorial loop antenna (CSLA) [1], which is a compact UWB omni- directional antenna with a very wide impedance bandwidth. However, its radiation pattern variations are less than desirable and in some applications a uni-directional antenna is required. The basic structure of the CSLA and a sample design for operation over the band 0.4 – 6 GHz will be presented. Then, the size reduction technique of the CSLA by folding it is introduced. The reduced size folded CSLA is shown to have the same good impedance matching as the original CSLA while having a smaller size. Its radiation pattern is found to be more directive at higher frequencies. And thus, combining it with a Vivaldi antenna, an even higher directivity could be achieved while maintaining the good input match. In order to further increase the directivity at lower frequencies, a corner reflector was added to the antenna. The directivity was simulated to be above 8 dBi at the lowest operating frequency and peaks to 15 dBi at the center frequency. Also, the direction of the maximum radiation remains constant with frequency, which is very important in Radar applications. 2. Omni-Directional Coupled Slot Antenna (CSLA) The basic structure of the CSLA is shown in Fig. 1. It consists of two sectorial loops that are magnetically coupled. It is fed using a coaxial cable at the circle center. The basic principle of operation of the CSLA is the strong frequency dependent magnetic coupling between the two sectorial loops, which causes the input impedance to be real and almost independent on the frequency. A sample CSLA was designed using Ansoft HFSS and fabricated. The fabricated structure is shown in Fig. 2 (a). The measurement and simulation results are shown in Fig. 2 (b). The antenna has a VSWR < 2 over the band 0.38 – 6 GHz (16:1 impedance bandwidth) and its height is less than quarter a wavelength at the lowest operating frequency. The antenna was fabricated on a 30 mils Rogers 4003 substrate. The dimensions of the antenna are: Rout = 150mm, α = 30 o and t = 1 mm. Figure 1: The CSLA Antenna Structure. Figure 2: (a) The fabricated CSLA and (b) its simulated and measured VSWR. 3. Folded CSLA Antenna In order to further reduce the antenna size, we folded the antenna by an angle β as shown in Fig. 3 (a). The simulation results of the input VSWR are shown in Fig. 3 (b). The simulation results show that the antenna maintains its good matching up to a fairly large folding angles. A sample antenna with a folding angle of 120 o is fabricated and measured. The measured results shows that the antenna has a very good matching over the same bandwidth as the original CSLA. The fabricated structure is shown in Fig. 4 (a) and its measured and simulated VSWR are shown in Fig. 4 (b). 5 β = 45 o 4 β = 90 o β = 135 o 3 β = 180 o (flat) VSWR 2 1 0 0 0.5 1 1.5 2 Frequency [GHz] (a) (b) Figure 3: The structure of the folded CSLA and its simulated VSWR for different folding angles, β. The antenna radiation pattern was simulated for a folding angle of 135 o as shown in Fig 5. From which, it can be seen that the radiation pattern is omni-directional at lower frequencies. However, it becomes more directive at high frequencies even for a 135 o folding angle. This suggests the usage of the antenna together with a back reflector, would improve the gain at the low frequencies without causing undesirable variations in the radiation pattern due to destructive interference between the direct and the reflected component at higher frequencies. However, since the antenna has a small front-to-back ratio, another modification has to be performed on the antenna to further favor the radiation in the forward direction before a back reflector can be added. (a) (b) Figure 4: (a) The Folded CSLA antenna and (b) it simulation and measurment results. One way to further improve the directivity of the antenna is to combine it with another UWB antenna. A very good candidate is the Vivaldi antenna [2] – [3], since it has the same polarization as the CSLA and its slot line feed is compatible with the CSLA. Several taper shapes were investigated, and it was found that a simple circular taper can provide very good radiation pattern characteristics while maintaining the excellent impedance bandwidth of the CSLA. A 120 o folded CSLA was combined with a circularly tapered Vivaldi. The fabricated structure is shown in Fig. 6 (a) and its measured and simulated VSWR are shown in Fig. 6 (b). The simulated H-plane radiation pattern of the combined structure at different frequencies is shown in Fig. 7. Comparing to the folded CSLA, significant improvement in the directivity and the front to back ratio can be observed 0.4 GHz 0.8 GHz 1.2 GHz 1.6 GHz 90 °°° 90 °°° 90 °°° 90 °°° 120 °°° 60 °°° 120 °°° 60 °°° 120 °°° 60 °°° 120 °°° 60 °°° 0dB 0dB 0dB 0dB 150 °°° -20 30 °°° 150 °°° -20 30 °°° 150 °°° -20 30 °°° 150 °°° -20 30 °°° -40 -40 -40 -40 180 °°° 0°°° 180 °°° 0°°° 180 °°° 0°°° 180 °°° 0°°° 210 °°° 330 °°° 210 °°° 330 °°° 210 °°° 330 °°° 210 °°° 330 °°° 240 °°° 300 °°° 240 °°° 300 °°° 240 °°° 300 °°° 240 °°° 300 °°° 270 270 270 270 °°° °°° °°° °°° Figure 5: The normalized H-plane radiation pattern of the Folded CSLA antenna for folding angles of 135 o. (Solid line is the copolar component and the dashed line is the crosspolar component). (a) (b) Figure 6: (a) The structure of the Vivaldi-CSLA antenna, (b) its measured VSWR and (c) its simulated directivity. 0.4 GHz 0.8 GHz 1.2 GHz 1.6 GHz 90 °°° 90 °°° 90 °°° 90 °°° 120 °°° 60 °°° 120 °°° 60 °°° 120 °°° 60 °°° 120 °°° 60 °°° 0dB 0dB 0dB 0dB 150 °°° -20 30 °°° 150 °°° -20 30 °°° 150 °°° -20 30 °°° 150 °°° -20 30 °°° -40 -40 -40 -40 180 °°° 0°°° 180 °°° 0°°° 180 °°° 0°°° 180 °°° 0°°° 210 °°° 330 °°° 210 °°° 330 °°° 210 °°° 330 °°° 210 °°° 330 °°° 240 °°° 300 °°° 240 °°° 300 °°° 240 °°° 300 °°° 240 °°° 300 °°° 270 270 270 270 °°° °°° °°° °°° Figure 7: The simulated normalized radiation pattern of the Vivaldi-folded CSLA antenna in the H-plane. (Solid line is the copolar component and the dashed line is the crosspolar component). 5. Vivaldi-CSLA with Corner Reflector By placing a corner reflector in the back of a Vivaldi-CSLA, a significant improvement in the antenna directivity can be obtained. The reflector spacing and angle was optimized to obtain a good directivity while maintaining a good input match. The corner reflector has a folding angle of 120o and is spaced 7.5 cm away from the antenna. The structure is shown in Fig. 8 (a) and its measured VSWR compared to the Vivaldi-CSLA is presented in Fig. 8 (b). The H-plane radiation pattern of the antenna is shown in Fig. 9. The antenna was found to have a very stable radiation pattern with the direction of maximum at an elevation angle of 30 o above the ground and independent on frequency. The apparent bad cross polarization performance of the antenna in the H-plane, is actually because the radiation pattern maxima is at an angle 30 o above the ground at which the cross polarization performance is very good as shown in Fig. 9 (b). (a) (b) (c) Figure 8: (a) The CSLA antenna backed with a folded ground plane and (b) the measured VSWR compared to the case without the reflector and (c) comparison of the simulated directivity of the three antennas. 0.5 GHz 1 GHz 1.5 GHz 2 GHz (a) (b) Figure 9: The normalized radiation pattern in the (a) H-plane and (b) E-plane of the Vivaldi-CSLA backed with a corner reflector at a distance d = 7.5cm. 6. Conclusion A number of ultra-wideband antennas based on the coupled sectorial loops antenna were presented. The size reduction of the original CSLA by folding is investigated. It was shown to give the same very good input matching while showing higher directivity near the high frequency end of the spectrum.