A Numerical Study of the Antipodal Vivaldi Antenna Design for Ultra Wideband Applications Adham R. Azeez 1, Taha A. Elwi 2 and Zaid A. Abed AL-Hussain 1 1 Department of Electrical Engineering /Al-Mustansiriya University, Baghdad, Iraq 2 Department of Communication Engineering, Al-Mammon University Collage, Baghdad, Iraq Abstract: In this paper, a numerical analysis is invoked to study the performance of a Corrugated-shaped Antipodal Vivaldi (CAV) antenna. The proposed antenna provides a wideband with an acceptable gain in the endfire direction. Nevertheless, the antenna is consistent of two copper layers wings separated by an FR4-Epoxy dielectric substrate with dimension of 50 38 mm 2. The corrugated antenna is feeding by used the electromagnetic coupling technique. The feeding structure is designed as L-geometry which is located at the middle. It is found that such antenna shows a wideband, S 11 < -10, ranges from 5 GHz up to more than 20 GHz and shows a frequency mode at 1.58 GHz. The radiation patterns are found mostly in the endfire with gain varying from 3.7 db to 6.6 db over the entire band. The performance of the proposed design is tested using HFSS software package. Keywords: CAV, HFSS, bandwidth, endfire. I. Introduction Due to the fast growth in the wireless communication systems with increasing the human requirements, it became a very necessary to realize a new version of antennas to perform a wideband of operating frequencies and cover a wide range of applications [1]. Therefore, the research was oriented to find an antenna that covers the most essential bands and taking in the consideration the antenna size reduction [2], high efficiency, acceptable gain[3], and easy to fabricate with low cost [3]. In 1979, a new type of antenna called Vivaldi antenna was introduced [4] of a periodic continuously scaled structure to perform unlimited bandwidth with high directivity in the endfire direction and easy to fabricate and embedding in the circuit boards. Such antennas sometimes are known as notch antennas or tapered slot antennas that consistent of substrate, feeding line and radiating structure wings [5]. The feeding technique that mostly used in such antennas is the electromagnetic coupling feeding technique [6]. The principle operation of such antenna is the traveling of the surface waves [6] where the radiating structure works as a guide down on the curved path of the flare along the antenna. The reason of the unlimited bandwidth is back to the separation distance of the exponential slot curve in the radiating structures [7]. After that, many improvements were proposed such as an antipodal Vivaldi antenna that was invented in [8]. The antipodal Vivaldi antenna is considered as a next generation of the Vivaldi antenna that invented to solve the feeding problem of the previous design in [4] and [5]. The antipodal Vivaldi antenna consists of two wings separated by dielectric substrate arranged in the opposite complementary back to back direction [9]. The used feeding technique in the antipodal Vivaldi antenna was microstrip line followed by a balanced strip line transition [10]. Such design showed many advantages compare with the design in [4] such as easy to design and fabricate. A twin line feeding line was introduced to increase the range of the bandwidth because there are no slot line width limitations [10]. The disadvantage of the antipodal Vivaldi antenna is the cross-polarization [11]. The next improves in the Vivaldi antenna is called balanced antipodal Vivaldi antenna [12]. In this design, the cross polarization problem was solved by adding extra
metallization and substrate layers. The extra layers are work to create two E-field vectors that start from the middle layer and each E-field vector goes to one layer [13]. The sum of these vectors gives E-field vector parallel to metallization and as result the cross polarization would be reduced significantly [14]. In this paper, the design of a corrugated antipodal Vivaldi (CAV) antenna is proposed. The antenna performance in terms of S 11, gain, directivity and radiation patterns is tested numerically. The rest of the paper is organized as follows. In Section 2, the antenna design and the geometrical details are presented. The numerical results are discussed in section 3. Finally, we conclude the paper in Section 4. II. Antenna Geometry Design This work show of a design of antipodal Vivaldi antenna based corrugated shape. The antenna is consisting of two copper layers which are printed on FR4- Epoxy substrate with relative permittivity, ε r =4.4, and dielectric loss tangent (tanδ=0.02). The substrate has dimension of 50 38 5 mm 3 as show in figure 1. (a) 50 mm (b) 38 mm Figure 1: The CAV antenna shape: (a) 3D View, (b) Top View. The copper layers (radiating structures) are characterized by corrugated shape with straight line in the bottom as seen in figure 2. The dimensions of the corrugated copper layers (L C W C ) are 50 19 mm 2 where the other dimensions are shown table 1. W CL L 1 L 2 L 4 L 3.t Figure 2: The Geometry of the proposed Corrugated Antipodal Vivaldi Antenna.
Table 1: Parameters of the Corrugated Copper Layer. Parameter W CL L 1 L 2 L 3 L 4 t Value (mm) 19 18 14 18 40 2 The main goal of the design the copper layers in corrugated shape is to increasing the antenna impedance gradually to achieve a good matching with medium. As for added a straight line in the bottom of the corrugated wings is to avoid any back lobe occurs in the radiation pattern. The feeding technique that used in this antenna is the electromagnetic coupling. The feed line is invoced in the middle of the substrate that characterized by L-shape and dimension of 14 12 mm 2 as shown in figure 3. This type of feeding is easy to design and match to the antenna. Figure 3: The location and dimensions of the L-shape feed line. III. Simulation Results In this section, the performance of the proposed Vivaldi antenna is obtained analytically and then discussed. The numerically results is obtained in terms of return loss (S 11 ), radiation pattern, directivity and gain. The return loss is showed a wideband of operating frequencies more than 15 GHz starting from 5 GHz to the beyond of the 20 GHz with S 11 under -10 db. Also, the antenna showed a first frequency mode at 1.58 GHz as shown in figure 4. Figure 4: The Return Loss (S 11 ) in db of the Proposed Antenna. The gain of the proposed model is presented in Figure 5. The corrugated antipodal Vivaldi antenna shows a gain changing from 3.7dB to 6.6 db with endfire radiation patterns. In
addition to the gain, the directivity is found. The result show varying in the directivity value starting from 4.7671 db up to 9.0434 db in the operating band as shown in figure 6. Figure 5: The Gain of the Corrugated Antenna. Figure 6: The Directivity of the Corrugated Antenna. Finally, the radiation pattern is obtained in frequencies 8.5 GHz, 10 GHz, 12.5 GHz, and 15 GHz, in order to cover the entire band and as seen in table 2. Table 2: The Radiation Pattern of the Corrugated Antenna. Frequency [GHz] 3D Radiation Pattern Frequency [GHz] 3D Radiation Pattern 8.5 12.5 10 15
IV. Conclusions A novel design based on CAV antenna is proposed in this paper for UWB applications. The antenna is designed to provide a wide range of frequencies using a corrugated geometry fed by an L-Shaped transmission line feed. The radiation patterns of the antenna are enhanced in the most frequencies of interest at the endfire radiations with sufficient gain for short and medium communication distance. Those achievements are obtained by moving the skew surface waves at the end direction of the antenna through adding the corrugation steps to the Vivaldi structure. Numerical simulations are invoked based FEM of HFSS formulations to evaluate the antenna performance. It is found the antenna shows resonant mode around 1.58 GHz and another matched band starts from 5 GHz and continues to more than 20 GHz. Nevertheless, the antenna shows gains in the endfire varying between 3.7 db to 6.6 db. References [1] T. A. Elwi, I. Imran, and Y. Alnaiemy, "A Miniaturized Lotus Shaped Microstrip Antenna Loaded with EBG Structures for High Gain-Bandwidth Product Applications", Progress In Electromagnetics Research C, volume 60, pp. 157-167, December 2015. [2] O. A. Ibrahim, T. A. Elwi, N. E. Islam, A miniaturized microstrip antenna based on sinusoidal patch geometry for implantable biomedical applications, 6 th Global Conference on Power Control and Optimization, AIP Conference Proceedings; volume 1499, issue 1, pp. 254, November 2012. [3] T. A. Elwi, H. M. Al-Rizzo, Y. Al-Naiemy, and H. R. Khaleel, Miniaturized microstrip antenna array with ultra mutual coupling reduction for wearable MIMO systems, 2011 IEEE International Symposium on Antennas and Propagation, July 2011. [4] P. G. Frayne and A. J. Leggetter, "Wideband measurements on Vivaldi travelling wave antennas,", IEE Colloquium on Multi-Octave Microwave Circuits, London, pp. 5/1-5/6, 8 Nov 1991. [5] E. Gazit, "Improved design of the Vivaldi antenna," IEE Proceedings H - Microwaves, Antennas and Propagation, vol. 135, no. 2, pp. 89-92, April 1988. [6] P. J. Gibson, "The Vivaldi Aerial,", 9th European in Microwave Conference,IEEE, Brighton, UK, pp. 101-105, 17-20 Sept. 1979. [7] R. C. Johnson, Antenna Engineering Handbook, 3rd ed.: McGraw-Hill Professional, Dec 1, 1992. [8] J. D. S. Langley ; P. S. Hall ; P. Newham, "Balanced antipodal Vivaldi antenna for wide bandwidth phased arrays," IEE Proceedings - Microwaves, Antennas and Propagation, IET, vol. 143, no. 2, pp. 97-102, April 1996. [9] R. Q. Lee and R. N. Simons, "Effect of curvature on tapered slot antennas," Antennas and Propagation Society International Symposium, 1996. AP-S. Digest, IEEE, vol. 1, Baltimore, MD, USA, pp. 188-191, 21-26 July 1996. [10] K. S. Yngvesson,T. L. Korzeniowski, Y. -S. Kim, E. L. Kollberg and J. F. Johansson, "The Tapered Slot Antenna-A New Integrated Element for Millimeter-Wave Applications," IEEE Transactions on Microwave Theory and Techniques, vol. 37, no. 2, pp. 365-374, Feb. 1989. [11] K. Yngvesson, D. Schaubert, T. Korzeniowski, E. Kollberg,T. Thungren, J. Johansson, "Endfire Tapered Slot Antennas on Dielectric," IEEE Transactions on Antennas and Propagation, vol. 33, no. 12, pp. 1392-1400, December 1985. [12] T. A. Elwi, Z. Abbas, M. Noori, Y. Al-Naiemy, E. Y. Salih, M. M. Hamed, "Conformal Antenna Array for MIMO Applications" Journal of Electromagnetic Analysis and Applications, vol.06 No.04(2014), Article ID:43960,7 pages. [13] T. A. Elwi, S. Al-Frieh, M. Al-Bawi, and M. Noori, No Frequency Reuse: Wearable Steerable MIMO Microstrip Antenna Array for Wearable Ad Hoc Applications, British Journal of Applied Science & Technology, volume 4, issue 17, pp. 2477-2488, April 2014. [14] T. A. Vu, M. Z. Dooghabadi, S. Sudalaiyandi, H. A. Hjortland, Ö. Næss, T. S. Lande and S. E. Hamran, "UWB Vivaldi antenna for impulse radio beamforming," in NORCHIP,IEEE, Trondheim, pp. 1-5, 16-17 Nov. 2009.