Design and Development of Tapered Slot Vivaldi Antenna for Ultra Wideband Applications

Transcription

1 Design and Development of Tapered Slot Vivaldi Antenna for Ultra Wideband Applications D. Madhavi #, A. Sudhakar #2 # Department of Physics, #2 Department of Electronics and Communications Engineering, RVR & JC College of Engineering, Chowdavaram, Guntur, Andhra Pradesh, India madhavids_ya@yahoo.com, 2 alapati_sudhakar@yahoo.com Abstract In this paper, the modelling and simulation of Vivaldi antenna which is a special type of tapered slot radiating element with an exponential flare profile is carried out. Excitation of radiating element is essential in the development of Vivaldi antenna. Vivaldi antenna consists of symmetrically etched slots in the metallization on the dielectric substrate and fed by wide band strip line to slot line transition. This antenna has been developed with reduced length using strip line to slot line transition for impedance matching. The Vivaldi antenna is modelled using HFSS software and thoroughly evaluated for various parameters like VSWR, directivity, gain, and radiation patterns. The results in terms of half power beam width, directivity, gain, and radiation patterns are obtained. The simulated results are in close agreement with the theoretical values. The designed Vivaldi antenna is capable of providing good impedance matching and stable directional radiation patterns across the UWB bandwidth. Keywords Antenna, Radiation, Ultra Wideband, Bandwidth, Directivity I. INTRODUCTION The Vivaldi antenna is a special type of tapered slot antenna belongs to the family of end-fire travelling wave antennas and consists of a tapered slot etched onto a thin film of metal. This is done either with or without a substrate of dielectric on one side of the metal film. This antenna provides symmetrical E-Plane and H-plane beam pattern and produces greater directivity, more gain and widely used in ultra wideband applications [-6]. An important step in the design of the antenna is to find suitable feeding technique for a slot line excited tapered slot antenna. The type of the feed that is used in the strip line which is unbalanced and the slot line is normally balanced, so a balun should be used here for the impedance matching purpose. Normally strip line is arranged perpendicular to the slot line and the balun is placed over it. The arrangement of the feed is quite complex in Vivaldi antenna while implementing arrays. The tapered slot antennas utilize a travelling wave propagating along the antenna structure because the phase velocity V ph is less than the light velocity in free space or V ph < C (and the limiting case when V ph = C). Therefore, they produce radiation in the end fire direction at the wider end of the slot in preference to other directions. The limiting case of V ph = C relates to the case of the antenna with air as the dielectric and consequently the beam width and side lobe level are considerably greater than with the change in the thickness, dielectric constant and taper design. These antennas have symmetric radiation pattern with side lobes. At different frequencies, the antenna radiates with constant wavelength. Thus the antenna is frequency independent and provides an infinite bandwidth of operation. As the wavelength varies, radiation occurs from a different section which is scaled in size in proportion to the wavelength and has same relative shape. This translates into an antenna with very wide bandwidth. As the antenna gets excited the travelling wave gets travelled and the pattern is radiated. When the impedance of the strip line matches the free space impedance, the pattern is radiated in the end fire direction [7-0]. Ideally the bandwidth of this antenna is infinite and so is used in ultra wide band applications. Theoretically, the Vivaldi antenna provides a bandwidth within the range of 2 GHz to 90 GHz and practically this is limited by the finite dimensions of the antenna and also by the transition from the feed to the slot line of the radiating element. The choice of dielectric substrate plays a major role in the simulation of the antennas. Normally these antennas include substrate and so effective dielectric constant plays a key role. 5

2 The typical dielectric constants range from 2.2 ε r 2. Normally used value is 2.2 in practical sense. Substrates with low relative dielectric constants and high thickness are used to provide wider bandwidth and better efficiency. However, smaller antenna size can be obtained with thin substrates of high dielectric constants. But this also results negatively on the efficiency and bandwidth. Therefore, the design trade-off is to chosen between good performance of the antenna and the size of the antenna. II. ANTENNA DESIGN For the proposed tapered slot antenna, an exponential taper is designed and implemented. In the Fig., shown above, R is the rate of opening of the tapered slot and the taper profile terminates on the two end points P (z, y) and P2 (z2, y2). The point P is the starting point of the slot line flare and is close to the uniform line from the cavity. The flare length L is the difference Z 2 Z, as shown in the Fig.. It is to be noted that when taper rate R becomes zero, the slope of the taper remains constant making the taper profile linear and the antenna is commonly referred to as the Linearly Tapered Slot antenna (LTSA). C C 2 y C e RZ C 2 Where y2 y RZ2 RZ e e RZ 2 ye y e RZ2 e e RZ 2 RZ Fig. Proposed Design with Feed A length L of the taper, 44mm, and the height H of the aperture, 42mm, are selected for this design. Increasing the length of the taper, keeping the aperture height constant, and the flare angle gets reduced correspondingly. It has been reported that, for small flare angles, the lowest operating frequency of the antenna reduces and subsequently enhances the bandwidth. For larger exponential opening rates R, the flare angle reduces, at the origin and the lower frequency of operation reduces and also exhibits significant improvement in the SWR [-3], at the upper end of the operating frequency band. An optimum value of R, 0.09, is selected for this antenna. 52

3 III. MODELING AND SIMULATION Using the above design procedure, from the values obtained, the antenna is modelled in Ansoft HFSS V2 to verify the results. Fig.2. Initial Design of Antenna in HFSS Step Drawing the exponential curves, slot line circular cavity, a Perfect Electric Conductor (PEC) is used as the patch surface and a perfect E boundary is used to represent a perfectly conducting surface in a surface and assigning RT duroid material to box of thickness 0.8mm, length and width of 35mm and 25.5mm respectively. All these are shown in Fig. 2. Fig.3. Feed of Antenna Step2 The strip line feed is designed and wave port is assigned. The wave port connects the input signal to the radiating patch geometry. The wave port is used for modeling strip lines and waveguide structures. The setup of wave ports is selected, depending on the type of solution needed, such as modal or terminal. Fig. 4 Antenna with Radiation Box Step3 A rectangular box, extending λ/4 on all the sides of antenna is designed and assigned the radiation boundary by selecting all the sides of the box, except the feed side, that is shown in Fig.4. These boundary conditions specify the behavior of the field at the object interfaces and the edges of the problem region. The excitations in HFSS software are used to indicate the electromagnetic field sources such as currents, voltages and charges on surfaces or objects in the design. 53

4 Fig.5. Vivaldi Antenna After making many adjustments the final optimized model obtained is shown in Fig. 5. The S at the input port of the antenna is shown in Fig.6. Fig.6. The Return Loss plot for Vivaldi Antenna 6GHz 9GHz (a) (c) 7GHz 0GHz (b) H-plane E-plane (d) Fig.7. Antenna Radiation Patterns at different operating Frequencies The radiation patterns of this antenna are shown in Figs.7. (a)-(d). The patterns are satisfactory, and the gain is very good. 54

5 IV. RESULTS AND CONCLUSIONS The main focus is to successfully design and model an ultra wide band exponentially tapered slot antenna and validate the design through simulation software. Figure 6 shows the simulated return loss between 6 to 8 GHz. The response is very similar to that of theoretical values. The UWB characteristic is confirmed in the measurement, with only slight shift of the upper edge frequency to 2 GHz. Figures 7 (a)-(d) show the simulated radiation patterns for the Vivaldi antenna at 6 to 0 GHz in both the E- and H-planes. The directional radiation pointing bore sight is observed. The half power beam widths are 30 0 in the E-plane and 42 0 in the H-plane. The results also show desired radiation performance with stable radiation direction, beam widths and gain over the UWB band width. The ultra-wide band exponentially tapered slot antenna has wide applications for direction finding, as a radiating element in linear and planar arrays in microwave band. REFERENCES [] E. A. Theodoru, M. R. Gorman, P.R. Rigg and F. N. Kong, Broadband pulse-optimized antenna, IEE proceeding microwaves, optics and antennas, vol.28, Iss.3, 24-30, 98. [2] M. Kanda, Transients in a resistively loaded linear antenna compared with those in conical antenna and a TEM horn, IEEE Transactions on Antennas & Propagation,vol. 28, No., 32-36, 980. [3] K.L. Shlager, G.S. Smith and J. B. Maloney, Accurate analysis of TEM horn antennas for pulse radiation, IEEE Transactions on Electromagnetic Compatibility.,vol. 38, No.3, pp ,996. [4] M.J. Ammann, Impedance bandwidth of the square planar monopole, Microwave and Optical Technology Letters, vol. 24, pp.85-87, [5] Z.N. Chen and M.Y.W. Chia, Impedance characteristics of trapezoidal planar monopole antenna, Microwave and Optical Technology Letters, vol.27, pp.20-22, [6] M.J. Ammann and Z.N. Chen, Wideband monopole antennas for multiband wireless systems, IEEE Antennas Propagation Magazine, vol.45, No.2, pp , [7] P.J. Massey and K.R. Boyle, Controlling the effects of feed cable in small antenna measurements, Proceedings of 2 th International Conference on Antennas and Propagation, 3 March to 3 rd April, Exeter, UK, Vol. 2, pp , [8] V.H. Rumsey, Frequency Independent Antennas, Academic press, 966. [9] H.G. Schantz and L. Fullerton, The diamond dipole: a Gaussian impulse antenna, IEEE AP Sympossium, July, Vol. 4, pp.8-3, 200. [0] Kraemer, R., Short-Range Wireless Communications; Emerging Technologies and Applications, John Wiley and Sons Ltd., [] J. Ravindranadh, A.Sudhakar, K.Padma Raju, Design of optimized reflector for enhancing the gain of helical antenna International Journal of Electronics & Telecommunication and Instrumentation Engineering(IJETIE), Vol 4, issue, pp , Sep 200. [2] C.Subba Rao, A.Sudhakar, Modeling and Analysis of Electrically Large Square Loop Antenna ANU Journal of Engineering & Technology, Volume,No, PP. 3-33, June 2009, India. [3] J.Ravindranadh, A.Sudhakar, K.Padma Raju, Analysis of Circularly Polarized L-band Helical Antenna ANU Journal of Engineering & Technology, Volume, No2, PP. 48-5, December 2009, India. 55