Abstractâ In this paper, the directivity enhancement mechanism of a dipole antenna placed in the Fabry-Perot type cavity is presented. The directivity of the ...
Terahertz Dipole Antenna in Fabry-Perot Cavity with two Side-walls to Enhance the Directivity Kumud Ranjan Jhaa and Ghanshyam Singhb a School of Electronics and Communication Engineering, Shri Mata Vaishno Devi University, Katra, Jammu and Kashmir -182301, India b Department of Electronics and Communication Engineering, Jaypee University of Information Technology, Waknaghat, Solan- 73251 India Abstract— In this paper, the directivity enhancement mechanism of a dipole antenna placed in the Fabry-Perot type cavity is presented. The directivity of the antenna has been increased upto 19 dBi at 610 GHz. To validate the analysis, the antenna has been simulated by using two different commercially available simulators CST Microwave Studio and Ansoft HFSS.
I.
F
INTRODUCTION
the wireless communication/surveillance system, the antenna plays a crucial role in determination of the overall performance of the system and successful transmission of the data. An importance of the electrical performances of antenna increases many fold when the system is placed in the lossy media. To combat the effect of lossy media, it is required to enhance the directivity and gain of the antenna in wireless communication system [1,2]. It is well known fact that the terahertz spectrum is comparatively lossy and electromagnetic wave experiences heavy attenuation in the medium in this spectrum. The attenuation increases with the increase in the frequency of the operation. However, there are many low loss atmospheric windows in which meaningful communication/surveillance system can be developed by using the enhanced gain/directivity terahertz antennas [3]. Recently, the Fabry-Perot type resonator has been used to enhance the directivity of the single source antenna in place of antenna array at microwave frequency regime of the electromagnetic spectrum [4,5]. In this paper, we have used the cavity with two side walls to enhance the directivity of the antenna in the terahertz frequency regime of the electromagnetic spectrum. OR
dimension of the unit cell of FSS is 320x50 μm.2. The FSS spacing is equal to 50 μm and 30 μm along x- and y- axis, respectively. The ground plane, FSS, sidewalls and dipole are made of copper of thickness 20 μm. The number of FSS used in this antenna is an array of 11x3 unit cells. The geometrical parameters of the unit cell as shown in Fig. 2 is a = 0.65λ0 , b = 0.101λ0 , c = 0.711λ0 , d = 0.203λ0 , g1 = 0.0305λ0 and g 2 = 0.0508λ0 . To analyze the unit-cell, the image theory is applied in which the ground plane is removed and the image of the structure is placed 4h distance away from the original unit-cell where h=h1=h2=125 μm, and structure is simulated with the waveguide module [5] where the electromagnetic wave is propagating in z-direction and the electric field is oriented in x- direction.
Fig. 2. Geometrical configuration of the unit cell.
II. GEOMETRIC CONFIGURATION AND ANALYSIS OF THE PROPOSED ANTENNA
The structure of the proposed antenna is shown in Fig. 1.
(a)
Fig. 1. Structure of the proposed antenna at terahertz frequency.
A simple dipole antenna of length 172 μm, and width 20 μm, is placed in the partial Fabry-Perot type cavity consisting of two sidewalls, ground plane, and frequency selective surface (FSS) on the top. The area of the ground plane is 1800x1800 μm2. The height of side walls are 255 μm. The
(b) Fig. 3. The scattering parameters (a) magnitude and (b) phase of waveguide model.
The scattering parameter of the equivalent cavity model of the FSS is shown in Fig. 3, which indicates that the resonance frequency of this cavity is equal to 610 GHz. The reason to consider two cavity walls only can be explained with the electric field distribution just above the FSS. The electric field distribution is shown in Fig. 4.
Fig. 5. Reflection co-efficient of proposed antenna in (dB).
Fig. 4. Distribution of electric field above the FSS.
From Fig. 4, it is seen that maximum of the electric field is oriented along x-axis. The x-axis oriented field is continuous at the lateral walls if placed along y-axis and electromagnetic wave would cross over these boundaries. On this way, the application of sidewalls along y-axis doesn’t contribute to the directivity enhancement. Apart from this, it is seen that field concentration is maximum at the centre in place of at the boundary. Since the maximum field is located at the centre of the cavity and aligned along x-axis, the reflection from the side-walls are minimum which reduces the chance of generation of higher mode within the intended frequency range. Partially, the open cavity reduces the complexity associated with feeding the dipole in the cavity. The TE011 mode is dominant in the cavity and the resonance frequency of the cavity can be calculated using following formulas [6]. f mnp = kx =
v 2π
kx 2 + ky 2 + kz 2
mπ nπ , ky = , kz = 2π f FSS με a b
(1)
where a and b denote the lengths of the cavity in the xand y-directions, v is the phase velocity, m and n are the integer numbers determining specific modes in the cavity. From Fig. 3, it is clear that the f FSS = 610 GHz. By using (1), the calculated resonance frequency of the cavity is equal to 615 GHz which is close to the resonance frequency of the FSS. To obtain the resonance frequency of the cavity antenna, we have simulated the antenna model proposed in Fig. 1 by two different simulators as CST Microwave Studio and Ansoft HFSS. The reflection coefficient (dB) of this antenna is presented in Fig.5. The directivity of the antenna at 610 GHz in the principle E- and H- plane is shown in Fig. 6. These results are comparable to each other and the directory of the antenna is equal to 19 dBi.
Fig. 6. Radiation pattern of the Fabry-Perot dipole antenna at 610 GHz.
III. CONCLUSION In this paper, a novel approach to enhance the directivity of the cavity type antenna with two sidewalls has been presented. Due to the application of two sidewalls, the generation of higher order mode is reduced and it provides a design flexibility to incorporate an appropriate feeding technique apart from the directivity enhancement up to 4 dB in comparison to without sidewalls. REFERENCES [1]
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K. R. Jha and G. Singh, “Dual-band rectangular microstrip patch antenna at terahertz frequency for surveillance system,” J. of Comp. Electronics, vol. 9, no. 1, pp. 31-41, 2010. K. R. Jha, S. V. R. K. Rao, and G. Singh, “Constructive interference in Yagi- Uda type printed trahertz antenna on photonic crystal substrate,” 33rd IEEE Sarnoff Symposium, Princeton, U. S. A., Apr. 12-14, 2010, pp. 1-5. M. J. Fitch and R. Ostiander, “Terahertz waves for communications and sensing,” John Hopkins APL Technical Digest, vol. 25, no. 4, pp. 348355, 2004. Z-Guo Liu, Z-Chen GE, and X-Yuan Chen, “Research progress on Fabry-Perot resonator antenna,” Int. J. Zhejiang Univ. Sci. A, vol. 10, no. 4, pp. 583-588, 2009. D. Kim and J. I. Choi, “Analysis of a high-gain Fabry-Perot cavity antenna with an FSS superstrate: effective medium approach,” Progress in Electromagnetics Research Lett., vol. 7, pp. 59-68, 2009. J. Ju, D. Kim, and J. Choi, “Fabry-Perot cavity antenna with lateral metallic walls for WiBro base station applications,” Electronics Lett., vol. 45, no. 3, pp. 141-142, 2009.
