Hybrid microstrip and carbon nanotubes based patch antenna for wireless applications

Ijaz Rashid, Dr. Panagiotis Kosmas Department of Engineering, Kings College London,

London, United Kingdom

Dr. Haider Butt Department of Engineering, University of Cambridge

Cambridge CB3 0FA UK

Abstract— In this we have looked at the concept of

introducing carbon nanotubes on the surfaces of the microstrip patch antennas. We examined the performance improvements in a patch antenna through finite difference time domain simulations to increase the efficiency of the antenna. The results suggest that carbon nanotubes lead to a higher gain due to their electrical properties. A high gain antenna with low power requirements resulted in achieving a higher overall bandwidth. The designed antenna’s gain, bandwidth and directivity are analyzed before and after introducing carbon nanotubes.

Keywords— Patch antenna, carbon nanotubes, radiation patterns, gain

I. INTRODUCTION Carbon nanotubes were first discovered by S. Iijima in

1991 [1] and have since then been the focus of enormous research. There are two main types of carbon nanotubes [2] which exist in stable states, single-walled carbon nanotubes (SWCNTs) and multiwall carbon nanotubes (MWCNTs). SWCNTs are structurally similar to a single graphite sheet wrapped into a cylindrical tube and MWCNTs comprise an array of such tubes concentrically nested like the rings of a tree trunk. MWCNTs are mostly metallic and are able to carry high current densities. Due to their high conductivities and aspect ratios they produce very strong electric fields, which makes them an efficient source of electrons, as has been demonstrated in field emission displays [3]. Also, it has been reported that the electric fields produced by a single MWCNT can be characterized as near Gaussian in shape [4, 5]. Arrays of MWCNTs have been used as electrodes in rectifiers and other electrical devices [6], as nano-electrodes for liquid-crystal photonic devices [7], solar cells to increase the efficiency [8], antenna arrays [9, 10] and metamaterials [11].

MWCNTs are normally grown as tangled masses by laser ablation or arc discharge. But most devices require the nanotubes with well-defined diameters and lengths. The exact positioning of the nanotubes is also vital for devices. MWCNTs of desired sizes can be grown at precisely determined locations by the process of plasma enhanced chemical vapour deposition (PECVD) [12, 13]. The PECVD method has the advantage of being controllable. In

it e-beam lithography is used to deposit nickel catalyst on silicon substrates, marking the locations of CNT growth. Fairly recently substrates other than silicon have also used for the CNT growth such as quartz and glass.

The size of the CNTs (diameter) is controlled by the nickel catalyst dots which are deposited through E-beam lithography. Typically a catalyst array of diameter 100 nm and thickness 5 nm is deposited on the silicon wafer which allows the growth of a single MWCNT of 50 nm diameter on each catalyst dot. The substrate is heated by dc current under vacuum of 10–2 mbar to 650 °C at a ramping rate of 100 °C per minute. This mild heating process is preferred to protect the catalyst dots from cracking. Ammonia gas is used in this process to etch the surface of the nickel catalyst islands. Acetylene is the mainly used carbon source, and is imported into the deposition chamber after the temperature in it has reached 690 °C, followed by a dc voltage of 640 V between the gas shower head and the heating stage to create plasma of 40 W in power. The growth process lasts for 10 to 15 min at 725 °C, which gives multiwalled carbon nanotubes of nearly 1 to 2 µm in height.

The theory behind the working of carbon nanotubes as dipole antennas has been extensively documented by researchers. In reference [14] a quantitative theory on the working of nano-wire or nanotubes based antennas is presented, including the discussion on their radiation resistance, the input reactance and resistance, and antenna efficiency, as a function of frequency and nanotube length. A transmission-line model was established for the carbon nanotube based dipole antennas. In this method, two parallel conductors form a transmission line, and the transmission-line parameters such as inductance, capacitance, and resistance are determined based on the line geometry and materials.

A vital concept about the working of carbon nanotubes as antennas was discussed and that the velocity of wave propagation will be different from the wave velocity in free space. The wave propagation occurs due to the collective oscillations of 1D or 2D electron density i.e., plasmons. Plasmonics is a newly emerged area of extensive research due to the advancements in nanotechnology and it deals with the generation, propagation and detection of electronic waves called surface plasmons (SPs). SPs are collective excitations of charges running as density fluctuations along a solid surface

978-1-4673-6195-8/13/$31.00 ©2013 IEEE

such as a metallic thin film. They are generated by electromagnetic fields that excite a metal interface. Due to this interaction between matter and radiation, the electromagnetic fields are confined to the surface and propagate along the interface [15, 16]. Therefore, the wave speed on the CNTs will be on the order of plasmon velocity vp, this is what distinguishes a nanotube antenna from a traditional antenna. Likewise, the wavelength of electromagnetic waves on the nanotube is plasmon wavelength λp, rather than λ which is the free space electromagnetic wavelength.

Similar work was also carried out by Dr. Hanson in which he investigated the fundamental properties of carbon nanotubes based dipole transmitting antennas [17]. He established a quantum mechanical conductivity model for the carbon nanotube antennas to calculate their input impedance, current profile, radiation efficiency, and the radiation pattern.

II. CHARACTERISATION OF MICROSTRIP ANTENNAS WITH CNT

The radiation effect of vertical multiwall carbon nanotube arrays incorporated with microstrip patch antennas has been experimentally studied by Qi Zhu [18]. By exciting CNTs with microstrip patch antenna, it has been observed that the radiation pattern and intensities are significantly modified. Each CNT is excited by the electric field along the axial direction at the surface of the microstrip patch antenna and behaves like a dipole antenna [18]. A conventional model of inset-fed square patch antenna [19, 20] is shown in Figure 1. The feeding method is microstrip feed line.

Figure 1: Conventional inset-fed square patch antenna [19, 20]

The conventional model shown in Figure 1 is modelled and analysed in by finite difference time domain method (FDTD). The geometry of the modelled antenna is shown as follows in Figure 2.

Figure 2: Geometry for conventional inset-fed square patch antenna

The geometry of the conventional inset-fed square patch antenna incorporated with carbon nanotubes array is shown in the Figure 3 below. The nanotubes were arranged in the form of a one dimensional array at the radiating edge of the patch antenna.

Figure 3: Geometry of a hybrid patch with an incorporated CNT array

III. RESULTS The FDTD simulations for both the antennas were

performed and their resultant antenna parameters are compared. A comparison is made between the return loss (S11 in dB) plots of both configurations at -10 dB loss as shown in Figure 4. The results clearly show an increase in bandwidth and a higher impedance matching is achieved for the second resonant frequency. The first plot shows a bandwidth of 50 MHz (4.855-4.805 GHz). The second plot shows a bandwidth of 165 MHz (4.765-4.6GHz). The resonant frequency is slightly different from the original patch as the CNTs have affected the impedance of the antenna. This factor can be overcome by slightly varying the width of the patch to adjust the impedance to resonate the antenna at the original resonating frequency.

Figure 4: S11 plots

The far field radiation patterns generated by the two antennas are also compared. Figure 5 shows the electric field patterns (constant phi 90) before and after incorporating CNTs on the patch. The maximum gain is originally around 0 dB. After introducing CNT array, it has been observed that the gain increased significantly more than 5dB.

Figure 5: E Theta Plots (constant phi 90)

Figure 6 shows the electric field patterns (at

constant theta 0) of the patch antenna before and after introducing CNT array. After incorporating CNT array, it has been observed that the magnetic field gain remained around the same value which is less than 0 dB.

Figure 6: E Phi Plots (at constant theta 0)

The plots in Figure 7 are the electric field patterns

(constant theta 90) of the patch antenna before and after introducing CNT array. After incorporating CNT array, it has been observed that the gain remained around the same value which is around 0 dB; however the directivity of the pattern is changed by 90 degrees. This is due to the dipole

antenna effect of the CNTs as explained in the previous section. A dipole antenna has a radiation pattern in the direction of 90 to 270 degrees. In this case, the radiated pattern is reflected by the CNTs in the same direction as the dipole which clearly verifies their behaviour as dipole antennas after incorporating them on the patch.

Figure 7: E Theta Plots (constant theta 90)

The presented work is still in progress and in future further different geometries of nanotubes will be incorporated on the patch antenna. The effects of different nanotube geometries and arrangements on the antenna parameters will be studied.

IV. CONCLUSION In conclusion, an improvement in efficiency is observed

in the performance of the microstrip patch antenna after introducing CNT array. The bandwidth increased significantly and a higher gain is achieved for the same antenna. These results are of great significance in the field of wireless communications where bandwidth and power requirements matter a lot. Future work includes fabrication and testing of the designed antennas for practical results. Further investigation can be done to minimize the level of back lobes. Furthermore, new designs can be modelled and investigated to verify the results achieved so far. The new designs can further be investigated by another computational method like finite element (FEM) method.

ACKNOWLEDGMENT This research work is done and submitted to the

Department of Engineering at Kings College London in partial fulfillment of the requirements for the degree of MSc Mobile and Personal Communications Engineering under the supervision of Dr. Panagiotis Kosmas.

REFERENCES [1] S. Iijima, "Helical microtubules of graphitic carbon," Nature, vol. 354, pp.

56-58, 1991.

[2] R. H. Baughman, A. A. Zakhidov, and W. A. de Heer, "Carbon Nanotubes--the Route Toward Applications," Science, vol. 297, pp. 787-792, August 2, 2002.

[3] C. Deuk-Seok, S. H. Park, H. W. Lee, J. H. Choi, S. N. Cha, J. W. Kim, J. E. Jang, K. W. Min, S. H. Cho, M. J. Yoon, J. S. Lee, C. K. Lee, J. H. Yoo,

K. Jong-Min, J. E. Jung, Y. W. Jin, Y. J. Park, and J. B. You, "Carbon nanotube electron emitters with a gated structure using backside exposure processes," Applied Physics Letters, vol. 80, pp. 4045-4047, 2002.

[4] W. I. Milne, K. B. K. Teo, M. Chhowalla, G. A. J. Amaratunga, S. B. , D. G. Hasko, H. Ahmed, O. Groening, P. Legagneux, L. Gangloff, J. P. Schnell, G. Pirio, D. Pribat, M. Castignolles, A. Loiseau, V. Semet, and V. T. Binh, "Electrical and field emission investigation of individual carbon nanotubes from plasma enhanced chemical vapour deposition," Diamond and Related Materials, vol. 12, pp. 422-428, 2003.

[5] X. Q. Wang, M. Wang, H. L. Ge, Q. Chen, and Y. B. Xu, "Modeling and simulation for the field emission of carbon nanotubes array," Physica E: Low-dimensional Systems and Nanostructures, vol. 30, pp. 101-106, 2005.

[6] Y. Chen, C. Liu, and Y. Tzeng, "Carbon-nanotube cold cathodes as non-contact electrical couplers," Diamond and Related Materials, vol. 12, pp. 1723-1728, 2003.

[7] T. D. Wilkinson, X. Wang, K. B. K. Teo, and W. I. Milne, "Sparse Multiwall Carbon Nanotube Electrode Arrays for Liquid-Crystal Photonic Devices," Advanced Materials, vol. 20, pp. 363-366, 2008.

[8] Z. Hang, C. Alan, A. Arman, Y. Yang, R. Nalin, B. Tim, H. Ibraheem, H. Pritesh, N. Arokia, and A. J. A. Gehan, "Arrays of Parallel Connected Coaxial Multiwall-Carbon- Nanotube-Amorphous-Silicon Solar Cells," Advanced Materials, vol. 21, pp. 3919-3923, 2009.

[9] Y. Wang, K. Kempa, B. Kimball, J. B. Carlson, G. Benham, W. Z. Li, T. Kempa, J. Rybczynski, A. Herczynski, and Z. F. Ren, "Receiving and transmitting light-like radio waves: Antenna effect in arrays of aligned carbon nanotubes," Applied Physics Letters, vol. 85, pp. 2607-2609, 2004.

[10] L. Ying and Z. Baoqing, "Properties of Carbon Nanotube Optical Antennae," International Journal of Infrared and Millimeter Waves, vol. 29, pp. 990-996, 2008.

[11] H. Butt, Q. Dai, R. Rajesekharan, T.D. Wilkinson, G.A.J. Amaratunga, "Plasmonic Band Gaps and Waveguide Effects in Carbon Nanotube Arrays Based Metamaterials," ACS Nano, 5 (11), 9138-9143, 2011.

[12]K. B. K. Teo, M. Chhowalla, G. A. J. Amaratunga, W. I. Milne, D. G. Hasko, G. Pirio, P. Legagneux, F. Wyczisk, and D. Pribat, "Uniform patterned growth of carbon nanotubes without surface carbon," Applied Physics Letters, vol. 79, pp. 1534-1536, 2001.

[13] M. Chhowalla, K. B. K. Teo, C. Ducati, N. L. Rupesinghe, G. A. J. Amaratunga, A. C. Ferrari, D. Roy, J. Robertson, and W. I. Milne, "Growth process conditions of vertically aligned carbon nanotubes using plasma enhanced chemical vapor deposition," Journal of Applied Physics, vol. 90, pp. 5308-5317, 2001.

[14] P. J. Burke, S. Li, and Z. Yu, "Quantitative theory of nanowire and nanotube antenna performance," IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2006.

[15] L. Qi, R. Rahul, S. Bindu, Q. William, M. R. Apparao, and K. Pu Chun, "Coupling of photon energy via a multiwalled carbon nanotube array," Applied Physics Letters, vol. 87, p. 173102, 2005.

[16] M. Dragoman and D. Dragoman, "Plasmonics: Applications to nanoscale terahertz and optical devices," Progress in Quantum Electronics, vol. 32, pp. 1-41, 2008.

[17] G. W. Hanson, "Fundamental transmitting properties of carbon nanotube antennas," Antennas and Propagation, IEEE Transactions on, vol. 53, pp. 3426-3435, 2005.

[18] Q. Zhu, W. Liu, H. Zhang, and H. Xin, "Experimental study of microwave radiation of carbon nanotube arrays," Applied Physics Letters, vol. 95, pp. 083119, 2009.

[19] D. M. Sheen, “Application of the Three-Dimensional Finite-Difference Time-Domain Method to the Analysis of Planar Microstrip Circuits,” IEEE Transaction of Microwave Theory and Techniques vol. 38, pp. 849-857, July 1990.

[20] R. Dehbashi, “New Compact Size Microstrip Antennas with Harmonic Rejection,” IEEE Antenna and Wireless Propagation Letters, vol. 5, pp. 395 – 398, December 2006.