Hardwired Design of Ultra-Wideband Reconfigurable MEMS Antenna

Hardwired Design of Ultra-Wideband Reconfigurable MEMS Antenna Hyungrak Kim 1, David Chung 1, Dimitrios E. Anagnostou 2, Young Joong Yoon 3, and John Papapolymerou 1 Georgia Institute of Technology, Atlanta, USA 1 South Dakota School of Mines and Technology, Rapid City, USA 2 Yonsei University, Seoul, Korea 3 ABSTRACT In this paper, a design of an ultra-wideband reconfigurable RF-MEMS antenna with multifunctional transition is shown. By the analysis of equivalent circuit model, it is validated that the multi-functional transition in the proposed antenna operates as both a wideband transition of a tapered slot antenna (TSA) and a wideband slot antenna. Therefore, the proposed antenna can provide ultrawideband reconfigurable characteristics with both broadside and endfire directions. I. INTRODUCTION The reconfigurable antenna can be defined as an antenna where various electrical and mechanical characteristics of the antenna, such as radiation patterns, directivities, and resonant frequencies can be by electrically and mechanically controlled. It has been more and more spotlighted with its much potential to RF wireless applications. Especially, for the reconfigurability of the antenna, electrical switches and mechanical actuators have been used, and recent trends moves towards the use of electrical switches since they provide long life time, easy design, low loss, and flexible pattern expansion. Also, as electrical switches, MEMS (Microelectromechanical Systems), PIN Diodes, and transistors have been used. Recently, MEMS switches are extensively considered for the reconfigurable antennas since they have better switch isolation, lower insertion loss and power consumption, and higher linearity than solid-state switches over a wide RF band [1]. Since the reconfigurable MEMS antenna was first introduced by E. R. Brown in 1998, many studies of the reconfigurable MEMS antenna have been proposed [2-5]. From these results, the full integration of the antenna and MEMS switch is realized, and frequency and pattern reconfigurable characteristics are also presented. However, these kinds of antennas were not suitable for UWB (Ultra-WideBand) [6-8] applications since they usually have narrow bandwidth due to the antenna structure and bias-line with narrow bandwidth. Therefore, studies of the reconfigurable MEMS antenna with ultra-wideband characteristic are still left as future work. For the next generation wireless communication systems, it can be expected that the integration of the reconfigurable technology and UWB technology will be a very important wireless communication theme because the reconfigurability has immense potential to the wireless communications, such as enhanced signal detecting, low noise environment, and size decrease of the RF front-end [9]. Recently, various reconfigurable antennas were proposed [4, 5, 10, 11]. Some antennas among them are integrated with MEMS switches. However, all these antennas have narrow bandwidth since they are microstripline type antennas with high Q. These kinds of antennas usually have external λ/4 bias-lines for DC power, and these bias-lines can maintain open input impedance at narrow bandwidth. Therefore, the bias-lines with ultra wide-bandwidth suitable for UWB applications should be previously designed. However, this design causes the problem of additional size increase and complex front-ends. To solve the above problems, a novel ultrawideband reconfigurable RF-MEMS antenna with multi-functional transition is required. As a first step, a hardwired design of an ultra-wideband reconfigurable RF-MEMS antenna with multifunctional transition is presented in this paper. This proposed antenna is the modified CPW (Coplanar Waveguide) based TSA [12-13] without any kinds of DC bias-lines and provides the novel reconfigurability with ultra wideband characteristic at both endfire and broad-side directions as one TSA with the modified design of the transition part. Therefore, the transition part of the TSA can be also used as a good radiator for broadside radiation. 1-4244-0330-8/06/$20.00 2006 IEEE

II. DESIGN OF THE HARDWIRED ANTENNAS A. Design for the prototype antenna The configuration of the prototype for the proposed antenna is shown in Fig. 1. Basically, this prototype is composed of 3 conductor planes, 2 slots, and CPW feed, and they are all separated. By the compositions of these ones, final radiations can be controlled to occur at the one of each slot. Slot 1 and slot 2 are for endfire radiation and broadside radiation, respectively. In this study, CPW is selected because it is suitable for the concept of no bias lines, and has been used widely for microwave monolithic and hybrid integrated circuits due to its advantages, such as lower dispersion, low profile, reduced size, and easy integration with other devices [14]. Also, Liquid Crystal Polymer (LCP) [15] substrate with conductor thickness of 5 um, substrate height of 4 mils, dielectric constant of 3, and loss tangent of 0.003 is used. Fig. 1. Configuration and simplified equivalent circuit of the prototype for the proposed antenna. Therefore, this proposed antenna structure has the advantages of both CPW and LCP. The simplified equivalent circuit of the prototype for the proposed antenna is shown in Fig. 1. This consist of a CPW feed, an endfire directional slot radiator, a broadside directional slot radiator, and ultra wideband slot transformer with virtual shorts. Here, Z slot, Z cpw, Z slot_tran, C r, C s, and G s are the impedance of the slot, the impedance of the CPW, the input impedance of the slot transformer, a capacitance of the radiator, a capacitance between conductor planes, and a radiation conductance, respectively. By this model, it can be verified that this prototype has the potential to do pattern reconfigurability for both endfire and broadside directions. B. Ultra-wideband antenna for the endfire radiation For the ultra-wideband characteristic with endfire radiation, the antenna is composed as shown in Fig. 2. To obtain this ultra-wideband CPW-to-slot transition, both the electrical length and characteristic impedance of the slotline and the size of the slot stub (slot 2) that compensates the mismatched admittance are optimized. Therefore, the RF signal fed from the CPW feed flows to the slotline connected with slot 1, and slot 2 operates as an ultra-wideband transformer shown in Fig. 2, where the input impedance of the slot transformer (Z slot_tran ) is seen as (open circuit) since the virtual short due to the double λ g /4 slotline stubs occurs at the edge of the slot 2. The length of slot 2 is approximately equal to λ g /2, and the two input parts of the slot 2 are excited with a 180 phase difference because the slotline to CPW excites a balanced mode in CPW [16]. Therefore, radiation is suppressed, and the RF signal from the CPW feed completely flows to the slotline connected with slot 1. To obtain this operation, the feed line should be connected with conductor 2 and separated from conductor 1, while conductors 2 and 3 are separated from each other. The simulation is performed by using Ansoft HFSS. The return loss of the proposed antenna for ultra-wideband endfire radiation is shown in Fig. 3. A bandwidth of 4.4:1 (3 ~ 13.1 GHz) for less than - 10 db is achieved.

Fig. 2. Configuration of the antenna for endfire radiation and its simplified equivalent circuit. C. Ultra-wideband antenna for the broadside radiation For the ultra-wideband characteristic with broadside radiation, the antenna is composed as shown in Fig. 4. To have the ultra-wideband broadside radiation, the wide slot radiator (slot 2) fed by the CPW feed line should have ultrawideband characteristic. Basically, this kind of wide slot antenna usually has ultra-wideband characteristic due to the gradual transition by the structural characteristic from the feed line to the radiator. Therefore, constant input impedance is maintained over an ultra wide-bandwidth [17]. To obtain this characteristic, the electrical length of the CPW line between slot 2 and the CPW feed line and the gradual transition of the slot 2 are optimized. Here, both modes of the slot transformer and tapered slot radiator (slot 1) do not exist as shown in Fig. 4 because conductor 2 and the antenna feed are connected with conductor 3 and conductor 1, respectively, while the antenna feed is separated from conductor. This leads to broadside radiation of the proposed antenna. Therefore, RF signal fed from CPW feed directly flows to slot 2, and radiates over ultra wideband. At the radiating edge of slot 2, a virtual open occurs since the two input parts of slot 2 are excited with equal phases. Therefore, the RF signal from CPW feed completely flows to the wide slot radiator (slot 2). The return loss of the proposed antenna for ultra-wideband broadside radiation is shown in Fig. 5. The simulated bandwidth is 3.1:1 (9.7 ~ 30 GHz) and more for less than -10 db. Fig. 3. Simulated return loss of the proposed ultrawideband antenna for endfire radiation.

Fig. 4. Configuration of the prototype for the proposed antenna. Fig. 5. Configuration of the prototype for the proposed antenna. III. CONCLUSION In this paper, an ultra-wideband antenna with multi-functional transition is proposed and analyzed. Through the multi-functional transition operated as both a wideband transition of a tapered slot antenna and a wideband slot antenna, it is verified that the proposed antenna can provide ultra-wideband pattern reconfigurable characteristics with both broadside and endfire directions. Therefore, both broadside and endfire radiation characteristics using only one antenna can be achieved without any broadside directional radiator. REFERENCES [1] G.M. Rebeiz, RF MEMS: Theory, Design, and Technology. New york: WILEY, 2003. [2] E.R. Brown, RF-MEMS switches for reconfigurable integrated circuits, IEEE Transaction on Microwave Theory and Techniques, Vol. 46, pp. 1868 1880, Nov. 1998. [3] D.E. Anagnostou,, G. Zheng, M.T. Chryssomallis, J.C. Lyke, G.E. Ponchak, J. Papapolymerou, and C.G. Christodoulou, Design, fabrication, and measurements of an RF-MEMS-based self-similar reconfigurable antenna, IEEE Transaction on Antennas and Propagation, Vol. 54, pp. 422 432, Feb. 2006. [4] C.W. Jung, Ming-jer Lee, Li, G.P., F. De Flaviis, Reconfigurable scan-beam single-arm spiral antenna integrated with RF-MEMS switches, IEEE Transaction on Antennas and Propagation, Vol. 54, pp. 455 463, Feb. 2006. [5] G. Huff and J.T. Bernhard, Integration of packaged RF MEMS switches with radiation pattern reconfigurable square spiral microstrip antennas, IEEE Transaction on Antennas and Propagation, Vol. 54, pp. 464 469, Feb. 2006. [6] R.J. Fontana, Recent system applications of short-pulse ultra-wideband (UWB) technology, IEEE Transaction on Microwave Theory and Techniques, Vol. 52, pp. 2087 2104, Sep. 2004. [7] D. Porcino and W. Hirt, Ultra-wideband radio technology: potential and challenges ahead, IEEE Communications Magazine, Vol. 41, pp. 66-74, July 2003. [8] H.G. Schantz, A brief history of UWB antennas, IEEE Aerospace and Electronic Systems Magazine, Vol. 19, pp. 22-26, Apr. 2004. [9] S. Zhang, G.H. Huff, J. Feng, and J.T. Bernhard, A pattern reconfigurable microstrip parasitic array, IEEE Transaction on Antennas and Propagation, Vol. 52, pp. 2773 2776, Oct. 2004. [10] S. Nikolaou, R. Bairavasubramanian, C. Lugo, Jr., I. Carrasquillo, D.C. Thompson, G.E. Ponchak, J. Papapolymerou, and M.M. Tentzeris, Pattern and frequency reconfigurable annular slot antenna using PIN diodes, IEEE Transaction on Antennas and Propagation, Vol. 54, pp. 439 448, Feb. 2006. [11] Xue-Song Yang, Bing-Zhong Wang, Weixia Wu, Pattern reconfigurable patch antenna with two orthogonal quasi-yagi arrays, IEEE Antennas Propagation Soc. Int. Symp., vol. 2B, pp. 617-620, July 2005. [12] 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 Transaction on Microwave Theory and Techniques, Vol. 37, pp. 365 374,Feb. 1989. [13] R.N. Simons, N.I. Dib, R.Q. Lee, and L.P.B. Katehi, Integrated uniplanar transition for linearly tapered slot

antenna, IEEE Transaction on Antennas and Propagation, Vol. 43, pp. 998 1002, Sep. 1995. [14] Ting-Huei Lin and Ruey-Beei Wu, CPW to waveguide transition with tapered slotline probe, IEEE Microwave and Guided Wave Letters, Vol. 11, pp. 314 316, July 2001. [15] D.C. Thompson, O. Tantot, H. Jallageas, G.E. Ponchak, M.M. Tentzeris, and J. Papapolymerou, Characterization of liquid crystal polymer (LCP) material and transmission lines on LCP substrates from 30 to 110 GHz, IEEE Transaction on Microwave Theory and Techniques, Vol. 52, pp. 1343 1352, Apr. 2004. [16] K.C. Gupta, Ramesh Garg, Inder Bahl, and Prakash Bhartia, Microstrip lines and slotlines. Norwood, MA: Artech house, 1996. [17] J.D. Kraus, Antennas. New york: McGraw-Hill, 1988.