Abstract â In this work, ohmic contact series cantilever. RF MEMS switches are used to reconfigure a planar dipole antenna in order to exhibit the same ...
An X-band Reconfigurable Planar Dipole Antenna Dimitrios E. Anagnostou, Guizhen Zheng1, Silvio. E. Barbin2, Michael T. Chryssomallis3, John Papapolymerou1, and Christos G. Christodoulou University of New Mexico, Albuquerque, NM, 87131, USA (1) Georgia Institute of Technology, GA, USA, (2) University of São Paulo, SP, Brazil, (3) Demokritus University of Thrace, Xanthi, 67100, Greece Abstract — In this work, ohmic contact series cantilever RF MEMS switches are used to reconfigure a planar dipole antenna in order to exhibit the same radiation pattern and good impedance matching at any frequency in the X-band range. The antenna is fabricated on a silicon substrate and the desired reconfigurability is achieved with four switches, each one shifting the resonant frequency by 900 MHz approximately. For the successful integration of the system, the antenna and the switches are fabricated from the same materials. Issues regarding the biasing of the switches and the antenna feed are resolved herein. Index Terms — Reconfigurable antenna, dipole, MEMS switches, planar antennas, X-band.
approximately 2.54 are used. The substrate is thick with respect to the frequency of operation. The RF-MEMS switches are ohmic contact series cantilever switches, with a 3 μm thick membrane made of gold for greater flexibility. The biasing is achieved with 9pin GSG probe pads that provide the necessary 17 Volts potential difference between the pull-down electrode and the membrane. The switches have been designed and fabricated at the Georgia Institute of Technology [2]. They demonstrated a high isolation (-15dB) and low insertion loss (-0.3 dB) for frequencies up to 40 GHz. The bias lines are made of high-resistive material. Sheet resistance Rs = 10Ȁȍ/sq, and each line is more than 20 squares long causing the currents to attenuate rapidly on them. The lines end in 150-picthed biasing pads, placed 3000 Pm from the top and bottom of the structure, in order to minimize interference with the bulky metallic probe heads. Finally, the antenna’s RF feed is achieved with CPW probes, through a wideband CPS to CPW transition [3] designed specifically to make possible the feed of any planar dipole-like antenna up to 40 GHz.
I. INTRODUCTION Dipole antennas have found extensive use in a plethora of applications in wireless systems. In this work, a reconfigurable dipole antenna is designed to exhibit the same radiation pattern at any frequency in the X-band range. Previous work in reconfigurable dipole antennas [1] has illustrated similar designs, but issues such as biasing of the switches, antenna feeding and impedance matching were not resolved. In this work, an attempt to illustrate the use of RF MEMS switches to enhance the frequency performance of a planar dipole antenna is presented. II. OVERVIEW OF A PLANAR DIPOLE ANTENNA AND RF MEMS SWITCHES A planar dipole antenna with a total length l, resonates at a frequency f which corresponds to Ȝg.= l/0.44, where Ȝg is the effective guided wavelength given as the average of the relative permittivities of the two dielectric media. The structural characteristics of the RF MEMS switch can be seen in Fig. 1, and the placement of a switch on the antenna in Fig. 2. Compatibility issues impose that the antenna is also fabricated on the same substrates. Thus a 1 μm thick silicon dioxide layer over 400 μm of silicon with an effective relative dielectric permittivity of
0-7803-9342-2/05/$20.00 © 2005 IEEE
654
Fig. 1.
Cross-section of cantilever RF MEMS switch.
Fig. 2.
Connection of a switch to antenna and bias lines.
III. RECONFIGURABLE DIPOLE ANTENNA DESIGN
previous state. Its resonance now occurs at 10.5 GHz, with a bandwidth from 10.1 GHz to 11 GHz. The antenna’s performance is shown in Fig. 5, where this 900 MHz shift of the bandwidth can be seen. By setting ‘on’ the second switch of each arm, another shift occurs, this time from 9.3 to 10.1 GHz. The simulation results are similar to the ones shown before and thus are not presented herein. The bandwidth from 8 to 9.3 GHz is too large to be covered with the addition of one switch only. Therefore, two more switches are placed in order to achieve full bandwidth coverage using the procedure previously described. The total possible bandwidth of this reconfigurable dipole antenna is illustrated in Fig. 6. Since all resonances are coming from an antenna with similar shape to the initial dipole, its pattern remains the same at any frequency in the X-band. The antenna’s simulated elevation pattern for the case of maximum deformation, which occurs when all four switches are ‘on’, is shown in Fig. 7.
A dipole antenna that can shift its resonant frequency to fit in any region of the X-band is designed. It can be used as an example that illustrates the application of RFMEMS switches in a reconfigurable antenna design. Since other planar antennas, such as the bowtie, have a broader bandwidth than planar dipole, this theoretical model may also be used to determine a ‘maximum number of switches’ to be used in order to achieve the desired reconfigurability. The goal is set to design a reconfigurable dipole antenna that can operate on demand at any frequency in the Xband. In this application the bandwidth of the planar dipole is approximately 8.5%. The arms of the dipole are connected with RF-MEMS switches to additional patches. The basic function of the switches is to conductively couple the additional metallic patches thus extending on-demand each arm’s length. When the switches are ‘off’ the patches couple capacitively to the main arms, thus they slightly increase the antenna’s bandwidth. When the switches are ‘on’, the additional patches are connected to the antenna’s arms via the continuous metallic path that each switch provides through its membrane. To avoid unnecessary discontinuities in the structure, the dipole is set to have the same width as the switches (110 μm). In other words it can be considered as an extension of the switch’s transmission line. To design this antenna, first the 12 GHz resonance is considered and an arm length of 2860 μm is found necessary. The antenna’s design is shown in Fig. 3, and a summary of its dimensions in Table 1. Its performance when all switches are ‘off’ is shown in Fig. 4. In order to achieve a set of resonances at sequential frequencies at the X-band, the length of the dipole needs to be successively prolonged by approximately 'L = 345 μm for each new resonance. Since the membrane of the switch is made of gold, it becomes a part of the radiating structure when the switch is set to ‘on’. Thus, the first 315 μm of this length can be contributed by the switch as it is equal to the lengths of the membrane and the transmission line connected to each switch. This additional short transmission line provides adequate space for the bias lines to connect from the side. Therefore only additional 30 μm long patch needs to be connected before each switch to provide the additional required length. A total of 4 switches are connected in each arm to provide the whole successive shift of the bandwidth from 12 GHz to 8 GHz (25 %). When the first switch is turned ‘on’, while the others are kept at their initial ‘off’ state, the antenna resonates at a frequency 900 MHz lower than its
DC pads
d Bias lines MEMS switches
L a
b
c Transition Dipole
DC pads Fig. 3. Topside view of the reconfigurable planar dipole antenna design
TABLE I ANTENNA DESIGN PARAMETERS Parameter Size A B C D L
655
5930 μm 8780 μm 210 μm 110 μm 4500 μm
Fig. 4.
Initial dipole’s reflection coefficient magnitude.
Fig. 7. Simulated radiation pattern of the planar dipole antenna at 8.5 GHz, when all switches are ‘on’.
IV. CONCLUSION
Fig. 5.
A reconfigurable planar dipole antenna was designed and the coverage of the entire X-band was achieved with the integration of ohmic contact cantilever series RFMEMS switches. To change the resonant frequency, one can simply scale the vertical length of the antenna. Future research on such antenna structures may include bandwidth broadening by mean of placing parasitic patches close to the antenna itself.
Antenna’s performance with one switch ‘on’
REFERENCES [1] J. Kiriazi, H. Ghali, H. Ragaie, H. Haddara, “Reconfigurable dual-band dipole antenna on silicon using series MEMS switches,” 2003 IEEE Antennas and Propagation Society International Symposium, vol. 1, pp. 403 – 406, 22-27 June 2003. [2] D. E. Anagnostou, G. Zheng, L. Feldner, M. T. Chryssomallis, J. C. Lyke, J. Papapolymerou and C. G. Christodoulou, “Silicon-etched Re-configurable SelfSimilar Antenna with RF-MEMS Switches,” 2004 IEEE APS/URSI International Symposium, Monterey, vol. 2, pp. 1804-1807, 20-26 June 2004. [3] D. E. Anagnostou, G. Zheng, M. T. Chryssomallis, J. C. Lyke, G. Ponchak, J. Papapolymerou, C. G. Christodoulou, “Design, Fabrication and Measurements of a Self-Similar Re-configurable Antenna with RF-MEMS Switches,” IEEE Transactions on Antennas & Propagation, Special Issue on Reconfigurable Antennas (submitted).
Covered bandwidth > 4 GHz (8-12 GHz) Fig. 6. The antenna’s performance at different configurations, illustrating the total covered bandwidth.
656
