NASA/ESA Conference on Adaptive Hardware and Systems
Reconfigurable MEMS Antennas Nakul Haridas1, Ahmet T. Erdogan1, Tughrul Arslan1, Anthony J. Walton1,2, Stewart Smith1,2, Tom Stevenson1,2, Camelia Dunare1,2, Alan Gundlach1,2, Jon Terry1,2, Petros Argyrakis1,2, Kevin Tierney1,2, Alan Ross1,2 and Tony O’Hara3 1 School of Engineering and Electronics, University of Edinburgh, Edinburgh, UK 2 Scottish Microelectronics Centre, Edinburgh, UK 3 MEMSSTAR, Point 35 Microstructures, Edinburgh, UK
[email protected] switches are also very promising devices as they will be able to replace a number of solid state circuits enabling better performance in terms of loss, isolation, linearity, power consumption, and compatibility with integrated circuits. MEMS devices are ideal for reconfigurable networks, antennas and subsystems. They have very low insertion loss and high Q up to frequencies of 120 GHz. They can be integrated on low dielectric constant substrates which is important for high performance tunable filters, high efficiency antennas, and low loss matching networks. MEMS devices offer very low loss switching and can be controlled using 10 to 120K Ω resistive lines. This means that the bias network for RF MEMS switches will not interfere and degrade antenna radiation patterns. As the bias network does not consume any power this is an important advantage for large antenna arrays. Two issues remain attached to MEMS switches that one should consider are electrostatic discharge sensitivity and hot switching, which happened due to high bias voltage or thermal effects which can permanently damage the switch. Power handling capabilities of RF MEMS switches are limits due to self actuation and stiction in the down state due to high incident RF power typically in the range of 10’s of mW for ohmic switches and up to 1W for capacitive switches [21]. MEMS are employed in many ways to achieve reconfigurability. The first is to change the shape of the effective radiating structure to alter the pattern or the frequency of operation. The second method employs MEMS to mechanically actuate the antenna, and change the orientation of the antenna with respect to the substrate or another radiating structure. The third method employs MEMS capacitive switches to modify the impedance of the antenna, which changes the resonant frequency of the radiating antenna. The fourth employs MEMS phase shifters.
Abstract This paper reviews the work carried out in the field of reconfigurable antennas, and in specific the reconfigurable MEMS (Micro-Electro-Mechanical Systems) antennas. The application of MEMS to antennas is studied and compared with the various implementations such as pattern reconfigurable MEMS antennas, mechanically actuated MEMS antennas, capacitive MEMS antennas and MEMS phased array antennas, as reported by research groups in the field. Finally a design is described, driven by the objectives of low power, high efficiency, linear operation and real time frequency and space diversity reconfiguration.
1. Introduction A reconfigurable antenna is one which alters its radiation, polarization and frequency characteristics by morphing its physical structure. Reconfigurable antennas with the ability to radiate more than one pattern at different frequencies are necessary in radar and modern communication systems. Many reconfigurable antennas concentrate on changing operating frequency while maintaining their radiation characteristics. However, the ability to change the radiation patterns while maintaining operating frequency could greatly enhance system performance. Manipulation of an antenna's radiation pattern can be used to avoid noise sources or intentional jamming, improve security by directing signals only toward intended users, serve as a switched diversity system, and expand the beam steering capabilities of large phased arrays. To date reconfigurable antennas have been realised using three major technologies: MEMS antennas, optical antennas, and holographic antennas; the later two being similar in nature by employing light or some bias field to create the desired shape of the radiating structure. MEMS antennas have gained popularity due to their higher linearity and as a result lower signal distortion when compared to semiconductor devices. MEMS
978-0-7695-3166-3/08 $25.00 © 2008 IEEE DOI 10.1109/AHS.2008.28
147
radiation pattern or changes the frequency of the antenna. The antenna is a single turn square microstrip spiral fabricated on a Duroid 5880 substrate as shown in figure 2. The outer end of the spiral is shorted to ground whilst the inner one forms the feed to the SMA probe. When used in the end fire (axial) mode, switch 1 is closed while switch 2 is open. This creates a single turn spiral antenna working at 3.7 GHz. With switch 1 open and switch 2 closed it reconfigures the antenna to the broadside configuration working at 6 GHz. Hence it successfully demonstrates the two features of a reconfigurable antenna using frequency and pattern reconfigurability.
2. Pattern Reconfigurable Antennas MEMS switches can be employed to connect different elements that make up the antenna structure, one such example is the work carried out by Yang et. al. [1]. They demonstrate the use of MEMS switches in patch antennas designed by 3rd order Hilbert curves designed to work at 10GHz as shown in figure 1(a). This antenna is connected to slots via MEMS switches and on actuating these switches the resonant frequency changes to 12.5GHz (See figure 1(b)).
Figure 1. (a) Layout of the original patch antenna, (b) Antenna being reconfigured by use of MEMS switches By having two slots to the side of the main patch antenna results in a number of different configurations in which the antenna can be reconfigured. These include one with the slots connected to the antenna, one with individual slots connected to the antenna, and one configuration where the slots are disconnected from the main patch. Results indicate that in each configuration a variation in both frequency and beam directivity is observed, which demonstrates the reconfigurable nature of the design.
Figure 3. Implementation of the rectangular spiral antenna by De Flaviis et. al. De Flaviis et. al. [6] extended this concept by employing a rectangular spiral antenna with a set of MEMS switches which were monolithically integrated and packaged on the same substrate. The antenna is printed on a PCB and fed through a via hole, which is placed at the centre of the antenna. This creates a right handed circularly polarized (RHCP) spiral, see Figure 3. The antenna is made up of multiple lines which are connected via the MEMS switches, and by activating these switches the overall length of the antenna is changed which modifies its radiation pattern. This is claimed by the authors to be the first implementation of a truly reconfigurable antenna, by fabricating both MEMS and the printed antenna on the same substrate.
Figure 2. Implementation of a single arm square antenna by Bernard et. al. Similar work reported by Bernard et. al. [2-5] consists of a single turn square spiral antenna working at a nominal frequency of 3.7 GHz. The antenna is provided with a set of commercially available MEMS SPST switches, which when employed either redirect the
148
This design employs two MEMS switches to demonstrate three different beam directions by changing the effective length of the antenna. This antenna is designed to work at 11 GHz and has three different spiral arm lengths. In this design the maximum beam directions is from 34˚ to 42˚ in its elevation angle and -29˚ to 14˚ in the azimuth angle, whilst the gain varies between 1.1 to 2.5 dBi when antenna is reconfigured between the three different lengths. Another very good example of a reconfigurable antenna is the pixel patch antenna designed again by De Flaviis et. al. [9]. They have proposed to use a pixel antenna concept which uses an array of individual antenna elements that can be connected via MEMS switches. A variety of patterns can be created by actuating the MEMS switches.
Figure 4. Layout of the reconfigurable antenna with MEMS switches As shown in Figure 4, the rectangular spiral antenna is fed through a coaxial feed. The spiral consists of five sections connected with four MEMS switches (S1-S4). The spiral arm length is increased following the right hand circular polarization for the radiation field. The operating frequency of the antenna is chosen to be 6 GHz, and switching on the different sections of the radiating structure creates a tilted beam. The maximum beam direction in the azimuth angle tilts from 18˚ to 104˚ and a maximum tilt angle of 30˚ in the elevation plane as the length of the antenna changes. The gain varies between 4-6 dBi depending on the length of the antenna. An alternative approach was designed and fabricated using a microstrip line. This converted the original RHCP to a left handed circularly polarized (LHCP) antenna [7, 8], as shown in Figure 5.
Figure 6. Implementation of pixel patch antenna
Figure 7. Pixel patch reconfigured for lower frequency of 4.1 GHz Frequency reconfigurability can be achieved by simply changing the size of the antenna and figures 6 and 7 illustrate this method. By selecting 25 pixels they
Figure 5. Microstrip fed antenna with MEMS switches
149
achieved an upper operating frequency of 6.4 GHz, whereas for the lower frequency of 4.1 GHz when all the 64 pixels were selected [9]. Linear polarization can be achieved by selecting antennas in a particular plane to act as a single radiating structure. Figures 8 and 9 illustrate how polarisation can be achieved selecting the pixels in either X or Y direction only.
produces the correct dimension for each desired frequency specification. The off-state of each pixel acts to produce a parasitic-loaded slot in the structure, which can then be combined with others in various geometries to produce the desired polarization.
Figure 10. Pixel patch reconfigured for RHCP radiation Figure 8. Reconfigurable pixel-patch antenna schematics with linear X Polarisation
Figure 11. Pixel patch reconfigured for LHCP radiation
Figure 9. Reconfigurable pixel-patch antenna schematics with linear Y polarization
3. Mechanically actuated MEMS Antenna
Circular polarization is obtained by introducing internal slots in the antenna geometry. By deactivating some patches creates parasitic elements in the pattern and accordingly RHCP or LHCP radiation is achieved. Figure 10-11 illustrates how we can obtain the circular polarizations. Careful layout of pixel elements and an on-/off-state algorithm allow the antenna to reconfigure its electrical size and shape, achieving both frequency and polarization diversity. The resulting variation in antenna geometry
This work demonstrates the ability to mechanically actuate the antenna with electrostatic force [17]. The antenna is suspended on a flexible spring. A bias voltage is applied between patch and antenna, creating an electrostatic force which attracts the patch towards the ground plate, as shown in figure 12 and 13. By varying the height of the antenna with respect to the ground plane the antenna operating frequency also changes. Its operating mode is equivalent to changing the relatively permittivity of antenna substrate.
150
Results show that, with the pull down voltage set to 76V for the patch, as the bias voltage is altered the resonant frequency of the antenna starts changing from 46.3 GHz to 38.8 GHz depending on the height of the patch.
shifts down from 16.05 GHz to 15.75 GHz as the actuation voltage is increased from 0 to 11.9 V as the height of the capacitive gap changes from 1.5 to 1.4 µm.
Figure 14. Implementation of capacitive MEMS antenna [18]
Figure 12. Layout of micromechanical patch antenna without bias voltage
5. MEMS Phased Array Antennas Phased array antennas find a lot of use in satellite communications both in terrestrial and space applications. Phased array antennas are being actively used for satellite tracking and on naval radars for surface detection and aircraft tracking.. The National Severe Storms Laboratory (NSSL) [11] applies phased array antenna for the investigation of all aspects of severe weather phenomenon like thunderstorms and tornadoes. Military and commercial satellites utilise phased array antennas for beam shaping and steering, making efficient use of the frequency spectrum and increasing space diversity of the antenna.
Figure 13. Layout of micromechanical patch antenna with bias voltage
4. Capacitive MEMS Antennas The work reported in [18] demonstrates a reconfigurable microstrip patch antenna that is monolithically integrated with RF MEMS capacitors for tuning the resonant frequency of the antenna. The structure consists of a patch antenna loaded with a coplanar waveguide (CPW) section attached to the antenna via microstrip to CPW transition as shown in figure 14. The reconfigurability in the resonant frequency of the antenna is provided with the aid of the MEMS bridges acting as a variable capacitor placed on the CPW stub. Varying the actuation voltage between the centre conductor and MEMS bridge metal, alters? the height of the bridges on the stub and hence modifies the loading capacitance. Thus, the CPW stub with bridges provides a variable load to the connected radiating edge, which results in change in the resonant frequency. The antenna is fabricated on a Pyrex 7740 glass substrate, and the MEMS bridge is suspended 1.5μm above the CPW. The resonant frequency of the antenna
Figure 15. Traditional phased array antennas MEMS can also be employed in phased array antennas as suggested in reference [10]. Traditional phased array antennas, see figure 15, are expensive because each antenna has its own transmit/receive Monolithic Microwave Integrated Circuits (MMIC) module including individual power amplifiers. In addition, the design of the
151
RF feed network and packaging system is complex at higher frequencies.
shifts of 0, 45, 90, 135,180, 225, 270 and 315˚, depending on the combination of the 3 bits used [13]. The implementation uses a 1 bit phase shifter of 30˚. With two sets of antennas connecting the individual phase shifter there are 3 possible combinations for beam steering. When both are in phase the beam is not steered, when left shifter is 0˚ and right is 30˚ the beam will steer left, and when the left is 30˚ and right is 0˚ the beam will steer right, see Figure 17. Hence this provides a beam steering capability. The work carried out at the University of Edinburgh employs an analog phase shifter which provides continuous variable phase shift from 0-306˚ [15, 16] figure 21. This is achieved by employing a distributed MEMS transmission line (DMTL) technique, which offers an alternative approach to the standard reflect-line or switched-line designs. These techniques have been used as a solution to obtain very wide band circuits. The concept is based on periodically loading a t-line with MEMS bridges (i.e. capacitance). A bias voltage is then applied between the MEMS bridge and coplanar waveguide (CPW) centre conductor, which varies the height of the bridge. This alters the distributed MEMS capacitance, resulting in a change in the loaded transmission line impedance and phase velocity, which in turn causes phase shift. Therefore, a structure with several MEMS bridges can act as a phase shifter when a bias voltage less than the pull-down voltage is applied [14, 15]. This results in an analog control of the transmission line phase velocity and, therefore, in a true-time delay (TTD) phase shifter. Another advantage of this design is that the phase shift is dependent on the frequency and by the varying the bias voltage, one can easily calibrate the phase shift for the desired frequency.
Figure 16. Improved phased array antenna design for low power and low cost A simpler solution, see figure 16, is to provide a single power amplifier and employ MEMS phase shifters which have very low insertion loss measured 0.1 dB at 25GHz and 0.18 at 40 GHz, some designs had insertion loss of 0.4 dB at 18 GHz and 0.7 dB at 34 GHz [19] compared to traditional GaAs MESFET switches with a 5.8 dB loss at 4GHz [20]. A practical implementation of such a phased array uses switched line MEMS phase shifter [12]. The switched line is a digital phase shifter and uses the delay line technique, and will provide a discrete set of phase shifts by employing MEMS switches to select the delay path along the transmission line. The relative phase shift is calculated by the delay the selected path will create, this kind of implementation is relatively easy to operate very much like a digital system by selecting a combination of these MEMS switches to provide the desired phase shift. However this works only for a fixed frequency and does not provide frequency reconfigurability in the antenna.
Figure 18 SEM image of a MEMS bridge As traditional MEMS these phase shifters can be fabricated on the same substrate as the antenna directly on the transmission line and depending on the number of elements in the array, the beam steering can be made finer. The phase shifters and antennas have been fabricated on silicon wafers and pictures of prototype systems are
Figure 17. Implementation of phased array antenna with switch line phase shifters For example, a 3-bit phase shifter is based on 45/90/180˚ set of delay networks and can provide phase
152
[2] G. Huff, J. Feng. D Zhang, J.T. Bernard, A Novel Radiation Pattern and Frequency Reconfigurable Single Turn Square Spiral Microstrip Antenna, IEEE Microwave and Wireless Components Letters, Vol. 13, No. 2, February 2003, pp. 5759. [3] H. Pan, J.T Bernhard, V.K. Nair, Reconfigurable SingleArmed Square Spiral Microstrip Antenna Design, IEEE International Workshop on Antenna Technology Small Antennas and Novel Metamaterials, 2006, March 6-8, 2006, pp. 180 – 183. [4] G.H. Huff, J.T. Bernhard, Integration of packaged RF MEMS switches with radiation pattern reconfigurable square spiral microstrip antennas, IEEE Transactions on Antennas and Propagation, Vol. 54, Issue 2, Part 1, Feb. 2006, pp. 464 – 469. [5] T.L. Roach, G.H. Huff, J.T. Bernhard, On the Applications for a Radiation Reconfigurable Antenna, Second NASA/ESA Conference on Adaptive Hardware and Systems (AHS 2007), 5-8 Aug. 2007, pp. 7 – 13. [6] C. Jung; M. Lee; G.P. Li, F. De Flaviis, Reconfigurable scan-beam single-arm spiral antenna integrated with RFMEMS switches, IEEE Transactions on Antennas and Propagation, Vol. 54, Issue 2, Part 1, Feb. 2006, pp. 455 – 463. [7] C. Jung; F. De Flaviis, Reconfigurable multi-beam spiral antenna with RF-MEMS capacitive series switches fabricated on rigid substrates, IEEE Antennas and Propagation Society International Symposium, Vol. 2A, 3-8 July 2005, pp. 421 – 424. [8] C. Jung; De F. Flaviis, RF-MEMS capacitive series switches of CPW and MSL configurations for reconfigurable antenna application, IEEE Antennas and Propagation Society International Symposium, Vol. 2A, 3-8 July 2005, pp. 425 – 428. [9] B.A. Cetiner, H. Jafarkhani, J. Qian, H.J. Yoo, A. Grau, F. De Flaviis, Multifunctional Reconfigurable MEMS Integrated Antennas For Adaptive MIMO Systems, University of California, Irvine, IEEE Communication Magazine, Vol. 42, Issue 12, Dec. 2004, pp. 62 – 70. [10] G.E. Ponchak, R.N. Simons, M. Scardelletti, Microelectromechanical switches for phased array antennas, IEEE Antennas and Propagation Society International Symposium, Vol. 4, 16-21 July 2000, pp. 2230 – 2233. [11] National Severe Storms Laboratory National Oceanic and Atmospheric Administration, February 2008, http://www.nssl.noaa.gov/research/radar/par.php [12] N. Kingsley, G.E. Ponchak, J. Papapolymerou, Reconfigurable RF MEMS Phased Array Antenna Integrated Within a Liquid Crystal Polymer (LCP) Systemon-Package, IEEE Transactions on Antennas and Propagation, Vol. 56, Issue 1, Jan. 2008, pp.108 – 118. [13] G.M. Rebeiz, “MEMS Phase Shifters”, in RF MEMS Theory, Design and Technology, John Wiley & sons Publication, 2003, pp. 259-297. [14] T.S. Ji, K.J. Vinoy and V.K. Vardhan, Distributed MEMS Phase shifters by microstereolithography on silicon substrates for microwave and millimetre wave application, Smart Materials and structures, Institute of Physics Publishing, PII: S0964-1726(01)30168-4, 2001. pp. 12241229. [15] N. Haridas, A.T. Erdogan, T. Arslan, M. Begbie, Adaptive Micro-Antenna on Silicon Substrate, First NASA/ESA
shown in figures 18-19. Two different types of bridges were fabricated, serpentine and planar; in order to characterise the operation of the phase shifter. The phase shifter, which will fulfill the requirements for low loss, high isolation, low power, robustness in design and ease of control, will be implemented into a phased array with a multi-frequency antenna [16] to provide true reconfiguration in directivity, frequency, beam shaping and steering.
Figure 19 Reconfigurable Antenna Array implementation
6. Summary This paper has briefly described various implementations of MEMS reconfigurable antennas. MEMS provide us unique advantages of low power, low insertion loss, higher linearity, lower signal distortion and ease of integration; compared to MMIC and solid state circuits. MEMS devices have been effectively used to create such a wide variety of reconfigurable antennas serving as either as switches, capacitors or phase shifters. They can be applied in a variety of applications to antenna technology to have a truly integrated solution for a reconfigurable antenna.
7. Acknowledgement The authors would like to acknowledge financial support from EPSRC (EP/C546318/1) and the Edinburgh Research Partnership in Engineering and Mathematics (Institute of Integrated Systems) for financial support.
8. References [1] X. Yang, B. Wang, Y. Zhang, A Reconfigurable Hilbert Curve Patch Antenna, IEEE Antennas and Propagation Society International Symposium, Vol. 2B, 3-8 July 2005, pp. 613 – 616.
153
Conference on Adaptive Hardware and Systems, (AHS 2006), 15-18 June 2006, pp. 43 – 50. [16] N. Haridas, A. El-Rayis, A.T. Erdogan, T. Arslan, MultiFrequency Antenna design for Space based Satellite Sensor Node, Second NASA/ESA Conference on Adaptive Hardware and Systems, (AHS 2007), August 5-8, 2007, Edinburgh. pp. 14-19. [17] H. Chen, Z. Shi, L. Wu, D. Guo, Frequency Reconfigurable Antenna with Micromechanical Patch, IEEE International Workshop on Anti-counterfeiting, Security, Identification, 16-18 April 2007, pp. 18 – 22. [18] E. Erdil, K. Topalli, M. Unlu, O.A. Civi, T. Akin, Frequency Tunable Microstrip Patch Antenna Using RF MEMS Technology, IEEE Transactions on Antennas and Propagation, Vol. 55, Issue 4, April 2007, Page(s):1193 – 1196. [19] S.P. Pacheco, L.P.B. Katehi, Microelectromechanical KBand Switching Circuits, 29th European Microwave Conference, Vol. 2. October 1999, pp. 45-48. [20] K. Purnell, A. Katz, A novel phase shifter using a GaAs MESFET in passive configuration, IEEE MTT-S International Microwave Symposium Digest, 1996., Volume 2, 17-21 June 1996 pp. 1197 – 1200. [21] J. Park, Fabrication and measurements of direct contact type RF MEMS switch, IEICE Electronics Express, Vol. 4, No. 10, 2007, pp. 319-325.
154
