MEMS Reconfigurable Vee Antenna


Research Projects

ABSTRACT
A new architecture for microelectromechanical system (MEMS) reconfigurable Vee-antennas was presented. The planar antenna structure can be dynamically reconfigured to steer the radiation beam or change the shape of the beam using electrically-controlled microactuators. The theoretical and experimental beam patterns were investigated for a 17.5-GHz MEMS Vee-antenna. The beam steering and shaping capabilities were also demonstrated.


Recent advances in millimeterwave systems and antennas allow integration of planar antennas with active and passive circuits on the same chip monolithically [1]. It is desirable to integrate beam-steering and shaping functionality into the systems so one can dynamically control the radiation beam directions and/or vary the beam shapes in many applications such as seekers for smart weapons, automobile and airplane radar, reconfigurable wireless and satellite communication networks, as well as space-borne remote sensing. Conventional electronic-scanning antennas, such as phased-array antennas, have the advantage of fast scanning. However, they require many phase shifters. Millimeterwave phase shifter circuits are expensive and have higher losses as the frequencies increase. Microelectromechanical system (MEMS) devices, which provide the advantages of fast actuation and low losses, are an attractive option for high-frequency systems [2]. As the frequencies increase to millimeterwaves and submillimeterwaves, the antenna sizes reduce to the scales of MEMS architectures. This allows the possibility of fast actuation to reconfigure antenna structures with low power-consumption by microactuators such as electrostatic side-drive micromotors [3], comb-drive microactuators [4] and scratch-drive actuators [5]. Furthermore, using metal-coated polysilicon and pure metal-to-metal contacts to construct two- or three-dimensional reconfigurable radiation and wave-guiding structures on the surface of devices allows the waves to propagate and be controlled in the air. This reduces the dielectric losses of millimeterwave signal in the substrate.
Previous efforts for integration of MEMS architectures and millimeterwave beam-steering components focused on quasi-optical systems [2,6] and true-time delay networks [2,7]. While recent development on MMICs achieved higher frequencies and higher powers, planar reconfigurable structures for antennas become essential for monolithic integration of millimeterwave devices and dynamic operation of low-cost wireless networks. In this work, a new architecture was proposed for planar reconfigurable antennas using multi-layer surface micromachining technology. The architecture can be further integrated with planar impedance tuners and other MEMS devices as integrated smart antennas.

ARCHITECTURE

Figures 1 and 2 show the concept and a photo of a MEMS reconfigurable Vee-antenna, respectively. The main structure of the device was fabricated using the three-polysilicon-layer surface-micromachining process offered by the MCNC (Microelectronics Center of North Carolina) MUMPS program. Post-processing including deposition and etching of dielectric material, metal and silicon was done at the University of Hawaii. The arms of the Vee-antenna are moveable through pulling or pushing by actuators. One end of the antenna arm is held by a rotational hinge locked on the substrate, which allows the arm to rotate with the hinge as the center of the circle. The antenna arms are pulled or pushed by support bars connected to the actuators with moveable rotation hinges on both ends. The moveable rotation hinges translate the lateral movement of the actuators to circular movement of the antenna arms. The antenna arms and the support bars are physically connected but electrically separated with dielectric material. The actuators used are scratch drive actuators [5] with a bushing height of 1mm and a distortion-plate size of 50µm x75µm. The scratch drive actuators are connected together with sidebars to increase motion force and lateral movement range. The sidebars are guided with overhang rails, which allow biasing in parallel to the actuators. The antenna arms are connected to contact pads through the fixed rotation hinges.

Each antenna arm can be controlled independently with forward-moving or backward-moving biases. When both antenna arms move in the same direction with a fixed Vee-angle, the antenna can be used to steer the radiation beam. When the Vee-angle changes, the radiation beam shape can be adjusted.

Figure 1: (a) The concept and (b) the cross section of a MEMS reconfigurable Vee-antenna.

Figure 2: Top view and details of a MEMS reconfigurable Vee-antenna.

THEORY AND MEASUREMENTS

A 3-GHz Vee-antenna model was constructed to investigate antenna performance and find the optimal antenna dimensions. The Vee-antenna was used as a receiving antenna, connected to an HP 8564E spectrum analyzer, and the radiation pattern was measured by rotating a transmitting horn antenna, excited by an HP 83592C sweeper. Experiments found that a narrow main beam and smaller back-scattering sidelobes can be achieved with an arm length of 1.5l and a Vee-angle of 75°. The measured E-plane and H-plane patterns, compared with the theoretical patterns, are shown in Fig. 3 and Fig. 4, respectively. The theoretical patterns in the E-plane are calculated using both travelling-wave and standing-wave theories for Vee-antennas [8]. For a travelling-wave long-wire Vee-antenna, theory predicts that an optimal angle of 82.5° will give a maximum directivity of 5.6 [8]. The estimated directivity using 3-dB E-plane and H-plane beamwidths in the 3-GHz model is 20.2.


Figure 3: E-plane co-pol and cross-pol patterns compared with the theoretical co-pol patterns for the 3-GHz model.

Figure 4: H-plane co-pol and cross-pol patterns compared with the theoretical pattern for the 3-GHz model.

Using scaled dimensions, 17.5-GHz MEMS reconfigurable Vee-antennas were fabricated and tested. The scratch drive actuators move in a step of 20nm per biasing pulse with voltages in the range of 70v - 120v. The received power by the Vee-antenna at the main beam is about -25dBm. Figure 5 shows the co-pol and cross-pol E-plane patterns and Fig. 6 shows the co-pol and cross-pol H-plane patterns, compared with theoretical patterns, respectively. The estimated directivity using 3-dB beamwidths is 37.9.


Figure 5: E-plane co-pol and cross-pol patterns compared with the theoretical patterns for a 17.5-GHz 75°-Vee-antenna.

Figure 6: H-plane co-pol and cross-pol patterns compared with the theoretical pattern for a 17.5-GHz 75°-Vee-antenna.

RECONFIGURABILITY AND INTEGRATION

The antenna arms were rotated 30° and 45° in the same direction by the microactuators, while the Vee-angle was kept at 75°, to demonstrate the beam steering functionality. Fig. 7 shows shifts of main beams at 30° and 48° for the 30°- and 45°-steering, respectively. The antenna arms were moved in the opposite directions to change the Vee-angles in order to demonstrate the beam shaping capability. The co-pol E-plane patterns were measured for 90°- and 120°-Vee-antennas, shown in Fig. 8, and compared with theoretical patterns. Only halves of the patterns were shown because of symmetry in the patterns. Comparison of Fig. 8 and Fig. 5 demonstrated that the Vee-antenna could be dynamically adjusted to reach the optimal directivity.


Figure 7: E-plane beam-steering patterns for a 17.5-GHz 75°-Vee-antenna.

Figure 8: Measured and theoretical E-plane patterns (in halves) for 90°- and 120°-Vee-antennas.
The architecture of MEMS reconfigurable Vee-antennas allows integration of MEMS planar impedance tuners and other MEMS devices. Fig.9 shows the concept. A sliding planar backshort on top of the planar transmission line, connected to the antenna feed point, forms a moveable short-circuit over a useful bandwidth. This allows a variation of the transmission line's electrical length by varying the position of the sliding planar backshort. The planar backshort has a cascade of several sections of short-circuit separated by quarter wavelengths on the sliding plate. Lubecke demonstrated a 3-dB improvement for the detector response over the untuned response at 100GHz using a 5-section planar backshort without electrically-controlled actuators [9]. Prototypes have been designed and fabricated, as shown in Fig. 9(b), to investigate the mechanical characteristics and the integration of sliding planar backshort plates with microactuators.
Figure 9: (a) The integration concept of a MEMS Vee-antenna with a MEMS planar impedance tuner. (b) Top view of the prototype for a planar sliding impedance tuner.


The dynamic reconfigurability of MEMS Vee-antennas was demonstrated, including beam steering and beam shaping capabilities. This demonstration is the first step toward integrated smart antenna systems where MEMS impedance tuners, MEMS passive elements, active devices and control circuits can be monolithically integrated on the same chip, and the antenna performance and functionality can be dynamically adjusted.


REFERENCES
[1] F. Schwering, "Millimeter Wave Antennas," Proc. IEEE, Vol.80, No.1, pp.92-102, Jan. 1992.
[2] E. Brown, "RF-MEMS Switches for Reconfigurable Integrated Circuit," IEEE Trans. Microwave Theory Tech., Vol.46, No.11, pp.1868, 1998.
[3] L. Fan, Y.-C. Tai and R. Muller, "IC-processed Electrostatic Micromotors," Sensors and Actuators, Vol. 20, pp.41-47, Nov. 1989.
[4] W. Tang, T. Nguyen and R. Howe, "Laterally Driven Polysilicon Resonant Microstructures," Sensors and Actuators, Vol. 20, pp.25, Nov. 1989.
[5] T. Akiyama and K. Shono, "Controlled Step-wise Motion in Polysilicon Microstructures," J. of MEMS, Vol.2, No.3, pp.106, Sept. 1993.
[6] J.-C. Chiao and D. Rutledge, "Microswitch Beam-Steering Grid," The Intl. Conference on Millimeter and Submillimeter Waves and Applications, San Diego, CA, Jan. 1994.
[7] N. Barker and G. Rebeiz, "Distributed MEMS True-time Delay Phase Shifters and Wide-band Switches," IEEE Trans. Microwave Theory Tech., Vol.46, No.11, pp.1881, 1998.
[8] C. Balanis, Antenna Theory, 2nd Ed., pp. 502, published by John Wiley & Sons, Inc., 1997.
[9] V. Lubecke, W. McGrath and D. Rutledge, "A 100GHz Coplanar Strip Circuit Tuned with a Sliding Planar Backshorts," IEEE Microwave and Guided Wave Letters, Vol.3, No.12, pp.441, 1993.



Created by J.C. Chiao