Sunan Liu, Ming-Jer Lee, Changwon Jung, G.-P. Li, and Franco De Flaviis. University of California, Irvine, Department of Electrical Engineering and Computer.
A Frequency-Reconfigurable Circularly Polarized Patch Antenna by Integrating MEMS Switches Sunan Liu, Ming-Jer Lee, Changwon Jung, G.-P. Li, and Franco De Flaviis University of California, Irvine, Department of Electrical Engineering and Computer Science, Engineering Gateway 2233, Irvine, CA, 92697, United States
A microstrip antenna with integrated RF mircoelectromechanical system (MEMS) switches is proposed and demonstrated to operate at dual frequencies with circular polarization. The switches are incorporated into the diagonally-fed square patch for controlling the operating frequency and a rectangular stub attached to the edge of the patch acts as the perturbation to produce the circular polarization at 6.69 GHz and 7.06 GHz.
I. Introduction Microstrip antennas are widely used in wireless communications because of their compatibility, low profile and low cost. In more demanding environments such as military aircraft, circularly polarized antenna is typically used because it mitigates the loss in most of the situations. The capability to select the frequency is essential for diverse missions. A simple idea to adjust the resonant frequency of an antenna is to reconfigure its geometrical structure. This has been made possible with the use of the microelectromechanical systems (MEMS) switches [1, 2]. The MEMS devices can outperform their semiconductor counterparts such as transistors and diodes in lower insertion loss [3], lower power during operation and higher Q, which inherently fits the antenna element. In this manuscript, a novel circularly polarized patch antenna using MEMS switches to reconfigure the resonant frequency is reported. An additional patch is placed near to the main patch. Actuating the MEMS switches can connect these two patches and thus increase the effective electrical length, affording a lower frequency operation. The variance of frequency ratio due to different number of switches is also discussed. Both simulation and experiments demonstrate that the proposed antenna has the same polarizations and similar patterns at both frequencies.
II. Antenna Structure Fig. 1 shows the schematic diagram of the proposed antenna with 10 switches integrated. A diagonal-fed square patch with length L1 is printed onto a quartz substrate (thickness = 1.58 mm, relative permittivity = 3.78 and width = 38.1mm). 10 switches are monolithically integrated into the patch with 1 mm spacing. The second patch with length L2 is placed as shown in Fig. 1. The perturbation stub attached at the square patch excites the circular polarization mode. Two quarter-wavelength lines having same length S1 are used as matching network at both frequencies. To provide isolation between the RF signal and the DC bias signal, an open stub is placed in parallel to the biasing line quarter-wave away from the center of the second patch.
y x Fig.1 antenna structure L1=9 mm, L2=10.3mm, G=0.3mm, S1=3.61mm, SW=1.2 mm, SL=2.4 mm The control voltage is applied between the coaxial center conductor and the bias pad. When the voltage is applied, all the switches are activated simultaneously, providing the connection between the two patches. When no voltage is applied, the switches are in the off state (up position), therefore only the main patch is active while the second patch serves as parasitic element, lowering the resonant frequency. As the switches are actuated along L1 and L2, the effective electrical length increases. Therefore the resonant frequency decreases while the relative phase between the two orthogonal modes (along L1 and L2) remains the same. By placing the switches on both sides of the antenna, we are able to keep the circularly polarized wave at both frequency operations.
(a) (b) Fig. 2. (a)The simulated return loss for different number of MEMS switches; (b) frequency ratio vs. number of switches While frequency selection may be realized by just one switch on each side (a pair of switches), a detailed study of the effect of different number of switches on the return loss and frequency ratio is also of interest. In Fig. 2a, we plot the simulated return loss for different number of switches (both ON and OFF). Note that the switches on both sides are symmetric with respect to the signal path and the spacing between two neighboring switches is fixed. Fig. 2a shows that the resonant frequency tends to follow the increase of the number of switches at ON state. In contrast, the resonant frequency shifts to lower frequency as the number of switches at OFF states increases. As a result, the frequency ratio decreases with the increase of the number of switches, which is shown in Fig. 2b.
The reason is that when the switches are on the state, fewer switches lead to longer electrical path for the current, which in turn lowers the resonant frequency and raises the frequency ratio. Only experimental results with five switches integrated will be reported in this manuscript.
III. Fabrication and Performance For the fabrication of the proposed antenna, we use quartz substrate, because it’s low dielectric constant and relatively low loss at the frequency operation. The microstrip antenna and RF MEMS switches are monolithically fabricated onto the same quartz substrate with RF MEMS switches to be part of the antenna. There are seven key process steps. 1) 1st metallization. Titanium and Gold are deposited onto the substrate by E-beam evaporation. 2) 1st metal patterning. Patterns are defined by lithography and chemical etching; 3) Dielectric layer deposition and patterning. SiN 1800 Å in thickness is deposited by plasma enhanced chemical vapor deposition (PECVD) and then patterned by lithography and reactive ion etching (RIE). 4) Sacrificial layer coating and patterning. A thick (5~6 um) photoresist (PR) layer is coated and patterned for defining posts. This PR layer serves as the sacrificial layer. 5) Switch definition. Electroplated Ni thin film and E-beam evaporated gold film (~5000 Å) gold are patterned by lithography. 6) Switch releasing. The PR sacrificial layer wet released in acetone. To minimize the stiction, boiling methanol is applied right after acetone releasing. 7) Ground plane metallization. Thick metal films (Ti/Au or Ti/Cu) are deposited on the backside of the quartz substrate as the ground plate. An optical microscopy of the released patch is shown in Fig. 3a.
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(b) Fig. 3 (a) released MEMS (b)Return loss (simulation and measurement) Fig. 3b shows the comparison between simulated and measured return loss for the proposed antenna. The antenna resonates at 6.69 GHz (Switch ON) and 7.06 GHz (Switch OFF), giving rise to a frequency ratio of 1.05. The small discrepancy between them is probably due to the fact that we did not include the SiN layer in the simulation due to the computational limitation. We implement switches as the perfectly conducting stub when in the ON state, while we remove such stubs when these switches are in the OFF states. A bias of 30 volts with neglectable current consumption is required to actuate the switches. The non-ideal contact for the switches at ON state may explain why the measured resonant frequency is higher than the simulated one as shown in the figure.
Meanwhile, the finite height of the MEMS switches during the OFF state introduces additional coupling between the two patches, lowering the resonant frequency further. The measured radiation patterns are shown in Fig. 4. Fig.4a shows radiation pattern when the switches are in the ON state, while the radiation pattern with switches in OFF states is shown in Fig. 4b, respectively. In both cases, the radiation pattern is Right Hand Circularly Polarized with axial ratio less than 3 dB in the main direction (perpendicular to the patch).
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(b)
Fig. 4 The radiation pattern in XZ plane at (a) 6.69GHz (b) 7.06 GHz
IV. Conclusion A novel microstrip antenna capable of changing frequency operation using RF MEMS switches is presented in this paper. The antenna can be used in applications requiring frequency diversity. Thanks to the high Q of the MEMS switches, and to the fact that the switches are monolithically integrated, the antenna performance is not degraded.
Acknowledgement The authors would like to thank Mark Beckman, Jianyuan Qian and Alfred Grau Besoli for useful discussions and the staff of the UCI’s Integrated Nanosystem Research Facility for their support. This work is currently supported by a grant from the Air Force Edwards, AFB (contract no FA9300-04-P-0042).
References [1] Z.J. Yao, S. Chen, S. Eshelman, D. Denniston and C. Goldsmith, “Micromachined low-loss microwave switches,” IEEE J. of Microelectromechanical Systems, vol. 8, no. 2, pp. 129-134, June 1999. [2] C. H. Chang, J. Y. Qian, B. A. Cetiner, F. De Flaviis, M. Bachman, and G.P. Li, "Low Cost RF MEMS Switches Fabricated on Microwave Laminate Printed Circuit Boards," Electronic Device Letters, 2003 [3] F. Yang and Y. Rahmat-Samii, “Patch antenna with switchable slot (PASS): Dual frequency operation,” Microwave Opt. Technol. Lett., vol. 31, no. 3, pp. 165–168, Nov. 2001.
