A research and development effort now underway is directed toward satisfying requirements for a new type of relatively inexpensive, lightweight, microwave antenna array and associated circuitry packaged in a thin, flexible sheet that can readily be mounted on a curved or flat rigid or semi-rigid surface. A representative package of this type consists of microwave antenna circuitry embedded in and/or on a multilayer liquid- crystal polymer (LCP) substrate. The circuitry typically includes an array of printed metal microstrip patch antenna elements and their feedlines on one or more of the LCP layer(s). The circuitry can also include such components as electrostatically actuated microelectromechanical systems (MEMS) switches for connecting and disconnecting antenna elements and feedlines. In addition, the circuitry can include switchable phase shifters described below.
LCPs were chosen over other flexible substrate materials because they have properties that are especially attractive for high-performance microwave applications. These properties include low permittivity, low loss tangent, low water-absorption coefficient, and low cost. By means of heat treatments, their coefficients of thermal expansion can be tailored to make them more amenable to integration into packages that include other materials. The nature of the flexibility of LCPs is such that large LCP sheets containing antenna arrays can be rolled up, then later easily unrolled and deployed.
Figure 1 depicts a prototype three- LCP-layer package containing two four-element, dual-polarization microstrip patch arrays: one for a frequency of 14 GHz, the other for a frequency of 35 GHz. The 35-GHz patches are embedded on top surface of the middle [15-mil (≈0.13-mm)-thick] LCP layer; the 14- GHz patches are placed on the top surface of the upper [9-mil (≈0.23-mm)- thick] LCP layer. The particular choice of LCP layer thicknesses was made on the basis of extensive analysis of the effects of the thicknesses on cross-polarization levels, bandwidth, and efficiency at each frequency.
The diagonal orientation of the microstrip patches in Figure 1 is not inherent in the LCP implementation: instead, it is part of an example design for a typical intended application in radar measurement of precipitation, in which there would be a requirement that both the 14- and the 35-GHz arrays exhibit similar orthogonal-polarization characteristics, including high degrees of polarization purity. The diagonal orientation helps in realizing a symmetrical feed network for both polarizations with similar impedance characteristics and radiation patterns. RF MEMS switches would be included in a production model but are not included in the prototype: Instead, to simplify computational simulation and testing, switching of polarizations is represented by the presence of hard-wired open and short circuits at switch locations.
Figure 2 is a plan view of a switchable phase shifter — in this case, one that can be switched between two different phase shifts. The device includes electrostatically actuated RF MEMS switches that are used to make and break connections to eight microstrip delay lines having different lengths (e.g., 1 wavelength versus 3/4 wavelength). Necessarily omitting details for the sake of brevity, each MEMS switch includes a microscopic flexible electrically conductive member that, through application of a suitably large DC bias voltage, can be pulled into proximity with microstrip conductors on opposite sides of the gap. The flexible member is covered to prevent direct electrical contact with the microstrip conductors, but the effect of the proximity is such as to enable substantial capacitive coupling of the microwave signal across the gap. The measured loss of the four-bit packaged phase shifter is only 0.24 dB per bit with a phase error less than 4° at 14 GHz. At the time of this reporting, this is the first package flexible organic RF MEMS multibit phase shifter ever documented.
This work was done by Dane Thompson, Ramanan Bairavasubramanian, Guoan Wang, Nickolas D. Kingsley, Ioannis Papapolymerou, Emmanouil M. Tenteris, Gerald DeJean, and RongLin Li of Georgia Institute of Technology for Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Semiconductors & ICs category.
Inquiries concerning rights for the commercial use of this invention should be addressed to
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Refer to LEW- 17980-1.