Antenna Electronically Steered Using MEMS Phase Shifters

April 1, 2008

Army Research Laboratory, Adelphi, Maryland

An experimental phased-array microwave antenna assembly includes an array of eight patch antenna elements connected to Microelectromechanical System (MEMS) phase shifters, by means of which the directional radiation pattern of the antenna can be controlled electronically. The antenna and the MEMS-based phase shifters were designed for a nominal operating frequency of 17 GHz. In addition, some 35-GHz MEMS phase shifters were designed, built, and tested. This work is part of a continuing effort to develop relatively inexpensive electronically steerable antennas.

The phase shifters are based on the concept of delay lines having various lengths selectable by use of electrostatically actuated MEMS switches. A reflection-delay-line phase-shifter architecture was chosen to make best use of the performance of the MEMS switches and to help minimize the overall dimensions of each phase shifter.

The phase shifters were fabricated on high-resistivity silicon substrates. Both two- and three-bit phase shifters were designed and built. Figure 1 shows one of the 17- GHz two-bit phase shifters as it looks before it is packaged. Each phase shifter includes a coplanar-waveguide (CPW) Lange directional coupler and two delay lines with shunt switches at the locations appropriate for switching the desired increments of phase. The switches are actuated in left-right pairs to obtain in ascending sequence phase increments of 0°, 90°, 180°, and 270°, respectively. Assuming that the delays along the two delay lines are equal, the microwave signals reflected from the closed switches add in phase with each other at the output terminal of the directional coupler with a phase shift equal to twice the delay associated with length of one delay line. The transmission line shown as a vertical line at the top, which is terminated in a CPW short circuit, is used in the 270° switch state.

In tests, the MEMS switches alone exhibited insertion loss <0.3 dB and isolation greater than 20 dB from DC to 40 GHz. The MEMS phase shifters were enclosed in packages that include external connectors for DC switching control signals and for the radio-frequency (RF) signals to be phase-shifted. The 17-dB MEMS phase shifters were tested and found to function with average insertion loss ≈2.5 dB and return loss >20 dB. The average insertion loss of the 35-dB phase shifters was found to be ≈3.3 dB.

Eight packaged two-bit, 17-GHz MEMS phase shifters, tested and selected to be functional and nearly identical in operational characteristics, were mounted on a dielectric board that holds the array of antenna patches and the associated feed array of microstrip transmission lines (see Figure 2). The phase shifters were connected to a switch panel to provide DC switching signals to the MEMS switches. In a test in which the array was operated in a receiving mode, the array functioned well, exhibiting steering of the radiation beam to five available positions: 0°, +25°, -25°, +55°, and -55°.

This work was done by Ronald G. Polcawich, Daniel Judy, Jeffrey S. Pulskamp, and Steve Weiss of the Army Research Laboratory.

ARL-0033

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