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Microelectromechanical Systems for Spacecraft Communications 151

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satellitebus. Thesesystemsrequiremanyswitchesforsignalroutingandredundancy.
In the past, they have been implemented by large electromechanical switches or by
power-hungry solid-state switches. MEMS offer a lightweight, low-power alternative
to such switches.
MEMS switches also enable ‘‘active aperture phase array antennas.’’ These
systems consist of groups of antennas phase-shifted from each other to take
advantage of constructive and destructive interference in order to achieve high
directionality. If the phase separations can be actively controlled, then such systems
allow for electronically steered, radiated, and received beams, which have greater
agility and will not interfere with the satellite’s position. An adaptive phase array
can also be used to combat a jamming signal by pointing a null toward the
interfering signal source. A key component in a phase array is the phase shifting
element that is associated with each individual antenna in the array. Such phase
shifters have been implemented with solid-state components. However, they are
power-hungry, and have large insertion losses and problems with linearity. In
contrast, phase shifters implemented with microelectromechanical switches have
lower insertion loss and require less power, especially in the range of 8 to 120 GHz. 7
This makes MEMS an enabling technology for lightweight, low-power, electronic-
ally steerable antennas for small satellites. Rebeiz has written a thorough review of
RF MEMS, which is recommended to anyone who has interest in the field. 8
The first microfabricated relay was designed by Kurt Petersen in the late
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1970s. He used bulk micromachining techniques to create a switch with an
actuation voltage of 70 V, 5 V of DC resistance in the closed state and a 10-ms
switching time. The most active groups currently in the field of microwave
switches are the Rockwell Science Center (RSC), Raytheon (begun at Texas
10–12 13–16
Instruments), Hughes Research Laboratories (HRL), the University of
17–19 12,20
Michigan, Cronos (which is also associated with the Raytheon effort),
21 22,23
OMRON corporation, and UCLA. RSC has flown its RF switches in space
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on a picosatellite.
8.2.1 MEMS SWITCH DESIGN AND FABRICATION
The basic MEMS switch is a suspended mechanical structure that moves when
actuated to vary the electrical impedance between two electrodes. To clarify
the language we will refer to two conducting plates of the switch that receive the
control voltage as ‘‘electrodes’’; one is stationary and the other is the moving
electrode. Then there is the ‘‘conducting bar’’ through which the signal will travel
(either to complete the path or to ground, depending on switch configuration). The
contacts are the points at which the conducting bar connects to the transmission
line. MEM switches can be classified by configuration, contacting mode, actuation
mechanism, and switch geometry.

8.2.1.1 Switch Configuration
As is illustrated in Figure 8.1, there are two general configurations for switches: series
and shunt. In a series configuration, the conducting bar sits along the signal path. The
on state is when the conducting bar is brought down, completing the path. In the shunt

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