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Microelectromechanical Systems for Spacecraft Communications 161

S in a b ∆f/2

S out d c ∆f/2
quadrature
hybrid

FIGURE 8.7 A schematic illustration of a reflection phase shifter.

in development than the switched-line and loaded-line types, but such phase
shifters have been demonstrated for 50 to 70 GHz. 56,57 Malczewski et al. 58 have
also demonstrated reflection-based X-band phase shifters based on Lange couplers.

8.4 OTHER RF MEMS DEVICES
MEMS technology is also used to create mechanical filters, variable capacitors
and inductors, all of which can be used in microwave and RF filter circuits.
Micromachining provides distinct advantages for all three types of components.
Microelectromechanical filters based on coupled microresonators have been dem-
59,60
onstrated for frequencies in the range of tens of kHz to tens of MHz. MEMS
filters are much smaller than SAW-based and crystal resonators, and are also more
easily integrated with other microwave systems.
In monolithic microwave integrated circuits (MMICs), inductors are implemen-
ted as planar spirals. Such devices require relatively large area and also suffer
from parasitic capacitances. Micromachining can be used to lift the structure off
the substrate in order to reduce parasitic capacitances, as well as to increase the
coupling surfaces in order to reduce overall dimensions. 61,62 Micromachining has
also been used to create tunable capacitors. 63,64 Such capacitors usually involve a
capacitor with a movable electrode that is positioned by electrostatic or electrother-
mal forces to achieve the desired capacitance level.

8.5 RF MEMS IN ANTENNA DESIGNS
8.5.1 ELECTRICALLY STEERED ANTENNAS

Being able to switch different antenna sections with a given phase shift has two
major applications in antenna design. In phased array antennas, multiple smaller
antennas are connected in a way such that the transmission from each smaller
antenna is phase-shifted from the others to take advantage of constructive and
destructive interference, thus controlling the radiation pattern of the antenna.

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