Page 205 - Satellite Communications, Fourth Edition

Antennas 185

highly conducting (ON) state and a highly resistive (OFF) state. MESFETs
utilize gallium arsenide (GaAs) substrates, and can be constructed
along with the line elements as an integrated circuit, forming what is
known as a monolithic microwave integrated circuit (MMIC). (MMICs
may also contain other active circuits such as amplifiers and oscillators).
Figure 6.36 shows four MESFETS integrated into a switched line phase
shifter.
The MEM switch is a small ON/OFF type switch that is actuated by
electrostatic forces. In one form, a cantilever gold beam is suspended over
a control electrode, these two elements forming an air-spaced capacitor.
The dimensions of the beam are typically in the range of a few hundred
microns (1 micron, abbreviated 1 m is 10 6 meters) with an air gap of
a few microns.
The RF input is connected to one end of the beam, which makes con-
tact with an output electrode when the beam is pulled down. The pull
down action occurs as a result of the electrostatic force arising when a
direct voltage is applied between the control electrode and the beam. The
voltage is in the order of 75 V, but little current is drawn. The power
required to activate the switch depends on the number of cycles per
second and the capacitance. In one example (see Reid, 2005), a voltage
of 75 V, capacitance of 0.5 pF and a switching frequency of 10 kHz
resulted in a power requirement of 14 W.
A MEM switch can also be constructed where the beam, fixed at both
ends, forms an air bridge across the control electrode (Brown, 1998). The
top surface of the control electrode has a thin dielectric coating. The elec-
trostatic force deflects the beam causing it to clamp down on the dielec-
tric coating. The capacitance formed by the beam, dielectric coating,

400-mm FETs
Gate pad

In Out

Figure 6.36 A MESFET switched line phase shifter.
(From http://parts.jpl.nasa.gov/mmic/3-IX.PDF)