MEMS-Switched Reconfigurable Antennas

Gabriel M. Rebeiz
University of Michigan, Ann Arbor, MI

Introduction:

Reconfigurable multi -band phased-array antennas are receiving a lot of attention
lately due to the emergence of RF MEMS (micro-electro-mechanical systems)
switches [1-6]. A MEMS-switched reconfigurable multi-band antenna is one that
can be dynamically reconfigured within a few microseconds to serve different
applications at drastically different frequency bands, such as communications at
L-band (1-2 GHz) and synthetic aperture radar (SAR) at X -band (8-12.5 GHz).
The Air Force also uses both ground- and airborne- moving target indication
(GMTI/AMTI) at these frequencies in order to detect moving targets such as
vehicles on the ground and low observables in the air.

The RF MEMS switch is attractive because it shows of achieving excellent
switching characteristics [2] over an extremely wide band (DC-40 GHz and
upwards). These switches can also be used to develop wideband phase shifters
[3]. Although there is currently a tremendous amount of research in RF MEMS
devices, reliability and packaging of the switches continue to be problematic. The
switches are also limited in their power handling capability.

In this work, we do not focus on the development of the MEMS switches
themselves. Rather, we want to use them as control elements in a reconfigurable
antenna. Since the actual MEMS switches were not available to us at the time of
this work, we simulated the MEMS switches using ideal OPEN and CLOSED
circuits. There is actually a great deal of work to develop optimal radiating
elements and feed structures in order to achieve the desired multi -band
performance.

Reconfigurable Patch Module (RPM):

We investigated the design and fabrication of a dual L/X-band reconfigurable
antenna. Microstrip antenna elements were chosen due to their inherent low-
profile, which is suitable for satellite and UAV applications. We used RT/duroid
5880 material with a dielectric constant of 2.2 and a loss tangent of 0.0009 at 10
GHz. Two material thicknesses were investigated: 0.062" and 0.125". The
material thickness must be chosen carefully, since it controls both the bandwidth
and array scanning performance. The thicker the material, the more bandwidth,
particularly at the low frequency end. However, if the substrate becomes too
thick, surface waves are generated and array scanning performance and efficiency
is lost.

Figure 1 shows a picture of the 3x3 RPM fabricated on 0.125" duroid. The
patches are 0.370" square and separated by 0.590" on center. Interconnecting tabs
are 0.050" wide and 0.085" long. The "reconfigurable" antenna was actually
fabricated as two separated prototypes (OPEN and CLOSED configurations) for
testing in the laboratory.

Figure 2 shows the measured return loss for the 3x3 RPM in Figure 1 for both the
CLOSED (L-band) and OPEN (S-band) configurations. We were able to achieve
1.2% impedance bandwidth for the L-band configuration and greater than 7%
bandwidth at X-band. This bandwidth was limited primarily by the substrate
thickness. Figures 3 and 4 show the measured radiation patterns at both L -band
and X -band. Computer simulation results for both the return loss and radiation
patterns agree well with the measurements, and will be shown in the talk.

Acknowledgement:

This work was supported by the US Air Force Research Laboratory
(AFRL/SNHA) under contract F19628-99-C-0056.