Reconfigurable MEMS Antennas

Nakul Haridas1, Ahmet T. Erdogan

1, Tughrul Arslan

1, Anthony J. Walton

1,2, Stewart Smith

1,2, Tom

Stevenson1,2

, Camelia Dunare1,2

, Alan Gundlach1,2

, Jon Terry1,2

, Petros Argyrakis1,2

, Kevin

Tierney1,2

, Alan Ross1,2

and Tony O’Hara3

1School of Engineering and Electronics, University of Edinburgh, Edinburgh, UK

2Scottish Microelectronics Centre, Edinburgh, UK

3MEMSSTAR, Point 35 Microstructures, Edinburgh, UK

N.Haridas@ed.ac.uk

Abstract This paper reviews the work carried out in the field of

reconfigurable antennas, and in specific the

reconfigurable MEMS (Micro-Electro-Mechanical

Systems) antennas. The application of MEMS to antennas

is studied and compared with the various

implementations such as pattern reconfigurable MEMS

antennas, mechanically actuated MEMS antennas,

capacitive MEMS antennas and MEMS phased array

antennas, as reported by research groups in the field.

Finally a design is described, driven by the objectives of

low power, high efficiency, linear operation and real time

frequency and space diversity reconfiguration.

1. Introduction

A reconfigurable antenna is one which alters its

radiation, polarization and frequency characteristics by

morphing its physical structure. Reconfigurable antennas

with the ability to radiate more than one pattern at

different frequencies are necessary in radar and modern

communication systems. Many reconfigurable antennas

concentrate on changing operating frequency while

maintaining their radiation characteristics. However, the

ability to change the radiation patterns while maintaining

operating frequency could greatly enhance system

performance. Manipulation of an antenna's radiation

pattern can be used to avoid noise sources or intentional

jamming, improve security by directing signals only

toward intended users, serve as a switched diversity

system, and expand the beam steering capabilities of large

phased arrays.

To date reconfigurable antennas have been realised

using three major technologies: MEMS antennas, optical

antennas, and holographic antennas; the later two being

similar in nature by employing light or some bias field to

create the desired shape of the radiating structure.

MEMS antennas have gained popularity due to their

higher linearity and as a result lower signal distortion

when compared to semiconductor devices. MEMS

switches are also very promising devices as they will be

able to replace a number of solid state circuits enabling

better performance in terms of loss, isolation, linearity,

power consumption, and compatibility with integrated

circuits.

MEMS devices are ideal for reconfigurable networks,

antennas and subsystems. They have very low insertion

loss and high Q up to frequencies of 120 GHz. They can

be integrated on low dielectric constant substrates which

is important for high performance tunable filters, high

efficiency antennas, and low loss matching networks.

MEMS devices offer very low loss switching and can be

controlled using 10 to 120K Ω resistive lines. This means

that the bias network for RF MEMS switches will not

interfere and degrade antenna radiation patterns. As the

bias network does not consume any power this is an

important advantage for large antenna arrays.

Two issues remain attached to MEMS switches that

one should consider are electrostatic discharge sensitivity

and hot switching, which happened due to high bias

voltage or thermal effects which can permanently damage

the switch. Power handling capabilities of RF MEMS

switches are limits due to self actuation and stiction in the

down state due to high incident RF power typically in the

range of 10’s of mW for ohmic switches and up to 1W for

capacitive switches [21].

MEMS are employed in many ways to achieve

reconfigurability. The first is to change the shape of the

effective radiating structure to alter the pattern or the

frequency of operation. The second method employs

MEMS to mechanically actuate the antenna, and change

the orientation of the antenna with respect to the substrate

or another radiating structure. The third method employs

MEMS capacitive switches to modify the impedance of

the antenna, which changes the resonant frequency of the

radiating antenna. The fourth employs MEMS phase

shifters.

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2. Pattern Reconfigurable Antennas

MEMS switches can be employed to connect different

elements that make up the antenna structure, one such

example is the work carried out by Yang et. al. [1]. They

demonstrate the use of MEMS switches in patch antennas

designed by 3rd order Hilbert curves designed to work at

10GHz as shown in figure 1(a). This antenna is

connected to slots via MEMS switches and on actuating

these switches the resonant frequency changes to

12.5GHz (See figure 1(b)).

Figure 1. (a) Layout of the original patch antenna, (b)

Antenna being reconfigured by use of MEMS switches

By having two slots to the side of the main patch

antenna results in a number of different configurations in

which the antenna can be reconfigured. These include

one with the slots connected to the antenna, one with

individual slots connected to the antenna, and one

configuration where the slots are disconnected from the

main patch. Results indicate that in each configuration a

variation in both frequency and beam directivity is

observed, which demonstrates the reconfigurable nature

of the design.

Figure 2. Implementation of a single arm square

antenna by Bernard et. al.

Similar work reported by Bernard et. al. [2-5] consists

of a single turn square spiral antenna working at a

nominal frequency of 3.7 GHz. The antenna is provided

with a set of commercially available MEMS SPST

switches, which when employed either redirect the

radiation pattern or changes the frequency of the antenna.

The antenna is a single turn square microstrip spiral

fabricated on a Duroid 5880 substrate as shown in figure

2. The outer end of the spiral is shorted to ground whilst

the inner one forms the feed to the SMA probe.

When used in the end fire (axial) mode, switch 1 is

closed while switch 2 is open. This creates a single turn

spiral antenna working at 3.7 GHz. With switch 1 open

and switch 2 closed it reconfigures the antenna to the

broadside configuration working at 6 GHz. Hence it

successfully demonstrates the two features of a

reconfigurable antenna using frequency and pattern

reconfigurability.

Figure 3. Implementation of the rectangular spiral

antenna by De Flaviis et. al.

De Flaviis et. al. [6] extended this concept by

employing a rectangular spiral antenna with a set of

MEMS switches which were monolithically integrated

and packaged on the same substrate. The antenna is

printed on a PCB and fed through a via hole, which is

placed at the centre of the antenna. This creates a right

handed circularly polarized (RHCP) spiral, see Figure 3.

The antenna is made up of multiple lines which are

connected via the MEMS switches, and by activating

these switches the overall length of the antenna is

changed which modifies its radiation pattern. This is

claimed by the authors to be the first implementation of a

truly reconfigurable antenna, by fabricating both MEMS

and the printed antenna on the same substrate.

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Figure 4. Layout of the reconfigurable antenna with

MEMS switches

As shown in Figure 4, the rectangular spiral antenna is

fed through a coaxial feed. The spiral consists of five

sections connected with four MEMS switches (S1-S4).

The spiral arm length is increased following the right

hand circular polarization for the radiation field.

The operating frequency of the antenna is chosen to be

6 GHz, and switching on the different sections of the

radiating structure creates a tilted beam. The maximum

beam direction in the azimuth angle tilts from 18˚ to 104˚

and a maximum tilt angle of 30˚ in the elevation plane as

the length of the antenna changes. The gain varies

between 4-6 dBi depending on the length of the antenna.

An alternative approach was designed and fabricated

using a microstrip line. This converted the original RHCP

to a left handed circularly polarized (LHCP) antenna [7,

8], as shown in Figure 5.

Figure 5. Microstrip fed antenna with MEMS

switches

This design employs two MEMS switches to

demonstrate three different beam directions by changing

the effective length of the antenna. This antenna is

designed to work at 11 GHz and has three different spiral

arm lengths. In this design the maximum beam directions

is from 34˚ to 42˚ in its elevation angle and -29˚ to 14˚ in

the azimuth angle, whilst the gain varies between 1.1 to

2.5 dBi when antenna is reconfigured between the three

different lengths.

Another very good example of a reconfigurable

antenna is the pixel patch antenna designed again by De

Flaviis et. al. [9]. They have proposed to use a pixel

antenna concept which uses an array of individual

antenna elements that can be connected via MEMS

switches. A variety of patterns can be created by actuating

the MEMS switches.

Figure 6. Implementation of pixel patch antenna

Figure 7. Pixel patch reconfigured for lower

frequency of 4.1 GHz

Frequency reconfigurability can be achieved by simply

changing the size of the antenna and figures 6 and 7

illustrate this method. By selecting 25 pixels they

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achieved an upper operating frequency of 6.4 GHz,

whereas for the lower frequency of 4.1 GHz when all the

64 pixels were selected [9].

Linear polarization can be achieved by selecting

antennas in a particular plane to act as a single radiating

structure. Figures 8 and 9 illustrate how polarisation can

be achieved selecting the pixels in either X or Y direction

only.

Figure 8. Reconfigurable pixel-patch antenna

schematics with linear X Polarisation

Figure 9. Reconfigurable pixel-patch antenna

schematics with linear Y polarization

Circular polarization is obtained by introducing

internal slots in the antenna geometry. By deactivating

some patches creates parasitic elements in the pattern and

accordingly RHCP or LHCP radiation is achieved. Figure

10-11 illustrates how we can obtain the circular

polarizations.

Careful layout of pixel elements and an on-/off-state

algorithm allow the antenna to reconfigure its electrical

size and shape, achieving both frequency and polarization

diversity. The resulting variation in antenna geometry

produces the correct dimension for each desired

frequency specification. The off-state of each pixel acts to

produce a parasitic-loaded slot in the structure, which can

then be combined with others in various geometries to

produce the desired polarization.

Figure 10. Pixel patch reconfigured for RHCP

radiation

Figure 11. Pixel patch reconfigured for LHCP

radiation

3. Mechanically actuated MEMS Antenna

This work demonstrates the ability to mechanically

actuate the antenna with electrostatic force [17]. The

antenna is suspended on a flexible spring. A bias voltage

is applied between patch and antenna, creating an

electrostatic force which attracts the patch towards the

ground plate, as shown in figure 12 and 13. By varying

the height of the antenna with respect to the ground plane

the antenna operating frequency also changes. Its

operating mode is equivalent to changing the relatively

permittivity of antenna substrate.

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Results show that, with the pull down voltage set to

76V for the patch, as the bias voltage is altered the

resonant frequency of the antenna starts changing from

46.3 GHz to 38.8 GHz depending on the height of the

patch.

Figure 12. Layout of micromechanical patch antenna

without bias voltage

Figure 13. Layout of micromechanical patch antenna

with bias voltage

4. Capacitive MEMS Antennas

The work reported in [18] demonstrates a

reconfigurable microstrip patch antenna that is

monolithically integrated with RF MEMS capacitors for

tuning the resonant frequency of the antenna. The

structure consists of a patch antenna loaded with a

coplanar waveguide (CPW) section attached to the

antenna via microstrip to CPW transition as shown in

figure 14. The reconfigurability in the resonant frequency

of the antenna is provided with the aid of the MEMS

bridges acting as a variable capacitor placed on the CPW

stub.

Varying the actuation voltage between the centre

conductor and MEMS bridge metal, alters? the height of

the bridges on the stub and hence modifies the loading

capacitance. Thus, the CPW stub with bridges provides a

variable load to the connected radiating edge, which

results in change in the resonant frequency.

The antenna is fabricated on a Pyrex 7740 glass

substrate, and the MEMS bridge is suspended 1.5μm

above the CPW. The resonant frequency of the antenna

shifts down from 16.05 GHz to 15.75 GHz as the

actuation voltage is increased from 0 to 11.9 V as the

height of the capacitive gap changes from 1.5 to 1.4 µm.

Figure 14. Implementation of capacitive MEMS

antenna [18]

5. MEMS Phased Array Antennas

Phased array antennas find a lot of use in satellite

communications both in terrestrial and space applications.

Phased array antennas are being actively used for satellite

tracking and on naval radars for surface detection and

aircraft tracking.. The National Severe Storms Laboratory

(NSSL) [11] applies phased array antenna for the

investigation of all aspects of severe weather

phenomenon like thunderstorms and tornadoes. Military

and commercial satellites utilise phased array antennas for

beam shaping and steering, making efficient use of the

frequency spectrum and increasing space diversity of the

antenna.

Figure 15. Traditional phased array antennas

MEMS can also be employed in phased array antennas

as suggested in reference [10]. Traditional phased array

antennas, see figure 15, are expensive because each

antenna has its own transmit/receive Monolithic

Microwave Integrated Circuits (MMIC) module including

individual power amplifiers. In addition, the design of the

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RF feed network and packaging system is complex at

higher frequencies.

Figure 16. Improved phased array antenna design for

low power and low cost

A simpler solution, see figure 16, is to provide a single

power amplifier and employ MEMS phase shifters which

have very low insertion loss measured 0.1 dB at 25GHz

and 0.18 at 40 GHz, some designs had insertion loss of

0.4 dB at 18 GHz and 0.7 dB at 34 GHz [19] compared to

traditional GaAs MESFET switches with a 5.8 dB loss at

4GHz [20]. A practical implementation of such a phased

array uses switched line MEMS phase shifter [12]. The

switched line is a digital phase shifter and uses the delay

line technique, and will provide a discrete set of phase

shifts by employing MEMS switches to select the delay

path along the transmission line.

The relative phase shift is calculated by the delay the

selected path will create, this kind of implementation is

relatively easy to operate very much like a digital system

by selecting a combination of these MEMS switches to

provide the desired phase shift. However this works only

for a fixed frequency and does not provide frequency

reconfigurability in the antenna.

Figure 17. Implementation of phased array antenna

with switch line phase shifters

For example, a 3-bit phase shifter is based on

45/90/180˚ set of delay networks and can provide phase

shifts of 0, 45, 90, 135,180, 225, 270 and 315˚, depending

on the combination of the 3 bits used [13].

The implementation uses a 1 bit phase shifter of 30˚.

With two sets of antennas connecting the individual phase

shifter there are 3 possible combinations for beam

steering. When both are in phase the beam is not steered,

when left shifter is 0˚ and right is 30˚ the beam will steer

left, and when the left is 30˚ and right is 0˚ the beam will

steer right, see Figure 17. Hence this provides a beam

steering capability.

The work carried out at the University of Edinburgh

employs an analog phase shifter which provides

continuous variable phase shift from 0-306˚ [15, 16]

figure 21. This is achieved by employing a distributed

MEMS transmission line (DMTL) technique, which

offers an alternative approach to the standard reflect-line

or switched-line designs. These techniques have been

used as a solution to obtain very wide band circuits. The

concept is based on periodically loading a t-line with

MEMS bridges (i.e. capacitance). A bias voltage is then

applied between the MEMS bridge and coplanar

waveguide (CPW) centre conductor, which varies the

height of the bridge. This alters the distributed MEMS

capacitance, resulting in a change in the loaded

transmission line impedance and phase velocity, which in

turn causes phase shift. Therefore, a structure with several

MEMS bridges can act as a phase shifter when a bias

voltage less than the pull-down voltage is applied [14,

15]. This results in an analog control of the transmission

line phase velocity and, therefore, in a true-time delay

(TTD) phase shifter. Another advantage of this design is

that the phase shift is dependent on the frequency and by

the varying the bias voltage, one can easily calibrate the

phase shift for the desired frequency.

Figure 18 SEM image of a MEMS bridge

As traditional MEMS these phase shifters can be

fabricated on the same substrate as the antenna directly on

the transmission line and depending on the number of

elements in the array, the beam steering can be made

finer.

The phase shifters and antennas have been fabricated

on silicon wafers and pictures of prototype systems are

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shown in figures 18-19. Two different types of bridges

were fabricated, serpentine and planar; in order to

characterise the operation of the phase shifter. The phase

shifter, which will fulfill the requirements for low loss,

high isolation, low power, robustness in design and ease

of control, will be implemented into a phased array with a

multi-frequency antenna [16] to provide true

reconfiguration in directivity, frequency, beam shaping

and steering.

Figure 19 Reconfigurable Antenna Array

implementation

6. Summary

This paper has briefly described various

implementations of MEMS reconfigurable antennas.

MEMS provide us unique advantages of low power, low

insertion loss, higher linearity, lower signal distortion and

ease of integration; compared to MMIC and solid state

circuits. MEMS devices have been effectively used to

create such a wide variety of reconfigurable antennas

serving as either as switches, capacitors or phase shifters.

They can be applied in a variety of applications to

antenna technology to have a truly integrated solution for

a reconfigurable antenna.

7. Acknowledgement

The authors would like to acknowledge financial

support from EPSRC (EP/C546318/1) and the Edinburgh

Research Partnership in Engineering and Mathematics

(Institute of Integrated Systems) for financial support.

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