This content has been downloaded from IOPscience. Please scroll down to see the full text.

Download details:

IP Address: 134.151.40.2

This content was downloaded on 25/08/2014 at 04:24

Please note that terms and conditions apply.

A MEMS-based tunable coplanar patch antenna fabricated using PCB processing techniques

View the table of contents for this issue, or go to the journal homepage for more

2007 J. Micromech. Microeng. 17 812

(http://iopscience.iop.org/0960-1317/17/4/019)

Home Search Collections Journals About Contact us My IOPscience

IOP PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERING

J. Micromech. Microeng. 17 (2007) 812–819 doi:10.1088/0960-1317/17/4/019

A MEMS-based tunable coplanar patchantenna fabricated using PCB processingtechniquesM Maddela1, R Ramadoss1 and R Lempkowski2

1 Department of Electrical and Computer Engineering, 200 Broun Hall, Auburn University,Auburn, AL 36849, USA2 Motorola, 1301 East Algonquin Road, Schaumburg, IL 60196, USA

E-mail: maddema@auburn.edu, ramadra@auburn.edu and bob.lempkowski@motorola.com

Received 7 January 2007, in final form 16 February 2007Published 16 March 2007Online at stacks.iop.org/JMM/17/812

AbstractIn this paper, a tunable coplanar rectangular patch antenna (CPA) designedusing a MEMS varactor is reported. The MEMS varactor is monolithicallyintegrated with the antenna on Duroid substrate using printed circuitprocessing techniques. Specifically, the MEMS varactor located at one ofthe radiating edges capacitively loads the CPA. The resonant frequency ofthe antenna is tuned electrostatically by applying a DC bias voltage betweenthe MEMS varactor and the actuation pad on the antenna. The movableMEMS varactor membrane deflects downward toward the actuation pad dueto an electrostatic force of attraction caused by the applied DC bias voltage.The deflection of the varactor membrane decreases the air gap, therebyincreasing the loading capacitance. The increase in the loading capacitanceresults in a downward shift in the resonant frequency of the CPA. The CPAis center fed at the second radiating edge using a 50 � CPW feed line. TheCPA operates in the frequency range from 5.185 to 5.545 GHzcorresponding to the down and up states of the varactor. The tunablefrequency range is about 360 MHz and the return loss is better than 40 dB inthe entire tuning range. In this tuning range, the required DC voltage is inthe range of 0–116 V.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

In recent years, there has been a considerable interest in thedevelopment of tunable antennas because of the increasingnumber of global wireless standards in close proximity to oneanother. A single tunable antenna would eliminate the needfor multiple antennas operating in various frequency bands.Several tunable printed antennas using varactor diode(s) werereported in [1–7]. The operating frequency of the antenna wasaltered by varying the DC bias applied to the varactor diode(s).With the advent of MEMS technology, there have beenconsiderable research efforts in the development of frequencytunable and reconfigurable (or switchable) antennas using RFMEMS devices [8–18]. In [10], a patch antenna operating at25 GHz was made tunable over a range of 1.6%, i.e., 400 MHz.

In [11], a tunable MEMS patch antenna using an electrostaticactuator with wrapped metallization was reported. The areaof the wrapped metallization was changed to obtain operatingfrequencies at 23.8 GHz and 12.4 GHz for 0 V and 150 V,respectively. In [12, 13], dual-band frequency reconfigurableplanar antennas integrated with RF MEMS switches werereported. In [14], a triple-band reconfigurable microstripantenna that employs two RF MEMS switches was reported.The antenna resonates at 1.8 GHz, 2.4 GHz and 1.9 GHz withreturn losses better than 10 dB when both the switches areOFF, only one switch is ON and both the switches are ON,respectively. In [15], a quadruple-band reconfigurable mini-nest patch antenna was developed using half u-slots integratedwith RF MEMS switches to achieve frequency bands such asGSM (0.9 GHz), GPS (1.57 GHz), DCS (1.8 GHz) and WLAN

0960-1317/07/040812+08$30.00 © 2007 IOP Publishing Ltd Printed in the UK 812

A MEMS-based tunable coplanar patch antenna fabricated using PCB processing techniques

(2.4 GHz and 5.2 GHz). The return losses were reported tobe in the range of 14–22 dB. RF MEMS switches were alsoused in conjunction with fractal antenna structures to obtaina reconfigurable array antenna structure. In [16], a multi-band frequency reconfigurable modified sierpinski gasket typefractal antenna was reported. Different configurations of thefractal antenna were achieved using RF MEMS switches toobtain operation in multiple frequency bands ranging from0.6 GHz to 1.95 GHz.

Recently, Printed Circuit Board (PCB) process compatibleMEMS-based frequency tunable and reconfigurable antennashave been reported in [17, 18]. The advantages of PCBcompatible MEMS technology include low cost, compatibilitywith organic laminate substrates, ease of integration withsurface mount components, suitability for batch fabricationin large panels and high volume manufacturing [19]. In [17],monolithic integration of RF MEMS switches with a diversityantenna on a PCB substrate was reported. In [18], a MEMS-based tunable circular microstrip patch antenna fabricated on aflexible Kapton polyimide film using printed circuit processingtechniques was reported by our group. A 6 mm diametercircular patch antenna was reported that was tunable from16.91 GHz at 0 V to 16.64 GHz at 165 V, for a tuning rangeof 270 MHz.

In this paper, we present a Coplanar Patch Antenna (CPA)made tunable using a MEMS varactor fabricated using printedcircuit processing techniques. CPAs have many advantagessuch as less radiation loss, less dispersion and uni-planarconfiguration. They also exhibit wider bandwidth and lowercross-polarization radiation as compared to microstrip patchantennas [1]. The concept of a coplanar antenna structure wasfirst introduced by Greiser [20]. The antenna consists of apatch surrounded by a closely spaced ground conductor, andis suitable for being fed using coplanar lines. Electromagnetic(EM) field simulations of the structure revealed that theantenna behaves more like a microstrip patch antenna thana loop slot antenna. Particularly, the resonant frequency ofthe antenna is primarily determined by the patch length (L) ofabout a half guide-wavelength instead of the total loop size.Electromagnetic simulations also show that the electric fielddistributions around the slots of the CPA resemble that of themicrostrip patch edges. The variation of the input impedanceof the CPA with the length of the patch exhibits a behaviorsimilar to that of a microstrip patch. This characteristic allowsimpedance matching of the CPA to the feed line by onlyadjusting the width (W) of the patch. In the following sections,design, simulation, fabrication and experimental results forthe PCB MEMS-based tunable coplanar patch antenna arediscussed.

2. Tunable antenna design and simulation

2.1. Antenna configuration

The Coplanar Patch Antenna (CPA) is based on the conceptof a half-wavelength, open-ended coplanar waveguide (CPW)resonator. The layout of the CPA fed at the radiating edge isshown in figure 1. It consists of an open-ended CPW resonatorsection (along the y-direction) fed by a CPW feed line. Thecharacteristic parameters of the CPA are length L, width W and

Figure 1. Coplanar patch antenna with CPW feed at the bottomradiating edge and a MEMS varactor mounted at the top radiatingedge.

slot S. The field distributions in the slots of the half-wavelengthresonator section (along the y-direction) are out-of-phase andhence these slots behave as non-radiating edges (NRE). On theother hand, the field distributions in the slots at the open ends(along the x-direction) are in-phase and therefore these slotsare referred as radiating edges (RE). The CPA is fed by a 50 �

CPW line connected to one of the radiating edges of the CPAas shown in figure 1. The antenna design parameters, such aslength L = 21 mm, width W = 30 mm and slot S = 2.2 mm,were taken from [7]. The CPW feed line width g = 0.1 mmand slot gap s = 1.642 mm are obtained by using the LineCalctool (a transmission line calculator) in Agilent ADS [21]. Inthe following sections, MEMS varactor design and frequencytuning characteristics of the antenna are discussed.

2.2. MEMS varactor

The Tunable Coplanar Patch Antenna (TCPA) has beendemonstrated in [7] by using a pin diode at one of theradiating edges. In this work, a MEMS-based TCPA hasbeen designed by integrating a MEMS varactor at one of theradiating edges as shown in figure 1. Figure 2(a) illustratesthe detailed cross-sectional view of the MEMS varactor andfigure 2(b) shows the top view of the MEMS varactor onthe antenna. An actuation pad is included in the antennametallization layer to facilitate electrostatic actuation of theMEMS varactor. An air gap is present between the MEMSvaractor membrane and the actuation pad on the antenna. TheMEMS varactor is controlled by a DC bias voltage. As the DCbias is increased, the movable MEMS varactor membraneis pulled toward the actuation pad due to electrostatic forceof attraction caused by the applied DC bias voltage. Thedeflection of the varactor membrane decreases the air gap,thereby increasing the capacitive load at the radiating edge ofthe CPA. The increase in the loading capacitance increases theeffective length of the CPA and hence results in a downwardshift in the resonant frequency of the CPA.

813

M Maddela and R Ramadoss

Air gap (~ 40 µm)

V Copper (9 µm)

Kapton film (~ 50 µm)

Spacer film (~ 50 µm)

BCB (7 µm)Top electrode (3 µm)

Anchor

Pad 1 Pad 2 Substrate (~ 760 µm)(a)

(b)

Figure 2. MEMS varactor configuration on the antenna: (a) schematic view and (b) top view (figure not to scale).

The close-up view of the MEMS varactor configurationon the antenna is shown in figure 2(b). The dimensions of thesuspension beams are LA = 1.4 mm, LB = 0.75 mm, LC =1.4 mm, LD = 1 mm and the flexure width is 0.2 mm. Fromfigure 3, the capacitance of the MEMS varactor is given by

Cu,dMEMS = C

u,d1 C

u,d2

Cu,d1 + C

u,d2

, (1)

where C1 is the capacitance between pad 1 and the MEMSelectrode and C2 is the capacitance between pad 2 and theMEMS electrode. Superscripts u and d represent the up anddown states, respectively.

By ignoring the fringe effects, the MEMS varactorcapacitances in the up and down states are given by

Cu,dMEMS = ε0εu,d

hu,d

(A1A2

A1 + A2

)(2)

where A1 = wel1 is the area of overlap between the MEMSelectrode and pad 1, A2 = wel2 is the area of overlap betweenthe MEMS electrode and pad 2, εu (= 1) is the relativedielectric constant of air, εd (= 2.65) is the relative dielectricconstant of the BCB dielectric layer, hu is the air gap measuredfrom the top surface of the dielectric to the MEMS electrodeand hd is the dielectric thickness of the BCB layer.

The capacitance ratio between the down state capacitance,Cd, and the up state capacitance, Cu, can be written as

ς = CdMEMS

CuMEMS

. (3)

Using equations (2) and (3), the capacitance ratio can beexpressed as

ς = εdhu

hd. (4)

From figure 2(a), the thickness of the spacer film is 2 mils(∼50 µm). It can be noted that the sum hu + hd + copperthickness (3 µm) should be equal to the spacer thickness. Ifthe dielectric thickness hd is chosen to be 7 µm, the air gap hu

can be calculated to be 40 µm. In this case, using equation(4), the capacitance ratio can be calculated to be 15.

Using equation (1), the MEMS varactor capacitance inthe down state is given by

CdMEMS = Cd

1Cd2

Cd1 + Cd

2

. (5)

For design purposes, Cd2 is assumed to be smaller than Cd

1 . Inthis case, from equation (2) the capacitance in the down stateis given by

CdMEMS ≈ ε0εdA2

hd. (6)

For a chosen MEMS electrode width we = 1 mm and overlaplength l2 = 0.45 mm, the capacitance Cd

MEMS can be calculatedto be 1.5 pF. Using equation (3), Cu can be estimated to be100 fF.

2.3. Equivalent transmission line model

The equivalent transmission line model for the tunable CPAcapacitively loaded by the MEMS varactor (along with theparasitic gap and open capacitances) is shown in figure 3.It should be noted that this model ignores discontinuityreactances at the feed junction. From the model, the totalcapacitance can be expressed as

Cu,dT = C

u,dMEMS + Cgap + Copen (7)

where Cgap is the capacitance between the actuation pad andpad 2 and Copen is the capacitance between pad 1 and pad 2.

814

A MEMS-based tunable coplanar patch antenna fabricated using PCB processing techniques

MEMS Varactor

CgapC1

C2

Pad 1

Pad 2

Copenopen

CPW Feed L+S

L+S ∆l

CMEMSC

Figure 3. Transmission line model for the tunable CPA with the MEMS varactor.

Figure 4. Simulated gap capacitance (Cgap) between the actuationpad (pad 1) and the ground plane (pad 2) for various gap widths (ga).

Superscripts u and d represent the up and down states,respectively.

The total capacitance loading the transmission line can bemodeled as an equivalent open-circuited line section of length�l. By equating the impedance of the capacitive load to anopen-circuited line section of length �l, we obtain

ZOC = j/ωCT = −jZ0 cot(β�l). (8)

The equivalent line length extension �l can be expressed as

�l = 1

βcot−1

(1

ωCTZ0

)= c

2πf√

εeffcot−1

(1

Z02πf CT

).

(9)

For a chosen slot width of S = 2.2 mm, the capacitanceCgap was obtained using CoventorWare [22] for various gapwidths (ga) in the range of 0.1–2.1 mm. In order to generouslyaccount for any misalignment between the MEMS electrodeand the actuation pad, the width of the actuation pad wa waschosen to be 2 mm. Figure 4 shows a plot of the variationof the gap capacitance Cgap as a function of the gap widthga. The gap value was chosen to be ga = 0.1 mm so as toobtain a long actuation pad that would provide high actuationforce and hence a reasonably low actuation voltage for PCBMEMS varactors. The corresponding Cgap is 2.04 fF. UsingCoventorWare [22], Copen was estimated to be around 9 fF.Substituting these values in equation (7), the total capacitancesin the up and down states of the MEMS varactor are calculatedto be 111 fF and 1.5 pF, respectively.

Using the aforementioned design parameters, an antennamodel including an actuation pad but without MEMS varactorwas built in Agilent ADS [21], and electromagnetic (EM)

Figure 5. Simulated return loss of the coplanar patch antennawithout and with the MEMS varactor (up and down states).

simulation was carried out. The simulated return loss of theantenna is shown in figure 5. From figure 5, it can be seen thatthe antenna resonates at 5.583 GHz with a return loss betterthan 45 dB. Also, using the LineCalc tool (a transmission linecalculator) in Agilent ADS [21] for the antenna parametersyields Z0 = 66.26 � and εeff = 1.198. After substitutingall these values in equation (9), the equivalent line lengthextensions (�l), in the up and down states, are calculated tobe 2.1 mm and 10 mm, respectively.

2.4. Frequency tuning characteristics

The effective length of the transmission line, Leff, can beexpressed as Leff = L + S + �l, where �l is the equivalentline length extension that accounts for the capacitive loading.The resonant frequency of the antenna can be expressed as afunction of the effective length, Leff, as

f = c

2Leff√

εeff. (10)

Assuming εeff remains constant, the ratio of the antennaresonant frequencies in the down and up states of the MEMSvaractor can be obtained from equation (10) as

fd

fu= L + S + �lu

L + S + �ld. (11)

The frequency ratio (FR) is defined as the ratio of the changein the antenna resonant frequency to the up state resonantfrequency:

FR = fu − fd

fu. (12)

Using these design parameters, a MEMS varactor modelwas created and included on top of the rectangular patch

815

M Maddela and R Ramadoss

(a) (b) (c)

Figure 6. Three layers of the MEMS tunable CPA: (a) substrate layer coated with BCB, (b) milled polyflon bonding film and (c) Kaptonfilm layer after DRIE.

antenna for EM simulation in Agilent ADS [21]. The returnlosses of the antenna for the up and down states are shownin figure 5. We can observe that as the air gap betweenthe MEMS varactor membrane and the substrate decreases,the capacitance increases and hence the resonant frequencydecreases. This antenna design is expected to be tunable inthe frequency range from 4.94 GHz (down state) to 5.44 GHz(up state). The return loss values are 28.85 dB and 44.67 dBat 4.94 GHz and 5.44 GHz, respectively. From thesimulated results, the frequency ratio was calculated to be0.09. In the following sections, fabrication and experimentalcharacterization of this antenna design are discussed.

3. Fabrication and assembly

The three layers that make up the configuration of the tunablecoplanar patch antenna are substrate, spacer and a polyimidefilm.

3.1. Substrate

A 2′′ × 2′′ RT/Duroid 6002 (30 mils thick with 9 µmcopper cladding, εr = 2.94) substrate was used. Coplanarwaveguide (CPW) lines are defined on the metallization layeron the substrate by photolithography and etching processes.Photosensitive BenzoCycloButene (BCB) is spin-coated onthe substrate and patterned to form the dielectric layer onthe bottom electrode metallization. Figure 6(a) shows aphotograph of the substrate with the patterned antenna, CPWline and BCB dielectric layer.

3.2. Spacer

The spacer layer provides the required spacing between thesubstrate and the polyimide layer. Hence, the thickness of thespacer film determines the up-position gap height. A 2 milsthick polyflon bonding film is used as the spacer film. Thisfilm is machined using a milling machine to create openingsfor the contact pads and the MEMS varactor in the polyimidefilm. A milling machine processed spacer film is shown infigure 6(b).

3.3. Polyimide film

A 2 mils thick flexible Kapton E polyimide film (εr = 3.1)with 100–150 A nichrome seed layer and 3 µm thick coppercladding is used. The Kapton polyimide film layer is used asthe MEMS structural layer because of its ability to withstand

Figure 7. Photograph of the fabricated MEMS tunable coplanarpatch antenna.

millions of mechanical flexing cycles. First, the electrode isdefined on the 3 µm thick copper cladding on the Kaptonfilm by photolithography and etching processes. Then, thefilm is machined using deep reactive ion etching (DRIE) tocreate slot openings required for the formation of a movablemembrane. These flexures separate the membrane from itsadjacent area and thus reduce the stiffness of the movablemembrane. The equipment used for the DRIE process is anSTS AOE (advanced oxide etcher). The configuration of thegases used for this etch in our facility is 8 sccm CF4 and35 sccm oxygen with 500 W of RF power. These processingparameters take about 70 min to etch a 2 mils thick Kaptonlayer. Finally, resistive bias lines are defined in the nichromeseed layer for DC biasing of the MEMS varactor. Thesebias lines prevent leakage of the RF signal from the MEMSvaractor to the DC supply. Figure 6(c) shows the photographof a processed Kapton film.

3.4. Thermo-compression bonding

Thermo-compression bonding is performed using a Carverpress consisting of two platens. The platens are heated usingheaters, which are controlled by a thermocouple. The fixtureconsists of two steel plates with alignment holes at fourcorners. The substrate forms the bottom most layer, the spacer

816

A MEMS-based tunable coplanar patch antenna fabricated using PCB processing techniques

Resistive bias line V

NA

Bias-T

MEMS Varactor

BCB

+ DC FR

VDC

RF

(bottom side of Kapton film)

DC

Duroid Substrate

Spacer

CPW dFee

Top Electrode

Figure 8. Biasing arrangement used for RF testing of the tunable CPA.

is the middle layer and the Kapton film forms the topmost layerin this structure. The different layers are aligned by aligningthe marks created on the three layers during the fabricationprocess described earlier. This unit is now placed between thepress platens. The bonding is performed at a pressure of 65 psi(a load of 165 lbs) and a temperature of 130 ◦C. Both pressureand temperature are maintained for 5 min during bonding.Before pressure is released, the assembly is cooled down tothe room temperature.

4. Experimental characterization

A photograph of the fabricated tunable coplanar patch antennais shown in figure 7. Figure 8 shows the biasing arrangementfor the MEMS-based tunable coplanar patch antenna. A bias-T(Picosecond model no. 5542-203) with a maximum DC biasvoltage rating of 100 V was used. The bias-T ensures that theapplied DC voltage does not leak into the RF input therebyproviding proper isolation between the RF test equipment andthe DC power supply. An SMA launcher was soldered to theCPW feed line for RF testing. To ensure rigidity of the MEMSdevices during testing, the substrate was mounted on a Luciteframe. Two types of measurements were taken to characterizethe antenna: MEMS varactor profile and RF measurements.

4.1. MEMS profile characterization

The WYKO optical profilometer was used to measure theprofile of the MEMS varactor in the CPA and the measuredprofile is shown in figure 9. It was found that the warpage isabout 5–10 µm across the MEMS varactor plate. The height ofthe MEMS varactor above the ground plane is about 100 µm.During lamination of the final design, spare Kapton pieceswere placed in the air gap between the BCB on the bottomelectrode (plate 1 and plate 2) and the Kapton membrane tohelp reduce warpage during bonding. The slight warpagecould be attributed to the thermal coefficient of expansionmismatch between the copper patch (17 ppm) and Kapton(16 ppm) polyimide film.

4.2. RF measurements

The return loss of the MEMS patch antenna was measuredusing an HP 8510C vector network analyzer (VNA). A DC bias

y

xx’

y’(a)

(b)

(c)

Figure 9. 2D profile of the MEMS varactor in the CPA measuredusing WYKO NT 2000 optical profiler: (a) top view of the MEMSvaractor membrane showing xx′ and yy′ reference planes, (b) xx′

profile of the MEMS varactor membrane and (c) yy′ profile of theMEMS varactor membrane.

voltage was applied to the patch, and return loss measurementswere taken for various bias voltages in the range of 0–116 V,

817

M Maddela and R Ramadoss

Table 1. Simulated and measured resonant frequencies and return losses for the antenna in the up and down states of the MEMS varactor.

Simulated Measured

Frequency (GHz) S11 (dB) Zin (�) Frequency (GHz) S11 (dB) Zin (�)

Up state 5.44 −28.85 47.51−2.486i 5.545 −40 49.53−1.045iDown state 4.94 −45 49.97−0.583i 5.185 −41 50.43−0.88i

Figure 10. Return loss results of the tunable MEMS antenna forvarious applied DC bias voltages.

at 5 or 10 V increments. Figure 10 shows the plot of thereturn loss measurements for various applied voltages. Itcan be observed that the resonant frequency of the antennashifts downward with increasing bias voltage. At 0 V, theMEMS varactor is in the ‘up’ state and the antenna resonates at5.545 GHz with a return loss of 40 dB. When the applied

0

30

60

90

120

150

180

210

240

270

300

330

-30

-20

-20

-10

-10

0

30

60

90

120

150

180

210

240

270

300

330

-30

-20

-20

-10

-10

0

30

60

90

120

150

180

210

240

270

300

330

-30

-20

-20

-10

-10

0

30

60

90

120

150

180

210

240

270

300

330

-30

-20

-20

-10

-10

H-plane

E-plane

0

30

60

90

120

150

180

210

240

270

300

330

-30

-20

-20

-10

-10

0

30

60

90

120

150

180

210

240

270

300

330

-30

-20

-20

-10

-10

0

30

60

90

120

150

180

210

240

270

300

330

-30

-20

-20

-10

-10

0

30

60

90

120

150

180

210

240

270

300

330

-30

-20

-20

-10

-10

(a)

(c)

(b)

(d )

H-plane

E-plane

Figure 11. Measured radiation patterns of the MEMS tunable antenna for various applied voltages: (a) H-plane co-polarization, (b) H-planecross-polarization, (c) E-plane co-polarization and (d) E-plane cross-polarization.

voltage is below 80 V, no significant frequency shift wasobserved. The antenna provides a downward shift in theresonant frequency as the applied DC bias is increased to110 V. When the DC bias voltage is increased to 116 V,the MEMS varactor reaches the ‘down’ state and the antennaresonates at 5.185 GHz with a return loss of 41.12 dB. Thetunable frequency range of the antenna between the up anddown states of the MEMS varactor is 360 MHz. The measuredfrequency ratio is 0.065. Table 1 shows the comparison ofthe simulated and measured resonant frequencies in the upand down states. The simulated results were obtained byEM simulation of the CPA (including the MEMS varactor) inthe HP-momentum available in the Agilent’s advanced designsystem (ADS) [21]. In the up state, the real parts of the inputimpedance obtained from the simulated and measured returnlosses are 47.51 � and 49.53 �, respectively. Hence theantenna exhibits a better impedance match to the 50 � feedline in the measured case when compared to the simulatedcase. From table 1, it can be noted that the simulated tuningrange is about 9% whereas the measured tuning range is about6.5%. The lower measured tuning range can be attributedto a slight decrease in the down state capacitance caused byfactors like the surface roughness of the RT/Duroid substrate

818

A MEMS-based tunable coplanar patch antenna fabricated using PCB processing techniques

and fabrication tolerances. Typical surface roughness for theRT/Duroid substrate is in the range of 1–2 µm. A similartrend was observed in [23] for RF MEMS switches fabricatedon RT/Duroid substrates.

The radiation patterns of the antenna have been measuredusing a Diamond Engineering 6000 Series Desktop AntennaMeasurement system (DAMS) (available from DiamondEngineering [24]). Measured radiation patterns for variousbias voltages of 0, 110, 113, 115 and 116 V are shown infigure 11. The null along the broad-side direction in the E-fieldco-polarization patterns can be attributed to the asymmetrycaused by the presence of the MEMS varactor and the feedline at the center of the radiating edges. It can be noted thatthere is no significant change in the overall patterns throughoutthe tuning range for various applied voltages.

5. Conclusions

A MEMS-based rectangular coplanar patch antenna tunablefrom 5.545 GHz at 0 V to 5.185 GHz at 116 V has beendemonstrated. The return loss of the CPA is better than40 dB over the tuning range of 360 MHz. In this work,an inexpensive tunable antenna fabricated using conventionallow-cost PCB processing techniques has been demonstrated.For demonstration of tunable antennas using low-cost PCBMEMS technology, a bi-directional coplanar patch antennawas chosen. The approach presented here could also be appliedto uni-directional planar antennas such as microstrip patchantennas. This tunable antenna is suitable for multi-mode,multi-band wireless communication systems.

Acknowledgment

This project is sponsored by the Motorola UniversityPartnership in Research (UPR) program, Motorola,Schaumburg, IL.

References

[1] Garg R, Bhartia P, Bahl I and Ittipiboon A 2003 MicrostripAntenna Design Handbook (Boston: Artech House)

[2] Bhartia P and Bahl I J 1982 Frequency agile microstripantennas IEEE Antennas Propag. Symp. Digest. 20 304–7

[3] Luxey C, Dussopt L, Le Sonn J-L and Laheurte J-M 2000Dual-frequency operation of CPW-fed antenna controlledby pin diodes Electron. Lett. 36 2–3

[4] Kermakar Nemai Chandra 2004 Shorting strap tunable stackedpatch PIFA IEEE Trans. Antennas Propag. 52 2877–83

[5] Liu Z D, Hall P S and Wake D 1997 Dual-frequency planarinverted-F antenna IEEE Antennas Propag. Symp. Digest.45 1451–8

[6] Jin Z and Mortazawi A 2003 An L-band tunable microstripantenna using multiple varactors IEEE Antennas Propag.Symp. Digest 4 22–7

[7] Holland B R, Ramadoss R, Pandey S and Agrawal P 2006Tunable coplanar patch antenna using varactor Electron.Lett. 42 319–20

[8] Brown E 1998 RF MEMS switches for reconfigurableintegrated circuit IEEE Trans. Microw. Theory Tech.46 1868

[9] Schaffner J H et al 2000 Reconfigurable aperture antennasusing RF MEMS switches for multi-octave tunability andbeam steering IEEE Trans. Antennas Propag. 1 321–4

[10] Simons R N, Chun D and Katehi L P B 2001 Reconfigurablearray antenna using microelectromechanical systems(MEMS) actuators IEEE Trans. Antennas Propag. 3 674–7

[11] Blondy P, Bouyge D, Crunteanu A and Pothier A 2006 A widetuning range MEMS switched patch antenna IEEEMicrowave Symp. Digest pp 152–4

[12] Lee A W M, Kagan S K, Wong M, Singh R S and Brown E R2003 Measurement and FEM modeling of areconfigurable-patch antenna for use in the widebandgapfiller satellite system IEEE Trans. Antennas Propag. 1379–82

[13] Kiriazi J, Ghali H, Ragale H and Haddara H 2003Reconfigurable dual-band dipole antenna on silicon usingseries MEMS switches IEEE Trans. Antennas Propag. 1403–6

[14] Onat S, Alatan L and Demir S 2004 Design of triple-bandreconfigurable micro-strip antenna employing RF-MEMSswitches IEEE Trans. Antennas Propag. 2 1812–5

[15] Zhang C et al 2006 Development of reconfigurablemini-nested patches antenna for universal wireless receiverusing MEMS IEEE Trans. Antennas Propag. pp 205–8

[16] Anagnostou D, Khodier M, Lyke J C and Christodoulou C G2002 Fractal antenna with RF MEMS switches for multiplefrequency applications IEEE Trans. Antennas Propag. 222–5

[17] Cetiner B A, Qian J Y, Chang H P, Bachman M, Li G P andDe Flaviis F 2003 Monolithic integration of RF MEMSswitches with a diversity antenna on PCB substrate IEEETrans. Microw. Theory Tech. 51 332–4

[18] Jackson R and Ramadoss R 2007 A MEMS-basedelectrostatically tunable circular microstrip patch antennaJ. Micromech. Microeng. 17 1–8

[19] Ramadoss R, Lee S, Lee Y C, Bright V M and Gupta K C2006 RF MEMS capacitive switches fabricated usingprinted circuit processing techniques J. Microelectromech.Syst. 15 1595–604

[20] Greiser J W 1976 Coplanar stripline antenna Microwave J.pp 47–9

[21] Advanced Design System 2005 Agilent Technologies, CA,USA

[22] CoventorWare 2005 Coventor, NC, USA[23] Ramadoss R, Lee S, Gupta K C, Lee Y C and Bright V M

2003 Fabrication, assembly, and testing of RF MEMScapacitive switches using flexible printed circuit technologyIEEE Trans. Adv. Packag. 26 248–54 (special section onMEMS/NEMS Packaging)

[24] Desktop Antenna Measurement System Diamond Engineering,CA, USA

819