Frequency Reconfigurable Microstrip Circular PatchArray Antenna Integrated with

RF-MEMS SwitchesNaveen Kumar Saxena and Dr. P.K.S. Pourush

Microwave Lab Department of Physics Agra College AgraPIN 282002 (U.P) India. Email: Nav3091@rediffmail.com

Fig: 1 Schematic top view ofa shunt micromechanical switch.

Table 1: Comparison ofRF switching devices

Meanwhile, the power handling of these devices is limitedmostly by current density limitations.

Port 2

GroundSuspended Membrane

Relays FET PIN MEMS

Switching Time >5 ms I-lOOns 1-100 ns 1-300 IlsResistance Rs 0.005 - 4-6 2-4 0.5 - 5(0) 0.01Insertion loss 0.01- 0.3 0.4- 2.5 0.3-1.2 0.2 - 0.5IGHz (dB)Isolation 1-30 50-80 20 - 30 20 - 30 20 - 50GHz (dB)Actuation Vol 12-30 3-50 3-5 5-100(V)Power consumed 400 0.05 - 0.1 5 - 100 0.05- 0.1(mW)Signal Power 100 - 1500 <10 <10 0.2 - 0.5(W)FOM (fs) N/A 500 110-220 0.01-12

3. Electrostatic operation of the mechanical motion of the RFMEMS devices requires negligible quiescent currentconsumption. Typical switching energy is approximately1OnJ. The main limitation of these switches is theirswitching speed. Microsecond switching precludes theiruse in high-speed applications such as transmit/receiveswitching. However, these speeds are more than sufficientfor a variety of applications including beam steering inphased antenna arrays. Following table 1 and 2 shows thecomparison of RF switching devices and performancerespectively [10].

1. The contact and spreading resistance associated withohmic contacts are eliminated, significantly reducing theresistive losses in the device. Instead, high conductivityfilms are used to fabricate metal structures that carry RFcurrents with ultra-low losses.

2. The removal of nonlinearities associated withsemiconductor junctions significantly improve thedistortion characteristics and power handling of the RFMEMS devices. RF MEMS switches exhibit nomeasurable harmonics or intermediation distortion.

I. INTRODUCTION

The capability to select the frequency is essential for diversemissions. A simple idea to adjust the resonant frequency of anantenna is to reconfigure the geometrical structure. This hasbeen made possible with the use of the RF-MicroElectromechanical System (MEMS) switches. The MEMSdevices can outperform their semiconductor counterparts suchas transistors and diodes in lower insertion loss, lower powerduring operation and higher Qwhich inherently fits the antennaelements [1-3].RF MEMS Switches: Micromechanical switches were firstdemonstrated in 1979 as electrostatically actuated cantileverswitches used to switch low frequency electrical signals. Sincethen, these switches have demonstrated useful performance atmicrowave frequencies using cantilever, rotary and membranetopologies [4-9]. These switches have shown that movingmetal contacts possess low parasitic at microwave frequencies(due to their small size) and are amenable to achieving low on­resistance (resistive switching) or high on-capacitance(capacitive switching). Micromechanical membrane switcheshave several advantages compared to FET or p-i-n diodeswitches. Eliminating the use of semiconductor p-n and metal­semiconductor junctions in radio frequency (RF) devicesserves three very useful functions.

Abstract- A microstrip reconfigurable array antenna withintegrated RF-MEMS switches is proposed which can operate atdual frequencies. The switches are incorporated to circular patchto control or change the frequency. The array of these circularpatches gives more directivity and scanning power, which makesit useful in many Communication Systems.

978-1-4244-1864-0/07/$25.00 ©2007 IEEE

Table 2: Microwave switches performance comparison

Device Type Ron (ohms) CoIT (F)FOM-CutoffFreq. (GHz)

GaAsMESFET 2.3 249 280GaAspHEMT 4.7 80 420GaAs p-i-n Diode 5.6 420 730Metal Membrane 0.4 35 >9000Capacitive

To reconfigure the structure additional patch ring is placedaround the main patch with MEMS switches. These switchesare cantilever based type. The actuated cantilever opens andcloses the gap between two ports or patches. A clearance of2mm above the surface of ports or patches, when openprovides 25dB isolation at 50 GHz. The isolation increases asfrequency decreases

Grounded

Feeding Point -------I~.l.r__---

Switches

Fig 1: Geometrical Structure of a single Microstrip Patch with Switches

In this communication, an array of 16 elements (4 x4)is proposed. Each element of this array is supposed to beconnected with parasitic patch with the help of RF-MEMSswitches. These switches [11, 12] are based on actuationprovided by thin film Lead Zirconate Titanate (PZT) depositedon thin cantilever.

II. ANTENNA STRUCTURE

Fig 2 shows the schematic diagram of the proposedantenna with 6 switches integrated. Here we have circularpatch of radius "a" is modeled on a Quartz substrate withdielectric constant=2.2 and thickness h=0.254mm. Themicrostrip array antenna and RF-MEMS switches aremonolithically onto the same dielectric substrate with RF­MEMS switches to be part of every antenna patch theseswitches are based on actuation, provided by thin film of leadzirconate (PZT) deposited on a thin cantilever. Switches aregenerally manufactured using standard photolithographytechniques in which PZT layer is deposited using sol-gelprocess. The cantilever is released using a combination of bulksilicon etching and deep reactive ion etching (DRIE) utilizing aSTS Multiplex system.

To obtain high gain, beam scanning or steeringcapabilities the patches are arranged in array configuration as

shown in fig 2.

Ground Plane

Fig 2: Geometry of Sixteen Patch Array Antenna

The resonant frequency of the antenna is evaluated by theclassical equation as follow [13].

Where

2h {(1fK" }K = aX [1 +-.- Jog .-.-) +1.7726 J1/3Tt~rK 2h"

Here "a" is the radius of the circular patch and D isthe distance of two patches on both directions, which cancontrol mutual coupling between patches. To obtain goodperformance, there are many feeding methods, such as CPW inthe ground feeding microstrip antenna [14], and CPW with stubpatch feeding slot antenna [15]. Considering impedancematching of patches coaxial feeding is preferred.

Using the pattern multiplication approach andneglecting mutual coupling between the elements, thenormalized form of the array factor for the present geometry isobtained and given as [16].

A F = 0.0625 sin {2 (kd x sin B co s ¢ + fJ x)}sin {0 .5 ( kd x sin B cos ¢ + fJ x ) }

sin {2 (kd y sin B sin t/J + Py )}

x-------:....--------=---sin {O.5 (kd y sin B sin t/J + P y )}

III. SIMULATION

When the control voltage is applied to the patch all theswitches are activated simultaneously, providing theconnection between main and parasitic patches. When novoltage is applied, the switches are in the off state (upposition), therefore only the main patch is active while thesecond patch serve as parasitic element, lowering the resonant

frequency. As the switches are actuated the electrical length isincreases, therefore the resonant frequency decreases while therelative phase remains same.

A bias voltage of 20-30 volts with negligible currentconsumption is required to actuate the switches. Due to non­ideal contact for the switches at on state the resonant frequencyis lower than the simulated one. Meanwhile the finite height ofthe MEMS switches during the OFF state introduces additionalcoupling between the two patches, lowering the resonantfrequency further. The simulated radiation patterns are shownin Fig 3(a,b) and 4(a,b), when switches are OFF and ONrespectively.

Some important parameters related to single patchcharacterization is listed in table 3 which shows thecomparison between when MEMS switches is OFF and ONrespectively.

90 8

o

270

Radiation Pattern of4*4 Araay

Fig 3(a) Graphs when switches are OFF

-j1.0

Smith Chart of S11 and S22

Fig 3(b) Graphs when switches are OFF

Table 3: Table of Parameters between Two States When Switches is OFF andON respectively

Switch is OFF Switch is ON

Radius (a) = 0.5046 cm Radius (a) = 0.5660 cm

Dielectric Constant = 2.2 Dielectric Constant =2.2

Subs Width = 0.254 mm Subs Width = 0.254 mm

Frequency = 10GHz Frequency = 9GHz

Comparison shows that after switches ON Radius,Radiation power and Resonant Frequency are considerablychanged. This analysis has been carried out using MATLAB7.1.

90120 60

\

\-'~

A

// \

150, I 30'-z

0

210 330

240 300270

Radiation Pattern of4*4 Array

Fig 4(a) Graphs when switches are ON

+j1.0

-j1.0

Smith Graph ofS11 and S22

Fig 4(b) Graphs when switches are ON

IV. CONCLUSION

A microstrip patch array antenna, capable of changingfrequency operation using RF-MEMS switches is presented.This type of antenna configuration is may be use in variouscommunication systems. This study is still I progress to explorefurther its application spectrum.

V. REFERENCES

[1] Z.1. Yao, S. Chen, S. Eshelman, D. Denniston and C.Goldsmith,"Micromachined low-loss microwave switches," IEEE J. ofMicroelectromechanical Systems, vol. 8, no. 2, pp. 129-134, June1999.

[2] C.H. Chang, 1. Y. Qian, B. A. Cetiner, F. De Flaviis, M. Bachman, andG.P. Li, "Low Cost RF-MEMS Switches Fabricated on MicrowaveLaminate Printed Circuit Boards," Electronic Device Letters, 2003

[3] F. Yang and Y. Rahmat-Samii, "Patch antenna with switchable slot(PASS): Dual frequency operation," Microwave Opt. Tee/mol. Lett.,vol. 31, no. 3, pp. 165-168, Nov. 2001

[4] K.E. Petersen, "Micromechanical membrane switches on silicon," IBMJ. Res. Develop., vol. 23, no. 4, pp. 376-385, July 1979.

[5] 1. 1. Yao and M. F. Chang, "A surface micromachined miniature witchfor telecommunications applications with single frequencies from DCup to 4 GHz," in Proc. Transducers'95, June 1995, pp. 384-387.

[6] S. Zhou, X. Sun, and W. N. Carr, "A micro variable inductor chipusing MEMS relays," in Proc. Transducers'97, June 1997, pp.1137-1140.

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C.Goldsmith,1. Randall, S. Eshelman, T. H. Lin, D. Denniston, S.Chen,and B. T. H. Norvell, "Characteristics of micromachined switches atmicrowave frequencies," in IEEE Microwave Theory Tech. Symp.,May 1996, pp. 1141-1144.

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C. H. Chang, 1. Y. Qian, B. A. Cetiner, F. De Flaviis, M. Bachman, andG.P. Li, "Low Cost RF-MEMS Switches Fabricated on MicrowaveLaminate Printed Circuit Boards," Electronic Device Letters, 2003[10]Garg, R., P. Bhartia, I. Bahl, and A. Ittipiboon, Microstrip AntennaDesign Handbook, Artech House, Norwood, MA, 2001.Saed, M. A., "Reconfigurable broadband microstrip antenna fed by a

coplanar waveguide," Process In Electromagnetics Research, PIER55, 227-239, 2005.Eldek, A. A., A. Z. Elsherbeni, and C. E. Smith, "Rectangular slotantenna with patch stub for ultra wideband applications and phasedarray systems," Process In Electromagnetics Research, PIER 53,227­237,2005.

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