An Adaptive Rectangular Microstrip Patch AntennaArray Element Using Photonic Controls

Randy L. HauptApplied Research Laboratory

Penn State UniversityPO Box 40

State College, PA 16804814-865-7299 x210hauptWieee.org

Abstract This paper shows how certain photoconductivematerials can be used to design an adaptive array elementwith a center frequency of 2 GHz. The resonant frequencyof the patch gradually shifts to a lower frequency as theconductivity off its gap filled with photonic materialincreases. The resulting increase in the reflection coefficientat the center frequency and decrease in gain acts as acontinuous amplitude weight. This approach offerscontinuous variation of the conductive portions of the patchrather than an "on" or "off' approach offered by theswitches. Varying the amplitude of the elements allowsdynamic control over the array sidelobe levels.

TABLE OF CONTENTS

1. INTRODUCTION......................12.PATCHDESIGN......................1

3.ANTENNA ARRAY DESIGN...................... 34. CONCLUSIONS......................5REFERENCES .....................5BIOGRAPHY .....................6

1. INTRODUCTION

Adaptive antennas have found use in many wirelessapplications. An adaptive antenna has the ability to changeits antenna pattern in order to enhance reception of a desiredsignal while minimizing undesired signals. The antennapatterns are manipulated by controlling the amplitude and/orphase of the signals received at each element in the array.Various antenna configurations and adaptive algorithms areoutlined in the literature [1].

Hardware requirements for an adaptive antenna can be quiteexpensive. The most elegant signal processing algorithmsrequire a receiver at each element with an associatedcalibration scheme. These approaches are derivatives of theApplebaum adaptive loop [2] and the least mean square(LMS) algorithm [3]. Signals from the elements are used toform a covariance matrix from which the adaptive weightsare derived. Another approach minimizes the total output11-4244-1488-1/08/$25.00 C 2008 IEEE.IEEEAC paper paper# 1 184, Version 3, Updated 15 Nov 2007

power of the array. It requires constraints on the hardwarephase shifters, amplitude weights, or number of adaptiveelements, so the desired signal is not nulled in the mainbeam. This approach has been implemented in a phase-onlyalgorithm [4] and amplitude and phase adaptive nullingusing a genetic algorithm [5]. The goal is to minimize thetotal output power while at the same time minimizing theperturbations to the main beam. Limiting the amount ofcontrols by using a subset of all the elements or leastsignificant bits of the weights prevents the algorithm fromplacing a null in the main beam and reducing the desiredsignal strength.

This paper explores the use of photoconductive materials inthe design of microstrip patch antennas for adaptive arrays.Making part of a patch antenna from photoconductivematerial allows the tuning of that element via an opticalsignal and hence controlling the reception of that signal atthe element. Normally, the adaptive weights are digitalamplitude and/or phase weights or the weights are appliedin software if a digital beamformer is used. A patch designis optimized for control at 2 GHz. The patch consists of arectangular perfect electric conductor (PEC) separated froma thin rectangular PEC section by a thin rectangular piece ofsilicon (see Figure 1). Five of these patches are placed in alinear array to demonstrate sidelobe control. Sidelobes arereduced through altering the conductivity of the small stripof silicon placed in the patch.

2. PATCH DESIGN

A simple pin-fed rectangular microstrip patch serves as thestarting point for this adaptive element. The majority of thepatch is made from a PEC with a small portion made fromphotonic material with variable conductivity. There areseveral different types of materials whose conductivity canbe changed using an electrical signal. Conductiveelectroactive polymers, such as polypyrroles andpolyanilines, have a conductivity that is proportional to an

1

applied electric potential. Conducting polymeric materialshave controllable conductivity at microwave frequencies[6]. A small dc potential applied across a poly(aniline)-silver-polymer electrolyte composite changes itsconductivity. These materials have been incorporated into aSalisbury screen to alter the radar cross section of largesurfaces [7]. Organic photoconductors have been used inphotocopying for many years [8]. They were not used incircuit design due to the low conductivity. Recent advancesin increasing the conductivity of organic photoconductorshave become popular for use in plastic circuits [9]. Sheetsof single wall carbon nanotubes have been designed toexhibit photoconductivity. They are not yet practical to usein circuits though [10]. Probably the most common type ofvariable conductive material is silicon. Silicon has a relativepermittivity of £r = 11.7 and an electrical conductivity thatvaries from an insulator to a good conductor, dependingupon the intensity of the optical source or the biasingcurrent. Silicon is used in the manufacture ofmany differenttypes of electronic devices, as well as photo voltaic cells. Ithas been used to reconfigure dipole antennas [11] as well aselectromagnetic bandgap surfaces [12]. Other applicationsinclude a reconfigurable Fresnel-zone plate antenna [13]and a reconfigurable reflectarray [14].

CST Microwave Studio [15] performs the simulations inthis paper. It uses the finite integration technique (FIT) withthe perfect boundary approximation and a multilevelsubgridding scheme. The time domain approach covered afrequency band from 1.7 to 2.3 GHz.

coordinate system is centered on the patch. The pin-feed islocated at x = 7.6 mm.

A graph of the amplitude of the return loss is shown inFigure 2 for the following conductivities: 0, 1, 2, 5, 10, 20,30, 50, 75, 100, 200, 500, and 1000 S/m. At 2 GHz, there isa distinct resonance when the silicon has no conductivity.As the conductivity increases, the resonance at 2 GHztransitions to a new resonance near 1.78 GHz. As a result,the s1l at 2 GHz increases from zero to 0.9. The amount ofpower delivered to the patch at 2 GHz reduces as theconductivity increases. Consequently, the photoconductivesilicon acts as an amplitude control to that element. Thisamplitude control could be useful in receive arrayapplications. Figure 3 shows the return loss as a function ofconductivity at 2 GHz. Since the conductivity is directlyrelated to the intensity of the optical source, then increasingthe optical intensity decreases the signal at the patch. Ineffect, this configuration is an optically controlledattenuator built into the patch element.

The microstrip patch model is shown in Figure 1. It consistsof a main rectangular patch made from a PEC that is58.7 x 39.4 mm. The substrate is a slab of opticallytransparent fused quartz with Er = 3.78 backed by a PECgroundplane. The substrate is 88.7x69.4mm and is 3 mmthick. To the right of the patch is a thin strip of silicon (G)58.7x2mm with Er = 11.7 . To the right of the silicon isanother thin strip of PEC (F) 58.7x 4.2 mm. An opticalsource illuminates the bottom of the small rectangular sliverof silicon from below. A laser or LED beneath thegroundplane can illuminate the silicon through small holesin the groundplane or by making the groundplane from atransparent conductor, such as indium tin oxide [16].Increasing the optical source intensity increases the siliconconductivity, thus promoting the flow of current from themain patch to the small rectangle. Enlarging the patchlowers the resonant frequency.

The dimensions for the patch and the location of the feedpoint were found using the numerical optimizationalgorithm in CST Microwave Studio. The optimization wasinitially done assuming the silicon has zero conductivity.The patch was designed to be resonant at 2 GHz. The lengthof the main patch and the location of the feed point alongthe x-axis were optimized to minimize |s11i at 2 GHz. The

0,41-

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Figure 1. Diagram of the adaptive patch.

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Figure 2. Plots of the magnitude of s1l for siliconconductivities of 0, 1, 2, 5, 10, 20, 30, 50, 75, 100, 200,500, and 1000 S/m.

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fteqtwncy (GHz)

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200 4001 600conldLiclvlitv (SAin)

Figure 3. Plot of the magnitude of s11 versus conductivity at2 GHz.

A three dimensional view of the antenna pattern is shown inFigure 4. The peak gain is 5.22 or 7.22 dB. Orthogonal cutsof the element pattern are shown in Figure 5. The gain andpattern are typical of a rectangular microstrip patch.

The first attempt at an adaptive patch did not have the smallPEC to the right of the silicon strip. This configuration didnot allow a large change in s 1 with a small change inconductivity, because the resonance did not move far from 2GHz.

Figure 4 Plot of the linear antenna gain at 2 GHz whenv= 0 . The peak gain is 5.22.

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+ 0

The signal strength received by an element is a function ofthe element gain and the s1l. Consequently the receivedsignal is proportional to the gain times the power notreflected by the element mismatch. Figure 6 is a plot of thegain times the portion of power not reflected (1-_s2).Significant attenuation is possible with modest increases insilicon conductivity.

-80 ,40 60 SO

Figure 5. Graphs of the element gain0=0 and 0=90'.

patterns (dB) for

3. ANTENNA ARRAY DESIGN

The next step places these adaptive elements in a smallarray. A five element linear array of the photoconductivepatches is shown in Figure 7. The spacing betweenelements is 75 mm or 0.51Z. If the silicon insets all have aconductivity of zero, then the array is uniform with a farfield pattern shown in Figure 8. This quiescent pattern has again of 12.81 dB and a relative peak sidelobe level of 13.84dB.

The element patterns of the uniform array are shown inFigure 9. The average gain of these patterns at boresight is6.14 dB. This average gain is over 1 dB less than theisolated element pattern. Mutual coupling also flattens thegain patterns and adds a ripple due to the finite length of thearray.

20 40 60conductivity (S/ii)

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Figure 6. Plot of the magnitude of s1l times patch gainversus conductivity at 2 GHz.

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Illuminating the silicon at each element with a differentoptical intensity produces a conductivity taper across thearray. Increasing the conductivity of the silicon in a patchdecreases the product of the patch gain times the powerdelivered to the patch. Thus, the conductivity taper inducesan amplitude taper. An array pattern with equal sidelobesresults when the conductivity has values of [16 5 0 5 16]S/m. The corresponding antenna pattern is shown in Figure10. It has a gain of 10.4 dB and a peak relative sidelobelevel 23.6 dB below the main beam. The element patternshave different gain patterns (Figure 11) than those of theuniform array. The gain of the elements with the siliconilluminated goes down as predicted. The general elementpattern shape also changes slightly due to the change incoupling.

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Figure 7. A five element linear arraypatches.

of photoconductive

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Figure 8. Far field pattern of a five element uniform array.

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--80 (60 40 20 0 20 40 60 80O(degrL4s)

Figure 10. The quiescent pattern is the dashed line and hasall the conductivities set to 0. The adapted pattern is thesolid line and has the silicon conductivities set to [16 5 0 516] S/m.

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0 60 40 20 0 20 4

= ~~~~~~0 (d X Ne,.es0

..

60t so

10

-15t-80 -|6)0 -40 -20 0 20 40 60 80

n (dege s)

Figure 9. The element patterns of the quiescent array. Thenumbers correspond to the elements in Figure 7.

Figure 11. The element patterns of the array that has thesilicon conductivities set to [16 5 0 5 16] S/m. The numberscorrespond to the elements in Figure 7.

A linear conductivity taper of [16 8 0 8 16] S/m reduces theinner sidelobes even farther. The resulting antenna patternin Figure 12 has a gain of 10.14 dB and a first sidelobelevel of 24 dB below the peak of the main beam. The

4

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corresponding element patterns appear in Figure 13. Theincreased conductivity at elements 2 and 4 reduced theirgains. Altering the conductivity at certain elements changesthe array pattern. It would be possible to switch the patternbetween uniform with high gain and low sidelobe withlower gain depending upon the interference present. Itwould also be possible to use an adaptive algorithm toadjust the conductivity of the patches to reject interferenceentering certain sidelobes. Figure 14 demonstrates theeffects on the array pattern of varying the conductivity atelement 2 from 0 to 500 S/m while keeping the conductivityat the other elements at 0 S/m. Sidelobes and nulls arechanged, but the gain remains relatively unchanged. Theminimum of s1l of element 2 moves to below 1.8 GHz whenits conductivity is 500 S/m. The minimum of s1l of elements1, 3, and 4 occur at about 1.98 GHz, while The minimum ofs1l of elements 5 stays at 2.0 GHz.

15

5

0

-1 5

I 0 SIMu.o X(J

010

O 4-0 40 =20 0 20

XfmMI ......

S6i0.............

.........................................

Figure 14. The array patterns associated with varying theconductivity at element 2 from 0 to 500 S/m.

4. CONCLUSIONS

This paper shows how to control the signal strengthreceived by a patch antenna by altering the conductivity ofpart of the patch. Photoconductive elements can be used toplace an amplitude taper on a linear array to lower thesidelobe level. This paper demonstrated thephotoconductive element concept through computermodeling. The relative peak relative sidelobe level of a fiveelement array was lowered by about ten dB through a linearconductivity taper.

-0 60 40 20 0 20 Optimizing the conductivity taper would make it more0 (:degJeg)efficient, so the main beam gain would not be reduced so

Figure 12. The quiescent pattern is the dashed line and has much. Also, optimization would produce a lower maximumall the conductivities set to 0. The adapted pattern is the sidelobe level. Adding more elements to the array wouldsolid line and has the silicon conductivities set to [16 8 0 8 reduce the effects of errors and allow lower sidelobe levels.16] S/m. Extensions to planar arrays are also possible.

1Ol r0 r .................... ,,,,,,,,,,,,AcKNOWLEDGEMENT

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Figure 13. The element patterns of the array that has thesilicon conductivities set to [16 8 0 8 16] S/m. The numberscorrespond to the elements in Figure 7.

This work was sponsored by Army CECOM under contractN00024-02-D-6604 DO-295.

REFERENCES

[1] R.A. Monzingo and T.W. Miller, Introduction toAdaptive Arrays, Raleigh, NC: SciTech Publishing,Inc., 2003.

[2] S.P. Applebaum, "Adaptive arrays," SyracuseUniversity Research Corporation Report SPL TR 66-1,Aug 1966.

[3] B. Widrow, et al., "Adaptive antenna systems," IEEEProc., Vol. 55, No. 12, Dec 1967, pp. 2143-2159.

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451.5

[4] C. A. Baird and G. G. Rassweiler, "Adaptive sidelobenulling using digitally controlled phase-shifters," IEEEAP Trans., Vol 24, No. 5, pp. 638-649, Sep 76.

[5] R.L. Haupt and H.L. Southall, "Experimental adaptivenulling with a genetic algorithm," Microwave Journal,vol. 42, no. 1, Jan 99, pp. 78-89.

[6] P.V. Wright, et.al., "Progress in smart microwavematerials and structures," Smart Mater. Struct. Jun2000, 9, (3), pp. 273-279.

[7] B. Chambers, "Surfaces with adaptive radar reflectioncoefficients," Smart Mater. Struct., Oct 1997, 6, (5), pp.521-529.

[8] S. Forrest, P. Burrows, and M. Thompson, "The dawnof organic electronics," IEEE Spectrum, Vol. 37, No. 8,Aug 2000, pp. 29-34.

[9] S.K. Moore, "Just one word plastics," IEEESpectrum, Vol. 39, No. 9, Sep 2002, pp. 55-59.

[10] S. Lu and B. Panchapakesan, "Photoconductivity insingle wall carbon nanotube sheets," Nanotechnology,Vol. 17, 2006, pp. 1843-1850.

[11] A.E. Fathy, et al., "Silicon-based reconfigurableantennas concepts, analysis, implementation, andfeasibility," IEEE MTT Trans., Vol. 51, No. 6, Jun 03,pp. 1650-1661.

[12]V.J. Logeewaran, et al., "Switching between positiveand negative permeability by photoconductive couplingfor modulation of electromagnetic radiation," Appl.Phys. A 87, 2007, pp. 209-216.

BIOGRAPHY

Randy L. Haupt is an IEEE Fellow andDepartment Head of ComputationalElectromagnetics and Senior Scientist atthe Penn State Applied RwsearchLaboratory. He has a Ph.D. in ElectricalEngineering from the University ofMichigan, MS in Electrical Engineeringfrom Northeastern University, MS inEngineering Management from Western

New England College, and BS in Electrical Engineeringfrom the USAF Academy. He was Professor andDepartment Head of Electrical and Computer Engineeringat Utah State University from 1999-2003. He was aProfessor of Electrical Engineering at the USAF Academyand Professor and Chair of Electrical Engineering at theUniversity ofNevada Reno. In 1997, he retired as a Lt. Col.in the USAF. Dr. Haupt was a project engineer for theOTH-B radar and a research antenna engineer for RomeAir Development Center. He was the Federal Engineer ofthe Year in 1993 and is a member of Tau Beta Pi, EtaKappa Nu, URSI Commission B, and ElectromagneticsAcademy. He served on the board of directors for theApplied Computational Electromagnetics Society and is onthe IEEE Antenna and Propagation Society AdministrativeCommittee. He has many journal articles, conferencepublications, and book chapters on antennas, radar crosssection and numerical methods and is co-author ofthe bookPractical Genetic Algorithms, 2 ed., John Wiley & Sons,2004 and Genetic Algorithms in Electromagnetics, JohnWiley & Sons, 2007. He has eight patents in antennatechnology.

[13]M. Hajian, G.A. de Vree, and L.P. Ligthart,"Electromagnetic analysis of beam-scanning antenna atmillimeter-wave band based on photoconductivityusing Fresnel-zone-plate technique," IEEE AP Mag.,Vol. 45, No. 5, Oct 2003, pp. 13-25.

[14] M.R. Chaharmir, J. Shaker, M. Cuhaci, and A.R.Sebak, "Novel photonically-controlled reflectarrayantenna," IEEE AP Trans., Vol. 54, No. 4, Apr 2006,pp. 1134-1141.

[15]CST Microwave Studio, Version 2006.05, April 19,2006.

[16] R.G. Gordon, "Criteria for choosing transparentconductors," MRS Bulletin, Aug 2000, pp. 52-57.

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