4. CONCLUSION

In this theoretical study, we have considered how materialswith real negative parameters can be physically realized inwide frequency bands. The use of conventional compositematerials does not give such a possibility because of thecausality requirements. One possible solution can be foundusing active inclusions in a particulate composite, forming ameta-material. It is seen that simple impedance invertercircuits are sufficient to create appropriate loads for smallmetal inclusions forming the composites. It is well known thatsuch circuits are easy to realize, and they are stable in widefrequency bands.

To realize a thin sheet of a negative material, there is noneed to fabricate a three-dimensional mixture of active‘‘molecules’’: one can use a planar array or conducting wiresloaded by similar active circuits. Such realizations can beused in experimental investigations of these exotic media,especially in the low-frequency end of the microwave fre-quency band.

As is obvious from the energy density formula for nondis-1 2 2Ž � � � � .persive isotropic media, W � � E � � H , it is not pos-2

sible to realize a medium with negative parameters since theenergy density is then negative. This means that any realiza-

Žtion will be dispersive and lossy so the parameters are.actually complex numbers . This restriction does not hold in

media with active inclusions because there are local powersources present. Here, we have shown that a possible realiza-tion needs only simple and well-known impedance invertercircuits. With the help of these devices, thin artificial layerswith the properties of negative media can be possibly real-ized, with no fundamental restrictions on the bandwidth andloss factor.

REFERENCES

1. D.R. Smith and N. Kroll, Negative refractive index in left-handedŽ .materials, Phys Rev Lett 85 2000 , 2933�2936.

2. J.B. Pendry, Negative refraction makes a perfect lens, Phys RevŽ .Lett 85 2000 , 3966�3939.

3. V.G. Veselago, The electrodynamics of substances with simulta-Ž .neously negative values of � and �, Sov Phys Usp 10 1968 ,

Ž509�514 originally published in Russian in Usp Fiz Nauk 92Ž . .1967 , 517�526 .

4. R.A. Shelby, D.R. Smith, and S. Schultz, Experimental verifica-Ž .tion of a negative index of refraction, Science 292 2001 , 77�79.

5. I.V. Lindell, S.A. Tretyakov, K.I. Nikoskinen, and S. Ilvonen, BWmedia�Media with negative parameters, capable of supportingbackward waves, Electromag Lab, Helsinki University of Tech-

Žnology, rep 366, May 2001; Microwave Opt Technol Lett sub-.mitted .

6. F. Auzanneau and R.W. Ziolkowski, Theoretical study of syn-thetic bianisotropic materials, J Electromag Waves Appl 12Ž .1998 , 353�370.

7. R.W. Ziolkowski, The design of Maxwellian absorbers for numer-ical boundary conditions and for practical applications usingengineered artificial materials, IEEE Trans Antennas Propagat

Ž .45 1997 , 656�671.8. S.A. Tretyakov and T.G. Kharina, The perfectly matched layer as

Ž .a synthetic material with active inclusions, Electromag 20 2000 ,155�166.

9. S.A. Tretyakov, F. Mariotte, C.R. Simovski, T.G. Kharina, andJ.-P. Heliot, Analytical antenna model for chiral scatterers: Com-parison with numerical and experimental data, IEEE Trans An-

Ž .tennas Propagat 44 1996 , 1006�1014.10. A.B. Williams, Electronic filter design handbook, McGraw-Hill,

New York, 1981.

11. I.V. Lindell, Methods for electromagnetic field analysis, Claren-don Oxford, England, 1992, pp. 90�92.

12. M.I. Kontorovich, M.I. Astrakhan, V.P. Akimov, and G.A. Fers-man, Electrodynamics of grid structures, Radio i Svyaz, Moscow,

Ž .1987 in Russian ; M.I. Kontorovich, V.Yu. Petrunkin, N.A.Esepkina, and M.I. Astrakhan, Reflection factor of a planeelectromagnetic wave reflecting from a plane wire grid, Radio

Ž .Eng Electron Phys 1962 , 222�231.13. V.V. Yatsenko, S.A. Tretyakov, S.I. Maslovski, and A.A. Sochava,

Higher order impedance boundary conditions for sparse wireŽ .grids, IEEE Trans Antennas Propagat 48 2000 , 720�727.

� 2001 John Wiley & Sons, Inc.

PATCH ANTENNA WITH SWITCHABLE( )SLOT PASS : DUAL-FREQUENCY

OPERATIONFan Yang1 and Yahya Rahmat-Samii11 Department of Electrical EngineeringUniversity of California at Los AngelesLos Angeles, California 90095-1594

Recei�ed 16 May 2001

ABSTRACT: A no�el kind of microstrip antenna, a patch antenna with( )a switchable slot PASS , is proposed in this paper. A slot is incorporated

into the patch, and a p-i-n diode is utilized to switch the slot on or off.This antenna can work at dual frequencies with a small and flexiblefrequency ratio and the same polarization. Experimental results �alidatethis concept. � 2001 John Wiley & Sons, Inc. Microwave Opt Tech-nol Lett 31: 165�168, 2001.

Key words: patch antenna; slot; switch; diode; dual frequency

1. INTRODUCTION

Microstrip antennas are widely used in wireless communica-tions due to their advantages of low profile, low weight, lowproduction cost, and conformability with RF circuitry. Dual-frequency operation is an important topic in the applicationof microstrip antennas. Typically, there are two main ap-proaches to solve this problem: one is to use stacked patches� �1, 2 , and the other is to activate different modes of the

� �patch 3 . The first approach incorporates a multilayeredpatch substrate that will resonate at different frequencies. Adisadvantage of this method is the increased height of theantenna. The second approach achieves dual-frequency oper-ation by activating two modes under the patch, such as theTM and TM modes or the TM and TM modes.10 30 10 01However, this approach has many limitations. For example,when the TM and TM modes are activated, the frequency10 30ratio is very large, approximately about 3. When the TM10and TM modes are activated, the two frequencies will have01different polarizations.

In this paper, we propose a novel method to realizedual-frequency operation that is a patch antenna with a

Contract grant sponsor: ARMY�MURIContract grant number: 442522-22995Contract grant sponsor: JPLContract grant number: 442520-79278

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 31, No. 3, November 5 2001 165

W

L

Ls

Ps Ws

(Xf,Yf)

x

yh

εr

Figure 1 Geometry of a dual-frequency patch antenna with aŽ .switchable slot PASS

Ž .switchable slot PASS . A slot is incorporated into the patch,� �and is perpendicular to the currents on the patch 4 . A p-i-n

diode is then positioned in the center of the slot to change� �the configuration of the slot 5, 6 . Under different modes of

the diode, the antenna will work at different frequencies.This kind of antenna is very simple considering that it hasonly one patch and a single feed. It is also worthwhile topoint out that the frequency ratio of this structure is smalland very flexible. Another attractive feature is that the an-tenna has the same polarizations and similar patterns at twofrequencies. The validity of this concept is demonstrated by

Ž .both finite-difference time-domain FDTD simulation andexperimental results. The fabricated antenna resonates at4.82 and 4.38 GHz, which has a frequency ratio of 1.10. Theradiation patterns at these two frequencies are also pre-sented, and they are very similar to each other.

2. ANTENNA STRUCTURE AND DESIGN

The antenna structure is shown in Figure 1. The patch size isL � W, and it is built on dielectric substrate with permittivity

Ž .� and thickness h. A probe located at X , Y is used tor f factivate the TM mode. A slot with length L , width W , and10 s sposition P is incorporated into the patch, and a p-i-n diodesis inserted in the center of the slot to control its status. Whenthe diode is in the OFF mode, the currents on the patch have

Ž .to flow around the slot, as shown in Figure 2 a , resulting in along current path length. Therefore, the antenna resonates ata low frequency. On the other hand, when the diode is in theON mode, the currents can go directly through the diode, as

Ž .shown in Figure 2 b . In this case, the length of the current

(a) (b)

Ž .Figure 2 Currents on the patch antenna. a Switch OFF; thecurrent path has a long length so that the antenna resonates at a

Ž .lower frequency. b Switch ON; the current path has a short lengthso that the antenna resonates at a higher frequency

4 4.5 5 5.530

25

20

15

10

5

0

Freq (GHz)

S11

(dB

)

Initial patchdiode OFF diode ON

Figure 3 Simulated S results of PASS under diode ON�OFF11modes, compared to a patch antenna without a slot. The frequencyratio of the two modes is 1.10

path is smaller so that the antenna has a higher resonantfrequency.

Ž .The finite-difference time-domain FDTD method is ap-plied to analyze the antenna properties. When the diode is inthe OFF mode, it is removed from simulation, and when thediode is in the ON mode, it is modeled as a metal tape. Theantenna is mounted on a finite substrate of 50 mm � 50 mmsize, 3 mm thickness, and � � 2.20. The patch parametersr

Ž .are listed below unit: millimeters :

Ž . Ž .L � W � 18, X , Y � 5, 9f f

L � 16, W � 1, P � 13.s s s

The initial patch without a slot works at 4.85 GHz, as shownin Figure 3. When the slot is incorporated into the patch andthe diode is turned off, the antenna resonates at 4.40 GHz.Once the diode is turned on, the antenna frequency shifts to4.75 GHz. In this way, one can control the antenna frequencyby turning the diode on or off. The effects of the slot lengthand slot position are studied, and are depicted in Figures 4and 5. It can be noticed from Figure 4 that the resonantfrequency will decrease when the slot length increases. Inall of these cases, the antennas have better matches than

0 5 10 154.3

4.4

4.5

4.6

4.7

4.8

4.9

Slot length (mm)

Fre

q. (

GH

z)

diode OFFdiode ON

ŽFigure 4 PASS resonant frequencies of both modes FDTD simu-.lated decrease with slot length increase

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 31, No. 3, November 5 2001166

0 5 10 154.3

4.4

4.5

4.6

4.7

4.8

4.9

Slot position (mm)

Fre

q. (

GH

z)

diode OFFdiode ON

ŽFigure 5 PASS resonant frequencies of both modes FDTD simu-.lated change with different slot positions

�10 dB. Another important feature in this figure is that theslot length effect on the diode OFF mode is more significantthan on the diode ON mode. According to this feature, onecan choose the appropriate slot length to obtain a suitablefrequency ratio, which means that the frequency ratio in thiskind of antenna is very flexible. As revealed in Figure 5, theslot position also has some effects on the antenna frequen-cies. The resonant frequency is the lowest when the slotposition is at 9 mm. In this case, the slot is located in thecenter of the patch where the currents are maximum, andthus the slot effect is the most significant. The slot positionalso affects the antenna match such that, as the slot getscloser to the feed probe, the match becomes worse due to thecoupling of the slot and probe.

3. EXPERIMENTAL RESULTS

Some experiments have been carried out to demonstrate thisPASS concept. Four antennas were built for comparison, andtheir parameters are the same as described in the previoussection, except that the thickness of the substrate RT�Duroid

Ž .5880 � � 2.20 is 3.18 mm. Figure 6 shows photos of theserŽ .four antennas: Figure 6 a is an initial patch antenna, Figure

Ž . Ž .6 b is a patch antenna with a switchable slot PASS , Figure

(a) (b)

(c) (d)

(2)

(1)

Ž . Ž . ŽFigure 6 Photos of four patch antennas. a Initial patch antenna. b PASS 1: p-i-n diode, 2: shorted quarter-wavelength strip for dc. Ž . Ž .bias . c Reference patch antenna for switch OFF mode. d Reference patch antenna for switch ON mode

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 31, No. 3, November 5 2001 167

4 4.5 5 5.530

25

20

15

10

5

0

Freq (GHz)

S11

(dB

)

Ant. ab, OFFb, ON Ant. cAnt. d

Figure 7 Measured S results of four patch antennas shown in11Figure 6. A 1.10 frequency ratio is achieved, and the PASS showssimilar resonant frequencies to reference patch antennas

Ž .6 c is a reference patch antenna for the switch OFF mode,Ž .and Figure 6 d is a reference patch antenna for the switch

ON mode. The last two are the same as the FDTD models. InŽ . Ž .Figure 6 b , a p-i-n diode HPND-4005 is inserted in the

center of the slot. To isolate the dc, the slot is cut the fullwidth of the patch, and two capacitors are soldered on theedges of the slot for RF continuity. A shorted quarter-wave-length strip is connected to the right edge of the patch forgrounded dc, while not affecting the RF. A 2 V voltage issupplied from the coax probe.

As predicted by the FDTD simulation, the dual-frequencyperformance can be observed in Figure 7. The PASS res-onates at 4.38 GHz when the p-i-n diode is in the OFF mode,and resonates at 4.82 GHz when the p-i-n diode is turnedON. The input match is better than �10 dB at the tworesonant frequencies, and the frequency ratio is only 1.10.The PASS results display similar resonant frequencies as thereference antennas c and d, which proves the validity of theFDTD model.

There are some differences, which may be a result of theresistance and capacitance of the p-i-n diode and two capaci-tors. The radiation patterns are measured and displayed inFigure 8. As revealed by the graph, the PASS with the diodesON and OFF have very similar patterns at two frequencies,and they are very close to the patterns of the patch antennawithout the slot. It is also noticed that, when the diode isturned ON, the antenna gain is about 1 dB lower than theother two cases, which may be attributed to the diode loss.

4. CONCLUSION

Ž .A novel patch antenna with switchable slot PASS is pre-sented in this paper. The antenna has a single patch, singlefeed, and operates at dual frequencies, with a low frequencyratio of 1.10. The two frequencies display the same polariza-tions and very similar radiation patterns. This antenna designis very promising for wireless communications, and can befurther applied to dual-band circularly polarized antennas,frequency-sweeping antennas, and antennas with switchableright-hand and left-hand circular polarizations.

ACKNOWLEDGMENT

The authors would like to thank Mr. James Sor for fabricat-ing the antenna with a p-i-n diode.

150 100 50 0 50 100 15030

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tern

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150 100 50 0 50 100 15030

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10

Angle (Degree)

Pat

tern

(dB

)

No slot diode OFFdiode ON

(a)

(b)

Ž .Figure 8 Comparison of measured radiation patterns. a E-planeŽ . Ž . Ž .xz-plane . b H-plane yz-plane

REFERENCES

1. J.S. Dahele, K.F. Lee, and D.P. Wong, Dual frequency stackedannular-ring microstrip antenna, IEEE Trans Antennas Propagat

Ž .AP-35 1987 , 1281�1285.2. F. Croq and D.M. Pozar, Multifrequency operation on microstrip

antennas using aperture coupled parallel resonators, IEEE TransŽ .Antennas Propagat 40 1992 , 1367�1374.

3. S. Maci, G.B. Gentili, and G. Avitabile, Single-layer dual fre-Ž .quency patch antenna, Electron Lett 29 1993 , 1441�1443.

4. X.-X. Zhang and F. Yang, The study of slit cut on the microstripantenna and its applications, Microwave Opt Technol Lett 18Ž .1998 , 297�300.

5. C. Luxey, L. Dussopt, J.L. Sonn, and J.M. Laheurte, Dual fre-quency operation of a CPW-fed antenna controlled by PIN diodes,

Ž .Electron Lett 36 2000 , 2�3.6. K.L. Virga and Y. Rahmat-Samii, Low profile enhanced-band-

width PIFA antennas for wireless communications packaging,Ž .IEEE Trans Microwave Theory Tech 45 1997 , 1879�1888.

� 2001 John Wiley & Sons, Inc.

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 31, No. 3, November 5 2001168