the insertion loss control is governed by the AlGaN barrier thick-ness. Following such a strategy, its is possible to develop nitride-based SAW oscillators with a thermal stability close to ST-quartzcrystal oscillators, but at higher frequencies and with higher elec-tromechanical coupling coefficients.

3. CONCLUSION

Voltage controlled SAW filters have been fabricated on AlGaN/GaN heterostuctures. The interaction between the 2DEG in theheterostructure and the SAWs is responsible for the insertion losscontrol in these filters. An AlGaN/GaN-based SAW delay-lineoscillator with analog control of the loop gain and the outputspectrum has been demonstrated. A temperature coefficient ofdelay of 17.5 ppm/°C has been obtained.

These nitride-based SAW filters can be directly integrated intoMMICs based on AlGaN/GaN high electron mobility transistors.

ACKNOWLEDGMENTS

Authors thank Z. Bougrioua and F. Omnes (CRHEA-CNRS, Val-bonne), and M. Eickhoff (WSI-TUM, Munchen) for providingsamples. This work is partially supported by the Spanish Ministe-rio de Educacion y Ciencia (TEC2005–07010-C02–01/TCM andTEC2004–05698-C02–01/MIC).

REFERENCES

1. O. Ambacher, Growth and applications of group III-nitrides, J Phys D:Appl Phys 31 (1998), 2653–2710.

2. Special issue on wide bandgap semiconductor devices, Proceedings ofthe IEEE, Vol. 90, June 2002.

3. J. Gualtieri, J.A. Kosinski, and A. Ballato, Piezoelectric materials foracoustic wave applications, IEEE Trans Ultrason Ferroelec Freq Con-trol 41 (1994), 53–59.

4. M. Shur, “http://www.ece.rutgers.edu/ ylu/research.html,” tech. rep.5. T. Palacios, F. Calle, E. Monroy, J. Grajal, M. Eickhoff, O. Ambacher,

and F. Omnes, High frequency SAW devices on AlGaN: fabrication,characterization and integration with optoelectronics, in Proceedingsof 2002 IEEE Ultrasonics Symposium, Munich, Germany, pp. 55–58,Oct. 2002.

6. A. Wixforth, Interaction of surface acoustic waves, electrons and light,Int J High Speed Electron Syst 10 (2000).

7. M. Rotter, W. Ruile, G. Scholl, and A. Wixforth, Novel concepts forGaAs�LiNbO3layered systems and their device applications, IEEETrans Ultrasonics Ferroelectrics Frequency Control 47 (2000), 242–248.

8. Y. Lu and N.W. Emanetoglu, Integrated tunable surface acoustic wavewith quantum well structure technology and system provided thereby.United States Patent No. US 6.559.736 B2, 6 May 2003. http://www.ece.rutgers.edu/ ylu/research.html.

9. F. Calle, J. Grajal, and J. Pedros, Active SAW devices on 2DEGheterostructures, Electron Lett 40 (2004), 1384–1386.

10. R.W. Rhea, Oscillator design and compute simulation, Noble Publish-ing, 1995.

11. H. Jeong, S. Kim, Y. Jung, H. Choi, J. Lee, and Y. Lee, Characteristicanalysis of SAW filters fabricated using gan thin films, Phys StatusSolid A 188 (2001), 247–250.

© 2008 Wiley Periodicals, Inc.

NOVEL DIPLEXER USING COMPOSITERIGHT/LEFT-HANDED TRANSMISSIONLINES

Xin Hu1,2 and Sailing He1,2

1 Division of Electromagnetic Theory, School of Electrical Engineering,Royal Institute of Technology, S-100 44, Stockholm, Sweden2 Centre for Optical and Electromagnetic Research, State KeyLaboratory of Modern Optical Instrumentation, Zhejiang University,Hangzhou 310027, China; Corresponding author: sailing@kth.se

Received 18 March 2008

ABSTRACT: A new type of notch filter based on a balanced compositeright/left-handed (CRLH) transmission line is proposed. Combining suchtwo notch filters with a Y-junction divider, a novel diplexer is designedand studied. The diplexer has a wide and flat-top pass-band and verygood isolation between the two outputs. Both theoretical analysis andexperimental results are given and they agree with each other. © 2008Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 2970–2973,2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23851

Key words: diplexer; composite right/left-handed (CRLH) transmissionline; notch filter

1. INTRODUCTION

In recent years left-handed materials (LHMs; also-called negativeindex media) with simultaneous negative permittivity and negativepermeability, which was first investigated by Veselago in 1960s[1], have gained much interest since their experimental realizations[2, 3]. Eleftheriades et al. have proposed the concept of utilizinginductor–capacitor (L–C) loaded planar transmission line (TL)grids to synthesize two-dimensional (2-D) isotropic “left-handed”media exhibiting a negative refractive index [4, 5]. In [6], a moregeneral model of left-handed transmission line metamaterials,composite right/left-handed (CRLH) transmission line, was pro-posed. Various types of applications based on left-handed TLs andCRLH TLs have been proposed, such as some couplers [7, 8],leaky wave antennas [9, 10], and filters [11].

Filters are used in various communication systems. Thus, it isof great interest to develop filters of new type (or low cost). In thisarticle, we propose a new type of notch filter based on CRLHtransmission line. The filter can be very compact and fabricatedvery easily. A diplexer at 0.9 GHz and 1.8 GHz is demonstratedbased on the proposed notch filter structures.

2. THEORY

The homogeneous model of a CRLH lossless TL is shown inFigure 1 [9]. It consists of a series impedance Z constituted byinductor LR in series with a capacitor CL and a shunt admittance Yconstituted by a capacitor CR in parallel with an inductor LL.

The propagation constant of the CRLH transmission line isgiven by

� �1

pcos�1�1 �

ZY

2 � (1)

where the series impedance Z and shunt admittance Y of the unitcell are defined by

Z��� � j�LR �1

j�CL, (2)

2970 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 11, November 2008 DOI 10.1002/mop

Y��� � j�CR �1

j�LL.

For the case of a balanced CRLH transmission line (i.e., LRCL �LLCR) [9] (this is the case considered in this letter), the propagationconstant � is 0 at frequency

�0 � 2�f0 �1

�4 LRCLLLCR

�1

�LLCR

�1

�LRCL

(3)

The characteristic impedance of the balanced CRLH TL is givenby

ZCRLH � �Z

Y� �LL

CL� �LR

CR(4)

Denote the ratio of LL and LR by K (i.e. K � LL/LR). Then we canobtain the following explicit expressions for LL, LR, CL, and CR

Figure 1 The lumped LC unit cell for the CRLH TL

Figure 2 Structure of the proposed notch filter

Figure 3 Simulated transmission response when f0 is set to 900 MHz,ZCRLH � 500 � and K � 10

Figure 4 Simulated transmission response of the proposed notch filter(a) for different values of ZCRLH (f0 is set to 900 MHz and K � 10); (b) fordifferent values of K (f0 is set to 900 MHz and ZCRLH � 200)

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 11, November 2008 2971

LL ��KZCRLH

�0, CL �

�K

ZCRLH�0(5a)

LR �ZCRLH

�0�K, CR �

1

ZCRLH�0�K(5b)

On the basis of such a balanced CRLH transmission line, we havedesigned a novel notch filter using the CRLH transmission line as ashunt branch between the input port (Port 1) and output port (Port 2),as shown in Figure 2. The characteristic impedance of the CRLH TLis ZCRLH, and the propagation constant is �, which equals 0 atfrequency f0. Hence, the input impedance of the shunt branch is

Zin � jZCRLHtan�n�p� (6)

where n is the number of the unit cells in the CRLH transmissionline (we use 3 unit cells below in all our simulation and experi-ments).

As Zin is 0 at frequency f0, the reflectance will become 1 and wecan achieve a notch filter performance at f0 (thus called notchfrequency hereafter). Here we set ratio K � 10, impedance ZCRLH

� 200 �, and frequency f0 � 900 MHz, and use the Agilent ADSsoftware to do the simulation. The simulated S21 parameter of thefilter is shown in Figure 3.

As we can see from Eq. (6) that Zin is proportional to theZCRLH, we can use a larger ZCRLH to achieve a narrower stopband.Figure 4(a) shows the bandwidth of the stopband, which becomesnarrower (as expected; however, the magnitude decreases mean-while) as ZCRLH increases.

If we fix the characteristic impedance and change the ratio ofLL and LR (i.e. K), we will see different frequency responses of thenotch filter in Figure 4 (b). The response will deteriorate forsmaller K, as some undesired pass bands appear.

Combining two proposed notch filters (at 2 different notchfrequencies) with a conventional Y-junction divider, we can con-struct a novel diplexer. The schematic diagram of the device isshown in Figure 5(a). The Y-junction between the input and outputports is used to achieve high isolation between the two outputports. The two branches of the Y-junction have the same charac-teristic impedances Z0 but different electrical lengths (with onebeing �/4 at f1 and the other �/4 at f2). We choose frequencies f1� 900 MHz and f2 � 1.8 GHz as the operating (central) frequen-cies of the diplexer, with K � 10 and ZCRLH � 200 �, and thecorresponding simulation results for S-parameters are shown inFigure 5(b). From Figure 5(a) one sees that the isolation is verygood and the insertion loss is very small (virtually zero withoutconsidering the material loss) at each operating frequency and thepass-band is relatively wide.

This design utilizes stopband performance of the filters, whileconventional diplexers are usually based on bandpass filters. Thediplexer structure shown in Figure 6(a) was constructed on a FR4substrate with dielectric constant �r � 4.4 and height h � 1.6 mm.The transmission lines in the Y-junction divider are designed tohave a characteristic impedance of 50 �. The values of the

Figure 5 (a) Schematic diagram of the novel diplexer and (b) thesimulated S-parameters with f1 � 900 MHz and f2 � 1.8 GHz

Figure 6 (a) The fabricated device of the proposed diplexer. (b) Themeasured S parameters of the fabricated diplexer

2972 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 11, November 2008 DOI 10.1002/mop

capacitors and inductors used in two different notch filters arelisted in Table 1.

All lab measurements were carried out on a HP8753C seriesvector network analyzer. The experimental results are shown in Fig-ure 6(b), which agrees with the simulation results in Figure 5(b).

3. CONCLUSION

In this article, we have proposed a new type of notch filter based ona shunt branch consisting of a balanced CRLH transmission line. Thefilter shows a very sharp stopband performance. On the basis of suchnotch filters, a novel diplexer has also been demonstrated. The di-plexer is compact, and has a relatively wide and flat-top pass-bandwith very good isolation. Unlike conventional diplexers (which areusually based on bandpass filters), this present diplexer utilizes stop-band performance of the filters. The new filter and diplexer structuresmay have potential applications in e.g. microwave circuits.

ACKNOWLEDGMENTS

The work was supported by the Swedish Research Council (VR)under Project No. 2006–4048 and the National Basic ResearchProgram (973) of China (NO.2004CB719802).

REFERENCES

1. V.G. Veselago, The electrodynamics of substances with simulta-neously negative values of � and �, Sov Phys Usp 10 (1968), 509–514.

2. D.R. Smith, W.J. Padilla, D.C. Vier, et al. Composite medium withsimultaneously negative permeability and permittivity, Phys Rev Lett84 (2000), 4184–4187.

3. R.A. Shelby, D.R. Smith, and S. Schultz, Experimental verification ofa negative index of refraction, Science 292 (2001), 77–79.

4. A.K. Iyer and G.V. Eleftheriades, Negative-refractive- index metama-terials supporting 2-D waves, In IEEE International Microwave Sym-posium Digest, Seattle, WA, Jun. 2–7, 2002, pp. 1067–1070.

5. G.V. Eleftheriades, A.K. Iyer, and P.C. Kremer, Planar negative-refractive-index media using periodically loaded transmission lines,IEEE Trans Microwave Theory Tech 50 (2002), 2702–2712.

6. A. Lai, C. Caloz, and T.Itoh, Composite right/left-handed transmissionline metamaterials, IEEE Macrowave Mag 9 (2004), 34–50.

7. I. Lin, C. Caloz, and T. Itoh, A branch-line coupler with two arbitraryoperating frequencies using left-handed transmission lines, IEEE-MTTInternational Symposium Digest Philadelphia, PA, Vol. 1, 2003,pp.325–327.

8. C. Caloz and T. Itoh, A novel mixed conventional microstrip andcomposite right/left-handed backward wave directional coupler withbroadband and tight coupling characteristics, IEEE Microwave Wire-less Compon Lett 14 (2004), 31–33.

9. L. Liu, C. Caloz, and T. Itoh, Dominant mode (DM) leaky-waveantenna with backfire-to-endfire scanning capability, Electron Lett 38(2000), 1414–1416.

10. S. Lim, C. Caloz, and T. Itoh, Metamaterial-based electronically-controlled transmission line structure as a novel leaky-wave antennawith tuneable angle and beamwidth, IEEE Trans Microwave TheoryTech 53 (2005), 161–173.

11. Horii, Y. Caloz, and C. Itoh, T, Super-compact multilayered left-handed transmission line and diplexer application, IEEE Trans Micro-wave Theory Tech 53 (2005), 1527–1534.

© 2008 Wiley Periodicals, Inc.

A COMPACT BROADBANDCIRCULARLY POLARIZED COMPOSITEANTENNA WITH WIDE BEAMWIDTH

Houjun Sun Lei Shi and Jingtao LiDepartment of Electronic Engineering, Beijing Institute of Technology,5 South Zhongguancun Street, Haidian District, Beijing, People’sRepublic of China; Corresponding author: forrest1981@gmail.com

Received 24 March 2008

ABSTRACT: A compact composite antenna applied to the satellite mov-able termination is presented in this article. The antenna unit is composedof a monofilar conical helix and a radial omnidirectional waveguide an-tenna with eight vertical metal rods. From the viewpoint of widening thecircularly polarized (CP) beamwidth in vertical plane and frequency band-width, these components are unified as a single composite antenna. Boththe simulated and measured results show that the broad CP beamwidth canbe obtained over the frequency range from 2.3 to 3.3 GHz. © 2008 WileyPeriodicals, Inc. Microwave Opt Technol Lett 50: 2973–2975, 2008;Published online in Wiley InterScience (www.interscience.wiley.com).DOI 10.1002/mop.23857

Key words: composite antenna; compact; circularly polarized; widebeamwidth

1. INTRODUCTION

The requirement for the circularly polarized (CP) antenna hasincreased with the growing demands of the telemeter, countermea-sures, and communication in the aircraft and satellite applications.A CP wave can be obtained using a planar antenna, such as a spiralor a microstrip patch. When the installation space is limited, a rodtype CP antenna is preferable, that is, a helix. It is well-known thatthe helix antenna has good performance in radiating CP wave [1,2]. However, the operating frequency bandwidth is not wideenough.

Recent communication systems often require a wide CP beam-width over a wide frequency band. The monofilar conical helix[3–6] with a decaying current distribution satisfies the require-ment. However, it has the disadvantage that the structure is notcompact in size. The CP beamwidth and the frequency bandwidthare also narrow.

This article presents a compact composite antenna, whichachieves wide bandwidth and radiates wide CP beamwidth. Theantenna consists of a monofilar conical helix and an omnidirec-tional waveguide antenna with eight vertical metal rods. The beamof the helix antenna covers axial direction while the beam of thewaveguide antenna covers horizontal azimuth. The design takesadvantage of the two antennas which have different radiationmechanism. As a result, the CP radiation beamwidth can beimproved. Moreover, the structure is compact by utilizing theuniform coaxial probe which is quite different from [7]. Finally,the return loss, radiation pattern, axial ratio, and gain of theantenna are studied. Reasonable agreement between simulationand experiment is obtained.

2. ANTENNA CONFIGURATION

Figure 1 shows the proposed composite antenna configuration. Thediameter Da of the antenna is 38 mm It consists of a monofilarconical helix antenna with height Hhe � 6.5 mm and a radialwaveguide with height Hw � 6.45 mm and diameter Dw � 30 mm.The apex angle of the cone helix is 20 � 150°. The helix antennais mounted through the centre-hole of the radial waveguide. Theradial waveguide is also excited by the coaxial probe. A circular

TABLE 1 Values of Capacitors and Inductors

LL CL LR CR

f1 � 900 MHz 31.3 nH 9.5 pF 3.3 nH 1 pFf2 � 1.8 GHz 15.6 nH 7.8 pF 1 nH 0.5 pF

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 11, November 2008 2973