PIERS ONLINE, VOL. 3, NO. 5, 2007 603

A New Type of Microstrip Coupler with ComplementarySplit-Ring Resonator (CSRR)

K. Y. Liu, C. Li, and F. LiInstitute of Electronics, Chinese Academy of Science, China

Abstract— A new type of microstrip Coupler with Complementary Split-Ring Resonator(CSRR) is proposed. The characteristic impedance of the even mode can be enhanced by etchingCSRR structure in the grounded plane of a conventional microstrip double-line coupler. Thisresults in a higher degree of coupling. Based on the analysis of the odd and even modes, a newtype of microstrip coupler was designed. The results of simulation and measurement show thatthe new coupler achieves a high degree of coupling (3 dB coupling) over a wide frequency band(38.1% relative bandwidth).

DOI: 10.2529/PIERS060906085815

1. INTRODUCTION

A well-known problem of the conventional microstrip parallel-coupled couplers is their difficulty toachieve tight backward coupling because of the excessive small lines-gap required [1]. Alternativecomponents include non-coupler-line couplers such as the branch-line or rat-race, however, thesecouplers are inherently narrowband (no more than 15% bandwidth) circuit. The Lange coupler [2]is a solution widely used in the MMIC industry for broadband 3-dB coupling, but it has thedisadvantage of requiring bonding wires. In the past few years, there has been a great interestin the emerging field of meta-materials and more specially left-handed (LH) structures. Couplersbased on LH transmission lines (TL) was proposed by some authors [3, 4]. This couplers, whichcomposed of one or two LH-TL constituted of series interdigital capacitors and shunt shorted-stub inductors, can provide arbitrary loose/tight coupling levels. But designing this coupler is toocomplicated.

Very recently, Complementary Split-Ring Resonator (CSRR), which is the negative image ofSplit-Ring Resonators (SRR) [5] (see Fig. 1), have been reported by some authors [6]. It has beendemonstrated that CSRR etched in the ground plane or in the conductor strip of planar transmissionmedia (microstrip or CPW) provide a negative effective permittivity to the structure. CSRR hasbeen successfully applied to the narrow band filters and diplexer with compact dimensions [7, 8].

(a) (b)

Figure 1: (a) Topology of the SRR, (b) Topology of CSRR (Metal regions are depicted in gray).

In this paper, a novel coupled-line backward coupler based on CSRR is presented. The charac-teristic impedance of the even mode can be enhanced by etching CSRR structure in the groundedplane of a conventional microstrip double-line coupler. This results in a higher degree of coupling.The results of simulation and measurement show that the new coupler achieves a high degree ofcoupling (3 dB coupling) over a wide frequency band (38.1% relative bandwidth).

PIERS ONLINE, VOL. 3, NO. 5, 2007 604

2. ODD AND EVEN MODES ANALYSIS

The electric fluxline of coupling microstrip of odd mode and even mode are showed in Fig. 2. Inodd mode, the E-field is asymmetric and continuous even in the presence of CSRR slot. Signal doesnot slow down just as the one without CSRR. In even mode, the E-field is discontinuous along themiddle line of coupler. The CSRR act as open circuit for the even mode.

(a) (b)

Figure 2: Electric fluxline of coupling microstrip. (a) odd mode, (b) even mode.

The odd and even modes S-parameters of the coupler of Fig. 3 were computed by full-wavesimulation, and are showed in Fig. 4. Table 1 shows the odd and even characteristic impedances

(a) (b)

Figure 3: (a) Structure of conventional directional coupler, (b) Structure of CSRR-based directional coupler.

Frequency (GHz)Frequency (GHz)

(a) (b)

Figure 4: Magnitude of the S-parameters for the two structures showed in Fig. 3 obtained by simulation.(a) odd mode, (b) even mode.

PIERS ONLINE, VOL. 3, NO. 5, 2007 605

Table 1: Odd and even impedance for the two structures showed in Fig. 3 at the central frequency (2.1GHz).

Zco Zce

Conventional coupler 38.7Ω 69.6 Ω

CSRR coupler 38.2 Ω 240Ω

at the frequency of 2.1 GHz computed from the odd and even S-parameters, using the generalformula

Zci = Z0

√(Πi − 1) / (Πi + 1), (i = e, o) (1)

withΠi =

(S2

21i − S211i − 1

)/ (2S11i) .

It is demonstrated that the characteristic impedance of the even mode can be enhanced by CSRRstructure. This results in a higher degree of coupling.

3. DESIGN OF CSRR-BASED 3DB DIRECTIONAL COUPLER

The structure of CSRR-based directional coupler is showed in Fig. 5. The parameter of substrate isεr = 2.65, h = 1.5mm. The characteristic impedance at the ports is set to Z0 = 50 Ω. The resultsobtained by full-wave simulation and measurement are showed in Fig. 6. The performances of the3-dB coupler are the following: 3.7 ± 0.5 dB backward coupling, return loss smaller than −18 dBand isolation better than 25 dB over the 1.7 to 2.5 GHz range (38.1% fractional bandwidth).

Figure 5: Structure of CSRR-based directional coupler.

(a) (b)

Figure 6: (a) Magnitude of the S-parameters for the coupler of Fig. 4 obtained bysimulation (Ansoft-Design)(b) Magnitude of the S-parameters for the coupler of Fig. 4 obtained by measurement.

PIERS ONLINE, VOL. 3, NO. 5, 2007 606

ACKNOWLEDGMENT

This work was supported by the National Natural Science Foundation of China under GrantNo. (60501018, 60271027), and the National Basic Research Program of China under Grant No.2004CB719800.

REFERENCES

1. Mongia, R., I. Bahl, and P. Bhartia, RF and Microwave Coupler-line Circuits, Artech House,Norwood, MA, 1999.

2. Lange, J., “Interdigital stipline quadrature hybrid,” IEEE Trans. Microwave Theory and Tech.,Vol. MTT-26, 1150–1151, 1969.

3. Caloz, C. and T. Itoh, “A novel mixed conventional microstrip and composite right/left-handedbackward-wave directional coupler with broadband and tight coupling characteristics,” IEEEMicro. Wireless Comp. Lett., Vol. 14, No. 1, 31–33, 2004.

4. Caloz, C., A. Sanada, and T. Itoh, “A novel composite right/left-handed coupled-line di-rectional coupler with arbitrary coupling level and broad bandwidth,” IEEE Trans. Microw.Theory and Tech., Vol. 52, 980–992, 2004.

5. Pendry, J. B., A. J. Holden, D. J. Robbins, and W. J. Stewart, “Magnetism from conductorsand enhanced nonlinear phenomena,” IEEE Trans. Microw. Theory Tech., Vol. 47, No. 11,2075–2084, 1999.

6. Falcone, F., T. Lopetegi, J. D. Baena, R. Marques, F. Martin, and M. Sorolla, “Effectivenegative-ε stop-band microstrip lines based on complementary split ring resonators,” IEEEMicrow. Wireless Compon. Lett., Vol. 14, No. 6, 280–282, 2004.

7. Bonache, J., I. Gil, J. Garcia-Garcia, et al., “Novel Microstrip Bandpass filters based on Com-plementary Split-Ring resonators,” IEEE Trans. Microw. Theory Tech., Vol. 54, No. 1, 265–271, 2006.

8. Bonache, J., I. Gil, J. Garcia-Garcia, et al., “Complementary split ring resonators for microstripdiplexer design,” ELECTRONICS LETTERS, Vol. 41, No. 14, 2005.