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IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 15, NO. 8, AUGUST 2005 521

A Novel Rat Race Coupler Designfor Dual-Band Applications

Kwok-Keung M. Cheng, Senior Member, IEEE,and Fai-Leung Wong, Student Member, IEEE

AbstractThis letter presents a novel and compact rat-racecoupler design that can operate at two widely separated frequencybands. It is accomplished by modifying the length and impedanceof the branch lines in the conventional structure, and the in-sertion of two additional shunt stubs. The proposed circuit alsofeatures planar structure and low insertion loss. For verification,the measured performance of a microstrip coupler operating at900/2000 MHz are shown.

Index TermsCouplers, dual band, microwave circuits.

I. INTRODUCTION

RAT-RACE coupler is one of the passive circuits that are

widely used in microwave and millimeter-wave applica-

tions. This device offers both in-phase and anti-phase relation-

ship at the designated outputs with equal power splitting, as well

as perfect isolation between the two input ports. Rat-race cou-

pler is widely employed in the design of balanced mixers for

obtaining good spurious signal rejection. However, due to the

inherent narrow-band nature of the traditional design, its appli-

cation to wide-band and multiband systems is thus limited. In

the past years, research was mainly focused on bandwidth ex-

pansion [1][3] and size reduction [4], [5] techniques.

In this work, a novel, planar, rat-race coupler design suitable

for dual-band operation is presented with explicit design for-

mulas. The proposed circuit also features compact layout and

low insertion loss performance. For demonstration, the experi-

mental results of a 900/2000-MHz rat race coupler implemented

using microstrip is included.

II. PROPOSEDDUAL-BANDCOUPLERDESIGN

Fig. 1 shows the basic configuration of the proposed dual-

band coupler, where and are the centre frequencies of the

lower and upper bands. In compared to the conventional design,

the major differences are the electrical length of the longer arm(equals to 450 evaluated at the average value of and ); the

branch-line impedance values (not 70.7 ); and the inclusion of

two additional shunt stubs with short-circuit termination.

Manuscript received February 8, 2005; revised March 30, 2005. The reviewof this letter was arranged by Associate Editor J.-G. Ma.

The authors are with the Department of Electronic Engineering,The Chinese University of Hong Kong, Shatin, Hong Kong (e-mail:kkcheng@ee.cuhk.edu.hk).

Digital Object Identifier 10.1109/LMWC.2005.852792

Fig. 1. Proposed dual-band coupler structure.

For the derivation of the branch-line impedance values, the

even- and odd-mode analysis (Fig. 2) is applied in conjunc-

tion with the input-output relationship (magnitude and phase)

of race-race coupler

(1)

(2)

(3)

(4)

(5)

Subsequently, a general design requirement for the proposed

configuration may there be formulated as

(6)

(7)

(8)

(9)

Note that , , and are the electrical lengths of

the corresponding branch-lines evaluated at the two operating

frequencies ( & ), where

(11)

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522 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 15, NO. 8, AUGUST 2005

Fig. 2. Even- and odd-mode topologies.

Fig. 3. Variation of line impedance versus frequency ratio.

From the above equations, one can deduce that a possible solu-tion for dual-band operation is simply given by

(12)

(13)

(14)

(15)

(16)

(17)

(18)

Fig. 3 shows the computed impedance variation as a func-

tion of the frequency ratio . However, due to the prac-

tical limit of impedance realization (30100 typically), these

curves suggested that the operating frequency of the proposed

coupler is in the range of (the theoret-

ical limit is ).

III. MEASUREMENTRESULTS

For verification, a dual-band micro-strip rat-race coupler op-erating at 900 and 2000 MHz was designed by (15) (18) and

Fig. 4. Measured return loss and port isolation.

Fig. 5. Measured insertion loss (in-phase outputs).

Fig. 6. Measured phase response (in-phase outputs).

Fig. 7. Measured insertion loss (anti-phase outputs).

validated using a circuit simulator. The optimum values of ,

, and were found to be approximately 44 , 52 ,

52 and 40 , respectively.

For experimental demonstration, a prototype (Fig. 9) was

constructed using Duriod substrate with dielectric constant of

3.38 and thickness of 0.8 mm. Scattering parameter measure-

ments were performed using HP8510C network analyzer overthe frequency range from 0.1 to 4 GHz. Fig. 48 shows the

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CHENG AND WONG: NOVEL RAT RACE COUPLER DESIGN 523

Fig. 8. Measured phase response (anti-phase outputs).

Fig. 9. Fabricated dual-band rat race coupler.

measured performance of the coupler in which the center fre-

quencies of the two operating bands were found to be slightly

shifted to 880 MHz and 1980 MHz.

Table I gives the overall performance of the coupler measured

at the two center frequencies. Return loss and port isolation of

better than 28 dB were observed. Moreover, the insertion loss

TABLE IMEASUREDPERFORMANCE OFRAT-RACECOUPLER

(including connector losses) was found to be at most 0.7 dB

higher than the loss-less case. The operating bandwidth of the

coupler is governed by the amplitude and phase mismatch re-

quirement to around 50 MHz.

IV. CONCLUSION

A new, compact, planar, rat race coupler design for dual-band

operation has been presented and experimentally verified.A prototype implemented using microstrip has been con-

structed and characterized with excellent measurement results.

Dual-band coupler design with much wider bandwidth and

band separation is under development.

REFERENCES

[1] D. Kim and Y. Naito,Broad-band design of improved hybrid-ring 3 dBdirectional coupler,IEEE Trans. Microwave Theory Tech., vol. 82, no.11, pp. 20402046, Nov. 1982.

[2] C.-H. Ho, L. Fan, and K. Chang,Broad-band uniplanar hybrid-ring andbranch-line couplers,IEEE Trans. Microwave Theory Tech., vol.41, no.12, pp. 21162125, Dec. 1993.

[3] H. Okabe, C. Caloz, and T. Itoh,A compact enhanced-bandwidth hy-brid ring using a left-handed transmission line section,in IEEE MTT-SInt. Symp. Dig., Jun. 2003, pp. 329332.

[4] T. Hirota, A. Minakawa, and M. Muraguchi,Reduced-size branch-lineand rat-race hybrids for uniplanar MMICs, IEEE Trans. MicrowaveTheory Tech., vol. 38, no. 3, pp. 270275, Mar. 1990.

[5] R. K. Settaluri,G. Sundberg,A. Weisshaar,and V. K. Tripathi, Compactfolded line rat-race hybrid couplers,IEEE Microw. Guided Wave Lett.,vol. 10, no. 2, pp. 6163, Feb. 2000.