Proceedings of Asia-Pacific Microwave Conference 2007

A New Branch-Line Hybrid Coupler with ArbitraryPower Division Ratio

Chilhyeun GwonDept. of Electrical and Communication System Engineering

Soonchunhyang UniversityAsan, Chungnam, Republic ofKOREA, 336-745

050713kch@hanmail.net

Inho NaDept. of Electrical and Communication System Engineering

Soonchunhyang UniversityAsan, Chungnam, Republic ofKOREA, 336-745

zxr8 1 @hanmail.net

Yongbeom KimDept. of Electrical and Communication System Engineering

Soonchunhyang UniversityAsan, Chungnam, Republic ofKOREA, 336-745

yb kim@ramrec. sch.ac.kr

Kwansun ChoiDept. of Electrical and Communication System Engineering

Soonchunhyang UniversityAsan, Chungnam, Republic ofKOREA, 336-745

cks 1329@sch.ac.kr

Jongsik LimDept. of Electrical and Communication System Engineering

Soonchunhyang UniversityAsan, Chungnam, Republic ofKOREA, 336-745

jslimgsch.ac.kr

Dal AhnDept. of Electrical and Communication System Engineering

Soonchunhyang UniversityAsan, Chungnam, Republic ofKOREA, 336-745

dahnkr@sch.ac.kr

Kwisoo KimDept. of Electrical and Communication System Engineering

Soonchunhyang UniversityAsan, Chungnam, Republic ofKOREA, 336-745

kwisoo@ramrec. sch.ac.kr

Abstract-This paper proposes a new branch-line hybrid couplerwith controllable power ratio at the output ports. It is structuredas two 900 branch-line hybrid couplers connected by a variableimpedance transmission line. The power division ratio is adjustedby controlling the impedance of the transmission line. Modelingthe transmission line as series and parallel resonators shows thatits impedance can be changed by changing the inductance andcapacitance. In practice, using a varactor diode facilitates ease indealing with the impedance of the transmission line and thus thepower division ratio of the coupler.

Keywords-Branch-line hybrid coupler; transmission line;arbitrary power division ratio; reflection type

I. INTRODUCTION

Advances in high-frequency wireless communications havenecessitated the use of several coexisting frequency bands, anddesign has focused on creating small, light-weight,

multifunctional radio frequency (RF) components. Withproper design, a component can be useful in more than oneapplication, without which different components for eachspecialized application would increase both the size and theoverall cost of the circuit. Some passive components, such asdividers, directional couplers, and filters based on fundamentalcircuit theory, are useful in multiple applications. The branch-line hybrid coupler is one such component; it can be used intransmitting and receiving systems to divide or combinesignals, or as a phase shifter. It is widely used in combinationwith other passive and active components, and its wide rangeof division ratios is due to its structure [1].

In this paper, the division ratio of the proposed newstructure is controlled by a variable transmission line. Thetransmission line can be modeled by its equivalent circuit of aseries and parallel L-C resonators [2]. By controlling thecapacitance of the resonator with a varactor diode, the

1-4244-0749-4/07/$20.00 w2007 IEEE.

impedance between the two couplers can be changed [3]-[4].The change in impedance causes a mismatch to occur, and thisin turn alters the division ratio [5]-[9]. In this paper, wesuggest a new branch-line hybrid coupler that uses thisconcept to adjust the division ratio of the output ports.

II. DESIGN THEORY

9003b h 90lh'

po rt4 pate 3

ZC1 ZL I ZC2

Figure 1. 90o branch-line hybrid coupler to connected Zv

Fig. n shows the proposed branch-line hybrid coupler. Allports are matched to 50 ohms. The structure has twoconventional 900branch-line hybrid couplers connected toeach other with a normal transmission line of impedance Z,.Wvhen Zv is 50 ohms, the circuit couples the power from port Ito port 3 without loss, while ports 2 and 4 are isolated. If Zv isany other value ohms, a mismatch occurs, and this causesreflection. The object is to control the amount of reflection bycontrolling Zv. This paper describes a branch-line hybridcoupler that has an output range of O to 3 dB. Fig. 2 shows anequivalent circuit as in Fig. I in which ZL1 represents Zv andthe second stage 90° branch-line hybrid coupler is ZC2. Thefirst stage 900 branch-line hybrid coupler is Zcl.

F ZLlIZCl (1)

ZL1 + ZC1

ZV 2

ZLI (2)C2

If Zcl and ZL1 are matched, the reflection coefficient is zero.When Zcl and ZL1 are mismatched, the reflection coefficienthas some constant value. Equation (1) shows that the reflectionratio can be controlled, and if the Zv is any other value ohms, a900 phase shift occurs at ports 3 and 4 due to the reflectedwave. Port 2 is isolated because of the 1800 phase difference.Thus, a variable ZL1 makes it possible to have any arbitrarypower division ratio. Zcl and ZC2 are the conventional 3-dBhybrid couplers matched to 50 ohms, and Zv is used to causethe mismatch between Zcl and ZC2.

* i/4_

. IZca zci2

z1

Figure 3. Simplified transmission line and second-stage 90° branch-line hybridcoupler model for determine input impedance

Fig. 2 can also be represented as Fig. 3. To calculate thetransmission-line impedance that causes the mismatch, ZL1 andZC2 are set to Z, and ZC2, respectively. Since Z, has anelectrical length of )/4, it can be expressed as (2). The hybridcoupler with an arbitrary power division ratio can be designedusing (1) and (2). For a OdB coupler, when the impedance ofthe transmission line connecting two couplers is set to 50 ohms,the signal flows only from port 1 to port 3. As the impedanceof ZV increases, the signal is reflected to the isolated port 4,giving that port has the same power division ratio as port 3.

yv= ,> C2=(oL2 -I

(OC

1

o L2 -yY(3)

(4)YV = cooC, -I -> C,=1 =+MA) co coo LI

The equivalent circuit of the transmission line withimpedance Z, using lumped elements for series and parallelresonators is shown in Fig. 4. Equations (3) and (4) are derivedfor a reactive value set for the center frequency. Fixing theinductance means the capacitance can be calculated, and thusthe impedance of the transmission line can be made variable bycontrolling the capacitance. Using a varactor diode makes iteasy to adjust the capacitance and thus the power division ratio.Table 1 shows the values of L and C required for powerdivision ratios in the 0-3dB range for different impedances.

L

L n

0 2 TU 2

Figure 4. Transmission-line equivalent 1T -type circuit

zz1

Figure 2. Reflection coefficient between Zcl with ZLI

OPv

Fzri%a --Iu

TABLE I. Elements values of connected transmission lines at 890MHz

50Q 85Q 121Qtransmission transmission transmission

line line line

Yv [mho] 0.02 0.012 0.008L1 [nH] 10 10 10

C1 [pF] 6.779 5.3 4.6758

L2 [nH] 25 25 25

C2 [pF] 1.99 3.263 9.512

III. SIMULATION AND MEASUREMENTS

A hybrid coupler using the variable transmission line wasconstructed and tested at its center frequency of 890 MHz. Thecircuit was built on a microstrip with a 31-mil substrate ofdielectric constant 4.8. Fig. 5 shows the new branch-line hybridcoupler structure. Tested results were compared to the idealperformance simulated with Ansoft Designer.

L2 C2

Port 1

Port 4

C2 L2

Port 2

L2 C

Port 3

Figure 5. The new branch-line hybrid coupler structureFigure 6. Measured and ideal performances of the new branch-line hybrid

coupler for (a) Zv= 502, (b) Zv= 85Q;, and (c) Zv= 1212

Fig. 6 shows the circuit simulation and measured s-parameter as functions of the impedance. Fig. 7 presents thecircuit as it was fabricated. Fig. 8 shows that a 900 phase shiftoccurs between the two output ports.

Frequency [MHz]

(a)

Figure 7. Photograph of the branch-line hybrid coupler as fabricated

a)

ctI

Frequency [MHz]

(b)

a)

ct

Frequency [MHz]

(c)

-a

P-

135-

90-

45,

a)t 0oa)

ct

P -45,

-90

-135

840 860 880 900

Frequency [MHz]

920 940

Figure 8. Measured phase difference between output ports of the 1212 circuit

As previously noted, a 900 phase difference occurs betweenports 3 and 4, and the power division ratio is regulated by thecapacitance of the connecting transmission line. The incidentpower does not flow to isolated port 2.

IV. CONCLUSION

In this paper, we proposed a new branch-line hybrid couplerwith an arbitrary power division ratio. The variabletransmission line was extracted to its equivalent circuitconsisting of series and parallel resonators. A varactor diodewas used to control the power division ratio in the 0-3dB range.

The coupler exhibited broadband characteristics, and a 900phase difference occurred at the output ports. A comparison ofsimulated and measured results showed that the proposeddesign concept is sound. Thus, this new coupler has potentialapplications in power dividers, phase shifters, and antennas, as

well as in many other systems.

ACKNOWLEDGMENT

This work has been financially supported by the Ministry ofEducation and Human Resources Development (MOE), theMinistry of Commerce, Industry and Energy (MOCIE), and theMinistry of Labor (MOLAB) through the fostering project ofthe Laboratory of Excellency.

REFERENCES

[1] D. M. Pozar, Microwave Engineering, John Wiley & Sons, 2005.[2] C.-S. Kim, J.-S. Park, D. Ahn, J.-B Lim, "Variable directional coupler

with LC resonator" IEE Electronics Letters, vol.36, no. 18, pp. 1557-1559,Aug. 2000.

[3] C.-S. Kim, C.-S. Yoon, J.-S. Park, D. Ahn, J.-B. Lim, and S.-I. Yang,"Design of the novel varactor tuned directional coupler," IEEE MTT-SInt. Microwave Symp. Dig., vol. 4, pp.1725-1728, 1999.

[4] E. A. Fardin, k. Ghorbani, A. S. Holland, "A varactor tuned branch-linehybrid coupler," Microwave conference proceedings, 2005. APMC 2005.Asia-Pacific conference proceedings, vol. 3, Dec. 2005.

[5] E. J. Wilkinson, "An N-way hybrid power divider," IRE Trans.Microwave Theory Tech., vol. MTT-8, pp.116 118, Jan. 1960.

[6] A. Tetsuo, J.-P. Hsu, "Analysis and synthesis of triplate branch-line 3dBcoupler based on the planar circuit theory" IEEE MTT-S Int. Microwavesymp. Dig., vol. 87, pp.207-210, Jun. 1987.

[7] Y.-B. Kim, H.-T. Kim, K.-S. Kim, J.-S. Lim, D. Ahn, "A branch linehybrid having arbitrary power division ratio and port impedances" 2006Asia-Pacific Microwave conference, vol.2, no.58, pp. 1376-1379, Dec.2006.

[8] J.-S. Bae, J.-S Lim, K.-S. Kim, J.-U. Kim, D. Ahn, "A new type of 3-way power division using an intentional mismatched termination" 2006Asia-Pacific Microwave conference, vol.2, no.3, pp. 909-912, Dec. 2006.

[9] K.-T. Kim, Y.-C. Chung, I. Tatsuo, D. Ahn "Reconfigurable PowerDivider and Combiner with Variable Power Ratio" IEEE MTT-S Int.Microwave Symp. Dig, TU3A-1, pp57-60, Jun. 2004.

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