IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 27, NO. 2, FEBRUARY 2017 141

A Compact Broadband Riblet-Type Three-WayPower Divider in Rectangular Waveguide

G. Arun Kumar, Member, IEEE, Bijit Biswas, Member, IEEE, and D. R. Poddar, Senior Member, IEEE

Abstract— A compact broadband three-way power dividerbased on a Riblet type coupler is proposed and designed inW-band. Coupling occurs among three adjacent waveguidesthrough narrow walls. Two cylindrical posts located in thecoupling region, help to achieve efficient coupling within shorterlength and also to reduce reflections. The tapering of the inputcentral waveguide improves the return loss of the power divider.The proposed power divider is validated with experimentalmeasurements and measured power division ratio is −5.2 dB± 0.25 dB. The return loss and isolation between the outputports is better than 20 dB over 88.75-97.5 GHz frequency range.The amplitude imbalance between the output ports is less than± 0.4 dB. The fractional bandwidth for the power divider is9.47% and the coupling length is 1.7λg. The simulated andmeasured results are in close agreement.

Index Terms— Broadband, fractional bandwidth (FBW),millimeter wave, power divider, short slot hybrid.

I. INTRODUCTION

POWER combiners and splitters are widely used atmicrowave and millimeter wave frequencies. They are

used in design of power amplifiers, beamforming networks,and for local oscillator (LO) power splitting [1], [2]. Manypower divider configurations in waveguide are available inliterature such as multi-aperture, rat-race, branch guide, magictee [3] etc. Riblet coupler is one such coupler which is widelyused due to its simple and compact design [4].

Waveguide three way power dividers are designed usingsix port branch waveguides [2], [5]–[7]. An E-plane branchlayout is utilized in these designs. Synthesis and numericalanalysis of the power divider is presented in [2], whereascompact multi-aperture power dividers are presented in [5]and [6]. Though, the simulated results show high isolation,good amplitude balance and compact size, the designs are notexperimentally validated. Generalized scattering matrix is usedto analyze a non symmetric six-port branch directional couplerin [7]. A six port Riblet type coupler is designed at Ku-bandfrequencies [8]. This coupler is based on coupling mechanismof higher order modes. A fractional bandwidth of 8.4% isachieved using the concept of multituning of input resonators,but it suffers from design complexity and the required couplinglength is about 3λg. A high efficiency Ka-band unequal power

Manuscript received August 12, 2016; accepted October 18, 2016. Date ofpublication February 1, 2017; date of current version February 10, 2017.

G. Arun Kumar and B. Biswas are with the Society for Applied MicrowaveElectronics Engineering and Research (SAMEER), Kolkata 700091, India(e-mail: g.arun@mmw.sameer.gov.in; bijit@mmw.sameer.gov.in).

D. R. Poddar is with Electronics and Telecommunication EngineeringDepartment, Jadavpur University, Kolkata 700032, India.

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LMWC.2016.2646999

Fig. 1. Schematic drawing of the power divider

divider for power combiner application is reported in [9].This power combiner is designed with two serially connectedidentical unequal branch power combiners. Though, it showsgood return loss and isolation characteristics, it suffers fromnarrow fractional bandwidth of 1.52%.

In this letter, a compact broadband three-way power divideris proposed based on a Riblet type configuration. Two cylin-drical posts placed symmetrically about the central axis areused for efficient coupling within shorter lengths and theyalso reduce the reflections from the metallic shims. Theseposts perturb the field in the coupling region to generateTE30 mode. The input return loss of the power divider isimproved by tapering the input metallic shims dividing thethree waveguides. The proposed power divider exhibits afractional bandwidth of 9.47% and the required couplinglength is reduced only to 1.7λg.

II. PROPOSED DESIGN OF POWER DIVIDER STRUCTURE

The structure of the proposed power divider is shownin Fig. 1. It consists of three waveguides separated by twometallic shims and coupling region is formed by introducingnarrow slots along the smaller waveguide dimension. Twocylindrical posts are located symmetrically in the couplingregion. The coupling region width is controlled by the match-ing elements present on its either sides. The matching ele-ments are so adjusted that modes higher than TE30 do notpropagate in the coupling region [10]. A linear taper in theinput waveguide is used near the coupling region to reducereflections. The power in the isolated ports is absorbed by thewaveguide matched terminations. The waveguide pyramidalterminations are designed using Eccosorb R© MF-117 materialand the return loss for the terminations is better than 35 dBwhen measured separately. The design parameters of the powerdivider are described in Fig.2.

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142

Fig. 2. Design parameters of the proposed structure (a) top view (b) sideview

Fig. 3. (a) E-Field distribution (b) H-Field distribution inside the proposedstructure

The power division mechanism due to the coupling of TE10and TEm0 modes is discussed in [8]. The thickness(t) ofmetallic shims are 19% of the terminal waveguide broaderdimension and they present an appreciable amount of reflec-tion for all the TEm0 modes in the coupling region [4], [11].Conventionally, these reflections can be reduced either byincreasing the coupling length or by shaping the metallic shimsor both [11]. In the present case, two cylindrical posts areintroduced to reduce the reflections, while keeping the cou-pling length unaltered. In Fig. 3, the electric and magnetic fielddistribution in the optimized power divider with cylindricalposts and linear taper is shown. The two cylindrical posts servethree purposes: a) generation of TE30 mode, which combineswith the TE10 mode to provide equal power division at thethree output ports, (b) reduction in reflections from the metallicshims resulting in broader bandwidth, (c) reduction in couplinglength compared to conventional power divider [8].

The above discussions are substantiated by observing thereturn loss characteristics of the power divider shown in Fig. 4.It is observed that the return loss for the basic power divideris less than 15 dB whereas it is better than 20 dB when twocylindrical posts are symmetrically placed about the centralaxis. The two posts help in reduction of reflections from themetallic shims and hence, there is an improvement in thereturn loss. The linear taper in the metallic shims is thenintroduced, which shifts the return loss characteristics towardslower frequency. The linear taper also reduces reflections andhence, the return loss improves further at the centre of thefrequency range.

III. SIMULATED AND MEASURED RESULTS

The simulations are carried out using Ansys HFSS. Theoptimized parameter values are as follows: L1 = 1mm;L2 = 1.82mm; L3 = 2.65mm; L4 = 8.1mm; L5 = 0.6mm;L6 = 0.9mm; L7 = 6.45mm; L8 = 0.4mm; L9 = 2.64mm;L10 = 7.1mm; a = 2.54mm; b = 1.27mm; h = 0.03mm;

Fig. 4. Return loss of the power divider with various configurations.

Fig. 5. Fabricated prototype of the power divider.

Fig. 6. Simulated and measured power division at the three output ports.

d = 0.6mm; t = 0.5mm; frad = 0.4mm. The position of thecylindrical posts are initially placed along the input metallicshims at a distance of λg/3 based on the E-field distributioninside the coupling region. Then, slight optimization iscarried out to get the required power division, return loss,isolation and bandwidth. To avoid internal reflections due tosharp edges of the matching element a fillet with a radiusof 0.4mm is used. For the proof of concept, a prototype isfabricated and its performance is experimentally tested. Thephotograph of the fabricated power divider is shown in Fig. 5.The measurements are performed after TRL calibration usingAgilent E8364B network analyzer with millimeter wave VNAextenders. Fig. 6. shows the simulated and measured powerdivision characteristics of the power divider. The return lossand isolation characteristics of the power divider are shownin Fig. 7. Both the simulation and measurement resultsare in close agreement. The measured power division ratio

IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 27, NO. 2, FEBRUARY 2017

ARUN KUMAR et al.: COMPACT BROADBAND RIBLET-TYPE THREE-WAY POWER DIVIDER IN RECTANGULAR WAVEGUIDE 143

Fig. 7. Simulated and measured return loss and isolation between ports ofthe power divider.

Fig. 8. Simulated and measured transmission phase between the three outputports.

TABLE I

COMPARISON OF THREE-WAY POWER DIVIDERS

is −5.2 dB ± 0.25 dB, while the return loss is betterthan 20 dB. The isolation among the three output ports isin excess of 20 dB over 88.75-97.5 GHz frequency range.The simulated and measured transmission phase of thethree output ports are shown in Fig. 8 and they are inclose agreement. The measured amplitude difference (AD)between the output ports is shown in Fig. 9 and is less than± 0.4 dB. The measured fractional bandwidth of the divideris 9.47%. Coupling length of the fabricated power divideris 7.1mm, which corresponds to 1.7λg at centre frequency.The deviations in the simulated and measured results maybe attributed to the fabrication tolerances. The taper andcylindrical post positions are also crucial for the return lossand isolation between port 3 and port 4. The comparison ofthe performance of the power divider with other reportedworks is shown in Table I. The proposed power divider showshigher fractional bandwidth and ease of fabrication comparedto [8] and [9].

Fig. 9. Simulated and measured amplitude difference between the threeoutput ports.

IV. CONCLUSION

A simple, compact three-way power divider based on Riblettype coupler is proposed and experimentally validated. Themanufacturing process is simple and cost effective. The powerdivider shows an improvement in bandwidth and reductionin coupling length without any degradation in the RF per-formance. Hence, the power divider may widely be used forpower combiner applications, beamforming networks and LOpower division at millimeter wave frequencies.

ACKNOWLEDGMENT

The authors would like to thank S. Ranade, Director,SAMEER, for her constant support and encouragement, A.Majumder, OIC, SAMEER Kolkata, for his help in presentingthis work in its final form, and S. Mondal and his mechanicalteam for providing precision mechanical fabrication.

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