2008 IEEE MTT-S International Microwave Workshop Series on Art of Miniaturizing RF and Microwave Passive Components

Design of Compact Orthomode Transducers

(Invited paper)

Yun Tao and Zhong-Xiang Shen*School of Electrical and Electronic Engineering, Nanyang Technological University,

50 Nanyang Avenue, Singapore 639798E-mail: *ezxshengntu.edu.sg

Abstract- This paper presents the design of compactorthomode transducers (OMTs). Analysis methods for waveguideOMTs, including the mode-matching method and hybridmethods, are briefly reviewed. Three types of compact OMTs aredescribed: a dielectric-filled OMT with reduced size, a planarOMT implementing using substrate integrated waveguide, and adual band Tx/Rx OMT with a compact diaphragm to improveisolation. The dual-band Tx/Rx and SIW OMTs are verified bymeasurements. Challenges for designing multi-band OMTs withcompact size are also pointed out.

full-wave methods are advocated. Then, three types ofcompact OMTs are described: the first one is a dielectric-filledOMT, which reduces the dimensions of the junction region;the second one is a planar circuits compatible OMTimplemented using substrate integrated waveguide (SIW), asimple quadruple ridged waveguide is employed to implementa compact transition between the normal waveguide and SIW;and the last one is a dual band Tx/Rx OMT, employing a verythin diaphragm to improve the isolation between the Tx andRx ports. The last two OMTs are verified by measurements.

I. INTRODUCTION

Orthomode transducer is a key component in modemsatellite communications, where the dual-polarization signalsare commonly involved. Besides the main performancecriteria, such as return loss, insertion loss and isolation, acompact design is preferred or sometimes mandatory becauseOMTs are often integrated with other system componentsunder the tight limitation of volume and weight, especially forspacecraft applications. However, the performance of theOMT, especially the bandwidth, is usually closely related tothe complexity of the structure. The symmetrical branches cansuppress the higher order modes [1] at a cost of noticeableincrease in size and complexity. A compact single branchscheme with image load [2] also requires complicatedmatching steps [3]. A compact-size high-isolation double-branch OMT is implemented based on a short circuitedcommon waveguide, but the two branches must keep adistance of half-wavelength for high isolation [4]. Double-septum OMT can also realise shorter common waveguide thannormal single-septum one [5].

In this paper, the compact design is not only restricted tominiaturize the size of OMT itself, but also refer to the OMTwhich is easy to integrate with other components. In this pointof view, the planar OMTs [6, 7] are more compact designscompared to their waveguide counterparts for they can beintegrated into planar circuits.The available analysis methods in the design of OMTs are

briefly reviewed. The efficient mode-matching buildingblocks are developed for our designs, and the hybridtechniques combining the mode-matching methods with other

II. ANALYSIS METHOD

Unlike other waveguide components, the design theory ofOMTs has not been fully established yet, nearly all of themstill require sophisticated designs for special applications.With the advent of commercial simulators, such as Ansoft'sHigh Frequency Structure Simulator (HFSS), CST and so on,microwave engineers can easily model almost any types ofOMTs though their design circle is long due to their lowcomputational efficiency. Therefore, the mode-matchingmethod is still preferred for its high efficiency in the design ofcompact OMTs. Furthermore, it forms the basis for hybridtechniques combining the mode-matching method with othernumerical methods.A. Mode-Matching Method

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Fig. 1. Double-plane step junction (left) and six-port junction (right)

The mode-matching method is most efficient for regularwaveguide structures; it also holds true for the waveguide OMTcase. Many OMTs have been analyzed based on modematching

978-1-4244-2877-9/08/$25.00 ©2008 IEEE 38 December 2008, Chengdu, China

techniques. Among all, two building blocks are essential toanalyze most of the rectangular OMT structures: a) thedouble-plane step junction; b) six-port junction, as shown in Fig.1. With several multi-port generalized scattering matrix (GSM)cascading programs, the rectangular waveguide OMTs can beefficiently analyzed under the following limitations: no ifurcation(septum), no discontinuities within junction (resonance) regions.B. Hybrid Methods

Although practical OMTs are usually not suitable to beanalyzed fully by the mode-matching method, most regions ofwaveguide OMTs can still be modeled by the two building blocksmentioned above. Therefore, a preferred solution is to analyzethese regions with the mode-matching method and the remainingregions with other full-wave methods, such as finite-elementmethod or boundary-element method, and then combine themtogether. These hybrid methods have been successfully employedin the design of waveguide components [8], and a broadbandtaper/branching OMT was reported in [9]. A hybridmode-matching boundary-element method (MMBEM) isdeveloped to analyze the quadruple corner-cut ridged waveguide[10], which can be used as a compact dual-polarizationimpedance transformer in our OMTs.

III. DIELECTRIC-FILLED OMT

The turnstile junction is widely used in the design of OMTs forits symmetry while its size is not compact. A straightforwardexpediency is to fill the waveguide with dielectrics. Because theinput and outputs still need to be compatible to the horn andstandard waveguide, only the junction region is filled and scaleddown approximately by the factor of r . The structureconsidered is shown in Fig. 2, where the common waveguide is of50 mmx50 mm, the branches are standard rectangular waveguideWR229. The dielectric constant of the filling material in thejunction region is 4.4. It is obvious from Fig. 2 that the branchesare retracted within the common waveguide, and the distancefrom apertures to the short circuited plane is also reduced.Therefore, the whole structure is more compact than the normalone.

Fig. 2. Structure of the dielectric-filled OMT

The mode-matching building blocks are employed to modelthis structure. One six-port junction with one short circuited planeand several step junctions are sufficient to represent the wholemodel. Simulated results are shown in Fig. 3. The resonance isobserved at 3.58 GHz, and the cross-pol in the common port isbetter than 40 dB. Better results can be achieved with moreimpedance matching steps in the branches.The effect of the filling material's dielectric constant is studied,

as shown in Fig. 4. The resonant frequency shifts down as thedielectric constant increases.

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Fig. 3. Scattering parameters of the dielectric-filled OMT

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Fig. 4. Influence of the filling material's dielectric constant on the resonantfrequency of OMT.

IV. SIW OMT

The dielectric-filled OMT has successfully reduced thedimension of OMT. However, its bandwidth is very limited andstill not compatible with other planar circuits.

In this section, a planar orthomode transducer is proposedusing substrate integrated waveguide (SIW) [11], as shown in Fig.5. The four arms of the turnstile junction are replaced by the SIW,because it can be viewed as an equivalent rectangular waveguidein the substrate. Therefore, it maintains high Q-factor, high powerhandling capacity compared to microstrip line [7]. However, thetransition between a square metallic waveguide to SIW has not

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been carefully studied [12] because the substrate is very thincompared to normal rectangular waveguide. For the turnstilejunction, the scattering element cannot be inserted in the substrateunlike the rectangular waveguide [13]. To achieve the impedancematching condition between the dual-polarized square waveguideand ultra-thin substrate integrated waveguide, four short ridgesare symmetrically inserted into the common waveguide as acompact transition. This is because the ridges can lower thecharacteristic impedance of the square waveguide and the ridgescan be part of the dual-polarized horn antenna [14]. Due to itsintrinsic broadband and dualpolarization characteristics, thequadruple-ridge waveguide is very suitable as a transitionbetween the square waveguide and SIW.

Although more complicated quadruple corner-cut ridgedwaveguide can be analysed by the proposed hybridmodematching boundary-element method (MM-BEM) [10],rectangular ridges are employed to simplify the design process.As the characteristics of quadruple-ridge waveguide is moresensitive to its length than to its width, the width can be fixed.Only the length and height are required for further optimization.Four SIW-microstrip taper convertors are connected to SIW latersimply for the ease of measurement.Measured results of a designed SIW-OMT are compared with

simulated ones, which exhibit good agreement, as shown in Fig. 6.Over the band 3.4 GHzA4.2 GHz, the return loss is better than 15dB and the isolation is better than 30 dB, while the length of theridges is only 22 mm.

This design can be more compact and exhibit broaderbandwidth with more delicately designed ridges, as reported in[14]. It is also possible to integrate the proposed OMT withdual-polarized antenna to forms a complete feed system.

V. DUAL BAND TX/RX OMT

A dual-band Tx/Rx OMT is to receive dual-polarized signals atone band and transmit single-polarized one at another band. Asthe Tx signal is very strong compared to the Rx ones, theisolation between them is critical to the performance of the wholesystem. The main challenge is to isolate the Tx and Rx ports withthe same polarization. The conventional solution is to connect afilter behind the Rx port to block the leakage of Tx signals, at thecost of increased size.

In our design, an ultra-thin irregularly shaped diaphragm isinserted in the waveguide to function as a compact bandstop filterwithin the OMT itself. The proposed structure is shown in Fig. 7,where Port 1 the common port connecting to a horn, Port 2 is forthe Tx signal, Ports 3 & 4 are for the Rx signals.

The irregularly shaped diaphragm is inserted in Port 4 toimprove the isolation between Ports 2 & 4. In the actualconfiguration, the diaphragm can be directly placed on the sidesurface of the common waveguide, as shown in Fig. 8. Therefore,a very compact design of the dual-band Tx/Rx OMT is achieved.The whole structure can be divided into five regions as follows.

1) Region I is the slot coupling for the vertical Rx signal.2) Region II is a narrowed waveguide that stops the vertical Rx

signal and passes the Tx and horizontal signals.3) Region III is the slot coupling for the horizontal Rx signal.

Fig. 5. Designed SIW-OMT (left: 3-D model, right: fabricated OMT)

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Fig. 6. Simulated and measured results of the SIW-OMT.

Fig. 7. Structure of the dual-band Tx/Rx OMT

Fig. 8. Fabricated dual-band Tx/Rx OMT (left: single, right: back-to-backconfiguration)

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4) Region IV is to match the Tx port.5) Region V is the irregularly shaped diaphragm.As the diaphragm at Port 4 has little effect on the regions II-IV,

the return loss at Ports 2 & 3 are firstly optimized by thepreviously introduced mode-matching building blocks withoutconsidering the diaphragm (Region V). The initial dimensions ofRegion I can also be obtained. Then the effect of an irregularlyshaped diaphragm inserted into a rectangular waveguide isinvestigated separately by a finite-element simulator. Thebandstop phenomenon is observed, as shown in Fig. 9. Thefabricated OMT is measured using a back-to-back configuration,as shown in Fig. 8. The Tx band covers from 13.7GHz to14.7GHz, the horizontal Rx band is from 10.7GHz to 12.7GHz,while the vertical one is from 10.4GHz to 11.8GHz, as shown inFigs. 10 ,11, and 12. The isolation improvement is over 15 dBusing the proposed diaphragm, as shown in Fig. 13.

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10 10.5 11 11.5 12 12.5 13 13.5 14 14.5 15Frequeny (GHz)

Fig. 11. S-parameters for the horizontal signals at the receiving band.

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Fig. 9. Bandstop phenomenon of an irregularly shaped diaphragm

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Fig. 10. S-parameters of the fabricated OMT at the transmitting band.

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Fig. 12. S-parameters for the vertical signals at the receiving band.

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Fig. 13. Isolation between transmitting port and vertical receiving port.

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VI. CONCLUDING REMARKS [15] Z. Shen and C. Feng, "A new dual-polarized broadband horn antenna,"IEEE Antennas and Wireless Propagation Letters, vol.4, pp.270-273,2005.

Several designs of compact orthomode transducers have beenpresented. The dielectric-filled OMT can greatly reduce the sizeof the common waveguide while resulting in a narrow bandwidth.The SIW OMT implements the output ports by low-cost printedcircuit board (PCB) technology, which can be further integratedinto other planar circuits. A compact bandstop irregularly shapeddiaphragm is employed in the dual-band Tx/Rx OMT to improvethe isolation between the Tx and Rx ports. However, severalchallenges for designing compact OMTs remain unsolved. Ageneral theory of designing OMTs has not appeared so far,especially for multiband OMTs. In the case of multi-band OMTs,there are more ports corresponding to different bands andpolarizations, which will result in an increased size. An efficientand versatile modeling technique is highly desirable for theanalysis and design of multi-band OMTs.

REFERENCES

[1] J. Uher, J. Bornemann, and U. Rosenberg, Waveguide Components forAntenna Feed Systems: Theory and CAD. Norwood, MA: Artech House,1993.

[2] W. Steffe, "A novel compact OMJ for Ku band Intelsat applications," inIEEE Antennas and Propagation Society International Symposium,Newport Beach, CA, 1995, pp. 152-155.

[3] M. Ludovico, B. Piovano, G. Bertin, G. Zarba, L. Accatino, and M[4] Mongiardo, "CAD and optimization of compact ortho-mode

transducers," IEEE Trans. Microwave Theory Tech., vol.47,pp.2479-2486, 1999.

[5] Y. Naofumi, M. Moriyasu, T. Masaru, and N. Hiromasa, "Design ofcompact-size high isolation branching OMT by the mode-matchingtechnique," in 26th European Microwave Conference, Prague, 1996,pp.848-851.

[6] J. A. Ruiz-Cruz, J. R. Montejo-Garai, J. M. Rebollar, C. E. Montesano,and M. J. Martin, "Very compact ortho-mode transducers with doubleseptum configuration," Microvave And Optical Technology Letters,vol.48, pp.765-767, 2006.

[7] R. W. Jackson, "A planar orthomode transducer," IEEE MicrowaveandWireless Compon. Lett., vol.11, pp.483-485, 2001.

[8] P. K. Grimes, 0. G. King, G. Yassin, and M. E. Jones, "Compactbroadband planar orthomode transducer," Electronics Letters, vol.43,pp.1146-1147, 2007.

[9] F. Arndt, J. Brandt, V. Catina, J. Ritter, I. Rullhusen, J. Dauelsberg, U.Hilgefort, and W. Wessel, "Fast CAD and optimization of waveguidecomponents and aperture antennas by hybrid MM/FE/MoM/FDmethods-state-of-the-art and recent advances," IEEE Trans. MicrowaveTheory and Tech. , vol.52, pp.292-305, 2004.

[10] F. Arndt, V. Catina, and J. Brandt, "MM/MoM CAD of broadbandtaper/branching OMTs utilizing stepped T-junctions," in 34th EuropeanMicrowave Conference, Amsterdam, 2004, pp.269-272.

[1 1] Y. Tao, Z. Shen, and G. Liu, "Analysis of quadruple corner-cut ridgedcircular waveguide by hybrid mode-matching boundary-elementmethod," in 13th Biennial IEEE Conference on Electromagnetic FieldComputation, Athens, 2008.

[12] D. Deslandes and K. Wu, "Integrated microstrip and rectangularwaveguide in planar form," IEEE Microwave and Wireless Compon.Lett., vol.11, pp.68-70, 2001.

[13] W. D'Orazio and K. Wu, "Substrate-integrated-waveguide circulatorssuitable for millimeter-wave integration," IEEE Trans. MicrowaveTheory and Tech., vol.54, pp.3675-3680, 2006.

[14] Y. Aramaki, N. Yoneda, M. Miyazaki, and T. Horie, "Ultra-thinbroadband OMT with turnstile junction," in IEEE MTT-S InternationalMicrowave Symposium Digest, 2003, pp.47-50.

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