1173

Computer Aided Design of Wideband Orthomode Transducersbased on the B0ifot Junction

Jorge A. Ruiz-Cruz*, Jose R. Montejo-Garai*, Jesu's M. Rebollar*,Carlos E. Montesano**, Maria J. Martin**, Margarita Naranjo-Masi**

* Departamento de Electromagnetismo y Teoria de Circuitos, Universidad Politecnica de Madrid,Ciudad Universitaria s/n. 28040 Madrid, Spain (jorgercgetc.upm.es)** EADS CASA-Espacio, Avda. Aragon 404, 28022 Madrid, Spain

Abstract A CAD tool based on the Boifot junction isproposed for the design of wideband OrthoMode Transducers(OMTs). It has two interesting features. First, the OMT routingbranches use well-known waveguide elements, such as truncatedE-plane bends, which can be efficiently analyzed by Mode-Matching (MM) methods and can achieve widebandperformance. Second, a MM technique is presented for the four-port Boifot junction, which provides its generalized multimodecharacterization. As a result, the whole OMT is segmented intodifferent regions, yielding an efficient CAD tool for its optimumdesign. The analysis method and the designed OMT have beenvalidated with other numerical methods and experimentalresults.

Index Terms - B0ifot junction, Wideband OrthomodeTransducer, Mode-Matching, Generalized Admittance Matrix.

I. INTRODUCTION

Waveguide OrthoMode Transducers (OMTs) are passivecomponents used in antenna feed systems to discriminate intodedicated ports the two orthogonal signals that can be eitherreceived or transmitted by a dual-polarization antenna [1].These devices are often designed for satellite communicationsystems, where the dual polarization is used to increase thetraffic capacity of the link. Other interesting applications arefound in low-noise receivers and radiometers at centimeterand millimeter wavelengths, where the use of waveguideOMTs can improve the performance of the radiostronomyreceiver systems [2],[3].

Regardless of the specific application, the OMT has arelevant impact in the entire antenna feed systemperformance, and its typical figures of merit are bandwidth,cross-polarization, isolation between ports and powerhandling capabilities. Moreover, the manufacturing process isanother important aspect, especially at the millimeterfrequency band. All these factors lead to many different OMTconfigurations [1],[4], where the symmetry of the structureplays an essential role and determines the symmetry of thehigher-order modes that will be excited in the OMT. Forinstance, the compact designs in [5],[6] are limited inbandwidth by the TE 1 /TM1 1 mode excitation at the squarecommon port, leading to a maximum 3400 fractionalbandwidth between these modes and the TE1O/TEO.

Lateral port

V

Fig. 1. OMT based on the B0ifot junction, with the proposed routingbranches. All the parts of the structure, including the four-port B0ifotjunction, are characterized with efficient Mode-Matching methods.

A bulkier configuration that in turns provides a broaderband performance was proposed by B0ifot et al. in [7]. TheOMT in Fig. 1 is based on that structure, where some slightmodifications have been introduced in the routing branches inorder to facilitate its analysis and design. The maincharacteristic of this structure (without considering the E-plane bend near the axial port) is the double xz- and yz-symmetry, which prevents the TEl1/TM11 generation at thecommon port. In fact, its first higher order modes are theTE12/TM12/TE21/TM21, which represent a 76.4%bandwidth with respect to the TElO/TEO1.

This paper describes the Computer Aided Design (CAD)tool of the OMT in Fig. 1, with special emphasis in two pointsi) the proposed routing branches and ii) the multimodecharacterization of the four-port B0ifot junction (see Fig. 2).The result is an efficient CAD method based on Mode-Matching (MM) techniques for the optimum design of thesecomponents.

0-7803-9542-5/06/$20.00 ©2006 IEEE

a)

V

z

X '! (

Axialport

Hei ght-transformer

-- E-planebesnd

septum

QDH

E-planebifurcation

\ Lateral

Fig. 2. Four-port B0ifot junction with metallic septum and metallicposts to perform the orthomode separation.

II. THEORY

A. Description ofthe OMT operation

The OMT in Fig. 1 is composed of the following elements:an axial branch, two lateral branches and a four-port junction,also called B0ifot junction. The rectangular axial port isplaced at the end of the axial branch, after a steppedtransformer in the y-direction and an E-plane bend. The lateralbranches connect the two lateral ports of the B0ifot junctionwith the lateral port of the whole OMT.The B0ifot junction further detailed in Fig. 2 has four ports:

two rectangular lateral apertures (3 and 4), one square

aperture at the common port (1) and another square apertureconnected to the axial branch (2). A number of full heightposts are placed close to ports 3 and 4.The principle of operation is explained according to Figs. 1-

3, assuming that either a vertical or horizontal polarization isexciting the square common port. In the first case, the modeswith Perfect Magnetic Wall (PMW) at the yz-plane andPerfect Electric Wall (PEW) at the xz-plane are excited. Theelectromagnetic symmetry is reversed for the horizontalpolarization and the considered modes have PEW at the yz-

plane and PMW at the xz-plane. The symmetry at the xz-planeimplicitly assumes that the E-plane bend in the axial branch isnot present in this analysis. In the actual component it isplaced at a distance far enough to prevent any higher modeinteraction (dtb in Fig. 3.a). Without this E-plane bend, thestructure has indeed double symmetry.The B0ifot junction acts very differently for the orthogonal

polarizations. When a vertical one (Fig. 3.a) is exciting thecommon port 1, it finds a septum perpendicular to the electricfield that should not affect significantly the propagation to 2.In principle, some signal could go to 3 and 4. This is avoidedby two combined effects. First, the modes at 3, 4 generated by

0) 0D

x 4 1WE-planebe nd

Fig. 3. Operating principle of the wideband orthomode for thevertical (a) and horizontal polarization (b).

this excitation are evanescent, so the vertical polarizationfinds two waveguides under cutoff in the two lateral branches.This is interpreted as a reactive load for the verticalpolarization that has to be matched. Second, the elements thatcontrol the matching are the full height posts. Intuitively, theyare placed in a position such that the vertical polarization sees

an equivalent lateral metallic wall.The horizontal polarization should not be affected much by

the posts as long as there are just a few of them (two in our

case) and they are small enough. A compromise between bothpolarizations must be found. When the horizontal polarizationfrom common port 1 is arriving to the septum region (Fig.3.b), it is split along the septum in two opposite phase fields.With a proper design of the septum profile, the signals at 3and 4 carry half the power of the exciting wave, with fieldsoriented in opposite sense. The lateral branches route thesignal to the lateral port, where they are recombined in phasein a final E-plane bifurcation.

B. Modeling ofthe routing branches

In the original structure [7], the routing branches of theOMT were waveguide tapers. The proposed scheme in Fig. 1

is made up of truncated E-plane bends. In principle, they are

easy to manufacture, can be also designed for broadbandoperation and can be dealt with efficient analysis tools. Forinstance, the lateral branches can be analyzed with theBoundary Contour Mode Matching method (BCMM) [8],[9].

1174

b)I z

H

1175

3,--I 1'

x~1

Fig. 4. Reference planes involved in the Generalized AdmittanceMatrix of the B0ifot junction (symmetries are taken into account).

PEW

R

rl

PEW/PMW

R-R'

Fig. 5. Reference planes involved in the Generalized AdmittanceMatrix of the B0ifot junction (symmetries are taken into account).

C. Modeling ofthefour-port B0ifotjunction

The analysis of the B0ifot junction begins with thecomputation of the Generalized Admittance Matrix (GAM) ofthe structure shown in Fig. 4 with reference planes at 1', 2', 3'.Since the junction is symmetric, the analysis only deals withone quarter of the junction. Two analysis are carried out(vertical and horizontal polarization), each one with itscorresponding PEW/PMW at the symmetry planes.

The GAM is computed using a method based on [10]-[12].It is obtained by solving a set of partial problems defined as

follows: in a port X a modal excitation is imposed while all theremaining apertures are short-circuited (closed by a PEW).Then, the magnetic field generated by the excitation at theposition of the short-circuited port is tested with its modes.This provides the elements of the GAM relating theinteraction between the modes of ports X and x.

The partial problems for the B0ifot junction are a set ofwaveguides with homogeneous cross section cascaded along a

longitudinal axis. Therefore, the resultant problems are a setof waveguide discontinuities which can be solved using MM.As an example, when the structure is excited by 3', with 1'and 2' closed by PEWs, the partial problem becomes a

cascading of ridge waveguides along the y-direction withdifferent ridge gaps and homogeneous ridge width (theseptum thickness). This problem is depicted in Fig. 5, where

the cut R-R' is shown as an example of waveguide arisingfrom the segmentation. In addition, the full-height metallicposts are handled in the same way, since they simply give riseto a trifurcation which is also easily addressed with MM.

Once the GAM of the junction at 1',2' and 3' has beencomputed, it can be cascaded with other elements representedeither with their GAM or their Generalized Scattering Matrix(GSM) counterpart. In this way, the waveguide discontinuitiesbetween 1-1', 2-2' and 3-3' are also convenient modeled byMM in order to obtain their GSMs. After the cascading, theGSM of the B0ifot junction with reference planes at 1, 2, 3 isobtained.

III. RESULTS

The OMT and CAD described in previous sections havebeen tested with the following design. The specifications thatmust fulfill the OMT are:

Table 1. Specifications of the OMTPolarization Frequency band Return loss lpl

(GHz) comm.portV and H 12.84-13.32 27 dBV and H 17.21-17.79 27 dB

The design is carried out in a modular way. The mostcritical part of the OMT is the B0ifot junction. To accomplishthe design goals, full-wave optimization is used, where thegoal function must take into account the return loss for bothpolarizations. The optimization variables are the septumprofile (width and lengths of a septum with 8 steps as theshown in Figs. 4-5) and the size and position of the metallicposts.The performance of the resultant junction computed by the

method above described (GAM method) is shown in Fig. 6.The response for both polarizations is better than 27 dB in aband from 12 GHz to 18.5 GHz, fitting the prescribedrequirements in the design subbands. The response is verysimilar to that obtained by the Ansoft HFSS software,validating the analysis method. The reference planes of thissimulation are 1, 2, 3 (see Fig. 4), which implies that theGAM of the B0ifot junction computed at 1', 2', 3' is enoughaccurate to allow subsequent cascading with other stepdiscontinuities.

After the B0ifot junction, the remaining elements of theOMT (truncated waveguide bends, bifurcations ...) aredesigned with an adequate margin to anticipate theperformance degradation by the connection of the differentparts. When they are considered individually, the parts aredesigned to have return loss at least 6 dB better than theshown in Table 1.The entire OMT is manufactured in three parts by

electroeroding: the lateral branches, the E-plane bend for theaxial port and the B0ifot junction that also includes the heighttransformer of the axial branch. Moreover, the lateralbranches are composed of two identical halves. The assemblyof all the involved elements is displayed in the inset of Fig. 6.

---- -- - - - -- - - - - ---

7-'k1 Z

1176

a 30

5-a0-

11 12 13 14 15 16 17 18 19Frequency (GHz)

Fig. 6. Simulation of the Boifot junction performance. Comparisonof the return losses obtained with the proposed method and the HFSS.

-5

-10

-15

m^ -20

a -25

_L -30

-35

-40

-45

-50CL.1 112.5 13 13.5 14 14.5 15 15.5 16 16.5 17 17.5 1E

Frequency (GHz)

Fig. 7. Experimental return losses of the OMT for both polarizations

The measured results of the entire OMT are shown in Fig.7, showing a return loss better than 27 dB in the designsubbands. The measured insertion losses for bothpolarizations are approximately 0.15 dB. In the OMT shownin Fig. 1, assuming that the axial arm E-plane bend is absent,the symmetry at both xz- and yz-planes isolates bothpolarizations completely, leading to perfect isolation betweenports. In the actual component, owing fundamentally tomisalignment in the manufacture, the measured isolationbetween ports is degraded to 55 dB.

IV. CONCLUSIONS

A CAD tool for the analysis and design of B0ifot junctionOMTs has been presented. The principal advantage of thisstructure is its wideband performance in return loss andisolation, consequence of its symmetrical properties.

An additional feature of this OMT relies on its power-handling capabilities. There are no irises in the structure,which improves its performance in terms of the multipactoreffect.The kernel of the OMT is the B0ifot junction. A MM

method has been presented for its characterization by meansof the GAM. The results have been checked with thoseobtained by using HFSS in order to validate the method. Theproposed routing branches can operate in a wideband and areamenable to be analyzed with efficient methods as theBCMM. The resultant CAD tool has been used for the designof a Ku-band prototype, whose experimental results havefulfilled the prescribed requirements.

REFERENCES

[1] J. Uher, J. Bornemann, and U. Rosenberg, Waveguidecomponents for antenna feed systems: Theory and CAD,Chapter 3, Boston, Artech House, 1993.

[2] E. J. Wollack, W. Grammer and J. Kingsley, "The B0ifotorthomode junction," Atacama Large Millimeter Array(ALMA) Report #425, 05/22/02. Available atwww.alma.nrao.edu/memos/html-memos/alma425/memo425.pdf

[3] G. Engargiola and A. Navarrini, "K-band orthomode transducerwith waveguide ports and balanced coaxial probes," IEEETrans. Microw. Theory Techn., vol. 53, no. 5, pp 1792-1801,May 2005

[4] A. M. B0ifot, "Classification of ortho-mode transducers," Europ.Trans. Telecommunications and Related Technologies, vol. 2,no. 5, pp. 503-510, Sept. 1991.

[5] M. Ludovico, B. Piovano, G. Bertin, G. Zarba, L. Accatino, andM. Mongiardo, "CAD and optimization of compact ortho-modetransducers," IEEE Trans. Microw. Theory Techn., vol. 47, no.12, pp. 2479-2486, Dec. 1999.

[6] J. M. Rebollar, J. Esteban, and J. De Frutos, "Asymmetricdouble-band orthomode transducer with high polarizationpurity", Microw. Opt. Techn. Lett., vol. 20, pp. 265-267, 1999.

[7] A. B0ifot, E. Lier, and T. Schaug-Pettersen, "Simple andbroadband orthomode transducer," IEE Proc. H, Microwaves,Ant. and Pro., vol. 137, no. 6, pp. 396.-400, Dec. 1990.

[8] J. M. Reiter and F. Arndt, "Rigorous analysis of arbitrarilyshaped H- and E- plane discontinuities in rectangularwaveguides by a full-wave boundary contour mode-matchingmethod," IEEE Trans. Microw. Theory Techn., vol. 43, pp. 796-801, Apr. 1995.

[9] J. A. Ruiz-Cruz, J. Esteban, and J. M. Rebollar, "An efficientboundary contour mode-matching method of H- and E-planejunctions by a fast fourier transform algorithm," IEE Microw.Ant. and Prop., vol. 150, no. 5, pp. 332-338, Oct. 2003

[10] E. D. Sharp, "An exact calculation for a T-junction ofrectangular waveguides having arbitrary cross sections," IEEETrans. Microw. Theory Tech., vol. 15, no. 2, pp. 109-116, Feb.1967.

[11] J.M. Rebollar, J. Esteban and J.E. Page, "Fullwave analysis ofthree and four-port rectangular waveguide junctions," IEEETrans. Microw. Theory Techn., vol. 42, no. 2, pp. 256-263, Feb.1994.

[12] C. Wang and K. Zaki, "Full-wave modeling of generalizeddouble ridge waveguide T-junctions," IEEE Trans. Microw.Theory Techn., vol. 44, no. 12, pp. 2536-2542, Dec. 1996.

Vertical Pol. (Measurements)Horizon. Pol. (Measurements)

u .