BROADBAND DUAL MODE COUPLERS WITH CYLINDRICAZ, POSTS IN RECTANGULAR WAVEGUIDES

M. G. Castello Branco CPqD / TELEBdS

13088-061 / S.Paulo - Brasil Rod. C ~ ~ ~ ~ p i n a s Mogi-Mirim, Km. 118,5 - Campinas

Luiz Costa da Silva Pontificia Universidade Cat6lica do Rio de Janeiro

R. Marques de S. Vicente, 225, Gvea - Rio de janeiro 22453-900 / Rio de Janeiro - Brasil

Abstract

Moment Method techniques associated with experimental protootype bench work have made possible to develop a m series of broadband dual mode couplers that can be applied to SmalE antenna feed vstem. Those couplers incorporate cylindrical posts which can be partially inserted in the main waveguide to act as a jilter, or can bejiclly inserted to reflect all the energy contained on one of the polarization components driving it to the coupling window.

1. INTRODUCTION

The increasing demand on new satellite services and spectrum utilization has made necessary the development of compact and low cost earth station antennas that can be operated in wider bandwidths and with fkequency reuse. These requirements led to new reflector and feed system design solutions, specially on the dual mode couplers and small diplexing structures which ~ p e a r in literature [1-4] with very few analytical and design approaches.

This paper presents the preliminary results of the development of a new family of broadband dual mode couplers which include a series of cylindrical posts inserted in the main rectangular waveguide that were optimized through Moment Method techniques used for the scattering matrix determination [SI.

Figure 1 shows a the dual mode coupler, with fully inserted cylindrical posts in the main waveguide. In figure 2 it is shown a different version of coupler, with posts partially inserted in the main waveguide, acting as a filter.

2. THEORETICAL FORMULATION

To determine the scattering matrix of the coupler shown in Fig. 1, the device was replaced by the equivalent structusre shown in figure 3, where conductive walls and equivalent magnetic cunent distributions were imposed in the coupling windows between the rectangular waveguides that constitute the coupler, and the cylindrical posts were replaced by electric current densities. In this way, the coupler was divides into three regions: one rectangular waveguide, one rectangular cavity and one rectangular waveguide telinated into a short.

The electric and magnetic current densities were determined by the moment method. The scattered fields, and consequently the scattering matrix, were obtained by applying d m c Green's functions techniques.

The magnetic current densities were expanded into a series of basis functions, being used, as basis functions, the modal electric fields for the rectangular waveguide with transverse dimensions identical to those of the windows. Only "E,, modes were considered, since, due to the smaU width of the windows, "E,, and modes, with m not equal to zero, do not contribute significantly to the fields.

0-78034165-1/97/$10.00 @ 1997 IEEE. 434 SBMOAEEE MTT-S IMOC'94 Proceedings

For the expansion of the electric current densities, two set of basis functions were considered for each post, generating fields TE and TM to the direction y, respectively (see figure 3).

It resulted

n L

+ 3 + Where A41 andM2 are the magnetic current densities over the lower and upper windows, respectively, J i is the electric current density induced in the i-th post, a ln, a z n , ynm, 5 nm are coefficients to be determined, a and b are the transverse dimensions of the main waveguide, R,, is the radius of the posts, L is the length of the windows, k, is the fiee space wavenumber 8 is the angular coordinate shown in figure 3, and a ,,, a z, a e are the unit vectors in the directions y, z and 8, respectively.

+ + +

To determine fhe fields generated by the electric current densities inside the waveguide, the method of images was applied, in a similar way as employed by one of the authors in the analysis of cylindrical posts in rectangular waveguides [5], avoiding series with slow convergence, that would result fiom the application of Green's functions to the determination of such fields. As a result of the choice of basis functions and techniques for the determination of the fields, all integrations inherent to the moment method, with exception of a simple one dimensional integral, were made analytically, resulting result a fast computer code, able to produce rigorous results, that was incorporated into optimktion schemes, for the optimized design of the couplers.

3. NUMERICAL AND EXPERlMENTAC RESULTS

As an example of application, a coupler with the geometry shown in figure 1 , with 6 cylindrical posts, was designed to operate in the frequency band of 10.7 to 14.2 GHz. Theoretical results for the return loss in the main waveguide are shown in figure 4. The return loss is better than 33 dl3 all over the frequency band. - -

The convergence of the mathematical model is illustrated in table I, that shows the variations in the values of the return loss at 10.7, 11.7 and 12.7 GHz, as the number of basis functions used to represent the magnetic current densities is changed.

For the geometry shown in figure 2, a prototype for a Ku band coupler was built, to pass the 12 GHz band and deviate the 14GHZ band to the coupling window. The position of the partly inserted posts, relative to the coupling window, was adjusted experimentally. The coupler achieved experimental results better than -25 dB of passband return loss (10.7 to 12.75 GHz) , and -45 dJ3 of cutband isolation (13.75 to 14.8 GHz).

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magnelx currents - 10 -32.77 -44.31 .- -47.13

12 -33 -03 -45.17 -46.74

14 -33.22 -45.81 -46.38 I I

Table I - Return Loss theoretical results at port 1 of a 6 posts dual mode Ku band coupler

References

[I] G. Broussaud, “Sur Quelques Perfectionnements Aux Circuits Hypefiequences Pour Faiceaux Hertziens”, Annales de Radio-Eletricit6, tome XTI, no 49, June, 1957. [2] RC. Johnson, F. L. Cain and E.N. Bod, “Dual Mode Coupler”, EEE Trans. on MTT, vol.15, pp 651-652, Sept., 1962. [3] J.Y. Wong. A Dual Polarization Feed Horn for a Parabolic Reflector”. The Microwave J o d , pp. 188-191, Sept. 1962. [4] G. Hy&, RW. Kreutel and L.V. Smith, “The Unattended Earth Terminal Multiple-beam Torus Anten”’, Comsat Tech. Rev., vol. 4, pp.231-262, Nov., 1974. [5] L. C. Silva and O.M.C.P. Filho, “Scattering Matrix of Cylindrical Posts Centered on the Walls of Rectangular Waveguides“, IEEE Trans. on M’TT, vo1.42, pp. 1198-1205, July.

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Figure 1 - Coupler prototype with fully inserted posts reflecting all the aligned field component

Figure 2 - Coupler prototype with partially inserted posts reflecting (as a filter) some frequency bands of the aligned field component

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b J2 'cc("

- 2

longitudinal section t a __+

cross section

1

Figure 3. Equivalent structure for the coupler shown in figure I

-30

Y

-40 v) v)

4: -45

t w -55 a

-60

10.7 11.0 11.3 11.6 11.9 12.2 12.5

FREQUENCY ( G H z )

12.8

Figure 4. Return loss (d33) as a function of frequency of a Ku band coupler of the type shown in figure

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