Microwave transition device between a strip line and a rectangular waveguide where a metallic link bridges the waveguide and a mode converter

For associating different technologies of a microstrip line and of a rectangular waveguide, for example on a ceramic, in a transition device including a mode transformer between the line integrated into a printed circuit board, and the waveguide, the board includes a housing containing the waveguide with a large sidewall coplanar and coaxial to the strip of the line and another large sidewall fixed onto a metallic layer of the board at the bottom of the housing. A linking metallic element bridges a mechanical tolerance gap between the transformer and one of the line and the waveguide. The transformer can be integrated into the board, or into the waveguide in a microwave component.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the entry into the United States of PCT Application No. PCT/EP2010/069007 filed Dec. 6, 2010 and claims priority from French Patent Application Number FR 0958684 filed Dec. 7, 2009, the entirety of each of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to passive components for microwave propagation. More particularly, it relates to a planar transition device between a conductive microstrip line and a component in rectangular waveguide technology.

The conductive microstrip technology offers the possibility to quite easily integrate microwave functions to frequencies of a few Gigahertz, including up to the C-band. Such a technology becomes more complex when used at higher frequencies, of about ten Gigahertz (Ku-band, K-band and Ka-band). Indeed, the radiating nature of a microstrip line requires conductors to be contained in a conductive 15 mechanical structure providing an electric shielding. The dimensions of such a mechanical structure should be smaller since the frequency is high.

SUMMARY OF THE INVENTION

Air waveguides are, by nature, not radiating structures, and are poorly adapted for integrating complex functions. As a result, waveguides are used for low loss devices or for high microwave powers. Replacing air by a dielectric with a relative permittivity higher than 1, allows the dimensions of the waveguide to be sufficiently reduced so as to allow a substrate integrated waveguide to be integrated into a microstrip line.

The article “Integrated Microstrip and Rectangular Waveguide in Planar Form” by Dominic Deslandes and Ke Wu, IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, Vol. 11, No. 2, February 2001, provides a solution to the transformation with no loss of the quasi-TEM propagation mode in the microstrip line into the electric transverse fundamental mode TE10of the waveguide. The transition device according to this article comprises one single thin dielectric substrate wherein there are integrated a microstrip line, a rectangular waveguide and a planar mode transformer between the line and the waveguide. The mode transformer provides, in addition to the transformation from the quasi-TEM mode into the TE10mode, the electric continuity between the line and the waveguide. On the face of the dielectric substrate supporting the strip of the line, the mode transformer comprises an isosceles trapezoid tapered conductive section having a small base merging into an end of the strip and a larger base merging into a central portion of the cross sectional edge of a first large sidewall of the waveguide. The other face of the dielectric substrate is fully covered with a conductive layer acting as a ground plane for the line and as a second large sidewall for the waveguide. The small longitudinal sidewalls of the waveguide are made either by two rows of metallized through holes or by two metallized grooves arranged in the dielectric substrate. Thus, the height (or the thickness) of the waveguide can be reduced with little influence on the propagation of the TE10 mode, allowing the waveguide to be integrated into the thin dielectric substrate of the microstrip line while reducing losses through radiation.

The structure of the transition device in the abovementioned article is used in European patent 1 376 746 81 for integrating a microwave filter in rectangular waveguide and a microstrip line on the same thin dielectric substrate.

An object of the invention is to associate, by means of a microwave transition device, a first technology of a microstrip line with a second technology of a waveguide different from the first one, while maintaining the advantages both of those technologies.

Accordingly, a transition device comprising a mode transformer between a conductive strip line integrated into a printed circuit board, and a rectangular waveguide, is characterized in that the board comprises a housing containing the waveguide having a large sidewall coplanar and coaxial to the strip of the line and another large sidewall fixed onto a metallic layer of the board at the bottom of the housing, and the device comprises a gap bridged by a metallic linking element and located between the mode transformer and one of the line and the waveguide.

The mode transformer is integrated into the dielectric substrate either of the board according to the first technology or of the waveguide according to the second technology. If the mode transformer is integrated into the dielectric substrate of the board, the gap and the metallic linking element are located between the mode transformer and an end of the waveguide. If the mode transformer is integrated into the dielectric substrate of the waveguide, the gap and the metallic linking element are located between an end of the strip line and the mode transformer. The gap results from a mechanical tolerance for introducing the structure of the waveguide into the housing of the board. The metallic linking element which can comprise one or more metallic sheet strips or one or more metallic wires, provides the electric continuity between the strip of the line and a large sidewall of the waveguide via the mode transformer that matches the impedances of the strip of the line and the larger sidewall of the waveguide while taking into consideration the mismatch created by the gap bridged by the linking element. The impedances are matched in the mode transformer by strip line segments having strip widths and thicknesses, i.e. the distances between the microstrip line and the ground plane, that increase by steps from the strip line to the waveguide, and having lengths approximately equal to one quarter of wavelength.

Whatever the embodiment of the transition device, the microstrip line technology, like that of a multilayer printed circuit board, and the manufacturing technology for the waveguide, like Substrate Integrated Waveguide (SIW) technology on a ceramic substrate, are maintained, imparting more flexibility in the choice of the characteristics of the line and the waveguide, more specifically the different dielectric relative permittivities of the board and the waveguide. In particular, the waveguide can be integrated into a microwave component having a ceramic substrate; the small sidewalls of the waveguide can each be constituted by rows of staggered metallized holes for reducing the losses through radiation.

This invention achieves low radiation, low loss and low weight microwave structures, while suppressing a large part of the metallic structure and is thus particularly valuable for airborne devices. It enables the association of a microstrip line with various rectangular waveguide structures, including very selective filters and couplers with high directivity. In particular, this invention is appropriate for implementing emitting or receiving heads, or network or electronic scanning antennas, operating at high frequencies up to about ten Gigahertz.

This invention also relates to a method for manufacturing a transition device comprising a mode transformer between a strip line integrated into a printed circuit board, and a rectangular waveguide. The method is characterized by the following steps:arranging in the board a housing having a bottom consisting in a portion of a metallic layer internal to the board,introducing the waveguide inside the housing so that a large sidewall of the waveguide be coplanar and coaxial to the line strip and another large sidewall of the waveguide be fixed onto the portion of the metallic layer, andforming and fixing a thin metallic linking element bridging a gap between the mode transformer and one of the line and the waveguide.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

According to an embodiment of the invention shown inFIGS. 1 to 4, a transition device is a passive microwave circuit between a microstrip line1integrated into a thin printed circuit board2of the multilayer PCB (“Printed Circuit Board”) type and a microwave component3with a rectangular waveguide structure between which a planar mode transformer4is arranged. In these figures, two transition devices symmetrical about the transversal plane of the microwave component3are arranged at the longitudinal ends of the component on the same board2. The component3is to be fitted on the board2for being adapted, to the size and propagation characteristics of the microstrip line1. The board2integrating the microstrip line1thus acts as a support for the component3.

The printed circuit board2is a microwave circuit and has a transverse section with a smaller thickness E (FIG. 4) compared to its width L (FIG. 1). The board comprises layers of dielectric substrate20between which internal metallic layers superimposed on a first face of the board. The internal metallic layers are a ground layer12for the line1and ground layers21,22and23under the layer12for the mode transformers4, as further described. The metallic layers12,21and22extend the whole width L of the board and to a depth b (FIG. 1) of the board equal to the height of the component3. The layer23located at the depth b and another metallic ground layer24arranged on a second face of the board2are separated by a layer of the substrate20with a thickness E-b (FIG. 1) and extend on the whole length and the whole width of the board. The layers23and24make up ground planes common to all the components supported by the board. The various layers12and21,22,23and24are connected therebetween by small metallized holes25perpendicular to the faces of the board.

As shown inFIGS. 1,2,3and5, the line1comprises a layer10of the substrate20, a rectilinear metallic strip11on the layer10at the level of the first face of the board and along the longitudinal axis X-X of the board, (FIG. 1) and a ground plane formed by the internal metallic layer12underlying the portion of the first face of the board supporting the strip11.

Between the metallic layers23and24of the board, other microwave devices (not shown) can be provided.

The substrate20is a dielectric with a low relative permittivity ∈r2. The width w of the strip11(FIG. 4) and the thickness e of the line, for example, of approximately E/12, are small, with respect to the width L of the board and the ground plane12, (FIG. 1) so that the microstrip line1is able to propagate a wave guided in the quasi-TEM mode in the range of centimetric waves, including for high frequencies from a few Gigahertz to about forty Gigahertz so as to cover, for example, all or part of the frequency of the Ku-, K- and Ka-bands. A large part of the power is propagated in the dielectric and a small part is propagated in air in the vicinity of the conductive strip11. The characteristic impedance Z1cof the microstrip line, typically of 50Ω, essentially depends on the width w of the strip and on the thickness e and the permittivity ∈r2of the selected dielectric substrate20.

As shown inFIGS. 1,2and5, on both sides of the conductive strip11, the line1is shielded by two metallic layers13extending symmetrically about the axis X-X, coplanar to the strip11on the first face of the board2and extending in parallel to the strip11at a predetermined distance of a few widths w of the strip11for confining the electric field lines toward the strip. The shielding layers13are connected to the ground layers12and21,22,23and24by metallized holes25.

The passive microwave component3is manufactured according to a Substrate Integrated Waveguide (SIW) technology with a waveguide31integrated into a dielectric substrate33with a rectangular section. As shown inFIGS. 1,2,3,4and6, the rectangular section of the waveguide comprises large sidewalls formed by two longitudinal metallic layers31sand31i(FIG. 3) on the large faces of the substrate33and small sidewalls formed by two pairs of peripheral longitudinal rows of staggered metallized holes321and322crossing the substrate33. The pairs of hole rows321and322are symmetrical about the longitudinal axial plane of the component3. The distance between two neighboring holes321,322in each row is substantially equal to the diameter of the holes and significantly less than the operating wavelength of the waveguide so as to minimize any loss through radiation. The width a (FIG. 6) of the waveguide is defined by the distance between the pairs of rows of metallized holes321-322depending on the dimensions of the holes and on the pitch between the holes. The height b (FIGS. 4,6) of the waveguide in the direction of the thickness E of the board2is defined by the distance between the metallic layers31sand31i. Alternatively, the waveguide31is replaced by a conventional waveguide31with a rectangular section having solid metallic sidewalls and filled with the dielectric substrate33. The SIW manufacturing technology of the component3uses in the shown embodiment a Low Temperature Cofired Ceramic (LTCC) method, wherein the dielectric substrate33is a ceramic with a relative permittivity ∈r2higher than the relative permittivity ∈r2of the dielectric substrate20in the board2and therefore, higher than that of the layer of substrate10in the microstrip line1.

In other variants of the transition device, the dielectrics of the substrate20of the board2and the substrate of the line1and of the substrate33of the waveguide31can be of the same nature and have an identical relative permittivities ∈r2and ∈r3.

In order to avoid propagation discontinuities and to facilitate the change of the quasi-TEM mode of the microstrip line to the TE10mode of the waveguide, the height b (FIG. 4) thereof is selected to be equal to the available thickness in the board2. To this end, a parallelepiped housing26is arranged in the board2in which is inserted, with a transversal play, the waveguide component3between the ends of the mode transformers4. The height of the housing26is equal to the height b (FIG. 4) of the waveguide and to the thickness between the metallic strip11of the microstrip line1and the internal metallic layer23. The external face of the large sidewall of the waveguide formed by the metallic layer31sis coplanar to the strip11of the line1, and the external face of the other large sidewall of the waveguide formed by the metallic layer31iis in mechanical and electric contact with the portion of the metallic layer23at the bottom of the housing. The portion of the board underlying the housing26with a thickness E-b (FIG. 4) between the metallic layers23and24is maintained for optionally integrating therein one or more microwave devices. The length of the housing26is substantially greater than the length of the waveguide31and of the component3so as to facilitate arranging it with a mechanical tolerance play. The width of the housing26can be equal to the width L (FIG. 1) of the board for easily machining the board. The width of the component3more than the width a of the waveguide31is generally at the most equal to the length L of the board2and is determined as a function of the cutoff frequency of the TE10mode in the waveguide which is a function of2a. For example, the ratio a/b is approximately 10 to 15 and the waveguide is thus flat. The component3with the waveguide31is centered in the housing26and fixed by brazing the metallic layer31ion the portion of the metallic layer23at the bottom of the housing26while carefully aligning the symmetry longitudinal axial plane of the waveguide with the longitudinal symmetry axis X-X of the strip11of the line1.

According to the illustrated embodiment, the passive microwave component3with a rectangular waveguide planar structure31is a bandpass microwave filter comprising six pairs of metallized holes34(FIGS. 1,4) crossing the dielectric substrate33and connected to the metallic layers31sand31i. The pairs of metallized holes34are arranged symmetrically about the longitudinal and transverse axial planes of the component. The arrangement of the holes34makes up inductive pillars depending on the frequency response of the filter. According to another example, the microwave component3is designed as a directive coupling device.

The propagation mode transformer4in a transition device connects facing ends of the strip11of the microstrip line1and the large sidewall31sof the waveguide31coplanar to the strip11, and connects the internal ground plane layer12of the microstrip line to the large sidewall31iof the waveguide31fixed to the metallic layer23at the bottom of the housing26. The mode transformer4progressively transforms, while minimizing losses, the quasi-TEM mode of the microstrip line1into a TE10guided mode of the waveguide31and matches the impedances thereof. The planar structure of the mode transformer is designed so as to make up a nearly perfect quadripole, having transmission parameters S12and S21the terminals of the quadripole being approximately equal to 1 and having reflection parameters S11and S22n the terminals of the quadripole approximately equal to 0, taking into consideration, in a practical situation, losses induced by imperfect conductors and dielectrics.

The mode transformer4can be integrated into the waveguide31, or even be integrated into the board2, as described hereinafter and shown inFIGS. 1 to 4. As the characteristic impedance of a microstrip line decreases when the ratio w/e increases, the mode transformer4comprises N microstrip line segments21-41to2N-4N symmetrical about the longitudinal plane of the line1having X-X as the axis. The number N is generally at least equal to 1 and depends on the manufacturing technology based on layers of the board2and on that of the microwave component3. The lengths of the segments of the mode transformer4are approximately equal to one quarter of the wavelength of the operating central frequency and allow for a progressive impedance transformation while minimizing interference reflections at the junctions between segments. The mode transformer4according to the illustrated embodiment comprises N=3 line segments21-41,22-42and2N-4N=23-43. The strip4N=43 the closest to the component3has longitudinal edges substantially collinear with the longitudinal internal solid edges of the waveguide31delimited by the large sidewall31sand the rows of metallized holes321. As shown in detail inFIG. 4, introducing with a transverse play the component3in the housing26of the board2creates two air gaps5of several tenths of a millimeter between the longitudinal ends of the component3, and thus of the waveguide31, and the longitudinal ends of the line segments2N-4N=23-43 of the mode transformers4. For each mode transformer4, a thin metallic linking element6with a length a bridges the respective gap5and is interposed at the level of the facing transversal edges of the strip4N=43 and the metallic layer31sof the waveguide for providing an electric continuity between such edges. The linking element6can be achieved by one thin metallic strip or several juxtaposed thin metallic strips, for example, being cut in a gold sheet or juxtaposed thin metallic wires54, (FIG. 7b) extending parallel to the axis X-X and having the ends brazed on the strip4N=43 and the layer31sso as to cover the gap on the width a (FIGS. 4,6). The bottom of the gap5is a small portion of the metallic ground layer23providing the electric continuity between the ground planes12,21,22and23of the line1and the line segments21-41,22-42and23-43, via the metallized holes25, and the metallic layer31iof the component3fixed on the underlying portion of the metallic ground layer23. Because of the transition between the microstrip-and-dielectric-line segment and the air-and-microstrip line and the transition between the air-and-microstrip line and the waveguide at the level of the air gap5, the lengths of the line segments are somewhat different therebetween and can be each somewhat lower than, equal to or somewhat higher than one quarter of the operating wavelength so as to compensate for interference effects including wave reflection at various transitions, in particular at the level of the gap5, and so as to bring back by the transformer4an impedance equal to the characteristic impedance Z1cof the line1, at the junction between the latter and the first line segment21-41.

As shown inFIGS. 1 and 2, the line segments21-41,22-42and23-43are shielded by symmetrical pairs of metallic layers47,48and49extending the shielding layers13. The shielding layers47,48and49are coplanar to the strips41,42and43on the first face of the board and extend in parallel along such strips at the predetermined distance of a few widths w (FIG. 5) of the strip11. The shielding layers47,48and49are connected respectively to underlying ground layers12and21to24by metallized holes25.

In a second embodiment, where the mode transformer is integrated into the waveguide31and thus, to the component3, the housing26arranged in the board is much longer. The arrangement of the line segments21-41,22-42and23-43with the shielding layers47,48and49and the width a (FIG. 6) of the waveguide remain. The strips41,42and43originate from the same metallic layer as the large sidewall31sof the waveguide and in electric continuity with the latter on the same face of the substrate33of the structure of the waveguide. The dimensions of the line segments having their metallic ground layers superimposed and integrated into the substrate33of the structure of the waveguide, that is then of the multilayer type, are modified as a function particularly of the relative permittivity ∈r3. The strip4N=43 the closest to the component3has still the width a (FIG. 6) of the waveguide31and is directly linked to the transversal end of the large sidewall31sof the waveguide. The air gap5is thereby suppressed between the line segment23-43and the waveguide31and replaced by an air gap as a result of the play required for introducing the monolithic assembly of the component with the two mode transformers in the housing of the board. The air gap is located between the end of the strip line1and the line segment21-41having the less wide strip and is bridged by a thin linking metallic element similar to the element6, but with a width w (FIG. 5), and brazed to the strips11and41.

The method for manufacturing a transition device comprises the following steps. Upon manufacturing the multilayer printed circuit board according to the illustrated embodiment, the mode transformer4is integrated into the board, or even in a second embodiment of this invention, the mode transformer is integrated into the waveguide structure of the component.

Then, the parallelepiped housing26is arranged in the board2at a depth equal to the height b (FIG. 3) of the rectangular waveguide31, for example, by means of a matrix having the dimensions of the housing upon compression of the layers of the dielectric substrate20superimposed and coated with various metallic layers while the board is being manufactured, so that a portion of the internal ground layer23makes up the bottom of the housing.

The rectangular waveguide31, or in particular the component3with a rectangular waveguide structure, is introduced with a longitudinal play and centered in the housing26so that the large sidewall31sof the waveguide become coplanar and coaxial to the strip11of the line1and the other large sidewall31iof the waveguide be fixed through brazing on the portion of the metallic layer23of the board at the bottom of the housing. The longitudinal play results from a mechanical tolerance for inserting the rectangular waveguide31, or in particular the component3, into the housing26.

Then a strip or a web of several side by side strips,52(FIG. 7a) cut from a metallic sheet, or a web of several side by side metallic wires54(FIG. 7b) having a width higher than the width of the gap5and a thickness similar to that of the metallic layers is presented on the gap5so as to form the thin linking metallic element6. The longitudinal ends of the linking metallic element are fixed on the edges of the gap5. For the embodiment illustrated in the figures, the linking metallic element6bridges the gap5between the mode transformer4integrated into the board2and the waveguide31, has a length equal to the width a of the waveguide, and has longitudinal ends brazed to the transversal edge of the widest strip43of the line segments21-41,22-42and2N-4N=23-43 of the mode transformer and to the transversal edge of the large sidewall31sof the waveguide. For the second embodiment, the linking metallic element6bridges the gap between the microstrip line1and the mode transformer4integrated into the waveguide structure31, has a length equal to the width w of the conductive strip11, and has longitudinal ends brazed to the cross-sectional edge of the strip11and to the transversal edge of the less wide strip41of line segments21-41,22-42and2N-4N=23-43 of the mode transformer.

For another embodiment, shown inFIG. 7a, the linking metallic element6bridges the gap5between them mode transformer4integrated into the board2and the waveguide31, and the linking metallic element6is formed by a web of several side-by-side strips52. For another embodiment shown inFIG. 7b, the linking metallic element6bridges the gap5between the mode transformer4integrated into the board2and the waveguide31, and the linking metallic element6is formed by a web of several side-by-side metallic wires54.