EP0223673B1 - Coupling device between an electromagnetic surface wave transmission line and an external microstrip transmission line - Google Patents

Description

The present invention relates to a coupling device between a surface wave line and a microstrip line. More specifically, it relates to a coupling device between a microstrip line, in which the field distribution is asymmetrical, operating in quasi-TEM mode, and an access line to a device, in which the field distribution is symmetrical, using an electromagnetic surface mode propagating in a thin core line, charged by ferrite parts, and polarized by a magnetostatic field, such as an insulator or non-reciprocal ferrite device.

An object of the invention is to make this non-reciprocal device integrable, and to do without the coaxial connectors which until now were used, because these are too bulky for integration.

A known technique, giving satisfactory results but delicate to implement and therefore not very usable, consists in integrating, on the two access lines to the surface wave device, a coaxial line element, in the form of a pearl. glass adapted to 50 ohms, which amounts to reconstructing the excitation system of the electromagnetic surface wave mode used in known devices. However, this glass bead introduces parasitic elements disturbing the adaptation of the device.

Furthermore, experience has shown that the direct connection of a microstrip line to the thin core line of a symmetrical surface wave device does not give good results, the insertion losses being too high.

The adaptation system, or coupling device according to the invention, consists in using, to make the transition between the thin core and the microstrip line, several line elements of short lengths and of small transverse dimensions compared to the length of wave, these elements being of different types and structures so as to achieve a gradual symmetry-asymmetry transition, in stages, the element closest to the thin core being necessarily symmetrical and of small transverse dimensions so as to impose a structure symmetric field of view at the access to the thin core line.

• - ondes de surface électromagnétiques,
• - triplaque
• - ligne microbande plus réactance
• - ligne microbande extérieure.

The symmetry-asymmetry transition is thus done in four stages, representing four modes:

• - electromagnetic surface waves,
• - triplate
• - microstrip line plus reactance
• - external microstrip line.

• - mode à onde de surface électromagnétique, symétrique
• - mode triplaque
• - mode microbande à deux diélectriques différents
• - mode microbande à air, dissymétrique.

More specifically, the invention relates to a device for coupling between a symmetrical electromagnetic surface wave line and an external microstrip line, the surface wave line, terminated by at least one microband access line, operating in a mode symmetrical field distribution, while the external microstrip line operates in an asymmetrical field distribution mode, this coupling device being characterized in that it comprises a plurality of line elements of short lengths and small transverse dimensions in front the wavelength of the signal, the nature and structure of these line elements providing a gradual transition between the symmetrical and asymmetrical modes in four stages:

• - symmetrical electromagnetic surface wave mode
• - triplate mode
• - microstrip mode with two different dielectrics
• - air microstrip mode, asymmetrical.

• - Figure 1 : vue en coupe d'un isolateur OSEL, connu,
• - Figure 2 : vue en plan d'un isolateur OSEL, connu,
• - Figure 3 : vue en coupe d'un dispositif de couplage d'une ligne microbande sur un isolateur OSEL, selon l'invention,
• - Figure 4 : vue en plan d'un dispositif de couplage d'une ligne microbande sur un isolateur OSEL, selon l'invention,
• - Figures 5 et 6 : vues en plan et en coupe des deux pièces qui assurent la transition en mode triplaque, dans le dispositif de couplage selon l'invention.

The invention will be better understood from the following description of an exemplary embodiment, this example being based, to be more precise in the description, on the case of an isolator called OSEL (electromagnetic surface waves), as well as on the attached figures which represent:

• - Figure 1: sectional view of a known OSEL insulator,
• - Figure 2: plan view of a known OSEL insulator,
• FIG. 3: section view of a device for coupling a microstrip line to an OSEL isolator, according to the invention,
• FIG. 4: plan view of a device for coupling a microstrip line to an OSEL isolator, according to the invention,
• - Figures 5 and 6: plan and sectional views of the two parts which ensure the transition in triplate mode, in the coupling device according to the invention.

The fact of choosing a non-reciprocal device such as an OSEL isolator to exhibit the invention does not limit the scope of the invention which applies more generally to electromagnetic surface wave devices and to the transitions between modes of distribution of symmetrical and asymmetrical fields. However, the prior description of an OSEL isolator will make it possible to better concretize the coupling device according to the invention.

• - deux plaques minces en ferrite 10 et 11,
• - une âme centrale très mince à profil spécial 12, placée entre les plaques en ferrite 10 et 11, - un aimant 13,
• - deux plaques en matériau absorbant 14 et 15, situées de part et d'autre de l'âme 12,
• - deux platines 16 et 17 en acier doux, rigides, servant simultanément de plans de masse (revêtement argenté) et de culasses pour refermer le circuit magnétique (représenté par des flèches).

An OSEL electromagnetic surface wave isolator is constituted according to the diagrams of FIG. 1 and of FIG. 2 which are to be considered simultaneously. This type of insulator is essentially made up, apart from its connection elements, by:

• - two thin ferrite plates 10 and 11,
• - a very thin central core with a special profile 12, placed between the ferrite plates 10 and 11, - a magnet 13,
• - two plates of absorbent material 14 and 15, located on either side of the core 12,
• - two rigid steel plates 16 and 17, serving simultaneously as ground planes (silver coating) and cylinder heads to close the magnetic circuit (represented by arrows).

All of these parts are made integral by clamping between the two cylinder heads 16 and 17, by means of screws whose holes appear in FIG. 2. In this figure, the cylinder head 16 as well as the ferrite plate 10 and the absorbent plate 14 are removed to reveal the internal structure of the insulator and the particular shape of the thin central core 12, which ends in two microstrips 18 and 19 of external access laughing, by coaxial connector, 50-ohm adapted glass bead or coupling device according to the invention.

When the ferrites are polarized by a magnetostatic field Ho normal to the plates, this type of structure supports modes of TEmo type of a particular nature, because we can admit that they are guided or confined between two "magnetic walls" defined by the parallel surfaces at Ho and resting on the edges of the central core 12.

For optimal operation, very wide band oscillators and receivers absolutely need a good adaptation, at least within their nominal operating band, and for most of them, within a certain range surrounding it, in order to avoid snagging by reaction or parasitic oscillations.

• - adaptation bien meilleure : rapport d'onde stationnaire maximum 1,25 (contre 1,5 pour les jonctions Y) dans la bande passante, et stable en phase,
• - adaptation quasi maintenue dans le reste de la bande, alors qu'une jonction Y se comporte comme un filtre passe-bande,
• - isolation supérieure à 18 dB contre 14 dB pour les jonctions Y.

OSEL electromagnetic surface wave isolators are best suited for non-reciprocal broadband ferrite devices. Compared to the only type of Y-junction isolator currently achievable (structure with 2 ferrites), they have the following advantages:

• - much better adaptation: maximum standing wave ratio 1.25 (against 1.5 for Y junctions) in the passband, and stable in phase,
• - adaptation almost maintained in the rest of the band, while a Y junction behaves like a bandpass filter,
• - insulation greater than 18 dB against 14 dB for Y junctions.

The use of this type of isolator in the new microwave systems using very flat amplifiers can be envisaged insofar as we know how to make an integrable transition between the OSEL mode, of TEoo type and the non-symmetrical quasi-TEM mode of the microstrip lines.

The problem posed by the connection of an OSEL isolator to a line of microstrip type therefore comes from the asymmetrical nature of the mode propagating on the microstrip lines.

The coupling device according to the invention has the merit of remaining continuous throughout the conductive cores in flat structure, therefore of reducing to their lowest expression the parasitic elements due to the discontinuities between the central core 12 and an external microstrip line.

This coupling device, according to the invention, is shown in section in Figure 3, while Figure 4 shows it in plan, mounted on an OSEL isolator and allows a better understanding of the design.

What is explained about one end 19 of the insulator is of course valid for the other end 18.

The benchmarks, preserved, make it possible to find in FIGS. 3 and 4 the constituents specific to the OSEL insulator of FIGS. 1 and 2.

In FIG. 3 appears - to the right of the figure - a fragment of the OSEL insulator, comprising a thin core 12, clamped between two thin ferrite plates 10 and 11, themselves clamped between two steel plates 16 and 17. The thickness of each of the plates 16 and 17 is sufficient for it to be possible to drill therein, longitudinally, a tapped hole for fixing the coupling device. The end 19 of the central core 12 protrudes from the insulator over a length of the order of 2.5 to 3 mm: it is on this end 19 that contact will be made with an external microstrip 9. L isolator further comprises, in known manner, two parts 7 and 8, placed between the ferrite plates 10-11 and the coupling device: these parts 7 and 8 are made of dielectric material of constant e ₂ and are used for the adaptation of the OSEL insulator.

The coupling device proper comprises the three parts marked 1, 2 and 3, and their respective mechanical supports 4, 5 and 6.

The part 1 is a part made of dielectric material of polytetrafluoroethylene type loaded with glass fiber, such as that known under the name of RT Duroid, but it can also be for example made of alumina or beryllium oxide. Its permittivity P-1 is the same as that of the support of the external microstrip piece 9 and that of the piece 2 which will be described later.

This part 1 has a T shape (see fig 5) and it is metallized on its two main faces, to give a ground plane 21 on one side and, after etching, a metallization 20 on the other face. The transverse branch 22 of the T has a length Li, a width ₁₁ , and a thickness of dielectric h _dl . The part 1 is attached to the OSEL insulator by its transverse branch 22, and the end 19 of the central core 12 comes to rest on the metallization 20.

According to a variant, represented by a dotted outline in FIG. 5, the etched metal track 20 may have an enlarged part. This enlarged part participates, with the dielectric part 3, in the adaptation in the transition between the symmetrical and asymmetrical modes.

• - même permittivité P-1
• - même longueur L₂
• - même largeur 1₂
• - même épaisseur de diélectrique h_d2
• - même forme
  que la branche transversale 22 de la pièce 1, mais elle est métallisée sur une seule face principale, en 23. La pièce 2 est accolée à l'isolateur OSEL par son côté le plus long, de sorte qu'elle correspond à la branche transversale 22 de la pièce 1. Mais la pièce 2 est posée par dessus l'extrêmité 19 de l'âme centrale 12, la métallisation 23 étant en contact avec ladite extrémité 19.

The part 2 is a tongue of dielectric material which has (see fig 6):

• - same permittivity P-1
• - same length L ₂
• - same width 1 ₂
• - same thickness of dielectric h _d2
• - same shape
  as the transverse branch 22 of the part 1, but it is metallized on a single main face, at 23. The part 2 is attached to the OSEL insulator by its longest side, so that it corresponds to the transverse branch 22 of the part 1. But the part 2 is placed over the end 19 of the central core 12, the metallization 23 being in contact with said end 19.

The part 3 is a parallelepiped of dielectric material of permittivity es, the thickness of dielectric of which is h _d3 and the width 1 ₃ , measured along the axis common to the end 19 of the core 12 and to the external microstrip 9 The dimensions of the part 3 are such that, when it is placed on the end 19 of the core 12, which constitutes a microstrip, it projects beyond this microstrip, in order to allow adaptation between the two microstrips 19 and 9. It is made of polytetrafluoroethylene, or alumina.

All of these three parts 1, 2 and 3 of dielectric material are mechanically held in place by three other non-magnetic metal parts, respectively 4, 5 and 6. These are for example brass or silver beryllium bronze, shade UBe 2.

The part 4 constitutes the support of the coupling device according to the invention. It is integral with the insulator, or more exactly with a plate 17, and ensures its correct mounting on a ground plane. This support 4 supports the part 1 of dielectric material, itself in contact by its etched metal track 20 with a first face of the microstrip 19 of the central core 12.

The part 5 is, like the support 4, integral with the insulator, and more exactly with the plate 16. This pressure part 5 holds in place the part 2 made of dielectric material, and presses it on a second face of the microstrip 19 of the central core 12, the metallization 23 of the part 2 being in contact with said microstrip 19.

The support 4 and the pressure piece 5 both have a housing which makes it possible to position the two dielectric pieces 1 and 2, and prevents their lateral sliding relative to the microstrip line 19.

The part 6 is a stirrup, secured to the support 4: it makes it possible to maintain the dielectric block 3, against the microstrip 19, and participates in the adaptation of the coupling device.

The dimensions of the dielectric and metallic parts, and in particular of the support 4, with respect to the microstrip 19, are such that they allow the end of an external microstrip 9 to be introduced into the housing provided in the support 4 for the part. 1. The microstrip 9 comprises a substrate, of permittivity ei, a metallization of a ground plane on a main face of the substrate, and the metallic track of the microstrip line on the other main face of the substrate: it is in the form of a tongue.

This external microstrip line 9 rests - when it is in place - by its ground plane on the support 4; it abuts against the dielectric part 1, and the microstrip line proper is in contact with the end 19 of the central core 12. The dielectric block 3 and the stirrup 6 support the end 19 of the core central 12 against the microstrip 9. So that the electrical contact is good, the end 19 is glued to the microstrip 9 by means of a conductive adhesive. In a variant, the end 19 can slide on the microstrip 9 during large temperature variations.

The nature (es) of block 3, and the dimensions of block 3 (1 ₃ ) and stirrup 6 (L ₄ ), measured along the axis of the microstrip line 19, allow, by adjustment, the compensation of the reactances discontinuity.

Figure 4 completes Figure 3, showing, seen in plan, an isolator provided with the coupling device according to the invention, as well as an external microstrip line about to be connected to the coupler. In order to better see the structure of the assembly, the insulator is cut at the level of the central core 12 and, for the coupler, the dielectric parts 2 and 3 as well as the metal parts 5 and 6 are removed.

It has been said that the coupling is obtained by using several line elements of small width and of small transverse dimensions compared to the wavelength, the type and the structure of these elements being different so as to achieve a gradual transition, in stages. , between the symmetrical or asymmetrical distribution of the fields. In this gradual transition, the isolator requires that the line element which is closest to it is symmetrical.

• - le plan de masse 21 de la première pièce diélectrique 1
• - la ligne microbande 20 en contact avec la ligne microbande
• - la métallisation 23 de la deuxième pièce diélectrique 2.

This is indeed the case for the three-ply line formed by:

• - the ground plane 21 of the first dielectric part 1
• the microstrip line 20 in contact with the microstrip line
• the metallization 23 of the second dielectric part 2.

• - le mode symétrique OSEL, à ondes de surface électromagnétiques, au niveau de l'isolateur 10 + 11 + 12 et de son adaptation 7 + 8
• - le mode triplaque au niveau de la branche transversale 22 de la première pièce diélectrique 1 et de la deuxième pièce diélectrique 2
• - le mode microbande et réactance au niveau du bloc diélectrique 3 et de l'étrier 6
• - le mode dissymétrique microbande au niveau de la microbande extérieure 9.

The coupling device according to the invention therefore ensures the transition between a device in which the distribution of the fields is symmetrical (OSEL) and a circuit in which it is asymmetrical by means of four stages in which the modes are different:

• - the symmetrical OSEL mode, with electromagnetic surface waves, at the level of the isolator 10 + 11 + 12 and its adaptation 7 + 8
• - the triple plate mode at the level of the transverse branch 22 of the first dielectric part 1 and of the second dielectric part 2
• - the microstrip and reactance mode at the level of the dielectric block 3 and of the stirrup 6
• - the asymmetric microstrip mode at the external microstrip 9.

Maintaining the width of the central core throughout the transition at values very close to that of the coupling level is an essential point of the transition. The dimensions (1 ₁ , 1 ₂ , 1 ₃ , 1 ₄ and h _d s) of the other parts are adjusted to maintain the necessary impedance level, that is to say generally close to 50 ohms.

• - que l'épaisseur h_L de la microbande 9 soit inférieure à l'épaisseur h_F des plaques de ferrite 10 et 11
  
• - que l'épaisseur h_L de la microbande 9 soit égale à l'épaisseur h_di et h_d2 des pièces diélectriques 1 et 2
  

For the realization of the coupler to give a good overall adaptation for the isolator and its transitions, for example a standing wave ratio ROS = 1.35 in a range between 6 and 18 GHz, a certain number of conditions are necessary. Some are mechanical:

• - that the thickness h _L of the microstrip 9 is less than the thickness h _F of the ferrite plates 10 and 11
  
• - that the thickness h _L of the microstrip 9 is equal to the thickness h _d i and h _d2 of the dielectric parts 1 and 2
  

• - la largeur 11 = 1₂ de la région triplaque (largeur des pièces diélectriques 1 et 2) doit être très inférieure au quart de la longueur d'onde dans le matériau diélectrique (101) de ces pièces, à la fréquence la plus haute
  
• - la longueur Li = L₂ de ces mêmes pièces 1 et 2 doit être inférieure à la moitié de la longueur d'onde dans ce même matériau, à la fréquence la plus haute
  
• - la largeur 1₃ du bloc diélectrique 3 doit être très inférieure au quart de la longueur d'onde dans le matériau diélectrique (es) du bloc 3, à la fréquence la plus haute
  

The others are linked to the wavelength λ in a material of permittivity _E , it being known that:

• - the width 11 = 1 ₂ of the triplate region (width of the dielectric pieces 1 and 2) must be much less than a quarter of the wavelength in the material di electric (101) of these parts, at the highest frequency
  
• - the length Li = L ₂ of these same parts 1 and 2 must be less than half the wavelength in this same material, at the highest frequency
  
• - the width 1 ₃ of the dielectric block 3 must be much less than a quarter of the wavelength in the dielectric material (es) of block 3, at the highest frequency
  

The invention has been explained by relying on the case of an OSEL isolator, and by describing and representing only one coupling device. It is obvious to a person skilled in the art that if the symmetrical device has more than one external connection, it is provided with the appropriate number of devices for coupling to an external microstrip line. For example, the insulator of FIG. 4 comprises in its embodiment a coupler on the end 18 of the central core and a coupler on the end 19. As a variant, the second access can be equipped with a corrector.

The coupling device according to the invention operates at least in the frequency range 6 - 18 GHz, with insertion losses of less than 1.6 dB and a standing wave ratio at the ports of less than 1.35.

It is applicable to all surface wave devices operating according to a symmetrical field distribution mode, provided that these devices are provided with at least one microstrip type access line.

The possible variations on the form, the nature of the materials or the construction, obvious to a person skilled in the art, form part of the field of the invention, specified by the following claims.

Claims (16)

1. A coupling device between a symmetrical electromagnetic surface wave transmission line and an external microstrip line, the surface wave transmission line being terminated by at least one access microstrip line (19) and operating according to a symmetric field distribution mode while the external microstrip line (9) operates according to an asymmetric field distribution mode, characterized in that the device comprises a plurality of line elements (1, 2, 3) of short length and small transverse size with respect to the signal wavelength, the kind and structure of these line elements (1, 2, 3) being such that a progressive transition between the symmet- tric mode and the asymmetric mode is obtained in four steps:

- a symmetric electromagnetic surface wave mode

- a three-plate mode (1+2)

- a microstrip mode having two different dielectric materials

- an asymmetric microstrip mode (9) in air.

2. A coupling device according to claim 1, characterized in that the three-plate line element comprises:

- a first component (1) made of dielectric material and being T-shaped, one main surface of which is metal coated and constitutes the earth plane (21) while another main surface is metal-coated and is engraved in order to constitute a microstrip (20) perpendicular to the transverse branch of the T,

- a second component (2) made of dielectric material and being tongue-shaped, one main surface of which is metal-coated (23),

- these two dielectric components (1, 2) being each applied against one surface of the access microstrip (19) of the surface wave line, namely the first component (1) in such a way that its engraved microstrip (20) is aligned with the axis of the access microstrip (19), and the second component (2) in such a way that its metal layer is perpendicular and in contact with the access microstrip (19).

3. A coupling device according to claim 2, characterized in that the first and second dielectric components (1, 2) are maintained in place and applied against the access microstrip line (19) by means of two metal components (4, 5) made of a non-magnetic material and integral with the surface wave device (16, 17), the first metal component (4) constituting a support for the first dielectric component (1) and for the access microstrip (19) and else the earth plane of the coupling device, and the second metal component (5) constituting a component urging the second dielectric component (2) against the access microstrip (19).

4. A coupling device according to claim 1, characterized in that the microstrip mode line element having impedance matching capacities comprises a third component (3) made of dielectric material and being block-shaped, this component being located on the end of the access microstrip (19) between the three- plane mode line element (1+2+4+5) and the external microstrip (9).

5. A coupling device according to claim 4, characterized in that the third dielectric component (3) is maintained in place by a third metal component (6) which is stirrup-shaped and is made of a non-magnetic material.

6. A coupling device according to claim 3, characterized in that the metal support (4) comprises a housing for positioning the first dielectric component (1) and the end of the external microstrip line (9), the latter abutting against the first dielectric component (1) and being in contact with the end of the access microstrip (19).

7. A coupling device according to claim 6, characterized in that the access microstrip (19) is glued onto the external microstrip line (9) by means of a conducting adhesive or is fixed in order to allow a sliding movement.

8. A coupling device according to claim 2, characterized in that the first and second dielectric components (1, 2) are made of a material having the same permittivity (e_i) as the dielectric material constituting the substrate of the external microstrip line (9), their dielectric layer thickness (h_di, h_d2) being the same as the thickness of the substrate (h_L) of the external line

9. A coupling device according to one of claims 4 or 5, characterized in that the permittivity (es) of the third dielectric component, its thickness (h_d3), its length (1₃) and the length (1₄) of the metal stirrup (6) are adjusted to ensure the matching of the impedances of discontinuity.

10. A coupling device according to claim 2, characterized in that it is necessary to respect the following formulae

Li and L₂ being the length of the first and second dielectric components (1, 2) respectively, measured perpendicularly to the access microstrip (19), and 11 and 1₂ being the widths of these components, λ_ε1 being the wavelength at maximum frequency in the dielectric material of permittivity ei.

11. A coupling device according to claim 4, characterized in that it is necessary to respect the following formula

1₃ being the length of the third dielectric component (3) measured along the axis of the access microstrip (19), and λ_ε3 being the wavelength at maximum frequency in the dielectric material of permittivity _E3.

12. A coupling device according to claim 1, characterized in that the three dielectric components (1, 2, 3) constituting line elements are made of polytetrafluoroethylene filled with glass fibers as far as the first and second components (1, 2) are concerned. 13. A coupling device according to claim 1, characterized in that the three dielectric components (1, 2, 3) constituting line elements are made of ceramic material such as alumina.

14. A coupling device according to one of claims 3 or 5, characterized in that the three metal components (4, 5, 6) are made of brass or of beryllium bronze.

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