EP0467818B1 - Transition element between electromagnetic waveguides, especially between a circular waveguide and a coaxial waveguide - Google Patents

Description

The field of the invention is that of transition elements between electromagnetic waveguides.

In the microwave domain, the waveguides are the elements ensuring the guided transmission of an electromagnetic signal for example between a source and a radiating element. The most common microwave signal transmission components are the rectangular guide, the circular guide, and the coaxial guide.

Transition elements are elements that are simply inserted between two guides of different types to change the transmission technology. Thus, there are transition elements making it possible to go from a technology from rectangular guide to coaxial guide, from rectangular guide to circular guide, from circular guide to coaxial guide, and vice versa.

For example, document US 2,981,904 (AJIOKA) describes a collinear transition between a rectangular waveguide and a coaxial waveguide. The objective of this transition is to provide a broadband transition performing the transformation from TE10 mode (in the rectangular guide) to TEM mode (in the coaxial guide). All the means used (edges and inclined block in particular) have a continuously variable section.

The most frequently used transitions are those allowing to pass from a rectangular or circular guide technology to a coaxial guide.

Circular guides are preferably used in certain frequency bands, because they have notable advantages: they are easier to produce than rectangular wave guides and their circular configuration allows them to be used as rotary joints ( in particular in the field of rotating antennas used for aerial and maritime surveillance) mechanically dissociating a fixed assembly from a mobile assembly, without creating discontinuity in the guided propagation.

The present invention specifically relates to the transitions between circular electromagnetic waveguides and the electromagnetic waveguides coaxial.

In a known manner, and as explained in particular in document US 2 207 845 (WOLFF), the transition from a circular guide to a coaxial guide takes place by the gradual appearance of an internal conductor (as shown in FIG. 2).

FIG. 2 represents a longitudinal section of a transition between a circular guide and a coaxial guide.

An electromagnetic wave propagates in a direction 24 in a circular guide 21 to which is connected a transition 22 of radius A comprising in its center a conical conductor 20. The conical conductor 20 constitutes one end of a circular conductor 23 of radius B forming a conductor center of a coaxial guide 25. The transition 22 constitutes one end of a coaxial waveguide 25. The coaxial guide 25 consists of two conductors 23, 26 of external A and internal radii B and a dielectric 27 allowing placing the internal conductor 23 coaxially inside the external guide 26. The dielectric can either completely fill the section between the internal conductor 23 and the external guide 26 over the entire length of extension of the coaxial guide, or consist of thin dielectric discs spaced apart and arranged regularly along the coaxial guide. The dielectric chosen must of course not disturb the transmission of waves carried out.

The progressive transition 22 is characterized by an angle α. Usually the value of the angle α is between 7 and 10 degrees, depending on the bandwidth and the standing wave ratio (R.O.S.) desired. The relationships between the R.O.S., the bandwidth and the angle α are such that the angle α must be small if a high bandwidth or an R.O.S. low (low mismatch, high transferred power).

Thus, so that the transitions carried out do not limit the bandwidth too much or cause too large reflections due to a mismatch, it is necessary to choose a low angle α, for a radius B of the constant central conductor, whence a relatively long transition length 22.

A significant transition length constitutes a non-negligible drawback, in particular in the case where it is not possible to accept a compromise on the transmission characteristics. So to maintain a cutoff frequency and bandwidth suitable, it is not always possible to reduce the radius B of the central conductor 23 to reduce the length of the transition 22.

Furthermore, the longer the transition 22, the greater its weight. This constitutes a major drawback, in particular in the case where such a transition 22 must be part of a device mounted on a satellite.

Another disadvantage of known transitions is that the end 28 of the conical part 20 of the central conductor 23 must absolutely be placed in the center of the circular waveguide 21, so as not to excite unwanted modes, in particular the TEM mode. (Magnetic Transverse) of the coaxial waveguide 25 which can propagate whatever the transmission frequency.

The present invention aims in particular to overcome these drawbacks.

More specifically, a first objective of the present invention is to implement a transition element between a circular electromagnetic waveguide and a coaxial electromagnetic waveguide of reduced length and mass compared to existing transitions, for a bandwidth and an equivalent adaptation.

A second objective of the present invention is to provide such a transition element ensuring conservation of the desired propagation mode or modes, and avoiding the excitation of unwanted modes. In particular, the invention aims not to excite, in the coaxial waveguide, the TEM mode.

Another objective of the invention is also to present a circular guide / coaxial guide transition element whose position of the central conductor is less critical than in the case of a conical central conductor end.

These objectives, as well as others which will appear subsequently, are achieved by means of a transition element for electromagnetic waveguides, of the type intended to ensure the transition between a circular waveguide and a coaxial waveguide comprising a central conductor, said transition element comprising a circular external guide cooperating with an internal conductor forming an end portion of the central conductor of said coaxial waveguide, said circular external guide being connected axially on the one hand to said waveguide circular and on the other hand to said coaxial waveguide, and said inner conductor having at least a section portion substantially constant and reduced compared to the section of the central conductor of the coaxial guide.

The use of such portions of substantially constant and reduced section (or "bearings"), instead of conventional means of continuously variable section, makes it possible to reduce the size of the transition by almost 50%, for bandwidths and an equivalent adaptation.

Advantageously, said inner conductor has essentially steep shoulders at the two ends of each of said portions. Thus, the problems of centering the inner conductor are much less crucial.

Said inner conductor can also have a conical or frustoconical leading edge.

Advantageously, said inner conductor is formed of a first end portion of circular section having an abrupt leading edge, of a second portion of circular section, of radius greater than the radius of said first end portion, said second portion having a first abrupt shoulder connecting to said first end portion and a second abrupt shoulder connecting to said central conductor of said coaxial waveguide.

According to a preferred embodiment of the present invention, said circular outer guide has a narrowing section of its inner diameter at said one or more portions of said inner conductor.

Preferably, said narrowing section has a constant reduced diameter over a length substantially centered on the leading edge of the end of said inner conductor.

Advantageously, said inner conductor has two consecutive portions, and said narrowing section of said outer guide extends approximately to the middle portion of the second portion of larger radius.

Preferably, said narrowing section has essentially abrupt shoulders at its two ends.

A particular application of the transition according to the invention resides in dual-band duplexers.

• la figure 1 est une représentation schématique d'un duplexeur bibande utilisant une transition entre un coupe circulaire et un guide coaxial.
• la figure 2 représente une coupe longitudinale d'une transition entre un guide circulaire et un guide coaxial du type existant;
• la figure 3 représente une coupe latérale d'une transition selon un mode de mise en oeuvre particulier de la présente invention.
• la figure 4 représente l'évolution du R.O.S. pour des fréquences de transmission allant de 3 à 4,5 GHz, pour une transition selon l'invention et une transition abrupte.

Other characteristics and advantages of the present invention will appear on reading the following description of an advantageous embodiment of the present invention, given by way of illustration and not limitation, and the appended drawings, in which:

• Figure 1 is a schematic representation of a dual band duplexer using a transition between a circular section and a coaxial guide.
• FIG. 2 represents a longitudinal section of a transition between a circular guide and a coaxial guide of the existing type;
• Figure 3 shows a side section of a transition according to a particular embodiment of the present invention.
• FIG. 4 represents the evolution of the ROS for transmission frequencies ranging from 3 to 4.5 GHz, for a transition according to the invention and an abrupt transition.

Figure 2 shows a longitudinal section of a transition of the existing type.

As previously described, the known transitions are of the progressive type and characterized by the value of the angle α. The cut-off frequency of the coaxial guide 25 increases when the rays A or B decrease and the ratio of the rays A / B decreases. Thus, the reduction in the angle α results in a longer transition length 22 if one wishes to keep a reasonably low cut-off frequency and therefore a large passband.

FIG. 3 represents a longitudinal section of a transition 30 circular guide 21 / coaxial guide 25 according to a preferred embodiment of the present invention.

• un guide extérieur circulaire 31 de rayon A présentant avantageusement une échancrure 32, ou section de rétrécissement de rayon R₁ et de longueur L₁, permettant de concentrer le champ électromagnétique;
• un guide intérieur constitué par un conducteur central 33 composé de deux paliers 34,35 de rayons respectifs R₂ et R₃ et de longueurs respectives L₂ et L₃ avec une transition abrupte 38 entre les paliers 34 et 35 et une seconde transition abrupte 39 entre le second palier 35 et la portion de conducteur central de rayon le plus important, cette portion de conducteur central formant l'extrémité du conducteur central 23 du guide coaxial 25.

The transition 30 shown can be broken down into two parts:

• a circular external guide 31 of radius A advantageously having a notch 32, or narrowing section of radius R₁ and of length L₁, making it possible to concentrate the electromagnetic field;
• an inner guide constituted by a central conductor 33 composed of two bearings 34,35 of respective radii R₂ and R₃ and of respective lengths L₂ and L₃ with an abrupt transition 38 between the bearings 34 and 35 and a second abrupt transition 39 between the second bearing 35 and the portion of central conductor with the largest radius, this portion of central conductor forming the end of the central conductor 23 of the guide coaxial 25.

The constriction section 32 is delimited by two shoulders 40 and 41, advantageously essentially abrupt, and is located at the level of the intermediate bearings 34, 35.

The leading edge 36 of the inner conductor 33 is advantageously abrupt and perpendicular to the direction of propagation 24 of the microwave wave. In this case, the position of the central conductor 33 is not as critical as in the case where the leading edge 36 is conical or frustoconical. Indeed, in the case where the leading edge 36 of the central conductor 33 is conical or frustoconical, it is absolutely necessary to place the leading edge in the center of the waveguide 21 under penalty of exciting propagation modes unwanted, for example the TEM mode of the waveguide which can propagate whatever the frequency of the propagated signal.

However, it is entirely possible to use a leading edge 42 of the conical central conductor 33, the correct positioning of the central conductor 33 being therefore essential for good propagation of the microwave wave. The attack front can also be tapered.

The use of an inner conductor 33 having a different number of bearings 34.35 is entirely conceivable, as is a different number of bearings on the outer guide at the level of the transition 30. The number of bearings is as a function of the desired bandwidth and of the geometry of the circular 21 and coaxial waveguide 25. The addition of additional transitions results in a longer length of the transition 30, without necessarily improving the ROS, the relationship linking the frequency and the speed of propagation of the wave in the guide not being linear for the TE₁₁ mode because of the dispersion.

The leading edge 36 is preferably located approximately in the middle of the narrowing section, but another position of the leading edge 36 with respect to this section is possible, depending on the transmission characteristics to be obtained.

Furthermore, a preferred embodiment of the present invention consists in that the narrowing portion 32 of the outer guide 31 extends approximately to the middle portion of the second bearing 35 of radius R₃.

The transition 30 can either constitute one end of the coaxial guide 25, which in this case can be secured (using fixing means not shown) with the circular guide 21, or be integrated into a monobloc assembly consisting of the circular guide 21, of the transition 30 and of the coaxial guide 25.

In the case of a structure with symmetry of revolution, only the modes TE _1X and TM _1X can be excited by a discontinuity for an excitation in mode TE₁₁ in the direction 24. The dominant mode is thus the mode TE₁₁ and the first higher mode is the TM₁₁ mode in the two waveguides.

If we consider for example a circular guide with radius A = 40 mm, its cut-off frequency is 2.198 GHz for TE₁₁ mode and 4.574 GHz for TM₁₁ mode. Similarly, a coaxial guide of radii 14 and 40 mm for the central conductor and the external guide respectively, has a cut-off frequency of 1.815 GHz for TE₁₁ mode and 5.989 GHz for TM₁₁ mode.

Thus, the propagation of the dominant mode TE₁₁ is theoretically possible for frequencies ranging from 2.198 GHz to 4.574 GHz. In practice, for the dispersion to be acceptable, the lower cut-off frequency is a little higher, on the order of 2.25 GHz. The bandwidth is therefore in practice from 2.25 to 4.5 GHz if the transition element is not taken into account.

The bandwidth is given by:
(F _a -F _b ) / F _b , with F _at the high frequency of F _b the low frequency.

For the configuration of a known transition as shown in Figure 2, respecting a bandwidth of 50% (3 to 4.5 GHz), with an ROS less than 1.12 (good adaptation of the transition), we obtains a transition size 22 of 100 mm for an A / B ratio = 2.85 (A = 40 mm and B = 14 mm) and an angle α of 8 degrees. The bandwidth is deliberately limited to 50% so as not to degrade the ROS

• bande passante équivalente (de 3 à 4,5 GHz, soit 50%);
• R.O.S. inférieur à 1,12 c'est à dire de valeur équivalente.

According to one embodiment of the invention, the following geometry is adopted: R₁ = 38.72 mm; R₂ = 5.94 mm; R₃ = 10.3 mm; L₁ = 52.48 mm; L₂ = 21.19 mm; L₃ = 22.66 mm; L₄ = 2.07 mm;
L4 being the distance between the connecting shoulder of the second bearing 35 to the central conductor 23 and the connecting bearing of the narrowing section 32 of the outer conductor 31 to the outer conductor 23 of larger radius. The following transmission characteristics are obtained with these values:

• equivalent bandwidth (from 3 to 4.5 GHz, ie 50%);
• ROS less than 1.12, i.e. equivalent value.

It can therefore be seen that the transition 30 according to the embodiment described has the same bandwidth and R.O.S. that a transition 22 as shown in Figure 2, with geometries of the inlet guides (circular guide 21) and outlet (coaxial guide 25) equal.

The main advantage of the present invention is that the length of the transition 30 having the characteristics set out above is no more than 54.55 mm (L₁ + L₄), a gain of 45.45% in size. By analogy with classic transitions, this length corresponds to an angle α of 14.45 degrees. In this case, the bandwidth is only 25% for an R.O.S. less than 1.12, which shows the advantage of using a "compact" transition 30 according to the invention. The R.O.S. remains the same regardless of the direction of propagation of the microwave wave (from the circular guide to the coaxial guide or from the coaxial guide to the circular guide).

In addition, the transition 30 being shorter, its mass is less than that of the known transitions. This notably favors the use of such a "compact" transition 30 in a device operating on a satellite.

Of course, additional bearings can be added and the dimensions of the various discontinuities (bearings of the inner conductor, notch of the outer guide, etc.) can be modified, depending on the result to be obtained (strip busy, ROS).

It is also possible to carry out the connection of successive bearings 34, 35 by oblique shoulders, the bearings of course remaining parallel to the direction 24 of propagation of the electromagnetic wave.

Figure 4 shows the evolution of R.O.S. for the TE₁₁ transmission mode, for a transition according to the invention and an abrupt transition.

The transmission frequency on the abscissa varies from 3 to 4.5 GHz (50% of the bandwidth in TE₁₁ mode).

Characteristic 50 represents the variation of R.O.S. in the case of a "compact" stepwise transition according to the invention between a circular guide and a coaxial guide. The dimensions of the previous lengths and radii are respected. We note that for a bandwidth of 50%, the R.O.S. remains below 1.12, whatever the transmission frequency, and notably passes through a minimum around 3.3 GHz.

Characteristic 51 is that of an abrupt transition between the same guides as before: the external radius of the coaxial waveguide is 40 mm and the radius of the circular guide also. The radius of the internal conductor of the coaxial guide is 14 mm and this conductor has a truncated end. Feature 51 has an R.O.S. constantly higher than 1.9, a minimum around 3.4 GHz and the R.O.S. increases considerably when the frequency passes beyond 4 GHz.

These results clearly show the advantage of using a "compact" step transition according to the present invention.

A particular application of the transitions between circular guides and coaxial guides lies in particular in the production of dual-band duplexers and bi-polarizations. The invention can in particular be applied to a dual band duplexer as shown diagrammatically in FIG. 1, using a transition between a circular guide and a coaxial guide.

As shown in Figure 1, such a device comprises a circular guide 10 integral with a transition 11 followed by a set 12 of two duplexers then of a coaxial guide 13. The coaxial guide 13 comprises in its center a conductive element 14 which extends all along the coaxial guide and its end 15 is located in the transition zone 11. The coupling of the duplexer part with waveguides (not shown) is produced by symmetrical slots.

In general, the polarization, horizontal or vertical, is not identical in the two frequency bands.

The excitation of the high band is done via a circular waveguide excited in TE₁₁ mode. The two polarizations can exist, depending on the excitation of the TE₁₁ mode in the circular waveguide.

For the low band, the excitation is carried out by coupling using a slot between a rectangular guide and the coaxial guide. It is necessary to use two symmetrical slots to excite the TE₁₁ mode of the coaxial guide. The excitation of the TEM mode which propagates regardless of the geometry of the guide and the working frequency cannot be carried out in this way. The separation of the rectangular input waveguide (not shown) into two identical rectangular guides for excitation of the symmetrical slots is carried out using a tee.

It is also possible to obtain the two polarizations according to the position of the two symmetrical slots. So that there is propagation of the wave towards the radiating element and not towards the circular guide 10, the radius of the circular guide 10 must constitute a short-circuit for all the frequencies of the low band.

The advantage of such a duplexer compared to a duplexer whose output is in a circular guide is that the bandwidth is greater in the case of the coaxial guide. The appearance of the higher modes occurs at higher frequencies in coaxial guide than in circular guide, provided that the radii of the two conductors of the coaxial guide (inside and outside) are properly chosen. In this case, the frequency spacing between the two bands can then be greater.

In the case of the implementation of the “compact” transition technology of the invention, the step transition makes it possible to obtain a low ROS, and the dual-band duplexer used therefore does not, in principle, require adaptation. A "compact" transition of the type of the invention finds application in many fields, in particular in that of duplexers, and generally whenever it is necessary to change from a circular waveguide transmission to a guide transmission coaxial, and vice versa.

Claims (9)

1. Transition element for electromagnetic waveguides, of the type intended to provide the transition between a circular waveguide (21) and a coaxial waveguide (25) comprising a central conductor (23), the said transition element comprising a circular outer guide (31) interacting with an inner conductor (33) forming an end portion of the central conductor (23) of the said coaxial waveguide (25), the said circular outer guide (31) being connected axially, on the one hand, to the said circular waveguide (21) and, on the other hand, to the said coaxial waveguide (25), characterized in that the said inner conductor (33) has at least one portion (34, 35) of cross-section which is substantially constant and reduced with respect to the cross-section of the central conductor of the coaxial guide (25).
2. Transition element according to Claim 1, characterized in that the said inner conductor (33) has essentially abrupt shoulders (38, 39) at the two ends of each of the said portions (34, 35).
3. Transition element according to Claim 1, characterized in that the said inner conductor (33) has a conical or frustoconical leading edge (42).
4. Transition element according to any one of Claims 1 and 2, characterized in that the said inner conductor (33) is formed by a first end portion (34) of circular cross-section having an abrupt leading edge (36), by a second portion (35) of circular cross-section, with radius (R₃) greater than the radius (R₂) of the said first end portion (34), the said second portion (35) having a first abrupt shoulder (38) for linking to the said first end portion (34) and a second abrupt shoulder (39) for linking to the said central conductor (23) of the said coaxial waveguide (25).
5. Transition element according any to one of Claims 1 to 4, characterized in that the said circular outer guide (31) has a narrowing cross-section (32) of its internal diameter in the region of the said portion or portions (34, 35) of the said inner conductor (33).
6. Transition element according to Claim 5, characterized in that the said narrowing section (32) has a constant reduced diameter (R₁) over a length (L₁) which is centred substantially on the leading edge (36) of the end of the said inner conductor (33).
7. Transition element according to Claim 6, characterized in that the said inner conductor (33) has two consecutive portions (34, 35), and in that the said narrowing section (32) of the said outer guide (31) extends approximately as far as the mid-part of the second portion (35) of larger radius (R₃).
8. Transition element according to any one of Claims 5 to 7, characterized in that the said narrowing section (32) has essentially abrupt shoulders (39, 40) at its two ends.
9. Dual-band duplexer, characterized in that it includes a transition element according to any one of Claims 1 to 8.

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