EP0296929B1 - Balanced microwave transmission line with two coplanar conductors - Google Patents

Description

The present invention relates to improvements to microwave transmission lines comprising two parallel and coplanar flat conductive tapes.

Such commonly used transmission lines are classified into two types, those called symmetrical lines and those called asymmetrical lines. A symmetrical line is formed by two linear metal strips having equal widths W and arranged parallel to each other at a predetermined distance G on a non-conductive substrate. An asymmetrical line consists of a first conductor in the form of a narrow flat metallic strip having a small width W, and of a second conductor in the form of a wide longitudinal conductive plane having a width 1 clearly greater than W and arranged parallel to the narrow conductive tape at a distance G therefrom on the same type of substrate.

For a given characteristic line impedance, the symmetrical line requires a ratio W / G, strip width over width of the gap between conductors, greater than that of the asymmetrical line. As a result, the symmetrical line has ribbons wider than that of the asymmetrical line and / or a narrower gap than that of the asymmetrical line. This dimensional characteristic of the symmetrical line is thus advantageous in that it uses less resistant conductive tapes, while reducing the line width. The symmetrical line is very often chosen when it is necessary to ensure symmetry of the electric and / or magnetic fields of the microwave wave which propagates in the line.

However, two major drawbacks inherent to the connection of the line and to the line's own resonances are to be considered when using a symmetrical line.

In general, use of the symmetrical line requires line end connections and external microwave components such as microwave source, load, probe, via coaxial, miniature or subminiature connectors. As is known, such a coaxial connector includes a slender central internal conductor having a small diameter and a cylindrical external conductor having a larger diameter and, therefore, provides an asymmetrical conductor structure. The differences in geometric shapes of the connector and the symmetrical line thus cause connection difficulties. These difficulties are in practice resolved by providing, at the end of the line to be connected, a small, substantially rectangular extremal conductive plane connected coplanarly with the end of one of the linear ribbons and constituting with the end of the other ribbon. a flat asymmetrical line portion. The extremal conductive plane is welded laterally to the external cylindrical conductor of the coaxial connector, and the projecting end of the internal conductor of the connector is welded to the end of said other strip of the line.

The second drawback of the symmetrical line consists of the appearance of relatively low parasitic frequencies of longitudinal resonance which limit the useful frequency band of the symmetrical line. Longitudinal resonances are by definition lower than transverse resonances which are in the range of very high frequencies. An experimental analysis of resonance shows that part of the microwave energy is neither transmitted nor reflected, but is radiated. Indeed, a symmetrical line has natural frequencies for which a standing wave can form and constitute a source of radiation.

The present invention aims to provide a microwave transmission line of the symmetrical line type with two narrow coplanar and parallel ribbons, offering the advantages of the symmetrical lines according to the prior art without the drawbacks of these, particularly with regard to the limitations due to frequencies. of resonance. In other words, a line according to the invention offers a useful frequency band significantly higher than a symmetrical line according to the prior art, for identical dimensions relating to the conductive tapes.

To this end, a symmetrical type microwave transmission line comprising
a first conductor in the form of a first flat narrow conductive tape extending over the entire length of the line and having first and second ends, and
a second flat conductor coplanar with the first conductor, said second conductor comprising
a second flat narrow conductive strip extending parallel to the first narrow strip between the first and second ends of the first narrow strip, and
first and second substantially rectangular end planar conductors, connected to the ends of the second narrow strip, and having sides substantially parallel to the first and second ends of the first narrow strip, respectively,
is characterized in that the second conductor comprises a wide longitudinal flat conductive strip extending coplanarly and parallel to the first and second narrow strips over the entire length of the line,
said wide conductive tape having ends connected to the first and second end plane conductors respectively so as to form in the second flat conductor a resonant cavity delimited by longitudinal sides of the second narrow tape and of the wide tape and by transverse sides opposite the planar conductors extremals.

The constitution of the resonant cavity by the presence of the wide longitudinal conductive ribbon connecting the ends of the second narrow ribbon through the small extremal planar conductors makes it possible to offer longitudinal resonance frequencies significantly higher than those of a symmetrical line only with two ribbons close conductors. Indeed, the appearance of standing waves at small resonant frequencies of the symmetrical line with only two ribbons is prevented when the dimensions of the cavity are suitably chosen.

In particular, the distance between the longitudinal wide ribbon and the second narrow ribbon defining the width of the cavity is chosen to be relatively large compared to the geometric characteristics of the line composed by the two narrow ribbons, namely the widths of the narrow ribbons and the width of the gap between these two ribbons. Under these conditions, the presence of the wide longitudinal conductive tape does not disturbs the characteristic impedance of the symmetrical line only negligibly.

If it is desired to raise the first cutoff frequency of the transmission line, the cavity is then divided into one or more sub-cavities by intermediate conductive tapes connected transversely to the second narrow ribbon and to the wide longitudinal ribbon.

Furthermore, the short circuits produced by the extreme planar conductors between the second narrow ribbon and the wide ribbon contribute to producing, with the ends of the first narrow ribbon, two end sections of asymmetrical line facilitating the connection of the transmission line to connectors. coaxial.

• la Fig. 1 est une vue de dessus d'une ligne de transmission hyperfréquence ayant une longue cavité résonnante;
• la Fig. 2 est une vue de côté de la ligne de la Fig. 1;
• la Fig. 3 est une vue de dessus d'une extrémité de la ligne de la Fig. 1 reliée à un connecteur coaxial;
• la Fig. 4 est une vue de côté de l'extrémité de ligne et du connecteur coaxial;
• la Fig. 5 est une vue de dessus d'une seconde ligne de transmission hyperfréquence ayant plusieurs sous-cavités résonnantes; et
• la Fig. 6 est une vue de dessus d'une troisième ligne de transmission hyperfréquence ayant deux sous-cavités résonnantes et des dimensions de rubans plans conducteurs identiques à ceux de la ligne de la Fig. 1.

Other advantages and characteristics of the invention will appear more clearly on reading the following description of several transmission lines according to the invention with reference to the corresponding appended drawings in which:

• Fig. 1 is a top view of a microwave transmission line having a long resonant cavity;
• Fig. 2 is a side view of the line in FIG. 1;
• Fig. 3 is a top view of one end of the line in FIG. 1 connected to a coaxial connector;
• Fig. 4 is a side view of the line end and of the coaxial connector;
• Fig. 5 is a top view of a second microwave transmission line having several resonant sub-cavities; and
• Fig. 6 is a top view of a third microwave transmission line having two resonant sub-cavities and the dimensions of flat conductive ribbons identical to those of the line in FIG. 1.

With reference to Figs. 1 and 2, a microwave transmission line comprises a first flat conductor 1 and a second flat conductor 2 which are fixed coplanarly on a plate of non-conductive material 3 such as a dielectric substrate. The conductors 1 and 2 are for example conductive ribbons screen printed on the wafer 3 and having the same thickness.

The first conductor 1 consists only of a narrow linear strip 11 having a uniform width W₁.

The second conductor 2 consists of a narrow linear strip 21 which has a width W₂ and which is parallel to the conductive strip 11, two rectangular transverse and extremal planes 22 and 23, and a longitudinal rectangular plane or wide parallel strip 24 with narrow ribbons 11 and 21. The four elements 21 to 24 making up the conductor 2 are delimited by hatching in FIG. 1 in order to differentiate them, although they form a one-piece conductor.

The ribbon 21 thus extends parallel to the ribbon 11 over the major part L of the length of the microwave line, in order to form a symmetrical line when the widths W₁ and W₂ are equal or substantially equal. The distance G between the two strips 11 and 21 is of the same order of magnitude as the widths W₁ and W₂ and, in general, less than said widths.

The extremal planes 22 and 23 have short sides 221 and 231 substantially parallel to the ends 12 and 13 of the first strip 11 and separated from these by interstices having widths g₂ and g₃ greater than the width G, so that the transitions between the ribbon 21 and the planes 22 and 23 provide recesses 212 and 213. The widths l₂ and l₃ of the extremal planes 22 and 23 are significantly greater than those W₁ and W₂ of the ribbons 11 and 12 in order to form sections of asymmetrical line at the ends of the microwave line. These two sections make it possible to connect the symmetrical line 11 + 21 to connection connectors for coaxial lines. In particular, the pairs of dimensions g₂ and l₂, and g₃ and l₃ which may be different, are adapted as a function of the characteristic impedances and therefore of the dimensions of the coaxial lines to be connected respectively.

As shown in Figs. 3 and 4, such a connector 4 to be connected to the end of the line comprising the plane 22, conventionally comprises a metallic central conductor 41, an metallic external cylindrical conductor 42 which is connected to the ground, and an insulator 43 filling the inside the conductor 42 around the internal conductor 41. One end 411 of the internal conductor 41 projects from a base face 44 of the connector 4 and is brazed collinearly on the corresponding end 12 of the first strip 11. A edge 222 of the extremal plane 22 perpendicular to the ribbon 11 is applied against the connection face 4 and is welded to the external conductor 42 to be grounded.

The ribbons 11 and 21 and the extreme conductive planes 22 and 23, without the conductive plane 24, together constitute a known microwave line of the type with symmetrical coplanar ribbons (11 and 21) and asymmetrical ends (11 and 22; 13 and 23).

According to the invention, the microwave line also comprises the wide and longitudinal rectangular conductive plane 24 having a predetermined width l₄. The plane 24 has a long side 241 which is parallel and opposite to a longitudinal side 211 of the second narrow strip 21 and which has ends 242 and 243 constituting second short longitudinal sides of the end conductive planes 22 and 23. Thus, in the earth conductor 2 appears a flat rectangular cavity 25 whose long sides are the opposite sides 211 and 241 of the strip 21 and the longitudinal plane 24 and whose short sides are large opposite sides opposite 223 and 233 of the extreme planes 22 and 23.

The length of the cavity 25 is equal to L, that is to say substantially less than that of the microwave line. The length L for a predetermined width D of the cavity defines a longitudinal resonance frequency of the cavity which inhibits any lower standing wave frequency due to the initial symmetrical line resonance 11 + 21. The cavity 25 thus behaves like a real low-pass filter whose cutoff frequency is equal to the lowest resonant frequency of the cavity.

As shown in Fig. 5, if it is desired to increase the cut-off frequency in order to eliminate other frequencies of longitudinal resonance from the symmetrical line, the length of the cavity L is subdivided into N preferably identical sub-cavities 25₁ to 25 _N each having a length substantially equal to L / N. Between two adjacent sub-cavities, for example 25 _n and 25 _{n + 1} , where n is an index varying from 1 to N, an intermediate narrow "wall" is provided consisting of a short transverse conductive strip 26 _n which is perpendicular to the strips narrow longitudinal 21 and to the longitudinal flat ribbon 24 and connected to them. The N-1 transverse ribbons 26₁ to 26 _N-1 of length D are thin and have a width t equal to or less than those W₁ and W₂ of the ribbons 11 and 12. Each transverse ribbon plays a role analogous to a shunt inductance between the conductors 21 and 24.

The number N and the dimensions, length L / N and width D, of the sub-cavities 25₁ to 25 _N are chosen so as to ensure the best filtering of the low resonance frequencies, that is to say parasitic longitudinal resonances of the symmetrical line. In practice, for a predetermined width D and a predetermined length L, the integer N can be selected so that the smallest resonant frequency of each of the sub-cavities is greater than the maximum frequency of the useful band of the signals to be transmitted.

However, according to other variants, the lengths of the sub-cavities are different, or more generally the dimensions of the sub-cavities are different in order to select resonant frequencies and therefore predetermined cut-off frequencies. For example, only with a single wall 26₁ and two sub-cavities 25₁ and 25₂ having somewhat different lengths, the microwave line behaves like a low-pass filter having a cutoff frequency equal to the smaller of the two resonant frequencies of the two sub-cavities 25₁ and 25₂ which is associated with the longer of the sub-cavities.

By way of a practical example, the results of comparative measurements are presented below between a symmetrical line 11 + 21 + 22 + 23 of known type, on the one hand, and two lines according to the invention comprising elements 11, 21, 22 and 23 identical to that of the symmetrical line and a longitudinal conductive plane of earth 24. One, L₁, of the two lines according to the invention has only one large cavity 25 as shown in FIG. 1, while the second line L₂ according to the invention comprises an intermediate thin strip 26₁ separating the cavity 25 at N = 2 identical sub-cavities 25₁ and 25₂, as shown in FIG. 6. The dielectric material used 3 was lithium niobate Li Nb O₃. The characteristic impedance of the balanced line is equal to 50 Ohms. The dimensions were as follows: L = 14 mm, W₁ = W₂ = 80 µm, G = 50 µm, g₂ = g₃ = 135 µm; D ≃ 1₂ = 1₃ = 1 mm; t = 30 µm; and l₄ = 1 mm. The measurements were made in the frequency band between 10 MHz and 6 GHz.

For the symmetrical line 11 + 21 + 22 + 23 according to the prior art and without an earth plane 24, the first longitudinal resonance appears around 1 GHz. For the line L₁ according to the invention, the first longitudinal resonance only appears at 2.5 GHz. The first one longitudinal resonance of the line L₂ with two sub-cavities is twice greater and is equal to approximately 5 GHz.

Claims (9)

1. Microwave transmission line of symmetrical type comprising :
   a first conductor (1) in the form of a first flat narrow conductive strip (11) extending over the entire length of the line and having first and second ends (12, 13), and
   a second flat conductor (2) coplanar with the first conductor (1), said second conductor comprising
   a second flat narrow conductive strip (21) extending parallel to the first narrow strip (11) between the first and second ends (12, 13) of the first narrow strip, and
   first and second planar end conductors (22, 23) substantially rectangular, connected to ends of the second narrow strip (21), and having sides (221, 231) substantially parallel to the first and second ends (12, 13) of the first narrow strip, respectively
   characterized in that the second conductor (2) comprises a longitudinal wide planar conductive strip (24) extending coplanar and parallel to the first and second narrow strips (11, 21) over the entire length of the line,
   said wide conductive strip (24) having ends (242, 243) connected to the first and second planar end conductors (22, 23) respectively thereby forming in the second flat conductor a resonant cavity (25) bounded by longitudinal sides (211, 241) of the second narrow strip (21) and of the wide strip (24) and by opposite transverse sides (223, 233) of the planar end conductors (22, 23).

2. Microwave transmission line according to claim 1, characterized in that the second flat conductor (2) comprises an intermediate conductive strip (26₁) connected transversely to the second narrow strip (21) and to the wide strip (24) in order to divide said cavity (25) into two resonant sub-cavities (25₁, 25₂).

3. Microwave transmission line according to claim 1, characterized in that the second flat conductor (2) comprises several intermediate conductive strips (26₁ à 26_N-1) connected transversely to the second narrow strip (21) and to the wide strip (24) in order to divide said cavity (25) into several resonant sub-cavities (25₁ to 25_N).

4. Microwave transmission line according to claim 1, characterized in that the lowest longitudinal resonance frequency of the cavity (25) is greater than a useful frequency band of signals to be transmitted by said line.

5. Microwave transmission line according to claim 2 or 3, characterized in that the lowest longitudinal resonance frequency of the sub-cavities (25₁ and 25₂ ; 25₁ to 25_N) is greater than the useful frequency band of the signals to be transmitted by said line.

6. Microwave transmission line according to any one of claims 2, 3 and 5, characterized in that the sub-cavities (25₁, 25₂ ; 25₁ to 25_N) are identical.

7. Microwave transmission line according to any one of claims 2, 3, 5 and 6, characterized in that an intermediate conductor (26₁ to 26_N-1) has a width (t) smaller than the widths (W₁, W₂) of the first and second strips (11, 21).

8. Microwave transmission line according to any one of claims 1 to 7, characterized in that the wide strip (24) and the cavity (25) have widths (l₄, D) substantially equal.

9. Microwave transmission line according to any one of claims 1 to 8, characterized in that the width (G) of the longitudinal interstice extending between the first and second narrow strips (11, 21) and the widths (W₁, W₂) of the first and second narrow strips (11, 21) are much smaller than the width (D) of the cavity (25).

Images