EP0035929B1 - High frequency multimode feed, and antenna comprising such a feed - Google Patents

Description

The present invention belongs to the field of multimode microwave sources, as well as to that of so-called monopulse antennas, which comprise such sources.

In monopulse antennas, several radiation patterns are implemented simultaneously and their shapes are directly involved in the overall performance of the radar system using such antennas. Monopulse techniques use several diagrams from the same aerial simultaneously; in so-called amplitude operation for example, there is on the one hand a diagram with even symmetry or diagram "sum serving as a reference and on the other hand, diagrams with odd symmetry or diagrams" difference giving signals of deviation in elevation and in bearing with respect to the axis of the antenna.

In so-called “phase” operation, the angular deviation signals are obtained by comparing the phase between two diagrams having the same amplitude law. It should also be noted that it is possible to switch from one operating mode to another via a system of couplers, so that in the following description, only the case of the amplitude exploitation will be considered.

In these various operating modes, the diagrams used are represented mathematically by orthogonal functions, which leads to the decoupling of the corresponding channels.

On the other hand, the various characteristics of radiation of these diagrams, characteristics which intervene directly in the performances of the system are not a priori independent, but are linked by stress relationships depending on the structure of the antenna. These characteristics are the gain and the level of the side lobes in the sum track and difference tracks, the slope in the vicinity of the axis and the level of the main lobes in the difference track.

For a given antenna structure, the problem posed amounts to seeking an optimization between the factors which have already been mentioned, while respecting between them the hierarchy imposed by the functions of the system considered. We deduce that any structure has an optimization domain, but precisely the conventional antenna structures have shown their limits in the case of monopulse techniques. In fact it has proved impossible in conventional monopulse antennas to independently control the sum diagrams and the difference diagrams, to carry out a correct control of the form of the law of illumination of the primary source, which is important, mainly in the construction of low noise antennas for radio astronomy and space telecommunications. The conventional monopulse technique has also shown its limits in the application to telecommunications antennas by tropospheric diffusion where the diversity between the sum and difference channels is carried out.

To overcome these limitations, we have developed what have been called multimode sources which have been used in antennas also called multimodes.

A multimode source also called moderator is capable by the structure which is given to it, of generating direct propagative modes with controllable phases and amplitudes making it possible to obtain a desired illumination in its opening.

In general, a moderator is a structure formed by waveguides comprising discontinuities at the level of which higher modes are generated. An exemplary embodiment is given in French patent application 2118 848.

A study of such moders can also be found in French patent 2418551, of which we will take Figure 1, which relates to a mixed multimode structure constituted by the union of a plane moder E and a plane moder H as Figure 1, representative of prior art.

Such a structure makes it possible to obtain independent control of the sum and difference diagrams in the plane E and in the plane H. However, such a control is not done simultaneously in the planes E and H but successively in these planes.

The structure of FIG. 1 is constituted by plane moderators ME1, ME2 placed side by side and separated by a common vertical partition. Each of these moders is excited by two pairs of guides 1, 10 and 2, 20 which receive the fundamental mode and which open into a guide 3, 30 of length L1 between the planes PO and P1. The plane PO is what is called the discontinuity plane at the level of which higher, propagative or evanescent modes are formed, the length L1 and the dimensions of the guides 3.30 being such that only the desired modes, in this case by example, the odd modes H11 and E11 and the even modes H12 and E12, propagate until the opening of the moder E thus constituted, that is to say the plane P1, the fundamental mode being the mode H10.

From the plane P1, there are modellers plane H which will achieve the desired distribution laws in the horizontal plane without distorting the distribution laws produced in the vertical plane by the plane moders E, ME1 and ME2. Metal lamellas 4.40-5.50-6.60 arranged horizontally in a guide 8.80 of length L2 extending the guides 3 and 30 beyond the plane P1, define four pairs of adjacent horizontal flat guides by their short side , which are excited according to the distribution laws defined by moderators ME1 and ME2. The horizontal strips extend beyond the plane P2 in a guide 7 having the shape of a horn of length L3.

The set between the planes P1 and P3 constitutes superimposed plane H planes, the plane P2 being the plane of discontinuity where higher modes are formed. The opening of the mixed structure, which is in the plane P3 radiates according to a global law of illumination, produces partial laws of illumination obtained in the vertical plane and in the horizontal plane.

Multimode sources conforming to that which has just been described are used in radar antennas, more particularly in tracking radars, but they have the drawback of having a large longitudinal bulk, which is a hindrance in the production of certain antennas for which increased performance, mainly in bandwidth, leads to an increase in inertia, detrimental to the operation of servomechanisms.

Studies have been undertaken by the Applicant to define multimode sources escaping the drawbacks which have just been reported and in particular a structure of a mixed moder plane E, plane H has been defined in which, in addition to a reduction in the dimensions of the source an increase in bandwidth has been obtained in the H plane.

Figure 2 gives a view of such a moderator, in which the increase in bandwidth is obtained by the presence in the opening of the horn 13 of metal bars or lamellae 14-15, 140-150, arranged so adequate parallel to the electric field of the emitted wave.

According to the invention, a multimode source structure is produced which escapes the drawbacks of the prior art, in which means defining an increase in the bandwidth of the transmitted signals are defined, mainly in the plane E.

According to the invention, the multimode structure comprises a waveguide element forming a cavity ending in a horn, at least four supply waveguides distributed so as to form two pairs of horizontal guides and two pairs of vertical guides, and a profiled obstacle located in the area where the feed guide-cavity junction takes place.

• la figure 2, une structure mixte du modeur plan E et H dans laquelle sont montrés des moyens visant à augmenter la bande passante dans le plan H ;
• la figure 3, un modeur plan E classique en coupe ;
• la figure 4, le modeur plan E de la figure 3 en plan ;
• la figure 5, des courbes représentant les modes présents à la sortie des guides d'alimentation du modeur ;
• la figure 6, la loi d'illumination dans le plan d'ouverture du modeur ;
• la figure 7, le modeur plan E suivant l'invention en coupe ;
• la figure 8, le modeur de la figure 7 en plan ;
• la figure 9, une vue en perspective du modeur plan E suivant l'invention ;
• la figure 10, une variante de l'obstacle suivant l'invention ; et
• la figure 11, un schéma aidant à comprendre le calcul de l'angle optimum de l'obstacle inséré dans le modeur.

Other advantages and characteristics of the invention will appear during the description of exemplary embodiments, given with the aid of the figures which, in addition to FIG. 1 using an embodiment of the prior art, represent:

• FIG. 2, a mixed structure of the plane moderator E and H in which are shown means aiming at increasing the bandwidth in the plane H;
• FIG. 3, a conventional plane moder E in section;
• Figure 4, the plane moder E of Figure 3 in plan;
• FIG. 5, curves representing the modes present at the output of the moder's power guides;
• Figure 6, the law of illumination in the moderator opening plane;
• Figure 7, the plane moder E according to the invention in section;
• Figure 8, the moderator of Figure 7 in plan;
• FIG. 9, a perspective view of the plane moder E according to the invention;
• Figure 10, a variant of the obstacle according to the invention; and
• FIG. 11, a diagram helping to understand the calculation of the optimum angle of the obstacle inserted in the moderator.

In the introduction to the present invention, it has been recalled with reference to an embodiment of a mixed moderator plane E, plane H pertaining to the prior art, the drawbacks that such a moderator has in use which must be made as the primary source of a multimode antenna, preferably tracking, for which an increase in performance is requested mainly in bandwidth. We have also recalled an embodiment of the Applicant providing a solution to the problem posed, firstly in saving weight and dimensions, making it possible to use the antenna in a location imposed in advance and of relatively small volume, in which the displacement of the reflector would tend to increase the inertia of the assembly, an increase acting in a detrimental manner on the servo-mechanisms in particular, and then in an increase in the bandwidth in the H plane.

Referring to Figures 3 and 4, we recall the constitution and operation of a plane moder E, as it appears in the embodiment of Figure 2, for example; this moderator comprises, seen in section in FIG. 3 and in plan in FIG. 4, four supply guides 9, 10, 90, 100 adjacent two by two along a wall 11 for the upper guides and 110 for the lower guides. These feed guides open into the cavity 12 at a plane P called discontinuity. The opening plane of the cavity is S. The dimensions a, b, c respectively represent the height of the supply guides, parallel to the electric field Ë, the height of the cavity 12, that is to say of the plane E considered and the width of the moderator. The four supply guides being supplied in phase in the fundamental mode TE10 (or H10) there is creation at the level of the discontinuity plane P of a hybrid higher mode EM12 composed of mode TE12 and mode TM12. FIG. 5 shows the diagrams of these modes in the plane P and FIG. 6, the law of illumination in the plane of the aperture S of the moderator, resulting from the superposition of the modes TE10 and EM12.

et qui est indépendant de la fréquence, aussi bien en amplitude qu'en phase.We know how to calculate the ratio p of the amplitudes of the electric field E from the hybrid mode EM12 to the fundamental mode, a ratio which is written:

and which is independent of the frequency, both in amplitude and in phase.

We also know how to calculate, in the plane S of the aperture of the plane moder E considered, the phase shift between the modes either:

It is noted that the phasing of the modes considered on the opening of the plane moder E, that is to say in the plane S is a function of the frequency. According to the prior art, by playing on the length L of the moderator, it can be arranged so that at the central frequency of the operating band, the differential phase shift is equal to π, which means that the rigorous phasing is only possible for a single frequency corresponding to the wavelength x. Obtaining a relatively wide bandwidth is thus not possible under good conditions because when one moves away from the central frequency of the band, the phase center of the source which constitutes the moder considered, varies ; located approximately at point G for the central frequency, it deviates from it beyond the plane S for decreasing frequencies and below the plane S for increasing frequencies. The variation of this phase center causes poor illumination at the aperture and a poor radiation pattern of the source with the appearance of large lateral lobes and an enlargement of the main lobe translating a loss of gain, for increasing frequencies and a narrowing for decreasing frequencies, in other words for a fixed direction (9 ₀ ), the width of the diagram varies with the frequency.

avec :

0 étant l'angle de la direction de rayonnement par rapport à la source. Cette formule permet de déterminer le diagramme de rayonnement primaire et définir les niveaux de recoupement sur le réflecteur éclairé par la source.We can give the mathematical expression of the radiation diagram in the plane E of the source of FIG. 3, that is:

with:

0 being the angle of the direction of radiation with respect to the source. This formula makes it possible to determine the primary radiation diagram and define the levels of overlap on the reflector illuminated by the source.

It is possible from the above to determine the conditions under which, according to the invention, the source constituted by the plane moder E will have an increased bandwidth, without having the drawbacks of the moderator of the prior art.

To widen the frequency operating band, it is therefore necessary that the amplitude of the radiated diagram in an angle 0 ₀ , undergo little variation as a function of the frequency. The study of relations 1 and 3 shows that, to obtain the phasing, the IPI ratio of the hybrid mode EM12 to the fundamental mode TE10 must increase with frequency.

The phasing of the modes EM12 and TE10 must remain constant in the opening of the moderator, and this throughout the band considered; the study of relation (2) shows that this constancy is obtained if the plane in which the EM12 hybrid mode generation takes place seems to move to the left in the figure when the frequency increases and therefore to the right in the case opposite.

Figure 7 shows in section, and Figure 8, in plan, the plane moder E according to the invention comprising means allowing it to fulfill the conditions set out in the foregoing.

In FIG. 7, almost all the elements which have been described in support of FIG. 3 are found. The references for these elements are therefore retained. This also applies to Figure 8.

The plane moder E according to the invention comprises a cavity 12, the opening of which is located in the plane S, behind which a plane moder H can be placed, which will constitute, with the plane moder E, a mixed microwave source, plane E, plane H ; in this cavity open, in the example described, four guides 9, 10, 90 and 100, adjacent in pairs, along a wall 11 for the guides in the upper position 9 and 10, 110 for the guides in the lower position 90 and 100. However, while in the moderator of the prior art the passage of the supply guides to the cavity 12 was done along a plane P known as of discontinuity, parallel to the electric field E, according to the invention , we have on a part of this plane P, between the upper and lower feeding guides, a profiled obstacle 17 whose shape and dimensions determine a different action depending on the frequency, on the modes created in the area where the 'obstacle. This shape is such that the obstacle projects inside the cavity 12 with a decreasing section.

FIG. 9 represents a side view, a preferred form of the obstacle 17 introduced into the moderator. According to the invention, this obstacle is a paving stone of trapezoidal cross section whose large base 18 is in the plane P, plane at which the moderator feed guides open, in the part located between the upper guides 9-10 and lower 90-100. The small base 19 is located at a distance 1 from the plane P, inside the cavity 12 and at a distance a, from the wall of the cavity, distance measured parallel to the electric field AND This distance is variable when going from the small to the large base.

The sides of the block 17, between the large and the small base determine an angle a with the direction D perpendicular to the plane P. The other dimensions of the moderator are as for that of the prior art b and c.

The operation of the source, which constitutes the plane moder E, according to the invention is as follows, which can be followed with reference to FIG. 7.

Given the shape of the obstacle, one of the bases of which is in the so-called discontinuity plane P, the higher modes, mainly the hybrid mode EM12, are not created at the level of the plane P, but in the different short-circuit planes. according to the frequencies at which we work.

Thus, in the low frequencies, the excitation plane of the hybrid mode EM12 is found in P _B , which happens to be the plane of the small base of the trapezoidal block 17. The phasing length is then L _B , length between the plane P _B and the plane of the opening S of the moderator. The mode report module has the following expression:

At high frequencies, the EM12 hybrid mode excitation plane is at P _H , the intermediate position between the P plane and the P _B plane. The phasing length is L _H , distance between the plane P _H and the plane of the opening S. The mode report module takes the following expression:

The conditions that have been set out for the moderator to operate at high bandwidth, that the mode ratio | β | increases with frequency and the displacement of the excitation plane of the hybrid mode EM 12 takes place to the left, that is to say towards the source, for increasing frequencies, causing L _H greater than L _B are thus fulfilled.

One can determine by calculation, an optimal value of the angle a so that the preceding conditions are realized in a broad band of frequencies, this angle a being able to vary theoretically between 0 and 90 °. To do this, we calculate the value of the modulus and of the argument of the expression p representing the ratio of the higher mode to the fundamental mode at the level of the discontinuity.

Figure 11 helps to understand how this calculation is done. This figure 11 reproduces Figure 7, in its upper part above the longitudinal axis zz of the moderator. Obstacle 17 is obviously only partially represented, its profile is identified by the letters CBA O '. The distance from the obstacle to the wall of the moder in which it is introduced, directly above the plane P is designated by a _o , while this distance above the plane P _B is designated by a _B , represented by the AO segment. A datum 8 is defined which represents the variation of the phase of the fundamental mode as a function of the frequency.

In the proposed calculation, the evanescent higher modes EM 14, EM 16, etc. will be neglected. which appear at the discontinuities in the planes P _B and P.

We first write the connection equations of the electric fields of the various propagative modes of the source.

et Xg longueur d'onde guidée est

The electric field upstream of the plane 00 'also called P _B , has for expression, for the mode TE 10: e ^ikδ in which

and Xg guided wavelength is

In an analogous manner the electric field downstream of the plane 00 'has for expression for the mode TE10 and the hybrid mode EM12 mode TE10 + mode EM12: A (1 + β cos πx / b) in which A is a normalization factor and β the mode ratio in complex form.

et β a pour expression

qui est de la forme |β|e^jψ. On déduit de l'expression (6) que le module de |β| croît avec la fréquence, que la phase varie avec la fréquence et que si le modeur a une longueur bien définie, telle que les différents modes se retrouvent en phase sur l'ouverture S, cette phase décroît, tendant à réduire la variation du déphasage différentiel entre le mode EM 12 et le mode TE 10 dans la bande de fréquence d'utilisation.If we integrate the expressions of the field in the 00 'plane, that is the P _B plane we have

and β has the expression

which is of the form | β | e ^jψ . We deduce from expression (6) that the modulus of | β | increases with frequency, that the phase varies with frequency and that if the moderator has a well-defined length, such that the different modes find themselves in phase on the opening S, this phase decreases, tending to reduce the variation of the differential phase shift between EM 12 mode and TE 10 mode in the operating frequency band.

The following tables give the results established for a conventional moderator and a moderator according to the invention.

The first table 1 gives in a first column the level of overlap NR and in a second column the differential phase shift Δϕ between the modes for, successively the high frequency F _H , the median frequency F _m and the low frequency F _B , this for a conventional moderator having a relative bandwidth of 10%, a value u = πb / λ sin θ ₀ between 3.3 and 3.7 and β≃ 0.8, the angle θ ₀ being the angle of overlap on the antenna reflector.

The second table II reports the results obtained with the moder according to the invention, which constitutes a broadband source.

It can be seen from these tables that for a conventional moderator (table I) the variation in the level of NR overlap is of the order of 3 dB, 3, for a relative band of 10% when one passes from high frequency to frequency low of the band, while for a moderator according to the invention (table II) this variation is reduced to 1 dB, 3, proving that the relative band is increased. Similarly, the differential phase shift goes from 40 ° for the conventional moderator to 16 ° for the moderator according to the invention, also an indication of an increased band. In fact the relative bandwidth is then of the order of at least 15%. We also deduce that for a value of the modulus of the mode ratio | β | between 0.8 and 8.88, the optimal value for the angle a is around 50 ° in a range of ± 10 °.

FIG. 10 presents a variant of the block 17 introduced into a moder E. This block has, with respect to the block described in support of the preceding figures, a modified profile. The latter 20 is no longer a line segment, but has a convex curvature, which can tend to exponential. The results obtained are of the same order as those of the version described, with a slight tendency to be superior; however, the mechanical production of such a block is a little more difficult.

A multimode microwave source has thus been described, constituted by a moder E having a relative passband increased compared to that of a conventional moderator. According to the invention, a multimode microwave source also constituted by a mixed moderator E plane, H plane, for which the bandwidths are increased both in the E plane and the H plane. Such a source is that of FIG. 2, in which the moderator E includes the block 17. FIG. 7 shows in section such a mixed source, according to the invention, the horn 13 with the metal bars 14, 140, 15 and 150 being disposed at the outlet of the cavity 12 of moderator E. The opening of the mixed source is designated by 16.

Claims (7)

1. A multimode microwave source comprising :

- on the one hand two pairs of waveguides (9, 10-90, 100), which are fed by a fundamental mode such as TE_1o, the two waveguides of one pair, of the height a, being adjacent along a wall (11 and 110) and the two pairs being disposed one above the other and separated by a distance which is not zero, and

- on the other hand a rectangular cavity (12) having a height b and a width c and receiving the two pairs of guides (9, 10-90, 100) at the level of a so-called discontinuity plane P, the waveguides constituting with the cavity an E plane moder, the distance between the two pairs of waveguides being (b-2a), characterized in that the lower walls of the waveguides (9, 10) of a first pair are connected inside the cavity to the upper walls of the waveguides (9, 100) of the second pair through a profiled obstacle (17) projecting into the cavity, the rectangular cross-section of the obstacle decreasing continuously from the discontinuity plane (P) between the guides and the cavity to the inner side of said cavity (12), thereby creating a variation of the phasing length (L) in the moder while the phase center (G) of the source remains in the plane (S) of the opening of the cavity (12) for a large band of operational frequency.

2. A source according to claim 1, characterized in that the rectangular cross-section of the profiled obstacle (17) decreases linearly and constitutes thus a brick of a rectangular cross-section and of a trapezoidal transversal section, the large base (18) of which being located in the discontinuity plane (P) and the small base (19) in a plane (P_B) which is parallel to the plane (P) and situated at a distance 1 therefrom inside the cavity (12), this plane (P_B) being the excitation plane of higher hybrid modes EM₁₂ at low frequencies.

3. A source according to claim 2, characterized in that the angle a built up in a transversal section by the not-parallel sides of the trapezoidal section of the profiled obstacle (17) with respect to a direction D which is perpendicular to the discontinuity plane (P) has an optimum value between 40 and 60°.

4. A source according to claim 1, characterized in that the rectangular cross-section of the profiled obstacle (17) varies continuously according to a convex curve (20) between the discontinuity plane P of the E-moder and the end of the obstacle located in an excitation plane (P_B) of the higher hybrid modes at low frequencies.

5. A multimode microwave source comprising an E-moder according to one of the claims 1 to 4, associated to a H-moder, characterized in that the E-moder opens on a horn (13) with increasing aperture in the direction of the H-plane and being shaped for the propagation of a higher mode such as H_ao ; that two vertical rods (14 and 15) are disposed in this horn (13) parallelly to the direction of the electrical field E and symmetrically with respect to the vertical plane passing through the phase center (G) of the source, these elements creating a higher even propagation mode of the type H₃₀, and that the microwave source presents a reduced total length, a large pass band in the E-plane and the H-plane and illumination laws in the E-plane and H-plane which may be controlled independently but simultaneously in the plane (S) of the aperture of the source.

6. A microwave source according to one of the claims 1 to 4, characterized in that the relative pass band is greater than or equal to 15 % in the E-plane as well as in the H-plane.

7. The use of a multimode microwave source according to one of the claims 1 to 6 in a microwave antenna.

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