EP0027067B1 - Planar bifilar antenna with transversal radiation and its use in radar aerials - Google Patents

Description

The invention relates to a flat two-wire antenna with transverse radiation and its application to radars overhead.

• - avoir une large bande passante en fréquence,
• - permettre un déplacement du centre de phase en fonction de la fréquence selon une loi linéaire de manière à pouvoir effectuer des mesures d'écartométrie par l'association d'au moins deux antennes similaires,
• - posséder un encombrement réduit.

In some detection or transmission equipment by electromagnetic waves, it is necessary to have an antenna meeting the following conditions:

• - have a large frequency bandwidth,
• - allow a displacement of the phase center as a function of the frequency according to a linear law so as to be able to carry out deviation measurements by the association of at least two similar antennas,
• - have a small footprint.

Antennas which meet these conditions at least partially are known in the prior art, which proposes either antennas of the equiangular spiral type wound on a cone, or of the log-periodic type or else, as in the American patent US-A-3 249 946 in the name of ER Flanagan, an antenna comprising a conductive line having folds inscribed in an envelope made up of two intersecting lines. In this last antenna, the phase center is mobile. However, the above-mentioned antennas, although having some of the conditions required, do not have transverse radiation making it possible to produce an antenna which can be accommodated in thin volumes.

To meet this condition of reduced bulk, the prior art has proposed planar antennas with double spiral nested; but this type of antenna, although broadband in frequency, has a fixed phase center relative to the wavelength used.

The present invention aims to define a two-wire planar antenna having all the conditions mentioned above and the radiation of which is in a transverse direction.

According to a main characteristic, the flat two-wire antenna with transverse radiation according to the invention, comprises two conductive lines arranged on two planes parallel symmetrically with respect to the median plane and each having N folds whose envelope is defined by straight lines forming an angle of value a predetermined constant, these two conductive lines differing only by a variation in the electrical length of the same conductive line at the folds, so that two elements of conductive lines belonging to the two planes and delimited by two folds successive are traversed by a phase current for the wavelength used.

• - la figure 1, un premier exemple de réalisation d'une antenne selon l'invention;
• - la figure 2, l'antenne de la figure 1 vue de race permettant ainsi une meilleure représentation des différents éléments;
• - la figure 3, un second exemple de réalisation d'une antenne selon l'invention vue de face;
• - la figure 4, une variante de réalisation de l'antenne des figures 1 et 2;
• - les figures 5 et 6, les emplacements d'un élément réflecteur adjoint à l'antenne selon l'invention;
• - les figures 7 et 8, deux exemples non limitatifs de groupement d'antennes selon l'invention.

Other advantages and characteristics of the invention will emerge from the description which follows of two nonlimiting exemplary embodiments given with the aid of the figures which represent:

• - Figure 1, a first embodiment of an antenna according to the invention;
• - Figure 2, the antenna of Figure 1 race view thus allowing a better representation of the different elements;
• - Figure 3, a second embodiment of an antenna according to the invention seen from the front;
• - Figure 4, an alternative embodiment of the antenna of Figures 1 and 2;
• - Figures 5 and 6, the locations of a reflective element attached to the antenna according to the invention;
• - Figures 7 and 8, two non-limiting examples of antenna array according to the invention.

A two-wire line whose conductors are sufficiently close to each other has a very weak radiation because the conductors being traversed by a current in phase opposition, their respective radiations cancel each other out.

If, by any means, one of the conductors has its electrical length increased relative to the other, the conditions for radiation will be created which will be maximum at the point where the conductors will be traversed by phase currents .

This method is used to create the radiation of the antenna object of the invention.

Figures 1 and 2 show a first embodiment of an antenna according to the invention seen in perspective and front view. It comprises a dielectric plate 10 limited by two planes P and P ₂ , the thickness of which is small, and on which are fixed on either side, and respectively, two conductive metal lines 1 and 2, for example by etching according to the known technique of printed circuits; these lines have N folds which are arranged so as to be inscribed in an opening angle α.

By way of nonlimiting example, FIG. 2 shows a possible geometry of the folds which here are such that they only have right angles and thus delimit elements of straight conductive lines, parallel and equidistant from each other. In this example, one of the conductive lines 2 is extended by a length 11.1 relative to the other conductive line 1 at each folding.

Under these conditions, if one feeds the two lines in phase opposition at point 0, vertex of the angle in which the folds are registered, one obtains from this point 0 a variation of the relative phase of the two conducting lines 1, 2 directly function of the distance to point 0 of the conductive line element considered. When this phase shift reaches 180 ° the two conductors are by run by electric currents in phase, therefore in maximum radiation condition.

So that the direction of maximum radiation is perpendicular to the plane of the antenna, that is to say that the radiation is transverse, it is necessary that two neighboring elements of conducting lines, for example in FIGS. 1 and 2, comprising the points A and B, are in phase.

• - différence de marche de λ/2 entre les lignes conductrices 1 et 2,
• - longueur moyenne des éléments de lignes conductrices contenant les points A et B égale à λ/2.

This assumes that the following two conditions are met simultaneously for a maximum radiation wavelength:

• - path difference of λ / 2 between the conducting lines 1 and 2,
• - average length of the conductive line elements containing the points A and B equal to λ / 2.

The consequence of these two conditions is that the phase center of the antenna thus formed deviates from the feed point 0 in proportion to the wavelength. In addition, in the example described in FIGS. 1 and 2, the line elements are equidistant; therefore the increases Δ1, at each folding, of the length of the second conductive line 2 are equal.

The value of α is not critical since, in combination with the distance d between two consecutive elements of the same conductive line, it only determines the interval Δλ of wavelength between the maximum radiations. Thus, if it is desired that the antenna has an almost continuous tuning, it is necessary that a and d are as low as possible, which of course increases the size of the antenna and its difficulty of realization. The value given to d is in any case limited below by the coupling phenomena between the elements of neighboring lines.

FIG. 3 shows a second embodiment of an antenna according to the invention for which the distance between two successive elements of the same conductive line varies in proportion to the distance of these line elements at the feed point 0. This variant in the geometry of the folds, implies that the variations Δ1 of each fold are proportional to the distance from the feed point 0 of the antenna.

These two examples of geometry by folding are not limiting. In particular, it is conceivable that the line elements delimited by the successive folds are not straight, or even parallel with respect to each other; moreover the distance between two successive line elements can vary according to any predetermined law, other than a linear law. Whatever the geometry of the folds chosen, they must however keep as an envelope an opening angle α constant. A preferred value of this angle α is 90 ° because it allows the association of four identical antennas on a square dielectric plate.

An elementary antenna as described above can be used for very wide frequency transmission and / or reception of a rectilinear polarized wave, for example in an electromagnetic detection device.

The combination of at least two of these antennas can be used as an aerial deviation meter; the phase difference between the signals received by two neighboring antennas is then measured, these being able to be supplied either in phase or in phase opposition as shown in FIGS. 7 and 8.

Arrays of several antennas of this type can be done in many different ways. For a couple of two antennas, it is possible, for example, to arrange them on the same plane and symmetrically with respect to a point or with respect to a straight line of this same plane as shown in the examples corresponding to FIGS. 7 and 8.

In order to obtain the transverse radiation in one direction, a plane 11 reflecting the electromagnetic waves, such as for example a metal plate, can be placed either parallel to this antenna at a distance close to λ _M / 4 where λ _M is the length mean wave of the antenna, either passing through the point 0 and making an angle β so that each conductive line element of order 1 is situated relative to the reflecting plane, at a distance equal to half the length electric Ai / 2 of the element considered α'order i, that is to say at a distance λi / 4 measured parallel to the direction of radiation, where λ _i is the wavelength corresponding to the resonance of this line element. These two variant embodiments are illustrated in FIGS. 5 and 6.

It is possible, as shown in FIG. 4, in an alternative embodiment, to suppress the variation Δ1 of the conductive line 2. The two conductive lines 1 and 2 then being perfectly symmetrical with respect to the median plane of the planes P and P _Z , a dielectric plate 12 of suitable index and thickness is fixed on one of the two lines 2 and covering it totally or partially so as to obtain an effect similar to the geometric elongation of one of the two conducting lines . In the case of the example chosen in FIG. 3, the dielectric plate then completely covers one face of the antenna so as to create an electrical elongation Δ1 varying proportionally to the length of the conductive line. In the case of the example given in FIG. 2 and illustrated by FIG. 4, where the increase Δl is proportional to the distance from point 0, the dielectric plate occupies only two angular sectors delimited by angles α "< α of vertex 0 and adjacent to the sides of the angle α.

A two-wire planar antenna with transverse radiation has thus been described, and its application to radar aerials.

Claims (11)

1. Plane two-wire aerial with transversal radiation comprising a first and second conducting line 1, 2 disposed facing each other on a first and a second parallel plane P and P₂ respectively, characterized in that the first, respectively second, conducting line 1, 2 presents a plurality of N folds, delimiting the conducting elements that are each disposed opposite a conducting element of the other line, form respectively a pair with the opposite facing conducting elements and which are contained in a sector of the first, respectively second, plane limited respectively by two secant straight lines constituting the enclosure of the first, respectively second, conducting line 1, 2 and in that each conducting element of the second line 2 has an electric length that is longer than that of the conducting element of the first line 1 situated facing opposite and which is such that at least two conducting elements situated facing opposite and forming a pair are through-crossed by currents in phase and that the average length of the conducting elements of the said pair and an adjacent pair at least is equal to λ/2, in which λ is the wave length of maximal radiation.

2. Plane two-wire aerial according to claim 1, characterized in that the increment of the electric length of each conducting element of the second line 2 with respect to the conducting element of the first line 1 situated facing opposite is realized, at each fold, by an increase (Δ1) of the length of the second conducting line 2 with respect to the first 4.

3. Plane two-wire aerial according to claim 1, characterized in that the increment of the electric length of each conducting element of the second line 2 with respect to the corresponding conducting element of the first line 1 situated facing opposite is realized by the presence of a plate made of dielectric material 12 covering completely or partially one face of the two-wire aerial.

4. Plane two-wire aerial according to any one of claims 1 to 3, characterized in that the conducting line elements, determined by N folds, are parallel.

5. Plane two-wire aerial according to any one of claims 1 to 4, characterized in that the conducting line elements, determined by N folds, are rectilinear.

6. Plane two-wire aerial according to claim 5, characterized in that the conducting elements of each line 1,2 are equidistant from one another and in that the increment of the electric length is constant for each conducting element of the second line 2.

7. Plane two-wire aerial according to claim 4, characterized in that, for each conducting line 1, 2 the distance between the two consecutive parallel conducting elements varying with the removal of the first of these line elements with respect to the output point 0 of the conducting line 1, 2 according to a predetermined function, the increment of the electric length of one of the conducting lines 2 varies according to this same function.

8. Plane two-wire aerial according to any of claims 1 to 7, characterized in that the first and second conducting lines 1, 2 are fixed on either side of a single dielectric plate 10.

9. Plane two-wire aerial according to any one of claims 1 to 7, characterized in that it comprises a plane reflecting surface 11 parallel to the first and second planes of the two-wire line and located at a distance close to a quarter of the average wave length of the aerial.

10. Plane two-wire aerial according to any one of claims 1 to 8, characterized in that it comprises a reflecting surface 11 passing through the point 0 and forming with the first or second plane P₁ or P₂ of the two-wire aerial an angle beta so that each conducting element of the line of the said plane P, and P₂ is distant from the said reflecting surface by a length substantially equal to half the electric length of said conducting element.

11. Utilization in a radar scanner of at least two two-wire plane aerials according to any one of claims 1 to 10, characterized in that the two-wire aerials are connected in parallel or in series or supplied separately in phase or in phase opposition.

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