EP0374008A1 - Over the whole spherical space electronically scanning antenna with random and reduced three-dimensional distribution of the antenna elements - Google Patents

Abstract

This antenna is a so-called "steric" antenna, that is to say consisting of a fixed grid (1) comprising a plurality of elementary antennae (3) with quasi-omnidirectional individual radiation, distributed according to a statistically isotropic random distribution law inside a surrounding volume of revolution (2), the average spacing between elementary antennae being considerably greater than a half wavelength of the minimum frequency to be received or emitted, and each elementary antenna being connected to an active module (6) comprising individually controllable dephasing means themselves connected to common distributing means (7). <??>The elementary antennae (3) consist of vertically oriented dipoles supplied by a supply line (4, 5) comprising a first section (4), extending horizontally between the respective dipole and a common vertical mast (9) coaxial with the surrounding volume of revolution (2), and a second section (5) extending inside this mast. <??>The antenna thus produced possesses vertical polarisation, advantageous in numerous cases of pattern (marine, secondary radars) with, by virtue of the use of simple dipoles, a very small equivalent reflecting surface, which renders it very poorly locatable. <??>Furthermore, the first sections (4) of the supply lines can, by virtue of this structure, readily constitute self-supporting means for mechanical support of the dipoles on the common vertical mast (9), for example constituted by the mast of a ship. <IMAGE>

Description

The present invention relates to an antenna with three-dimensional coverage and electronic scanning, of the random rarefied volume array type.

Several types of antennas are known which make it possible to obtain three-dimensional coverage (most often hemispherical or quasi-hemispherical coverage) from a configuration of fixed elements combined with electronic scanning, that is to say antennas in which the shape of the radiation diagram is modified (in particular, the pointing of a main lobe) by playing on the individual, adjustable phase shifts, of the various elements constituting the network.

The most commonly used configuration, in practice, for producing such an antenna, consists in distributing the various elementary antennas of the array on one or more reflecting surfaces, such as for example the surface of a cylinder or a plurality of differently oriented panels.

These antennas of the so-called "area array" type are however not in all respects satisfactory. Indeed :
- The cylindrical surface network has the disadvantage of poor coverage for relatively large elevation angles, that is to say when one approaches the direction of the zeith;
- the multi-panel antennas make it possible to remedy this drawback, by placing the various panels (generally four in number) on the faces of a trunk pyramid, which makes it possible to obtain a relatively satisfactory hemispherical coverage.

However, these multi-panel antennas are relatively expensive because each panel, therefore each antenna of the network, works only in a single quadrant (in the case of a four-panel antenna).

Indeed, for a given direction of the main lobe, only one of the four panels is used, the elementary antennas of the other three panels contributing in no way to the formation of the beam in this direction.

In this way, to have complete azimuth coverage, it is necessary to quadruple the number of antennas and phase-shifting modules, correspondingly increasing the cost of the assembly.

Another type of antenna with three-dimensional coverage and electronic scanning is known in which, unlike multi-panel or cylindrical surface antennas, all of the elementary antennas of the array participate in the formation of the beam and contribute to the gain of the antenna, whatever whatever the direction of the main lobe.

These antennas are the so-called "steric" or "solid" antennas in which, unlike surface antennas, the elementary antennas are no longer distributed on the surface of a given plane or volume, but inside d 'a volume (usually a sphere).

The elementary antennas are distributed in this volume as irregularly as possible, so as to minimize the mutual coupling between elementary antennas and thus attenuate the network lobes as much as possible; this condition is obtained by distributing the antennas in the volume according to a statistically isotropic random distribution law, and on the other hand by providing an average spacing between elementary antennas which is notably greater than half a wavelength.

We thus speak of "random rarefied network". In such networks:
-the rarefaction makes it possible to save on the number of radiating elements for a given dimension of the network, that is to say for a given opening of the beam. It also makes it possible to greatly reduce the coupling between sources, which are frequent causes of degradation in the performance of network antennas; and
- the hazard makes it possible to eliminate the lobes of the network inherent in regular structures with large steps.

Such an antenna has in particular been described in DE-A-28 22 845.

More specifically, this document describes a so-called crow's nest antenna, that is to say an antenna formed by a network in which the elementary antennas are open loops or ^“ turnstile ^” antennas, radiating on a horizontal polarization and placed at the top of vertical coaxial feed lines.

Although it appears as a theoretically very interesting solution of antenna with three-dimensional coverage and electronic scanning, this type of antenna, although proposed for more than ten years, has so far been the subject only of experimental realizations , without effective application to the various fields where such a type of antenna would prove to be particularly desirable: air defense, marine, radar for weapons systems, secondary radars for aviation, etc.

In fact, first of all, the length of the coaxial lines, the longest of which have a length at least equal to twice the radius of the envelope sphere makes the system mechanically fragile and requires, if we want to have the desired precision of positioning of the different loops inside the sphere and sufficient overall rigidity, to provide additional mechanical means such as nylon threads holding the semi-rigid power cables in position and / or drowning the entire network in a mass of foam (polyurethane foam for example).

In addition to the difficulties of mechanical implementation, in the latter case the presence of foam plays the role of thermal insulator which prevents the evacuation of calories in the event that the antenna is used in emission, which limits this solution to antennas receiving or transmitting at low power, the problem of caloric evacuation not being resolved.

A second drawback, also linked to the long length of the supply lines, is the own phase shift introduced by them, phase shift which can vary in significant proportions depending on whether it is a short line or a long line and it will be necessary to compensate to avoid the appearance of phase faults independent of the direction pointed.

These drawbacks, both mechanical and electrical, related to the long length of the supply lines are all the more troublesome as the dimensions of the sphere are large relative to the wavelength. However, as the fineness of the beam (opening angle of the main lobe) is directly linked to the dimension (expressed in wavelengths) of the sphere, we are thus led to limit the performance of the system with regard to its fineness of beam.

Third, such a network is very "visible" in terms of radar signature, due to the use of loops or turnstile antennas; however the use of such types of elementary antennas is inevitable because, by nature, a network requires antennas having, in amplitude as in phase, a quasi-omnidirectional diagram in azimuth.

Fourth, this known type of antenna is limited, due to its structure, to operation essentially in horizontal polarization.

However, many applications absolutely require vertical polarization, for example antennas for radar on board ships (vertical polarization eliminates indeed the effects of reflection on the sea), or even antennas for secondary radars, in particular IFF radars (Identification Friend or Foe: friend / enemy identification).

These different reasons explain why, despite its obvious theoretical advantages and the need for an antenna with three-dimensional coverage and electronic scanning in many fields of application, this known type of antenna has so far not exceeded the experimental stage.

The present invention relates to a steric type antenna (that is to say of the “random rarefied volume network” type explained above) which overcomes all of the aforementioned drawbacks, while keeping a simple, robust and therefore inexpensive structure. to achieve.

This antenna is, in itself known, made up of a fixed network comprising a plurality of elementary antennas with quasi-omnidirectional individual radiation distributed according to a statistically isotropic random distribution law inside an envelope volume of revolution, the average spacing between elementary antennas being notably greater than half a wavelength of the minimum frequency to receive or transmit, each elementary antenna being connected to individually controllable phase-shifting means themselves connected to common distributor means.

Characteristically of the invention, the elementary antennas consist of vertically oriented dipoles supplied by a supply line comprising a first section, extending horizontally between the respective dipole and a common vertical mast coaxial with the volume envelope of revolution, and a second section extending inside this mast and leading to the distributor means.

The volume envelope of revolution can in particular be a sphere.

Very advantageously, the first sections of the supply lines constitute means, self-supporting, of mechanical support of the dipoles on the common vertical dish.

Compared to a crow's nest antenna, the length of the sections of the supply lines which form the self-supporting means is considerably reduced: the maximum length of these is at most equal to the radius of the sphere (more precisely, it is equal to the radius of the sphere minus the radius of the central cylinder), while in the crow's nest structure of the prior art described above, this length was at least twice the radius of the sphere.

Given the reduced length, it is no longer necessary to drown the network in a foam or to provide auxiliary holding means.

From the radioelectric point of view, the central mast only moderately disturbs the radiation diagram, and in any case has no effect on the isotropy in azimuth of the beam, because of its axial position; in other words, the non-uniformity introduced by the central cylinder will be essentially a non-uniformity in site, where one accepts very well a degradation of the performances of the network in the vicinity of the zenithal region.

In addition, in the case of a radar for the navy, the central tube may advantageously be constituted by a mast of the ship or by a similar superstructure element, which makes it much easier to find a suitable location for the antenna and makes the mast neutral from a radioelectric point of view, a particularly appreciable advantage on ships, where the superstructure elements close to the antenna always bring significant disturbances to the diagram.

Furthermore, the structure of the antenna makes it easy to place, on the supply line, the active modules inside the vertical mast and therefore close to the elementary antennas, which increases their efficiency all the more.

Finally, due to the use of simple dipoles as elementary antennas, the network can be made practically invisible in terms of radar signature by choosing very thin wires for the dipole production, therefore having an equivalent surface. extremely weak reflective (unlike the loops or turnstiles of the prior art).

• - la figure 1 montre, de façon schématique, un mode de réalisation de l'antenne selon les enseignements de la présente invention,
• - la figure 2 est une variante de réalisation de la figure 1, dans laquelle les modules actifs sont placés dans le mât central, au voisinage de leurs antennes élémentaires respectives associées,
• - les figures 3 et 4 sont des diagrammes donnant le gain en fonction de l'angle de site du réseau selon l'invention.

Other characteristics and advantages of the invention will appear on reading the detailed description below, made with reference to the appended drawings in which:

• FIG. 1 schematically shows an embodiment of the antenna according to the teachings of the present invention,
• FIG. 2 is an alternative embodiment of FIG. 1, in which the active modules are placed in the central mast, in the vicinity of their respective associated elementary antennas,
• - Figures 3 and 4 are diagrams giving the gain as a function of the angle of elevation of the network according to the invention.

In Figures 1 and 2, there is shown schematically the network according to the invention.

It will be noted that, for the clarity of the drawing, the respective proportions between the length of the different elementary antennas (half a wavelength), their relative spacings (of the order of several wavelengths), the diameter of the volume envelope (of the order of several wavelengths or several tens of wavelengths) and the diameter of the central mast (of the order of a wavelength or of a fraction of wavelength) have not been met.

Furthermore, as will be seen below, the different dimensions which have just been indicated can vary in significant proportions as a function of the performance desired for the network (gain, fineness of the beam, etc.).

The structure essentially comprises a network 1 formed of a plurality of elementary antennas formed of simple vertical dipoles 3, distributed randomly inside an envelope volume 2, in accordance with the principles of random rarefied networks, which have been explained more high.

The dipoles 3 are each connected by a clean supply line 4,5 to an active module 6.

(By ^“ active module ^” is meant an electronic module comprising at least one individually controllable phase shifting circuit, but which may further comprise amplifier, filtering circuits, transmission means, reception means, etc. depending on the functions assumed by the antenna and the types of signals it may be required to transmit or receive).

The different active modules 6 all lead to an antenna distributor 7 itself connected to the transmission and / or reception circuits 8.

The supply lines of each dipole consist of two sections 4 and 5.

The first section 4 is essentially horizontal to be transparent (from the radioelectric point of view), taking into account the vertical polarization provided by the antenna.

Furthermore, from the mechanical point of view, this first section 4 has an essentially rigid structure in order to play, in addition to its role of supplying the dipole 3, a role of mechanical support for this dipole on a central mast 9.

The second section 5 of the supply line runs inside the mast 9.

The mast 9 is made of a material forming radioelectric shielding, so that the sections 5, which are generally vertical, do not disturb the antenna pattern, the direction of polarization of which is also vertical.

In the embodiment of FIG. 1, the active modules 6 are placed at the end of section 5 of the supply line, near the distributor 7 (generally located at the base of the antenna or at the base of the mast ).

On the other hand, in the embodiment of FIG. 2, the active modules 6 are placed inside the mast 9, at the end of the horizontal section 4.

Although this second configuration requires an increase in the diameter of the mast 9 in order to accommodate the active modules of the various elementary antennas, it has the advantage of minimizing the distance between each elementary antenna and its associated active module, thus allowing a significant improvement. antenna performance, both from the point of view of the signal / noise ratio and of the disturbances introduced by the proper phase shifts of the supply lines.

In a variant not shown, the active modules can also contain transmission and reception means. In this case, they are positioned, for example, in the same way as the phase shifting means 6 shown in Figures 1 and 2, the dispensing means no longer appear in this case.

The vertical mast 9 can (in particular in the embodiment of FIG. 1) have a very small diameter (less than a wavelength) and consequently only bring a minimal gene to the quasi-hemispherical diagram of each elementary antenna.

All the elements of the network can be placed in free space, or inside a protective radome, or even be drowned in an appropriate material such as a polyurethane foam (although this solution, as indicated above, is not satisfactory from the point of view of heat dissipation when the network is used in transmission).

The envelope volume 1, in its simplest form, is a sphere.

This form is not, however, limiting and one could also consider other forms of envelope volume, that is to say with a principal height / diameter ratio which is different from 1, as soon as these forms are forms of revolution.

This choice in fact depends on the fineness of the beam desired as a function of the elevation angle: a spherical volume corresponds to a substantially uniform beam whatever the elevation angle, while a flattened shape, close to that of a disc, will obtain the fineness of the beam mainly for large angles of elevation.

In other words, it is the apparent outline of the envelope volume, seen from the target, which will determine the fineness of the beam.

Regarding the number of dipoles in the network, the relative average spacing between them and the diameter of the envelope volume, these parameters can vary significantly depending on the desired performance.

Basically:
- the number of elementary antennas determines the gain of the antenna in the chosen direction: the greater the number of elementary antennas, the higher this gain;
- and the diameter of the sphere determines the fineness of the beam: the larger the sphere, the more the beam, in the determined direction, is eroit; typically for a fine beam, of the order of 1 ° opening at -3 dB, it is necessary to provide a sphere having a diameter of the order of 70 wavelengths.

Figures 3 and 4 illustrate the performance obtained with a network produced according to the teachings of the invention, comprising 377 sources distributed with an average mesh of 3 wavelengths and an average random deviation of ± 1.5 wavelength.

On the two diagrams, the gain G has been plotted as a function of the elevation angle, the azimuth angle being in the two figures fixed at 60 °).

Figure 3 corresponds to a pointing of the beam at a site angle of 0 °, while Figure 4 corresponds to a pointing to a site angle of 60 °.

A beam width l of -3 dB of 2.52 ° in the first case and 2.56 ° in the second case is obtained. In this second case, the excellent performance of beam finesse will be emphasized, although there is both a high elevation angle (60 °) and a high azimuth angle (also 60 °). Note also the absence of variation of the maximum gain (point A) in one case and in the other, which reveals an excellent isotropy in site.

The antenna according to the invention lends itself to numerous applications, among which one can indicate:
- radars on board ships, where there is typically a need for both hemispherical coverage and vertical polarization to eliminate the effects of reflection on the sea,
- IFF radars and tracking radars for weapon systems, for which a continuous rotation of the beam is ill-suited. Indeed, once the threats are located, it is necessary to be able to exchange information sequentially in a plurality of well-defined directions, likely to extend over the entire horizon and with significant angles of site, directions in which it is desirable to be able to access selectively without having to scan the entire horizon, as is currently the case with continuously rotating radars.

Claims (7)

1. An antenna with three-dimensional coverage and electronic scanning, of the random rarefied volume array type, consisting of a fixed array (1) comprising a plurality of elementary antennas (3) with quasi-omnidirectional individual radiation distributed according to a law of random distribution statistically isotropic inside an envelope volume of revolution (2), the average spacing between elementary antennas being notably greater than half a wavelength of the minimum frequency to receive or transmit, each elementary antenna being connected to individually controllable phase shifting means, characterized in that the elementary antennas (3) consist of vertically oriented dipoles and supplied by a supply line (4,5) comprising a first section (4), extending horizontally between the dipole respective and a common vertical mast (9) coaxial with the volume envelope of revolution (2). 2. The antenna of claim 1, wherein the volume envelope of revolution (2) is a sphere. 3. The antenna of claim 1, wherein the first sections (4) of the supply lines constitute means, self-supporting, of mechanical support of the dipoles on the common vertical mast (9). 4. Antenna according to claim 1 in which the feed line consisting of the first section (4) comprises a second section (5) extending inside the mast (9) and leading to the individually controllable phase shifting means. 5. Antenna according to claim 1 in which each elementary antenna is connected to an active module (6) comprising the phase shifting means. 6. The antenna of claim 5, in which the active modules (6) are placed, on the supply line, inside the vertical mast (9). 7. Antenna according to claim 1, in which the dipoles of the elementary antennas (3) are fine wires.

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