EP0367656B1 - System for the integration of IFF sum and difference channels in an antenna of a surveillance radar - Google Patents

Description

The invention relates to radar surveillance antennas and, more particularly, in such antennas a system for identifying targets by coded interrogations, the antenna of which is associated with the radar surveillance antenna.

Radars can detect the presence of objects or targets and determine some of their characteristics such as their distance, altitude, speed. However, they do not make it possible to determine, in time of war, if the target is friendly or enemy. For such a determination, a system is used which "interrogates" the targets by sending them coded signals which are detected by the latter; the targets can then send coded signals to the interrogating system which indicate its category. A target that does not "respond" properly to coded signals is considered an enemy.

Such an interrogator / answering system, better known by the Anglo-Saxon abbreviation I.F.F. for "Identification Friend or Foe", is widely used in peacetime because it allows a radar operator to easily identify the aircraft with which it is in radio and radar contact by asking it to transmit a specific coded signal. This coded signal appears in a particular form on the radar screen near the corresponding radar signal.

For obvious reasons, the antenna of the I.F.F. is carried by the radar antenna and this results in a very bulky and heavy assembly.

To remedy this problem, it has been proposed to use a single antenna for the two radar functions and IFF such an antenna is for example produced using a so-called primary source of radar signals which illuminates a reflector. The primary source is associated with dipoles which emit IFF signals and also illuminate the radar reflector. A solution of this type is described in particular in European patent application EP-A-0 025 739 which relates to an antenna comprising a primary radar source constituted by a horn, radiating elements arranged in the horn to form the sum signal IFF and radiating elements arranged on either side of the opening of the horn to form the IFF difference signal. Such a solution is not entirely satisfactory because the IFF channel cannot be optimized while the level of the cross-polarized signals is too high to comply with certain technical standards imposed by the administrative authorities in the aeronautical field.

The aim of the present invention is therefore a system for integrating the sum and difference channels I.F.F. in a radar surveillance antenna which does not have the aforementioned drawbacks and which meets the standards imposed.

Furthermore, known, in particular from European patent application EP-A-0 018 476, radiating elements which can be excited in two crossed polarization directions.

Another object of the invention is, by using radiating elements having this property in an integrated radar / IFF antenna, to take advantage of the existence of cross-polarized signals to effect compensation so as to reduce the level of the spurious signals in cross polarization.

According to the invention, there is therefore provided a system for integrating the sum and difference channels I.F.F. in a radar surveillance antenna, as defined in the claims.

• la figure 1 est une vue schématique de face de la source primaire du radar montrant, selon l'invention, la position des éléments rayonnants de la source primaire des voies I.F.F. par rapport à la source primaire du radar ;
• la figure 2 est une vue en coupe d'un élément rayonnant de la source primaire I.F.F. suivant la ligne II-II de la figure 3 ;
• la figure 3 est une vue en coupe de l'élément rayonnant de la source primaire I.F.F. suivant la ligne III-III de la figure 2 ;
• les figures 4a et 4b sont des schémas indiquant les combinaisons des signaux dans la voie somme I.F.F. ;
• les figures 5a et 5b sont des schémas indiquant les combinaisons des signaux dans la voie différence I.F.F. ;
• la figure 6 est un schéma montrant un exemple de réalisation du neutrodynage de la voie somme I.F.F.,
• la figure 7 est un schéma montrant un exemple de réalisation du neutrodynage de la voie différence I.F.F., et
• la figure 8 représente des courbes de diagrammes d'antennes qui permettent de montrer les résultats obtenus par la présente invention.

Other characteristics and advantages of the present invention will appear on reading the following description of a particular embodiment, said description being made in relation to the accompanying drawings in which:

• FIG. 1 is a schematic front view of the primary source of the radar showing, according to the invention, the position of the radiating elements of the primary source of the IFF channels with respect to the primary source of the radar;
• Figure 2 is a sectional view of a radiating element of the primary source IFF along line II-II of Figure 3;
• Figure 3 is a sectional view of the radiating element of the primary source IFF along line III-III of Figure 2;
• FIGS. 4a and 4b are diagrams indicating the combinations of the signals in the IFF sum channel;
• FIGS. 5a and 5b are diagrams showing the combinations of the signals in the difference channel IFF;
• FIG. 6 is a diagram showing an exemplary embodiment of the neutralization of the sum IFF channel,
• FIG. 7 is a diagram showing an exemplary embodiment of the neutralization of the difference channel IFF, and
• FIG. 8 represents curves of antenna diagrams which make it possible to show the results obtained by the present invention.

The invention applies to a surveillance radar antenna which comprises a primary source and a reflector which is illuminated by the signals emitted by the primary source. The reflector has the shape of a paraboloid with double curvature and the primary source is slightly displaced compared to the focal point of the paraboloid.

Such an antenna is often called an offset primary source or an offset reflector.

The primary source is produced using a horn 1 of the "tulip" type (FIG. 1) which is connected to the radar transmitter by a waveguide provided with a polarizer so as to obtain a circular polarization of the radar signal emitted. This horn can also propagate TE₁₀ mode in vertical polarization and TE₀₁ mode in horizontal polarization.

According to the invention, the Somme I.F.F. is obtained using two identical radiating elements 3 and 4 placed in the horn 1 while the Difference I.F.F. is obtained using four radiating elements 5, 6, 7 and 8, identical to elements 3 and 4 but placed two by two on either side of the horn 1.

The elements 3 and 4 are arranged in the top 9 and bottom 10 walls of the horn and are inclined relative to the plane of the opening of the horn. The elements 5 to 8 are arranged in a plane parallel to that of the opening of the horn 1.

Each radiating element 3 to 8 consists, as shown in Figures 2 and 3, of a rectangular cavity 11 of a metallic material which has a bottom 12 and four sides 13, 14, 15 and 16. The cavity is closed by a cover 17 which is made of dielectric material. The inner wall of the cover is coated with a metallic layer 18 of rectangular shape.

The cover 17 and metal layer 18 assembly constitutes a so-called guiding plate.

The bottom 12 of the box is coated with a dielectric layer 19 surmounted by a metallic layer 20 of rectangular shape in which four slots 21, 22, 23 and 24 are formed, arranged in a cross relation to each other. The microwave signals are applied to the cavity 11 via the slit plate 20 which is connected at two points 25 and 26 to respective coaxial lines 27 and 28. The point 25 is located in alignment with the horizontal slits 22 and 24 while point 26 is located in alignment with the vertical slots 21 and 23.

The dielectric layer 19 and metal layer 20 assembly constitutes a so-called radiating plate.

Corners of the rectangular slit plate 20 are terminated by metal tabs 29 and 30 which serve to perfect the adaptation by adjusting their width and their length. The whole forms a cavity which radiates energy on one side, side 17.

When the microwave signal is applied at point 25, the electric field vector 31 is horizontal (horizontal polarization). On the other hand, when the microwave signal is applied at point 26, the electric field vector 32 is vertical (vertical polarization). In the following description, point 25 of the radiating elements will be referenced by the letter H associated with a numerical index. Similarly, point 26 of the radiating elements will be referenced by the letter V associated with a numerical index. The numerical indices 1 and 2 have been assigned respectively to the radiating elements 3 and 4, the numerical indices 3 and 4 have been assigned respectively to the radiating elements 5 and 8 and the numerical indices 5 and 6 have been assigned respectively to the radiating elements 6 and 7 .

To obtain the Somme I.F.F. in vertical polarization, the points V₁ and V₂ of the radiating elements 3 and 4 are excited using a hybrid ring circulator 33 (Figure 4-a) so as to propagate the TE₁₀ mode in the horn 1 vertical polarization.

For this, the circulator 33 has four input / output terminals B₁, B₂, B₃ and B₄ which are respectively connected to the signal source I.F.F., at point V₁, at point V₂ and at a load C₁. Thus, an I.F.F. applied in B₁ is divided into two phase signals which appear on terminals B₂ and B₃. This operating mode is used on transmission.

As the circulator operates reciprocally, phase signals received at V₁ and V₂ have their sum S _V which appears at the terminal B₁. This operating mode is used at reception.

To obtain the Sum IFF channel in horizontal polarization, the points H₁ and H₂ are respectively connected to the terminals B₂ and B₃ of a hybrid ring circulator 34. The signal Sum S _H in horizontal polarization is then obtained on terminal B₁. Terminal B₄ is connected to the load impedance.

To obtain the IFF Difference channel, the lateral radiating elements 5, 6, 7 and 8 are used and the following connections are made which will be described in relation with the figures 5-a and 5-b. The outputs V₃ and V₄ of the radiating elements 5 and 8 are grouped to be connected to the terminals B₂ of a hybrid ring circulator 35. Likewise, the outputs V₅ and V₆ of the radiating elements 6 and 7 are grouped to be connected to the terminal B₄ of circulator 35. The difference signal D _{V is} then collected in vertical polarization on terminal B₁. As for the terminal B₃, it is connected to a load.

To obtain the signal Difference D _H in horizontal polarization, the outputs H₃ and H₄ of the radiating elements 5 and 8 are grouped to be connected to the terminal B₂ of a circulator in hybrid ring 36 (figure 5-b). Similarly, the outputs H₅ and H₆ of the radiating elements 6 and 7 are grouped to be connected to the terminal B₄ of the circulator 36. The signal Difference D _H is then collected on the terminal B₁. Here too, the terminal B₃ is connected to a load.

The description which has just been made in relation to FIGS. 1 to 5 shows that it is possible, by implementing the invention, to produce an I.F.F. integrated into a radar antenna of the double curvature reflector type with offset primary source.

The description which follows, in relation to FIGS. 6, 7 and 8, will show that it is possible, by implementing other aspects of the invention, to reduce the level of cross polarization in the two channels Sum and Difference by combining the signals received on the IFF antenna described above.

To this end, a process of neutralizing or mixing the signals received on the different Sum and Difference channels is implemented. Figure 6 gives the block diagram of neutrodynage on the Somme track and Figure 7 gives the block diagram of neutrodynage on the Somme the Difference way.

It will be recalled that a "shifted" reflector which is illuminated by a primary radiation pattern of the even type gives a radiation pattern of the odd type in cross polarization. On the other hand, if the reflector is lit by an odd type primary radiation diagram, the radiation diagram of the reflector will be even in cross polarization.

In the case of the Somme I.F.F. in vertical polarization, the radiation pattern in cross polarization is even. To decrease its level, it is proposed to mix the sum sum I.F.F. in vertical polarization an odd type primary diagram in horizontal polarization so as to obtain an even type radiation diagram which is subtracted from the radiation diagram in cross polarization. We can then adjust the level of cross polarization of the sum sum I.F.F. by adjusting the amplitude and the phase of the primary diagram of the odd type in vertical polarization.

In the particular embodiment of FIG. 6, the primary diagram which is used is that of the Difference channel in horizontal polarization. For this, the terminals H₃ and H₄ of the radiating elements 5 and 6 are connected to the terminal B₂ of the circulator 36 while the terminals H₅ and H₆ of the radiating elements 6 and 7 are connected to the terminal B₄ of the circulator 36. The difference signal D _H is obtained on terminal B₁ and is applied to a phase shifter 37. The phase difference difference signal D ′ _H is mixed with the signal of the sum channel using a coupler 38. By suitably choosing the value of the phase shift in the phase shifter 37, a significant reduction in the level of cross-polarization is obtained in the sum IFF channel. In FIG. 8, the curve 39 represents the radiation diagram of the Somme channel.

In the absence of the neutralization according to the invention, the radiation diagram in cross polarization is given by the curve 40. With the neutralization according to the invention, the radiation diagram in cross polarization is given by the curve 41, which represents a improvement of ten decibels.

To decrease the level of cross polarization in the Difference channel, we use the diagram of the Sum channel in horizontal polarization to mix it after appropriate phase shift with the diagram of the difference channel in vertical polarization. Figure 7 gives the diagram of a particular embodiment in which the terminals V₃ and V₄ of the radiating elements 5 and 8 are connected to the terminal B₂ of the circulator 35 while the terminals V₅ and V₆ of the radiating elements 6 and 7 are connected at terminal B₄ of circulator 35. The Difference signal D _V is supplied by terminal B₁ and is applied to a coupler 39. Furthermore, the terminals H₁ H₂ of the radiating elements are connected respectively to terminals B₂ and B₃ of circulator 34 and the sum signal S _H is supplied by the terminal B₁. The signal S _H is phase shifted in a phase shifter 40 to obtain a signal S ′ _H which is applied to the coupler 39. By modifying the phase of the signal S _H , it is possible to adjust the level of the cross polarization of the Difference channel and to obtain a significant decrease of the order of ten decibels.

Claims (5)

1. System for integrating the IFF sum and difference channels in a radar surveillance antenna, the said antenna including a primary source of the horn type (1) which illuminates a reflector of the offset type, the IFF primary source including two first radiating elements (3, 4) arranged in the horn (1) to form the source of the sum channel, and second radiating elements (5, 6, 7, 8) arranged in twos on either side of the horn (1) to form the source of the difference channel, characterized in that the said first and second radiating elements each comprise an input/output port (25) into a first polarization direction and an input/output port (26) into a second crossed polarization direction, first combiner means (33, 35) connecting the said ports in the said first polarization direction of the first radiating elements (3, 4) respectively to give a sum channel signal in the first polarization direction and second radiating elements (5 to 8) to give a difference channel signal in the first polarization direction and second combiner means (34, 36) connecting the said ports in the said second polarization direction of the first radiating elements (3, 4) respectively to give a sum channel signal in the second polarization direction and second radiating elements (5 to 8) to give a difference channel signal in the second polarization direction, and in that there is furthermore provision for third combiner means (37, 38) for combining the said sum channel signal, delivered by the said first combiner means, in the first polarization direction, with the said difference channel signal, delivered by the said second combiner means, in the second polarization direction, in order to provide a neutrodyned IFF sum channel signal so as to obtain a reduction in the unwanted signals of the IFF sum channel under crossed polarization.
2. Integration system according to Claim 1, characterized in that it furthermore comprises fourth combiner means (39, 40) for combining the said difference channel signal, delivered by the said first combiner means, in the first polarization direction, with the said sum channel signal, delivered by the said second combiner means, in the second polarization direction, in order to provide a neutrodyned IFF difference channel signal so as to obtain a reduction in the unwanted signals of the IFF difference channel under crossed polarization.
3. Integration system according to Claim 2, characterized in that the third and fourth means each comprise a phase shifter (37; 40) inserted into the channel of the signal in the second polarization direction and a coupler (38; 39) for mixing the said signal in the first polarization direction with the said phase-shifted signal in the second polarization direction.
4. Integration system according to Claim 1, 2 or 3, characterized in that each radiating element consists of a resonant cavity which comprises a rectangular metal box (13), the bottom of which includes a radiating conducting plate (20) resting on a dielectric layer (19) and the cover of which consists of a conductive plate (18) which is carried by a dielectric layer (17) and which faces the radiating conductive plate (20).
5. Integration system according to Claim 4, characterized in that the radiating conductive plate (20) include slots (21, 22, 23, 24) arranged in a cross and associated with terminals (25, 26) for input/output of the electrical signals.

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