FR2551921A1 - METHOD FOR REDUCING THE AMPLITUDE OF REFLECTED HYPERFREQUENCY ENERGY IN THE AXIS OF AN AIR, RADAR AND AERIAL OF COMMUNICATION STATION WITH SATELLITE USING SUCH A METHOD - Google Patents

Abstract

THE INVENTION MAINLY CONCERNS A METHOD FOR REDUCING THE AMPLITUDE OF THE HYPERFREQUENCY ENERGY REFLECTED IN THE AERIAL AXIS OF RADARS, AND RADAR USING SUCH A METHOD. THE INVENTION CONCERNS MORE PARTICULARLY THE REDUCTION OF THE RADAR EQUIVALENT SURFACE OF AERIALS OF REMOTE-POINT TRACKING RADARS. THE INVENTION MAINLY CONSISTS OF PLACING A FILTER 6 INCLINED WITH RESPECT TO THE AXIS OF THE RADAR AERIAL. THUS THE RADAR INCIDENT 4 IS REFLECTED OFF THE AXIS 3 OF THE AERIAL 11. THE INVENTION APPLIES MAINLY TO THE PROTECTION OF OBJECTIVES AGAINST MISSILES CONTAINING ACTIVE SELF-DIRECTOR RADAR AS WELL AS AGAINST AIRCRAFT.

Description

192 1

  METHOD FOR REDUCING THE AMPLITUDE OF HYPERFREQUENCY ENERGY REFLECTED IN THE AXIS OF AN AIR, RADAR

  AND AERIAL OF COMMUNICATION STATION WITH SATELLITE USING SUCH A METHOD

  The invention mainly relates to a method for reducing the amplitude of the microwave energy reflected in the axis of an aircraft including a radar and the radar using such a method. Objective defense increasingly involves "seeing without being seen". Thus, it is essential to collect as much information as possible about enemy devices, while minimizing the possibility for an enemy device to collect information on its own device Sometimes these two requirements of the modern defense are contradictory Thus, the aerial of a radar 15 of known type, receiving in its axis radar microwave radiation, returns on the same axis, and therefore towards the emitter-receiver of this electromagnetic radiation, this radiation with a very important amplitude In what follows, we will say

  that it presents a large radar equivalent area.

  The invention proposes to place in a radar aerial with a non-zero inclination with respect to its axis, a frequency-selective filter. Such a filter reflects the radiation of the enemy radar, according to the laws of Descartes, without appreciably affecting the radiation of the radar. o it is placed, insofar as they work at different frequencies Thus, the energy returned in the axis of

  the air, towards the enemy radar, is considerably diminished.

  The invention mainly relates to a method for reducing the amplitude of the microwave energy reflected in the axis of an overhead plane from an incident wave, characterized in that a selective filter is placed on the antenna. axis of the aerial and inclined relative thereto, the selective filter reflecting the microwave energy incident, and not substantially affecting the microwave energy

issued by said air.

  The invention mainly relates to a radar characterized in that it comprises a source of microwave radiation and an aerial, the aerial having a frequency selective filter,

  disposed on the axis of the aerial and inclined with respect to the latter.

  The invention will be better understood by means of the following description and the appended figures, given as non-standard examples.

  limiting devices, among which: FIG. 1 is a sectional view of a radar of known type; FIG. 2 is a sectional view of a radar according to the invention; FIG. 3 is a perspective view of a filter used for

radar according to the invention.

  In Figures I to 3, the same references have been used to designate the same elements.

  In FIG. 1, a radar I of conventional type can be seen. The radar 1 receives, on an axis 3 of its aerial, an incident electromagnetic radiation 4. As illustrated by the line 41, the radiation 4, arriving on the 1, the radiation is reflected on the source 5 Thus the reflected radiation 14 lies in the axis of the aerial I with a very large amplitude. The axis of the aerial 1 is proportional to: S x G x R 2, S being the area of the opening of the aerial, G 25 being the gain in the axis of the aerial at the frequencies of the enemy radar ,

  R 2 being the reflection coefficient in power of the source 5.

  This is likely to be very troublesome, for example in the case where an aircraft or a missile would move towards a fixed radar overhead This is even more troublesome in the case of using a telepointfinder, effec30 killing an angular pursuit for example Thus, in this case, the axis 3 of the aerial of the radar 1 is always directed towards the

missile radar.

  FIG. 2 shows a radar 11 according to the invention. In this exemplary embodiment, the radar 11 is a telepointing tracking radar. The telepointing radar 11 tracks the movement of a missile 2. A main lobe 7 of the radar 11 illuminates the missile 2 and the missile 2 is permanently on the axis 3 of the aerial radar 1 l If the missile 2 has an active homing device, 5 it emits electromagnetic radiation 4 which arrives in the axis 3 of the radar 11 In order not to increase the quantity of electromagnetic energy reflected on the radar 11 that will capture the homing mechanism of the missile 2, a filter 6 has been placed inside the aerial of the radar 11. The filter 6 is such that It reflects the radiation 4, without affecting the radiation emitted by the radar 11 The normal to the filter 6 is not confused with the axis 3 Thus an axis 8 of the reflected radiation 40 forms an angle G with the axis 3 L angle of the normal to the filter 6 and the axis 3 is sufficient po ur no energy is returned by the filter 6 towards the missile 2 The angle between the axis 3 of the aerial 15 of the radar 11 and the normal of the filter 6 is for example 15, which

  corresponds to 4 = 30 Such a value of 4 takes into account the fact that the radiation 40 not being focused by the radar 11 is scattered Such a filter 6 makes it possible to reduce, for example by 20 decibels, the gain G along the axis 3 from the air 11 to the frequencies of the enemy radar.

  It is extremely easy to adapt a filter 6 on radars already in service, because it does not involve major modification thereof. In FIG. 3, an embodiment of the filter 6 can be seen. The filter 6 must reflect the radiation 4 of FIG. 2, without appreciably affecting the radiations of the main lobe 7 emitted by the source 5 of FIG. of course, that the radar according to the invention works with a frequency different from that of the missile 2 self-directioner. For example, the telepoint-finder uses the X-band while using a filter 6 which can reflect the

  energies included in the bands KU, KA and W In such a case, the filter 6 can be either of the low-pass type or of the band-pass type.

  Advantageously, the filter 6 is a dichroic mirror. A dichroic mirror of the low-pass type has been described in the patent filed by the Applicant on December 18, 1981, published under the number 2,518,828. Such a filter comprises three low-loss dielectric blades. , 12, 13 separated by two networks of metal rectangles

44 and 15.

  The filter 6 can be permanently set, for example in radon time, in front of the source 5 In an alternative embodiment, there is a set of filters 6 The filter 6 is set up according to for example the theater d operation, and therefore according to the

  Radiation frequency band of the opposing missile.

  In another variant embodiment, where a set of filters 6 is also available, one of these filters 6 is put in place in real time in front of the source 5, after the detection of an electromagnetic radiation 4 by means not illustrated So the filter 6

  chosen will be best suited to reflect this radiation.

  The invention is not limited to radar aerials The invention

  applies each time it is in the presence of a device focusing and reflecting the electromagnetic radiation of an adverse radar The method according to the invention applies in particular to radio-relay transmitters, as well as antennas 20 connection with telecommunication satellites.

255 1 921

Claims (9)

  A method for reducing the amplitude of a microwave energy reflected in the axis of an aerial, from an incident microwave energy, said aerial being capable of emitting microwave energy, characterized in that the a selective filter (6) is placed on the axis (3) of the aerial and inclined with respect thereto, the selective filter reflecting the incident microwave energy (4), and not appreciably affecting the energy   microwave emitted by the air.   Process according to Claim 1, characterized in that the filter (6) is a dichroic mirror.   3 Method according to any one of the preceding claims, characterized in that the filter (6) is a low-pass filter.   4. Process according to any one of the preceding claims, characterized in that the filter (6) is a passband filter.   Radar characterized by the fact that it comprises a source of   microwave radiation and an aerial, the aerial comprising a filter (6) frequency selective, disposed on the axis of the air and inclined relative to the latter.   Radar according to Claim 5, characterized in that the   filter (6) is arranged in front of the source (5).   Radar according to claim 5 or 6, characterized by the fact   the filter (6) is a dichroic filter. 255 1 92 1   Radar according to Claim 8, characterized in that   it is a telepointing tracking radar.   9 Aerial satellite communication station, characterized in that it comprises, placed in front of the ray source 5 nement, a selective filter (6) arranged on the axis and inclined relative to this one.