FR2678438A1 - LINEAR NETWORK ANTENNA. - Google Patents

Abstract

The present invention relates to a linear array antenna composed of a radiating element formed by an alignment of annular slots (21) excited by at least one buried distribution element (14), included in the radiating element, and therefore invisible from the radiating face of said antenna; so as to avoid any parasitic radiation from said distributor. Application in particular to the field of space telecommunications.

Description

Linear array antenna The invention relates to a linear array antenna to

  high efficiency Such an antenna is particularly advantageous in the

  Synthetic Aperture Radar (SAR) range used in the space field.

  The antennas used for such radars must have the following properties: a high gain to minimize the power emitted by the radar a very thin lobe in the azimuth plane (parallel to the orbit), of variable width to maintain a constant swath on the ground in the plane of elevation (perpendicular to the trace); a very high sweep in the elevation plane to be able to access any site very quickly (surveillance mission) and aim for the same low and high incidence test sites at 24 hours15 intervals (observation of the vegetation, separation of soil and leaf effects); a low sweep in the azimuth plane to follow the same zone in order to increase the synthetic length of the antenna and to obtain a resolving power of a few meters parallel to the orbit; two orthogonal linear polarizations (horizontal H and vertical V) to separate the backscatter from the soil and that from the vegetation cover; a very low level of cross polarization on the antenna scan range;

  radiating elements of cost and mass the lowest possible given their large number on the antenna.

  These specifications can only be maintained with a large direct radiation antenna consisting of a very large number of planar radiators. Indeed, the theory of grating lobes shows that as soon as the maximum angle of misalignment of the antenna (Max) exceeds a few degrees, the distance d between the phase control points on the antenna must check the relation d 15 1 + sin O max sin (Bw / 2)

  With Bw width at the base of the main lobe of the antenna network when it is pointed according to the normal to the antenna.

  If this condition is not respected, lattice lobes appear in the radiation pattern. Their level is higher than the one requested for the side lobes, which decreases the gain of the antenna and creates ambiguities. Such a condition has the following consequences. For the elevation scan, the pitch should be close to A / 2. For the azimuth scanning, the small misalignment is compatible with a step close to A However, the antenna having a great length, to generate a

  lobe end in the plane this condition leads to a prohibitive number of sources.

  Three solutions are then possible to reduce the number of commands: a rarefied network which breaks the periodicity of the law of illumination and consequently destroys the lobes of network but causes a

very high loss of profit.

  a network with non-regular pitch, obtained by progressively increasing the distance between phase controls when moving away from the center of the antenna This solution, in order not to lower the gain of the antenna, 25 requires the realization of many types different radiating elements, which prohibits the cutting of the antenna into sub-panels all identical and greatly increases the cost. a network formed of sub-networks whose illumination law in the azimuth plane must be as close as possible to that of a uniform opening of the same length. In this case, for a target in the axis, the lobes of The subnetworks of the antenna are canceled by the first zeros of the subnet diagram. This condition is no longer satisfied during a low azimuth scan. The maximum azimuth angle of depiction is then determined as the maximum length. 3 - subnetworks to keep network lobes at a lower level

to that of the lateral lobes.

  Similarly, it is found that, if no azimuth misalignment is specified, a sub-network of length equal to that of the antenna suffices. In practice, the length of the sub-networks is limited by the size of the panels of the antenna. antenna (1 to 3 meters) to be compatible

  with the encumbrance under the caster of the launcher. Currently, all of these specifications are not required for the radiant elements of synthetic aperture radars.

  Indeed: The slotted guides, as described in the article entitled "The planar array antennas for the European remote sensing satellite ERS-1" by Robert Peterson and Per Ingvarson published in "Proceedings of IGARSS 1988" (pages 289-294) ), used in particular on ERS-1 (1991), RADARSAT (1995) and SIR-C (X-SAR), despite their small losses do not allow to easily achieve the bipolarization because it would be necessary to insert two different guides radiating one in polarization H and the other in V polarization in the pitch of a very small network because of the high scanning in elevation Moreover, if azimuth scanning is specified, to limit the level of the grating lobes, these guides must be cut into small stretches which makes noticeably more

  This technology is of interest only for radars having a C-band polarization or in higher frequencies, without scanning in the azimuth plane.

  The radiating elements printed on honeycomb strips L and S, as described in the article entitled "SEASAT and SIR-A microstrip antennas" LR Murphy published in "Proceedings of Workshop on printed antennas technology" (Las Cruces; 1979, pages 18-1 to 18-20), used on SEASAT (1978), SIR-A (1981), SIR B (1984) and J. ERS (Japanese 1992) are lighter than the guides and allow a bipolarization however their linear losses are high This limits the length of the sub-networks to a few wavelengths (10> maximum); these losses do not allow to obtain a

  uniform illumination on the subnet.

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  The purpose of the linear array antenna of the invention is to satisfy all these specifications.

  The invention proposes, in fact, a linear array antenna composed of a radiating element formed of an alignment of annular slots excited by at least one distribution element, characterized in that this distribution element is a buried distributor, included in the radiating element, and therefore invisible from the radiating face of said antenna; in order to avoid any parasitic radiation from said splitter An antenna of this kind is provided for radiating (or receiving) simultaneously in two linear polarizations, or for radiating (or receiving) simultaneously in two circular polarizations (right and left). In an advantageous embodiment using only three levels of conductors, said antenna comprises an integrated radome and a splitter stage Advantageously said antenna comprises only substrates of small thickness, and therefore of low mass, spaced apart by conductive spacers without mechanical continuity in the plane of the In order to avoid differential expansion with the substrates, said antenna advantageously comprises shielding, between polarizations of a sub-network and between adjacent sub-networks, radiating elements and splitters so as to avoid any coupling between them. this. In a particular embodiment of the invention, the antenna is a broadband antenna using resonant disks etched on

  the splitter stage, resonating at a frequency close to that of the annular slots.

  In another particular embodiment of the invention; the antenna is a dual-band antenna using resonant disks

  engraved, on the splitter stage, resonant at a frequency away from that of the annular slots.

  Such an antenna has many advantages; It makes it possible to obtain in particular: small losses, - a low level of cross polarization on the scanning range, a use in bipolarization: the network antenna according to the invention can radiate (or receive) simultaneously in two linear or circular polarizations (right and left); a wide sweeping range in elevation; a significant length of subnets; a high gain; similar misalignment losses in both

polarizations.

  The features and advantages of the invention will emerge

  moreover from the description that follows, as an example not

  FIG. 1 illustrates an exploded view of a sub-network section of an embodiment of the antenna according to the invention; FIGS. 2 and 3 illustrate two exploded views of sections of sub-networks of embodiments of two variants of the antenna according to the invention; FIGS. 4 and 5 respectively show the radiation patterns for the horizontal and vertical polarizations, FIGS. 7 and 8 illustrate two other variants of FIG.

the antenna according to the invention.

  The antenna according to the invention is a linear array antenna composed of an alignment of annular slots excited by at least one buried distribution element, included in the radiating element, and therefore invisible from the radiating face of said antenna; in order to avoid any parasitic radiation coming from said splitter. Such an antenna is provided for radiating (or receiving) simultaneously in two linear polarizations; or to radiate (or

  receive) simultaneously in two circular polarizations (right and left).

  FIG. 1 illustrates a sub-network section of such an antenna which comprises: a lower half-shell 10 of conductive material (aluminum, metallized carbon fiber, or conductive plastic for example) in which the cavity 11 of the radiating elements and the duct 12 of the splitters of the two polarizations; a very thin first substrate (for example of about 0.1 mm thick), having a coefficient of expansion close to that of the half-shell material, on which are engraved, on the upper face, the H and V splitters 14; an upper half-shell 15 in which the duct 16 of the splitters of the two polarizations is formed and pierced (17) the upper part of the cavity; a second substrate 18 identical to the first substrate, an upper face 19 of which is fully decoupled, and Another 20 contains the annular engraved slots 21 This substrate is intended to center the annular slots above the cavities and to provide a function of rad 8 me.15 For operation "suspended plate" the two half-shells 10 and 15 must be connected to the same potential

  is provided by the first substrate through the metallization of the surfaces facing the two half-shells and the presence of a large number of metallized holes located on the periphery of the cavities, 20 connecting these two surfaces.

  Such an antenna has been realized and tested on sub-networks of 10, 12 and 24 annular slots The adequacy between the specifications of the sub-network and its various constituent elements is given below; such a sub-network makes it possible to obtain: low losses: the distributor in triplate technology suspended on a very thin substrate and with a low loss tangent with silver walls perfectly ensures this function. The radiating element is produced in the same technology. a low level of cross polarization on the scanning range: Thanks to the use of a triplate technology, the parasitic radiation of the lines and bends of a splitter engraved on the radiating plane is avoided. A difference diagram is generated on the polarization Crossed, thanks to symmetrical splitters with respect to the axis of the network This makes it possible to cancel the already low level of cross polarization of the annular slots over the entire 7-scan domain. a use in bipolarization: the excitation of the annular slots is obtained by two orthogonal linear polarizations

  or by two circular polarizations by inserting a 3 d B coupler at two 900 out of phase outputs at the center of the radiating sub-array.

  a wide range of scanning in elevation: The use of channels for the splitters allows a very great compactness on the

  subnetwork without risk of coupling between polarizations The pitch in the elevation plane is thus considerably reduced and allows a large misalignment in elevation.

  a significant length of the sub-networks: The low losses of the splitter ensure an almost uniform illumination in amplitude and

  In the phase of the sub-network Its length is limited only by the modification of the law of illumination over the bandwidth of the sub-network.

  a high gain: a perfect adaptation of the directivity of the radiating element to that of the mesh of the grating is obtained thanks to the use of a concentric cavity whose diameter makes it possible to very precisely adjust this magnitude. between 6 d Bi for quasi-decoupled slots of the cavity whose perimeter, at resonance, corresponds to the guided wavelength and 9 d Bi for slots resonant to al, 5> Thus covers gg the domain of square meshes from 0.56 to 0.8 to required for such networks.25 similar misalignment losses in both polarizations: The annular cavity slots have a

  quasi-symmetry of revolution In the plane of elevation, the diagram is thus invariant with the polarization, which guarantees the same losses on the scanning domain.30 of the sub-networks of low cost and mass.

  A particularly advantageous embodiment of the antenna according to the invention is shown in FIG. 2. To obtain a low mass, the antenna consists of three substrates, about 0.1 mm thick, whose functions are as follows: A lower substrate 25 having: a locally coppered lower face 26 (zone 27) around the RF supply connectors 28 and 29, bonded to the structural panel; * a fully coppered upper face (except RF power saving), used as a ground plane; * metallized holes 31 connecting the two surfaces. an intermediate substrate 32 having: a copper-plated underside 33 on the surfaces facing the partitions formed by conductive spacers 41; * a coppered upper surface 34 on the surfaces 42 opposite the partitions and on the distribution circuits H and V; * metallized holes 44, providing the electrical connection between the two ground planes, which are arranged vertically partitions formed by the spacers 41; (The holes 36 serve for welding the webs of the connectors 28 and 29 supplying the distributors H and

V 35);

  an upper substrate 37 having a fully coppered lower face 38 except at the locations of the annular slots 39 and serving as a ground plane for the triplate; * a top face 40 fully decoupled and making

radome function.

  These three substrates 25, 32 and 37 are spaced apart by the conductive spacers 41, for example made of aluminum, which delimit the cavities of the annular slots and the splitter channels. The assembly can be carried out in a single operation by soldering in the oven after dep 8 t of a solder preform at the interfaces between the spacers and the copper substrates, or by tinning the spacers. The materials and methods used in such an embodiment ensure that very good performance in terms of cost and mass of the sub-networks.30 At the thermal level, the spacers of small dimensions and without continuity in the plane of the network make it possible to use materials with a low coefficient of expansion, compatible for example with a structural panel made of With a high azimuth offset, the 9-sub-gratings are much shorter, allowing for an antenna with slightly higher ohmic losses. A variant of the previous antenna is shown in FIG. 3. The antenna then consists only of two parts: a lower part 45 made of conducting material: the ground plane is formed by a thin substrate 43 on which are soldered the spacers 46; an upper portion 47 of dielectric material; It is a substrate which contains on the upper face 48 the annular slots 49 and on the lower face 50 the polarization splitters 51 and 52 and the exciters 53 slots The continuity of the cavity of the annular slots in the substrate is provided by holes

  metallized 54 which connect the spacers to the ground plane of the slots. The distribution of the distributors is by coaxial probe through two metallized holes 55 and 56 for the continuity of mass between the probe and the triplate.

  This variant of the invention retains all the properties of the antenna shown in FIG. 1. Only the ohmic losses are slightly increased in favor of a simplification of embodiment. This increase in losses is entirely acceptable for short sub-networks. (<10 A) is due to the use of: a "suspended triplate" splitter where the microwave energy propagates essentially in a vacuum because the lines are supported by a very thin substrate (about 0.1 mm) Of an "inverted microstrip" splitter where the propagation is largely in the thicker dielectric substrate (about 1 mm). The invention thus makes it possible, in any of these embodiments, to form a synthetic space radar antenna.

  opening comprising a grating of 1.33 x 10 m 2 composed of subpanels of 0.66 x lm 2.

  The elevational sweep requiring a step between the sub-arrays of about 0.7 to 5.3 GHz, no azimuth sweep being specified, to avoid an additional splitter stage supplying short radiating elements, - Bipolarized sub-networks of the length of the panel (about 1 m) are therefore best suited The invention makes it possible to meet all these specifications with sub-networks of 24 annular slots. The antenna thus performs the functions of radiator and distributor on a single level The lower and upper spacers are made by aluminum half-shells assembled by screws. The excitation of the two polarizations is carried out by miniature coaxial probes placed near the center of the network. The radiation diagrams for the horizontal and vertical polarizations are given in FIGS. 4 and 5. The normal polarization diagrams 60 and 61 in the azimuth planes are very close to those of an equi-amplitude equi-amplitude aperture as shown by FIG. the indicators at "*" at i 3,220 which correspond to the first nulls of such an opening. The crossed polarization diagrams 62 and 63 in the axis are less than 30 d B / max over the entire bandwidth. the antenna, the gains of the two polarizations must be compared to the theoretical directivity D of the mesh of the network given by the formula 41 TS

  D = 10 log 2 = 22, ld B; S being the surface of the antenna.

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  In horizontal and vertical polarizations, respectively, losses of 1.5 d B and 2.5 d B are obtained with respect to this magnitude. These losses include: The excitation errors of the 24 slots vis-à-vis a law equi-amplitude and equi-phase; Losses due to cross polarization; Ohmic losses;

  Loss of maladjustment as input.

  These results make it possible to judge the interest of the invention

  for the realization of linear subnets of antennas.

  The invention makes it possible to obtain a double-resonator antenna, as shown in FIGS. 6 and 7: In order to increase the bandwidth, or to make the antenna operate at two spaced apart frequencies, it is possible to place at the end as shown in FIGS. 2 and 3, in the same plane as the latter, disks 65 whose diameter is adjusted so that they resonate at a frequency F 2 different from the Fl resonance frequency of the annular slot above. If F 2 is close to Fl, then the antenna can be operated over a wide band. If F 2 is distant from Fl, a two-band antenna is obtained; each of the two distributors being adapted to a particular band and exciting one of the two annular slots. Such an embodiment is compatible with the two variants of the invention shown above: In the suspended triplate variant, the additional annular slots are etched either on the upper face or on the lower face of the upper substrate. In Figures 6 and 7 the variants taken into account are respectively those illustrated in Figures 2 and 3; the upper substrate 37 'shown in FIG. 6 being provided on its upper surface 67 with slots 69; metallized holes 66 being provided to allow the slit mass-triplate mass bonding. It is understood that the present invention has been described and shown only as a preferred example and that it will be possible

  replace its constituent elements by equivalent elements without departing from the scope of the invention.

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Claims (7)

  1) Linear array antenna composed of a radiating element formed by an alignment of annular slots (21) excited by at least one distribution element (14), characterized in that this distribution element is a buried distributor included in the radiating element , and therefore invisible from the radiating face of said antenna, so as to avoid any   parasitic radiation from said distributor.   2) network antenna according to claim 1, characterized in that said antenna is provided for radiating (or receiving) simultaneously in two linear polarizations.   3) network antenna according to claim 1, characterized in that said antenna is provided for radiating (or receiving) simultaneously   in two circular polarizations (right and left).   4) A network antenna according to claim 1, characterized in that it comprises an integrated radome and a splitter using only three driver levels.   Network antenna according to any one of the claims   previous, characterized in that it comprises only substrates of small thickness spaced by conductive spacers without mechanical continuity in the plane of the network so as to avoid the   differential dilations with substrates.   6) Network antenna according to any one of the claims   previous, characterized in that it comprises shielding between polarizations of a sub-network and between adjacent sub-networks, radiating elements and splitters so as to avoid any coupling between them.   7) A network antenna according to claim 1, characterized in that it comprises: a lower half-shell (10) of conductive material in which is formed the cavity (11) of the radiating elements and the conduit (12) of the splitters of the two polarizations; a very thin first substrate (13) having a coefficient of expansion close to that of the half-shell material, on which are engraved on the upper face the horizontal and vertical splitters; vertical (14).   13 - an upper half-shell (15) in which is formed the conduit (16) of the splitters of the two polarizations and pierced (17) the upper portion (17) of the cavity; a second substrate (18) identical to the first substrate, one upper face (19) of which is fully decurved and the other (20) containing the annular engraved slots (21); the two half-shells (10 and 15) being connected to the same potential Which is ensured by the first substrate thanks to the   metallization of the surfaces facing the two half-shells and the presence of a large number of metallized holes located on the periphery of the cavities, connecting these two surfaces.   8) A network antenna according to claim 1, characterized in that it comprises: a lower substrate (25) having: a lower surface (26) locally coppered (zone 27) around connectors (28 and 29) RF power supply and glued to the structural panel; an upper face (30) fully coppered (except RF power saving) and used as a ground plane; metallized holes (44) connecting the two surfaces; an intermediate substrate (32) having: a coppered underside (33) on the surfaces facing the partitions formed by conductive spacers (41); an upper face (34) coppered on the surfaces (35) facing these partitions and distribution circuits H and V; metallized holes (44) providing the electrical connection between the two ground planes; an upper substrate (37) having: a fully coppered underside (38) except at the locations of the annular slots (39) and serving as a ground plane for the triplate; An upper face (40) fully decentered and acting rad 6 me; These three substrates (25, 32 and 37) being spaced apart by conductive spacers (41) delimiting the cavities of the slots   annular and splitter channels.   14 - 9) An array antenna according to claim 1, characterized in that it comprises: a lower portion (45) of conductive material; The ground plane being formed by a thin substrate on which are brazed spacers (46); An upper portion (47) of dielectric material which is a substrate which contains on the upper face (48) the annular slots (49) and on the underside (59) the polarization splitters (51 and 52) and the exciters (53). The continuity of the cavity of the annular slots in the substrate is provided by metallized holes (54) which connect the spacers to the substrate.   ground plane of the slots; The distribution of the distributors is done by coaxial probe through metallized holes (55 and 56) for the continuity of mass between the probe and the plate. 10) An array antenna according to any one of the preceding claims, characterized in that said antenna is a   broadband antenna using resonant disks (65) etched on the resonant distributor stage at a frequency close to that of the annular slots. 11) Network antenna according to any one of claims 1 to 6, characterized in that said antenna is a dual-band antenna   using resonant disks (65) etched on the splitter stage resonating at a frequency remote from that of the annular slots; one splitter exciting the resonant disc and the other the slot annular.