FR2540297A1 - Microwave antenna having two reflectors - Google Patents

Abstract

The subject of the invention is a microwave antenna having two reflectors. The auxiliary reflector 2 of this antenna is generated by an elliptical arc MN rotating about an axis of revolution ZZ% on which one end M of the said arc is located and its first focal point F1, generating a surface of revolution, the point M of which is directed towards the principal reflector 1. The principal reflector is generated by the rotation about the same axis of revolution ZZ% of a semi-parabola ST having as the focal point F2 the second focal point of the elliptical arc. Applications to radar and telecommunication antennas.

Description

MICROWAVE ANTENNA WITH TWO REFLECTORS

  The present invention relates to a microwave antenna

two reflectors.

  In many applications, it is necessary that an antenna provides a beam of radiation of a few degrees of opening (one or more) at half power, has a good performance, a good standing wave ratio (ROS) and presents

  good polarization characteristics.

  Antennas belonging to the prior art are known which

  partially meet the conditions set out aboveo One can thus cite an antenna comprising only a single parabolic reflector with a primary source placed at the focus of the reflector; when the beam emitted by this antenna has only a few degrees of aperture at half power, the primary source forms an important mask for the radiation of the antenna and in

disrupts operation.

  Mention may also be made of antennas with two reflectors of the

  Cassegrain or Gregory type which respond, but still partial-

  lernent, under the conditions mentioned above.

  It will be recalled that an arena of the Cassegrain type, one of which

  description can be found on pages 10-13 to i 0-15 of Merrill's book

  I Skolnik "Radar liandbook" published in 1970 by Mc Graw Hill Cy, is an antenna with two reflectors a reflector known as principal, concave, generally parabolic and a reflector known as auxiliary, convex,

  generally hyperibolic, with a primary hyper-

  frequency, usually placed near the top of the main reflector

  cipal which illuminates the auxiliary reflector; this reflects the beam back to the main reflector which, in turn, returns to

  space a cylindrical bundle, axis parallel to the axis of the anienne.

  In a Gre type antenna; -gory e rfllect-eur at: iiaire est

a portion of e J 1 ipso Tde.

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  In a Cassegrain or Gregory antenna, the auxiliary reflector

  liaire forms a mask for the radiation of the main reflector, thus disrupting the functioning of the antenna This phenomenon is of course all the more accentuated as the auxiliary reflector is of large dimensions However, if the latter is small iimensions, it must be placed near the primary source and it then reflects a significant part of the incident energy towards this source: this results in a high ROS ratio which, as is known, leads to a deterioration of the yield and of the ellipticity rate in the case of circular polarization This is detrimental in particular in the field of telecommunications where such an antenna is used for the eradication of links

  transmission-reception with a satellite.

  The object of the invention is an antenna with two reflectors which generally meets the conditions set out above thanks to the choice, for the auxiliary reflector, of a shape such that, according to the laws of geometric optics, the ROS is zero whatever the dimensions of the primary source (provided that they are smaller than those of the auxiliary reflector); this configuration has for other

  advantage of authorizing the use of large sources.

  More precisely, the invention relates to a micro- antenna.

  wave with two reflectors; one of the reflectors, called main reflector

  pal, is generated by a half-parabola rotating around an axis of

  revolution parallel to the axis of the half-parabola; the other of the

  flectors, known as the auxiliary reflector, is generated by an elliptical arc rotating around the same axis of revolution, one of the focal points of the ellipse being coincident with that of the parabola and the other focal point being located on the axis of revolution between the two reflectors, one end of the elliptical arc also being located on the axis of

  revolution, the antenna being adapted to receive a source of

  radiated energy, placed between the two reflectors so that its

  phase center is confused with the second focus of the elliptical arc.

  Other advantages and characteristics of the invention have appeared to be

  tront in the couirs of the following description, or born as

  of nonlimiting example and illustrated by the appended drawings, which represent: FIG. 1a, a diagram of the meridians of the reflectors of

  the antenna according to the invention and FIG. 1b, its radius diagram-

ment;

  FIG. 2, a flowchart showing the operations performed

  cut to conform the reflectors; Figure 3, an embodiment of a complete antenna according to the invention, with its power supply;

  FIG. 4, an alternative embodiment of the auxiliary reflector

  binder used in the antenna according to the invention;

  Figure 5, another variant of this auxiliary reflector.

  In these different figures, the same references refer to

relate to the same elements.

  In FIG. 1, the auxiliary reflectors are therefore diagrammed

  and main antenna, as well as the primary source.

  One of the reflectors, called the auxiliary reflector, 2, is generated

  by the rotation, around an axis of revolution ZZ ', of an arc of el-

  lipse MN whose concavity is turned towards the other, called the main reflector, 1, the axis ZZ 'being chosen so that the first focal point F 1 of the ellipse is located on this axis and the second focal point F 2 of l 'ellipse, outside this axis One end M of this arc of ellipse is located on the axis of revolution ZZ' while its other end N is located on an axis SN, parallel to the axis of revolution ZZ 'Cet axis SN passes through the second focal point F 2 of the arc of ellipse Generated by the arc of ellipse MN rotating around the axis of revolution ZZ ', the auxiliary reflector 2 is in the form of a solid of revolution, with elliptical generators with a vertex M having the appearance of a

  point facing the concavity of the main reflector 1.

  The focal point F 2 is moreover the focal point of a parabolic arc ST, S preferably being the apex of the parabola to which belongs

  the arc ST and SN the axis of this parabola; the rotation of the half

  parabola ST around the axis of revolution ZZ 'generates the main reflector l The main reflector thus appears as a sort of torus of axis ZZ', the generator circle of which would be replaced by a

half-dish of axis SN.

  The axis SN and its symmetric S'N 'with respect to the axis of revolution-

  tion ZZ 'define the central zone 4 of the antenna.

  In this central part 4, in front of the auxiliary reflector 2, there is a primary source 3 of microwave waves, by

  example constituted by a waveguide 31 terminated by a cor-

  net 32 The phase center of source 3 must coincide with the

  first focus (F 1) of the ellipse arc MN.

  o A ray from F 1 and striking the auxiliary reflector 2 at a point A is reflected along AF 2 and strikes the main reflector 1 at B; the latter reflects the radius F 2 B along BC, parallel to the axis of revolution ZZ 'Another radius F 1 D, very slightly inclined on the axis of revolution ZZ', strikes the auxiliary reflector 2 in D, at 1 S proximity of the point M and is reflected along DF 2, striking the main reflector 1 at E; this one reflects the ray following EG, parallel to ZZ 'As for the extreme ray F 1 N, it is reflected following

NS then following SN.

  We thus observe that, according to the laws of geometrical optics

  there is no energy returned by the auxiliary reflector 2

  in the central part 4 of the antenna, whatever the dimensions

source 3.

  However, the application of the laws on diffraction of radiated energy shows that a very small part of the energy emitted by the

  source 3 is actually returned to it by the auxiliary reflector

  liaire 2 But this part of energy being very weak, it does not affect

  not significantly the operation of the antenna.

  It thus appears that the shapes chosen according to the invention, for the meridians of the reflectors, appreciably improve the standing wave ratio. In addition, the dimensions of the auxiliary reflector 2 being limited to the central part 4 of the antenna, it can be seen that '' there are no (or few) losses due to reflections from the

  radiation from the main reflector I on the auxiliary reflector 2.

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  It should also be noted that the shape of the main reflector between S and S 'is indifferent, since it is not (or very little) brought

to receive energy there.

  In Figure ib, there is shown the law of illumination of the antenna according to the invention, which has a recess 6 in the part

  central (4) of the antenna, due to the mask.

  According to an alternative embodiment of the antenna according to the invention, an optimization of this antenna is carried out on experimental bases, consisting in adjusting the shape of the reflectors; this operation is called "conformation" of the reflectors For this, starting from the shapes of the reflectors which have been defined, we slightly deviate from it, first of all for the auxiliary reflector which we conform so that it lights up according to a law optimal in amplitude the main reflector This, then, is shaped to correct

  phase errors due to the previous conformation.

  Figure 2 describes more precisely the different stages which

  are performed to optimize the antenna.

  Starting from the auxiliary reflector 2 defined at the start (step 7), we calculate (step 8) the radiation diagram it gives, which we compare (step 9) to the ideal diagram sought. The result of the comparison of the diagrams allows us to calculate (step 10) the deformations that must be given to the auxiliary reflector, deformations which are carried out during step 11 The process is iterative, that is to say the calculation of the radiation diagram obtained after qu the surface of the auxiliary reflector has been deformed (step 8) and a comparison (step 9) of the newly obtained diagram with the desired ideal diagram, etc. is repeated. With regard to the main reflector, the phase radiated by the conforming auxiliary source-reflector assembly is calculated and

  makes the necessary corrections.

  FIG. 3 represents an embodiment of an antenna according to the invention, with its air antenna

  In this figure, we find the primary source 3, the reflec-

  main sensor I and the auxiliary reflector 2.

  We defined in the above the shape to be given to the reflectors I and 2 of the antenna so that it responds with the best approximation to the conditions which have been given. We then indicated how, from the data provided, we optimized the antenna by calculating the deformations to be made to the surfaces of the reflectors. Optimization can be further improved by using, for the

source 3, a corrugated cornet.

  It is recalled that the term corrugated horn means a horn having transverse grooves on its internal face, a comime described for example in French patent no. 2 255 716 or the article by VU published in INT J ELECTRONICS (1973, vol 34) entitled "Corrugated horn as highperformnance mornopuise feed", or the article published in PROC IEE (vol 118, n 9, Sept 71) entitled "Propagation and radiation behavior of corrugated feeds" This type

  of cornet has the advantage of having radius diagrams-

  identical in planes E and H, characterized by very low lateral lobes, a width of diagram almost constant as a function of frequency and a position of the phase center

  practically independent of frequency.

  However, as is also known, a corrugated cornet only works satisfactorily when it is relatively large: for example, a conventional cornet usually has an opening of the order of 2 or 3 times the length medium wave (X) of antenna operation, while a corrugated horn has an opening which can be between 4 or 5 X and 10 X.

  There thus appears another advantage of the antenna according to the invention.

  tion which is to allow the use of corrugated cones: their large size is not a drawback here because of the very small amount of energy which is, by construction, likely to be

returned to primary source.

  In one embodiment of the invention, a conical corrugated horn is used, with grooves whose depth is of the order

  of X / 4, X being the mean wavelength of the frequency band

  quence used These grooves are narrow relative to their depth, that is to say of the order of X / 10, and relatively close to each other, their distance being small compared to the length

  wave (of the order of X / i O also).

  The antenna according to the invention is completed by a means for supporting the auxiliary reflector 2 from the horn 32 This support means is a skirt 12 of revolution, made of a dielectric material This skirt has a shape, for example, conical It is fixed by any suitable means 5 (glue, screws, etc.) to the auxiliary reflector 2 and is supported on the horn 32 by a thread 13 The skirt 12 can thus be moved precisely and, with it, the auxiliary reflector 2, which facilitates focus adjustment The thread

  can of course be replaced by an equivalent system, sliding

health for example.

  This method of attachment by a skirt 12, made possible by the fact that the dimensions of the horn 32 are relatively large by

  compared to those of reflector 2, has a number of advantages

  Firstly, the entire antenna system retains the symmetry of revolution and the auxiliary reflector 2 is not held by metal supports which risk bringing disturbances to the electromagnetic field. Next, the fixing of the skirt to the horn also seals the primary source Finally, as

  mentioned above, the focus adjustment is facilitated.

  FIG. 4 shows an alternative embodiment of the antenna according to the invention, in which the auxiliary reflector is corrugated in a manner analogous to that which has been described for the horn 3 of

Figure 2.

  In this figure, there is therefore shown the reflector 2, the face receiving the radiated energy is generated by the rotation about the axis ZZ 'of the arc of the ellipse MN. On this face, concentric grooves 14 have been produced whose center is carried by the axis ZZ '

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  and the dimensions (depth, width and pitch) are substantially

those of the horn 3.

  Such an embodiment makes it possible to ensure continuity of the corrugated horn, while preserving for the radiation diagram of the auxiliary reflector the symmetry of revolution which it had when leaving the horn. FIG. 5 represents another alternative embodiment of the antenna according to the invention, in which the auxiliary reflector 2 comprises at least one band 16, in the form of a ring deposited on the surface of the reflector, the number and thickness of which and the distance from the center M of the reflector 2 are determined experimentally in order to confer on the waves reflected by the reflector 2 towards the source 3 phase shifts such that these waves cancel each other -or at least are minimized at the entry of the guide 31 d food

source 3.

  Finally, according to a variant not shown, it is possible to add to the center of the auxiliary reflector 2 a metal part having the same function as the preceding rings and being

dimensioned in the same way.

  For example, a telecom antenna has been produced.

  munitions produced according to the principles of the invention, which has the characteristics: diameter of the main reflector 1500 mm, diameter of the auxiliary reflector 210 mm, beam opening at half power: 1.5 degrees, gain factor: -1.35 d B, level of the side lobes -17 d B,

ROS: 1.1,

  noise temperature: 40 K for the antenna pointed at 15 degrees

above the horizon.

Claims (9)

  1 microwave antenna with two reflectors, characterized by   one of the reflectors, called the main reflector (1), is generated   dré by a parabola arc (ST) rotating around an axis of revo-   lution (ZZ ') parallel to the axis (SN) of the parabola; that the other of the reflectors, called the auxiliary reflector (2), is generated by an elliptical arc (MN) rotating around the same axis of revolution (ZZ '), one of the focal points (F 2) of the ellipse being merged with that of the parabola and the other focal point (F 1) being located on the axis of revolution (ZZ ') between the two reflectors (1, 2), one of the ends (M) of the arc ellipse (MN) being also located on the axis of revolution (ZZ '), the antenna being adapted to receive a radiated energy source, placed between the two reflectors so that its phase center is   confused with the second focus (F 1) of the elliptical arc).   2 antenna according to claim 1, characterized in that the other of the ends (N) of the elliptical arc (MN) is placed on the axis (SN) of the parabola (ST), one of the ends (S) of Pl'arc de   parabola (SN) being the top of the parabola.   3 antenna according to claim 2, characterized in that the source is placed inside the cylinder defined by the rotation of the axis (SN) of the parabola arc (ST) around the axis of revolution (ZZ ').   4 antenna according to one of the preceding claims, charac-   terified by the fact that the source (3) comprises a corrugated horn (32).   5 antenna according to one of the preceding claims, charac-   terized in that it comprises a skirt (12) in a dielectrical shape   rod, revolving around the axis of revolution (RZ) fixed to the r, 3 auxiliary flector (2), which it supports% and to the source (3) to which it is fixed by a system allowing adjustment of the   focusing by displacement of the auxiliary reflector (2).   6 Antenna according to one of the preceding claims, charac-   sorted out by the fact that it is optimized by conformation of the auxiliary (2) and main (1) reflectors by modifying a part of the surface of the auxiliary reflector (2) only to obtain an optimal law of illumination of the reflector main (1) and, on the other hand, the surface of the main reflector (G) to correct the phase errors introduced by the modification of the surface of the reflector auxiliary (2).   7 antenna according to one of the preceding claims, charac-   terrified by the fact that the auxiliary reflector (2) is corrugated.   8 antenna according to one of claims I to 6, characterized   by the fact that the source comprising a waveguide (31) supplied   as a horn (32), the auxiliary reflector (2) carries conductive rings (16) whose number and dimensions are such that the waves reflected by the auxiliary reflector (2) towards the source (3) cancel each other out substantially at the entrance to the guide waves (31).   9 antenna according to one of claims I to 7, characterized   by the fact that, the source (3) comprising a waveguide (31) supplying a horn (32), the auxiliary reflector (2) carries in its center at least one metal part whose dimensions are such that the reflected waves by the auxiliary reflector (2) towards the source (3) cancel each other substantially at the level of the entry of the waveguide (31).