FR2839206A1 - Mechanically scanned antenna for car radar has feed in motor rotated dielectric tube with internal cylinder axis offset in angle - Google Patents

Abstract

An mechanically scanned antenna has a waveguide array or other feed (2) in a motor driven cylindrical dielectric tube (1) with non parallel external (20) and internal (30) envelope axes. Includes Independent claims for the use of a dielectric lens along the front of the tube and construction of the tube from discrete prisms.

Description

CAMUSAT CENT D'UT 100223

  The present invention relates to an antenna performing mechanical scanning. It applies in particular to radars in the millimeter range at low cost. It applies for example for

  radars fitted to motor vehicles.

  To receive or transmit electromagnetic signals, an antenna has at least one beam. This beam actually delimits a 0 space inside which the signals are received or received with a gain

  minimum given. This beam covers only a small part of the space.

  Thus, to carry out a detection it is necessary to scan the space with the antenna beam. There are several known solutions for performing this sweep. A first solution is mechanical, it consists in giving

  the antenna a rotational movement along one or two axes of space.

  This solution has in particular all the drawbacks associated with mechanical implementation. These disadvantages include the lack of flexibility, speed and robustness. It should be noted that this solution is further complicated by the need to use microwave rotating joints. These seals are used to pass a microwave signal

  from one fixed sub-assembly to another rotating sub-assembly.

  A second known solution is electronic. This solution

  so-called electronic scanning uses networks of phase shift modules.

  : 5 Each module applies a phase shift to a transmitted or received wave. The scanning is obtained by playing on the phase shifts of the modules. This solution is costly to implement, in particular because of the number

  necessary electronic modules and their associated commands.

  An object of the invention is in particular to allow a robust and economical beam scanning. To this end, the subject of the invention is an antenna comprising at least one illuminator and a tube made of dielectric material delimited by an external envelope and an internal envelope whose axes are not parallel. The illuminator is placed inside the tube, the tube having a rotational movement relative to the illuminator. The main advantages of the invention are that it allows agile scanning, that it avoids the use of rotating joints, that it is

  economical and that it is simple to implement.

  Other characteristics and advantages of the invention will appear.

  with the aid of the description which follows made with reference to the appended drawings which

  represent: - Figure 1, an embodiment of an antenna according to the invention comprising a tube of dielectric material rotating around an illuminator; - Figure 2, a first possible embodiment of the aforementioned tube; - Figures 3a, 3b and 3c, an illustration of the deflection of the antenna beam resulting from the prism effect of the tube; FIG. 4, an illustration of the beam scanning obtained by the rotation of the tube around the illuminator; - Figures 5a, 5b and 5c, an illustration of the deformation of the antenna beam; - Figures 6a and 6b, a sectional view and a partial perspective view of a dielectric tube used in an antenna according to the invention; z - Figure 7, another possible embodiment of an antenna according to the invention further comprising a compensation lens; - Figures 8a and 8b, another embodiment of the dielectric tube; - Figure 9, an embodiment of a compensation lens; - Figure 10, an embodiment of an antenna according to the invention equipped with means for rotating the tube; - Figure 11, an embodiment of an antenna according to the invention o the tube is composed of discrete prisms; - Figure 12, an exemplary embodiment of the elementary prism making up the aforementioned tube; FIG. 13, an example of a radiation diagram resulting from this latter embodiment; - Figure 14, an example of transition between two elementary prisms making up the tube; - Figure 15, another embodiment of an elementary prism; - Figure 16, the aforementioned exemplary embodiment comprising by

  elsewhere a convergence lens.

  Figures 1a and 1b show a possible embodiment 1s of an antenna according to the invention. This antenna comprises at least one tube made of dielectric material 1 and an illuminator 2. The illuminator 2 is placed inside the tube 1. The tube is in the form of a dielectric log hollowed out internally along an axis not parallel to the axis of the log. In other words, the tube is delimited by an outer casing and a

  inner envelope whose axes 20, 30 are not parallel.

  The illuminator 2 is centered on the axis 20 of the log, or eccentric with respect to this axis. The tube having a relative rotational movement relative to the illuminator, the wave emitted by the latter is deflected. The rotation of the tube around the illuminator thus causes a mechanical scanning of the beam of 2s the illuminator. The tube exerts the effect of a prism in rotation around the illuminator. In one embodiment, the illuminator is fixed and the tube rotates around the illuminator. The illuminator has for example a height hA The illuminator 2 is for example of the slot wave microwave guide type. It can be centered on the axis of the tube or eccentric with respect to this axis. FIG. 1b illustrates such a type of illuminator comprising for example several guides with slits in parallel, with its beam 3. The number of guides used plays in particular on the width of the antenna beam. The wider the emission source, here the slotted guides, the narrower the beam. To make a narrow antenna beam, it is therefore necessary to increase the number of guides in parallel. The illuminator can of course be

  produced with means other than slotted guides.

  Figure 2 shows a first embodiment of the dielectric tube 1. In this embodiment, the tube is hollowed out internally so that it has an outer cylinder 21 and an inner cylinder 22. These cylinders in fact respectively form the outer surfaces 21 and inner 22. The axis 30 of the inner cylinder is not coincident with the axis 20 of the outer cylinder and it is not parallel to it. The outer surface of the tube 1 being cylindrical with a circular base, it is therefore

  circularly dug along an axis not coincident with the axis 20 of the tube.

  The illuminator is for example centered on this axis 20 which is also the axis of the outer cylinder 22. In a given longitudinal section passing through the axis 20 of the tube, the generatrices of the outer and inner cylinders make an angle a between them. This angle varies as a function of the angular position of the longitudinal section around the axis 20 of the tube. Figure 2 shows an embodiment where the outer 21 and inner 22 cylinders defining the tube have a circular base. It is of course possible to consider other types of cylinders, or another type of hollow shape, as soon as the angle a of the abovementioned generatrices varies according to their angular positions, at

  less in the part of the tube which meets the beam 3 of the illuminator.

  Figures 3a, 3b and 3c show the deflection of the beam by rotary prism effect in an antenna according to the invention. For the record, FIG. 3a illustrates the effect of a prism 10 on an electromagnetic radiation forming a beam 31. This radiation is emitted by an illuminator 2. It

  could similarly be emitted by a target, to be received by the illuminator.

  The prism, made of a dielectric material with a relative constant úr greater than 1, deflects the beam 31. The incidence face and the beam exit face form an angle aO between them. For a small angle aO, the deflection D of the beam is approximately given by the following relation: D = aO (n- 1) (1) o n = FIG. 3b illustrates the case of an antenna according to the invention. More particularly, it represents, by a view in partial longitudinal section, the illuminator 2 opposite a part of the tube 1. This longitudinal section is taken in a plane passing through the beam 3 of the illuminator at a given instant. In this plane, the tube forms a prism for the beam 3 of the illuminator. It therefore in particular has the effect of deflecting the beam 3 by an angle D ,. This deviation is a function of the angle a that the generators 21, 22 of the outer and inner cylinders make between them. The rotation of the tube around the illuminator causes a deformation of the prism seen by the

beam 3.

  FIG. 3c illustrates this phenomenon always by a view in partial longitudinal section. It shows the illuminator 2 in the same position as in Figure 3b but the tube 1 having been rotated. The prism produced by the tube in the plane of incidence of the beam is modified with respect to the instant in FIG. 3b. In particular, the generators 21, 22 form between them an angle a2 different from the previous angle a. The beam consequently undergoes a different deflection D2. In the example of FIGS. 3b and 3c, the angle a2 is greater than the angle a. The deviation D2 is therefore greater

at the deviation of.

  FIG. 4 illustrates the scanning of the antenna beam according to the invention. It shows in particular the position of the beam as a function of the relative position of the tube 1 relative to the illuminator. The prism encountered by the beam of the illuminator evolves between an angle prism aO to an angle prism -aO, for example for a rotation of T. This angle is that made by the generatrices of the outer 21 and inner 22 surfaces. which cut the beam 3 of the illuminator. By a rotation of, the prism encountered then takes again for example the angle aO. Between these two values aO and

  -aO, the angle a of the prism varies continuously, and for example linearly.

  In particular, it passes through the value O which corresponds to an absence of deflection of the beam. During the rotation of the tube around the illuminator

  the beam 3 therefore scans the space between a deflection angle Dmax and Dmax.

  If the axis 31 of the tube placed in the plane of deposit, a scan in deposit

  antenna beam is thus obtained.

  Figures 5a, 5b and 5c show that the antenna beam can undergo a slight deformation. Figure 5a illustrates the cause by a partial sectional view of the beam 3 which attacks the tube 1. This sectional view is taken in a plane perpendicular to the axis 20 of the tube. Compared to the views of Figures 3a, 3b, 3c and 4, assuming by way of example these latter views in the reservoir plan, Figure 5a is then seen in the site plan. In this plane, it can be seen that the upper 51 and lower 52 surfaces of the bundle will pass through different thicknesses of tubes, due in particular to the relative positions of the inner and outer cylinders which delimit the tube. In fact, the upper part 51 will meet an equivalent prism different from that encountered by the lower part 52. These two upper parts 51 and lower 52 will therefore undergo different deviations in site as they pass through the tube. This variation in deflection obviously acts in the same way, to a lesser extent, on intermediate parts of the beam. In another plane parallel to that of the section plane of FIG. 5a, the variation in thickness of the tube would be different. The variation in deflection in elevation of the beam would therefore be different. This results in a deformation in elevation of the beam as illustrated in particular by FIGS. 5b and 5c. More particularly, these figures show a transverse view of the antenna beam 3 cut by a plane of field 53. The deformation varies in particular as a function of the relative position of the tube 1 in rotation relative to the illuminator 2. FIG. 5b represents the shape of the beam 3 at an instant to. FIG. 5c represents the shape of this same beam at an instant to increased by a rotation time of the tube. Between Figures 5b and 5c, the direction of the deformation has reversed. The deformation

  of the beam in fact causes a depointing in bearing.

  To limit these beam aberrations, it is possible to modify the relative dimensions of the illuminator 2 and the internal radius of the dielectric tube 1, illustrated in particular by FIG. 6a in a sectional view of an antenna according to the invention. In particular, one can play on the shape of the beam, or antenna lobe. The angle of the antenna beam, or lobe, at 3dB denoted 03dB is given by the following relation: 03dB h (2) o is the length of the transmitted wave and hA the height of

the illuminator 2.

  To limit the aberrations, it is then necessary that the illuminator and the tube respect the following relationship:

2 TO R (3)

  o R is the radius of the inner cylinder of the tube.

  In this condition, the maximum deflection of deformation éMax, that is to say in elevation in the example considered, is given approximately by the following relation: 63Max 2 L (4)

  where L is the length of the illuminator.

  One constraint to take into account concerns the thickness

  maximum tube eMaX and the corresponding maximum prism angle AMaX.

  This thickness eMaX corresponds to the angular difference of the two generators 61, 62 of maximum angle AMaX as illustrated in FIG. 6b. A first generator 61 generates the outer envelope and the second 62 generates the inner envelope. These two generators define the maximum angle prism AMaX. The angle a between the two generatrices 61, 62 varies as a function of their angular position R according to the following relation: x = Max R (5) Referring to relation (1), the deflection D of the antenna beam in function the angular position (3R is then given by the following relation: D = Max OR (n-1) (6)) r The deformation of the beam in elevation causes a deviation D 'defined by the following relation: D = (-OR + -) (n-1) (7)

  OR 03dB is the beam angle at 3dB in elevation.

  The depointing in bearing AD = D '- D is therefore: 0 D = - (n-1) .- (8) T 2 If we note 03GdB the angle to 3dB in bearing of the antenna beam, we limit for example the depointing AD so that: oG, .D <3dB (9) Which implies according to the relation (8) that: Max (n-1) 3dB <3dB (10) Considering that O3GdB = L o L is the length of the tube and that 03dB = h according to the relation (2) the relation (10) implies: AMax 2 L (n-) (11) In addition, eMaX = L.tg (AMax), the angle AMaX being relatively small, it follows that in an approximate way we can consider that eMax = L.AMaX. The relation (11) then implies: eM <_ A (12) The relation (4) shows that a way to limit the deformation is to limit the height hA of the illuminator. However, the more the height of the illuminator decreases, the more the beam widens. Such a solution is then

  not favorable for applications requiring fine beam scanning.

  The embodiment of FIG. 7 overcomes this drawback. In this embodiment, a convergence lens 71 is arranged along the tube 1, in the scanning field of the antenna beam. This lens is made of dielectric material, for example having a

  same relative constant sr. It remains fixed relative to the illuminator 2.

  The thickness of the lens varies so as to compensate for variations in the thickness of the tube. It then partially compensates for the deformations of the beam. The thickness of the lens 71 is for example maximum in its center, arranged opposite the center of the illuminator, and decreases by going towards its two longitudinal edges 72, 73. This solution partially compensates for the beam deformations, because the lens cannot exactly compensate for variations in thickness of the rotating tube. Compensation is possible for a fixed position of the tube. However, from one angular position to another, the variation in thickness changes for each cross section. Referring to FIG. 7, we see that on 2n, the thickness decreases from a maximum thickness eMaX to a minimum thickness emjn then increases to the maximum thickness eMax. The lens therefore in fact achieves an average compensation. Depending on the desired antenna beam width, this solution can advantageously be

  combined with the previous one which plays on the height of the illuminator.

  FIGS. 8a and 8b show another possible embodiment of the dielectric tube 1. In this embodiment, the effect of a rotary prism is preserved, that is to say that the tube always comprises an outer envelope 21 and an envelope interior 22 whose axes are not parallel as in the previous embodiment. The inner envelope 22 no longer forms a cylinder, circular for example, but has a discontinuity. More particularly, for a given section as illustrated in FIG. 8b, the thickness of the tube increases by a minimum thickness emjn to a maximum thickness eMaX substantially over an angle of 2. The thickness increases by example linearly. For a given cross section, the law of variation e of the thickness is then for example of the type e = C / radian, where C is a constant. An advantage compared to the previous embodiment is in particular that it makes it possible to carry out practically complete compensation for the deformation of the beam. Indeed, it suffices to create a lens the thickness of which varies with the same law of variation, but inversely. More particularly, when the tube is in rotation, the beam 3 of the illuminator meets the thicknesses of the tube and of the lens which vary in opposite ways. For example, if

  the thickness of the tube varies according to e, the thickness of the lens varies according to -e.

  FIG. 9 illustrates an exemplary embodiment of such a lens 91.

  This is placed along the tube 1 in the scanning field of the antenna beam, the tube being produced in accordance with FIGS. 8a and 8b. The dielectric constant of the material making it up is for example the same as that of the tube 1. By considering two given cross sections 92, 93, the variation in thickness of the lens e = C / radian is the same for the two sections. These cross sections are taken perpendicular to the axis of rotation of the tube. The differences in thickness of these two sections 92, 93 are due to their initial differences e '', e2 in a base plane 94 of the lens perpendicular to these sections 92, 93. The thickness of this base plane 94 follows for example the law of variation of thickness of the tube from emjn to eMax FIG. 10 shows an exemplary embodiment showing means 101 for rotating the tube 1 around the illuminator. These means 101 are for example a direct current motor whose rotor is mechanically integral with the tube to drive the latter in rotation around the illuminator. The illuminator is for example fixed on a support 102. This support 102 comprises for example a waveguide which conducts microwave waves between transmission and reception means, not shown, and the illuminator. Advantageously, an antenna according to the invention avoids the use of rotating microwave joints due to the fixity of the illuminator. This results in simplicity and economy of implementation. In an exemplary embodiment, for operation at 94 GHz, the dielectric tube 1 has a length of 3 cm. The radius of the inner cylinder of the tube is 3 cm. The relative dielectric constant sr is of the order of 3.5. The drive motor is a direct current motor with a nominal speed of 5780 rpm. Under these conditions, an angular scan of + 15 is obtained with a speed of the order of 2.5 per millisecond. It is also possible to use a stepper motor. The

  tube 1 can be produced by molding.

  FIG. 11 shows another possible embodiment of an antenna according to the invention, in particular another example of embodiment of the tube 1. The view is a view in transverse section taken in a plane perpendicular to the axis 20 of the 'illuminator 2. This embodiment is of the type of that of Figures 8a and 8b, but the thickness no longer varies continuously. It varies discontinuously, by level. In other words, the tube is composed of discrete prisms 110, or elementary, of dielectric material, for example juxtaposed and arranged around the illuminator 2,

  for example along a circular envelope. Each prism has an angle aj.

  This angle is different from one prism to another. The angle aj takes a maximum value amaX for a first prism 111 then decreases with the following prisms. This variation is illustrated in particular in FIG. 11 which shows 2s that the thicknesses of the prisms in the section plane of the figure decrease by an eDmaX value, corresponding to the angle prism 111

  maximum amaX by going to the other 110 prisms juxtaposed.

  The location of the last prism 112, juxtaposed with the first prism 111,

  can for example correspond to an empty space.

  FIG. 12 shows an elementary prism 110 facing the axis 20 of the illuminator. The length of the prism, more particularly its direction of deflection of the beam is substantially parallel to this axis 20. The juxtaposition of the prisms on a circular line results in the fact that the lateral faces 113, 114 of a prism are not parallel. To avoid any

  perturbation, these faces are for example outside of radiation 3.

  In the embodiment of Figure 11, the tube is driven by a stepper motor so that it takes discrete positions. At each step, the tube is positioned so that the beam 3 of the illuminator

  meets an elementary prism 110.

  Figure 13 shows an overview of the radiation pattern obtained by scanning the beam. The diagram is not continuous due to the discrete positions of the tube 1. The widths of the elementary prisms are calculated so that the detection troughs 130 correspond to the beam angle at 3dB, fl3B The tube 1 of FIG. 11 is for example obtained by molding. In particular, as illustrated in Figure 14 the transition 141 from one prism to another

is for example rounded.

  FIG. 15 shows another possible embodiment of an elementary prism 110. This embodiment makes it possible in particular to

  reduce the overall thickness and therefore the size and weight of a prism.

  The prism 110 is composed, in this embodiment, of a succession

  of prisms 151 of length dp = 2 / f o f is the center frequency of the wave.

  z5 Figure 16 shows, in a schematic view perpendicular to the axis 20 of the illuminator, an exemplary embodiment o a lens

  convergence 161 is placed in the field of the beam 3 of the illuminator.

  The illuminator and rotary tube 1 assembly is for example placed in a box 162 closed by the lens 161. This lens is for example a zoned lens, which advantageously makes it possible to reduce the overall thickness

of the lens and therefore its weight.

  An advantage of the embodiment of an antenna according to the invention illustrated by FIGS. 12 and following is in particular that it makes it possible to eliminate, totally or in part, deformations of beams

  likely to appear in deposit.

  The invention can advantageously be used for all radar applications in the millimeter range, requiring agile and rapid beam scanning. It thus advantageously applies to radars fitted to automobiles, in particular for the purpose of anti-collision. It also applies to surveillance radars, for example from airports or other public or sensitive places. It also applies advantageously

for LMDS type applications.

Claims (21)

  1. Antenna, characterized in that it comprises at least one illuminator (2) and a tube of dielectric material (1) delimited by an outer envelope (21) and an inner envelope (22) whose axes (20,) do not are not parallel, the illuminator being placed inside the tube, the   tube having a rotational movement relative to the illuminator.   2. Antenna according to claim 1, characterized in that   the outer casing (21) is cylindrical.   3. Antenna according to any one of the claims   previous, characterized in that the inner casing (21) is cylindrical.   4. Antenna according to any one of the claims   previous, characterized in that a lens of dielectric material (71) is arranged along the tube (1) in the scanning field of the beam (3) of the illuminator.   5. Antenna according to claim 4, characterized in that the thickness of the lens (71) is maximum at its center and decreases by going towards its two edges (72, 73).   6. Antenna according to either of claims 4 or 5,   characterized in that the dielectric material of the lens (71) has a dielectric constant substantially identical to the dielectric constant of the tube.   7. Antenna according to any one of claims 1 or 2,   characterized in that for a given section of the tube the thickness of the tube increases from a minimum thickness emjn to a maximum thickness eMaX on a angia substantially 2.   8. Antenna according to claim 4, characterized in that the thickness increases linearly.   9. An antenna according to any one of claims 7 or 8,   characterized in that a lens of dielectric material (91) is arranged along the tube (1) in the scanning field of the beam (3) of the illuminator.   10. Antenna according to either of claims 7 to 9,   characterized in that the dielectric material of the lens (91) has a dielectric constant substantially identical to the dielectric constant of the tube.   11. An antenna according to any one of claims 7 to 10,   characterized in that the lens (91) has a thickness which varies with the   same law of variation as the thickness of the tube, in reverse.   12. An antenna according to any one of claims   previous, characterized in that the maximum angular difference AMaX of the generators (61, 62) of the outer and inner envelopes of the tube (1) satisfies the following relationship: T h AMax 2 L (n-1) o hA and L represent respectively height and length   of the illuminator (2), and n = 4 :, sr being the dielectric constant of the tube (1).   13. An antenna according to any one of claims   previous, characterized in that the maximum thickness eMaX of the tube satisfies the following relation:) T hA Max 2 (n-1) o hA represents the height of the illuminator (2), and n = :, úr   being the dielectric constant of the tube (1).   14. An antenna according to any one of claims   previous, characterized in that the tube (1) is driven by a motor (101).   15. Antenna according to claim 1, characterized in that the tube (1) is composed of discrete prisms (110, 111), each prism having   an angle aj, this angle being different from one prism to another.   16. An antenna according to claim 15, characterized in that the   prisms (110, 111) are arranged along a circular envelope.   17. An antenna according to any one of claims 15 or 16,   characterized in that the prisms are juxtaposed. .., ,,,, -..   18. An antenna according to any one of claims 15 to 17,   characterized in that the angle aj takes a maximum value amaX for a   first prism (111) then decreases with the following prisms.   19. An antenna according to any one of claims 15 to 18,   characterized in that the tube is driven by a stepping motor, the tube being positioned at each step so that the beam (3) of the illuminator   meets an elementary prism (110, 111).   20. Antenna according to any one of claims 15 to 19,   characterized in that a prism (110) is composed of a succession of   prisms (151) of length dp = 2 / f where f is the center frequency of the wave.   21. An antenna according to any one of claims   previous, characterized in that the illuminator (2) is a waveguide