Optical: systems and elements – Lens – With graded refractive index
Reexamination Certificate
2001-08-22
2003-04-15
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With graded refractive index
C359S654000, C343S91100R, C343S91100R, C264S044000
Reexamination Certificate
active
06549340
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a focussing device comprising a Luneberg lens and its manufacturing process. It relates more particularly to a focussing device comprising a Luneberg lens comprising a homogeneous volume of dielectric.
It is known, from French patent applications No. 98/05111 and No. 98/05112 filed on Apr. 23, 1998 in the name of the applicant to use a Luneberg lens in satellite-signal receivers, especially for the tracking of nongeostationary satellites.
In theory, the lens must be composed of a given number of layers of dielectrics high enough to approach the ideal model of refractive index variation characteristic of the Luneberg lens. The refractive index n relating to a layer and the relative dielectric constant E (or permittivity) corresponding to it are therefore related by the equation: n=E
½
. However, increasing the number of layers is limited in practice by strict manufacturing tolerances which are incompatible with a mass production process. For small-sized lenses, typically having a diameter of less than 40 cm for transmissions in the Ku band, one solution to this problem is to opt for a lens having a single layer of homogeneous dielectric.
For the purpose of reducing the overall size of the lens, it is therefore necessary to increase the density, with the disadvantageous consequence of increasing the weight of the lens. There must therefore necessarily be a compromise between the size of the lens and its weight. These volume and weight constraints impose a well-defined density range on the dielectric. For example, for a lens 35 cm in diameter, the allowed density is typically between 0.3 g/cm
3
to 0.8 g/cm
3
.
It is known, from the prior art, to use, as dielectric, a compound comprising expanded polystyrene filled with high-density granules, ceramic or metal granules for example, in order to increase its density and to shift it toward the desired density range.
However, this type of compound does not allow there to be complete homogeneity of the granules in the compound, and therefore does not guarantee a homogeneous density within the volume of the lens. In addition, the compound obtained is expensive.
SUMMARY OF THE INVENTION
It is an object of the invention to remedy these drawbacks.
For this purpose, the subject of the invention is a focussing device comprising a Luneberg lens comprising a homogeneous volume of dielectric, characterized in that the dielectric comprises a granular agglomerate defined by a homogeneous granule size distribution of thermoplastic granules, at least one plurality of these granules being welded together by granule boundaries in order to keep said volume consolidated.
Thus, since the granules are placed one with respect to another in such a way that each of them is in contact with at least one other, the existence of solid granule boundaries makes it possible to fulfill the function of a binder between the various granules and to generate a compact assembly of granules.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
According to one embodiment, said plurality of granules is included at least within an outer layer of said homogeneous volume, said outer layer being relative to the outer surface of the volume extended toward the inside of said volume to a predetermined depth, preferably of the order of a multiple of a received and/or transmitted half-wavelength. Consequently, the outer layer serves to keep the granules of material under pressure within the outer layer. In addition, the thickness of the outer layer defined as a multiple of a received and/or transmitted half-wavelength optimizes, from the electromagnetic standpoint, the exchange of signals with the outside of the lens and at the same time allows this lens to act as a radome.
Preferably, said plurality of granules is uniformly distributed within said volume. Thus, it is possible to ensure density homogeneity over the entire volume.
Advantageously, the permittivity &egr;
r
of the lens of material composed of thermoplastic granules with a permittivity &egr;
r0
is connected with a fill factor F, denoting the ratio of the volume actually occupied by the granules to the total volume of said volume, by the equation:
&egr;
r
=[(1+2F) &egr;
r0
+2(1−F)]/[(1−F) &dgr;
r0
+2+F].
French patent applications No. 98/05111 and No. 98/05112 describe in particular primary sources close to the focussing surface of the lens. According to one embodiment, the focal length of the lens depends on the refractive index n of the lens and on the accepted phase variation over the aperture of the radiation pattern of the lens, the refractive index of the lens being given by:
n=&egr;
r
½
In order to allow the radius of the lens to be increased and thus to decrease the focal length of the latter without increasing the phase variation over the aperture of the radiation pattern of the lens, at least one additional layer covers said homogeneous volume of dielectric, said additional layer also comprising an agglomerate of thermoplastic granules of a material differing from that of the granules of said volume and having a density less than that of said volume. In this way, by creating a gradation in the refractive indices from the outside of the lens toward the inside of the latter, it is possible to approach the ideal model of a Luneberg lens.
According to one embodiment, the granules of said volume are composed of polystyrene. Thus, the density of the lens is within the desired density range.
According to one embodiment, the granules of the second layer are composed of polypropylene.
In addition, the processes for converting thermoplastics (injection molding, thermoforming, rotomolding and compression molding) do not allow parts to be produced with a thickness greater than about fifteen millimeters. Furthermore, these processes entail density variations within the parts and, because of the phenomenon of material shrinkage, deformations and geometrical variations appear within the parts. These problems may, in particular, interfere with the proper operation of a lens as described above and implemented according to said processes.
It is also an object of the invention to solve these drawbacks and, more particularly, to provide a process for manufacturing a Luneberg lens comprising a homogeneous volume of dielectric.
For this purpose, the subject of the invention is a process for manufacturing a Luneberg lens comprising a homogeneous volume of dielectric, comprising a step of forming said volume, characterized in that the volume comprises a granular agglomerate defined by a homogeneous granule size distribution of thermoplastic granules and in that said process comprises the following steps:
a step of heating the volume in order to raise the temperature of at least one outer layer of the volume to a transition temperature between the softening temperature of said material and the melting point of the material, the outer layer representing the outer layer of the volume extended into the volume to a predetermined depth and the transition temperature being defined by a phase change toward a viscous phase of at least part of said outer layer over said depth;
a step of cooling said outer layer in order to harden said outer layer.
Thus, this hardened outer layer allows the material within the layer to be kept under pressure. The fact of not completely melting the thermoplastic allows the initial density of the thermoplastic to be maintained. Furthermore, using a thermoplastic is inexpensive.
Advantageously, during the heating step, the temperature is raised in order to melt at least the outer layer of the granules contained in said outer layer of the volume for the purpose of forming viscous granule boundaries binding the granules of the outer layer of the volume. Thus, the granules of the outer layer, arranged against one another and leaving room for voids forming an open porosity, are consolidated by the solidification, during the cooling, of the viscous granule boundaries encapsulating said gra
Allaire Christian Yves
Hirtzlin Patrice
Louzir Ali
Pagnier Luc
Akiyama Kuniyuki
Epps Georgia
Spector David N.
Thomson Licensing S.A.
Tripoli Joseph S.
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