DIELECTRIC MATERIAL FOR ANTENNAS

Abstract: This invention relates to the fabrication of dielectric material suitable for dielectric antennas such as Luneburg-type lens antennas and to an according antenna system. It is an object of the present invention to produce dielectric material with a low weight and the possibility of adjusting a dielectric constant quite easily. According to the invention dielectric pieces, which collectively form the dielectric material, have a hollow shape. By the variation of the thickness, the effective dielectric constant can be varied. The antenna system according to the invention includes a virtuel lens antenna with extended reflecting means. The material produced according to this invention can preferably be used for the production of Luneburg-type lenses.

Description: _.i§i___--_li_.__3_i_L_.li_.i_.Ϊ2I__lD_-_.DD§_. The present invention relates to a production method of dielectric material suitable for dielectric antennas such as Luneburg-type lens antennas and to an according antenna. Dielectric antennas are well known, e . g from US 4 531 129. In this patent the use of Luneburg antennas and appropriate feed ' s are described for the use a satellite broadcasting receiver system to receive microwave signals. Such antennas can also be used as part of a transmitter system. Several combinations of di electrical antennas with different kinds of feeders are descr bed in the European patent applica¬ tion 91400179.9. Several methods for the fabrication of Luneburg-type lenses have been described in the European patent applications 90403051.7 and 91401444.4. Vi rtual dielectric antennas, e.g. Luneburg-t pe antennas are known from the article "Vi rtual Source Luneburg Lenses"; G. D. PI. Peeler et al., IRE TRANSACTIONS - ANTENNAS AND PROPAGA¬ TION, July 1954. In this article there are also vi rtual source lenses described, in which reflecting means, realized as one or more reflectors, are arranged. D electric lenses can be produced by dielectric material e.g. with voids, which are created in solid plas c material by he ispherical indentations in plastic sheets which are verti¬ cally stacked and fixed together. Such a method is known from the article "Artif cial dielectrics uti lizing cyl ndrical and spherical voids"; Proceedings of the IRE, Vol. 44, pp 171 - 174, 1956. It is an object of the present invention to provide a new method for the fabrication of dielectric antennas with re¬ duced production costs. This can be realised by the method according to claim 1. Dielectric antennas can be e.g. Luneburg-type lens antennas homogenous-type lens antennas, Eaton-Lippmann lens antennas, or thelike. The said antennas may be shaped e.g. spherical, hemispherical, quarter sphere, cylindrical or the like. The present invention is based on the following principle. A dielectric material composed of individual pieces will appear essentially homogeneous (contin ous) to signals whose frequency is such that their wavelength is greater than, the dimension of the pieces. This means, that at microwave fre¬ quencies of about 10 Ghz, normally used for direct broadcast¬ ing from a satellite (wavelength is in the range of about 2.5 cm), pieces of several mi Iimeters in di ens on may be used to fabricate the dielectric material. By changing their packing density by adjustment cf the piece shape the dielectric constant of the resulting material can be changed. The relative dielectric constant Er is related to the density of the material by Er = 0,4*Ero Cc/ o + 0,6* ( 1 + CEro - 1 ) *d/do ) , ( 1 ) where do is the density of the material used to fabricate the pieces, d is the density of the resulting material, Ero is the dielectric constant of the pieces. According to the present invention pieces of dielectric mate¬ rial are packed in a given shape, whereby a dielectric body with the desi red resulting d electric constant can be pro¬ duced, which forms a dielectric antenna or is part of the antenna to be produced. Because of thei r symmetry spherical pieces appear the same to signals approaching from any d rection. For mult i satellite applications therefore, spheres are the preferred shape and have an associated packing density of approximately 0,6. Assuming the spheres are made from polysterene for example (do is appro imately 1,05 g/cm ) the result ng material would 3 have a density of 0,63 g/cm . Lighter densities could be achieved by using hollow spheres or indentations en thei r surface. Since the pieces could be formed accurately using plastic molding techniques the dielectric constant value could be accurately achieved in this way. If the size of the dielectric p eces is very small with re¬ spect to the wavelength (that means less than about 1 /10 of the wavelength), thei r symmetry becomes less important and the spheres could be replaced by other shapes, e.g. like tubes, of either solid or hollow form. This is advantageous because raw plastic material for injection molding is usually supplied in small approximately round shaped pieces typically 3 mi li meters long and 3 mi l i meters in diameter. These raw forms can be used immediatel to achieve medium density mate¬ rial. Assu ing that there is a virtual lens antenna, e.g. a hemi¬ spher cal, a quartersphere Luneburg-type antenna, or thelike, ith a reflecting plate located at the flat sides, the loss of an incoming wave increases th an increasing angle of incidence. The use of preferred kinds of virtual dielectric lens anten¬ nas with used reflecting means, e.g. a reflecting metal plate, which are extended outside the boundaries of the lens, has the advantage that the loss in gain with an angle of incidence not perpend cular to cne of the reflecting means can be decreased. To receive the according wave a feeder horn or an endfi re or ε backfire helical antenna or thelike can be used. Using an antenna outside the boundaries cf the lens the system is more flexible for receiving waves from several directions, because the feeds have a greater physical separation and do not cause aperture blockage. Using an antenna which is integrated in the lens the antenna system is more compact. Further characteristics, advantages and details of the present invention will be explained with the aid of the fol¬ lowing embodiment and accompanying drawings, where Fig. I shows some dielectric pieces, which collectively form a dielectric body; Fig. 2 shows another kind of a dielectric piece; Fig. 3 shows a radome cover containing dielectric pieces; Fig. 4 shows a hemispherical Luneburg lens which is state of the art; Fig. 5 shows a hemispherical Luneburg-type antenna according to a first preferred embodiment of a lens antenna to be produced; Fig. 6 shows a quartersphere Luneburg-type antenna according to a further preferred embodiment of a lens antenna to be produced; Fig. 7 shows a top view of di f ferent kind of dielectric lenses and in principle according runs of an inci¬ dent wave. Fig. 1 shows some dielectri pieces 10, which collectively form a dielectric block, which itself has the shape of a dielectric antenna to be proαuced, from which such an antenna may be separated (cut), or which is a part of the said anten¬ na. The pieces 10 may be fixed to eachot er, e.g. by glueing, or may just lay near or on eachother. In this embodiment the pieces 10 are shaped as hollow spheres with a diameter d less than the wavelength of a radia ion to be received or to be transmitted. As it is given by equation ( 1), the resulting dielectric constant Er depends on the d electri c constant of the dielectric material 11, wh ch forms the pieces 10, and the relat onship of densities d and do. In this embod ment do is given by the thickness t of the dielectric material 11 com¬ pared to the diameter h of the hollow inter or 12. For the production of a Luneburg-type antenna a varia on of the resulting dielectric constant in dependence on the diame¬ ter cf the Luneburg lens is required. This variation can be achieved e.g. by using different kinds of pieces 10, which collectively form the lens or by producing different kinds of blocks, e.g. shaped as a shell or the like, which collective¬ ly form the lens. Each of these blocks can be produced by pieces 10 identical inside of each block or with a variation in thickness t, diameter d or by using pieces 10 with a ncn- spherical shape. Fig. 2 shows another type of dielectric pieces 10' . In this embodiment the shape of piece 10 ! shows some indentations. By the number or the size of these indentations the resulting dielectric constant Er can be varied. Referring to fig. 3, it may be mentioned, that normally a microwave lens antenna must be protected b a radome cover, which may also serve as a matching layer to reduce the amount of reflected signal at the lens surface. Normally this radome cover is made of plastic and may be used as a vessel 13 to contain the unbound plastic pieces 10, 10' respect vely. The vessel 13 shown in fig. 3 is used for the fabrication of a hemispherical Lens. It is evident, that Large volume lenses can be produced in this way without long cooling cycles since no heat forming process is needed to produce a Large volume shape other than to form the radome cover 13. Instead of using spherical or hemispherical shaped vessels, which can be used later as radcme cover, other kinds of ves¬ sels are possible. Other preferred types have the shape of a quarter sphere or a pyramid. Some of these pyramids may col¬ lectively form the dielectric antenna to be produced. For a variation of the dielectric constant inside of each block, a tool with a similiar function as a ice-cream shape- maker (scoop) may be used. As an extension, multiple shell Lenses could be made by em¬ ploying several thin solid shells to seperate bands of differ¬ ent density pieces. The said shells can be removed after positioning the pieces. It may be also mentioned, that the medium density plastic material is necessary for Luneburg and homogeneous Lens appli¬ cations where dielectric constants in the range 1 - 2,5 are typically required. For values close to approxi ately 2 to 2,5 solid plastic material can be used. In principle values in the range of approximately 1,8 to 2 can be achieved using low density poLyprope Lene and finally values in the range of approximately 1 to 1,15 can be achieved using foamed plastic materiaL. The intension of this invention is to provide a method for obtaining accurate Large volumes of dielectric material with a dielectric constant in the range of about 1,15 to 1,8, which does not requ re large cooling cycles. In a variation of the embodiments described the pieces 10, 10' respectively could be made from a mixture of a fi rst material with a h gh dielectric constant, such as cera ic material, ferroelectric ceramics, or thelike, and a second material ith a low dielectric constant, such as foamed plas¬ tic material, e.g polyethelene. Thereby a lighter weight dielectric material can be produced. Additionally the result¬ ng dielectric constant may be varied more accurately. It is another variation of the said embodiments to connect or to form the dielectric pieces 10, 10' respecti ely in such a way that lines or sheets of pieces are formed. These lines or sheets may contain pieces with the same dielectric constant or ith various dielectric constants. As an extension, sheets containing hollow spherical indenta¬ tions may be used collectively to form material containing spherical voids simi lar to the material shown in fig. 1. These lines or sheets may be arranged in such a way that the shape of the antenna or the block to be produced is formed and that a variation of the dielectric constant is achieved as desi red. For the fabrication of a hemi-spherical Luneburg antenna, first a small round sheet with the effective dielectric con ¬ stant Er1 of about 2,0 may be taken and laid on a core locat¬ ed n the center of the lens. On this fi rst sheet a second one ith a dielectric constant Er2 (smaller than Er1) is laid, where the size is a little b t larger than the one of the first sheet. This method s continued to a last sheet (n) w th an effective dielectric constant Ern of about 1,0 and ε quite big size. If Lines of pieces are used, the effective dielectric con¬ stant inside cf each line may be constant or may vary. With an appropriate variation each line may start in the center point of a (hemi-)spheri ca I Luneburg lens and end at its surface. Such Lines can also be used for the fabrication of other parts, as pyramids, which collectively form the lens to be produced. By using Lines or sheets it is easy to handle the dielectric pieces and many hollow spheres can be realized at the same time. Versions of the embodiments concerning the fabrication method nay include at Least one of the following variations: - the variation of the dielectric constant E with radius may have such 5 slope that the value n of the refraction 2 index (E = n ) at the center of a sphere is n = 1,7...1,35. Thereby a parallel incoming wave is focussed outside of the surface of a spherical-type Lens; - the the parts 10 can be smaller than the wavelength of a wave to be received. Fig. 4 shows the principle of a virtual dielectric Lens to be produced. A hemisperical Luneburg lens 110 with a radius R has a reflecting plate 111. A first part 112a of a beam 112 to be received is refracted by the Lens 110 and reflected by the plate 111 in such a way that it is focused in the focal point 113. The Location of this focal point is determined by the angle of incidence be relative to the perpendicular line 114 of the plate 111 and by the profile of the refraction index of the Lens 110. As this Lens 110 is a Luneburg Lens with a radius R, the refraction index n is given by here r is the actual point. Near the fecal point 113, which can also be inside or outs de of the boundaries of the lens 110, a non-shown antenna i s Located, which can be realized e.g. as feeder horn or helical antenna . As can be seen from fig. 4, a second part 112b of the beam 112 to be received passes outside of the lens 110. The por¬ tion of the first part 112a of the beam decreases ith in¬ creasing angle be and the portion of the second part 112b increases with increasing angle be. This shows that there is a loss of effective antenna area with angle be hence a Loss in antenna gain. Fig. 5 shows a fi rst develop ent of the antenna system of fig. 4. Means with the same function as those i fig. 4 are marked with the same reference numbers and they wi ll be ex¬ plained only as far as it is necessary for the understanding. The main difference of this embodiment compared to the ar¬ rangement of fig. 4 is that the reflecting plate 111 is ex¬ tended outside of the boundaries of the lens 110 by the lengh I. Thereby the second part 112b of the beam can also be re¬ flected by the plate 111 and refracted by the lens 110 in such a way that it is essentially also focussed in the focal poi nt 113. The extension length I of the plate 111 which is necessary depends on the range of the scan angle be reαui red according to I = R * ( (1 / cos be) - 1 ) . An embodiment has been realized as a hemi-sphericaL Luneburg- type Lens with a radius R of 15 cm and an extension length I of 15 cm. Measurements with a wave of 12 Gh∑ showed that the gain using this extension could be increased by about 2 dB at an angle of incidence be of about 62.5 degrees. Fig. 6 shows a three-dimensional sketch of another embodiment of an antenna to be produced. The main difference compared to the antenna system of fig. 5 is that a quartersphere Lens is used which can be shaped as a orange slice. The beam 112 to be received is transmitted from a satellite 115 and focussed by the quarter sphere lens 116 in the focal point 113 where a feeder horn 17 is provided. This lens 116 has twc reflector plates 111 and 111a respecti ely. These reflectors 111, 111a can have the same size as the according adjacent flat side of the quarter sphere lens 116, but they may also be extended in horizontal direction and/or in verti¬ cal direction. Thereby Losses can be reduced and the antenna gain can be maintained with scan angle, sim liar to the con¬ struction of fig. 5. It is preferred to extend the reflectors more in the upper haLf than in the Lower half of the antenna. This is due to the elevation angle of satellites to be received being not equal to 0 degrees, but typically about 30 degrees. Fig. 7 shows a top view of a spherical lens 120, the hemi¬ spherical Lens 110 and the quartersphere Lens 116 and in principle according runs of the incident ray 112. Having the full sphere Lens 120, which can be a Luneburg-type Lens, ho ogenoustype lens- or thelike, without any of the • reflectors 111 and 111a the wave 112 is focussed at a focal point 113a hich is on the opposite side compared to the direction cf ncidence of wave 112. Using half of the lens 120 which means the he isperical lens 110 including the reflector 111 the ave 112 is focussed at focal point 113b. Using a quarter of Lens 120 which means the quarter sphere lens 116 including the reflectors 111 and 111a the wave 112 is focussed at focal point 113c. It may be mentioned that the focal points 113 may be outside of the boundaries of the lenses 120, 110, 116 when using a homogeneous type lens with a relative dielectric constant Er where Er is about or less than 2 or en using a Luneburg-type lens but w th a modified varia¬ t on of the refraction index n wi h radius r as given by equation (2) . Versions of the antennas to be produced may include at least one of the following variations: instead of a hemi-spherical or a quartersphere lens just a lens with a conical or pyrami de-like shape may be used. In this case it is preferred that the shape of the according reflecting means is varied in such a manner that it covers at least one of those sides of the lens which are not penetrated by the wave to be received. One or more of these reflecting means can be extended; a homogeneous-type lens may be used, which means that the refraction index may be essentially constant through ¬ out the lens; the antenna system according to the nvent on may also be used as transmitter antenna; the antennas presented by this patent application may also be produced according to other fabrication methods, e.g. as presented by the European patent applications 904C3051.7 and 91401444.4. By the present invention a fabrication method for dielectric Lens antennas and according antenna systems are presented, where the said Lens is realized as a virtual source lens by using reflecting means. These reflecting means are preferably extended outside the boundaries of the Lens, whereby the gain cf the antenna system can be increased. The maximum operating frequency for the antenna of a pre¬ ferred embodiment is 12,75 Ghz and a void diameter of 8 mm and separation of 1 cm were chosen. The plastic material employed was polysterene with a dielectric constant of 2,44. This resulted in an effective dielectric constant of 1,52. The querter sphere lens is well suited to ulti satellite reception because it provides the necessary scan range with minimum volume.

Claims: C l a i m s 1. Method for the production of dielectric material suit¬ able for dielectric antennas, resulting dielectr c con¬ stant of said dielectric material may be constant or variable, characterized by compos ng dielectri c pieces (10) having dimensions and/or distances to eachother less than the wavelength of a radiation to be received or to be transmitted. 2. Method according to claim 1, characterized by vary ng the resulting dielectric constant by a variation of the size of the dielectric pieces. 3. Method according to claim 1 or 2, characterized in that said dielectric pieces are mace from materials ith different dielectric constants. 4. Method according to claim 3, characterized in t at a first material is used, which is cerami c-like, and a second material is used, which is plastic-Like. Method according to one of the claims 1 to characterized in that the dielectric pieces are hollow. Method according to one of the claims 1 to 5, characterized in that at least some of the dielectric pieces used have a shape showing indentations. ethod according to one of the claims 1 to 6, characterized in that said dielectric pieces are in a vessel at least during the production process. 8. Dielectric lens antenna system including a virtual source dielectric Lens (110) with according reflecting means (11), character sed in that the reflecting means (111) are extended outside the boundaries of the lens (110). 9. Antenna system according to claim 8, characterised in that the virtual source dielectric lens (110) is a Luneburg-type or homogeneous-type lens. 10. Antenna system according to one of the claims 8 or 9, characterised in that the shape of the virtual source dielectric lens (10) is hem spheri ca L-Li ke, quartersphere-li e, pyramide-li ke, or coni ca L-Li e. 11. Antenna system according to one of the claims 7 to 10, characterised in that it may be used for receiving and/or transmitting.

Inventors: FUJIMOTO MASAHIRO, HARRISON DAVID, FUJIMOTO, MASAHIRO, HARRISON, DAVID
Applicants: THOMSON CONSUMER ELECTRONICS, THOMSON CONSUMER ELECTRONICS S.A.
Application year: 1992
Main classification: H01Q19
IPC codes: H01Q15, H01Q19