RADOME STRUCTURE FOR CIRCULAR POLARIZATION ANTENNAS

DESCRIPTION

Field of the invention

[0001] The present invention relates to a wall structure of a protective enclosure for antennas, i.e. a radome. This structure is particularly suitable for use with circular polarization antennas operating at frequencies higher than 18- 20 GHz, for example radar, satellite, GPS antennas and the like.

Technical problem - prior art

[0002] The case is considered of a circularly polarized electromagnetic wave propagating along a line having a given propagation direction. In a circularly polarized wave, in stationary conditions, the electric field vector has the same magnitude in each point and a direction changing in a plane perpendicular to the propagation direction. As shown in Figs. 1 and 2, relating to a right-handed wave 1 , the electric field vector E of a circularly polarized wave can be resolved into its components E _x ,E _y on mutually perpendicular planes tp,p2 passing through a line 2 along which wave 1 propagates, in particular, it can be resolved in its horizontal and vertical components on corresponding horizontal and vertical planes tt-i,p2. These components oscillate describing sinusoids 3,4 on two planes tp,p2, said components having a same amplitude Eo and being shifted by 90° with respect to each other.

[0003] As shown in Fig. 2, while traveling through a wall portion 10 orthogonal to the propagation direction, both electric field components E _x , E _y of a circularly polarized wave are attenuated by the same amount, and remain shifted of 90° with respect to each other. In fact, in this case, both components E _x , E _y are contained in a plane T tangential at passage point 5, therefore they interact in the same way with wall portion 10. Therefore, after travelling through the latter, the circularly polarized electromagnetic wave 1 is even minimally attenuated, since this is a requirement for a radome, but is still circularly polarized. [0004] On the other hand, as shown in Fig. 3, if the propagating direction of electromagnetic wave 1 is not orthogonal to wall 10 at passage point 5, two components Ex, E _y are attenuated differentially with respect to each other. In fact, if for instance propagation direction 2 is horizontal and plane T” tangent to the radome at passage point 5 is vertical, but forms an angle different from 90° with propagation direction 2 if observed along the vertical direction, the vertical component E _y is attenuated in the same way as in the orthogonal traveling through of Fig. 2, whereas the horizontal component Ex is attenuated by a higher amount and, moreover, is also delayed to a higher extent, and remains therefore shifted by more than the original 90° angle, as it was before. For this reason, by travelling through the wall, electromagnetic wave 1 loses its circular polarization, and turns into an elliptically polarized wave 1’. More in general, in a situation not shown in Fig. 3, if plane t tangential to radome at passage point 5 is in anyhow oriented in the space, the two components E _x , E _y are in general attenuated and delayed differentially with respect to each other, therefore the circular polarization can be lost, if the two components E _x , E _y are attenuated by amounts very different with respect to each other.

[0005] In the case of an electromagnetic wave that is emitted/received by an adjustable antenna and that passes through the wall of a radome housing the antenna, the polarization change depends on the azimuth and elevation angles which defining the orientation of the antenna, which affect together both components E _x ,E _y .

[0006] This phenomenon, briefly indicated hereinafter as “distortion” of a polarized wave, is a problem, since the circular polarization is used for transmitting data that become no more understandable if the wave distortion exceeds a determined limit during its travel. In particular, in the satellite communication systems devices are provided that cut off a communication process if the aberration ratio between the magnitudes of the two components E _x ,E _y exceeds a predetermined limit value, typically corresponding to 1 dB.

[0007] This problem occurs, in particular, if the radome has a wall profile very different from a sphere. This is the case when a plurality of adjustable antennas is arranged within a same radome, which happens more and more often in on board installations, in order to limit the costs and room requirements for protecting the antennas. As shown in Figs. 4 and 5, a radome 9 of this type has normally an elongated shape in a top plan view, and comprises low-curvature wall portions 10, in particular, in the direction of change of to the azimuth angle Q of each antenna 50,55. Antennas 50,55 are configured to rotate horizontally about their axes 51 ,56, within an azimuth excursion ±0 ^* about a central position in which 0=0. In this case, the incidence angle, indicated here as a or b, i.e. the angle that propagation direction 2 of electromagnetic wave 1 forms with a line 6 normal to wall portion 10 of radome 9 in its point of incidence 5 with wall portion 10 is also 0. The azimuth excursion ±0 ^* is normally about ±50°, but circularly polarized wave 1 distortion problems arise unacceptably for angles as small as |0| > 15°-20°.

[0008] The same problem can also arise for elevation angles f higher than a certain limit, typically for |cp| > 15°-20°, in particular in the case of walls having low curvature cross sections.

[0010] Moreover, this problem is even more complicated at high electromagnetic wave frequencies, with inacceptable outcomes, in particular, if the frequency is higher than 18-20 GHz. As well known, for a given material, the higher frequency of the electromagnetic wave, the more the electromagnetic wave is attenuated, along with its electric field components. In the last years, radar and telecommunications antennas operating at frequencies higher than these values have become more and more common, as in the case of the Ka band, and normally up to 35-40 GHz.

[0009] The need is therefore felt of a radome wall structure that limits the above-mentioned distortion affecting a circularly polarized electromagnetic wave at the passage through the wall of a radome, in particular if the wave frequency is higher than 18-20 GHz, when the incidence angle is wider than a determined value, typically 15°-20°.

[0011] US 3,576,581 describes a radome having a conical shape for a sharp front portion of an aircraft, in which a parabolic dish antenna points the vertex of such cone. The radome wall has a conical structure comprising a plate of dielectric material and an array of strips of dielectric material arranged along some of the cone’s generation lines. The strips are supported by an edge of the plate and are arranged on spaced planes parallel to the incidence plane of at least one of two mutually orthogonal components of the wave. In an embodiment, the document exemplifies the case of a radome having the shape, still axisymmetric, of a cone, in which the strips are circumferential rings arranged coaxially to the cone and regularly spaced apart from one another.

[0012] Also US 3,871 ,001 relates to a radome having a rounded conical shape for the front portion of an aircraft, in which a parabolic dish antenna points the vertex of such cone. The wall structure is conceived for minimally affecting a circularly polarized electromagnetic wave travelling therethrough, and for allowing de-icing warm air inner circulation. To this purpose, the structure comprises two hollow sheet-like elements coupled on each other, each enclosing transversal channels separated from one another by parallel inner walls, which are mounted so that the inner walls of a sheet-like element are at a predetermined angle with respect to the inner walls of the other sheet. This angle is preferably a right angle in the regions where a zero wave incidence angle is expected, and advantageously deviates from the right angle as this incidence angle increases.

[0013] JP 2004/023645 A describes a radome wall structure comprising protrusions extending along variously-shaped lines at least on the outer surface of the wall, so as to prevent a steady water film to be formed in case of rain. This is used to limit communication jamming when a radome-housed antenna is used.

[0014] WO 2017/192330 describes a radome structure with a modular wall comprising a plurality of layers is provided, wherein the layers have protrusions and recesses to assist a coupling between them.

Summary of the invention

[0015] It is therefore a feature of the present invention to provide a radome wall structure that limits the differential attenuation of the vertical and horizontal components of the electric field and magnetic field vectors associated to a circularly polarized electromagnetic wave when passing through said wall, so as to substantially maintain the circular polarization of the electromagnetic wave.

[0016] It is a particular feature of the invention to provide such a structure that reduces this differential attenuation below 1 dB for higher azimuth angles set between 15° and 50°, and for frequencies of the electromagnetic wave set between 18 and 40GHz.

[0017] These and other objects are achieved by a radome according to independent claim n.1 , and are also reached by a method for making a radome according to independent claim n.15. Exemplary embodiments and modifications of the radome and of its manufacturing method are defined in respective dependent claims.

[0018] In one aspect of the invention, a radome comprises:

a covering having a base wall layer;

a positioning space of an adjustable transceiving element of an antenna within the covering,

said covering comprising a wall portion facing said positioning space,

the wall portion comprising:

a central region, facing the positioning space according to a central incidence angle set between a first limit angle and a second limit angle, said central region comprising the base wall layer;

at least one pair of peripheral regions arranged at opposite sides of the central region and facing the positioning space according to a peripheral incidence angle wider in absolute value than the first and the second limit angles,

whose main feature is that each peripheral regions comprises said base wall layer, on which an auxiliary wall layer is arranged comprising first strips having a first relative dielectric constant alternate to second strips having a second relative dielectric constant lower than the first relative dielectric constant.

[0019] In another aspect of the invention, a method for making a radome comprises the steps of:

defining a covering;

defining a positioning space of an adjustable transceiving element of an antenna within the covering,

defining, in the covering, a wall portion facing the positioning space definition , in the wall portion, of: a central region, facing the positioning space according to a central incidence angle set between a first limit angle and a second limit angle;

at least one pair of peripheral regions arranged at opposite sides of the central region and facing the positioning space according to a peripheral incidence angle wider in absolute value than the first and the second limit angles,

making a base wall layer in the central region and in the peripheral regions;

whose main feature is that it comprises a step of

making, in each peripheral regions, an auxiliary wall layer above the base wall layer, the step of making the auxiliary wall layer comprising a step of forming first strips having a first relative dielectric constant alternate to second strips having a second relative dielectric constant lower than the first relative dielectric constant

wherein the auxiliary wall layer is absent in the central region.

[0020] This way, in the case of a circularly polarized electromagnetic wave that passes through the central region of a mainly vertical radome wall, for example for an azimuth change narrower than a predetermined angle, for example a _0, since the incidence angle of the wave is not very different from zero, it occurs that both electric field vector components, i.e. E _x ,E _y , according to two orthogonal planes, of the electric field vector, i.e. E, travel only through the base wall layer and in identical proportions. In this case, they are attenuated by a same amount, i.e. they differ from each other by less than 1 dB, and have therefore an aberration ratio, e.g. Ex/E _y not very different from one.

[0021] In the case, instead, of a circularly polarized electromagnetic wave that passes through the peripheral regions, for example, for an azimuth change wider than a certain angle, it occurs that both components, i.e. E _x ,E _y , according to two orthogonal planes, of the electric field vector, i.e. E, travel through both the base wall layer and the auxiliary wall layer.

[0022] When traveling through traveling through base wall layer, the components are attenuated differently from each other, i.e. they differ from each other, for example, by more than 1 dB , since one of them, e.g. Ex, travels through a higher wall thickness due to the inclination of the wave with respect to the wall, proportionally to the inverse of cosine of the incidence angle. In this case, the aberration ratio Ex/E _y is very different from one.

[0023] For instance, by horizontally arranging both the first strips, which have a first dielectric constant, and the second strips, which have a second relative dielectric constant lower than the first dielectric constant, the component that is less attenuated by the base layer, due to the wide azimuth angle, i.e. Ey, is more attenuated by the first strips and, therefore, the component that is more attenuated by base layer due to the wide azimuth angle, i.e. Ex, is less attenuated by the first strips, which are mainly parallel to it. This way, the attenuation of Ex due to the wall incidence angle, is compensated by the corresponding attenuation of Ey, due to the direction of the first and of the second strips.

[0024] The same reasoning applies, oppositely, when the elevation angle is wider than a predetermined angle, for example, a ₀ .

[0025] In particular, said first limit angle and said second limit angle are selected in such a way that an electromagnetic wave emitted/received by said transceiving element, by traveling through said central region, is attenuated less than a value corresponding to 1 dB, which is normally selected as the limit value within which satellite communications can be maintained, and beyond which the communications are cut off.

[0026] The second strips can be manufactured in the same low relative dielectric constant material as the second wall layer.

[0027] In an exemplary embodiment, the first strips extend along respective planes parallel to one another. In particular, the first strips are arranged horizontally. This makes it possible to optimize the performances in terms of aberration ratio even for very wide azimuth angles. The azimuth angle is the most critical among the angles defining the orientation of the transceiving element of the antenna, i.e. azimuth and elevation, since its excursion is larger than the excursion of the elevation angle and, in particular, if the wall portion has a strongly curved cross section.

[0028] In another exemplary embodiment, the first strips extend along respective planes orthogonal to the base wall layer. [0029] In an exemplary embodiment, the pairs of peripheral regions comprise: a pair of lateral peripheral regions, at which the first strips are arranged horizontally;

a pair of upper and lower peripheral regions, at which the first strips are arranged vertically;

wherein the central region has a convex shape, according to the curvature of the wall portion, of size depending on the first and the second limit angles, as well as on the wall portion curvature.

[0030] In an exemplary embodiment, the second strips consist of air. In this case, the first strips can be inner protrusions of the radome, in particular, if the wall comprises the above-mentioned first and second layers.

[0031] The first strips are preferably made of a dielectric material, for example of a fiberglass-reinforced plastic, and have, in this case, a thickness set between 0.1 mm and 3 mm, and a depth or height, measured transversally to the wall portion, set between 0.1 mm and 50 mm.

[0032] Preferably, the first strips have a depth, measured transversally to the wall portion, set between 1 mm and 50 mm.

[0033] Preferably, the first strips are arranged at a mutual distance (d) set between 1 mm and 50 mm.

[0034] Preferably, the relative dielectric constant of the first strips is set between 2 and 10.

[0035] The auxiliary wall layer, in particular the first strips, can have features that vary as the incidence angle increases, the incidence angle being the angle according to which a corresponding part of the peripheral regions faces the respective adjustable antenna. This variation can concern at least one among the following features: depth, i.e. height of the first strips; mutual distance between the first strips; dielectric constant of the first strips. This way, walls are obtained that can specifically correct the distortion of the circularly polarized wave caused by the inclination of the wave propagation line with respect to the radome wall, i.e. by the incidence angle. This is described more in detail hereinafter, for each of the above-mentioned features.

[0036] In an exemplary embodiment, the depth of the first strips increases as the peripheral incidence angle increases. In particular, the depth of the first strips can increase continuously as the incidence angle increases, according to which the wall portion faces the positioning space of a respective transceiving element, or can increase stepwise at predetermined incidence angles, or even portions can be provided in which the depth changes continuously and portions at which it changes stepwise.

[0037] As an alternative, or in addition, in other exemplary embodiments, the mutual distance between the first strips decreases as the peripheral incidence angle increases. In particular, also in this case, the mutual distance between the first strips can change, decreasing continuously as the incidence angle increases, according to which the wall portion faces the positioning space, or can decrease stepwise at predetermined incidence angles, or even portions can be provided in which the mutual distance changes continuously and portions at which it changes stepwise.

[0038] As an alternative, or in addition, in other exemplary embodiments, the relative dielectric constant increases as the peripheral incidence angle increases. In particular, also in this case, the relative dielectric constant of the first strips can change, increasing continuously as the incidence angle increases, according to which the wall portion faces the positioning space, or can increase stepwise at predetermined incidence angles, or even portions can be provided in which the relative dielectric constant changes continuously and portions at which it changes stepwise.

[0039] For example, the first strips are made in a mixture of a resin and of a dielectric filler, which is present at a concentration that increases along the first strips as the peripheral incidence angle increases.

[0040] In an exemplary embodiment, the auxiliary wall layer also comprises third support strips for supporting the first strips. In particular, in case of walls of radome comprising the first outer wall layer and the second, low relative dielectric constant-wall layer, as described above, the third support strips can be made in the same material as the second wall layer.

[0041] [15]ln an exemplary embodiment, the auxiliary wall layer comprises a panel comprising a support plate or back plate from which the third strips protrude, starting from a first edge thereof. Such panel can be connected to a surface of the wall portion, typically to the inner surface, by the third strips, at a second edge thereof, or by the second strips.

Brief description of the drawings

[0042] The invention will be now shown with the following description of its exemplary embodiments, exemplifying but not limitative, with reference to the attached drawings in which:

Fig. 1 diagrammatically shows how propagates the electric field vector of a circularly polarized electromagnetic wave;

Fig. 2 shows the electric field vector of the wave of Fig. 1 resolved in its two horizontal and vertical components, when passing through a wall orthogonal to the wave propagation direction;

Fig. 3 shows the electric field vector of the wave of Fig. 1 resolved in its two horizontal and vertical components, when passing through a wall that is not orthogonal to the wave propagation direction;

Fig. 4 is a diagrammatical perspective view of a an elongated radome, for housing a plurality of adjustable antennas;

Fig. 5 is a cross sectional view taken along a plane parallel to the base of the radome of Fig. 4, with two differently oriented antennas;

Fig. 6 is a central longitudinal sectional view of a radome as in Figs. 3 and 4, with wall portions according to one exemplary embodiment of the invention, in which the wall portion has a curved cross section;

Fig. 7 is a cross sectional view taken along a plane parallel and close to the base of the radome of Fig. 6, with two differently oriented antennas;

Fig. 8 is a cross sectional view of the radome of Fig. 7, with a wall portion according to one exemplary embodiment of the invention;

Fig. 9 is a cross sectional view of the radome of Fig. 7, with a wall portion according to another exemplary embodiment of the invention;

Fig. 10 is an enlarged view of a detail of the wall portion shown in Fig. 8; Fig. 1 1 is an enlarged view, as shown in Fig. 10, of a detail of the wall portion shown in Fig. 8, according to another exemplary embodiment of the invention;

Fig. 12 is an enlarged view of a detail of the wall portion shown in Fig. 9; Fig. 13 is an enlarged view, as shown in Fig. 12, of a detail of the wall according to another exemplary embodiment of the invention;

Fig. 14 is a longitudinal sectional view of a radome as in Figs. 6 and 7, with a wall portion according to another exemplary embodiment of the invention;

Figs. 15 and 16 are cross sectional views of the radome of Fig. 14;

Fig. 17 is a partial cross sectional view taken along a plane parallel and close to the base of the radome of Fig. 14;

Fig. 18 is a longitudinal sectional view of a radome as in Figs. 6 and 7, with a wall portion according to another exemplary embodiment of the invention;

Figs. 19 and 20 are cross sectional views of the radome of Fig. 18;

Figs. 21 and 21 A are cross sectional views of a radome as in Figs. 6, 14, 18, to show more in detail the wall structure at a central region and at a side region, respectively, of a wall portion;

Fig. 22 is a longitudinal sectional view of a radome as in Figs. 3 and 4, with wall portions according to one exemplary embodiment of the invention, in which the wall portion has a substantially planar cross section;

Fig. 23 is a cross sectional view of the radome of Fig. 22, taken along a plane parallel to the basis and close to the top of the radome;

Fig. 24 shows a detail of the cross sectional view of Fig. 23;

Fig. 25 shows a detail of the cross sectional view of Fig. 23, as shown in Fig. 24, according to another exemplary embodiment;

Fig. 26 is a cross sectional view of the radome of Fig. 23, with a wall portion according to one exemplary embodiment of the invention;

Fig. 27 is a cross sectional view of the radome of Fig. 25, with a wall portion according to another exemplary embodiment of the invention;

Fig. 28 is an enlarged view of a detail of the wall portion shown in Fig. 26; Fig. 29 is an enlarged view, as shown in Fig. 28, of a detail of the wall portion shown in Fig. 26, according to another exemplary embodiment of the invention;

Fig. 30 is an enlarged view of a detail of the wall portion shown in Fig. 27; Fig. 31 is an enlarged view, as shown in Fig. 30, of a detail of the wall portion shown in Fig. 27, according to another exemplary embodiment of the invention;

Figs. 32 and 33 show a detail of the cross sectional view of Fig. 23, in an exemplary embodiment according to Figs. 27, 30 and 31 ;

Figs. 34 and 35 are cross sectional views of a radome of the type of Fig. 22, but with the depth of the first strips changing as the peripheral incidence angle changes;

Figs. 36 and 37 are cross sectional views of a radome of the type of Fig. 22, but with the distance between the first strips changing as the peripheral incidence angle changes;

Fig. 38 is a cross sectional view of a radome as shown in Fig. 22, to show more in detail the wall structure;

Fig. 39 is a cross sectional view of a radome according to the invention, in which support strips are provided, according to one exemplary embodiment, for the strips having higher relative dielectric constant;

Fig. 40 is a cross sectional view of a radome according to the invention, in which a support panel is provided, according to one exemplary embodiment, for strips having higher relative dielectric constant;

Fig. 41 is a cross sectional view of a detail of what is shown in Fig. 40;

Fig. 42 is a diagram that shows how the aberration factor depends on the azimuth angle, this trend being obtained by tests with a conventional radome wall and with a wall having the structure of the peripheral regions according to the invention;

Fig. 43 is a diagram showing how the aberration factor depends on the azimuth angle in the case of a radome wall according to the invention;

Figs. 44 and 45 diagrammatically show the apparatus used for carrying out the measurements given in the diagram of Fig. 42, with a conventional wall portion and with a wall portion having the structure of the peripheral regions of a wall portion according to the invention, respectively;

Fig. 46 is a cross sectional view of the panel of the test value of Fig. 45. Detailed of the invention

[0043] In order to show the invention, reference is made to a radome 100 that has an elongated shape along to a vertically arranged axial plane 101. This shape is similar to the shape of a radome 9 of Figs. 4 and 5, arranged to house a plurality of receiving and/or transmitting adjustable antennas 50,55, i.e. it includes a plurality of positioning spaces 18,19 of adjustable transceiver elements 52,57, and in any case is not a limitation for the shape of the radome of the invention.

[0044] As Figs. 6 and 7 show, radome 100 has a covering 17 comprising a plurality of wall portions 10, in particular in the case shown in the figure, four wall portions 10.

[0045] Each wall portion 10 faces a respective positioning space 18,19 of adjustable transceiver elements 52,57 of antennas 50 or 55. Only two antennas 50,55 are shown, whose transceiver elements 52,57 have different azimuth orientations. Each wall portion 10 comprises a central region 1 1 and a pair of adjacent peripheral regions 12 and 13, opposite to each other with respect to central region 11. Regions 1 1 , 12 and 13 are defined in radome 100 according to the position of the positioning space 18,19 corresponding to wall portion 10, in particular, in this case, according to the azimuth orientation of transceiving element 52,57 of adjustable antenna 50,55. In facts, central region 1 1 faces respective positioning space 18,19 according to a central incidence angle a, in this case equal to azimuth angle, set between a first predetermined limit angle -ao and a second predetermined limit angle +ao, whereas the two peripheral regions 12,13 face positioning spaces 18,19 according to respective peripheral incidence angles b wider in absolute value than the first and to the second limit angles ±ao, respectively.

[0046] The expressions“wall” portion or“central region” or“peripheral”“facing /towards positioning spaces according to an incidence angle” a or b mean that transceiver elements 52,57 have a predetermined transmission/reception direction, i.e. the direction of a line 2,2’ along which the wave emitted/received by transceiver elements 52,57 propagates, and mean that this direction forms angle a or b with the normal to wall portion 10, in a point 5 at which each electromagnetic wave 1 travels through wall portion 10. In the case of an antenna 50,55 equipped with a parabolic reflector, i.e. a reflector having the shape of an elliptical paraboloid, this transmission/reception direction depends in a well-known way on the direction the antenna according to two azimuth Q and elevation f angles.

[0047] As intersection point 5 it is considered the intersection of a line, indicated as propagation line 2,2’, at the centre of the beam of electromagnetic waves emitted/received by the antenna, i.e. a line that passes through a source of electromagnetic signals assimilated to a point. For instance, in said case of antennas 50,55 equipped with parabolic reflector with transceiver elements 52,57 located at the focuses of the paraboloids, as propagation lines 2,2’ of the electromagnetic signals lines are considered that pass through the focuses of the paraboloids and that have the direction of the respective beam emitted/received by transceiver elements 52,57.

[0048] In other words, central region 1 1 is defined within an angle 2a whose bisector line is a line that passes through one of transceiving elements 52,57 assimilated to a transmission point arranged on rotation axis 51 ,56 of antenna 50,55 and normal to a plane tangential to passage point 5 of wall portion 10. In particular, if wall portion 10 is locally planar at the point or in the zone of minimum distance between transceiving element 52,57 and wall portion 10, this bisector line is a line passing through transceiving element 52,57 of antenna 50,55 and normal to wall portion 10.

[0049] Covering 17 comprises a base wall layer 14, both at central regions 1 1 , both at peripheral regions 1 1 ,12 of wall portions 10. The structure of base wall layer 14 can be of the type conventionally known in the art of manufacturing radomes, for example as described hereinafter with reference to Figs. 21 and 21 A.

[0050] With reference to Figs. 8 and 9, wall portions 10 of radome 100 have a curved cross section, typically with a concavity towards the inside of radome 100.

[0051] As show more in detail Figs. 10-13, according to the invention, also peripheral regions 12,13 of wall portion 10 facing each positioning space 18,19 of a respective transceiving element 52,57 have a base wall layer 14 and an auxiliary wall layer 60 arranged on said base wall layer 14. Auxiliary wall layer 60 comprises first strips 61 having a first relative dielectric constant e-i, a thickness si and a height or protrusion length h with respect to base wall layer 14 of wall portion 10. First strips 61 arranged in alternance to second strips 62 having a second relative dielectric constant 82 lower than the first relative dielectric constant, a thickness S2 preferably larger than thickness si and a height or protrusion length from base wall layer 14, in this case, but not necessarily, identical to height or protrusion length h of first strips 61.

[0052] This auxiliary wall layer 60 is missing in central region 11 of wall portion 10.

[0053] As still shown in Figs. 10-13, first strips 61 and second strips 62 can have an orientation comprising directions substantially parallel to base wall layer 14 of wall portion 10, in particular first and second strips 61 ,62 can extend along respective planes comprising said directions parallel to base wall layer 14 of wall portion 10.

[0054] More in detail, strips 10 can extend from base wall layer 14 of wall portion 10, for example from an inner wall layer as conventionally used in the art of manufacturing radomes.

[0055] As still shown in Figs. 8-13, first and second strips 61 ,62 can be parallel to each other.

[0056] In the exemplary embodiment of Figs. 8, 10 and 1 1 , the strips can also extend on horizontal planes.

[0057] In another exemplary embodiment of Figs. 9, 12 and 13, first and second strips 61 ,62 can extend perpendicularly to base wall layer 14 of wall portion 10.

[0058] With reference to Figs. 10 and 13, second strips 62 can comprise air. In other words, auxiliary wall layer 60 can comprise first strips 61 in the form of protrusions extending from base wall layer 14 of wall portion 10 arranged at a mutual predetermined distance, for example at a same distance along a cross section of wall portion 10, as shown in the figures, in this case, strips 61 can be projections of an inner layer of the wall of the radome, for example made in the same material as the inner wall layer. [0059] Relative dielectric constant ei of first strips 61 is preferably set between 2 and 10, whereas relative dielectric constant 82 of second strips 62 is preferably set between 1 and 1.2.

[0060] Thickness si of first strips 61 is preferably set between 0.1 mm and 3 mm, whereas thickness S2 of second strips 62 is preferably set between 1 and 50 mm.

[0061] In particular, the first strips can be made of a dielectric material of relative dielectric constant ei set between 2 and 10, for example of fiberglass- reinforced plastic, whose thickness si is set between 0.1 mm and 3 mm.

[0062] The depth or height or protrusion length h of first strips 61 and preferably also of second strips 62 can be set between 1 mm and 50 mm, whereas relative dielectric constant 82 of second strips 62 is preferably set between 1 and 1.2.

[0063] Figs. 14-17 show a radome 100 according to one exemplary embodiment of the invention, in which the distance d between first strips 61 decreases as peripheral incidence angle b increases. According to one exemplary embodiment, not shown, distance d between first strips 61 can decrease continuously within a range of values of peripheral incidence angle b, or can change stepwise, i.e. discontinuously, at least once from a first upper value di to a lower value d2 when the incidence angle, still referred to the positioning space 18,19, becomes higher in absolute value than predetermined values -bi and +bi of the incidence angle, as indicated in Fig. 17. In particular, Fig. 15 relates to a cross section of wall portion 10 in a region where |b|<|-bi|, and where distance d between strips 61 has an upper value d-i, while Fig. 16 it relates to a cross section of wall portion 10 in a region where |b|>|-bi |, and where the distance between strips 61 has a value d2 shorter than d-i. In another exemplary embodiment, not shown, distance d can decrease continuously in at least one range of values of b, and stepwise at predetermined values of b. Even if Figs. 14-17 refer to the case where second strips 62 consist of air, this exemplary embodiment can refer even to the case where second strips 62 are made of a material of relative dielectric constant lower than first strips 61 ;

[0064] Figs. 18-20 show a radome 100 according to one exemplary embodiment of the invention, in which depth h of first strips 61 increases as peripheral incidence angle b increases. According to one exemplary embodiment, not shown, height or depth h can increase continuously within a range of values of peripheral incidence angle b, or can change stepwise, i.e. discontinuously, at least once from a first lower value hi to an upper value h2 when the incidence angle, still referred to the positioning space 18,19, becomes higher in absolute value than predetermined values -bi and +bi of the incidence angle, as indicated in Fig. 17. In particular, Fig. 19 relates to a cross section of wall portion 10 in a region where |b|<|-bi |, and where the depth of strips 61 has a lower value hi , while Fig. 20 it relates to a cross section of wall portion 10 in a region where |b|>|-bi |, and where the depth of strips 61 has a value h2 higher than h-i. In another exemplary embodiment, not shown, depth h can increase continuously in at least one range of values of b, and stepwise at predetermined values of b. Even if Figs. 18-20 refer to the case where second strips 62 consist of air, this exemplary embodiment can refer even to the case where second strips 62 are made of a material of relative dielectric constant lower than first strips 61.

[0065] According to one exemplary embodiment, not shown, of the invention, relative dielectric constant e of first strips 61 increases as peripheral incidence angle b increases. In particular, relative dielectric constant e can increase continuously within a range of values of peripheral incidence angle b, or can change stepwise, i.e. discontinuously, at least once from a first lower value ei to an upper value e2 when the incidence angle, still referred to the positioning space 18,19, becomes higher in absolute value than predetermined values -bi and +bi of the incidence angle, as indicated in the figures. In another exemplary embodiment, not shown, relative dielectric constant e can increase continuously in at least one range of values of b, and stepwise at predetermined values of b. even if Figs. 18-20 refer to the case where second strips 62 consist of air, this exemplary embodiment can refer even to the case where second strips 62 are made of a material of relative dielectric constant lower than first strips 61.

[0066] For instance, first strips 61 are made in a mixture of a resin and of a dielectric filler, which is present at a concentration that increases along first strips 61 as peripheral incidence angle b increases. Such dielectric filler is to be understood in a broad meaning, i.e. it can be a particle filler in a resin matrix or in a composite matrix of a resin and of a glass tissue, such as quartz or other, or can even be the substrate itself of a prepreg type composite article. As an alternative, variable dielectric constant strips can be provided by making strips 61 in a prepreg type composite with different substrates, arranged so that the dielectric constant increases as peripheral incidence angle b increases.

[0067] As shown in Figs. 21 and 21 A, base wall layer 14 of central region 1 1 and of a peripheral region, in this case of a lateral region 13, respectively, can comprise an outer first wall layer 15 and a second inner layer 16.

[0068] Outer wall layer 15 is manufactured in such a way to have a high mechanical resistance, but also a resistance against weather conditions, and is normally made of a polymeric material reinforced by stiffening fibres, in particular it is a layer of a prepreg material. The polymeric material of outer layer 15 can be a cross-linked thermosetting polymer, i.e. a resin preferably selected among the families of the epoxy resins, of the cyanate ester resins and of the phenolic resins, and has advantageously a relative dielectric constant set between 2.8 and 3.5, in particular the relative dielectric constant is set between 2.8 and 3.1 , whiles the stiffening fibres can be glass fibres, in particular glass S, quartz fibres, ceramic fibres, polypropylene fibres, polyethylene fibres, ceramic fibres or also aramid fibres.

[0069] The second layer of inner wall 16, besides strengthening the radome, is highly transparent to electromagnetic waves, and is normally made of a solid cell material, made for instance of fiberglass-reinforced plastic, in particular a honeycomb material, a polyurethane foam, a structural or syntactic foam, a material comprising at least one phenolic paper sub-layer, not shown.

[0070] Auxiliary wall layer 60 can be connected to inner layer 16 of wall portion 10 by one of the methods described hereinafter, with reference to Figs. 39 and 40.

[0071] Figs. 22 and 23 show radome 100 according to another exemplary embodiment of the invention. Even in this case, as in Figs. 6 and 7, wall portions 10 face positioning space 18,19 of respective transceiver elements 52,57 which, in this case, are not shown for the sake of simplicity. Each wall portion 10 comprises a central region 1 1 and two pairs of peripheral regions 12,13 and 22,23 arranged two by two at opposite sides of central region 1 1. Even in this case, as in Figs. 6 and 7, central region 1 1 and peripheral regions 12,13,22,23 are defined in radome 100 according to the position of respective positioning space 17,18 but, unlike what is shown in Figs. 6 and 7, in this case, regions 1 1 ,12,13,22,23 of wall portion 10 are defined according to both azimuth and elevation orientation of transceiver elements 52,57.

[0072] More in detail, the peripheral regions comprise a pair of lateral peripheral regions 12 and 13, generally azimuth-shifted with respect to the respective adjustable antennas, where auxiliary layer 60 can be made according to one of the exemplary embodiments shown for radome of Fig. 6, with first and second strips 61 ,62 extending horizontally, and also comprise a pair of upper and lower peripheral regions 22 and 23, generally elevation-shifted with respect to the respective adjustable antennas, with first and second strips 71 ,72 extending vertically.

[0073] In particular, the expressions“generally azimuth-shifted” or“generally elevation-shifted with respect to” an adjustable antenna can mean that the azimuth angle is respectively wider or narrower than the elevation angle.

[0074] Central portion 1 1 has a convex shape depending on the curvature of wall portion 10, and contains the points of wall portion 10 where the incidence angle is narrower in absolute value than |±ao|. Also the size of central portion 11 depend on the curvature of wall portion 10, and on the value of first and second predetermined limit angles ±ao, which depend in turn on the frequency of the electromagnetic signals emitted/received by transceiver elements 52,57. In particular, this shape can be approximated to an ellipse or to a circle.

[0075] The exemplary embodiment of Figs. 22 and 23 is preferred, in particular, when wall portions 10 have a linear longitudinal cross section, in particular when they comprise substantially planar portions, the exemplary case to which reference is made, describing these figures and the subsequent Figs. 24-37.

[0076] Figs. 24 and 25 show longitudinal sections of wall portions 10, at an upper peripheral region 23, in two exemplary embodiments in which second strips 71 are made of a material, for example a material normally used for the inner layer of the radome, or comprise air, respectively. [0077] Figs. 26-31 show a cross sectional view of a wall portion according to different exemplary embodiments similar to the embodiments shown in Figs. 8- 13 for case of curved wall portions 10.

[0078] Figs. 32 and 33 relate to the case in which first and second strips 71 and 72 are oriented perpendicularly to base wall layer 14 of wall portion 10, according to different exemplary embodiments corresponding to the ones of Figs. 24 and 25.

[0079] Figs. 34,35 and 36,37 relate to exemplary embodiments of the radome in which distance d and depth h, respectively, of first and second strips 61 ,62, in particular of first strips 61 , increases as the incidence angle increases, similarly to what is shown in Figs. 15,16 and 19,20, respectively, for the radome of Figs. 6 and 7. Also this variation relates to the incidence angle changing due to an azimuth displacement, a similar reasoning applies to elevation rotation, i.e. for first and second vertical strips 71 ,72.

[0080] Fig. 38 shows the two-layer 15,16 structure in the case of a planar wall or of a wall having a linear cross section, similarly to wall portion 10 described with reference to Fig. 21 A.

[0081] As Figs. 39 and 40 show, auxiliary wall layer 60 can comprise third support strips 63 for first strips. In the exemplary embodiment of Fig. 39, support strips 63 can protrude from base layer 14 of wall portion 10, in particular they can protrude from inner wall layer 16, more in particular, can be made in the same material as inner layer 16. In another exemplary embodiment of Fig. 40, auxiliary wall layer 60 can also comprise a panel 65 in which support strips 63 protrude from a back plate 66 starting from an own first edge 67. Moreover, third support strips 63 are connected to the surface of base layer 14, in particular they are connected to inner layer 16, by an own second edge 68 opposite to first edge 67. Even in this case, support strips 63 and also support back plate 66 can be made in the same material as inner layer 16, above indicated, so as to obtain a high transparency to electromagnetic waves.

[0082] Even if the exemplary embodiments of auxiliary wall layer 60 of Figs. 39 and 40 are shown only for the case of Fig. 22, in which wall portions 12,13 have a substantially linear cross section, in particular they extend on planes, and are shown only for wall portions azimuth-shifted with respect to axes 51 ,56 of respective antennas 50,55, these exemplary embodiments also apply for wall portions 22,23 elevation-shifted with respect to antennas 50,55 and for wall portions 12,13 having curved cross section, as in Figs. 6-21.

[0083] With reference to Figs. 42-46, irradiation tests are now described of a conventional first radome wall portion 30 and of a radome wall portion 35 having the structure of a peripheral region 12,13 of wall portion 10 (Fig. 6) according to the invention.

[0084] Conventional wall portion 30 comprises a wall layer 31 made of a prepreg material, in this case a fiberglass-reinforced plastic, and a second layer 32 arranged on to the first layer and made of a solid cell material, in this case polystyrene. Wall portion 35 is obtained by arranging a 500 mm x 500 mm panel 65 on second wall layer 32 side of conventional wall portion 30. Panel 65 is made of polystyrene, and comprises a 20 mm thick back plate 66, from which 0,6 mm thick and 14 mm deep third strips 63 protrude perpendicularly and parallel to each other, so as to form recesses 69 between adjacent third strips 63. first 0,6 mm thick strips 61 of dielectric material are arranged on the parallel sides of recesses 69, while the remaining portion of recesses 69 is filled with polystyrene to form second strips 62.

[0085] Wall portions 30 and 35 have been arranged vertically on a base frame 43 rotatably arranged about an own vertical axis 44, as Figs. 44 and 45 show. A transceiver antenna 41 and a receiving antenna 42, both standard horn antennas, are arranged facing each other in diametrically opposite positions with respect to rotation axis 44, and are electrically connected to a control unit 40, comprising an Agilent Network Analyzer model PNA E8364B associated to a computer. Control unit 40 and transceiving antenna 41 are configured to generate and emit polarized circularly electromagnetic waves 1 of frequency set between 26.5 and 40 GFIz, in which the electric field vector is known by its components Ex and E _y . Control unit 40 and receiving antenna 42 are configured to receive circularly/elliptically polarized electromagnetic waves 1’ of the same frequency, and to measure the intensity of components Ex’ and E _y ’ of electric field vector E’. Control unit 40 is also arranged to provide an aberration ratio value, expressed in dB, based on the ratio between the magnitudes of components Ex’ and E _y ’. [0086] A plurality of measurements were carried out and to determine how the aberration ratio changes as the orientation of conventional wall portion 30 and of the wall portion according to the invention 35 changes. More in detail, the angles formed by the normal to wall portions 30 and 35 and the line connecting transceiver and receiving antennas 41 , 42, corresponding to azimuth angle Q of transceiving antenna 41 , was caused to change between -60° and +60°, and the field was measured in a substantially continuous way.

[0087] The measurements results are given in the diagram of Fig. 42 versus azimuth angle Q, indicating by a dashed line and a full line the results related to conventional wall portion 30 and to wall portion 35 according to the invention, respectively. From the diagram of Fig. 42, it is observed that the aberration ratio for wall portion conventional 30 is lower than 1 dB for azimuth angles set between about -20° and about +20°, while it is higher than 1 dB outside of this range. On the other hand, the aberration ratio for wall portion 35 according to the invention is higher than 1 dB for azimuth angles within a neighbourhood of 0, and is lower than 1 dB outside of this neighbourhood, in particular for values of azimuth angle Q in absolute value higher than 18°-20°. The plots of the aberration ratio for the two types of wall portions 30,35 meet at values of azimuth angle Q of about ±18°.

[0088] Therefore, a wall portion consisting of a central region having the same structure as conventional wall portion 30, i.e., in particular, one without parallel strips 61 , for azimuth values of transceiving antenna 41 in absolute value lower than 18°, for instance, and elsewhere consisting of peripheral regions having the same structure as the wall portion according to the invention, i.e. without parallel strips 61 , has aberration ratio values below 1 dB, for any azimuth angle Q, as shown in Fig. 43, this value being the practical limit beyond which a communication process involving circularly/elliptically polarized waves becomes troublesome, according to what previously explained in connection with the technical problem solved by the present invention.

[0089] The measurements relate to the case of a planar vertical wall and of an electromagnetic wave 1 emitted at a fixed elevation angle, however these results can be qualitatively extended, with similar results in connection with the aberration ratio, to the case of curved walls and/or walls at an angle with respect to the horizontal as occurs in the radome structures according to the prior art, for instance, in a radome as shown in Fig. 4 for housing a plurality of adjustable antennas 50,55 within respective angular fields, and can also qualitatively extended to the case of variable elevation angles, as further tests and measurements have confirmed.

[0090] The foregoing description exemplary specific embodiments will so fully reveal the invention according to the conceptual point of view, so that others, by applying current knowledge, will be able to modify and/or adapt for various applications such embodiment without further research and without parting from the invention and, accordingly, it is therefore to be understood that such adaptations and modifications will have to be considered as equivalent to the specific embodiment esemplified. The means and the materials to realise the different functions described herein could have a different nature without, for this reason, departing from the field of the invention. It is to be understood that the phraseology or terminology that is employed herein is for the purpose of description and not of limitation.