WO2020083478A1

WO2020083478A1 - Beam waveguide antenna system

Info

Publication number: WO2020083478A1
Application number: PCT/EP2018/079112
Authority: WO
Inventors: Roberto Giusto
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2018-10-24
Filing date: 2018-10-24
Publication date: 2020-04-30

Abstract

The invention relates to a beam waveguide system (8) for providing a beam of radiation and focusing the beam to a focal point of an antenna (4, 5). The beam waveguide system (8) comprises: a feeder assembly (6) configured to provide a primary beam of radiation generated as a spherical wave having its origin at a phase center of the feeder assembly (6); a first mirror (1) arranged and configured to receive the primary beam from the feeder 10 assembly (6), wherein the first mirror (1) defines a primary focal point on a first optical axis; and a second mirror (2) arranged and configured to receive the beam from the first mirror (1), wherein the second mirror (2) defines a secondary focal point on a second optical axis; wherein the primary focal point of the first mirror (1) coincides with the phase center of the feeder assembly (6) and the secondary focal point of the second mirror (2) 15 coincides with the focal point of the antenna (4, 5). Moreover, the invention relates to an antenna system (50) comprising such a beam waveguide system (8).

Description

DESCRIPTION

BEAM WAVEGUIDE ANTENNA SYSTEM

TECHNICAL FIELD

Generally, the present invention relates to the field of equipment for wireless

communications. More specifically, the present invention relates to a microwave beam waveguide antenna system and a method of operating such an antenna system.

BACKGROUND

Antenna systems providing a high gain of more than 47dBi are generally based on a main reflector dish with a diameter larger than 120 wavelengths. As the beam width of the antenna is usually smaller than 0.5 degrees, the pointing of the antenna beam requires tight precision. However, antennas with large reflectors are exposed to substantial wind loads and, in case, they are mounted on high towers or masts, their pointing is affected by deflections, sways and vibrations of their support structures. By way of example, an E- band microwave backhaul antenna, operating at 71 -86 GHz with a reflector dish diameter of 2 feet (660mm), i.e. about 170 wavelengths, performs 50dBi of gain and a beam width as narrow as 0.4 degrees such that one degree of deflection or sway of its supporting structure causes an unacceptable antenna de-pointing with 10dB of gain loss and a resulting downgrading of the backhaul radio-link antenna gain to 40dBi.

Approaches for pointing the beam of large diameter antennas with very high accuracy have been developed for satellite telecommunication ground stations or for large radio telescopes antennas using several different types of beam waveguide feed systems. The use of a beam waveguide feed for a large antenna station has several advantages over a conventional feed, because the large antenna dish can be mechanically moved, without using rotary joints, for repointing the antenna beam in any direction of a very wide angular sector. Consequently, the equipment room for such large antennas can be stationary and can host large transmitting and very complex receiving equipment.

In principle, a beam waveguide based on reflecting mirrors is frequency independent and, moreover, it enables the repointing of multiband beams in a limited angular sector by means of the suitable motion of such secondary mirrors, while leaving the main reflector dish stationary. Finally, the beam waveguide system is also suitable for orthogonal polarization frequency re-use applications, while mirror configurations comprising offset sections of paraboloids may create depolarization, enhancing the detrimental effect of inherent cross-polarization, due to an imperfect feedhorn, and the effects of

misalignments within the overall four-reflector beam waveguide system.

Conventional beam waveguide feed systems have the disadvantage of high complexity, making them unaffordable for low-cost microwave backhaul antennas. Moreover, the repointing speed of the antenna beam is usually very slow, such that fast repointing of microwave backhaul antennas and relevant beacon-tracking are not viable for

compensating the highest frequencies of vibrations of masts or towers up to frequencies as high as 5 Hz.

Thus, there is a need for an improved microwave beam waveguide antenna system and a method of operating such an antenna system.

SUMMARY

Embodiments of the invention are defined by the features of the independent claims. Further advantageous implementations of the embodiments are defined by the features of the dependent claims.

According to a first aspect the invention relates to a beam waveguide system for providing a beam of radiation and focusing the beam to a focal point of an antenna, in particular a dual-reflector antenna. The beam waveguide system comprises: a microwave feeder assembly configured to provide a primary beam of radiation having its origin at a phase center of the feeder assembly; a first mirror arranged and configured to receive the primary beam from the feeder assembly, wherein the first mirror defines a primary focal point on a first optical axis; and a second mirror arranged and configured to receive the beam from the first mirror, wherein the second mirror defines a secondary focal point on a second optical axis. The primary focal point of the first mirror coincides with, i.e. is located at the phase center of the feeder assembly and the secondary focal point of the second mirror coincides with, i.e. is located at the focal point of the antenna.

The primary beam of radiation may be generated as a spherical wave. Thus, an improved low complexity and low cost beam waveguide system for an antenna system is provided. As will be described below, embodiments of the invention enable a full antenna beam steering within a limited angular sector, while the main reflector dish and the microwave radio equipments are stationary, allowing to minimize the gain loss and other radiation pattern degradations due to the beam steering, and allowing to maximize the repointing speed of the antenna beam steering for enabling high speed automatic tracking of a microwave beacon by means of a simple conical scan of the antenna beam.

In a further possible implementation form of the first aspect, the first mirror is rotatable about a first rotation axis orthogonal to the first optical axis and the second mirror is rotatable about a second rotation axis orthogonal to the second optical axis, wherein the second rotation axis is substantially orthogonal to the first rotation axis.

In a further possible implementation form of the first aspect, the beam waveguide system further comprises a support system configured to support and rotate the first mirror about the first rotation axis and to support and rotate the second mirror about the second rotation axis.

In a further possible implementation form of the first aspect, the support system comprises a first electric actuator for rotating the first mirror about the first rotation axis and a second electric actuator for rotating the second mirror about the second rotation axis, wherein first electric actuator comprises a first rotor being fixed to the first mirror and a first stator being fixed to the static support system and wherein the second electric actuator comprises a second rotor being fixed to the second mirror and a second stator being fixed to the static support system.

In a further possible implementation form of the first aspect, the beam waveguide system further comprises a microwave collimator arranged and configured to receive the beam from the second mirror and to collimate the beam towards the antenna. The microwave collimator can be fixed to the support system.

In a further possible implementation form of the first aspect, the microwave collimator comprises a rotationally symmetric collimating lens with an equivalent focal length of about 2 /₃, a quasi-parabolic gridded mirror collimator with a focal length of about 2 f₃ or a dual-reflector collimator system comprising two quasi-parabolic mirrors with a focal length of each of the two quasi-parabolic mirrors of about 4 f₃, wherein /₃ denotes the focal length of a parabolic approximation of a focal surface defined by set of positions described by the secondary focal point of the second mirror , when the first mirror is rotated about the first rotation axis and/or the second mirror is rotated about the second rotation axis. This surface is also known as Petzval's locus.

In a further possible implementation form of the first aspect, the first mirror and/or the second mirror is a quasi-parabolic mirror.

In a further possible implementation form of the first aspect, the first mirror and the second mirror are arranged such that the primary focal point of the first mirror coincides with the phase center of the feeder assembly and the secondary focal point of the second mirror coincides with the focal point of the antenna. In particular, the first mirror and the second mirror may be arranged in an offset Dragonian configuration so that the above condition is fulfilled.

In a further possible implementation form of the first aspect, the feeder assembly is arranged relative to the first mirror in a side-fed configuration or a front-fed configuration.

In a further possible implementation form of the first aspect, the feeder assembly comprises a multiband feeder assembly or a single-band feed horn.

In a further possible implementation form of the first aspect, the first mirror has a first focal length fa and the second mirror has a second focal length f₂ different from the first focal length

In a further possible implementation form of the first aspect, the ratio between the first focal length and the second focal length f₂ obeys the following equation:

wherein denotes the offset angle of the first mirror and q₂ denotes the offset angle of the second mirror relative to the first optical axis or the second optical axis.

In a further possible implementation form of the first aspect, the beam waveguide system further comprises a frequency selective screen arranged between the feeder assembly and the first mirror. In a further possible implementation form of the first aspect, the frequency selective screen is transparent at frequencies of the primary beam provided by the feeder assembly. However, said frequency selective screen behaves like a reflecting mirror at other frequencies of the primary beam provided by the multiband feeder assembly.

According to a second aspect the invention relates to an antenna system, comprising: a dual-reflector antenna defining an antenna axis and an antenna focal point; and a beam waveguide system according to the first aspect of the invention for providing a beam of radiation and focusing the beam to the focal point of the dual-reflector antenna.

In a further possible implementation form of the second aspect, the dual reflector antenna comprises a sub-reflector and a main reflector, wherein the microwave collimator is configured to collimate the beam towards the sub-reflector.

In a further possible implementation form of the second aspect, the dual reflector antenna is a bi-focal or multi-focal dual reflector antenna.

In a further possible implementation form of the second aspect, the dual reflector antenna comprises: a quasi-parabolic main reflector defining a focal point on an antenna axis; and a Cassegrain sub-reflector arranged on the antenna axis at a location between a vertex of the quasi-parabolic main reflector and the focal point of the quasi-parabolic main reflector, wherein the Cassegrain sub-reflector defines a virtual focal point on the antenna axis; or a Gregorian sub-reflector arranged on the antenna axis at a location between a vertex of the quasi-parabolic main reflector and the focal point of the quasi-parabolic main reflector, wherein the Gregorian sub-reflector defines a real focal point on the antenna axis.

In a further possible implementation form of the second aspect, the dual reflector

Cassegrain or Gregorian antenna is made of an axially displaced quasi-parabolic main reflector such that its focus is spread in a ring locus (instead of being a simple focal point) and of an axially displaced sub-reflector matching the ring focus of this main reflector and the secondary focal point defined by the second mirror of the beam waveguide system. According to a third aspect the invention relates to a method of providing the beam waveguide system configured to provide a beam of radiation and focusing the beam to a focal point of an antenna. The method comprises the steps of: providing a microwave feeder assembly configured to provide a primary beam of radiation generated as a spherical wave having its origin at a phase center of the feeder assembly; providing a first mirror arranged and configured to receive the primary beam from the feeder assembly, wherein the first mirror defines a primary focal point on a first optical axis and wherein the primary focal point of the first mirror coincides with the phase center of the feeder assembly; and providing a second mirror arranged and configured to receive the beam from the first mirror, wherein the second mirror defines a secondary focal point on a second optical axis and wherein the secondary focal point of the second mirror coincides with the focal point of the antenna.

Details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description, drawings, and claims.

Generally, embodiments of the invention improve on the complex design of conventional systems allowing minimizing the total number of components of the beam waveguide system. Thus, in embodiments only two secondary mirrors are required, instead of conventional four-mirror configurations. Indeed, two mirrors is the minimum possible configuration of offset sections of paraboloids, enabling the required antenna beam full steering within a limited angular sector.

Moreover, embodiments of the invention are more compact due the folded-configuration of the two offset mirrors, which is "fully compensated", i.e. depolarization-free, as it is equivalent to an axis-symmetric optical configuration. According to embodiments of the invention the first and the second mirror are "side-fed" by the feedhorn in a very compact assembly. Moreover, the proper design of the relevant offset sections of paraboloids enables optical magnification, which, in turn, allows reducing the dimensions of the feedhorn according to the magnification factor. Thus, the beam waveguide system according to embodiments of the invention can achieve high performances, while being enclosed in a minimum volume, for the following main reasons: (1 ) the mirror assembly can be folded, similarly to oysters, having their two valves slightly open; (2) the preferred "side-fed" allocation of the feedhorn is very compact; and (3) the dimensions of the feedhorn can be further reduced by the magnification factor relevant to the offset mirrors optical setup.

Moreover, embodiments of the invention address alignment complexity not only in consequence of the small number of parts, but also because both the first and second mirrors are firmly hinged with high precision along their axis of rotation. Moreover, embodiments of the invention provide a fast repointing or tracking, because the first and second mirror can be firmly hinged near their center of mass, which, in turn, can deal with very high frequency mechanical vibrations of each mirror along its own axis of rotation. The axis of rotation of each mirror is orthogonal and independent with respect to the axis of the other mirror, such that one of them corresponds to steer the azimuth while the other steer the elevation beam pointing of the antenna. Moreover, in order to get either fastest motion or highest frequency of vibration of these two mirrors, their moments of Inertia can be minimized by design methods and by choice of suitable materials.

In embodiments of the invention the beam waveguide system is essentially based on the following quasi-optical elements, namely two mirrors, in particular quasi-parabolic rotatable mirrors, which can be arranged together with a microwave feeder in a configuration previously known as "Side-Fed Dragonian Antenna", as well as optionally a collimator, which is static and can be fixed to the antenna support structure. According to embodiments of the invention the collimator performs with negligible loss, negligible frequency dispersion and negligible depolarization at the microwave frequencies of operation of the antenna system. The first and second rotatable mirror enables a free repointing of the microwave antenna beam within a required angular sector. For embodiments of the invention the speed of the beam steering may achieve very high values for the following reasons. Firstly, each mirror is rotatable along one axis near its center of mass, thus it is firmly hinged to the antenna support structure and its moment of Inertia can be minimized by design and by choice of suitable lightweight manufacturing materials. Secondly, each mirror can be integrated together with the "rotor" of the electric actuator for rotating the mirror itself, while the corresponding "stator", which can include the heaviest parts of the actuator, can be fixed to the antenna support structure.

Thus, embodiments of the invention provide a high-performance dual-reflector antenna system capable of high-speed beam steering and of fast tracking of a microwave beacon signal comprising the following components: two rotatable mirrors, a static collimator and a multiband feeder assembly (or, alternatively, a single-band feedhorn); as well as a static sub-reflector and a static main reflector, as typical components needed for a large antenna with a dual-reflector configuration, such as a Cassegrain or Gregorian configuration. Conventionally, the side-fed configuration made of an hyperboloidal sub reflector together with a paraboloidal main reflector has been used for the design of dual offset reflectors known as "Dragonian Antennas" or "fully-compensated" dual-offset antennas, because they achieve minimum cross-polarization and very low spill over losses. "Dragonian antennas", either with a side-fed configuration or a front-fed configuration, also provide a wide-angle capability of beam steering, because they are intrinsically free of astigmatism-coma aberrations.

Embodiments of the invention allow exploiting the feeder magnification and the quasi- optical lowest-loss propagation for illuminating the sub-reflector with extremely high efficiency and extremely low spill over. These feeder magnification characteristics allow low- scattering of microwave energy at the edge of the sub-reflector, thus the adoption of a small-diameter sub-reflector is viable, enabling low-blockage and very low sidelobes in the radiation pattern envelope (RPE).

For embodiments employing a frequency selective screen or surface (FSS) the FSS is a planar low-loss and low-cost component, which enables nearly perfect dual-sharing of previous advantages at two different microwave frequencies also in case the ratio of frequencies is equal or even lower than 2. In case the ratio of frequencies is larger than 2.5, the FSS component is not required, because a low-complexity coaxial feeder can be placed at a focal point F1. Indeed, when the ratio of frequencies reduces from 2.5 to 2 or even to smaller value, the sidelobes of this feeder are increasing more and more only at the lower frequency of operation. It is noted that these high sidelobes could reduce the feeder efficiency with a direct impact on the antenna gain and efficiency at the lower frequency of operation, but without affecting the "class4 RPE" compliance. Indeed, these high sidelobes would not hit the first mirror and, thus, their microwave energy could be absorbed by some absorbing material placed around the edges of the first mirror.

However, these sidelobes, as resulting from the "spatially filtering" by the first mirror, will advantageously not have an impact on the overall“class4 RPE” performance of the antenna other than an affordable gain loss. BRIEF DESCRIPTION OF THE DRAWINGS

In the following embodiments of the invention are described in more detail with reference to the attached figures and drawings, in which:

Fig. 1 A shows a perspective side view of several components of an antenna system according to an embodiment of the invention comprising a beam waveguide system according to an embodiment of the invention;

Fig. 1 B shows a bottom view of several components of an antenna system according to an embodiment of the invention;

Fig. 1 C shows a bottom view of several components of an antenna system according to an embodiment of the invention;

Fig. 2A shows a side view of several components of an antenna system according to an embodiment of the invention;

Fig. 2B shows a side view of several components of an antenna system according to an embodiment of the invention;

Fig. 2C shows a side view of several components of an antenna system according to an embodiment of the invention;

Figs. 3A, 3B and 3C show exemplary ray distributions in antenna systems according to embodiments of the invention;

Fig. 4A shows a cross-sectional side view of several components of a beam waveguide system according to an embodiment of the invention;

Fig. 4B and 4C show cross-sectional side views of the mirrors of the beam waveguide system of figure 4A;

Fig. 5A shows a perspective view of the beam waveguide system of figure 4A;

Fig. 5B and 5C show side views of the beam waveguide system of figure 4A; Fig. 5D and 5E show perspective views of the mirror supports of the beam waveguide system of figure 4A;

Fig. 6 shows a schematic view of several components of an antenna system according to an embodiment of the invention comprising a beam waveguide system and an axially displaced dual reflector according to an embodiment of the invention; and

Fig. 7 is a flow diagram showing an example of processing steps of a method for providing a beam waveguide system according to an embodiment of the invention.

In the following identical reference signs refer to identical or at least functionally equivalent features.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following description, reference is made to the accompanying figures, which form part of the disclosure, and which show, by way of illustration, specific aspects of embodiments of the invention or specific aspects in which embodiments of the present invention may be used. It is understood that embodiments of the invention may be used in other aspects and comprise structural or logical changes not depicted in the figures. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.

For instance, it is understood that a disclosure in connection with a described method may also hold true for a corresponding device or system configured to perform the method and vice versa. For example, if one or a plurality of specific method steps are described, a corresponding device may include one or a plurality of units, e.g. functional units, to perform the described one or plurality of method steps (e.g. one unit performing the one or plurality of steps, or a plurality of units each performing one or more of the plurality of steps), even if such one or more units are not explicitly described or illustrated in the figures. On the other hand, for example, if a specific apparatus is described based on one or a plurality of units, e.g. functional units, a corresponding method may include one step to perform the functionality of the one or plurality of units (e.g. one step performing the functionality of the one or plurality of units, or a plurality of steps each performing the functionality of one or more of the plurality of units), even if such one or plurality of steps are not explicitly described or illustrated in the figures. Further, it is understood that the features of the various exemplary embodiments and/or aspects described herein may be combined with each other, unless specifically noted otherwise.

Figure 1 A shows a perspective side view of several components of an antenna system 50 according to an embodiment of the invention comprising a beam waveguide system 8 according to an embodiment of the invention. A corresponding bottom view is shown in figure 1 B. The beam waveguide system 8 is configured to provide a beam of radiation and focusing the beam to a focal point of an antenna defining an antenna axis and an antenna focal point of the antenna system 50.

In the embodiments shown in the figures, the antenna of the antenna system 50 is a dual reflector antenna comprising a main reflector 5 and a sub-reflector 4 defining an antenna axis and an antenna focal point. In an embodiment, the dual reflector antenna 4, 5 is a bi focal or multi-focal dual reflector antenna 4, 5.

In an embodiment, the dual reflector antenna 4, 5 comprises a quasi-parabolic main reflector 5 defining a focal point on the antenna axis and a Cassegrain sub-reflector 4 arranged on the antenna axis at a location between a vertex of the quasi-parabolic main reflector 5 and the focal point of the quasi-parabolic main reflector 5, wherein the

Cassegrain sub-reflector 4 defines a virtual focal point on the antenna axis. Alternatively, a Gregorian sub-reflector can be used instead of the Cassegrain sub-reflector, wherein the Gregorian sub-reflector 4 defines a real focal point on the antenna axis.

The beam waveguide system 8 shown in figures 1 A and 1 B comprises a microwave feeder assembly 6 configured to provide a primary beam of radiation generated as a spherical wave having its origin at a phase center of the microwave feeder assembly 6. The microwave feeder assembly 6 can comprise a multiband feeder assembly or a single band feed horn.

Moreover, the beam waveguide system 8 comprises a first mirror 1 , in particular first quasi-parabolic mirror 1 and a second mirror 2, in particular second quasi-parabolic mirror 2.

The first mirror 1 is arranged and configured to receive the primary beam from the feeder assembly. The first mirror 1 defines a primary focal point on a first optical axis and can be rotatable about a first rotation axis orthogonal to the first optical axis. The primary focal point of the first mirror 1 coincides with the phase center of the feeder assembly 6.

The second mirror 2 is arranged and configured to receive the beam from the first mirror 1. The second mirror 2 defines a secondary focal point on a second optical axis and can be rotatable about a second rotation axis orthogonal to the second optical axis, wherein the second rotation axis is substantially orthogonal to the first rotation axis. The secondary focal point of the second mirror 2 coincides with the focal point of the antenna 4, 5.

As a result of the optional rotational motions of the first mirror 1 and the second mirror 2 combined together, the beam of the antenna 4, 5 can be steered in any direction with reduced aberrations and losses.

In an embodiment, as illustrated in the figures, the first mirror 1 and the second mirror 2 are arranged in an offset Dragonian configuration such that the primary focal point of the first mirror 1 coincides with the phase center of the feeder assembly 6 and the secondary focal point of the second mirror 2 coincides with the focal point of the antenna 4, 5.

In an embodiment, the feeder assembly 6 can be arranged relative to the first mirror 1 in a side-fed configuration (as illustrated in the figures) or a front-fed configuration.

Moreover, the beam waveguide system 8 can comprise a microwave collimator 3 arranged and configured to receive the beam from the second mirror 2 and to collimate the beam towards the antenna 4, 5 of the antenna system. For allowing steering the antenna beam to increasing angles, the microwave collimator 3 can minimize any spill over such that the maximum antenna efficiency and gain are actually achieved. As will be described in more detail below, the microwave collimator 3 can be a static component fixed to an antenna support system, which can also support the other components of the beam waveguide system as well as the sub-reflector 4 of the dual-reflector antenna 4, 5. For very small steering angles the microwave collimator 3 advantageously is essentially "transparent", i.e. not degrading the antenna performance. Thus, in an advantageous embodiment, the microwave collimator 3 is depolarization-free and/or provides minimal losses at the frequencies required for the antenna operation. This can be provided by the microwave collimator 3 implemented as a rotationally symmetric microwave lens 3 having both faces properly shaped in order to minimize the spill over losses at the edges of the sub-reflector 4 during antenna beam steering. Corresponding embodiments are shown, for instance, in figures 2C, 4A, 5A to 5C.

Alternatively, the microwave collimator 3 can be implemented as a "one-gridded-mirror", as illustrated in the embodiments of, for instance, figures 1 A, 1 B, 3A and 3B, which can be advantageous for single linear polarization operation.

According to a further alternative, the microwave collimator 3 can be provided by a "two- mirror assembly", as illustrated in the embodiment of, for instance, figure 3C.

The embodiment of the microwave collimator 3 shown in figures 2C, 4A, 5A to 5C, i.e. the "shaped-lens collimator embodiment", has the advantage that microwave collimator 3 has an advantageous position between the vertex of the main reflector 5 and the tip of the sub-reflector 4 such that the edges of this microwave collimator lens 3 can be further extended and shaped for creating a protective radome (not shown in figure 2C for the sake of clarity and not visible in other figures), i.e. a rotationally symmetric waterproof enclosure made of non-reflective materials, which isolates all components of the beam waveguide system 8 from the external environment.

The embodiment of the microwave collimator 3 shown in figure 3C, i.e. the "two-mirror collimator" embodiment, can be advantageous when dealing either with multiband operation or with very high Terahertz frequencies, because the frequency dispersion and the loss of these mirrors are much lower than the ones of a shaped-lens made of dielectric materials, which usually are neither dispersion-free nor perfectly transparent at microwave frequencies.

The embodiment of the collimator 3 shown in figures 1 A, 1 B, 3A and 3B, i.e. the "one- gridded mirror collimator" embodiment, can provide for a third reflection in the beam waveguide system 8 so that an extremely compact 3D-folded assembly can be achieved, as illustrated, for instance, in figures 1 A and 1 B.

In an embodiment, the beam waveguide system 8 further comprises a support system configured to support and rotate the first mirror 1 about the first rotation axis and to support and rotate the second mirror 2 about the second rotation axis. A corresponding support system is illustrated in figures 4A, 5A to 5E. As will be appreciated, in the embodiments illustrated in figures 4A, 5A to 5E the microwave collimator 3 is implemented as a shaped-lens collimator 3. As can be taken from figures 4A, 5A to 5E the feedhorn 6 is fixed to the static support system of the beam waveguide system 8, while the mirrors 1 and 2 are rotatable.

In an embodiment, the support system comprises a first electric actuator for rotating the first mirror 1 about the first rotation axis and a second electric actuator for rotating the second mirror 2 about the second rotation axis. The first electric actuator can comprise a first rotor being fixed to the first mirror 1 and a first stator being fixed to the static support system. Likewise, the second electric actuator can comprise a second rotor being fixed to the second mirror 2 and a second stator being fixed to the static support system. As already mentioned above, in an embodiment, also the microwave collimator 3 can be fixed to the static support system.

In the embodiments shown in figures 4A, 5A to 5E the first mirror 1 is integrated together with an electric actuator, whose "rotor" is made of two coils 101 and 102, while the "stator 1 1 1 is static and fixed to the support structure. For the second mirror 2 with the relevant coil 201 the hinges 221 and 222 enabling a "one-axis" rotation as well.

Thus, figures 4A, 5A to 5E illustrate: the rotatable quasi-parabolic first mirror 1 with relevant hinge 121 , hinge 122 and actuator coils 101 , 102; the rotatable quasi-parabolic second mirror 2 with relevant hinge 221 (corresponding hinge 222 not shown) and coils 201 , 202; the stator 21 1 , 212 of the electric actuator belonging to the second mirror 2; the shaped-lens microwave collimator 3; and the feedhorn 6.

As already described above, in an embodiment, the microwave collimator 3 comprises a rotationally symmetric collimating lens. According to an embodiment this lens can have an equivalent focal length of about 2 /₃, wherein /₃ denotes the focal length of a parabolic approximation of a focal surface (also known as Petzval's locus) defined by the set of positions described by the secondary focal point of the second mirror 2, when the first mirror 1 is rotated about the first rotation axis and/or the second mirror 2 is rotated about the second rotation axis.

In case the microwave collimator 3 is implemented as a quasi-parabolic gridded mirror collimator, according to an embodiment the collimator can have a focal length of about 2 h - In case the microwave collimator 3 is implemented as a dual-reflector collimator system comprising two quasi-parabolic mirrors, according to an embodiment the focal length of each of the two quasi-parabolic mirrors is about 4 f₃.

In an embodiment, the first mirror 1 has a first focal length and the second mirror 2 has a second focal length f₂ different from the first focal length

In an embodiment, the ratio between the first focal length fa and the second focal length f₂ obeys the following equation:

wherein denotes the offset angle of the first mirror 1 and q₂ denotes the offset angle of the second mirror 2 relative to the first optical axis or the second optical axis. The corresponding spatial arrangement of the first and second mirror 1 , 2 with respect to each other is also known as "fully-compensated" offset configuration.

Figures 2A and 6 illustrate the operation of the beam waveguide system 8 implemented respectively in a Cassegrain antenna system 50 and in an Axially Displaced Gregorian dual-band antenna with frequency selective screen (FSS), when there is no beam steering. In this special case, the beam waveguide 8 does not comprise a microwave collimator, as can be taken from figures 2A and 6. As illustrated in the embodiment of figure 2A, the feedhorn 6 is positioned at the primary focal point 10 of the first mirror 1 (defining a focal length f ) and the second mirror 2 is oriented with respect to the first mirror 1 so that the beam from the feedhorn 6 is collimated to the secondary focal point 22 of the second mirror 2.

In embodiments without the collimator 3 the virtual phase center of the Cassegrain antenna 4, 5 corresponds to the secondary focal point 20 of the second mirror 2 (defining a focal length f₂) such that the magnification factor is f f₂ (which means that the virtual feeder is enlarged, that is magnified by this scaling factor f_x/f₂ and results more directive than the real feeder 6 placed in the primary focal point 10 of the first mirror 1 ). In figure 2B the second mirror 2 has been rotated a few degrees about its center of mass such that the virtual phase center of the feeder 6 is displaced from the Cassegrain virtual focus 20, as shown in figure 2A, to the new focus position 21 of the second mirror 2. Moreover, figure 2B depicts also the proportional deviation and spill over of the rays 31 , while no deviation and spill over was affecting the rays 30 depicted in figure 2A. Figure 2C illustrates how the microwave collimator 3 allows recovering the spill over of rays 32, while the position of the focus 22 does not change with respect to the previous focus position illustrated in figure 2B, thus keeping the same antenna beam deviation.

In an embodiment, the respective diameters of the first mirror 1 and the second mirror 2 can be designed preferably as large as about 20 wavelengths. As already described above, the mirrors 1 , 2 can be offset sections of quasi-paraboloids and their focal lengths can be chosen according to a "fully-compensated" offset-configuration of the "side-fed" type. Thus, embodiments of the invention provide a highly compact "2-mirror" beam waveguide system 8, which can be folded similarly to the two valves of a slightly open oyster. Such embodiments provide a sufficient quasi-optical magnification of the microwave feedhorn 6 and a further saving of space. Each mirror is movable with high accuracy, as it can be firmly hinged along its rotation axis, which is orthogonal to the rotation axis of the other mirror. Thus, very high speed alternate rotations or high frequency mechanical vibrations of the first and second mirror 1 , 2 said two mirrors can be sustained, as can be taken from the embodiments shown in figures 4A, 5A, 5B, and 5C.

Figure 6 shows a further embodiment of the beam waveguide system 8 and the antenna system 50. In the embodiment shown in figure 6 the beam waveguide system further comprises a frequency selective screen or surface arranged between the feeder assembly 6 and the first mirror 1 . This feeder assembly 6 is made of two feedhorns operating at two different frequencies. The frequency selective screen can be transparent at frequencies of the primary beam provided by one of these feeder and can be reflective at frequencies of the other feeder belonging to the assembly 6. In other words, for a dual-frequency operation the frequency selective screen can be perfectly transparent at the frequency of the microwave feeder 6 placed in the primary focus F1 , while it is perfectly reflecting at the frequency of the microwave feeder 6 placed in the other primary focus F1 ', i.e. for the other operation frequency. With this configuration, based on FSS, the primary focus of mirror 1 has been provided with two feeders operating at different frequency bands in the same virtual point.

The FSS can be a planar low-loss and low-cost component, which allows advantageous operation at two different microwave frequencies also in case the ratio of frequencies is equal or even lower than 2. In case the ratio of frequencies is larger than 2.5, the FSS component is not required, because a low-complexity coaxial feeder 6 can be placed at the focal point F1. Indeed, when the ratio of frequencies reduces from 2.5 to 2 or even to smaller value, the sidelobes of this feeder 6 are increasing more and more only at the lower frequency of operation. It is noted that these high sidelobes could reduce the feeder efficiency with a direct impact on the antenna gain and efficiency at the lower frequency of operation, but without affecting the "class4 RPE" compliance. Indeed, these high sidelobes would not hit the first mirror 1 and, thus, their microwave energy could be absorbed by some absorbing material placed around the edges of the first mirror 1 .

However, these sidelobes, as resulting from the "spatially filtering" by the first mirror 1 , will advantageously not have an impact on the overall“class4 RPE” performance of the antenna system 50 other than an affordable gain loss. Finally, as far as the dual reflector antenna embodiment is concerned, figure 6 shows the axially displaced main and sub reflector Gregorian configuration, such that the secondary focal point of mirror 2 is transformed in a“ring focus” by the axially displaced sub-reflector in order to properly match the“ring focal locus” of the axially displaced main reflector.

Figure 7 is a flow diagram showing an example of processing steps of a method 700 for providing the beam waveguide system 8 according to an embodiment of the invention.

The method 700 comprises the steps of: providing 701 the microwave feeder assembly 6 configured to provide a primary beam of radiation generated as a spherical wave having its origin at a phase center of the feeder assembly 6; providing 703 the first mirror 1 arranged and configured to receive the primary beam from the feeder assembly 6, wherein the first mirror 1 defines a primary focal point on a first optical axis and wherein the primary focal point of the first mirror 1 coincides with the phase center of the feeder assembly 6; and providing 705 a second mirror 2 arranged and configured to receive the beam from the first mirror 1 , wherein the second mirror 2 defines a secondary focal point on a second optical axis and wherein the secondary focal point of the second mirror 2 coincides with the focal point of the antenna 4, 5.

In the following, some further aspects of the invention will be described as design steps, which can be advantageously used in designing the beam waveguide assembly 8 according to embodiments of the invention and in particular for selecting the focal length /₃ of the microwave collimator 3. For designing the beam waveguide assembly 8 according to embodiments of the invention computer-aided procedures can be used. First step: design a Cassegrain antenna comprising the sub-reflector 4 and the main reflector 5 for sustaining low scanning loss in the required steering angle interval using a bi-focal/multi-focal computer-aided procedure.

Second step: design the feeder assembly 6 for multi-band or for single-band operation, as required.

Third step: select/determine the magnification factor associated with the quasi-optical rotatable mirrors 1 and 2 taking into account the mechanical clearance required for rotating these mirrors and the clearance due to the dimensions of the mouth of the feeder assembly 6.

Fourth step: revise all previous design steps 1 to 3 until convergence is reached for the shaped-surfaces of the sub-reflector 4, the main reflector 5, the first mirror 1 , and the second mirror 2, preferably providing the best performances of steered beams in the required angular sector and keeping the dimensions of the feeder assembly 6 as small as possible.

Fifth step: compute the focal length f₃ of the parabolic approximation of the Petzval’s locus defined by the F2 focal points, i.e. 20, 21 , 22, while the rotatable mirrors 1 and 2 are moved for steering the antenna beam in the required angular sector.

Sixth step: either as reference design or for single-polarization operation, a parabolic-grid single-mirror collimator 3 can be preferred, as depicted, for instance, in figures 3A and 3B, whose focal length can be selected as 2 f₃. For dual-polarization operation a shaped- lens collimator 3 as depicted in figure 2C can be used, whose shaped-surfaces create an equivalent focal length equal to 2 f₃. Alternatively, choose a two quasi-parabolic mirrors collimator 3, as depicted in figure 3c, where each of said mirrors has a focal length equal to 4 - /₃.

Seventh step: revise the design of the selected collimator 3 until optimization and compliance of the performances for all beam-steering conditions of the antenna system 50 is reached.

While a particular feature or aspect of the disclosure may have been disclosed with respect to only one of several implementations or embodiments, such feature or aspect may be combined with one or more other features or aspects of the other implementations or embodiments as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms "include", "have", "with", or other variants thereof are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term "comprise". Also, the terms "exemplary", "for example" and "e.g." are merely meant as an example, rather than the best or optimal. The terms "coupled" and "connected", along with derivatives may have been used. It should be understood that these terms may have been used to indicate that two elements cooperate or interact with each other regardless whether they are in direct physical or electrical contact, or they are not in direct contact with each other.

Although specific aspects have been illustrated and described herein, it will be

appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific aspects shown and described without departing from the scope of the present disclosure. This application is intended to cover any adaptations or variations of the specific aspects discussed herein.

Although the elements in the following claims are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those elements, those elements are not necessarily intended to be limited to being implemented in that particular sequence.

Many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the above teachings. Of course, those skilled in the art readily recognize that there are numerous applications of the invention beyond those described herein. While the present invention has been described with reference to one or more particular embodiments, those skilled in the art recognize that many changes may be made thereto without departing from the scope of the present invention. It is therefore to be understood that within the scope of the appended claims and their equivalents, the invention may be practiced otherwise than as specifically described herein.

Claims

1. A beam waveguide system (8) for providing a beam of radiation and focusing the beam to a focal point of an antenna (4, 5), wherein the beam waveguide system (8) comprises: a feeder assembly (6) configured to provide a primary beam of radiation having its origin at a phase center of the feeder assembly (6); a first mirror (1 ) arranged and configured to receive the primary beam from the feeder assembly (6), wherein the first mirror (1 ) defines a primary focal point (20) on a first optical axis; a second mirror (2) arranged and configured to receive the beam from the first mirror (1 ), wherein the second mirror (2) defines a secondary focal point on a second optical axis wherein the primary focal point (20) of the first mirror (1 ) coincides with the phase center of the feeder assembly (6) and the secondary focal point of the second mirror (2) coincides with the focal point of the antenna (4, 5).

2. The beam waveguide system (8) of claim 1 , wherein the first mirror (1 ) is rotatable about a first rotation axis orthogonal to the first optical axis and the second mirror (2) is rotatable about a second rotation axis orthogonal to the second optical axis, wherein the second rotation axis is substantially orthogonal to the first rotation axis.

3. The beam waveguide system (8) of claim 2, wherein the beam waveguide system (8) further comprises a support system configured to support and rotate the first mirror (1 ) about the first rotation axis and to support and rotate the second mirror (2) about the second rotation axis.

4. The beam waveguide system (8) of claim 3, wherein the support system comprises a first electric actuator for rotating the first mirror (1 ) about the first rotation axis and a second electric actuator for rotating the second mirror (2) about the second rotation axis, wherein first electric actuator comprises a first rotor being fixed to the first mirror (1 ) and a first stator being fixed to the support system and wherein the second electric actuator comprises a second rotor being fixed to the second mirror (2) and a second stator being fixed to the support system.

5. The beam waveguide system (8) of any one of the preceding claims, wherein the beam waveguide system (8) further comprises a collimator (3) arranged and configured to receive the beam from the second mirror (2) and to collimate the beam towards the antenna (4, 5).

6. The beam waveguide system (8) of claim 5 when dependent on claim 3 or 4, wherein the collimator (3) is fixed to the support system.

7. The beam waveguide system (8) of claim 6, wherein the collimator (3) comprises a collimating lens with an equivalent focal length of about 2 /₃, a quasi-parabolic gridded mirror collimator with a focal length of about 2 f₃ or a dual-reflector collimator system comprising two quasi-parabolic mirrors with a focal length of each of the two quasi parabolic mirrors of about 4 /₃, wherein /₃ denotes the focal length of a parabolic approximation of a focal surface defined by a set of positions described by the secondary focal point of the second mirror (2), when the first mirror (1 ) is rotated about the first rotation axis and/or the second mirror (2) is rotated about the second rotation axis.

8. The beam waveguide system (8) of any one of the preceding claims, wherein the first mirror (1 ) and/or the second mirror (2) is a quasi-parabolic mirror.

9. The beam waveguide system (8) of any one of the preceding claims, wherein the first mirror (1 ) and the second mirror (2) are arranged such that the primary focal point of the first mirror (1 ) coincides with the phase center of the feeder assembly (6) and the secondary focal point of the second mirror (2) coincides with the focal point of the antenna (4, 5).

10. The beam waveguide system (8) of any one of the preceding claims, wherein the feeder assembly (6) is arranged relative to the first mirror (1 ) in a side-fed configuration or a front-fed configuration.

1 1. The beam waveguide system (8) of any one of the preceding claims, wherein the feeder assembly (6) comprises a multiband feeder assembly or a single-band feed horn (6).

12. The beam waveguide system (8) of any one of the preceding claims, wherein the first mirror (1 ) has a first focal length fa and the second mirror (2) has a second focal length f₂ different from the first focal length

13. The beam waveguide system (8) of claim 12, wherein the ratio between the first focal length and the second focal length f₂ obeys the following equation:

wherein denotes the offset angle of the first mirror (1 ) and q₂ denotes the offset angle of the second mirror (2) relative to the first optical axis or the second optical axis.

14. The beam waveguide system (8) of any one of the preceding claims, wherein the beam waveguide system (8) further comprises a frequency selective screen arranged between the feeder assembly (6) and the first mirror (1 ).

15. The beam waveguide system (8) of claim 14, wherein the frequency selective screen is transparent and reflective at different frequencies of the primary beam provided by the feeder assembly (6).

16. An antenna system (50), comprising: a dual reflector antenna (4, 5), defining an antenna axis and an antenna focal point; and a beam waveguide system (8) according to any one of the preceding claims for providing a beam of radiation and focusing the beam to the focal point of the dual reflector antenna (4, 5).

17. The antenna system (50) of claim 16, wherein the dual reflector antenna (4, 5) comprises a sub-reflector (4) and a main reflector (5) and wherein the collimator (3) is configured to collimate the beam towards the sub-reflector (4).

18. The antenna system (50) of claim 16 or 17, wherein the dual reflector antenna (4, 5) is a multi-focal, in particular bi-focal dual reflector antenna (4, 5).

19. The antenna system (50) of any one of claims 16 to 18, wherein the dual reflector antenna (4, 5) comprises: a quasi-parabolic main reflector (5) defining a focal point on an antenna axis; and a Cassegrain sub-reflector (4) arranged on the antenna axis at a location between a vertex of the quasi-parabolic main reflector (5) and the focal point of the quasi-parabolic main reflector (5), wherein the Cassegrain sub-reflector (4) defines a virtual focal point on the antenna axis; or a Gregorian sub-reflector (4) arranged on the antenna axis at a location between a vertex of the quasi-parabolic main reflector (5) and the focal point of the quasi-parabolic main reflector (5), wherein the Gregorian sub-reflector (4) defines a real focal point on the antenna axis.

20. The antenna system (50) of claim 19, wherein the focal point of the quasi-parabolic main reflector (5) is spread into a ring locus and wherein the Gregorian sub-reflector (4) matches the secondary focal point of the second mirror (2) of the beam waveguide system (8) to the ring locus of the quasi-parabolic main reflector (5).

21. A method (700) of providing a beam waveguide system (8) configured to provide a beam of radiation and focusing the beam to a focal point of an antenna (4, 5), wherein the method (600) comprises: providing (701 ) a feeder assembly (6) configured to provide a primary beam of radiation generated as a spherical wave having its origin at a phase center of the feeder assembly (6); providing (703) a first mirror (1 ) arranged and configured to receive the primary beam from the feeder assembly (6), wherein the first mirror (1 ) defines a primary focal point on a first optical axis, wherein the primary focal point of the first mirror (1 ) coincides with the phase center of the feeder assembly (6); and providing (705) a second mirror (2) arranged and configured to receive the beam from the first mirror (1 ), wherein the second mirror (2) defines a secondary focal point on a second optical axis and wherein the secondary focal point of the second mirror (2) coincides with the focal point of the antenna (4, 5).