EP1414110A1

EP1414110A1 - Steerable antenna system with fixed feed source

Info

Publication number: EP1414110A1
Application number: EP20020292622
Authority: EP
Inventors: Eric Amyotte; Martin Gimersky; Jean-Daniel Richerd
Original assignee: EMS Technologies Canada Ltd
Current assignee: EMS Technologies Canada Ltd
Priority date: 2002-10-23
Filing date: 2002-10-23
Publication date: 2004-04-28

Abstract

A C-shaped frame (22) rotatably mounted on a support structure (12), supports a hyperbolic sub reflector (20), a dual frequency dual circular polarization feed source (30) and a parabolic reflector (40) along source axis (A), and a planar reflector (50) along beam axis (B) perpendicular to the source axis. The stepper motors (24,52) rotate the frame along source and beam axes, respectively.

Description

FIELD OF THE INVENTION

The present invention relates to the field of antennas and is more particularly concerned with steerable antenna systems for transmitting and/or receiving electromagnetic signals.

BACKGROUND OF THE INVENTION

It is well known in the art to use steerable (or tracking) antenna systems to communicate with a relatively moving target. Especially in the aerospace industry, such steerable antennas preferably need to have a high gain, low mass, and a high reliability. One way to achieve such an antenna system is to provide a fixed feed source, thereby eliminating performance degradations otherwise associated with a moving feed source. These degradations include losses due to mechanical rotary joints, RF cable connectors, flexible waveguides, long-length RF cables associated with cable wrap units mounted on rotary actuators or the like.
Also, such steerable/tracking antennas should be designed such as to avoid a so-called keyhole effect, which is a physical limitation due to the orientation of the antenna rotation axis and caused by a limited motion range of an actuator or the like. This effect forces the antenna to momentarily disrupt communication when reaching the physical limitation to allow for the actuators to reposition before resuming the steering, thereby seriously affecting the communication capabilities of the entire antenna system.
US Patent 6,043,788 granted on March 28, 2000 to Seavey discloses tracking antenna system that is substantially robust and includes a large quantity of moving components that reduce the overall reliability of the system. Also, the steering angle range of the system is limited by the fixed angle between the boresite of the offset paraboloidal reflector and the kappa axis determined by the distance between the offset ellipsoidal subreflector and the offset paraboloidal reflector; a wide range requiring a large distance there between, resulting in a large antenna system that would not be practical especially for spaceborne applications.

SUMMARY OF THE INVENTION

It is therefore a general object of the present invention to provide an improved steerable antenna system with a fixed feed source that obviates the above-noted disadvantages.
An advantage of the present invention is that the steerable antenna system with a fixed feed source enables beam steering over a full spherical (4π steradians) angular range with minimum blockage from its own structure, whenever allowed by the supporting platform.
A further advantage of the present invention is that the steerable antenna system with a fixed feed source enables tracking of a remote station without any keyhole effect over any hemispherical coverage (2π steradians).
Yet another advantage of the present invention is that the steerable antenna system with a fixed feed source has a high gain, an excellent polarization purity and/or low sidelobes.
Still another advantage of the present invention is that the steerable antenna system with fixed feed source has simple actuation devices as well as locations of the same.
Another advantage of the present invention is that the fixed-feed source steerable antenna system can be so positioned with a first actuator as to enable tracking of a same orbiting remote station using only a second actuator when the orbit passes in proximity to the zenith of the system location.
A further advantage of the present invention is that the fixed-feed source steerable antenna system can be mounted on either an orbiting spacecraft or a fixed station and track a ground station or an orbiting spacecraft respectively, or be mounted on a spacecraft and track another spacecraft.
According to the present invention, there is provided a steerable antenna system for reflecting an electromagnetic signal between a fixed feed source and a moving target, the feed source mounting on a support structure and defining a source axis, the antenna system comprises: a frame mounting on the support structure; a hyperbolic subreflector for transmitting and receiving the electromagnetic signal to and from the feed source respectively, the hyperbolic subreflector defining a first focal point and a second focal point, the hyperbolic subreflector mounting on the frame so that the feed source is located at the first focal point; a parabolic reflector for transferring the electromagnetic signal from and to the hyperbolic subreflector respectively; the parabolic reflector defining a parabolic focal point and a beam axis, the parabolic reflector mounting of the frame so that the parabolic focal point is common with the second focal point; a planar reflector for transferring the electromagnetic signal from and to the parabolic reflector respectively, the planar reflector defining a normal axis, the planar reflector rotatably mounting on the frame so that the normal axis intersects the beam axis with a predetermined angle for transferring the electromagnetic signal to and from the target respectively; and a first rotating member for rotating the planar reflector about the beam axis, thereby having the antenna system to steer at the target.
Typically, the antenna system includes a second rotating member mounted on the supporting structure and rotatably supporting the frame, the second rotating member rotating the frame relative to the support structure about the source axis.
In one embodiment, the antenna system includes a controller controlling rotation of the first and the second rotating members; thereby controlling the antenna system to steer at the target.
Typically, the first and the second rotating members allow for the antenna system to steer at the target anywhere within a full spherical angular range.
Typically, the beam axis is co-planar, preferably perpendicular, with the source axis, thereby defining an antenna plane.
Typically, the predetermined angle is a 45-degree angle, thereby reflecting the electromagnetic signal from the parabolic reflector within a signal plane perpendicular to the beam axis.
Typically, the first and the second rotating members being first and second rotating actuators respectively, preferably first and second stepper motors.
Alternatively, the present invention provides for a steerable antenna system for transmitting or receiving an electromagnetic signal to/from a moving target, the antenna system comprises: a fixed feed source mounting on a support structure and defining a source axis; a frame mounting on the support structure; a hyperbolic subreflector for transmitting and receiving the electromagnetic signal to and from the feed source respectively, the hyperbolic subreflector defining a first focal point and a second focal point, the hyperbolic subreflector mounting on the frame so that the feed source is located at the first focal point; a parabolic reflector for transferring the electromagnetic signal from and to the hyperbolic subreflector respectively; the parabolic reflector defining a parabolic focal point and a beam axis, the parabolic reflector mounting of the frame so that the parabolic focal point is common with the second focal point; a planar reflector for transferring the electromagnetic signal from and to the parabolic reflector respectively, the planar reflector defining a normal axis, the planar reflector rotatably mounting on the frame so that the normal axis intersects the beam axis with a predetermined angle for transferring the electromagnetic signal to and from the target respectively; and a first rotating member for rotating the planar reflector about the beam axis, thereby having the antenna system to steer at the target.
Typically, the feed source is a dual frequency dual circular polarization feed source.
Other objects and advantages of the present invention will become apparent from a careful reading of the detailed description provided herein, with appropriate reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the annexed drawings, like reference characters indicate like elements throughout.

Figure 1 is a plan view of an embodiment of a steerable antenna system with a fixed feed source according to the present invention mounted on a support structure with the feed source axis parallel to the same, elevation and cross-elevation angles of zero and 180° respectively;
Figure 2 is a side view taken along line 2-2 of Fig. 1;
Figure 3 is a side view taken along line 3-3 of Fig. 1;
Figure 4 is a schematic perspective illustration showing the steering motion of the embodiment of Fig. 1 under activation of both actuator members for steering at relatively moving target such as an orbiting spacecraft or the like; and
Figure 5 is a partially sectioned side view of a second embodiment of a steerable antenna system with a fixed feed source according to the present invention, showing the system mounted on a support structure with the feed source axis perpendicular to the same.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to the annexed drawings the preferred embodiments of the present invention will be herein described for indicative purpose and by no means as of limitation.
Referring to Figs. 1 to 3, there is shown an embodiment 10 of a steerable antenna system with a fixed feed source according to the present invention mounted on a support structure 12 for transmitting and/or receiving an electromagnetic signal 14 to and/or from a target T relatively moving or orbiting around the same. The antenna system 10 includes a fixed RF (Radio Frequency) or the like feed source 30, preferably including a horn 32 connected to a conventional waveguide 34 or the like, secured to the support structure 12 and having a source axis A pointing at a hyperbolic subreflector 20 secured to a frame member 22 that is rotatably mounted on the structure 12, preferably secured to a planar platform P. The generally C-shaped frame 22 also supports a parabolic reflector 40 and a flat reflector 50, rigidly and rotatably mounted thereon, respectively.
The subreflector 20 is so oriented as to have its first F1 and second F2 focal points (or focus) in common with the focal point of the feed source 30 and the parabolic reflector 40, respectively. The latter is so oriented as to reflect (or transfer) the signal 14 received from the subreflector 20 to the flat reflector 50 along a beam axis B and vice-versa. Preferably, the feed source 30, subreflector 20, parabolic reflector 40 and flat reflector 50 all lie within a same antenna plane or elevation plane E. Accordingly, the source A and beam B axes are co-planar, and preferably perpendicular to each other, for the antenna system 10 to be as compact as possible.
A first rotating member 52, preferably a first rotating actuator such as a stepper motor or the like, mounted on the frame 22 rotates the flat reflector 50 preferably about the beam axis B; as illustrated in Fig. 1 with the flat reflector 50 shown in solid and dashed lines to reflect the signal 14 to the right and left hand side, respectively. A second rotating member 24, preferably a second rotating stepper motor actuator, mounted on thestructure 12 rotates the frame 22 along with the subreflector 20, the parabolic 40 and flat 50 reflectors about the source axis A. The flat reflector 50 is preferably elliptic in shape in order to provide a circular projected aperture along the beam axis B and the direction of the target T, in these two positions.
A controller member 60 is preferably connected to the motors 52, 24 via a first 64 and a second 62 encoders (or the like) respectively to control the rotation of the same; thereby controlling the system antenna 10 to steer at the target T, preferably anywhere within a full spherical angular range.
The normal axis C of the flat reflector 50 preferably makes a forty-five degree (45°) constant angle α relative to the beam axis B to reflect the signal 14 coming from the parabolic reflector 40 within a signal plane or cross-elevation (x-elevation) plane X perpendicular to the elevation plane E and parallel to the source axis A. Consequently, the projection of the flat reflector 50 perpendicular to both the output signal 14 direction and the beam axis B is circular as shown in Fig. 2 and 3, respectively.
Accordingly, the first 52 and second 24 motors are the x-elevation and elevation motors adjusting the reference x-elevation angle wand elevation angle ϕ of the antenna system 10 respectively. Similarly, the source A and beam B axes are the elevation and x-elevation axes respectively.
Although the antenna system 10 can steer in the 4π steradian full spherical angular range (ϕ = 0° to 360°; ω = 0° to 360°), it preferably operates over a half spherical angular range (ϕ = 0° to 180°; ω = 0° to 360°) above the platform P since the latter is obviously generally solid and opaque to RF signals. Only the portion of the frame 22 extending to support the flat reflector 50 provides small or negligible blockage and interference that might affect the antenna output signal or antenna gain when the flat reflector 50 is oriented toward the same (over a small x-elevation angle range of ω = 0° to ± 20° approximately), depending on its actual geometry and the frequency of the signal 14.
Since the source axis A is parallel to the platform P, both the elevation motor 24 and the horn 32 are mounted on respective brackets 16, 18 of the structure 12 to allow for the frame 22 to clear the same during its rotational displacement about the source axis A, as seen in Figs. 2 and 3. Furthermore, the actual shapes of the horn 32, subreflector 20, parabolic reflector 40 and flat reflector 50 are determined to maximize the overall electrical antenna gain as it would be obvious to anyone having ordinary skill in the art, also considering its performance in all other aspects such as mechanical, power, reliability, cost, manufacturability, etc.
Preferably, the feed source 30 is a dual frequency dual circular polarization feed source or any other suitable electromagnetic signal source.
In a preferred embodiment of the antenna system 10 of the present invention, the platform P represents a spacecraft Earth facing panel and the target T is a ground station on the Earth surface; the spacecraft orbiting around the Earth (or any other planet or the like). Alternatively, the antenna system 10 could be a ground station steering at an orbiting spacecraft to transmit and/or receive signal to/from the same.
The antenna system 10 of the present invention mounted on an orbiting spacecraft can also be used to communicate with a similar antenna system 10 mounted on another orbiting spacecraft, whereby the two antenna systems 10 would continuously steer at each other while the two spacecraft are moving in their respective orbits.
Obviously, the controller member 60 can simultaneously drive the two motors 24, 52 to have the antenna system 10 sequentially and continuously steering at a moving target in any desired direction.
Referring to Fig. 4, there is shown a schematic perspective sequential illustration of the steering coverage of the antenna system 10 (shown in dashed lines) of the present invention with the rotational displacement ω of the output signal 14 (shown by all the coplanar arrows in dashed lines) about the x-elevation axis B to form the x-elevation plane X, and the rotational displacement ϕ of both elevation E and x-elevation X planes about the elevation axis A to substantially cover the full spherical angle around the antenna system 10. The motion being represented in Fig. 4 by three different displacements of the elevation E₁, E₂, E₃ and x-elevation X₁, X₂, X₃ planes by the corresponding respective rotation angles ϕ₁, ϕ₂, ϕ₃ about the source axis A.
When the antenna system 10 has to track a moving target T for a short period of time over a relatively small angular range, it is possible for the controller 60 to properly position the antenna system 10 using the elevation motor 24 such that only the x-elevation motor 52 is used for the tracking itself of the target T, considering that the path of the target T essentially remains within a same plane, the x-elevation plane X, as seen by the antenna system 10.
Referring to Fig. 5, there is shown a second embodiment 10a of the antenna system positioned with the elevation source axis A generally perpendicular to the platform P. In this case, the bracket 18a is substantially reduced down to a simple mounting bracket connected to the horn 32 that points upward at the subreflector 20, thus limiting the run of the waveguide 34 connecting thereto, and the signal losses associated therewith. The bracket 16a is also reduced down to a simple support for the elevation motor 24a itself supporting the rotating frame 22a. The elevation motor 24a is preferably hollowed to enable the fixed horn 32 to be centered and point at the subreflector 20 without being affected by the rotation induced by the same 24a to the frame 22a.
Although the steerable antenna system has been described with a certain degree of particularity, it is to be understood that the disclosure has been made by way of example only and that the present invention is not limited to the features of the embodiments described and illustrated herein, but includes all variations and modifications within the scope and spirit of the invention as hereinafter claimed.

Claims

A steerable antenna system (10) for reflecting an electromagnetic signal (14) between a fixed feed source (30) and a moving target (T), said feed source (30) mounting on a support structure (12) and defining a source axis (A), said antenna system comprising:
- a frame (22) mounting on the support structure (12);

- a hyperbolic subreflector (20) for transmitting and receiving the electromagnetic signal (14) to and from said feed source (30) respectively, the hyperbolic subreflector (20) defining a first focal point (F1) and a second focal point (F2), said hyperbolic subreflector (20) mounting on said frame (22) so that the feed source (30) is located at said first focal point (F1);

- a parabolic reflector (40) for transferring the electromagnetic signal (14) from and to said hyperbolic subreflector (20) respectively; said parabolic reflector (40) defining a parabolic focal point and a beam axis (B), said parabolic reflector (40) mounting of said frame (22) so that said parabolic focal point is common with said second focal point (F2);

- a planar reflector (50) for transferring the electromagnetic signal (14) from and to said parabolic reflector (40) respectively, said planar reflector (50) defining a normal axis (C), said planar reflector (50) rotatably mounting on said frame (22) so that said normal axis (C) intersects said beam axis (B) with a predetermined angle (α) for transferring the electromagnetic signal (14) to and from the target (T) respectively; and

- a first rotating member (52) for rotating said planar reflector (50) about said beam axis (B), thereby having said antenna system (10) to steer at the target (T).
A system (10) as defined in claim 1, including a second rotating member (24) mounted on the supporting structure (12) and rotatably supporting said frame (22), said second rotating member (24) rotating said frame (22) relative to the support structure (12) about the source axis (A).
The antenna system (10) of claim 2, including a controller (60) controlling rotation of the first (52) and the second (24) rotating members; thereby controlling the antenna system (10) to steer at the target (T).
The antenna system (10) of claim 2 or 3, wherein the first (52) and the second (24) rotating members allow for the antenna system (10) to steer at the target (T) anywhere within a full spherical angular range.
The antenna system (10) of claim 1, 2, 3 or 4, wherein said beam axis (B) is co-planar with the source axis (A), thereby defining an antenna plane.
The antenna system (10) of claim 5, wherein the beam axis (B) is perpendicular to the source axis (A).
The antenna system (10) of claim 6, wherein the planar reflector (50) is of a generally elliptical shape so as to provide circular projections along the beam axis (B) and a direction of the target (T).
The antenna system (10) of claim 1, 2 or 6, wherein the predetermined angle (α) is a 45-degree angle, thereby reflecting the electromagnetic signal (14) from the parabolic reflector (40) within a signal plane perpendicular to the beam axis (B).
The antenna system (10) of claim 1, 2 or 8, wherein the support structure (12) includes a generally planar platform (P), said frame (22) mounting on the supporting structure (12) so that the source axis (A) is substantially parallel to the planar platform (P).
The antenna system (10) of claim 1, 2 or 8, wherein the support structure (12) includes a generally planar platform (P), said frame (22) mounting on the supporting structure (12) so that the source axis (A) is substantially perpendicular to the planar platform (P).
The antenna system (10) of claim 3, wherein the controller (60) includes a first encoder (64) and a second encoder (62) mounting on the first (52) and the second (24) rotating members respectively for providing feedback of a position of the respective rotating member (52, 24) to the controller (60).
The antenna system (10) of claim 3, wherein the controller (60) simultaneously drives the first (52) and the second (24) rotating members to have the antenna system (10) steering in a desired direction.
The antenna system (10) of claim 12, wherein the controller (60) provides commands to the first (52) and the second (24) rotating members so that said antenna system (10) automatically steers at the moving target (T).
The antenna system (10) of claim 1, wherein the first (52) and second (24) rotating members are first and second rotating actuators respectively.
The antenna system (10) of claim 14, wherein the first (52) and second (24) rotating actuators are first and second stepper motors respectively.
A steerable antenna system (10) for transmitting or receiving an electromagnetic signal (14) to/from a moving target (T), said antenna system comprising:
- a fixed feed source (30) mounting on a support structure (12) and defining a source axis (A);

- a frame (22) mounting on said support structure (12);

- a hyperbolic subreflector (20) for transmitting and receiving the electromagnetic signal (14) to and from said feed source (30) respectively, the hyperbolic subreflector (20) defining a first focal point (F1) and a second focal point (F2), said hyperbolic subreflector (20) mounting on said frame (22) so that said feed source (30) is located at said first focal point (F1);

- a parabolic reflector (40) for transferring the electromagnetic signal (14) from and to said hyperbolic subreflector (20) respectively; said parabolic reflector (40) defining a parabolic focal point and a beam axis (B), said parabolic reflector (40) mounting of said frame (22) so that said parabolic focal point is common with said second focal point (F2);

- a planar reflector (50) for transferring the electromagnetic signal (14) from and to said parabolic reflector (40) respectively, said planar reflector (50) defining a normal axis (C), said planar reflector (50) rotatably mounting on said frame (22) so that said normal axis (C) intersects said beam axis (B) with a predetermined angle (α) for transferring the electromagnetic signal (14) to and from the target (T) respectively; and

- a first rotating member (52) for rotating said planar reflector (50) about said beam axis (B), thereby having said antenna system (10) to steer at the target (T).
A system (10) as defined in claim 16, including a second rotating member (24) mounting on said supporting structure (12) and rotatably supporting said frame (22), said second rotating member (24) rotating said frame (22) relative to said support structure (12) about said source axis (A).
The antenna system (10) of claim 16 or 17, wherein said feed source (30) is a dual frequency dual circular polarization feed source.