EP0274979A1

EP0274979A1 - System for mechanically steering an airborne antenna

Info

Publication number: EP0274979A1
Application number: EP87730138A
Authority: EP
Inventors: Mohamed Abdelrazik; John Durant Martin; Boyd Lee Corcoran
Original assignee: E Systems Inc
Current assignee: Raytheon Co
Priority date: 1986-11-06
Filing date: 1987-11-05
Publication date: 1988-07-20
Also published as: CA1294703C

Abstract

A helical-element antenna is steered within a positioning envelope greater than hemispherical. The system for steering the helical antenna includes a supporting frame (22) having an azimuth member (26) with a longitudinal axis coinciding with the azimuth axis (28) around which the antenna rotates. Further, the supporting frame includes an elevation member (30) that is integral with the azimuth member and has a longitudinal axis displaced from the azimuth axis. An interface fitting (51, 52) rotatably mounts the antenna to the elevation member. The supporting frame is rotatably mounted to a pedestal base that has a plane perpendicular to the azimuth axis. To position the antenna about the azimuth axis, an azimuth steering unit (36) is energized to rotate the supporting frame 360 degrees around the azimuth axis. For positioning the antenna about the elevation axis (32), an elevation steering unit (56) rotates the interface fitting and the antenna through a gear coupling about the elevation axis. The total rotation excursion about the elevation axis is typically 180 degrees.

Description

TECHNICAL FIELD

This invention relates to a system for mechanically steering, with reference to an azimuth axis- and an elevation axis, an airborne high gain antenna; and more particularly to a system for mechanically steering an airborne antenna with reference to non-orthogonal azimuth and elevational axes.

BACKGROUND ART

Heretofore, a number of systems have been developed to non-mechanically steer an airborne antenna of a communication system. These previously developed systems have been less than satisfactory because of degradation of antenna performance parameters such as: gain, axial ratio, beam width, and sidelobe levels, to illustrate a few examples. These parameters were noted to be degraded as a function of the steering angle of such non-mechanically steered systems. Further, early non-mechanical steered systems had limited coverage of the total field of view from a given position.
In accordance with the present invention there is provided a system for mechanically steering an airborne antenna that provides for more than hemispherical coverage as the antenna is differentially positioned about non-orthogonal azimuth and elevational axes. Mechanically steering the antenna provides the advantage of minimizing or eliminating the degradation of the important antenna figures of merit.
The antenna system of the present invention meets the technical requirements of satellite networks with which the antenna may interface. For example, the antenna steered by the system of the present invention finds utility in communication with a satellite system for air traffic control, passenger telephone and telex services, airline communications, and navigational communications, all over either secure or clear transmission links.
Typically the antenna positioned by the system of the present invention comprises a radiating helical element that is designed to maximize antenna gain and minimize axial ratio. The element itself is surrounded by a metal cone for decreasing the beam width of the helical element with the resulting advantage of increasing the gain of the antenna. In a conventional communication system, the helical antenna element interfaces to a diplexer, a low noise amplifier, and a high power amplifier.
Although not limited thereto, the steering system of the present invention finds application for mounting an antenna on the vertical stabilizer of a Boeing 747 type aircraft. Also, the steering system finds utility for mounting an antenna on the fuselage of many presently operating aircraft. In all applications, a radome protects the antenna and the positioning system from the airborne environment, and provides an installation with a desired aerodynamic shape to minimize drag.

DISCLOSURE OF THE INVENTION

In accordance with the present invention, there is provided an antenna/pedestal assembly for an airborne communication system including an antenna positionable with reference to an azimuth axis and an elevation axis. The antenna includes a radiating helical element with a metal cone mounted to surround the helical element thereby decreasing the band width and increasing the gain of the radiating element. This assembly of the radiating element and the metal cone are mounted to a pedestal to be positionable thereby about the azimuth axis and the elevation axis. The pedestal includes an azimuth member having a longitudinal axis coinciding with the azimuth axis of the system, said azimuth member rotatable about the azimuth axis, and an elevation member integral with the azimuth member and having a longitudinal axis non-orthogonally positioned with reference to the azimuth axis, the elevation member mounted for rotation about the elevation axis.
Further in accordance with the present invention, there is provided a system for mechanically steering, with reference to an azimuth axis and an elevation axis, an airborne high gain antenna. To support and articulate the antenna, the system comprises a support frame, a pedestal base ring, an azimuth steering unit and an elevational steering unit. Specifically, the support frame comprises a differential mount which includes an azimuth member having a longitudinal axis coinciding with the azimuth axis of the system and an elevation member integral with the azimuth member and having a longitudinal axis differentially displaced from the azimuth axis and coinciding with the elevation axis of the system. Further, the system includes means for rotatably mounting the support frame to the pedestal base ring. Also included within the system is a means for rotatably mounting the high gain antenna with reference to the elevation member of the support frame.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawing in which:

FIGURE 1 is a pictorial view of a system for mechanically steering an airborne antenna in accordance with the present invention;
FIGURE 2 is a side view, partially cut away of the system of FIGURE 1 showing the antenna/pedestal assembly for the antenna of FIGURE 1;
FIGURE 3 is a schematic illustration of the movement of the antenna around the azimuth and elevational axes;
FIGURE 4 is a block diagram of an aeronautical high gain antenna system including the antenna/pedestal assembly of FIGURE 2; and
FIGURE 5 is a block diagram of a single element helical antenna system for use with the pedestal assembly of the present invention.

DETAILED DESCRIPTION

Referring to FIGURE 1, there is shown a pictorial view of a steerable/antenna and pedestal assembly in accordance with the present invention including a single helix antenna element 10 surrounded by a metal cone 12 that function to decrease the beamwidth of the helical element and therefore increase the gain of the antenna. The helical element 10 is supported in the metal cone 12 by crossbracing supporting rods 14 where each of the supporting rods is made from a composite non-metallic material. Supported on the metal cone 12 are electronic components of the antenna system including a diplexer 16, a low noise amplifier 18 and a power amplifier (not shown). The high power amplifier is located either on the metal cone 12 or in the interior of an aircraft when the system is mounted to an aircraft. These electronic components are interconnected into an antenna system such as illustrated in FIGURE 5, to be described.
The antenna element is mechanically steered by a differentially mounted pedestal including a pedestal base ring 20 to which is rotatably mounted a support frame 22.
Referring to FIGURE 2, there is shown the differentially mounted pedestal including the pedestal base ring 20 to which is rotatably mounted by means of a bearing 24 the support frame 22. The support frame 22 includes an azimuth member 26 having a longitudinal axis coinciding with the azimuth axis 28 of the antenna system. Integrally formed with azimuth member 26 is an elevation member 30 having a longitudinal axis coinciding with the elevation axis 32 of the antenna system. As illustrated in FIGURE 2, as an example, the angular displacement between the azimuth axis 28 and the elevational axis 32 is 52.5 degrees providing an elevation pointing range of 105 degrees, from -15 degrees to +90 degrees. The angle of displacement between the azimuth axis and the elevation axis is selected to provide the desired elevation pointing as the antenna 10 is rotated about the azimuth axis 28 and the elevation axis 32.
In one embodiment of the present invention, the antenna element 10 rotates about the elevational axis 32 from a position of -15 degrees to a position of +90 degrees relative to the plane of the base ring 20.
Attached to the azimuth member 26, is a motor support 34 to which is mounted an azimuth steering unit 36 comprising a position encoder 44 and a drive motor having a drive and sprocket 38. An azimuth drive cogged belt 40 engages the drive sprocket 38 and also engages a fixed sprocket 42 of the pedestal base ring 20. Energization of the azimuth steering drive unit causes the entire support frame 22 including the azimuth member 26 to be rotated with reference to the pedestal base ring 20 around the azimuth axis 28. The support frame 22 is free to rotate 360 degrees with reference to the base ring 20.
To limit and reference to a key position of the azimuth member 26 with reference to the pedestal base ring 20, an azimuth limit switch including a Hall-effect sensor 46 and a vane 48 is fixed to the pedestal ring 20 and the azimuth member 26. The position of the azimuth axis is determined by monitoring the output on an azimuth encoder 44 by counting and storing pulse data relative to the azimuth reference key identified by the limit switch. Subsequent to the arrival at the reference key position, azimuth feedback signals from the azimuth encoder 44 are applied to an antenna control unit to digitally control energization and rotational displacement of the azimuth steering unit 36.
Integral with the elevation member 30 is an elevation bearing housing 50 that includes bearing members (one shown 51) for rotatably supporting an antenna/pedestal interface fitting 52. The antenna/pedestal interface fitting 52 includes a hollow bearing internal to the bearing member and a U-shaped bracket 54 attached to the outer surface of the metal cone 12.
Supported by the elevation bearing housing 50 is an elevation steering unit 56 for rotatably driving a pinion gear 58 that engages a driven gear 60. The driven gear 60 is secured to the antenna/pedestal interface fitting 52 such that energization of the elevation steering unit 56 causes rotation of the metal cone 12 and the supported antenna element 10 around the elevation axis 32. To limit and reference to a key position of the antenna element 10 with reference to the elevation axis 32, there is provided an elevation limit switch assembly including a Hall-effect position sensor 64 mounted to the elevation member 30 and a sensor actuating vane 66 mounted to the antenna/pedestal interface fitting 50. Elevation feedback signals from an elevation encoder 62 are applied to the antenna control unit for monitoring the actual position of the elevation axis referenced to the elevation limit switch assembly.
Typically, the antenna and pedestal assembly of the present invention is designed for installation on the vertical stabilizer of a Boeing 747 type aircraft, or on the fuselage of other aircraft. In any installation, the antenna and pedestal assembly is enclosed within a radome 68 to protect the assembly from the airborne environment and provide the desired aerodynamic configuration to minimize drag forces.
Additional components of the system illustrated in FIGURE 2 include the diplexer 16 and the low noise amplifier 18 attached to the outer surface of the metal cone 12. These various electronic components are interconnected to the helical antenna by means of an element connector 70. Such a connector and interconnections between the antenna element 10 and the various electronic components are part of a conventional installation and interconnection system.
Referring to FIGURE 3, there is schematically illustrated the antenna/pedestal assembly of FIGURE 2 for positioning the antenna 10 with reference to the azimuth axis 28 and the elevation axis 32. Shown in dotted outline are various positions of the antenna 10 as it rotates about the elevation axis 32. As illustrated, the antenna 10 may be positioned in elevation from approximately -15 degrees to +90 degrees with reference to the plane of the base ring 20. In any of the positions illustrated, the antenna is also positionable about the azimuth axis 28 by rotation of the support frame 22 with reference to the base ring 20. As previously discussed, the antenna 10 is rotatable through 360 degrees around the azimuth axis 28. This combined rotational envelope provides pointing coverage which exceeds a hemispherical configuration and is achievable by the mechanical pedestal element of the present invention. The desired position for the antenna 10 is determined by the antenna control unit to be described with reference to FIGURE 4.
Referring to FIGURE 4, there is shown a block diagram of the antenna/pedestal assembly for an antenna system of FIGURES 1 and 2 including an antenna control unit 70. This control unit receives positioning information for position control of the antenna 10 on an input line 72. Also coupled to the antenna control unit are relative receive signal strength inputs on input line(s) 76. These relative strength signals are received from the helical antenna electronic components to position the antenna 10 to maximize received signal strength.
In addition to position control signals for the pedestal steering units 36 and 56, the antenna control unit 70 outputs antenna status information on a line 80.
Functionally, the antenna control unit -70 operates to provide elevation command signals on line(s) 82 to the elevation steering unit 56 and azimuth command signals on line(s) 84 to the azimuth steering unit 36. In FIGURE 4 these command signals are shown applied to the pedestal represented by a functional block identified by the reference numeral 86. Also applied to the pedestal 86 are RF input signals to the antenna 10 and RF output signals received by the antenna.
As previously explained, the position of the azimuth member 26 and the elevation member 30 is monitored by means of encoders 44 and 62, respectively. Feedback signals from these encoders are applied by means of lines 88 and 90 to the antenna control unit 70.
Also illustrated in FIGURE 4 is the radome 68 provided with controlled cooling by means of a conduit 92. Cooling of the radome 68 is conventional and further description is not deemed necessary for an understanding of the present invention.
In operation, the antenna control unit 70 receives the various input signals which are evaluated and processed for differential coordinate conversion to determine the required rotation at the azimuth axis 28 and the elevational axis 32 to achieve the desired pointing angles of the antenna 10. Azimuth command signals are generated and applied to the azimuth steering unit 36 and elevation command signals are applied to the elevational steering unit 56. The respective steering units are engerized until the desired position for the antenna is identified by means of the feedback signals from the encoders 44 and 62. Thus, the antenna control unit 70 along with the steering units 36 and 56 are part of a servo control system including a feedback loop provided by the encoders 44 and 62.
Referring to FIGURE 5, there is shown a block diagram of the antenna system where the single element helical antenna 10 is interconnected to electronic components of the system. Radiating helical elements of the antenna 10 are connected to the diplexer 16, which in the receive mode, applies an RF input to a low noise amplifier 18. In a transmit mode, the diplexer 16 receives RF output signals from the power amplifier 94. In accordance with conventional antenna systems, the low noise amplifier 18 is connected to a receiver and the power amplifier 94 is connected to a transmitter. A further description of such a receiver and transmitter is not considered necessary to understand the present invention and will not be further described.
Although the invention has been described in detail, the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the invention being limited only to the terms of the appended claims.

Claims

1. A system for mechanically steering with reference to an azimuth axis and an elevation axis an airborne antenna of a communication system, comprising:
a supporting frame including an azimuth member having a longitudinal axis coinciding with the azimuth axis of the system and an elevation member integral with the azimuth member and having a longitudinal axis displaced from the azimuth axis as the bisector of the included angle of desired elevation coverage, coinciding with the elevation axis of the system;
a pedestal base;
means for rotatably mounting the support frame to the pedestal base;
an azimuth steering unit for rotatably positioning the support frame with reference to the pedestal base;
interface means for rotatably mounting the antenna to the elevation member of the support frame at the elevation axis; and
an elevational steering unit for positioning the interface means with reference to the elevational member.

2. A system for mechanically steering an airborne antenna as set forth in Claim 1 wherein the angle of the longitudinal axis of the elevational member with respect to the longitudinal axis of the azimuth member is selected to position the antenna to cover an area greater than hemispherical.

3. A system for mechanically steering an airborne antenna as set forth in Claim 1 wherein the angle of the longitudinal axis of the elevation member with respect to the longitudinal axis of the azimuth member is selected to point the antenna from -15 degrees to +90 degrees in elevation and 360 degrees in azimuth relative to the plane of the pedestal base.

4. A system for mechanically steering an airborne antenna as set forth in Claim 1 wherein said azimuth steering unit includes an azimuth position encoder for generating a feedback signal to monitor the position of the azimuth member with reference to the pedestal base.

5. A system for mechanically steering an airborne antenna as set forth in Claim 1 including an elevation position encoder for generating a feedback signal to monitor the position of the antenna with reference to the elevational member.

6. A system according to one of Claims 1-6, characterized by:
the support frame having the elevation member non-orthogonally mounted to the azimuth member and having a longitudinal axis non-orthogonal to the azimuth axis; and by
an antenna control unit responsive to antenna position signals and generating steering control signals to the azimuth steering unit and the elevation steering unit.

7. A system for mechanically steering an airborne antenna as set forth in Claim 6 wherein the control unit includes means responsive to the relative strength of RF signals received by the antenna to generate a component of the steering control signals.

8. A system for mechanically steering an airborne antenna as set forth in Claim 6 wherein said antenna control unit includes means responsive to navigational and altitude information signals from an aircraft avionics system to generate a component of the steering control signals.

9. A system for mechanically steering an airborne antenna as set forth in Claim 6 wherein the angle of the longitudinal axis of the elevation member with respect to the longitudinal axis of the azimuth member is selected to position the antenna to cover an area greater than hemispherical.

10. An antenna/pedestal assembly for an airborne communication system as set forth in one of Claims 1-9, characterized by:
an antenna positionable with reference to an azimuth axis and an elevation axis and including:
a radiating helical element;
a metal cone mounted to surround said helical element to decrease the beamwidth and increase gain of the element.

11. An antenna/pedestal assembly for an airborne communication system as set forth in Claim 10 wherein said antenna further includes means for mounting said radiating helical element into said metal cone.

12. An antenna/pedestal assembly for an airborne communication system as set forth in Claim 10 wherein said antenna further includes means for mounting electronic components of a communication system to the exterior surface of said metal cone.

13. An antenna/pedestral assembly for an airborne communication system as set forth in Claim 10 further including a protective cover enclosing said antenna and said pedestal.