Note: Descriptions are shown in the official language in which they were submitted.
26949-169
SYSTEM FOR MECHANICALLY STEERING AN AIRBORNE ANTENNA
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.
,,
~L2~ 703
BA~KGROUND 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
lS 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
12~70~
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.
03
DISCLOSURE OF THE INVE~TION
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 mo~nted 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 lncludes 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 a~is 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
~L2~3~7(~3
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 lonqitudinal 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 sup~ort 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.
7~3
--6--
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 l 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 l showing the antenna/pedestal assembly for the
antenna of FIGURE l;
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.
7~3
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 functions 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
7~3
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 o 105 degrees, from -lS
degrees to ~90 degrees. The angle o~ 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 -lS degrees to a position of +90 degrees relative
to the plane of the base ring 2~.
Attached to the azimuth member 26, is a motor suppo{t 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 3~ and also engages a fixed sprocket 42 of the
pedestal base ring 2~. 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.
. .
~2~ 3
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 ~rom 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)
; 15 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.
Supp~rted 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
3~7~3
--10--
position of the antenna element 10 with reference to the
elevation a~is 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
assemhly.
10Typically, 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 o
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
10 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 descrihed 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 recei~e signal strength
inputs on input line(s) 76. These relative strength signals
~l2~D~7~3
are received ~rom 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
; 10 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 llnes 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
`3
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 com~and signals are applied to
the elevational steering unit 56. The respective steering
units are engerized until the desired positio~ 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 steerinq 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 ~F 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.
~ ~ . ., - . . . - . .
:IL2~7~3
-14-
Althouqh 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.
