Communications: radio wave antennas – Antennas – Wave guide type
Reexamination Certificate
2000-05-15
2001-10-23
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
Wave guide type
C343S782000
Reexamination Certificate
active
06307523
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of Radio Frequency (RF) communication, and, more particularly, to tracking targets with an antenna.
BACKGROUND OF THE INVENTION
Communication from a stationary or vehicle-based antenna to a skyborne target such as a polar orbiting satellite, a geosynchronous satellite, or an airplane, etc., may require automatic tracking. Various types of reflector antennas are known, and various techniques of generating offset beams, for example, conical scanning, sequential lobing, and single channel monopulse, have proven to be acceptable for automatic tracking of targets.
Typical conical scanning involves the principle of generating an offset beam about the focal axis (tracking axis or boresight) by using a single feed element which is offset and rotated about the focal axis. There are many variations of conical scanning. In general, a conical scan can be produced either electrically (by varying electrical parameters of the antenna) or mechanically (by rotating a component of the antenna).
A Cassegrain reflector antenna includes a parabolic dish for focusing the received signal onto a secondary reflector, or sub-reflector, having a hyperbolic profile, which refocuses the signal onto a feed, located in the center of the parabolic dish. A ring focus reflector antenna is similar to the Cassegrain except that dish is a section of a parabola and the feed is located offset from the center of the dish. The ring focus design is valued for its excellent RF performance in gain, side lobes and efficiency. In the Cassegrain or ring focus configurations, the sub-reflector may be rotated at high speed so as to obtain a mechanical conical nutation within a certain angle of rotation which defines the scan region covered by the antenna beam.
The conical scan can determine errors in the positioning of the antenna, since the amplitude of the received signal will be modulated by the mechanical nutation imparted by the rotating element. When an optimal position has been determined for the antenna, an antenna positioning device, typically an elevation over azimuth actuator, is used to finally position the antenna structure. The primary advantage of conical scanning is its low implementation cost, relative to monopulse.
Single Channel Monopulse (SCM) utilizes a feed, in typically four or five element configurations, and a combining network to generate a reference signal and azimuth and elevation difference signals of a monopulse feed. The azimuth and elevation difference signals are biphase modulated and sequentially coupled to the reference signal. The resultant signal is similar to conical scanning signals because the combined reference and difference signal produces an offset beam relative to the focal axis. However, monopulse tracking feeds are relatively expensive and may not be retrofitted to existing antennas.
SUMMARY OF THE INVENTION
In view of the foregoing background, it is therefore an object of the invention to provide a low cost tracking technique that can be implemented on existing and new antenna systems.
This and other objects, features and advantages in accordance with the present invention are provided by an antenna apparatus comprising an antenna feed, a radio frequency (RF) receiver connected to the antenna feed, and a main reflector for reflecting received RF signals to the antenna feed along a received signal path. Also, a sub-reflector is preferably positioned in the received signal path for reflecting received RF signals from the main reflector to the antenna feed. A body in the received signal path has at least one predetermined characteristic to modulate received RF signals. A tracking controller is connected to the RF receiver for generating an antenna pointing control signal based upon modulated received RF signals.
The body may at least partially block received RF signals and not block other RF signals.
Also, the body is preferably movable in the received signal path between the main reflector and the sub-reflector and may be rotatably mounted adjacent the sub-reflector. The body may modulate received RF signals by at least partially blocking received RF signals in a predetermined pattern in the received signal path. The apparatus may include a drive for rotating the body, with a counterweight for the body connected to the drive. The tracking controller preferably controls the speed of the drive. Also, a position sensor may be connected to the tracking controller for sensing a position of the body. The tracking controller may also determine an antenna pointing error, wherein the controller also generates the antenna pointing control signal based upon the antenna pointing error. The main reflector may comprise a parabolic dish having the antenna feed positioned at an apex thereof, or the main reflector may comprise a parabolic section-shaped dish having the antenna feed positioned offset from an apex thereof.
Features and advantages in accordance with the present invention are also provided by a method of tracking a target with the antenna apparatus including the steps of modulating received RF signals by at least partially blocking received RF signals in a predetermined pattern in the received signal path. Preferably, the method includes modulating received RF signals by moving a body in the received signal path, and generating an antenna pointing control signal based upon modulated received RF signals. Again, the body has at least one predetermined characteristic to modulate received RF signals, which may comprise at least partially blocking received RF signals, and not blocking other RF signals.
The antenna may further include a sub-reflector positioned in the received signal path for reflecting received RF signals from the main reflector to the antenna feed, and thus the body may preferably be movable in the received signal path between the main reflector and the sub-reflector. Moving the body may comprise rotating the body with a motor, and controlling the speed of the motor. Also, the method may include sensing a position of the body. An antenna pointing error may also be determined and the antenna pointing control signal may be generated based upon the antenna pointing error.
REFERENCES:
patent: 2572088 (1951-10-01), Young et al.
patent: 3611394 (1971-10-01), Parker
patent: 5025493 (1991-06-01), Cook et al.
patent: 5194874 (1993-03-01), Perrotta et al.
patent: 5351060 (1994-09-01), Bayne
Albert David Samuel
Daffron, III William Charles
Green Dennis Arnold
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Clinger James
Harris Corporation
Wong Don
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