|Title of Invention||
A METHOD FOR SIMULTANEOUSLY GENERATING PENCIL BEAM AND SHAPED BEAM FROM A SINGLE SHAPED REFLECTOR
|Abstract||The invention relates to concept of generating pencil beam from a shaped reflector that is shaped for some arbitrary coverage beam. The invention finds extensive application in the communication satellite antennas where a spot beam (pencil beam) is required to be generated in addition to the shaped beam from a same shaped reflector. The present invention finds wide application in the reflector surface distortion compensation. This invention reduces complexity over the existing art of compensating surface distortions. The invention describes the single shaped reflector with one feed for shaped beam and one for generating pencil beam away from shaped beam. One feed is placed at focus for generating shaped beam and second feed is placed such that it generates the pencil beam, which is equivalent to that generated from a same size parabolic reflector. The invention also provides technique to generate a composite beam, which has two isolated regions, one being large arbitrary shaped coverage and other is small coverage area. (Fig 1)|
The present invention relates to a method of generating a spot beam or a pencil beam from a shaped reflector in addition to a shaped beam from the same shaped reflector.
BACKGROUND OF THE INVENTION
A pencil beam is a beam of radiant energy in the form of a narrow cone or a cylinder. In the field of communication satellite antennas a pencil beam is a satellite downlink beam which covers only a very small geographic area, perhaps no more that a couple of hundred kilometers across, as distinct from shaped beams which tend to cover whole countries or regions. Pencil beams are used by domestic satellites to deliver certain transponder signals to geographically well-defined areas.
Traditionally parabolic dish antennas, when fed by a single radio frequency feed at the focus generate pencil beams. US 2,825,063 and US 2,929,064 are some of the basic prior art patents that describe such parabolic antenna systems that generate pencil beams. US patent 6,411,262 shows a shaped reflector antenna system that is used to generate beams for wide coverage areas, such as for the whole of the United States of America.
One prior art solution for generating pencil beam is shown in Figure-2, in which a parabolic reflector (6) with surface distortion is illuminated by multiple feed array (7) with different excitation coefficient achieved by beam forming network (8). Beam forming network (8) essentially consist of phase shifter and unequal power divider network. This increases complexity and loss when number of feeds increases.
Communication satellite antennas have requirements of generating shaped beam, which is conforming to an arbitrary shaped geographic coverage. Shaped reflector antennas are the most viable antenna solution to accomplish this requirement. Many times two isolated coverage, one large arbitrary shaped coverage and other
isolated small coverage region are required to be covered. In such cases single shaped reflector can accomplish such requirement but at the cost of gain reduction over both the regions. This problem of gain reduction can be overcome by employing a separate reflector, which can provide beam with high gain over island region, and taking fraction of power from the main land beam. However, this solution requires penalty of additional mass and real estate requirement on the spacecraft. The present invention provides effective solution to such requirements.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a method for generating pencil beam from a shaped reflector for some arbitrary coverage beam in addition to a shaped beam from the same shaped reflector.
This object is achieved by providing a first feed for the shaped beam and a second feed for generating the pencil beam away from the shaped beam. The first feed is placed at focus for generating shaped beam and the second feed is placed such that it generates the pencil beam, which is equivalent to that generated from a same size parabolic reflector. The invention also provides a technique to generate a composite beam that has two isolated regions, one being a large arbitrary coverage and the other being a small coverage area.
The invention therefore relates to a method for simultaneously generating pencil beam and shaped beam from a single shaped reflector said reflector being shaped for some arbitrary coverage comprising the steps of: providing an elliptical projection shaped reflector; placing a first feed at the focal point of the reflector; placing a second feed away from the focal plane; wherein the placement of second feed depends upon the phase error introduced by the shaped reflector in the aperture plane for generating pencil beam.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be discussed in greater detail with reference to the accompanying figures. Figure-1 shows shaped reflector with feed arrangement for shaped beam and
pencil beam (reflector surface distortion compensation) Figure-2 shows prior art of compensating reflector surface distortion Figure-3 shows surface deviations contours of shaped reflector from parent paraboloid
Figure-4 shows gain contour of shaped reflector
Figure-5 shows comparison of shaped beam and high directivity pencil beam Figure- 6 shows predicted results of simultaneous generation of shaped beam and
pencil beam at 11.575 GHz Figure- 7 shows measured results of simultaneous generation of shaped beam and
pencil beam at 11.575 GHz
In the present invention, as shown in Figure 1, the reflector (1) is a shaped reflector, which is shaped for arbitrary coverage as shown in Figure-3. First feed (2) is placed at the focal point of the reflector (1). Second feed (3) is placed away from the focal plane (4). The placement of second feed (3) depends upon the phase error introduced by the shaped reflector (1) in the aperture plane (5) for generating pencil beam.
The pencil beam is generated from the shaped reflector when the aperture field has uniform phase distribution. This generates high directivity pencil beam. In order to compensate the phase error introduced by shaped reflector (1) in the aperture plane (5), the focal plane (4) feeds are to be accordingly matched.
The iso-level contours of deviations (9) of shaped reflector profile from the parent paraboloid are shown in Figure-3. It shows that the surface deviations are quite
large in terms of the wavelength where 25mm is wavelength at central frequency.
The shaped beam gain contours (10) generated from this shaped reflector (1) is shown in Figure-4. Using the knowledge of feed phase pattern and reflector profile for computing aperture plane phase distribution, the first feed (2) is displaced from the focal plane (4) such that it generates focal plane field which in-turn realizes nearly uniform phase distribution in the aperture plane (5) of the shaped reflector (1). This field generates high gain pencil beam (12) as shown in Figure-5 with the comparison of shaped beam pattern cut (11). Shaped beam pattern cut (11) shows wide beam with lower gain achieved from shaped reflector (1) and first feed (2). Pencil beam pattern cut (12) shows pencil beam generated from the same shaped reflector (1) using second feed or displaced feed (3).
Comparison as shown in Figure-5 indicate that pencil beam (12) is having nearly 10 dB higher gain compared to that of shaped beam (11). . This indicates that second feed (3) has compensated (nullified) the effect of shaped surface profile and generated pencil beam equivalent to parabolic reflector of nearly same size.
Figure-6 illustrates simultaneously generated pencil beam (13) and shaped beam (14) gain contours (predicted performance) indicating two isolated coverage with different size and coverage gain.
Figure-7 shows the measured gain contours of shaped reflector for pencil beam (15) and for shaped beam (16) achieved from the invention implemented with shaped reflector of dimension 2.2m x 2m (elliptical projection) and at central frequency of 11.575 GHz.
The present invention is also extended for compensation of reflector surface distortion of large unfurlable reflector antenna in which feed is kept linearly movable on a bracket with flexible cable or flexible waveguide. The feed displacement depends
upon the order of reflector surface distortion. Applications
This antenna finds strong application in communication satellite antennas where shaped reflector is required to provide additional pencil beam for covering isolated islands areas. The invention provides simple solution for achieving surface distortion compensation in large unfurlable antennas. The invention also finds application wherein composite beam is required from two far away separated coverage areas.
Index of Reference numerals
Part no. Description
1 Shaped Reflector
2 Feed for shaped beam placed at Focal point
3 Feed for compensating phase errors due to shaped ref lector to generate pencil beam
4 Focal plane of shaped reflector
5 Aperture Plane of shaped reflector
6 Parabolic reflector with surface distortion
7 Feed array
8 Beam forming network
9 Surface deviation contours of shaped ref lector and parent paraboloid
10 Shaped beam pattern cut
11 Pencil beam pattern cut
12 Predicted gain contour of shaped reflector for shaped beam
13 Predicted gain contour of shaped reflector for pencil beam
14 Measured gain contour of shaped reflector for shaped beam
15 Measured gain contour of shaped reflector for pencil beam
The important specifications of the antenna are given below, Frequency Transmit-
Coverage Area - Two coverages 1) Shaped wide coverage, 2) Spot beam
Coverage Gain - 31.5 dBi over coverage-1, and 40 dBi over coverage-2
1. A method for simultaneously generating pencil beam and shaped beam from a
single shaped reflector said reflector being shaped for some arbitrary coverage
comprising the steps of:
providing an elliptical projection shaped reflector;
placing a first feed at the focal point of the reflector;
placing a second feed away from the focal plane; wherein the placement of
second feed depends upon the phase error introduced by the shaped reflector in
the aperture plane for generating pencil beam.
2. The method as claimed in claim 1, wherein a high directivity pencil beam is
generated when the aperture field has uniform phase distribution.
3. The method as claimed in claim 1, wherein in order to compensate the phase error
introduced by the shaped reflector in the aperture plane, the focal plane fields are
4. The method as claimed in any one of the preceding claims, wherein the first feed is
displaced from the focal plane, based on the feed phase pattern and reflector profile
for computing aperture plane phase distribution, such that it generates focal plane
field which in-turn realizes nearly uniform phase distribution in the aperture plane
of the shaped reflector.
5. A method for simultaneously generating pencil beam and shaped beam from a
single shaped reflector as hereinbefore described with reference to figures 1, 3 to 7.
|Indian Patent Application Number||1876/CHE/2007|
|PG Journal Number||35/2013|
|Date of Filing||22-Aug-2007|
|Name of Patentee||INDIAN SPACE RESEARCH ORGANISATION|
|Applicant Address||ISRO HEADQUARTERS, ANTARIKSH BHAVAN, NEW BEL ROAD, BANGALORE-560 094.|
|PCT International Classification Number||G03F 7/00|
|PCT International Application Number||N/A|
|PCT International Filing date|