Title:
OFFSET PARABOLIC REFLECTOR ANTENNA
United States Patent 3696435
Abstract:
In accordance with the disclosure, two or more main beams of an offset parabolic reflector antenna are directed without producing second order aberrations by placing their respective feeds so that their phase centers reside at mathematically defined locations.
US Patent References:
ANTENNA ARRAY AT FOCAL PLANE OF REFLECTOR WITH COUPLING NETWORK FOR BEAM SWITCHING
Korvin - March 1971 - 3569976
ANTENNA ARRAY AT FOCAL PLANE OF REFLECTOR WITH COUPLING NETWORK FOR BEAM SWITCHING
Korvin - March 1971 - 3569976
Application Number:
05/092400
Publication Date:
10/03/1972
Filing Date:
11/24/1970
View Patent Images:
Export Citation:
Assignee:
Bell Telephone Laboratories, Incorporated (Murray Hill, NJ)
Primary Class:
Other Classes:
343/840
International Classes:
H01Q19/13; H01Q19/10; H01Q19/14
Field of Search:
343/776,778,779,781,786,840,854
Primary Examiner:
Lieberman, Eli
Claims:
What is claimed is
1. A directive antenna for producing at least two main beams, said antenna comprising
2. A directive antenna for producing at least two main beams, said antenna comprising
3. A scanning antenna comprising
1. A directive antenna for producing at least two main beams, said antenna comprising
2. A directive antenna for producing at least two main beams, said antenna comprising
3. A scanning antenna comprising
Description:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to offset parabolic reflector antennas.
2. Description of the Prior Art
An offset antenna has an advantage over a symmetrical antenna in that blocking of the antenna aperture by the antenna feed is not present. Elimination of such blockage results in better control of unwanted sidelobes when transmitting or receiving and, when transmitting, of unwanted radiation back into the feed. These advantages are discussed in U.S. Pat. No. 2,644,092, issued to J. R. Risser on June 30, 1953; U.S. Pat. No. 3,407,404 , issued to J. S. Cook et al. on Oct. 22, 1968; and U.S. Pat. No. 3,500,427, issued to S. Landesman et al. on Mar. 10, 1970.
Each of the above-mentioned patents discloses an offset parabolic reflector having a single feed and a radiation pattern having a single main beam. Antennas having multiple feeds to provide multiple independent main beams are, on the other hand, sometimes desirable. U.S. Pat. No. 3,406,401, issued to L. C. Tillotson on Oct. 15, 1968, for example, shows the use of a multiple feed, spherical reflector antenna for satellite communication use. To the best of applicant's knowledge, however, a low aberration multiple feed antenna which includes the advantages offered by the offset parabolic reflector antenna in the on-axis position has not been available.
SUMMARY OF THE INVENTION
An object of the invention is to provide multiple feeds in an offset parabolic reflector antenna in such a manner as to produce multiple main beams having relatively low aberration levels.
This and other objects of the invention are achieved in an offset parabolic reflector antenna by locating each of two or more feeds so that their respective phase centers reside at mathematically defined locations with respect to the offset parabolic reflector. In particular, multiple main beams having substantial zero second order aberrations are achieved by locating two or more feeds so that their phase centers reside substantially on a line unique to the particular reflector.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 shows a side-elevation view of an embodiment of the invention; and
FIG. 2 and 3 show end-elevation and top views, respectively, of the embodiment.
DESCRIPTION OF THE DISCLOSED EMBODIMENT
The side-elevation view of Fig. 1 includes a reflector 10. This reflector comprises an off-axis sector of a paraboloid of revolution formed by the intersection of the paraboloid and a right angle cone whose vertex is located at the focal point of the paraboloid. In FIG. 1 the outline of the paraboloid of which reflector 10 is a sector is shown by a broken line 11 while its focal point and vertex are identified by symbols 12 and 13, respectively. In the end-elevation view shown in FIG. 2, reflector 10 has a circular projection. In the top view shown in FIG. 3, reflector 10 has an elliptical projection. Although an off-axis sector formed in this manner has advantages from a radiation pattern standpoint, off-set sectors of other configurations may be used in practicing the invention.
Reflector 10 and subsequently mentioned feeds may be positionally related through the use of three reference planes. The first plane is a focal plane 14 which includes focal point 12 and, furthermore, is perpendicular to the paraboloid line of symmetry which passes through focal point 12 and vertex 13. The second of the planes is a symmetrical plane 15 which includes the paraboloid line of symmetry and, furthermore, bisects reflector 10. The last of the three planes is referred to as asymmetrical plane 16. This plane passes through focal point 12 and is mutually perpendicular to the other planes.
Two feeds F 1 and elevation F 2 are schematically represented by a pair of dots in the drawing. These feeds may comprise horns or other conventional feeds directed so as to subtend solid angles with reflector 10. Furthermore, because of the reciprocal nature of antennas, they may be used for either transmitting or receiving.
Feeds F 1 and F 2 are positioned so that their phase centers are displaced to the vertex side of focal plane 14 by perpendicularly measured distances substantially equal to z 1 and z 2, respectively, and to either side of symmetrical plane 15 by perpendicular distances substantially equal to x 1 and x 2, respectively. On the other hand, the phase centers lie substantially in asymmetrical plane 16. Furthermore, feed F 1 subtends a substantially solid angle with reflector 10. As shown in FIG. 3, the direction of the main beam for feed F 1 makes an angle β 1 with respect to the paraboloid line of symmetry. Similar but unidentified angles having a subscript of two applied thereto relate to feed F 2 .
In accordance with the invention, second order aberrations in the radiation patterns are substantially eliminated when the values of x and z for each feed are substantially defined by the following:
x = mf sin α (1) z = f (m cos α - 1) (2)
where f is the distance from focal point 12 to vertex 13,
and
m and α satisfy the expressions: ##SPC1##
where angle A is the angle between the axis of the right angle cone and the line of symmetry and angle B is one half of the angle sustained in symmetrical plane 15 by reflector 10 with focal point 12.
(The above relationships are derivable from a spherical coordinate system centered about a point located on the paraboloid line of symmetry and at two focal lengths from the vertex, where the focal length is, of course, the distance between focal point 12 and vertex 13. The angle α is the angle between the line of symmetry and the line drawn between the coordinate system center and the phase center of the feed. Furthermore, m is the ratio of the distance of the phase center from the coordinate system center to the focal distance.)
An embodiment of the invention for operation at 16 GHz was constructed and successfully operated. The embodiment parameters are:
Antenna aperture diameter = 48.4λ, Focal length = 40.0λ, Angle A = 47.5° , and Feeds displaced to either side of the line of symmetry by α = 4.2° , where λ equals the wavelength at 16 GHz.
Measurements showed that the main beams produced by the feeds were displaced to either side of the line of symmetry by β = 3.4° . There was some broadening of the beams, mainly below the 10 db level, and also higher first sidelobes. However, these sidelobes were at the 30 db level. For all practical purposes, there was no significant increase in sidelobe levels for off-axis positions in comparison to the on-axis, focal-point position.
It should be noted that scanning is possible by moving feeds F 1 and F 2 through an arc defined by a continuous set of values for x and z. If only one main beam is desired, one of the feeds may be eliminated.
Further appreciation of the invention may be obtained by selecting the values of angles A and B so that:
sin A/(cos A + cos B) = 1 (5)
When this is done, expressions (3) and (4) become:
Ratio m therefore determines the values of angle α and angle β as follows:
m α β 1.000 0 0 1.001 1.810 0.906 1.005 4.040 2.024 1.010 5.696 2.869 1.015 6.956 3.517 1.020 8.010 4.065 1.025 8.930 4.548 1.030 9.754 4.986 1.035 10.500 5.300
1. Field of the Invention
This invention relates to offset parabolic reflector antennas.
2. Description of the Prior Art
An offset antenna has an advantage over a symmetrical antenna in that blocking of the antenna aperture by the antenna feed is not present. Elimination of such blockage results in better control of unwanted sidelobes when transmitting or receiving and, when transmitting, of unwanted radiation back into the feed. These advantages are discussed in U.S. Pat. No. 2,644,092, issued to J. R. Risser on June 30, 1953; U.S. Pat. No. 3,407,404 , issued to J. S. Cook et al. on Oct. 22, 1968; and U.S. Pat. No. 3,500,427, issued to S. Landesman et al. on Mar. 10, 1970.
Each of the above-mentioned patents discloses an offset parabolic reflector having a single feed and a radiation pattern having a single main beam. Antennas having multiple feeds to provide multiple independent main beams are, on the other hand, sometimes desirable. U.S. Pat. No. 3,406,401, issued to L. C. Tillotson on Oct. 15, 1968, for example, shows the use of a multiple feed, spherical reflector antenna for satellite communication use. To the best of applicant's knowledge, however, a low aberration multiple feed antenna which includes the advantages offered by the offset parabolic reflector antenna in the on-axis position has not been available.
SUMMARY OF THE INVENTION
An object of the invention is to provide multiple feeds in an offset parabolic reflector antenna in such a manner as to produce multiple main beams having relatively low aberration levels.
This and other objects of the invention are achieved in an offset parabolic reflector antenna by locating each of two or more feeds so that their respective phase centers reside at mathematically defined locations with respect to the offset parabolic reflector. In particular, multiple main beams having substantial zero second order aberrations are achieved by locating two or more feeds so that their phase centers reside substantially on a line unique to the particular reflector.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 shows a side-elevation view of an embodiment of the invention; and
FIG. 2 and 3 show end-elevation and top views, respectively, of the embodiment.
DESCRIPTION OF THE DISCLOSED EMBODIMENT
The side-elevation view of Fig. 1 includes a reflector 10. This reflector comprises an off-axis sector of a paraboloid of revolution formed by the intersection of the paraboloid and a right angle cone whose vertex is located at the focal point of the paraboloid. In FIG. 1 the outline of the paraboloid of which reflector 10 is a sector is shown by a broken line 11 while its focal point and vertex are identified by symbols 12 and 13, respectively. In the end-elevation view shown in FIG. 2, reflector 10 has a circular projection. In the top view shown in FIG. 3, reflector 10 has an elliptical projection. Although an off-axis sector formed in this manner has advantages from a radiation pattern standpoint, off-set sectors of other configurations may be used in practicing the invention.
Reflector 10 and subsequently mentioned feeds may be positionally related through the use of three reference planes. The first plane is a focal plane 14 which includes focal point 12 and, furthermore, is perpendicular to the paraboloid line of symmetry which passes through focal point 12 and vertex 13. The second of the planes is a symmetrical plane 15 which includes the paraboloid line of symmetry and, furthermore, bisects reflector 10. The last of the three planes is referred to as asymmetrical plane 16. This plane passes through focal point 12 and is mutually perpendicular to the other planes.
Two feeds F 1 and elevation F 2 are schematically represented by a pair of dots in the drawing. These feeds may comprise horns or other conventional feeds directed so as to subtend solid angles with reflector 10. Furthermore, because of the reciprocal nature of antennas, they may be used for either transmitting or receiving.
Feeds F 1 and F 2 are positioned so that their phase centers are displaced to the vertex side of focal plane 14 by perpendicularly measured distances substantially equal to z 1 and z 2, respectively, and to either side of symmetrical plane 15 by perpendicular distances substantially equal to x 1 and x 2, respectively. On the other hand, the phase centers lie substantially in asymmetrical plane 16. Furthermore, feed F 1 subtends a substantially solid angle with reflector 10. As shown in FIG. 3, the direction of the main beam for feed F 1 makes an angle β 1 with respect to the paraboloid line of symmetry. Similar but unidentified angles having a subscript of two applied thereto relate to feed F 2 .
In accordance with the invention, second order aberrations in the radiation patterns are substantially eliminated when the values of x and z for each feed are substantially defined by the following:
x = mf sin α (1) z = f (m cos α - 1) (2)
where f is the distance from focal point 12 to vertex 13,
and
m and α satisfy the expressions: ##SPC1##
where angle A is the angle between the axis of the right angle cone and the line of symmetry and angle B is one half of the angle sustained in symmetrical plane 15 by reflector 10 with focal point 12.
(The above relationships are derivable from a spherical coordinate system centered about a point located on the paraboloid line of symmetry and at two focal lengths from the vertex, where the focal length is, of course, the distance between focal point 12 and vertex 13. The angle α is the angle between the line of symmetry and the line drawn between the coordinate system center and the phase center of the feed. Furthermore, m is the ratio of the distance of the phase center from the coordinate system center to the focal distance.)
An embodiment of the invention for operation at 16 GHz was constructed and successfully operated. The embodiment parameters are:
Antenna aperture diameter = 48.4λ, Focal length = 40.0λ, Angle A = 47.5° , and Feeds displaced to either side of the line of symmetry by α = 4.2° , where λ equals the wavelength at 16 GHz.
Measurements showed that the main beams produced by the feeds were displaced to either side of the line of symmetry by β = 3.4° . There was some broadening of the beams, mainly below the 10 db level, and also higher first sidelobes. However, these sidelobes were at the 30 db level. For all practical purposes, there was no significant increase in sidelobe levels for off-axis positions in comparison to the on-axis, focal-point position.
It should be noted that scanning is possible by moving feeds F 1 and F 2 through an arc defined by a continuous set of values for x and z. If only one main beam is desired, one of the feeds may be eliminated.
Further appreciation of the invention may be obtained by selecting the values of angles A and B so that:
sin A/(cos A + cos B) = 1 (5)
When this is done, expressions (3) and (4) become:
Ratio m therefore determines the values of angle α and angle β as follows:
m α β 1.000 0 0 1.001 1.810 0.906 1.005 4.040 2.024 1.010 5.696 2.869 1.015 6.956 3.517 1.020 8.010 4.065 1.025 8.930 4.548 1.030 9.754 4.986 1.035 10.500 5.300