Compact frequency reuse antenna
United States Patent 3898667
A compact antenna system that permits orthogonally polarized frequency reuse operation is achieved by two overlapping parabolic reflectors. Each of the reflectors has a reflecting surface comprised of parallel, reflecting, conductive elements with the reflecting elements of one reflector polarized orthogonally to the reflecting elements in the other. Each reflector has an associated feed copolarized with the reflecting elements of the particular reflector. The two reflectors are overlaid without coinciding their respective focus points so that cross-polarized fields generated by the parallel elements in the surface of each reflector from its associated feed are scattered away from the copolarized beam of each reflector.
US Patent References:
Scanning antenna system for horizontally and vertically polarized waves
Van Staaden - July 1961 - 2991473
Variable aperture antenna system
Fink - August 1966 - 3267472
Scanning antenna system for horizontally and vertically polarized waves
Van Staaden - July 1961 - 2991473
Variable aperture antenna system
Fink - August 1966 - 3267472
Application Number:
05/439871
Publication Date:
08/05/1975
Filing Date:
02/06/1974
View Patent Images:
Export Citation:
Assignee:
RCA Corporation (New York, NY)
Primary Class:
Other Classes:
343/840
International Classes:
H01Q19/17; H01Q21/06; H01Q25/00; H01Q19/10; H01Q19/12
Field of Search:
343/756,779,837,838,840,897
Primary Examiner:
Lieberman, Eli
Attorney, Agent or Firm:
Norton, Edward Troike Robert J. L.
Claims:
What is claimed is
1. A compact antenna arrangement for communicating electromagnetic waves with a first and second polarization separated by 90° from one another comprising:
2. The combination claimed in claim 1, wherein said reflectors are mounted with the vertex of each separated so that the focal axis of said first and second reflectors are parallel.
3. The combination claimed in claim 1, wherein said first and second directions are the same.
4. The combination of claim 1, wherein one of said reflectors overlaps substantially half of the other reflector.
5. The combination of claim 1, wherein said reflectors are formed of parallel metal wires embedded in dielectric.
6. An arrangement for communicating electromagnetic waves with a first and second polarization separated by 90° from one another comprising:
7. The combination claimed in claim 6, wherein the region near the vertex of each of said reflectors is overlapped.
1. A compact antenna arrangement for communicating electromagnetic waves with a first and second polarization separated by 90° from one another comprising:
2. The combination claimed in claim 1, wherein said reflectors are mounted with the vertex of each separated so that the focal axis of said first and second reflectors are parallel.
3. The combination claimed in claim 1, wherein said first and second directions are the same.
4. The combination of claim 1, wherein one of said reflectors overlaps substantially half of the other reflector.
5. The combination of claim 1, wherein said reflectors are formed of parallel metal wires embedded in dielectric.
6. An arrangement for communicating electromagnetic waves with a first and second polarization separated by 90° from one another comprising:
7. The combination claimed in claim 6, wherein the region near the vertex of each of said reflectors is overlapped.
Description:
BACKGROUND OF THE INVENTION
This invention relates to a compact frequency reuse antenna system and more particularly to an antenna system which achieves frequency reuse by orthogonally polarized sources and reflectors.
With the ever increasing demand for more frequency spectrum, it is imperative that greater use be made of the allocated spectrum. This is particularly true in satellite communications where the coverage area is a substantial portion of the earth's surface. It is also desirable on a cost basis to achieve more channel space per frequency spectrum. A practical way of achieving spectrum reuse is by communicating with waves of orthogonal polarization. Isolation of the orthogonally polarized waves can be achieved by separating a great distance from each other the feeds and the associated reflectors communicating these orthogonally polarized waves. However, in addition to the desirability of spectrum reuse in satellite communication antennas particularly, there is a need for a highly compact antenna. This is particularly true of the satellite antenna itself. Providing a compact frequency reuse antenna in view of such factors as discussed above poses a problem.
BRIEF DESCRIPTION OF INVENTION
Briefly, a compact frequency reuse antenna system for communicating electromagnetic waves of the same frequency with a given orthogonal polarization separation is achieved by two reflectors having a given contour and resultant focus points. Each of the reflectors comprises parallel reflecting elements, with the elements of one reflector oriented in a first direction to reflect waves polarized in a first direction and with the elements of the other reflector oriented in an orthogonal direction to reflect waves polarized in an orthogonal direction. A first antenna feed is located at the focus of the first reflector for communicating electromagnetic waves copolarized with the elements of the first reflector, and a second antenna feed is located at the focus of the second reflector for communicating electromagnetic waves copolarized with the elements of the second reflector. The reflectors are mounted in an overlapping manner with the focus points of the reflectors sufficiently separated so that cross-polarized fields generated by the parallel elements at the surface of the respective reflectors are scattered away from the copolarized beam of each reflector.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
A more detailed description follows in conjunction with the following drawings wherein:
FIG. 1 is a front elevation drawing of a satellite antenna system mounted to the satellite according to one embodiment of the present invention.
FIG. 2 is a side elevation view of the arrangement shown in FIG. 1 as taken along line 2--2.
FIG. 3 is a front view of a paraboloid of revolution illustrating the portion of the paraboloid of revolution used in the antenna system of FIG. 1.
FIG. 4 is a front elevation drawing illustrating a general placement of two of the reflectors in FIG. 1 relative to each other.
FIG. 5 is a sketch illustrating the operation of a first pair of reflectors in the antenna system shown in FIG. 1.
FIG. 6 is a sketch illustrating a portion of a few of the vertical elements in one reflector and a vector diagram of the copolarized wave applied to these elements.
FIG. 7 is a sketch illustrating a portion of a few of the horizontal elements in one reflector and a vector diagram of the copolarized wave applied to these elements.
FIG. 8 is a sketch illustrating the operation of another pair of overlapping reflectors and associated feeds in the antenna system shown in FIG. 1.
Referring to FIG. 1, a satellite antenna system 10 is shown mounted to satellite structure 11. The antenna system 10 includes reflectors 12, 13, 15 and 17 and waveguide feed horns 19a, 19b, 19c and 19d. The reflectors 12, 13, 15 and 17 are mounted to satellite structure 11 by the support posts 12a, 13a, 13b, 15a, 17a and 17b. Support posts 13b and 17b are hidden from view by horns 19b and 19d in FIG. 1. Referring to the side elevation view of reflectors 15 and 17 in FIG. 2, it is seen that reflector 15 is supported by post 15a extending through reflector 15 and by fixing the end 16 of reflector 15 to one end of posts 17a and 17b. The reflector 15 is fixed to the posts 15a, 17a and 17b by suitable bonding. The reflector 17 is supported by the two posts 17a and 17b extending through the reflector 17 and by fixing the end 16a to posts 15a at a point just slightly rearward of the reflector 15. The post 17b extends only through an edge portion of reflector 17. The reflector 17 is fixed to the posts 15a, 17a and 17b by suitable bonding. Posts 12a, 13a and 13b similarly mount reflector 12 forward of reflector 13 and mount reflector 13 rearward of reflector 12. One suitable material for such support posts in space is a low thermal expansion material known as "GFEC" which letters stand for graphite fiber epoxy composite.
The reflectors 12, 13, 15 and 17 are each portions of a paraboloid of revolution. The portion 21 of the paraboloid of revolution illustrated in FIG. 3 is used for reflector 15. The portion 21a is used for reflector 17. The reflectors 12 and 13 use similar but symmetrical portions of a paraboloid of revolution as shown by dashed lines. The long edge of each of the portions intersects the vertex of the paraboloid. The portion 21 overlaps over half of portion 21a. The vertex V is along the long edge of the portions 21 and 21a. The vertex V is midway along the long edge of portion 21a and about one third up from the end of portion 21. The portion selected is near the center of the paraboloid of revolution to permit more closely spaced overlapping of the reflectors and to minimize excitation of cross-polarized fields.
Referring to FIG. 1, it is seen that the reflectors 12 and 13 overlap each other and the reflectors 15 and 17 overlap each other. The reflector 12 overlaps about one half of reflector 13. Similarly, reflector 15 overlaps about one half of reflector 17. The reflectors are overlapped so that the long edge of the forward reflector overlaps the long edge of the associated rearward reflector. Referring to FIG. 4, for example, reflector 15 overlaps reflector 17 with the long edge overlapped. The vertex point 23 for reflector 15 as shown in FIG. 4 is aligned and spaced from the vertex point 25 of reflector 17. The reflectors 15 and 17 overlap at their more flat ends near the vertex of each with essentially even symmetry. In other words, if one were to place a mirror at the middle of the overlap and cover the upper half, the mirror would reflect from the lower half what would substantially be at the covered upper half. The same is true with respect to the placement of reflectors 12 and 13, with reflector 12 forward or extending further away from structure 11 than reflector 13. The vertex point 23a for reflector 12 is aligned and spaced from the vertex point 25a of reflector 13.
The reflectors 12, 13, 15 and 17 are formed of parallel conductive elements as represented in part by the parallel lines in FIGS. 1 and 4. The parallel elements of reflectors 12 and 17 are represented by vertically oriented parallel lines 27 and 29, respectively. The parallel elements of reflectors 13 and 15 are represented by the horizontally oriented parallel lines 31 and 33, respectively. The elements that make up the reflectors 13 and 15 are oriented orthogonal to the elements that make up the reflectors 12 and 17. The parallel elements forming the reflectors may be provided by a plurality of closely spaced parallel wires embedded in a low dielectric plastic base. The wires are laid in such a manner that viewed from a great distance along the axis of the generating paraboloid they appear everywhere parallel to themselves and to an electric field generated by an associated copolarized feed.
The feed horn 19a is designed to couple signals over a given wide frequency band and is polarized to communicate vertically polarized waves. This feed horn 19a is mounted by support 51 extending from satellite structure 11 to the horn 19a as shown in FIG. 1. The feed horn 19a is positioned with the aperture of the horn 19a at the focus point of the copolarized reflector 12. The feed horn 19a is further oriented to optimize the illumination of reflector 12.
The feed horn 19b is designed to couple signals over the same frequency band as feed horn 19a but is polarized to communicate horizontally polarized waves. This feed horn 19b is mounted by support 53 extending from satellite structure 11 to the horn 19b to position the aperture of the horn 19b at the focus point of copolarized reflector 13. The feed horn 19b is further oriented to optimize the illumination of reflector 13.
The feed horns 19c and 19d are polarized to communicate horizontally and vertically polarized waves, respectively. The horns 19c and 19d are designed to couple signals over the same frequency band. This frequency band may be the same as the given frequency band or may be another frequency band. In the preferred embodiment horns 19a, 19b, 19c and 19d are designed to communicate signals over the same frequency band. The sub-frequency bands or channels within this wide frequency band communicated by horns 19a and 19b in this preferred arrangement differ from that communicated by horns 19c and 19d. The horns 19a and 19b communicate the odd numbered channels for example and horns 19c and 19d communicate the even numbered channels. This minimizes the problems associated with multiplexing these signals. Similarly the horn 19c is mounted by support 55 at the focus point of copolarized (horizontal) reflector 15, and the horn 19d is mounted by support 57 at the focus point of copolarized (vertical) reflector 17. FIG. 2 illustrates more clearly how the supports 55 and 57 are mounted between the horns 19c and 19d and structure 11. The feed horns 19a, 19b, 19c and 19d are coupled to the transmitter and receiver circuitry located within the structure 11 by waveguides such as waveguides 59 and 61 illustrated in FIG. 2 extending between feed horns 19c and 19d, respectively, and structure 11. The feed horns 19c and 19d are further oriented to optimize illumination of reflectors 15 and 17, respectively. The vertically oriented elements of reflectors 12 and 17 pass horizontally polarized waves and reflect the vertically polarized waves. The horizontally oriented elements of reflectors 13 and 15 pass vertically polarized waves and reflect the horizontally polarized waves. In the overlapping region 18 and 20 in FIG. 1, both horizontally and vertically polarized waves are reflected.
Referring to the illustrative sketch of FIG. 5, operation of the antenna system is considered in the case of transmitted vertically polarized waves toward reflector 12. These waves radiated toward the reflector 12 by horn 19a located at the focus f 1 of reflector 12, are represented in FIG. 5 by dashed lines 26. The waves are intercepted by reflector 12 having vertical elements and are directed in phase to the antenna aperture (collimated) whereupon a radiated beam in a given direction of arrow 28 is provided. The vertex 23a of the reflector 12 is along one edge 12d of reflector 12 as shown in FIG. 1. Since the horn 19a is located at the focus of reflector 12 and in line and forward of the vertex 23a, the horn 19a provides low blockage of the vertically polarized waves. The reciprocal operation takes place with respect to the received in phase waves at the antenna aperture. These in phase waves at the aperture are reflected to horn 19a. In the case of horizontally polarized waves transmitted to the reflector 13 by horn 19b located at the focus f 2 of reflector 13, these waves represented by dashed lines 36 in FIG. 5 are intercepted by horizontally polarized reflector 13 and are directed in phase at the antenna aperture (collimated) whereupon in the illustrated example a radiated beam in the same given direction of arrow 28 is provided. Since the vertex 25 a of the reflector 13 is along one edge of reflector 13 and the horn 19b is at the focus f 2 in line with the vertex, the horn 19b provides low blockage of the horizontally polarized waves.
The vertically polarized waves represented by dashed lines 26 passing through the reflector 12 at the common region 18 generate a cross (horizontally) polarized field 26h at the reflelctor 12 which field passes on to reflector 13 whereupon they are reflected. See FIG. 5. Although considerable skill may be extended in an effort to achieve perfectly parallel vertical elements in reflector 12 represented by lines 27 in FIG. 1, some degree of cross (horizontally) polarized field is generated by reflector 12 when intercepting the vertically polarized waves. This may be explained in part by a slight misalignment of the parallel elements relative to the polarization of the waves near the edge 22 furthest from the long edge 12d intersecting the vertex 23a. FIG. 6 is an enlarged view of a portion of a few elements 27 represented by lines located near edge 22 of reflector 12 and an exaggerated vector diagram of the copolarized wave 27a applied to these elements and the associated vector components. The misalignment of wave vector 27a relative to the lines 27 is greatly exaggerated for purposes of illustration. With the misalignment of the reflector elements 27 relative to the polarization of the incident wave along vector 27a, there is a horizontal vector component 27b of the wave in addition to a vertical vector component 27c. This misalignment of the applied field is due in part to a polarization change in the incident wave as the wave makes a greater angle with respect to the focal axis (increases as the wave is off the focal axis). The result is that with an applied field to these reflectors a cross (horizontal) field is produced. This is represented in FIG. 5 by dashed lines 26h.
The reflectors 12 and 13 are overlapped near the vertex of each to lessen this misalignment effect at the overlapped region and therefore minimize this effect. In addition, by displacing the focal points f 1 and f 2 of the two reflectors 12 and 13 a sufficient distance D, as shown in FIG. 5, the cross-polarized field 26h at the horizontally polarized reflector 13 (associated with the generated cross-polarized field 26h) is squinted away from the beam direction 28 associated with the vertically polarized reflector and at the same time is partially de-collimated.
In like manner, a vertically polarized field generated at the horizontally polarized conducting wires 31 in reflector 13 are scattered away from the horizontally polarized beam. Although considerable effect may be taken to achieve perfectly, parallel, horizontal elements in reflector 13 represented by wires or lines 31 in FIG. 1, some degree of cross (vertically) polarized field (represented in FIG. 5 by dashed lines 36v) is generated at reflector 13 when intercepting the horizontally polarized waves from feed horn 19b. This is due to a slight misalignment of the reflector elements relative to the polarization of the wave as viewed by the waves near the edge 22a in FIG. 1. FIG. 7 is an enlarged view of a portion of a few of the elements represented by lines 31 located near the edge 22a of reflector 13 and an exaggerated vector diagram of the copolarized wave applied to these reflector elements 31. It is to be noted that with the misalignment of the reflector elements 31 relative to the polarization of the incident wave along vector 31a, there is a vertical vector 31b in addition to a horizontal vector 31c. This misalignment is due in part to polarization change in the incident wave as the wave makes a greater angle with respect to the focal axis. The result is that a cross (vertical) field 36v is produced. By the displacement of the focal points f 1 and f 2 of the two reflectors 12 and 13 the sufficient distance D as discussed previously and illustrated in FIG. 5, this cross-polarized field 36v at reflector 13 is squinted away from the main horizontally polarized beam in the direction 28. In the case of received horizontally polarized waves generating cross (vertical) polarized waves at reflector 13, these cross (vertical) polarized waves are squinted away at reflector 13 from either of the horns 19a or 19b.
In the previously described arrangement with the focus f 1 spaced from focus f 2 by ten inches and with the vertex 23a spaced from vertex 25a so that the focal axis of reflector 12 is parallel to the focal axis of reflector 13, the unwanted cross-polarized wave is scattered 20 degrees from the desired direction of the communication signal (direction 28 in FIG. 5). With this antenna system in a satellite orbit such a scattering of 20° is sufficient to direct the squinted wave away from earth.
Referring to the sketch of FIG. 8 and considering the case of horizontally polarized waves transmitted to reflector 15 by horn 19c located at the focus f 3 of reflector 15, these waves 46 intercepted by reflector 15 are collimated and directed as a beam in for example the same direction 28 as the waves reflected by reflectors 12 and 13. In the case of vertically oriented waves communicated to reflector 17 by horn 19d located at the focus f 4 of reflector 17, these waves 56 intercepted by reflector 17 are collimated and directed as a beam in for example the same direction 28 as the waves reflected by reflectors 12, 13 and 15. The vertex of each of the reflectors 15 and 17 is located on one edge of these reflectors as shown in FIG. 1. The feed horn 19c is located at the focus f 3 of the reflector 15 and the feed horn 19d is located at the focus f 4 of reflector 17. Since the vertex is along one edge of each of the reflectors low blockage of the received or reflected waves occurs.
The vertically oriented waves 56 from horn 19d passing through the common region 20 shown in FIG. 1 pass through the reflector 15 to reflector 17 whereupon they are reflected. As discussed above, some degree of cross (horizontally) polarized waves indicated by dashed lines 56h are excited and they are squinted away as discussed previously due to the sufficient separation D of the focus points f 3 and f 4 of the two reflectors 15 and 17. In the case of horizontally polarized waves 46 passing through the reflector 15 at the common region 20 they generate cross (vertically) polarized fields 46v which are passed on to reflector 17. As mentioned previously, these cross-polarized fields are squinted away due to the sufficient separation of the focus points f 3 and f 4 of the two reflectors. The reciprocity theory of antennas applies in the operation of the antenna structure described herein. Therefore whatever happens in the transmission mode described previously applies in reverse in the reception mode.
This invention relates to a compact frequency reuse antenna system and more particularly to an antenna system which achieves frequency reuse by orthogonally polarized sources and reflectors.
With the ever increasing demand for more frequency spectrum, it is imperative that greater use be made of the allocated spectrum. This is particularly true in satellite communications where the coverage area is a substantial portion of the earth's surface. It is also desirable on a cost basis to achieve more channel space per frequency spectrum. A practical way of achieving spectrum reuse is by communicating with waves of orthogonal polarization. Isolation of the orthogonally polarized waves can be achieved by separating a great distance from each other the feeds and the associated reflectors communicating these orthogonally polarized waves. However, in addition to the desirability of spectrum reuse in satellite communication antennas particularly, there is a need for a highly compact antenna. This is particularly true of the satellite antenna itself. Providing a compact frequency reuse antenna in view of such factors as discussed above poses a problem.
BRIEF DESCRIPTION OF INVENTION
Briefly, a compact frequency reuse antenna system for communicating electromagnetic waves of the same frequency with a given orthogonal polarization separation is achieved by two reflectors having a given contour and resultant focus points. Each of the reflectors comprises parallel reflecting elements, with the elements of one reflector oriented in a first direction to reflect waves polarized in a first direction and with the elements of the other reflector oriented in an orthogonal direction to reflect waves polarized in an orthogonal direction. A first antenna feed is located at the focus of the first reflector for communicating electromagnetic waves copolarized with the elements of the first reflector, and a second antenna feed is located at the focus of the second reflector for communicating electromagnetic waves copolarized with the elements of the second reflector. The reflectors are mounted in an overlapping manner with the focus points of the reflectors sufficiently separated so that cross-polarized fields generated by the parallel elements at the surface of the respective reflectors are scattered away from the copolarized beam of each reflector.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
A more detailed description follows in conjunction with the following drawings wherein:
FIG. 1 is a front elevation drawing of a satellite antenna system mounted to the satellite according to one embodiment of the present invention.
FIG. 2 is a side elevation view of the arrangement shown in FIG. 1 as taken along line 2--2.
FIG. 3 is a front view of a paraboloid of revolution illustrating the portion of the paraboloid of revolution used in the antenna system of FIG. 1.
FIG. 4 is a front elevation drawing illustrating a general placement of two of the reflectors in FIG. 1 relative to each other.
FIG. 5 is a sketch illustrating the operation of a first pair of reflectors in the antenna system shown in FIG. 1.
FIG. 6 is a sketch illustrating a portion of a few of the vertical elements in one reflector and a vector diagram of the copolarized wave applied to these elements.
FIG. 7 is a sketch illustrating a portion of a few of the horizontal elements in one reflector and a vector diagram of the copolarized wave applied to these elements.
FIG. 8 is a sketch illustrating the operation of another pair of overlapping reflectors and associated feeds in the antenna system shown in FIG. 1.
Referring to FIG. 1, a satellite antenna system 10 is shown mounted to satellite structure 11. The antenna system 10 includes reflectors 12, 13, 15 and 17 and waveguide feed horns 19a, 19b, 19c and 19d. The reflectors 12, 13, 15 and 17 are mounted to satellite structure 11 by the support posts 12a, 13a, 13b, 15a, 17a and 17b. Support posts 13b and 17b are hidden from view by horns 19b and 19d in FIG. 1. Referring to the side elevation view of reflectors 15 and 17 in FIG. 2, it is seen that reflector 15 is supported by post 15a extending through reflector 15 and by fixing the end 16 of reflector 15 to one end of posts 17a and 17b. The reflector 15 is fixed to the posts 15a, 17a and 17b by suitable bonding. The reflector 17 is supported by the two posts 17a and 17b extending through the reflector 17 and by fixing the end 16a to posts 15a at a point just slightly rearward of the reflector 15. The post 17b extends only through an edge portion of reflector 17. The reflector 17 is fixed to the posts 15a, 17a and 17b by suitable bonding. Posts 12a, 13a and 13b similarly mount reflector 12 forward of reflector 13 and mount reflector 13 rearward of reflector 12. One suitable material for such support posts in space is a low thermal expansion material known as "GFEC" which letters stand for graphite fiber epoxy composite.
The reflectors 12, 13, 15 and 17 are each portions of a paraboloid of revolution. The portion 21 of the paraboloid of revolution illustrated in FIG. 3 is used for reflector 15. The portion 21a is used for reflector 17. The reflectors 12 and 13 use similar but symmetrical portions of a paraboloid of revolution as shown by dashed lines. The long edge of each of the portions intersects the vertex of the paraboloid. The portion 21 overlaps over half of portion 21a. The vertex V is along the long edge of the portions 21 and 21a. The vertex V is midway along the long edge of portion 21a and about one third up from the end of portion 21. The portion selected is near the center of the paraboloid of revolution to permit more closely spaced overlapping of the reflectors and to minimize excitation of cross-polarized fields.
Referring to FIG. 1, it is seen that the reflectors 12 and 13 overlap each other and the reflectors 15 and 17 overlap each other. The reflector 12 overlaps about one half of reflector 13. Similarly, reflector 15 overlaps about one half of reflector 17. The reflectors are overlapped so that the long edge of the forward reflector overlaps the long edge of the associated rearward reflector. Referring to FIG. 4, for example, reflector 15 overlaps reflector 17 with the long edge overlapped. The vertex point 23 for reflector 15 as shown in FIG. 4 is aligned and spaced from the vertex point 25 of reflector 17. The reflectors 15 and 17 overlap at their more flat ends near the vertex of each with essentially even symmetry. In other words, if one were to place a mirror at the middle of the overlap and cover the upper half, the mirror would reflect from the lower half what would substantially be at the covered upper half. The same is true with respect to the placement of reflectors 12 and 13, with reflector 12 forward or extending further away from structure 11 than reflector 13. The vertex point 23a for reflector 12 is aligned and spaced from the vertex point 25a of reflector 13.
The reflectors 12, 13, 15 and 17 are formed of parallel conductive elements as represented in part by the parallel lines in FIGS. 1 and 4. The parallel elements of reflectors 12 and 17 are represented by vertically oriented parallel lines 27 and 29, respectively. The parallel elements of reflectors 13 and 15 are represented by the horizontally oriented parallel lines 31 and 33, respectively. The elements that make up the reflectors 13 and 15 are oriented orthogonal to the elements that make up the reflectors 12 and 17. The parallel elements forming the reflectors may be provided by a plurality of closely spaced parallel wires embedded in a low dielectric plastic base. The wires are laid in such a manner that viewed from a great distance along the axis of the generating paraboloid they appear everywhere parallel to themselves and to an electric field generated by an associated copolarized feed.
The feed horn 19a is designed to couple signals over a given wide frequency band and is polarized to communicate vertically polarized waves. This feed horn 19a is mounted by support 51 extending from satellite structure 11 to the horn 19a as shown in FIG. 1. The feed horn 19a is positioned with the aperture of the horn 19a at the focus point of the copolarized reflector 12. The feed horn 19a is further oriented to optimize the illumination of reflector 12.
The feed horn 19b is designed to couple signals over the same frequency band as feed horn 19a but is polarized to communicate horizontally polarized waves. This feed horn 19b is mounted by support 53 extending from satellite structure 11 to the horn 19b to position the aperture of the horn 19b at the focus point of copolarized reflector 13. The feed horn 19b is further oriented to optimize the illumination of reflector 13.
The feed horns 19c and 19d are polarized to communicate horizontally and vertically polarized waves, respectively. The horns 19c and 19d are designed to couple signals over the same frequency band. This frequency band may be the same as the given frequency band or may be another frequency band. In the preferred embodiment horns 19a, 19b, 19c and 19d are designed to communicate signals over the same frequency band. The sub-frequency bands or channels within this wide frequency band communicated by horns 19a and 19b in this preferred arrangement differ from that communicated by horns 19c and 19d. The horns 19a and 19b communicate the odd numbered channels for example and horns 19c and 19d communicate the even numbered channels. This minimizes the problems associated with multiplexing these signals. Similarly the horn 19c is mounted by support 55 at the focus point of copolarized (horizontal) reflector 15, and the horn 19d is mounted by support 57 at the focus point of copolarized (vertical) reflector 17. FIG. 2 illustrates more clearly how the supports 55 and 57 are mounted between the horns 19c and 19d and structure 11. The feed horns 19a, 19b, 19c and 19d are coupled to the transmitter and receiver circuitry located within the structure 11 by waveguides such as waveguides 59 and 61 illustrated in FIG. 2 extending between feed horns 19c and 19d, respectively, and structure 11. The feed horns 19c and 19d are further oriented to optimize illumination of reflectors 15 and 17, respectively. The vertically oriented elements of reflectors 12 and 17 pass horizontally polarized waves and reflect the vertically polarized waves. The horizontally oriented elements of reflectors 13 and 15 pass vertically polarized waves and reflect the horizontally polarized waves. In the overlapping region 18 and 20 in FIG. 1, both horizontally and vertically polarized waves are reflected.
Referring to the illustrative sketch of FIG. 5, operation of the antenna system is considered in the case of transmitted vertically polarized waves toward reflector 12. These waves radiated toward the reflector 12 by horn 19a located at the focus f 1 of reflector 12, are represented in FIG. 5 by dashed lines 26. The waves are intercepted by reflector 12 having vertical elements and are directed in phase to the antenna aperture (collimated) whereupon a radiated beam in a given direction of arrow 28 is provided. The vertex 23a of the reflector 12 is along one edge 12d of reflector 12 as shown in FIG. 1. Since the horn 19a is located at the focus of reflector 12 and in line and forward of the vertex 23a, the horn 19a provides low blockage of the vertically polarized waves. The reciprocal operation takes place with respect to the received in phase waves at the antenna aperture. These in phase waves at the aperture are reflected to horn 19a. In the case of horizontally polarized waves transmitted to the reflector 13 by horn 19b located at the focus f 2 of reflector 13, these waves represented by dashed lines 36 in FIG. 5 are intercepted by horizontally polarized reflector 13 and are directed in phase at the antenna aperture (collimated) whereupon in the illustrated example a radiated beam in the same given direction of arrow 28 is provided. Since the vertex 25 a of the reflector 13 is along one edge of reflector 13 and the horn 19b is at the focus f 2 in line with the vertex, the horn 19b provides low blockage of the horizontally polarized waves.
The vertically polarized waves represented by dashed lines 26 passing through the reflector 12 at the common region 18 generate a cross (horizontally) polarized field 26h at the reflelctor 12 which field passes on to reflector 13 whereupon they are reflected. See FIG. 5. Although considerable skill may be extended in an effort to achieve perfectly parallel vertical elements in reflector 12 represented by lines 27 in FIG. 1, some degree of cross (horizontally) polarized field is generated by reflector 12 when intercepting the vertically polarized waves. This may be explained in part by a slight misalignment of the parallel elements relative to the polarization of the waves near the edge 22 furthest from the long edge 12d intersecting the vertex 23a. FIG. 6 is an enlarged view of a portion of a few elements 27 represented by lines located near edge 22 of reflector 12 and an exaggerated vector diagram of the copolarized wave 27a applied to these elements and the associated vector components. The misalignment of wave vector 27a relative to the lines 27 is greatly exaggerated for purposes of illustration. With the misalignment of the reflector elements 27 relative to the polarization of the incident wave along vector 27a, there is a horizontal vector component 27b of the wave in addition to a vertical vector component 27c. This misalignment of the applied field is due in part to a polarization change in the incident wave as the wave makes a greater angle with respect to the focal axis (increases as the wave is off the focal axis). The result is that with an applied field to these reflectors a cross (horizontal) field is produced. This is represented in FIG. 5 by dashed lines 26h.
The reflectors 12 and 13 are overlapped near the vertex of each to lessen this misalignment effect at the overlapped region and therefore minimize this effect. In addition, by displacing the focal points f 1 and f 2 of the two reflectors 12 and 13 a sufficient distance D, as shown in FIG. 5, the cross-polarized field 26h at the horizontally polarized reflector 13 (associated with the generated cross-polarized field 26h) is squinted away from the beam direction 28 associated with the vertically polarized reflector and at the same time is partially de-collimated.
In like manner, a vertically polarized field generated at the horizontally polarized conducting wires 31 in reflector 13 are scattered away from the horizontally polarized beam. Although considerable effect may be taken to achieve perfectly, parallel, horizontal elements in reflector 13 represented by wires or lines 31 in FIG. 1, some degree of cross (vertically) polarized field (represented in FIG. 5 by dashed lines 36v) is generated at reflector 13 when intercepting the horizontally polarized waves from feed horn 19b. This is due to a slight misalignment of the reflector elements relative to the polarization of the wave as viewed by the waves near the edge 22a in FIG. 1. FIG. 7 is an enlarged view of a portion of a few of the elements represented by lines 31 located near the edge 22a of reflector 13 and an exaggerated vector diagram of the copolarized wave applied to these reflector elements 31. It is to be noted that with the misalignment of the reflector elements 31 relative to the polarization of the incident wave along vector 31a, there is a vertical vector 31b in addition to a horizontal vector 31c. This misalignment is due in part to polarization change in the incident wave as the wave makes a greater angle with respect to the focal axis. The result is that a cross (vertical) field 36v is produced. By the displacement of the focal points f 1 and f 2 of the two reflectors 12 and 13 the sufficient distance D as discussed previously and illustrated in FIG. 5, this cross-polarized field 36v at reflector 13 is squinted away from the main horizontally polarized beam in the direction 28. In the case of received horizontally polarized waves generating cross (vertical) polarized waves at reflector 13, these cross (vertical) polarized waves are squinted away at reflector 13 from either of the horns 19a or 19b.
In the previously described arrangement with the focus f 1 spaced from focus f 2 by ten inches and with the vertex 23a spaced from vertex 25a so that the focal axis of reflector 12 is parallel to the focal axis of reflector 13, the unwanted cross-polarized wave is scattered 20 degrees from the desired direction of the communication signal (direction 28 in FIG. 5). With this antenna system in a satellite orbit such a scattering of 20° is sufficient to direct the squinted wave away from earth.
Referring to the sketch of FIG. 8 and considering the case of horizontally polarized waves transmitted to reflector 15 by horn 19c located at the focus f 3 of reflector 15, these waves 46 intercepted by reflector 15 are collimated and directed as a beam in for example the same direction 28 as the waves reflected by reflectors 12 and 13. In the case of vertically oriented waves communicated to reflector 17 by horn 19d located at the focus f 4 of reflector 17, these waves 56 intercepted by reflector 17 are collimated and directed as a beam in for example the same direction 28 as the waves reflected by reflectors 12, 13 and 15. The vertex of each of the reflectors 15 and 17 is located on one edge of these reflectors as shown in FIG. 1. The feed horn 19c is located at the focus f 3 of the reflector 15 and the feed horn 19d is located at the focus f 4 of reflector 17. Since the vertex is along one edge of each of the reflectors low blockage of the received or reflected waves occurs.
The vertically oriented waves 56 from horn 19d passing through the common region 20 shown in FIG. 1 pass through the reflector 15 to reflector 17 whereupon they are reflected. As discussed above, some degree of cross (horizontally) polarized waves indicated by dashed lines 56h are excited and they are squinted away as discussed previously due to the sufficient separation D of the focus points f 3 and f 4 of the two reflectors 15 and 17. In the case of horizontally polarized waves 46 passing through the reflector 15 at the common region 20 they generate cross (vertically) polarized fields 46v which are passed on to reflector 17. As mentioned previously, these cross-polarized fields are squinted away due to the sufficient separation of the focus points f 3 and f 4 of the two reflectors. The reciprocity theory of antennas applies in the operation of the antenna structure described herein. Therefore whatever happens in the transmission mode described previously applies in reverse in the reception mode.