BACKGROUND Q£ IH£ INVENTION

This invention relates to antennas which are suitable for spacecraft and are capable of transmitting separate electromagnetic signals independently at different polarizations and/ more particularly, ^' to a construction of an antenna having plural reflectors of substantially concentric curvature spaced apart by electrically insulating material and reflecting dual polarizations via a polarized subreflector for coupling to a pair of feed horns.

Spacecraft carry antennas which form a part of a communication link between the spacecraft and communication stations on the earth. The antennas serve to focus and to direct electromagnetic energy along the communication link. Various communication channels are employed in a communication link, which channels may be separated from each other in frequency and in polarization of the electromagnetic energy. Of particular interest, herein, is the construction of

antennas to provide for separation of the communication channels by polarization of the electromagnetic energy.

Typically, such antennas include a feed horn and a reflector. The physical arrangement of the horn and the reflector are determined in large measure by the configuration of the spacecraft, as well as by limitations on weight and requirements for rigidity.

As a result of these requirements, it is usually necessary to offset the position of a feed horn from a central axis of symmetry of an antenna reflector. To preserve a direction of polarization of the electromagnetic energy, and to prevent the generation of a cross-polarized component, a reflector may be constructed with a reflecting surface -having parallel, electrically conductive elements in the form of a screen.

In the case wherein two reflecting surfaces are to be employed for the reflection of electromagnetic waves of differing polarization, such as two cross-polarized waves, it is desireable to construct a single supporting structure for both of the reflecting surfaces, thereby to conserve overall weight of the antenna structure. Such a sharing of support structure requires a positioning of feed horns such that their respective polarized electromagnetic waves impinge upon the desired reflectors for directing the waves of the respective polarizations in the desired directions. In addition, the assembly of reflectors must be configured

in a fashion such that the presence of one reflector does not interfere with the propagation of electromagnetic energy between a second reflector and the feed horn associated therewith.

A problem arises in that modes of construction which are presently available for a composite antenna structure having plural reflectors entails a greater weight to the support structure than is desirable.

SUMMARY Q£ lfl£ INVENTION

The aforementioned problem is overcome and other advantages are provided by a mode of construction of an antenna system having plural reflectors positioned on a common support in spaced-apart relation while permitting independent illumination of the reflectors by their respective feeds. In accordance with an important feature of the invention, the configuration of the reflectors on the common support provides a reduction in overall weight of the antenna system while retaining a desired level of rigidity in the reflectors to ensure accurate formation of beams of electromagnetic energy.

The reflector support is formed as a shell of honeycomb material transparent to the electromagnetic energy. The inner and the outer surfaces of the shell are provided with a curvature suitable for forming a beam, a typical surface of curvature being a paraboloid formed by

rotation of a parabola about an axis of the surface. Along the inner surface of the shell there are provided strips of electrically conductive material oriented parallel to each other for reflection of electromagnetic energy wherein the electric field is parallel to the. electrically-conductive strips. On the outer surface of the shell there is provided a second set of electrically conductive strips which are parallel to each other and are oriented perpendicularly to the electrically-conductive strips on the inner surface of the shell. The strips on the outer surface of the shell reflect electromagnetic radiation having an electric field perpendicular to the electric field of radiation reflected from the strips on the inner surface of the shell. Also, the strips on the inner surface of the shell are sufficiently narrow so as to permit propagation of the course polarized radiation to the outer surface with substantially no attenuation by the strips on the inner surface. The strips on the inner surface of the shell serve to form an inner reflector, and the strips on the outer surface of the shell serve to form an outer reflector of the electromagnetic energy.

It is desired to make the shell as thin as possible, consistent with sufficient rigidity for maintaining dimensional stability of the reflectors. The shell can be made uniformly thin by arranging the reflectors such that their axes coincide. Also, the focal lengths of the two reflectors are substantially the same so as

to permit a substantially uniform spacing between the two reflectors. This minimizes the total volume of the honeycomb structure and, therefore, minimizes the weight of the supporting structure of the two reflectors.

In order to illuminate the two reflectors with cross- polarized radiation, two feed horns are employed, the two horns transmitting radiation toward the concave sides of the reflectors via a subreflector assembly having a polarization sensitive surface, such as a set of parallel, electrically-conductive strips, which reflect polarization in .one plane while being transmissive to polarization in a perpendicular plane. With such a configuration of subreflector, the two feed horns can be oriented perpendicularly to each other, with an element of the subreflector positioned between the horns so as to electrically isolate the two horns from each other. Also, the horns can be individually positioned at the focal points of the respective reflectors. This permits the linearly polarized waves of the individual horns to propagate via their respective reflectors for forming the desired transmitting and receiving beams of radiation. Also, the use of polarization-sensitive elements in both the reflectors and the subreflector assembly provides for a double filtering of polarization to ensure purity of the respective communication channels and minimization of any cross-coupling of electromagnetic energy between the two communication channels.

BRIEF DESCRIPTION OP THE DRAWING

The foregoing aspects and other features of the invention are explained in the following description, taken in connection with the accompanying drawing wherein:

Fig. 1 shows a v iew of an antenna system of the invention including a first embodiment of a feed structure, a portion of a reflector assembly being cut away to show reflective screens on both the front and back surfaces of a composite reflector;

Fig. 2 shows a v iew of an antenna system of the invention, similar to the view of Fig. 1, but including a feed structure in accordance with a second embodiment of the invention;

Fig. 3 is an enlarged fragmentary view of a composite reflector of Figs. 1 and 2 showing .the laying of tapes of strips of ref l ecting mater ia l in orthogonal directions on opposed surfaces of the compos ite reflector;

Fig. 4 is a sectional view of a portion of tape taken along the line 4-4 in Fig. 3;

Fig. 5 is a top plan view, shown diagrammatical ly, of the antenna system of Fig. 1 , the f ront ref lecting

surface of the composite reflector being a horizontally polarized screen;

Fig.6 is a perspective view, shown diagrammatically, of the antenna system of Fig. 1;

Fig. 7 is a side view of the antenna system of Fig. 1, shown diagrammatically and with the composite reflector in sectional view;

Fig. 8 is a view taken along a boresight axis of the composite reflector of the antenna system of Fig. 1, shown diagrammatically;.

Fig. 9 is an enlarged fragmentary view, encircled by the line 9-9 in Fig. 8 of a front surface of the composite reflector of Fig. 1 showing metallic strips which form a front reflecting surface;

Fig. 10 is a top plan view of the antenna system of Fig. 2, shown diagrammatically and wherein a front reflecting surface of the composite antenna has been removed to show vertically oriented reflecting strips of a vertically polarized screen on the back surface of the composite reflector;

Fig. 11 is a perspective view of the antenna system of

Fig. 2, shown diagrammatically and wherein the front reflecting surface has been removed to expose the vertically polarized reflecting screen on the back

surface of the composite reflector;

Fig. 12 is a side elevational view of the antenna system of Fig. 2, shown diagrammatically with the reflector being sectioned, and wherein the front reflecting surface of the composite reflector has been removed to show the vertically polarized reflecting screen on the back surface of the composite reflector;

Fig.13 is a front view of the antenna system of Fig. 12 taken along a boresight axis of the composite reflector, shown diagrammatically, and wherein. a front reflecting surface of the composite reflector has been deleted to show the vertically polarized reflecting screen on the back surface of the composite reflector; and

Fig. 14 is an enlarged fragmentary view, encircled by the line 14-14 in Fig. 13, of the vertically polarized screen on the back surface of the composite reflector.

DETAILED DESCRIPTION

With ref erence to Figs. 1 and 2, there are shown two embodiments of an antenna system of the invention. In the embodiment of Fig. 1, an antenna 20 comprises a reflector assembly 22 and a feed assembly 24 supported by a base 26. The reflector assembly 22 includes a composite reflector - 28 having a front reflecting

surface 30 and a back reflecting surface 32 which are spaced apart from each other by a supporting structure in the form of a dish 34. The dish 34 is secured within an encircling yoke 36 by flanges 38, the yoke 36 upstanding from the base 26 and holding the composite reflector 28 with the front reflecting surface 30 facing the feed assembly 24.

The feed assembly 24 includes two horn feeds 40 and 42 with a polarized screen 44 placed between the two feeds

40 and 42. The feed 42 is located on the side of the screen 44 which faces the reflector assembly 22 while the feed 40 is located on the opposite side of the screen 44. A transceiver 46 is connected by wires 48 to the horns 40 and 42, the transceiver 46 being of well-known configuration and having separate channels for the transmission and reception of signals of the horns 40 and 42. The screen 44 has been partially cut away to facilitate the showing of the horn 42; also, the yoke 36 has been partially cutaway to more clearly show the structure of the composite reflector 28.

In the embodiment of Fig. 2, an antenna 50 is constructed with the same general configuration as the antenna 20 of Fig. 1. The antenna 50 includes a reflector assembly 52 which has the same construction including the composite reflector 28 as the reflector assembly 22 of Fig.1, and a feed assembly 54 which is an alternative embodiment of the feed assembly 54. The reflector assembly 24 and the feed assembly 54 are

supported and positioned relative to each other by a base 56, the feed assembly 54 being positioned in front of the reflector assembly 52.

In the feed assembly 54, the horns 40 and 42 are positioned in a side-by-side relation relative to the general direction of propagation of radiation between the feed assembly 54 and the reflector assembly 52, rather than the front to back arrangement of the feeds 40 and 42 in Fig.1. In Fig.2, the feeds 40 and 42 are connected by the wires 48 to the transceiver 46 as in Fig. 1. The base 56 and the yoke 36 have been partially cut away in Fig. 2 to facilitate a showing of the components, of the antenna 50. The feed assembly 54 comprises two intersecting polarized screens 58 and 60, the direction of polarization of the screen 60 being crossed relative to the direction of polarization of the screen 58. The screens 58 and 60 are positioned between the horns 40 and 42.

Figs. 3 and 4 show further details in the construction of the composite reflector 28 of Figs. 1 and 2. The dish 34 is formed of a core 62 of foamed plastic material sandwiched between a front face sheet 64 and a back face sheet. 66. Strips of tape 68 comprising a backing layer 70 with uniformly spaced apart metallic strips 72 thereon are placed on the sheets 64 and 66. The strips of tape 68 are arranged in a side-by-side array to form the front reflecting surface 30 on the front face sheet 64, and the back reflecting surface 32

on the back face sheet 66. The backing layer 70, the face sheets 64 and 66, and the core 62 are electrically insulating and transparent to electromagnetic radiation. The overall thickness of the composite reflector 28 is less than an inch, typically, with the thickness of the core 62 being in the range of 1/4 inch to 1/2 inch. The application of the tape 68 on the front side of the reflector 28 is in the horizontal direction to provide a horizontal polarization to the front reflecting surface 30. The application of the tape 68 to the back side of the reflector 28 is in the vertical direction to provide a vertical polarization • to the back reflecting surface 32.

A suitable material for the fabrication of the core 62 is a foamed polycarbonate having a honeycomb structure, the material being commercially available in sheets such as that known as Kevlar. The front and the back face sheets 64 and 66 may be formed of a cloth impregnated with a resin. The cloth is positioned initially on a mandrel to attain the desired shape of the reflecting surfaces 30 and 32 after which the resin is cured to provide a rigid structure to the sheets 64 and 66. The shape of each reflecting surface is preferrably a conic section of revolution, such as a parabaloid. The honeycomb structure of the core 62 is readily flexed so as to permit a bending of the core 66 to match the shape of the front face sheet 64 and the back face sheet 66. Adhesive is applied to both the front and the back surfaces of the sheet of material of

the core 62, whereupon the sheet of core material 62 is placed between the face sheets 64 and 66 and bent to mate therewith/ after which the adhesive is cured to construct a rigid lightweight structure, which structure is the dish 34 which supports the front and the back reflecting surfaces 30 and 32.

A suitable material for the backing layer 70 of the tape 68 is a polycarbonate such as Kapton. The backing layer 70 may have a thickness of one mil. The metallic strips 72 are formed by securing a sheet of metal, such as copper, to the backing layer 70. The width of the tape 68 is typically 2.25 inches as is the width of the metallic sheet. The dep~th of the metallic sheet is 0.2 mils, typically. The metallic strips 72 are much narrower than the width of the tape 68, the metallic strips 72 having a width of typically 15 mils with a spacing on center lines of 45 mils. This leaves a spacing of 30 mils between edges of adjacent ones of the metallic strips 72. The metallic strips 72 of copper are formed from a copper sheet by a process of etching the copper sheet or foil after securing the copper to the backing layer 70. The thickness of the face sheets 64 and 66 is approximately 7 mils. The foregoing dimensions of the metallic strips 72 are suitable for operation of the antenna system of the invention for electromagnetic radiation in the frequency range of 4 - 14 GHz (gigahertz). A suitable adhesive for securing the tape 68 to the face sheets 64 and 66 is an epoxy glue. Suitable resins and epoxies

are available for curing at temperatures slightly above room temperature, such as 125 degrees fahrenheit.

In the construction of the composite reflector 28, it is desirable to have each of the metallic strips 72 appear as straight lines when viewed along a boresight axis of the reflector 28. However, due to the curvature of the reflecting surfaces, when the strips 72 are viewed at another direction the strips 72 appear to have a slight curvature. Accordingly, to compensate for the curvature of the reflecting surfaces, thereby to make the strips 72 appear straight as viewed along the boresight', it is desirable to introduce. a slight curvature to the metallic strips 72 as they are etched upon each strip of the tape 68. The amount of curvature varies slightly from strip to strip ^' of the tape 68, depending upon the positions which the respective strips of tape are to occupy within the reflector 28.

In the operation of the invention, the horizontal orientation of the metallic strips 72 in the front reflecting surfaces 30 provides for a reflection of electromagnetic waves wherein the electric field lies in the horizontal direction, while permitting a transmittance of electromagnetic waves having an electric field oriented in the vertical plane. Thus, the front reflecting surface 30 is essentially transparent to vertically polarized radiation allowing such radiation to pass through the front reflecting

surface 30 to be reflected from the back reflecting surface 32 wherein the metallic strips 72 are oriented in the vertical direction. In this way, the two reflecting surfaces 30 and 32 operate independently of each other for indepedently reflecting the horizontally and the vertically polarized radiations.

The feed assemblies 24 and 54 cooperate with the dual polarization capability of the composite reflector 28 by separately transmitting horizontally and vertically polarized radiation from the horns 40 and 42 respectively to the front and the back reflecting surfaces 30 and 32.

In the case of the feed assembly 24, the screen 44 is formed of vertically oriented metallic strips 74 mounted on a substrate 76 which is transparent to electromagnetic radiation. The spacing and width of the strips 74 is the same as that of the strips 72 in the reflector 28. The substrate 76 may also be formed of a polycarbonate. The screen 44 is transparent to horizontally polarized radiation from the horn feed 40 while reflecting vertically polarized radiation from the horn feed 42. Thus, radiation from the feed 40 propagates through the screen 44 to reflect off the front reflecting surface 30 of the reflector 28. Vertically polarized radiation from the feed 42 is reflected by the screen 44 towards the reflector 38 to be reflected from the back reflecting surface 32.

In the case of the feed assembly 54, the screen 60 is provided with horizontal metallic strips 78 disposed on a substrate 80. The screen 58 is provided with vertical metallic strips 82 on a substrate 84. The two substrates 80 and 84 are fabricated of material, such as a polycarbonate, which is transparent to electromagnetic radiation. The screen 58 is transparent to the horizontal radiation of the feed 40 while reflecting the vertically polarized radiation from the feed 42. The screen 60 is transparent to the vertically polarized radiation of the feed 42 while reflecting the horizontal polarized radiation of the feed 40. The radiation f.rom the feed 40 is reflected by the screen 60, a portion of the reflected radiation passing through the screen 58, the horizontally polarized radiation reflected by the screen 60 impinging upon the reflector 28 to be reflected from the front reflecting surface 30. Similarly, vertically polarized radiation of the feed 42 is reflected by the screen 58, a portion of the reflected radiation

-propagating through the screen 60, the vertically polarized radiation reflected by the screen 58 propagating through the front reflecting surface 30 of the reflector 28 to be reflected by the back reflecting surface 32. Thereby, the antenna 50 of Fig.2 and the antenna 20 of Fig. 1 are seen to provide for independent transmission and focusing of their respective horizontally polarized radiations and their respective vertically polarized radiations. The operations of the antennas 20 and 50 are reciprocal

such that independent operation at either polarization can be obtained for both received and transmitted radiation signals.

The arrangement of the metallic strips 72 in the front reflecting surface 30 is shown in Figs. 5-9. These figures also show the arrangement of the metallic strips 74 on the screen 44. In addition, these figures show the front-to-back arrangement of the feeds 40 and 42 and the point of intercept 86 of each of the front and the back reflecting surfaces 30 and 32 upon a line 88. While the hatching in Figs. 5-8 is directed to only one of the feed assemblies, namely the feed assembly 24 of the antenna 20, the showing of the horizontally arranged metallic strips 72 of the front reflecting surface 30 applies to both the antennas 20 and 50 of Figs. 1 and 2. Also, the enlarged fragmentary view of Fig. 9 applies to both the antennas 20 and 50 of Figs. 1 and 2.

Figs. 10-14 show the vertically oriented metallic strips 72 of the back reflecting surface 32 of the reflector 28 in both the antennas 20 and 50 of Figs.1 and 2. Figs.10 - 13 also show the arrangement of the components of the feed assembly 54 of the antenna 50 of Fig.2. In the Figs.10 - 14, the view of the metallic strips 72 of the reflector assembly 52 corresponds to that which would be seen by vertically polarized radiation, it being understood that the front reflecting surface 30 as well as the components of the

dish 34 are transparent to the vertically polarized radiation. The side-by-side arrangement of the feeds 40 and 42, as well as the arrangement of the screens 58 and 60 of the feed assembly 54 are readily seen in the views of Figs.10 -13.

Rays of radiation may propagate from distant points towards the reflector, or in the reverse direction, along a path 90 shown in Figs. 7 and 12. Vertically polarized radiation received along the path 90 is focused by the back reflecting surface 32 to a focus at the mouth of the horn feed 42. Horizontally polarized radiation received along the path- 90 is focused by the front reflecting surface 30 to a focus located at the mouth of the horn feed 40. The two focuses are located on a line 88 in Fig. 7, and in Fig. 12 are located equidistant from the line 88. The arrangement of the reflector 28 as depicted in Figs. 7 and 12 is convenient for installation on board satellites or space stations circumnavigating the earth because the location of the reflector 20 relative to the feed horns 40 and 42, as well as to the subreflectors provided by the screens 44, 58, and 60, allows for incoming radiation to clear the satellite and the base 26 or 56 of the antennas 20 or 50. Also, the use of a single supporting structure in the form of the dish 34 for both of the reflecting surfaces 30 and 32 significantly reduces the weight of the reflector 28, thereby to facilitate deployment of the antennas 20 and 50 in a satellite. In a typical installation, the focal length

of the reflector 28 may be as high as 100 inches, or more if desired. The diameter of the reflector 28 may be in the range of 80 - 90 inches.

It is to be understood that the above described embodiments of the invention are illustrative only, and that modifications thereof may occur to those skilled in the art. Accordingly, this invention is not to be regarded as limited to the embodiments disclosed herein, but is to be limited only as defined by the appended claims.