Microwave Antenna Feed Structure

The present invention relates generally to microwave antennas and waveguides and, more particularly, to the use of a novel feed structure for a microwave reflector antenna containing a waveguide and a feed horn integral with the waveguide.

A parabolic or other suitably shaped reflector is a well known device for the transmission or reception of electromagnetic energy. When employed as a transmitting antenna, a feed horn located at the focus of the reflector directs microwave energy toward the reflecting surface of the reflector. The surface of the reflector then serves to reflect the waves from the feed horn into space in the form of plane waves. Conversely, when employed as a receiving antenna, a microwave reflector reflects plane waves from space toward a feed horn located at the focus of the reflector. Whether operating in the mode of a transmitter or receiver, the feed horn is typically connected by means of a waveguide to a transmission line originating behind the surface of the reflector. The waveguide is appropriately curved so as to minimize interference with microwave energy passed between the feed horn and the reflector. Typically, the step of bending the waveguide in the prior art requires the use of an internal mandrill to avoid deforming the interior cross section of the waveguide. Nevertheless, bending of the waveguide creates imperfections in the interior cross section of the waveguide which contribute to energy losses in the reflector system. Energy losses may also be caused by imperfections in the waveguide, feed horn or reflector. Prior art feed horn assemblies further contribute to energy losses in that their waveguide and feed horn frequently consist of multiple components which are joined together by a brazing process resulting in an imperfect interface between the components. As a result of the above imperfections and associated energy losses, feed systems known in the art must commonly undergo an extensive tuning process before they may be operated efficiently.

The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.

In accordance with one aspect of the present invention, there is provided a microwave antenna consisting of a reflector and a feed structure for transmitting or receiving microwave energy to or from the reflector. The feed structure is comprised of a waveguide and a feed horn integral with an output end of the waveguide. The waveguide includes an inner surface having a rectangular cross section and an outer surface having a generally circular cross section.

In accordance with another aspect of the present invention, there is provided a method of manufacturing a feed structure for a microwave reflector antenna. The method includes a first step of forming a metal waveguide with an inner surface having a rectangular cross section and an outer surface having a generally circular cross section adapted to be bent with minimal resulting deformation of the rectangular inner surface of the waveguide. A externally threaded cylindrical input section is formed at one end of the waveguide which is adapted to be connected to an internally threaded hub connected to a reflector. A feed horn with a circular output aperture is then formed at an output end of the waveguide by machining a rectangular to circular transition within the inner surface of an output section of the waveguide. Finally, the metal waveguide is bent into a curved shape so that the feed horn is adapted to be directed toward the reflecting surface of a microwave reflector. The bending step is accomplished with minimal deformation of the rectangular inner surface of the waveguide.

The foregoing and other advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:

FIG. 1a is a sectional view of an assembled feed structure for use with a microwave reflector embodying the present invention;

FIG. 1b is an exploded sectional view of the feed structure of FIG. 1a;

FIG. 1c is a typical section view of the feed horn portion of the feed structure of FIG. 1a;

FIG. 2 is a sectional view illustrating the rectangular inner surface and generally circular outer surface of the waveguide portion of the feed structure embodying the present invention;

FIG. 3 is a sectional view of one feed horn for use in the feed structure of FIG. 1a; and

FIG. 4 is a sectional view of another feed horn for use in the feed structure of FIG. 1a.

While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

Turning now to the drawings and referring initially to FIG. 1a and 1b, a feed structure embodying the present invention is illustrated and generally designated by a reference numeral 10. Although the following description of the operation of the feed structure 10 will assume that the feed structure 10 is being used in a transmission mode for delivering microwave energy to a reflector 11, it should be understood that the feed structure 10 may also be used in a receive mode for receiving microwave energy from a refector 11. The feed structure 10 is constructed of a waveguide 12 having an input section 14, intermediate section 16, and output end 18. As shown in FIG. 2, the waveguide 12 has an inner surface 20 with a generally rectangular cross section. The waveguide 12 further includes an outer surface 24 with a generally circular cross section which is designed to be bent with minimal resulting deformation of the rectangular inner surface 20 of the waveguide 12. Although the waveguide 12 shown in FIG. 2 has a rectangular inner surface 20, it should be appreciated that the internal dimensions of waveguide 12 may be provided in any configuration capable of supporting the propagation of electromagnetic energy. According to one embodiment of the invention, the waveguide 12 is made of aluminum, but again it should be appreciated that the waveguide 12 may be made of any other material capable of supporting the propagation of electromagnetic energy. Referring again to FIG. 1a and 1b, the input section 14 of the waveguide 12 has an input end 26 which is adapted to be connected to an external transmission line (not shown). After connecting to an external transmission line, microwave energy may be propagated through the waveguide 12 in the direction of the arrows 30 when in the transmission mode, passing through an opening 34 of a hub 32 and continuing along the waveguide 12 toward the intermediate section 16 and output end 18.

The hub 32, which may be made of aluminum, is provided with an internally threaded bore 80 which corresponds with a threaded cylindrical input section 14 of waveguide 12. The input end 26 of the waveguide 12 is inserted into the threaded bore and rotated so that the input section 14 of the waveguide 12 becomes threadedly engaged within the threaded bore 80 of the hub 32 and extends at least partially through the length of the hub 32. The relative position of the waveguide 12 to the reflector 11 can thereby be adjusted by the user to optimize performance of the antenna by simply rotating the input section 14 of the waveguide 12 a desired distance into the threaded bore 80. This feature provides a significant improvement over antenna feed structures known in the art because it reduces the need to subsequently tune the antenna. Once the optimal position is found, a conventional fastener may be used to fix the rotational position of the input section 14 of the waveguide 12 relative to the hub 32. The input end 26 may extend all the way through the hub 32 such that it protrudes out of the opening 34 at the rear of the hub, in which case the input end 26 may be machined off so as to provide a consistent electrical interface. An 0-ring (not shown) may be provided within a retaining region 82 for enhancing the seal of the input section 14 within the hub 32.

At the output end 18 of the waveguide 12, there is provided a feed horn 35 integral with the output end 18 of the waveguide 12 having an inner surface generally designated by dashed lines 38. Because the feed horn 35 is integral with the waveguide 12, imperfections in the interface between the waveguide 12 and the feed horn 35 are minimized. As the horn geometry may be machined accurately, no brazing or heating is required and the need for tuning is minimized. The intermediate section 16 is bent such that the output of the feed horn 35 is located approximately at the focus of the reflector 11 and directed toward its reflecting surface 36. As portrayed in FIG. 1c, a window 39 is placed about the output of the feed horn 35 in order to protect the feed horn 35 and waveguide 12 from moisture and other environmental elements. Bending of the intermediate section 16 minimizes distortion of the rectangular inner surface 20 of the waveguide 12 and minimizes the need for using an internal mandrill, thereby providing a significant advantage over waveguides known in the art.

Referring again to FIG. 2, the rectangular inner surface 20 and exterior surface 24 of the waveguide 12 according to one embodiment of the invention will be described in greater detail. A Cartesian coordinate system centered at the interior of the waveguide 12 is included to facilitate the foregoing description. The rectangular inner surface 20 of the waveguide 12 is formed between two parallel faces 40 and 42 which intersect upper and lower faces 44 and 46 oriented at right angles to the faces 40 and 42. As illustrated in FIG. 2, the faces 40 and 42 have a cross-sectional length 2b and the shorter faces 44 and 46 have a cross-sectional length 2a. With reference to the Cartesian coordinate system, face 40 intersects the x axis at (a, 0) and intersects shorter faces 44 and 46 at (a, b) and (a, -b), respectively. Face 42 intersects the x axis at (-a, 0) and intersects shorter faces 44 and 46 at (-a, b) and (-a, -b), respectively. Faces 44 and 46 intersect the y axis at (0, b) and (0, -b), respectively. The exterior surface 24 of the waveguide 12 has a generally circular cross-sectional shape defined by two opposing convex surfaces 52 and 54 oriented outside faces 40 and 42 and intersecting the x axis at (c, 0) and (-c, 0). Dashed lines 48 and 50 extending through the corners of the rectangular interior surface 20 intersect the opposing convex surfaces 52 and 54 at points 56, 58, 60 and 62. The wall thickness of the waveguide 12 defined by the distance between the exterior surface 24 and the rectangular inner surface 20 of the waveguide 12 is less at points 56, 58, 60 and 62 than it is at any other point along the exterior surface 24. This enables the waveguide 12 to be bent with minimal resulting deformation of the rectangular inner surface 20 of the waveguide 12. The exterior surface 24 of the waveguide 12 further includes opposing locating surfaces 64 and 66 which intersect the opposing convex surfaces 52 and 54. The locating surfaces 64 and 66 are parallel flat surfaces which intersect the y axis at points (0, d) and (0, -d) respectively. The locating surfaces 64 and 66 are parallel to the short faces 44 and 46 of the rectangular inner surface 20 of the waveguide 12 so that a user may ascertain the orientation of the waveguide 12 by viewing its exterior surface 24.

Turning now to FIG. 3, there is illustrated a feed horn 35 according to one embodiment of the present invention. A feed horn by definition is a transition section of a feed assembly where, in the transmission mode, the electrical energy emerges from the waveguide to free space. Conversely, in the receive mode, a feed horn serves to transition electrical energy from free space to the waveguide. Accordingly, although the following description will refer to operation of the feed horn 35 in a transmission mode for delivering microwave energy to a reflector, it should be understood that the feed horn 35 may also be operated in a receive mode for receiving microwave energy from a reflector. As waves propagate through the waveguide 12 in the direction of the arrows 30, they encounter the feed horn 35 which is integral to the output end 18 of the waveguide 12. The feed horn 35 is manufactured by machining the rectangular inner surface 20 of an output section of waveguide 12 to form an inner area 68 defined within the boundaries of tapered walls 38. The inner area 68 of the feed horn 35 flares outwardly from the output end 18 of the waveguide 12 and terminates at a circular output aperture 70, thus forming a smooth tapered rectangular to circular transition between the output end 18 of the waveguide 12 and the output aperture 70 of the feed horn 35. The circular output aperture 70 is preferably located at the focus of a reflector (not shown in FIG. 3), so that waves exiting the feed horn 35 through the circular aperture 70 are directed toward the reflecting surface of the reflector and reflected into space in the form of plane waves.

Referring now to FIG. 4, there is illustrated a feed horn 35 according to another embodiment of the present invention. Again, while the following description will refer to operation of the feed horn 35 in a transmission mode for delivering microwave energy to a reflector, it should be understood that the feed horn 35 may also be operated in a receive mode for receiving microwave energy from a reflector. As waves propagate in the direction of arrows 30 and reach the output end 18 of waveguide 12, they encounter a series of outwardly expanding steps 74a, 74b and 74c, each having a progressively increasing cross sectional area. The output aperture 76 at the end of the series of steps 74a, 74b and 74c has a circular cross section adapted to be placed at the focus of a reflector substantially as described above. The number of steps 74 may be varied as needed to provide an efficient stepped transition between the rectangular inner surface 20 of waveguide 12 and the circular output aperture 76.

While the present invention has been described with reference to one or more particular embodiments, those skilled in the art will recognize that many changes may be made thereto without departing from the spirit and scope of the present invention. Each of these embodiments and obvious variations thereof is contemplated as falling within the spirit and scope of the claimed invention, which is set forth in the following claims.