Patent Publication Number: US-6700549-B2

Title: Dielectric-filled antenna feed

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to antenna feeds and, more particularly, millimeter wave frequency feeds adapted for low f/D reflectors. 
     One type of antenna known as a reflector antenna uses a contoured reflective surface to generate a highly directive far field antenna pattern. A small waveguide aperture feed antenna is typically placed at the focus of the reflector in order to illuminate the same. The desired directivity properties determine the relative dimensions of the reflector. A common parameter describing the geometric properties of a reflector antenna is f/D, which is the ratio of the focal length (f) to the diameter (D). The smaller f/D, the thinner or more compact the reflector antenna assembly can be made. 
     However, as one decreases f/D, the beamwidth of the illuminating feed must be increased proportionally in order to properly illuminate the reflector surface. For example, it is generally accepted that the reflector surface must receive energy from the feed in such a way that the energy level at the reflector edges is only about 10 decibels (dB) lower than the energy level at the center of the reflector. 
     One can obtain a broader beamwidth by decreasing the aperture size of the waveguide feed. Fundamentally, the lowest frequency of propagation in such a feed increases with decreasing rectangular waveguide width or circular waveguide diameter. The cutoff frequency of the dominant propagating mode is the waveguide&#39;s lowest frequency of operation. In summary, as the feed beamwidth is broadened and the aperture size is decreased, the cutoff frequency of the aperture will increase. Consequently, at a particular frequency, the maximum feed antenna beamwidth is limited, and along with it, the minimum obtainable f/D of the reflector antenna. 
     In addition, as the desired operating frequency increases into the millimeter-wave range and the aperture size decreases, it becomes difficult to physically machine the aperture and other small structures related to controlling the resulting electromagnetic waves. 
     SUMMARY OF THE INVENTION 
     The present invention is an electromagnetic energy feed formed from a section of open-ended, dielectric filled waveguide. The dielectric fill material used is a solid, processable (e.g., machinable), low-loss material that can be shaped as desired. 
     The dielectric material used to fill the waveguide lowers the cutoff frequency of the dominant electromagnetic mode compared to the same waveguide filled only with air. This allows one to increase the beamwidth when compared to a similar sized, but air-filled only waveguide section. 
     A broadening of the beamwidth of approximately 10% over an air-filled-only feed has been observed with the propagating mode cutoff frequency set low enough to maintain a good input match. These attributes were achieved for a feed designed to operate in a millimeter wave frequency band at approximately 60 GigaHertz (GHz). 
     One preferred material for use as the dielectric is Rexolite®. Other suitable materials could be used as long as their properties are stable with temperature and easily processable, i.e., they can be machined or shaped to the desired size to fill the waveguide. 
     The dielectric filled section is preferably provided as a solid fill of the interior dimension of the waveguide. However, even a partial filling of the waveguide can also be used to provide increased beamwidth. 
     The preferred embodiment uses a circular-type filled waveguide. However, other waveguide shapes, such as rectangular, may be used as well. 
     A quarter-wave choke slot may be used to encircle the dielectric-filled waveguide section. The choke slot may be used to match beamwidths in the electrical (E) and magnetic (H) planes. Because the aperture diameter of the dielectric-filled feed is smaller, a ridge between the circular waveguide and the choke slot may be thickened compared to that of an air-filled feed, making the choke slot easier to fabricate for a dielectric-filled feed than for an air-filled feed. 
     According to other optional aspects of the present invention, a protruding dielectric portion or tip may be used for efficient power transfer at the free space side of the feed. In this arrangement, the tip diameter is chosen to provide maximum power transfer with specific dimensions depending upon the dielectric constant of the dielectric fill. The length of the tip is chosen to be about one-quarter of the wavelength of the expected frequency of operation. In effect, the tip provides a single step, quarter wave transformer to match the feed aperture to free space. 
     Adaptations may also be made at the waveguide end of the feed. In particular, circular waveguide is not commonly used to construct microwave system components because of its reduced dominant-mode bandwidth compared to rectangular waveguide. Therefore, in a preferred embodiment, the input side of the feed uses a quarter wavelength waveguide transition (e.g., transformer). The transformer matches the field configuration of the circular waveguide used for the feed to the rectangular waveguide used to carry the signal. 
     In a preferred embodiment, the transformer is an annular ring of dielectric material. In this arrangement, the cross-sectional dimension of the annular ring transformer is chosen depending upon the interior dimension of the rectangular waveguide and the dielectric constant of the feed fill material. The dielectric ring provides an inhomogeneous, quarter wave matching section, functioning much the same as the tip used at the free space end. 
     It should be understood that the tip at the free space end and the annular ring at the input are specific embodiments of matching sections chosen for ease of machining. They can be interchanged or take other forms in other embodiments. For example, a dielectric tip can be used on the waveguide side, and an annular ring may be used on the free space side. 
     In a preferred embodiment, metal bosses are placed at the free space end of the waveguide adjacent the protruding tip. The bosses protect the protruding tip, for example, during handling of the feed while manufacturing an antenna assembly. Without the bosses, the protruding tip might otherwise be prone to breakage. The bosses are dimensioned and positioned in such a way that they do not interfere with the electromagnetic radiation properties of the feed. 
     Finally, the feed may be used with different types of reflectors, including standard parabolic metallic reflectors, transreflectors, and the like. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
     FIG. 1 is a perspective view of an antenna feed according to certain principles of the present invention. 
     FIG. 2 is a cross-sectional perspective view of the feed. 
     FIG. 3 is a perspective view of an embodiment of the exploded view of an antenna assembly according to certain principles of the present invention. 
     FIG. 4 is a cross-sectional view of the antenna assembly illustrating techniques for producing a collimated output beam according to certain principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     FIG. 1 is a perspective view of a dielectric-filled feed  100  made from a cylindrically shaped housing  120  containing a circular waveguide  140  that is filled with a section of solid dielectric material  150 . Typically, the housing  120  is made from solid aluminum plated with gold. However, any suitable conductive material can be used to form the housing  120 . 
     Generally, the solid dielectric material  150  is selected to have an index of refraction of less than 10. In one application, the dielectric material  150  is Rexolite®, preferred for its low-loss. Rexolite is a registered trademark of C-Lec Plastics, Inc. of Beverly, N.J., Rexolite can be easily machined to the general shape of the interior of the waveguide and to provide the structure of the protruding tip  105 . 
     The illustrated embodiment uses a cylindrically shaped housing  120  and waveguide  140 . It should be noted, however, that other waveguide shapes can be used, for example, a rectangularly shaped waveguide in a rectangular housing. 
     Antenna feed  100  can also include a choke for enhancing the proper illumination of a reflector, as will be described below. More specifically, housing  120  can be machined at one end to include a choke slot  110  formed by an outer choke slot ridge  135  and inner choke slot ridge  130 . The choke slot  110  is typically a quarter wavelength deep. 
     A protruding tip  105  is preferably formed as part of the feed from the same dielectric material that is used for the fill  150 . For example, a single-piece construction of dielectric material  150  can be machined to form a protruding tip  105  that is smaller in diameter than dielectric  150  filling the waveguide  140 . The tip  105  typically protrudes into free space a quarter wavelength beyond the free space end of the waveguide  140 . 
     In one embodiment, the tip  105  is a cylinder extending 45 thousandths of an inch (mils) beyond the end aperture of waveguide  120 . In this embodiment, the tip  105  can be machined to a diameter of 57 mils. Based on these dimensions, the antenna feed  100  can generally operate in a millimeter wave frequency range of about 57 to 64 GigaHertz (GHz). 
     Although the embodiment shown uses a cylindrically shaped protruding tip  105 , other shapes such as a rectangular protruding tip  105  can be used according to certain principles of the present invention. 
     The dielectric-filled waveguide also makes it possible to produce an easier-to-machine inner choke slot ridge  130 . More specifically, inner choke slot ridge  130  can now be 30 mils in thickness versus 15 mils that may otherwise be necessary to achieve certain operating characteristics without the dielectric material  150 . 
     A half-power angular beamwidth of one embodiment of the antenna feed  100 , including the dielectric material  150  filling and the protruding tip  105 , is approximately 68 degrees. Without dielectric material  150  filling waveguide  140 , the maximum half-power angular beamwidth is limited by the increasing waveguide cutoff frequency to about 62 degrees. Thus, more than a 10% increase in half-power angular beamwidth is achievable using the techniques according to certain principles of the present invention. 
     FIG. 2 is a more detailed cross-sectional view of the feed  100 . In a transmit direction, the feed  100  guides microwave energy presented at waveguide end  205  to launch an RF signal into a free space end via protruding tip  105 . In a receive direction, RF energy can be received from free space at protruding tip  105  and coupled to waveguide  205 . 
     More particularly, the feed  100  consists of the cylindrical waveguide  140 , filled with the dielectric material  150  at a free space end  155 . The protruding tip  105  serves as a transformer to efficiently couple electromagnetic energy between free space and the waveguide  140 . In addition, choke slot  110  and inner and outer choke slot ridges  130  and  135  are shown in this cross-sectional view, as previously described in connection with FIG.  1 . 
     In this embodiment for operation at approximately 60 GHz, the waveguide section  140  may have an interior diameter of 83 mils and length, L 1 , of 218 mils. 
     Choke slot  110  can be 48 mils deep, while outer surface of inner choke slot ridge  130  can be 143 mils in diameter. Consequently, inner choke slot ridge  130  can have a wall thickness of about 30 mils. 
     In this arrangement, the outer choke slot ridge  135  can have an inner diameter of about 223 mils. 
     The outer diameter of the various elements of the feed are not as critical, but are preferably as small as possible to keep the cross-sectional area small to minimize reflector blockage. 
     Also evident in the view of FIG. 2 is the waveguide end  200  of the feed  100  and, in particular, how it couples to a section of waveguide  205 . Generally, waveguide  205  may be any suitable microwave system waveguide such as a WR-15 rectangular-type waveguide. The waveguide section  205  may, in a preferred embodiment, be coupled to the feed  100  via a matching section, also called herein a transformer  207 . 
     As shown, the transformer  207  may consist of an annular ring section of dielectric material  210 . The properties of the dielectric material section  210  are chosen to act as a transition between the air filled region of the waveguide  205  and the dielectric fill  150  of the waveguide section  140 . Specifically, transformer section  207  along a length L 2 , can be formed as an inhomogeneous, quarter wave matching waveguide section. One particular preferred shape is a ring of dielectric  210  that includes a cylindrically shaped hollow section  263 . The hollow section  263  may have a length L 2  that is a quarter wavelength long. 
     The exact shape of the transformer section  207  may be different depending upon different applications. For example, the hollowed section  263  in the dielectric ring  210  may be conically shaped. Other transitional shapes may be possible, such as, for example, providing alternate sections of dielectric and air filled areas within the transition region presented by the transformer  207 . 
     In a transmit direction, where energy flows from a waveguide  205  into free space through the tip  105 , the transformer section  207  may be used to ensure that energy is more efficiently coupled through the antenna feed  100  rather than being reflected back into the waveguide  205 . In a receive direction, energy received from the free space at the tip  105  is more efficiently coupled into the waveguide  205  through the use of transformer section  207 . 
     In this embodiment, the transformer section  207  may have an inner diameter machined to 105 mils, with the hollow region  263  in the dielectric ring  210  being formed at a diameter of 39 mils and length of 59 mils. 
     Waveguide  205  can be a standard WR-15 rectangular waveguide having dimensions of 148 mils by 74 mils. The 148 mil width is standard for WR-15 with sharp corners; this width increases to 164 mils when the cross sectional shape has full-radiused ends for ease of machining. Either of the two structures can be used in this invention. Circular, partially circular, elliptical and other shaped waveguides can be used in lieu of rectangular waveguide. 
     In general therefore, the invention provides a feed as an open ended dielectric filled waveguide  140 . In a preferred embodiment, the waveguide section  140  is a circular waveguide operating in the dominant TE 11  mode. The dielectric  150  is chosen to lower the cutoff frequency of the dominant mode of the waveguide section  140 . This permits the electrical size of the aperture of the output and at the free space end  155  to be minimized in size. This, in turn, increases the available beamwidth, as compared to a waveguide section  140  that does not have the filling dielectric  150 . The dielectric filling  150  can be partial, but a solid fill is a specific preferred embodiment and is most likely the easiest to machine to the desired dimensions. 
     Furthermore, the choke slot  110  is dimensioned to equalize E- and H-plane beamwidths. In particular, the choke slot  110  may be chosen to control the resulting beamwidth in the E-plane. This is desirable for optimum illumination of the reflector accompanying the feed as is well known in the art. 
     In general, the diameter of the tip  105  is chosen to provide a maximum power transfer and will depend upon the dielectric constant of the filling material  150  chosen. 
     Although the free space to dielectric end  155  uses a tip-type matching section  105  and the feed  100  to waveguide transformer  207  uses an annular ring type matching section  210 , it should be understood that different matching sections can be used to serve the same purpose in each of the various positions. For example, a dielectric tip surrounded by air could be used at the waveguide end  200  of the device and, similarly, an annular ring of dielectric can be used at the free space end  155 . 
     In practice, we have found it useful to also provide extensions or bosses  300  around the periphery of the tip  105  as best shown in FIG.  3 . The bosses  300  prevent damage of the tip  105  during assembly operations for the feed  100 . 
     If the bosses  300  are utilized, one must be careful to ensure that their dimensions and positions are such that they do not interfere with the electromagnetic radiation properties of the feed  100 . For example, the bosses  300  should be positioned well clear of the E-plane axis of the feed  100 . In the illustrated embodiment for 60 GHz operation, a boss horizontal spacing, L 4 , of 160 mils, at boss vertical spacing, L 5  of 50 mils, at boss vertical dimension L 6  of 65 mils, and at boss depth L 7  of 65 mils may be used. 
     The bosses may be manufactured through the addition of simple manufacturing steps during location of the feed  100 . In particular, one vertical milling cut and three horizontal milling cuts may be used to form the four bosses  300  from a solid ring of metal surrounding the tip  105  and choke slot  110 . 
     FIG. 4 is a cross-sectional view showing the feed  100  and how it may be used with a reflector  400 . As previously mentioned, the antenna feed  100  can be advantageously used in a number of different devices, most particularly antenna devices that use a parabolic reflector to produce a collimated beam of radio frequency energy, transmitting or receiving such a collimated beam. 
     Reflector  400  may preferably be of a parabolic shape. The parabola has a normal equation which may be represented as 
     
       
           y=SQRT (4 *fx ) 
       
     
     where SQRT denotes the square root function, f is the desired focal length of the antenna, and x is the direction normal to the reflector plane. That is, x is the distance in the direction of a horizontal line  300  formed between the center line of the feed  100  and reflector  400 —and y is in a direction normal to x. 
     In one application, the reflector  400  is dimensioned to have a diameter, D, such that its aspect ratio f/D is 0.33, and its operating frequency is around 57-64 GHz. 
     The reflector  400  may be center fed as shown in FIG.  4 . However, other uses of the feed  100  are possible. For example, the reflector  400  may be a type of transreflector that actually consists of a thermoplastic dome having a parallel wire grating formed thereon. Such a transreflector is shown in U.S. Pat. Nos. 6,246,381 and 6,006,419, each of which are assigned to Telaxis Communications Corporation, the assignee of the present invention. 
     It should be understood that other configurations of the feed  100  and reflector  400  are possible. For example, the feed  100  may be used in an off axis feed arrangement whereby the feed is not aligned along the same center axis  300  of the reflector as shown in FIG.  4 . 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.