Abstract:
A converter for converting circularly polarized waves in a round waveguide into linearly polarized waves in a rectangular waveguide has two probes and a transmission line pattern all of which are formed of metal foil on a thin dielectric film board. The board is flexible and extends across the open ends of the two waveguides. The first probe has a square shape and the transmission line has two conductor arms connected to adjacent sides of the square. The other ends of the conductor arms are connected to the second probe with an impedance matching resistor formed on the film board between the two arms. One conductor arm is one-quarter wavelength longer than the other to convert from circularly polarized waveguide transmission to microstrip transmission.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a polarity converter for a parabolic antenna of the kind used in receiving satellite broadcasts or the like. 
     2. Description of the Background 
     In order to make it easier to install a parabolic antenna for receiving electromagnetic waves transmitted by a broadcast satellite, that is, in order to allow the receiving parabolic antenna to be installed without taking the polarity of the electromagnetic waves into consideration, such electromagnetic waves are typically transmitted from the satellite with a circular polarization. Thus, it is necessary to convert the electromagnetic waves with a circular polarization into ones with a linear polarization in order to efficiently transform the electromagnetic waves into an electrical signal. For this reason, a polarity converter is required when using a parabolic antenna. 
     FIG. 6 shows a typical configuration of a conventional polarity converter, in which a waveguide 2 is connected to a feedhorn 1, which has a circular cross section. A dielectric substance 6 is attached across the inside of a portion 3 of the waveguide 2 that is closest to the feedhorn 1. The dielectric substance 6 is fixed at an angle at a point along the length of the waveguide 2 on a diametrical line of the waveguide portion 3, the cross section of which is circular as described above. This dielectric substance 6 is used for converting the circular polarity of the received electromagnetic wave into a linear polarity. 
     A portion 5 of the waveguide 2 at the stage farthest from the feedhorn 1 is designed so that it is rectangular in cross section to facilitate the transmission of the electromagnetic waves with the linear polarity. A waveguide portion 4 between the portions 3 and 5 is a transition part of the waveguide 2 at which the circular cross section is gradually transformed into a rectangular cross section. Thus, the waveguide portion 4 linking the portions 3 and 5 to each other has a cross section which is a transition between the other two. 
     The conventional polarity converter is designed as a three-dimensional structure for converting circular-polarity electromagnetic waves into linear-polarity electromagnetic waves. As a result, the conventional polarity converter has several problems, such as large size and high cost to manufacture. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     Addressing these problems, it is an object of the present invention provide a design for a small-size and low-cost polarity converter for use in receiving satellite broadcast signals. 
     According to an aspect of the present invention a polarity converter is provided that comprises a first probe installed at an end of a first waveguide typically having a circular cross section, two conductor branches stretched out from the first probe in directions different from each other to constitute a waveguide to microstrip conversion portion in conjunction with the first probe, and a second probe installed at an end of a second wave guide typically having a rectangular cross section, wherein the length of one of the conductor branches is one-fourth wavelength of the received electromagnetic wave longer than that of the other and both branches are connected to the second probe at their other ends to form, together with the first probe, a unitary conductor pattern on a thin, flexible, dielectric film board. 
     The manner in which the above and other objects, features, and advantages are provided by the present invention is set forth in the following description and drawings, in which like reference numerals represent the same or similar elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view of a typical configuration of a parabolic antenna including a polarity converter as provided by the present invention; 
     FIG. 2 is an elevation in cross section of a configuration of the polarity converter according to an embodiment of the present invention; 
     FIG. 3 is a diagram showing the conductor patterns formed on a film board of the embodiment shown in FIG. 2; 
     FIG. 4 is a diagram showing a conductor pattern according to another embodiment of the present invention; 
     FIG. 5 is an elevation in cross section showing a connection of two waveguides according to an embodiment of the present invention; and 
     FIG. 6 is a diagram showing the configuration of a conventional polarity converter known in the prior art. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The elevational view of FIG. 1 shows a configuration that implements a parabolic antenna for a satellite broadcast receiver/transmitter making use of the polarity converter provided by the present invention. As shown in FIG. 1, a reflector 12 is installed on top of a support pole 11, and a polarity converter 13 is fixed at the position to which electromagnetic waves reflected by the reflector 12 are converged. The polarity converter 13 is connected to a signal converter unit 15 by a waveguide 14. 
     With the reflector 12 directed toward a broadcast satellite, circular-polarity electromagnetic waves transmitted by the broadcast satellite are reflected by the reflector 12 and converged to the polarity converter 13. The circular-polarity waves entering the polarity converter 13 are transformed into linear-polarity waves that are then guided by the waveguide 14 to the converter unit 15. Subsequently, the converter unit 15 converts the linear-polarity waves into an electrical signal that is finally output to a tuner (not shown). 
     FIG. 2 shows the polarity converter 13 of FIG. 1 cross section in which feedhorn 21 is circular, so that it transmits the incoming circular-polarity waves reflected by the reflector 12. The other end of circular feedhorn 21 is connected to a waveguide 22, also having a circular cross section. Electromagnetic waves coming from the feedhorn 21 propagate along the inside of the waveguide 22 toward the other end of the waveguide 22. An end plate 24 is attached at the end of the waveguide 22 so as to form a space 23 between the end plate 24 and the end of the waveguide 22. A film board 25 is fixed in the space 23 between the end plate 24 and the end of waveguide 22. The end plate 24 extends beyond the end of the waveguide 22 so that the space 23 continues to the side opposite the waveguide 22 where the waveguide 14 having a rectangular cross section is arranged. 
     FIG. 3 shows the electrical conductor pattern formed on the film board 25, which pattern is typically formed of aluminum foil. The film board 25 is very thin and flexible and is formed of a flexible dielectric material such as polyester, polyethylene, or polyolefin. A probe 31, branches 32 and 33, a link 34 and another probe 35 are formed as a single, unitary pattern on the film board 25. 
     The conductor pattern may be formed on the film board 25 by applying a thin aluminum film to one surface of the polyester film board 25 and then etching away the unwanted aluminum to result in the desired pattern, such as shown in FIG. 3. Alternatively, the specified pattern can be directly deposited by sputtering or evaporating aluminum onto the dielectric film board 25. Because the film board 25 is usually formed of transparent material, such as polyester, the ends of the two waveguides 14 and 22 are shown in FIG. 3. Specifically, the round end of the circular waveguide 22 can be seen adjacent probe 31 and the rectangular end of the rectangular waveguide 14 can be seen adjacent probe 35. 
     The conductor branches 32 and 33 constitute a suspended line or microstrip 42 in conjunction with the link 34, which is also a portion of microstrip. The probe 31 serves as a converter 41 for converting from waveguide transmission to suspended line or microstrip transmission. on the other hand, the other probe 35 serves as a reverse converter 43 for converting the suspended line or microstrip transmission back into waveguide transmission. 
     As used herein, suspended line means a kind of microwave conductor, like microstrip or coaxial cable, that has an axial conductor, as opposed to a waveguide microwave conductor that does not have an axial conductor. Waveguides typically operate in the transverse electrical mode (TE) or the transverse magnetic mode (TM), with rectangular waveguides operating in the TE mode and circular waveguides operating in the TM mode. Because of the axial conductor, the microstrip or suspended line operates in a transverse electrical and magnetic mode (TEM). Thus, the mode conversion operation of the two probes 31 and 35 is seen and, moreover, the conversion operation of links 32, 33, and 34 from the TM mode of probe 31 through the TEM mode and back to the TE mode is appreciated. 
     The probe 31 has a generally rectangular shape and is fixed at a location in the end space 23 corresponding to the end of waveguide 22, that is, in the path of the waves exiting the waveguide 22. The branches 32 and 33 are connected respectively to two adjacent sides of the rectangular shaped probe 31, which are perpendicular to each other. In addition, the length of the transmission line of the branch 32 is made one-fourth of a wavelength (λ) longer than the length of the branch 33, where λ is the wavelength of the electromagnetic wave of interest being received. The other ends of the branches 32 and 33 are joined to each other by the link 34, which is further connected to the probe 35. The probe 35 is fixed in the space 23 at a location corresponding to the beginning end of rectangular waveguide 14, that is, in the path of the waves entering the waveguide 14. A printed resistor 36 is fixed at the juncture between the branches 32 and 33. As such, a Wilkinson-type compound circuit is formed. Resistor 36 can be a carbon resistor that is printed directly onto the polyester film board 25 and that connects the edges of conductor branches 32 and 33, and resistor 36 acts as a terminator for performing impedance matching. 
     In Japan, electromagnetic waves transmitted by a broadcast satellite have a circular polarity rotating in the clockwise direction. The electromagnetic wave is a resultant of two component fields that have directions perpendicular to each other. The phase of one of the component fields lags behind the other by 90 degrees. The conductor branch 32, which has a transmission path one-fourth of a wavelength (λ) longer than that of the other conductor branch 33, detects the component with the 90-degree leading phase, as shown by an arrow A in FIG. 3. Note that λ is the wavelength of received electromagnetic waves at the frequency of interest as described previously. On the other hand, the conductor branch 33, which has a transmission path one fourth of a wavelength (λ) shorter than that of the other conductor branch 32, detects the component with the 90-degree lagging phase denoted by an arrow B in FIG. 1. The component being conducted by the conductor branch 32 arrives at the link 34 with its phase lagging by 90 degrees behind that of the component conducted by the branch 33, because the transmission path of the former is one-fourth of a wavelength (λ) longer than that of the latter. Accordingly, due to the effects of conductor branches 32 and 33 at the link 34 the phase of both the two components will be the same. As a result, the probe 35 that is connected to the link 34 outputs linear-polarity waves that propagate through the waveguide 14 to the converter unit 15. At the converter unit 15, the linear-polarity electromagnetic waves are finally converted into an electrical signal. 
     The polarity rotating directions are used to suppress interference between two broadcast satellites which are relatively close to each other. In Japan, electromagnetic waves are transmitted with a polarity rotating in the clockwise direction as described earlier. If Korea, a neighboring country, also launches a broadcast satellite, for example, an attempt must be made to avoid radio interference in Japan by electromagnetic waves transmitted from the broadcast satellite of Korea and vice versa. Such interference can be avoided by making the polarity of the electromagnetic waves transmitted by the broadcast satellite of Korea, for example, rotate in the opposite or counter-clockwise direction. 
     According to the principle of operation described above, however, the antenna receives not only electromagnetic waves having a polarity rotating in the clockwise direction, but also will receive those with a polarity rotating in the counter-clockwise direction as well. In order to suppress the electromagnetic waves having a polarity rotating in the counter-clockwise direction, the printed resistor 36 is employed. By inserting the printed resistor 36, which performs an impedance match, only the electromagnetic waves with a polarity rotating in the clockwise direction are passed through. It should be noted that if it is desired to receive the electromagnetic waves with a polarity rotating in the counter-clockwise direction instead of those with a polarity rotating in the clockwise direction, the film board 25 is installed reversed in the left-to-right direction, that is, with branch 32 on the right and branch 33 on the left relative to the A and B orientation of FIG. 3. 
     FIG. 4 shows another embodiment for the microstrip conductor pattern formed on the film board 25 that includes a filter 53 comprising protrusions or stubs 51 protruding in the horizontal direction and small-diameter paths 52 formed as thin pipes in the vertical direction. The stubs 51 and the small-diameter paths 52 function as capacitive and inductive components, respectively. By combining the capacitive and inductive components, a filter having the desired characteristics can be implemented integrally with the polarity converter as a single conductor pattern on the film board. 
     The film board 25 is extremely thin, having a typical thickness of 0.1 millimeters, so that it is highly flexible. Accordingly, the film board 25 can be easily bent to the form shown in FIG. 5. As a result, the position of the input waveguide 22 relative to that of the output waveguide 14 can be freely adapted to meet any particular requirement. In the embodiment of FIG. 5, the positions of the waveguides 14 and 22 are set so that their respective longitudinal axes form a right angle of substantially 90 degrees. Note that the dielectric substance 6 employed in the conventional polarity converter shown in FIG. 6 has a thickness on the order to 3 mm. Thus, unlike the film board 25, such a substance is difficult to bend. 
     As described above, the polarity converter provided by the present invention comprises a first probe, a suspended line or microstrip transmission line, and a second probe all of which are formed as a single, unitary device. By installing the first and second probes in first and second waveguides, respectively, not only can electromagnetic waves with a circular polarity be thereby converted into those having a linear polarity with ease, but the polarity converter itself can also be made small in size and can be manufactured at a low cost. 
     Having described preferred embodiments with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments and that various changes and modifications could be effected by one skilled in the art without departing from the spirit or scope of the novel concepts of the invention, as defined in the appended claims.