Abstract:
An antenna system that employs an antenna element for both transmit and receive functions, where a dual band polarizer is used to convert linearly polarized signals to circularly polarized signals and vice versa for two frequency bands. The dual band polarizer includes a waveguide including corrugated structures extending from opposing sidewalls, where ridges in the structures extend perpendicular to the propagation direction of the signal. The height of the ridges taper from a lowest height at the ends of the waveguide to a largest height at the middle of the waveguide. The corrugated structures interact with the field components of the signal in the direction perpendicular to the ridges that cause that component to be delayed relative to the field components parallel to the ridges so that the signal changes accordingly and maintains the same magnitude.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to an antenna system employing a dual band frequency polarizer and, more particularly, to a satellite antenna system employing a dual band frequency polarizer, where the polarizer includes a waveguide having opposing corrugated structures that operate to convert a linearly polarized signal to a circularly polarized signal for a satellite downlink and convert a circular polarized signal to a linearly polarized signal for a satellite uplink, and vice versa. 
     2. Discussion of the Related Art 
     Various communications systems, such as certain telephone systems, cable television systems, internet systems, military communications systems, etc., make use of satellites orbiting the Earth to transfer signals. A satellite uplink communications signal is transmitted to the satellite from one or more ground stations, that retransmits the signal to another satellite or to the Earth as a satellite downlink communications signal to cover a desirable reception area depending on the particular use. The uplink and downlink signals are typically transmitted at different frequency bands. For example, the uplink signal may be transmitted at 30 GHz band and the downlink signal may be transmitted at 20 GHz band. The satellite is equipped with antenna systems including a number of antenna feeds that receive the uplink signals and transmit the downlink signals to the Earth. 
     For most of these satellite communications systems, one antenna system is provided for receiving the uplink signals and another antenna system is provided for transmitting the downlink signals. Each antenna system typically employs an array of antenna feed horns and one or more reflectors to collect and direct the signals. The uplink and downlink signals are circularly polarized so that the orientation of the reception antenna can be arbitrary relative to the incoming signal. To provide signal discrimination, one of the signals may be left hand circularly polarized (LHCP) and the other signal may be right hand circularly polarized (RHCP), where the signals rotate in opposite directions. Polarizers are employed in the antenna systems to convert the circularly polarized signals to linearly polarized signals suitable for propagation through a waveguide with low signal losses, and vice versa. 
     Because there are important weight and real estate limitations on a satellite, it is desirable to use the same antenna system for both transmitting the downlink signals and receiving the uplink signals. Because the uplink and downlink signals are at different frequency bands, the feed horns would have to be designed to transmit and receive the signals at both the uplink and downlink frequency bands. It would also be necessary to employ a dual band polarizer that could effectively convert the downlink signal from a linearly polarized signal to a circularly polarized signal and convert the uplink signal from a circularly polarized signal to a linearly polarized signal. However, known polarizers are only optimized for a single frequency band, making them unsuitable for polarizing signals of different frequency bands. 
     High frequency polarizers employing corrugated profiles are known in the art for converting a linearly polarized signal to a circularly polarized signal, and vice versa. For example, see U.S. Pat. No. 4,228,410 issued Oct. 14, 1980 to Goudey et al. However, the known corrugated polarizers of this type are not dual band polarizers that are able to polarize signals at two different frequency bands. 
     What is needed is a polarizer for an antenna system capable of transmitting a satellite downlink signal and receiving a satellite uplink signal, that is able to effectively provide polarization conversion in two separate frequency bands. It is therefore an object of the present invention to provide such a polarizer and antenna system. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, an antenna system is disclosed that employs antenna elements for both transmit and receive functions and a dual band polarizer to convert linearly polarized signals to circularly polarized signals and circularly polarized signals to linearly polarized signals for two separate frequency bands. The dual band polarizer is a waveguide device that includes a corrugated structure extending from opposing sidewalls, where ridges in the structures extend transverse to the propagation direction of the signals. The width of the ridges, the spacing between the ridges and the number of ridges are selected so that the polarization conversion is optimized for two frequency bands. Additionally, the height of the ridges taper from a lowest height at the ends of the waveguide to a largest height at the middle of the waveguide to minimize reflections. The corrugated structures interact with the field components of the signal in the direction perpendicular to the ridges to cause that component to be delayed relative to the field component parallel to the ridges, so that the polarization of the signal is changed accordingly. 
     Additional objects, advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an antenna system employing a dual band polarizer, according to an embodiment of the present invention; 
     FIG. 2 is a perspective view of a the dual band polarizer used in the antenna system shown in FIG. 1, according to the invention; 
     FIGS.  3 ( a )- 3 ( c ) are graphs showing frequency versus return loss, frequency versus axial ratio, and frequency versus cross-polarization, respectively, for a satellite uplink signal within the frequency range of 28-30 GHz that has been polarized by the polarizer of the invention; and 
     FIGS.  4 ( a )- 4 ( c ) are graphs showing frequency versus return loss, frequency versus axial ratio and frequency versus cross-polarization, respectively, for a satellite downlink signal within the frequency range of 18.3-20.2 GHz that has been polarized by the polarizer of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following discussion of the preferred embodiments directed to a dual band polarizer for use in an antenna system is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. For example, the antenna system described below that employs the dual band polarizer of the invention is described in connection with a satellite communications system. However, as will be appreciated by those skilled in the art, the dual band polarizer has application for other communications systems other than satellite communications systems. 
     FIG. 1 is a block diagram of an antenna system  10  employing a dual band polarizer  12 , according to the invention. The antenna system  10  also includes a dual band feed horn  14  that receives a satellite uplink signal at a particular frequency band, for example, 28-30 GHz or 40 GHz, and transmits a downlink signal at another frequency band, for example, 18.3-20.3 GHz. Only a single feed horn is shown in the antenna system  10 , with the understanding that the antenna system  10  would include an array of feed horns arranged in a desirable manner depending on the particular application. The horn  14  is shown as a square or rectangular feed horn, but is intended to represent any feed horn operable in dual frequency bands having any suitable shape, including circular or elliptical shapes. The antenna system  10  may also employ reflectors and the like for collecting and directing the uplink and downlink signals, depending on the particular application. By using the antenna system  10 , separate antenna systems are not needed for the satellite uplink and downlink signals, and therefore valuable space on the satellite can be conserved and the weight of the spacecraft can be reduced. 
     The satellite uplink and downlink signals are circularly polarized so that the orientation of the antenna element relative to the signal can be arbitrary. However, the use of linearly polarized signals is desirable in the antenna system so that they can propagate through waveguides without significant attenuation. Therefore, polarizers are necessary after the feed horn to convert the downlink signal from a linearly polarized signal to a circularly polarized signal, and for converting the uplink signal from a circularly polarized signal to a linearly polarized signal. According to the invention, the dual band polarizer  12  performs this function for both the uplink and downlink frequency bands, either separately in time or simultaneously. Particularly, circularly polarized signals received on the satellite uplink by the dual frequency feed horn  14  are converted to a linearly polarized signal by the polarizer  12 , and the linearly polarized signals to be transmitted on the satellite downlink are converted to circularly polarized signals by the polarizer  12  before being sent to the feed horn  14 . It has not heretofore been known in the art to provide a polarizer that can perform this function satisfactorily in two separate frequency bands. 
     The linearly polarized uplink signal from the polarizer  12  is sent to a waveguide diplexer  16  that directs the signal to reception circuitry  18  within the satellite communications system. Likewise, linearly polarized downlink signals from transmit circuitry  20  are sent to the diplexer  16  that directs the downlink signals to the polarizer  12  for transmission. The diplexer  16  can be any known waveguide device that is suitable for the purposes described herein, as would be well understood to those skilled in the art. 
     FIG. 2 is a perspective view of the polarizer  12 . In this embodiment, the polarizer  12  is a hallow, rectangular waveguide  22  that includes a first corrugated structure  24  extending from one sidewall  26  of the waveguide  22 , and a second corrugated structure  28  extending from an opposing sidewall  30  of the waveguide  22 . The corrugated structures  24  and  28  are identical, and therefore only the corrugated structure  28  will be described herein with the understanding that the corrugated structure  24  is the same. The corrugated structure  28  includes a plurality of parallel ribs  32  defining spaces  34  therebetween. The width of the ribs  32  and the width of the spaces  34  remain constant along the length of the waveguide  22 . The height of each of the ribs  32  from the wall  30  is such that the corrugated structure  28  has a tapered configuration from one end  38  of the waveguide  22  to a center of the waveguide  22 , and from the center of the waveguide  22  to an apposite end  40  of the waveguide  22 . Particularly, the height of the ribs  32  proximate the ends  38  and  40  are at their lowest, and the height of the ribs  32  get progressively taller in a sequential manner towards the center of the waveguide  22 . In this embodiment, the center rib  42  has the largest height. This tapering of the height of the ribs  32  significantly eliminates reflections of the signal that may occur from discontinuities within the waveguide  22 . The other opposing side walls  44  and  46  of the waveguide  22  are smooth. 
     The signals enter the waveguide  22  through both ends  38  and  40 . Because the waveguide is symmetric, the circularly polarized signal from the feed horn  14  or the linearly polarized signal from the diplexer  16  can enter either end. The signal propagating through the waveguide  22  has orthogonal E x  and E y  field components. The E-field component (E x ) that is perpendicular to the ribs  32  interacts therewith and is delayed relative to the E-field component (E y ) that is parallel or transverse to the ribs  32  and does not interact with the ribs  32 . In other words, the spaces  34  between the ribs  32  act as waveguides that create a phase delay between the E x  and E y  field components. This delay causes the signal to rotate if the input signal is linearly polarized. The length of the waveguide  22  is selected so that the E-field components end up out of phase by 90 degrees at the output end creating circular polarization, and have the same magnitude. The orientation of the E x  and E y  field components relative to the ribs  32  determines which way the signal will rotate and whether the signal will be an RHCP or an LHCP signal. In a specific design, the E-field components of the linearly polarized downlink signal are oriented at an angle 45 degrees relative to perpendicular sides of the waveguide  22 . 
     Alternately, the ribs  32  can speed up the E-field component that interacts with the ribs  32  to also create a phase discrepancy between the field components. When the circularly polarized signal is coming into the waveguide  22  from the opposite direction, the delay caused by the ribs  32  matches the phases of the E-field components so that by the time they reach the opposite end of the waveguide  22 , they are in phase with each other, and have the same magnitude, making the signal linearly polarized. 
     The dimensions of the waveguide  22  and the dimensions and spacing of the ribs  32  and the numbers of ribs  32  are selected so that the lowest fundamental mode of the signal propagates through the waveguide  22 , and the phase relationship between the E-field components are 90 degrees apart, as described above. These parameters are also dependent on the speed of the signal propagating through the waveguide  22  that is also frequency dependent. For dual band polarization conversion, these dimensions are selected so that the higher frequency band, here 30 or 40 GHz, will be polarized in the desirable manner. Then, the dimensions are optimized for the lower frequency band, here 20 GHz. In other words, the dimensions of the waveguide  22  are selected so that the components of the E-field are 90 degrees out of phase with each other for the high frequency, and then these values are slightly varied relative to each other to make the E-field components of the lower frequency band to also be 90 degrees out of phase with each other. The E-field components also have the same magnitude. This design criteria is possible because the lower frequency band is a subset of the higher frequency band. In the known corrugated structure polarizers, the spacing between the ribs was typically selected to be one-quarter of a wavelength of the center of the frequency band of interest. Typically only a few corrugations were necessary to perform the polarization conversion. However, in the design disclosed herein, that operates in two bands, the number of corrugations required is greater, typically on order of more than five. 
     In a particular design for the frequency bands discussed herein, the width of the walls  26 ,  30 ,  44  and  46  of the waveguide  22  are 0.456 inches, the thickness of the ribs  32  is 0.018 inches, the space  34  between the ribs  32  is 0.073 inches, the number of ribs  32  and the number of spaces  34  between the ribs  32  is thirty-nine and the length of the waveguide  22  is 1.802 inches. These parameters provide the desired polarization conversion for the uplink and downlink frequency bands of known satellite communication systems. For other frequency bands, these parameters will be different and optimized accordingly. 
     To show that the polarizer  12  provides good performance for the uplink and downlink frequency bands being discussed herein, FIGS.  3 ( a )- 3 ( c ) give performance criteria for the downlink frequency band and FIGS.  4 ( a )- 4 ( c ) give performance criteria for the uplink frequency band. Particularly, FIG.  3 ( a ) shows the frequency versus return loss in dB, FIG.  3 ( b ) shows the frequency versus axial ratio in dB, and FIG.  3 ( c ) shows the frequency versus cross-polarization in dB. As is apparent, the performance is suitable for the downlink signal. Likewise, FIG.  4 ( a ) gives frequency versus return loss in dB, FIG.  4 ( b ) gives frequency versus axial ratio in dB and FIG.  4 ( c ) gives frequency versus cross-polarization in dB. As is also apparent, suitable performance is also provided for the uplink frequency band. 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit scope of the invention as defined in the following claims.