Patent Publication Number: US-6714165-B2

Title: Ka/Ku dual band feedhorn and orthomode transduce (OMT)

Description:
The present invention relates to a dual band feedhorn and orthomode transducer (OMT) for use with a terrestrial satellite parabolic reflector. 
     TECHNICAL BACKGROUND 
     Ideally, a dual band feedhorn should be capable of simultaneously illuminating an offset parabolic reflector (with an F/D ratio of about 0.5) at two frequencies, e.g. the Ku and Ka band. The antenna beams produced at both bands should be centred along the same boresight axis. This requires the use of one single feed for both bands. 
     The main function of the OMT is to provide isolation between the signals at two frequencies, for example the Ka and Ku bands. The OMT should be capable, for instance, of simultaneously transmitting both polarisation directions (vertical and horizontal) of the Ku band from the feedhorn to the Ku band port, and be capable of transmitting one of both polarisation directions (vertical or horizontal) of the Ka band from the Ka band port to the feedhorn. This means there are two possible versions of the OMT depending on the Ka band polarisation direction. 
     U.S. Pat. No. 5,003,321 describes a dual frequency feed which includes a high frequency probe concentrically mounted with a low frequency feed horn. A concentric circular waveguide has a first turnstile junction mounted adjacent the throat of the low frequency feed, which branches into four substantially rectangular, off axis waveguides extending parallel to the central axis of the waveguide. These waveguides and the low frequency signals conducted through them are then recombined in a second turnstile junction which is coaxial with the low frequency feed, high frequency probe and first turnstile junction. The high frequency feed is introduced in between two of the four parallel off-axis waveguides. The known device is split longitudinally. This split results in complex joining and sealing surfaces at the end of the low frequency feed horn and at the position where the high frequency probe is lead off axis. 
     SUMMARY OF THE INVENTION 
     The present invention may provide a dual band, higher and lower frequency range transducer with a circular coaxial waveguide feed, a first junction for connection of a lower frequency range outer waveguide of the coaxial waveguide feed to at least two rectangular or ridge waveguides offset from the longitudinal axis of the transducer, a second junction for connection of the at least two rectangular or ridge waveguides to a further waveguide and a third junction for connecting an inner waveguide of the coaxial waveguide feed to a higher frequency range waveguide, characterised in that the transducer is formed from at least two parts joined across a first plane perpendicular to the longitudinal axis and including a part of the higher frequency range waveguide within the join. By “higher and lower” frequency is meant that there is a frequency difference between the higher and lower ranges. Typically, there is no overlap between the ranges. 
     Preferably, a water seal is provided in the plane of the first join. Preferably, all of the junctions include impedance matching devices. A feed horn may be attached to the coaxial feed. The feed horn preferably has corrugations. The first and second junctions may be provided by further parts which are joined to the other parts along planes parallel to the first plane. The horn is preferably sealingly attached to the first junction part along a plane parallel to the first plane. Preferably, a dielectric rod antenna is located in the inner waveguide at the end facing the horn. The end of the inner waveguide is preferably provided with a device for preventing backscattering from the rod antenna. The device is preferably a flare opening outwards towards the horn. 
     The transducer of the present invention allows the attachment of a higher frequency waveguide to the inner waveguide of the coaxial waveguide such that the higher frequency waveguide extends at an angle to the longitudinal axis of the transducer. The higher frequency waveguide extends at substantially 90° to the longitudinal axis of the waveguide. This distinguishes the present invention over those dual band transducers which extract both higher and lower frequency range waveguides parallel to the longitudinal direction. 
     The present invention will now be described with reference to the following drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block diagram of an OMT and feed in accordance with an embodiment of the present invention. 
     FIG. 2 is a schematic front-end view of the embodiment of FIG.  1 . 
     FIG. 3 is a schematic longitudinal section at 45° to the vertical of an embodiment of an OMT and feed in accordance with the present invention. 
     FIG. 4 is a schematic longitudinal vertical cross-section of the embodiment according to FIG.  3 . 
     FIGS. 5 to  8  shows various views of a first to a fourth part  50  of an OMT in accordance with an embodiment of the present invention. 
     FIGS. 5 a  to  5   f  show respectively,  5   a : a cross-sectional side view taken vertically through the first part  50 ;  5   b : a view of the sealing face to the second part  60  looking towards the horn;  5   c : a side view;  5   d : a view of the face which is attached to the horn;  5   e : a side view; and  5   f : a cross-sectional view through the first part  50  taken along a 45° line to the vertical in FIG. 5 b  and passing through the centre line of the transducer. 
     FIGS. 6 a  to  6   h  show respectively,  6   a : a cross-sectional side view taken vertically through the second part  60 ;  6   b : a view of the sealing face to the third part  70  looking towards the horn;  6   c : a side view;  6   d : a view of the face which is attached to the first part  50 ;  6   e ; a side view;  6   f : is a cross-sectional view taken on a horizontal line in FIGS. 6 b ;  6   g : is a side view; and  6   h : a cross-sectional view through the second part  60  taken along a 45° line to the vertical in FIG. 6 b  and passing through the centre line of the transducer. 
     FIGS. 7 a  to  7   h  show respectively,  7   a : a cross-sectional side view taken vertically through the third part  70 ;  7   b : a view of the face which is sealed to the second part  60 ;  7   c : a side view;  7   d : a view of the face which is attached to the fourth part  80 ;  7   e : a side view;  7   f : is a cross-sectional view taken on a horizontal line in FIGS. 7 b ;  7   g : is a side view; and  7   h : a cross-sectional view through the third part  70  taken along a 45° line to the vertical in FIG. 7 b  and passing through the centre line of the transducer. 
     FIGS. 8 a  to  8   f  show respectively,  8   a : a cross-sectional side view taken vertically through the fourth part  80 ;  8   b : a view of the sealing face to the third part  70 ;  8   c : a side view;  8   d : a view of the face which is attached to the LNB;  8   e : a side view; and  8   f : a cross-sectional view through the fourth part  80  taken along a 45° line to the vertical in FIG. 8 b  and passing through the centre line of the transducer. 
     FIG. 9 is a schematic cross-section of a feed horn for use with the embodiment of FIGS. 5 to  8 . 
     FIG. 10 is a schematic cross-section of an inner waveguide for use with the embodiment of FIGS. 5 to  9 . 
     FIG. 11 is a schematic cross-section of an antenna rod for use with the inner waveguide of FIG.  10 . 
     FIG. 12 shows radiation patterns of a 75 cm diameter offset reflector antenna equipped with a dual frequency band feed/OMT in accordance with the present invention: curve A shows a Ku band azimuth co-polar pattern at 11.2 GHz, curve B shows a Ku band azimuth cross-polar pattern at 11.2 GHz. 
     FIG. 13 shows radiation patterns of a 75 cm diameter offset reflector antenna equipped with a dual frequency band feed/OMT in accordance with the present invention: curve A shows a Ku band elevation co-polar pattern at 11.2 GHz, curve B shows a Ku band elevation cross-polar pattern at 11.2 GHz. 
     FIG. 14 shows radiation patterns of a 75 cm diameter offset reflector antenna equipped with a dual frequency band feed/OMT in accordance with the present invention: curve A shows a Ka band azimuth co-polar pattern at 29.734 GHz, curve B shows a Ka band azimuth cross-polar pattern at 29.734 GHz. 
     FIG. 15 shows radiation patterns of a 75 cm diameter offset reflector antenna equipped with a dual frequency band feed/OMT in accordance with the present invention: curve A shows a Ka band elevation co-polar pattern at 29.734 GHz, curve B shows a Ka band elevation cross-polar pattern at 29.734 GHz. 
    
    
     DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS 
     The present invention will be described with reference to certain embodiments and drawings but the present invention is not limited thereto but only by the attached claims. 
     FIG. 1 shows a schematic block diagram of an OMT and feed  1  in accordance with the present invention. It includes a feed horn  3  with feed aperture  4  and an OMT  2 . The OMT  2  in accordance with an embodiment of the present invention is equipped with a first port  5  for a first frequency, e.g. the Ka band, normally used for (but not limited to) transmit and a second port  7  for a second frequency, e.g. the Ku band, normally used for (but not limited to) receive. Both ports  5 ,  7  preferably have standard interfaces allowing connection to a Ka band transmitter module and a standard Ku band LNB (low noise block downconverter) respectively. 
     FIG. 2 shows a schematic front view of the OMT and feed  1  as when looking into the feed aperture  4 . This and the following figures present the case of the OMT and feed construction for horizontal polarisation in the Ka band. The case for vertical polarisation in the Ka band is obtained by rotating 90 degrees around the feed centre axis  6 . 
     FIG. 3 show a schematic view of a longitudinal cross section of the OMT and feed  1  in any of the planes at 45 degrees to the vertical longitudinal plane. The OMT and feed  1  is made of conductive material such as a metal and comprises a corrugated horn section  11  having corrugations  36 , a transition region  12  from a circular waveguide  21  to a coaxial waveguide  22  and an impedance matching section including a dielectric rod antenna  28  for beam forming the high frequency central waveguide  24 , a coaxial waveguide section  13  in which a low frequency circular concentric waveguide  23  surrounds the central on-axis high frequency circular waveguide  24 , a first coaxial waveguide H-plane turnstile junction  14  with four rectangular or ridge waveguide ports  25 , an interconnection section  15  for four rectangular or ridge waveguides  26  having two E-plane bends  33 , a second circular waveguide H-plane turnstile junction  16  with 4 rectangular or ridge waveguide ports  27 , and a circular waveguide  17  with a circular waveguide interface  35  (Ku band). 
     Preferably, the exposed end of the inner waveguide  24  facing the horn  11  has a tube flare  29  which flares outwards in the direction of the horn  11 . This flare  29  reduces entry of high frequency signals into the low frequency feed. Preferably, the first and second turnstiles  14  and  16  have impedance matching devices  30  and  32 , respectively, which may be in the form of steps. 
     FIG. 4 shows a schematic cross section of the OMT  2  in the vertical plane. The end of the high frequency waveguide  24  remote from the horn  11  has a circular waveguide ( 24 ) to rectangular or ridge waveguide ( 41 ) transition  37 , an H-plane waveguide bend  39  and a rectangular waveguide interface  40  (Ka band). The transition  37  preferably has an impedance matching device  38  such as a step and the bend  39  preferably has an impedance matching device  42 . 
     Ku Band Operation 
     The corrugated feedhorn  11  collects the incoming spherical wave from a reflector dish (not shown) and converts this wave into a TE11 mode, propagating in the circular waveguide section  21  at the mouth of the horn  11 . The dielectric rod antenna  28  is made of a material with low permittivity, and its presence will not significantly affect this propagation nor will it affect significantly the radiating properties of the corrugated horn  11 . 
     At the transition  12  from circular  21  to coaxial waveguide  22  the signal is forced to propagate in between the outer and inner tubes  23 ,  24  as the diameter of the inner tube  24  is sufficiently small (and hence the cut-off frequency of the circular waveguide formed by this tube sufficiently high) to prevent propagation at Ku band down this tube. The signal propagates into the coaxial waveguide  22  formed by the outer and inner tubes  23 ,  24  according to the TE11 mode. Optional additional steps  9  in the diameter of the outer tube  23  provide matching of the discontinuity formed at the circular to coaxial waveguide transition  12  transition. 
     The coaxial waveguide section  13  terminates into an H-plane turnstile waveguide junction  14  with 4 rectangular waveguide branches  26 . Depending on the polarisation of the incoming signal, the signal will be divided between the two pairs of branches  26 , each pair collocated in the same 45 degrees plane. The signal will be divided equally between the two branches  26  constituting a pair. 
     The four rectangular waveguide branches  26  are connected with E-plane bends  33  and interconnection sections  15  to another H-plane turnstile junction  16  which collects the signal, coming from the 4 branches  26 , and combines it into a circular waveguide  17 . The polarisation of the signal coming out of the circular waveguide section  17  will be the same as the polarisation of the original signal going into the coaxial waveguide section  13  because the 4 rectangular branches  26  have the same length. 
     The received signal, independent of polarisation, is then obtained at the circular waveguide interface  35 . 
     A single polarisation embodiment of the OMT and feed  1  in accordance with the present invention may be obtained by omitting one pair of the rectangular waveguide branches  26  and replacing the second H-plane turnstile junction  16 , with an E-plane rectangular waveguide T-junction. The interface  35  is replaced by a rectangular waveguide port. 
     Ka Band Operation 
     The Ka band transmit signal is launched into the rectangular waveguide port  40 , via an H-plane waveguide bend  39 . It is routed to an H-plane transition  37  from rectangular to circular waveguide, including a matching step  38 . This transition forces the signal into the inner tube  24 , where it will propagate in the circular TE11 mode. The circular waveguide formed by this inner tube  24  serves as a launcher for the dielectric rod antenna  28 . 
     The dielectric rod antenna  28  is excited in the hybrid HE11 mode of cylindrical dielectric waveguide. A flare  29  at the end of the inner tube  24  is provided in order to reduce the back radiation from the dielectric rod antenna  28 , and also in order to launch the desired HE11 mode. The dielectric rod antenna  28  has two tapered ends, one tapered end to provide matching towards the circular waveguide  24 , and one tapered end to provide matching towards free space. 
     The dielectric rod antenna  28 , supporting the HE11 mode, radiates in a way similar to a corrugated feed horn, with identical radiation patterns in the E and H planes and low cross polarisation levels, and serves to illuminate the reflector dish. 
     The beamwidth of the dielectric rod antenna  28  is arranged to be smaller than the flare angle of the corrugated feedhorn  11  and the radiation from the dielectric rod antenna  28  will not significantly interact with the corrugated feedhorn  11 . The amount of radiation from the dielectric rod antenna  28  that is backscattered by the corrugated feedhorn  11  into the coaxial waveguide  13  will therefore be small. For this reason and also because the back radiation from the dielectric rod antenna  28  is limited by the flare  29 , a high amount of isolation is obtained at Ka band between the transmit waveguide port  40  and the receive waveguide port  35 . 
     Mechanical Arrangement and Sealing 
     The OMT and feed embodiments described above can be realised using a number of mechanical parts that can be easily machined or manufactured by other methods such as a casting process. The design therefore allows large-scale production. The basic OMT  2  can be realised with 4 mechanical parts. The OMT  2  is split transversely to the longitudinal axis  6  of the OMT  2 . 
     FIG. 5 shows the first part  50  which may be generally of quadratic section. This part  50  corresponds to the coaxial waveguide section  13  and turnstile junction  14 , and also includes the first set of the bends  33 . The outer surface of the tube  23  is formed by the inner surface  51 . The four E-bends  33  may be formed at 90° to each other from steps  52  or may be flat (two bends at 180° for the single polarisation alternative). The feed horn section  11  (see FIG. 9) is attached sealingly onto surface  53 . A first groove  54  may be arranged easily to accept a sealing ring such as a conventional “O” ring for sealing to the second part  60 . 
     FIG. 6 shows the second part  60  which may be generally of quadratic section but may have any suitable shape. Part  60  corresponds to half of the interconnection section  15  and half of the transition  37 . The inner tube  24  shown in FIG. 10 is attached to the second part  60  on side  62 , for instance in a circular recess  67 . The first part  50  is attached sealingly to the side  62 . Four rectangular (or ridge) waveguide branches  26  are distributed at 90° intervals around the longitudinal axis  6  (two branches at 180° for the single polarisation alternative). The impedance matching device  30  may be provided by a series of steps  63  on side  62 . The other major surface  61  includes a groove  64  which forms one half of the high frequency waveguide  41 . The impedance matching device  39  may be provided by a step  65 . A groove  66  may be provided for accepting a sealing ring, e.g. a conventional “O” ring for sealing to third part  70 . 
     FIG. 7 shows the third part  70  which may be of generally quadratic section but the present invention is not limited thereto. This part  70  corresponds to half of the interconnection section  15  and half of the transition  37 . This part  70  includes an H-plane waveguide bend  39  and a waveguide port  40 . The second part  60  is attached sealingly to the side  71 . Four rectangular (or ridge) waveguide branches  26  are distributed at 90° intervals around the longitudinal axis  6  (two branches at 180° for the single polarisation alternative). The branches  26  mate with the same branches in second part,  60 . The impedance matching device  32  may be provided by a stud  73  and optionally a series of steps  74  on side  72 . The side  71  includes a groove  75  which forms the other half of the high frequency waveguide  41  with groove  64  of second part  60 . The impedance device  38  is formed by a step  76 . 
     FIG. 8 shows the fourth part  80  which may be of generally quadratic section but the present invention is not limited thereto. This part  80  corresponds to the circular waveguide section  17  and second turnstile junction  16 . It also includes the second set of four waveguide bends  33  arranged at 90° to each other (two bends at 180° for the single polarisation alternative). The outer surface of the circular waveguide  17  is formed by the inner surface  81 . The four E-bends  33  may be formed from steps  82  or may be flat. The low frequency interface (LNB) is attached sealingly onto surface  83 . A first groove  84  may be arranged easily to accept a sealing ring such as a conventional “O” ring for sealing to the third part  70 . 
     The first to fourth parts  50 - 80  may attached to each other by bolts through suitable bolt holes. The corrugated feedhorn  11  and the outer tube with the matching section  12  can be realised in a single piece as shown in FIG. 9. A groove  85  is provided for a sealing ring such as an “O” ring seal to first part  50 . An impedance matching device  86  may be provided, e.g. steps in the diameter. An insulating plate (not shown) may be fitted into the wide end of the horn  11  to prevent rain, snow or moisture entry. 
     The inner tube  24  may be formed from a single tube with flared end (FIG.  10 ). The antenna rod  28  (FIG. 11) may be made as a light forced fit in the end of tube  24 . 
     All parts  50 - 80  and the horn  11  can be bolted together. The parts  50 - 80  as well as horn  11  may be made by matching, casting or a similar process. The design also allows for inclusion of sealing rings, especially rubber “O” ring seals in between the parts in order to make the OMT+feed assembly waterproof. In particular, the provision of a join plane between the second and third parts  60 ,  70  allows a convenient way of forming the high frequency waveguide  41  in a well-sealed manner without seals of complex geometry. 
     Performance 
     The performance results on a transducer in accordance with the present invention are summarised in tables 1 and 2. Test methods are according to internationally accepted standards such as ETSI EN 301 459 V1.2.1 (2000-10). Test made with a parabolic reflector were made using a visiostat reflector with aperture diameters of 75×75 cm (diameters of equivalent antenna aperture in plane perpendicular to parabolic axis) with a focal length of 48.75 cm, an offset angle of 39.95° (angle between bore-sight axis of feed and parabolic axis), a subtended angle of 74° (angle from focus subtended by reflector edge) and a clearance (distance between reflector edge and parabolic axis) of 2.5 cm. 
     FIGS. 12 to  15  are graphical representations of antenna patterns of a 75 cm reflector antenna with an OMT/feed in accordance with the present invention. The test results depend upon the diameter of the antenna dish which has been chosen as 75 cm as this is a common used standard size. The dish was from visiostat as described above. Better results can be obtained with a larger diameter dish, hence comparative results should be normalised to a 75 cm dish. Each test result given below, either individually or in combination, represents a technical feature of a transducer in accordance with an embodiment of the present invention. In particular, the present invention includes technical features provided by a combination of test results in accordance tables 1 and/or table 2. 
     
       
         
           
               
               
               
             
               
                          TABLE 1 
               
               
                   
               
             
            
               
                 Ka/Ku band feed-Horn OMT 
                   
                   
               
               
                 Ku frequency band 
                 10.7-12.7 
                 GHz 
               
               
                 Ka frequency band 
                 29.5-30 
                 GHz 
               
               
                 Ka band port i/p return loss 
                 at least 22 over frequency 
                 dB 
               
               
                   
                 range 
               
               
                 Ku band port i/p return loss 
                 at least 12 over frequency 
                 dB 
               
               
                   
                 range 
               
               
                 Ka band to Ku band isolation 
                 at least 35 over frequency 
                 dB 
               
               
                   
                 range 
               
               
                 Ka band loss 
                 ≦0.2 over frequency range 
                 dB 
               
               
                 Ku band loss 
                 ≦0.2 over frequency range 
                 dB 
               
               
                 Ka band co-polar radiation 
                 8-10 
                 dB 
               
               
                 pattern, feed taper 
               
               
                 Ka band co-polar radiation 
                 ≦±20 over frequency 
                 ° 
               
               
                 pattern, phase error 
                 range 
               
               
                 Ku band co-polar radiation 
                 8-12 
                 dB 
               
               
                 pattern, feed taper 
               
               
                 Ku band co-polar radiation 
                 ≦±20 over frequency 
                 ° 
               
               
                 pattern, phase error 
                 range 
               
               
                 Ka band peak cross-polar 
                 ≧18 over frequency range 
                 dB 
               
               
                 level 
               
               
                 Ku band peak cross-polar 
                 ≧19 over frequency range 
                 dB 
               
               
                 level 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                          TABLE 2 
               
               
                   
               
               
                 Performance of 75 cm offset reflector antenna with Ka/Ku band feed 
               
               
                 OMT* 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
            
               
                 Ku band performance @ 11.2 GHz 
               
            
           
           
               
               
               
            
               
                 3 dB beamwidth 
                 2.3 
                 ° 
               
               
                 Cross polar discrimination 
                 at least 25 
                 dB 
               
               
                 (XPD) within the 1 dB contour 
               
               
                 Off-axis antenna gain relative 
                 at least 16 over frequency 
                 dB 
               
               
                 to on-axis maximum @ 2.5° 
                 range 
               
               
                 from main beam axis 
               
               
                 First sidelobe maximum 
                 at least 27 over frequency 
                 dB 
               
               
                 relative to on-axis maximum 
                 range 
               
               
                 @ 4° from main beam axis 
               
               
                 Antenna efficiency 
                 at least 65 
                 % 
               
               
                 Ka band performance @ 11.2 GHz 
               
               
                 3 dB beamwidth 
                 0.9 
                 ° 
               
               
                 Cross polar discrimination 
                 at least 20 over frequency 
                 dB 
               
               
                 (XPD) within the 1 dB contour 
                 range 
               
               
                 Off-axis antenna gain relative 
                 at least 28 over frequency 
                 dB 
               
               
                 to on-axis maximum @ 1.8° 
                 range 
               
               
                 from main beam axis 
               
               
                 First sidelobe maximum 
                 at least 17 over frequency 
                 dB 
               
               
                 relative to on-axis maximum 
                 range 
               
               
                 @ 1.3° 
               
               
                 from main beam axis 
               
               
                 Antenna efficiency 
                 at least 64 
                 % 
               
               
                   
               
               
                 *these results are for plastic moulded reflector antenna with encapsulated metallic grid, slightly better results are obtained with solid aluminium reflectors  
               
            
           
         
       
     
     While the present invention has been shown and described with reference to preferred embodiments it will be understood by those skilled in the art that various changes or modifications in form and detail may be made without departing from the scope and spirit of the invention.