Patent Publication Number: US-9431715-B1

Title: Compact wide band, flared horn antenna with launchers for generating circular polarized sum and difference patterns

Description:
GOVERNMENT CONTRACT 
     The Government of United States of America has rights in this invention pursuant to a U.S. Government contract. 
    
    
     BACKGROUND 
     1. Field 
     This invention relates generally to a flared antenna feed horn and, more particularly, to a flared antenna feed horn that includes a flared outer conductor, a microstrip-to-coaxial transition TE 11  sum mode launcher and a TE 12  difference mode launcher. 
     2. Discussion 
     For some communications applications, it is desirable to have a broadband system, namely, operation over a relatively wide frequency range, typically greater than 1.5:1. In some reflector based systems, it is desirable to have a feed with a small foot print, making it suitable for illuminating very low focal length to diameter ratio reflector lens. 
     In certain communications systems, signal tracking between the receiver and transmitter is achieved using a sum and difference radiation pattern. A sum pattern provides a broadside peak radiation pattern and a difference pattern provides a broadside null radiation pattern. In this case, two electromagnetic propagation modes, particularly the transverse-electric (TE) modes TE 11  and TE 12 , are needed to realize a sum and difference within the same frequency range. System performance requirements may include a large instantaneous RF bandwidth and a small physical footprint, as well as other requirements. 
     A critical element to achieve the signal tracking feature, while meeting system specifications is the feed antenna. To meet desired size constraints, a smaller aperture size is usually required, such as that of an antenna feed horn. However, the cut-off frequency of the TE 12  difference mode of an antenna feed horn is about twice the cut-off frequency of the TE 11  sum mode, where the cut-off frequency of a particular mode is the lowest frequency that the mode can propagate. It is known in the art to load such a feed horn with a dielectric to lower the cut-off frequency of a particular mode. In addition to realizing the necessary modes for generating the sum and difference modes, ample signal from the feed horn must be transmitted or received. Namely, for a small aperture relative to the operating wavelength feed horn, there exists a significant impedance mismatch between the dielectric and free space resulting in significant signal loss. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric view of a coaxial flared antenna feed horn; 
         FIG. 2  is a cross-sectional view of the feed horn shown in  FIG. 1 ; 
         FIG. 3  is a cut-away, bottom isometric view of the feed horn shown in  FIG. 1 ; 
         FIG. 4  is an illustration showing circularly polarized signal excitation for a TE 11  sum mode; 
         FIG. 5  is an illustration showing circularly polarized signal excitation for a TE 12  difference mode; 
         FIG. 6  is a block diagram of a beam forming network for the TE 12  difference mode launcher for the feed horn shown in  FIG. 1 ; 
         FIG. 7  is a block diagram of a beam forming network for the TE 11  sum mode launcher for the feed horn shown in  FIG. 1 ; and 
         FIG. 8  is a cut-away, isometric view of a coaxial flared antenna feed horn including a coplanar waveguide TE 12  difference mode launcher. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the invention directed to a broadband coaxial flared antenna feed horn providing sum and difference mode signals is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
       FIG. 1  is an isometric view,  FIG. 2  is a cross-sectional view and  FIG. 3  is a cut-away, bottom isometric view of a coaxial flared antenna feed horn  10  having the appropriate dimensions for providing certain antenna feed horn parameters and performance characteristics, for example, a height of about 2.2 inches, a diameter of about 0.66 inches, an operational frequency band of 17-53 GHz with a bandwidth ratio (BWR) of 3.12:1, a half-power beam width less than 70° over the band, and dual cross-polarization less than 15 dB. The conductive layers and dielectric materials discussed herein can be any suitable conductor, such as copper, and dielectric material. 
     The feed horn  10  includes a dielectric substrate  12 , such as Rogers Duroid, having, for example, a relative dielectric constant ∈ r =3. A conductive finite ground plane  14  is deposited on a top surface of the substrate  12  and is in electrical contact with an outer cylindrical ground conductor  16  defining a flared feed horn chamber  18  therein. A lower slightly tapered portion  22  of the conductor  16  is electrically coupled to the ground plane  14 , where the taper of the portion  22  provides an impedance mismatch for a backward propagating mode at the location where the outer conductor  16  transitions to the ground plane  14 . The tapered portion  22  transitions into a centered tapered portion  24  at interface  26  and the tapered portion  24  transitions into a uniform cylindrical portion  28  at transition  30 , where an end of the cylindrical portion  28  defines an aperture  32  of the feed horn  10 . The tapered portion  24  allows a gradual transition from the input of the horn  10  to the aperture  32 . The length of the tapered portion  24  is adjusted to match the aperture impedance to the input waveguide impedance for the desired 3.12 to 1 bandwidth performance. The flared angle of the tapered portion  24  is small to avoid a large quadratic phase error on the aperture  32  that causes low aperture efficiency. 
     An embedded conductor  34  is provided within the chamber  18  and is coaxial with the ground conductor  16 , where the embedded conductor  34  includes a lower conical section  36  having an opposite taper to the tapered portion  22  and having a length from the ground plane  14  to the transition  26 , and an upper cylindrical section  38  that extends from the conical section  36  to the aperture  32  of the horn  10 , and where the embedded conductor  34  can be a solid conductive piece or be hollow. The taper of the conical section  36  prevents higher order modes from propagating into the beam forming circuitry discussed below. A conical dielectric layer  42  is provided around the conical section  36 , as shown. 
     Four microstrip feed lines  46  positioned at 90° relative to each other are deposited on a bottom surface of the substrate  12  opposite to the ground plane  14 . In this non-limiting embodiment, four separate microstrip lines  48  are connected to the feed lines  46  and extend through the substrate  12  to be electrically connected to a lower end of the conical section  36  of the embedded conductor  34 . Excitations signals applied to the microstrip lines  46  are properly phased to excite the TE 11  sum mode in the horn  10 , which generates a circularly polarized sum pattern. It is noted that although the invention as described herein employs microstrip lines for mode launching, other embodiments may employ other types of signal lines that provide the desired E-field profile. The conical section  36  provides part of a microstrip-to-coaxial mode transformer or mode launcher that allows a signal on the microstrip feed lines  46  propagating in the microstrip transmission mode to be converted to the coaxial transmission mode. Particularly, the mode transformer or launcher section converts the coaxial TE 11  sum mode to a quasi-TEM microstrip mode, where the mode transformer section essentially acts as a transition from the coaxial mode to the microstrip mode. The radius of the embedded conductor  34  is gradually increased in such a way that the coaxial modal field lines resemble that of a microstrip field. This allows wide band impedance matching between the mode launcher and the feed horn  10 . 
     Eight equally spaced electrical coaxial signal launchers  50  are coupled to the uniform section  28  of the outer conductor  16  and provide signal launchers for the TE 12  difference mode, where the signal launchers  50  each include a center signal pin  52  being a center conductor of a coaxial line extending into the chamber  18  that receive an excitation signal, and where the signal launchers  50  would be coupled to coaxial signal lines (not shown). The difference mode is selected as the TE 12  mode because that mode is the most appropriate mode for producing difference patterns with circular polarization. A portion of the TE 12  modal power that initially travels downward in the horn  10  reflects back from the tapered portion  24 . For some frequencies the reflected power is out-of-phase with the outward horn power. As a result a severe impedance mismatch occurs for the TE 12  difference mode launchers. To address this mismatch problem, a low loss dielectric strip  54  is formed on an inside surface of the uniform portion  28  just above the transition  30  that reduces the intensity of the reflected waves and as a result a complete mismatch for the TE 12  difference mode signal launchers does not occur. 
     In order to generate propagation of the TE 11  sum mode as described, a constant amplitude phase changing excitation signal is applied to the microstrip lines  46 . To illustrate this,  FIG. 4  shows a signal excitation system  64  including electrical terminals  66  representing the lines  46  provided at positions 0°, 90°, 180° and 270° around an outer conductor  68  and to which the TE 11  sum mode excitation signal is selectively applied in rotation. 
     In order to generate propagation of the TE 12  difference mode as described, a constant amplitude phase changing excitation signal is applied to the signal launchers  50 . To illustrate this,  FIG. 5  shows a signal excitation system  70  including electrical terminals  72  representing the signal launchers  50  provided at positions 0°, 90°, 180°, 270°, 0°, 90°, 180° and 270° around an outer conductor  74  and to which the TE 12  difference mode excitation signal is selectively applied in rotation. 
     Any suitable excitation circuitry can be used to generate the signals for the TE 12  difference mode and the TE 11  sum mode.  FIG. 6  is a block diagram of a beam forming network  80  that provides the excitation signals to the mode launcher for the TE 12  difference mode as one non-limiting example, where phased controlled output signals on lines  82  are provided to each one of the signal launchers  50 . A right hand circularly polarized signal (RHCP) and a left hand circularly polarized (LHCP) signal are applied to the input ports of a 90° hybrid coupler  84  that provides a 90° phase shift between the signals. The phase shifted output signals from the 90° hybrid coupler  84  are provided to two 180° baluns  86  that each provide 180° phase shifted signals to phase delay (PD) devices  88  that provide the 0°, 90°, 180°, 270°, 0°, 90°, 180° and 270° phase shifted signals to the TE 12  difference mode launcher, such as shown in the system  70 . 
       FIG. 7  is a block diagram of a beam forming network  90  that provides the signals to the microstrip lines  46  for the TE 11  sum mode launcher. The beam forming network  90  includes a 90° hybrid coupler  92  that receives an RHCP signal and an LHCP signal and provides a 90° phase shift between these signals. The phase shifted output signals from the 90° hybrid coupler  92  are provided to two 180° baluns  94  that provide the 0°, 90°, 180° and 270° phase shifted signals to the TE 11  sum mode launcher, such as shown in the system  64 . 
     Although the horn  10  includes the signal launchers  50  that are excited to launch the TE 12  difference mode, it will be clear to those skilled in the art that other signal excitation techniques can be employed to give the desired E-field profile for the TE 12  difference mode. To illustrate another example,  FIG. 8  shows a cut-away, isometric view of a feed horn  120  similar to the feed horn  10 , where like elements are identified by the same reference number. The feed horn  120  includes a grounded coplanar waveguide (CPW)  122  mounted to the cylindrical portion  28  proximate the aperture  32 , as shown, that operates as the TE 12  difference mode launcher instead of the signal launchers  50 . The CPW  122  includes eight excitation pins  124  having a general “teardrop” shape, where the teardrop shape is by way of a non-limiting example to provide improved bandwidth where other shapes may be applicable. The CPW  122  includes an upper conductive layer  126 , a lower conductive layer  128  and a center ground plane  130 , where the center ground plane  130  is electrically isolated from each of the signal pins  124 . A top dielectric layer  132  is sandwich between the top conductor  126  and the ground plane  130  and a bottom dielectric layer  134  is sandwich between the ground plane  130  and the lower conductive layer  128 . Each of the conductive layers  126  and  128  and the ground plane  130  end at an outside surface of the conductor  16  and are electrically coupled thereto. The signal pins  124  extend through the outer wall of the conductor  16  and are electrically isolated therefrom. The dielectric layers  132  and  134  also extend through the conductor  16  into the chamber  18 . Any suitable signal line, such as coaxial cable (not shown), can be electrically coupled to the signal pins  124 , where the outer conductor of the coaxial cable would be electrically coupled to the ground plane  130  and the center conductor of each coaxial cable would be electrically coupled to one of the pins  124 . 
     The foregoing discussion disclosed 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 and scope of the invention as defined in the following claims.