Abstract:
The present invention provides apparatus and methods for a ridge horn antenna that exhibits improved directivity and main lobe of the radiation pattern at the high end of the frequency range for which its gain remains usably high, while providing a relatively low VSWR across the frequency range of operation.

Description:
PRIORITY CLAIM  
       [0001]     This application claims priority of U.S. Provisional Application Ser. No. 60/496,175, filed on 08/19/2003. 
     
    
     TECHNICAL FIELD OF THE INVENTION  
       [0002]     The present invention relates to the field of RF antennas and, in particular, dual ridge horn antennas.  
       BACKGROUND OF THE INVENTION  
       [0003]     Among the simplest and probably most widely used antennas is the horn, with applications including use as a feed element for dish antennas, reflectors and lenses, as elements of phased array antennas, for calibration and gain measurements of other antennas and devices, and for electromagnetic compatibility (EMC) testing. The widespread applicability of horns arises from its relative simplicity, ease of construction, ease of excitation, versatility, large gain and performance.  
         [0004]     Horn antennas are essentially flared waveguides that produce a uniform phase front larger than the waveguide itself. A commercially available horn antenna is the Model 3115, manufactured by ETS Lindgren. See http://www.ets-lindgren.com/. A three dimensional view of this antenna is shown in  FIG. 1 .  FIG. 2  shows a bottom view, side view and rear view of the Model 3115. This antenna comprises a connection assembly  1000 , an upper plate  1100 , a lower plate  1200 , and side plates  1001  and  1002 . The dimensions shown are nominal dimensions for the ridged horn antenna designed for operation in the 1 to 18 giga-Hertz (gHz) frequency band. Thus, upper and lower plates  1100 ,  1200  are nominally 9.63 inches wide at the wide end  1210  of the flare and 3.63 inches at the narrow end  1220  (bottom view). Upper plate  1100  and lower plate  1200  are each at an angle of ±13 degrees, 14 minutes from the horizontal, extending 6 inches from connection assembly  1000 . This is referred to herein as a pyramidal horn since the horn formed by the plates is flared in both the E-plane and the H-plane. Connection assembly  1000  provides a connection  1050  to couple power to the antenna from a coaxial cable (not shown). A threaded stud  1003  is provided for mounting the antenna.  
         [0005]      FIG. 1  also shows a ridge  1250  attached to lower plate  1200 . A second ridge  1150  of identical contour is attached to upper plate  1100 . A side view and an edge view of a ridge  1150  or  1250  are shown in  FIG. 3 . The ridge exhibits a nominal edge thickness of 0.3550 inches and a nominal length of 7.5 inches. The ridge also exhibits a curvature or flare with nominal coordinates in inches as follows:  
                                                                                                     X   0.000   0.5000   1.000   1.500   2.000   2.500   3.000   3.500       Y   0.000   0.000   0.016   0.032   0.049   0.085   0.133   0.200                    X   4.000   4.500   5.000   5.500   6.000   6.500   7.000       Y   0.290   0.422   0.605   0.875   1.265   1.855   2.695                    
 At its widest point, the ridge is  1 . 66  inches wide. Further, the ridge termination  1151  coincides with the end  1210  of a plate  5100 ,  5200 . 
 
         [0007]     The implementation of ridges  1150  and  1250  vastly extends the usable bandwidth of the basic horn antenna. Adding ridges to the horn antenna increases its bandwidth by lowering the cut off frequency of the dominant mode, while raising the cut off frequency of the next higher order mode. A gain pattern for the Model 3115 antenna is shown in  FIG. 4 , which shows a substantial gain over the frequency range between one and eighteen gHz. The Voltage Standing Wave Ratio (VSWR) for this frequency range is shown in  FIG. 5 , and the half power beam width is shown in  FIG. 6 .  
         [0008]     A typical normalized radiation pattern of the ridge horn antenna is shown in  FIGS. 7, 8  and  9 , corresponding to 3, 12, and 17 gHz respectively. The preferred pattern is one in which the maximum power is delivered on the main axis (zero degrees), with monotonically decreasing power over a wide angular sector off the main axis. As shown in  FIGS. 7, 8 , and  9 , as frequency increases, the main lobe of the antenna pattern becomes narrower and side lobes increase in power. Moreover, as reported in a recent technical journal, when the frequency of operation increases, the amplitude of off-axis side lobes increases and eventually surpasses the on-axis power. See IEEE Transactions on Electromagnetic Compatibility, Vol. 45, No. 1, February 2003, pages 55-60, Bruns, et.al.  
         [0009]     Thus, although the standard ridged horn antenna provides usably high gain over a very broad frequency range, its directivity deteriorates at the high frequency end of that range. This is undesirable in most applications especially when the ridged horn antenna is used for calibration, gain measurements, or EMC testing. For EMC Immunity or susceptibility measurements it is also desirable to have the main lobe of the pattern wide enough to illuminate the equipment being tested, the narrow beam of the 3115 antenna is not well suited for this purpose. Improvement of an antenna&#39;s directivity without an increase in the VSWR within the frequency range of operation is difficult. Thus, what is needed is a ridged horn antenna that exhibits improved directivity at the high end of the frequency range for which its gain remains usably high, while providing a relatively low VSWR across the frequency range of operation.  
       SUMMARY OF THE INVENTION  
       [0010]     Accordingly, the present invention presents methods and apparatus for directivity enhancement of a ridged horn antenna that overcome limitations of the prior art. More particularly, a ridged horn antenna, and method of design there for, is presented that exhibits superior directivity at the high end of the frequency range for which its gain remains usably high, while providing a relatively low VSWR across the frequency range of operation.  
         [0011]     According to an aspect of the invention, ridges of a ridged horn antenna are provided that exhibit a pronounced curvature extending beyond the end of the plates that form the flared horn.  
         [0012]     According to another aspect of the invention, the curvature of a ridge exhibits an arc that is tangent to a line perpendicular to a surface of the plate to which the ridge is affixed.  
         [0013]     According to another aspect of the invention, the curvature of a ridge exhibits an acute arc that terminates on a surface of the plate to which it is affixed, the arc being tangent to a line perpendicular to a surface of the plate.  
         [0014]     According to another aspect of the invention, an aperture of smaller dimension and a smaller antenna length are achieved.  
         [0015]     According to another aspect of the invention the side plates of the pyramidal horn structure are eliminated as they affect the behavior of the main beam.  
         [0016]     The foregoing has outlined rather broadly aspects, features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional aspects, features and advantages of the invention will be described hereinafter. It should be appreciated by those skilled in the art that the disclosure provided herein may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Persons of skill in the art will realize that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims, and that not all objects attainable by the present invention need be attained in each and every embodiment that falls within the scope of the appended claims.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
         [0018]      FIG. 1  is a 3-dimensional view of a prior art ridged horn antenna.  
         [0019]      FIG. 2  shows a bottom, side and rear view of the prior art ridged horn antenna of  FIG. 1 .  
         [0020]      FIG. 3  shows the side and edge view of a ridge employed in the prior art ridged horn antenna of  FIG. 1 .  
         [0021]      FIGS. 4, 5 , and  6  are charts of the gain, VSWR, and Half Power Beam width versus frequency, respectively, for the prior art antenna of  FIG. 1 .  
         [0022]      FIGS. 7, 8 ,  9  show the normalized radiation patterns for prior art ridged horn antenna of  FIG. 1  for 3, 12, and 17 gHz, respectively.  
         [0023]      FIG. 10  is a 3-dimensional view of a preferred embodiment of the ridged horn antenna of the present invention.  
         [0024]      FIG. 11  shows an aperture view of a preferred embodiment of the ridged horn antenna of the present invention.  
         [0025]      FIG. 12  shows a side view of a preferred embodiment of the ridged horn antenna of the present invention.  
         [0026]      FIG. 13  shows a top view of a preferred embodiment of the ridged horn antenna of the present invention.  
         [0027]      FIG. 14  shows a side view and edge view of a preferred embodiment of a ridge for the present invention.  
         [0028]      FIGS. 15 , and  16  are charts of the gain, and VSWR versus frequency, respectively, for the preferred embodiment of the present invention.  
         [0029]      FIGS. 17, 18 ,  19  show the normalized radiation patterns for prior art ridged horn antenna of  FIG. 1  for 3, 12, and 17 gHz, respectively.  
         [0030]      FIG. 20  shows a comparison between a ridge of the prior art and a ridge of a preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]     A 3-dimensional view of a preferred embodiment of a ridged horn antenna  5000  of the present invention is shown in  FIG. 10 . An aperture view is shown in  FIG. 11 . The embodiment comprises an upper plate  5100  and a lower plate  5200  that are affixed to a cavity assembly  5001 . Cavity assembly  5001  is preferably rectangular in cross-section and is open at one end. Affixed to upper plate  5100  is an upper ridge  5150  and affixed to lower plate  5200  is a lower ridge  5250 .  
         [0032]     A side view of the preferred embodiment of antenna  5000  is shown in  FIG. 12 , comprising upper plate  5100 , lower plate  5200 , upper ridge  5150 , lower ridge  5250 , and cavity assembly  5001 . The angle between upper and lower plates  5150  and  5250  is nominally 41.97 degrees in the preferred embodiment as shown in  FIG. 12 . In a side  6010  of cavity assembly  5001  is a fitting  6020 , such as a precision type N jack, to receive a coaxial cable (not shown) to deliver RF power to antenna  5000 . The center conductor  6030  of the coaxial feed inserts through a hole in lower ridge  5250 , through the gap  6040  between upper and lower ridges  5250  and  5150 , and terminates in upper ridge  5150 .  
         [0033]     Upper plate  5100 , upper ridge  5150 , and cavity assembly  5001  are shown in  FIG. 13 , which is a top view of the preferred embodiment of antenna  5000 . Also shown is a tuning tongue  7100  for higher order mode suppression. The dimensions of the tongue are nominally 800 mils long by 620 mils and being 15 mils in thickness, the tongue has a notch centered on the width that is 320 mils by 700 mils deep for this embodiment. Directly behind the tongue is a smaller interior cavity formed in the inner rear wall of cavity assembly  5001  for additional control over the characteristics of the antenna, such as for example reducing the VSWR of the antenna. The dimensions of this interior cavity are 163 mils deep by 800 mils by 500 mils.  
         [0034]     Further, extending from the rear  7200  of cavity assembly  5001  is a threaded stud  7300  for centering and mounting antenna  5000 , as well as indexing pins  7400  for alignment. Note, as indicated in  FIG. 13 , that upper and lower ridges  5150  and  5250  extend beyond the edges  5175  of upper and lower plates  5100  and  5200 , respectively.  
         [0035]      FIG. 14  is a side view and an edge view of a ridge  5150  or  5250  of the present invention. Each ridge exhibits a nominal edge thickness of 0.266 inches and a nominal length of 6.486 inches. The ridge also exhibits a curvature or flare with nominal coordinates in inches as follows:  
                                                                               X   0.249   0.679   1.395   1.750   2.110   2.473   2.841   3.215   3.592   3.983   4.780       Y   1.102   1.268   1.516   1.639   1.748   1.848   1.936   2.007   2.071   2.100   2.117                    
         [0036]     for coordinates extending to a point where the tangent to the curve is parallel to a plate;  
                                                                           X   5.083   5.399   5.571   5.750   6.047   6.179   6.3   6.423   6.474   6.486       Y   2.112   2.073   2.018   1.943   1.759   1.609   1.426   1.235   1.040   0.838                  
 
         [0037]     for coordinates extending to a point where the tangent to the curve is vertical; and  
                                                               X   6.436   6.342   6.021           Y   0.648   0.515   0.447                      
 
 for coordinates extending to the plate edge. 
 
         [0039]      FIGS. 15 , and  16  are charts of the gain, and VSWR versus frequency, respectively, for the preferred embodiment of the present invention. Clearly, comparing  FIGS. 4 and 15 , and  FIGS. 5 and 16 , a smoother gain curve is achieved by the present invention and a substantial improvement in gain is obtained at the highest frequency of operation, without a substantial sacrifice in VSWR.  
         [0040]      FIGS. 17, 18 ,  19  show the normalized radiation patterns for the preferred embodiment of the present invention for 3, 12, and 17 gHz, respectively. Comparing these to the corresponding plots for the prior art antenna shown in  FIGS. 7, 8 , and  9 , a clear and substantial improvement in the main lobe is achieved. At 17 gHz the side lobe level has been reduced while the 3 dB beamwidth has been improved making the antenna more suitable for immunity EMC testing.  
         [0041]     Shown in  FIG. 20  is a comparison of ridgel  150  of the prior art Model 3115 and the ridge  5150  of the preferred embodiment of the present invention. Note that the angle of the prior art flare measured from the horizontal to the plate, φ 1 , is about 13 degrees, whereas for the preferred embodiment, the angle of the flare measured from the horizontal to the plate, φ 2 , is about 21 degrees.  
         [0042]     Expressing the dimensions of the preferred embodiment in terms of fractions of a wavelength at the lowest frequency of operation, λ L , in this instance, 1 gHz with λ L =11.811 inches, we have as follows: 
        the length, L=6.977 inches, of the antenna is about 0.591λ L , compared to L=8.13 inches=0.688λ L  for the Model 3115;     the aperture width, W=6.949 inches, of the antenna is about 0.588λ L , compared to W=9.63 inches=0.82λ L  for the Model 3115; and     the aperture height, H=6.036 inches, of the antenna about 0.511λ L , compared to H=6.25 inches=0.53λ L  for the Model 3115.        
 
         [0046]     Expressing the dimensions of the preferred embodiment in terms of fractions of a wavelength at the highest frequency of operation, λ H , in this instance, 18 gHz with λ H =0.656 inches, we have as follows: 
        the length, L=6.977 inches, of the antenna is about 10.63λ H , compared to L=8.13 inches=12.39λ H  for the Model 3115;     the aperture width, W=6.949 inches, of the antenna is about 10.59λ H , compared to W=9.63 inches=14.68λ H  for the Model 3115; and     the aperture height, H=6.036 inches, of the antenna is about 9.20λ H , compared to H=6.25 inches=9.52λ H  for the Model 3115.        
 
         [0050]     Note that although the angle of the flare formed by upper and lower plates  5150  and  5250  of the preferred embodiment is much greater than the corresponding angle for the Model 3115, the aperture height, H, is about the same for both antennas, yet the antenna length and width has been shortened considerably in the present invention compared to the prior art.  
         [0051]     Thus, although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. The invention achieves multiple objectives and because the invention can be used in different applications for different purposes, not every embodiment falling within the scope of the attached claims will achieve every objective.  
         [0052]     Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.