Patent Publication Number: US-4370660-A

Title: Broadband elliptic sheet antenna

Description:
BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an elliptic monopole radiator; 
     FIG. 2 shows an elliptic dipole radiator; 
     FIG. 3 shows the details of the electrical connection to the FIG. 1 antenna; 
     FIG. 4 is a graph of the measured SWR for the FIG. 1 antenna; 
     FIG. 5 is a graph of the measured input impedance for the FIG. 1 antenna. 
    
    
     DESCRIPTION 
     (i) The Experimental Model 
     The elliptic sheet antenna may be used in either a monopole or a dipole configuration. In the monopole case the antenna is an elliptic sheet of eccentricity 0.8, mounted normal to a reflecting plane with its major axis parallel to that plane; the antenna is fed through a coaxial line, FIG. 1. In the dipole case, the antenna consists of two coplanar elliptic sheets of eccentricity 0.8 with collinear minor axes, the two sheets being slightly separated to accommodate a balanced feeding line, FIG. 2. The tested experimental model was a monopole elliptic sheet antenna 1 mm thick made of brass, with major and minor axes of 10 and 8 cms, respectively. The monopole was mounted above the center of a circular sheet of copper 140 cms in diameter. A coaxial feed cable coming from below the reflecting plane penetrates through a hole at its center to feed the monopole thereabove. Details of the antenna feed and input region are shown in FIG. 3. The device shown below the reflecting plane is just a General Radio 50Ω cable connector type 874-C58A with a slight modification above M--M. In that region the GR inner conductor is replaced by another one of diameter 1.75 mms and a concentric cylindrical shell of teflon is inserted as shown. The so-modified GR cable connector is cut at the level of the upper surface of the reflecting plane, leaving the upper threaded parts of the inner conductor fits through a nut N welded to the elliptic sheet with one of its sides coinciding with the elliptical perimeter. The antenna is separated from the reflector plane by a teflon washer 0.85 mm thick. 
     Now the signal generator is connected to the feeding device via a GR patch-cord and a precision 50Ω slotted line GR LB-900. The patch cord is so selected from a set of GR 874-R20A, R22A, cords as to have standing wave ratio (SWR) less than 1.07 in the measuring frequency range. 
     (ii) Performance 
     The standing-wave ratio and impedance measurements were in the frequency range 0.4-4.5 GHz (height to wavelength ratio H/λ from 0.107 to 1.2) for the elliptic sheet monopole described above; the results are shown in FIGS. 4 and 5, respectively. For normalization the figs, show the SWR and Z versus frequency as well as versus the antenna height-to-wavelength ratio (H/λ). 
     When used in DIPOLE configuration, the impedance scale of FIG. 5 is multiplied by 2 while the SWR characteristics apply for a 100Ω feeding line. 
     (iii) Comparative Performance Figures 
     (a) Triangular antenna with 70° apical angle (having approximately same maximum horizontal and vertical dimensions) in the antenna height range from 0.35 wavelength and above. 
     
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                   Triangular                                             
                           Elliptic                                       
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Maximum resistance R.sub.max                                              
(ohms)               164       54                                         
Minimum resistance R.sub.min                                              
(ohms)               77        42                                         
R.sub.max /R.sub.min 2.130     1.286                                      
Maximum reactance |X| (ohms)                            
                     46        4                                          
Maximum reactance/resistance                                              
ratio                37.7%     8%                                         
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     (b) Helical antenna in its axial mode (1.7:1 frequency range) 
     
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                        Elliptic                                          
                Helical (0.706-1.2λ)                               
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SWR               &lt;1.5      &lt;1.18                                         
Maximum Resistance R.sub.max                                              
(ohms)            220       50                                            
Minimum Resistance R.sub.min                                              
(ohms)            90        43.5                                          
R.sub.max /R.sub.min                                                      
                  2.4       1.149                                         
Reactance Fluctuation                                                     
(ohms)            +5 to +40 -2 to +2.5                                    
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     (c) A Typical Log-periodic Dipole Array operating in a 2:1 frequency range. 
     
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             Log-periodic                                                 
                      Elliptic (0.6-1.2λ)                          
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Feeder Impedance                                                          
               110 Ohms   50 Ohms                                         
Standing wave                                                             
ratio          1.2-2.5    1.015-1.1215                                    
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