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   Antenna 3 dB angle.
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   Form:
   theta3dB = BeamWidth3dB( d, f, fIllumination )
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   ------
   Inputs
   ------
   d             (1,1)   Aperture
   frequency     (1,:)   Frequency (GHz)
   fIllumination (1,1)   Illumination factor (deg)
                        
   -------
   Outputs
   -------
   theta3dB      (:)     3 dB angle (deg)
                        
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	  References:	Maral, G. and M. Bousquet. (1998.) Satellite Communications
               Systems. Wiley. pp. 17-18.
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