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   Antenna 3 dB gain.
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   Form:
   g = Gain3dB( theta3DB, fIllumination, eff )
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   ------
   Inputs
   ------
   theta3DB      (1,1)   3 dB Angle (deg)
   fIllumination (1,1)   Illumination factor (deg)
   eff           (1,1)   Efficiency
                        
   -------
   Outputs
   -------
   g             (:)   Gain 
                        
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	  References:	Maral, G. and M. Bousquet. (1998.) Satellite Communications
               Systems. Wiley. pp. 18
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