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   Antenna gain.

   Type AntennaGain to run a demo.
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   Form:
   g = AntennaGain( d, frequency )
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   ------
   Inputs
   ------
   d             (:)   Antenna data structure
                         .type             (1,:) Type
                         .area             (1,1) Area
                         .effIllumination  (1,1) Illumination efficiency
                         .effSpillover     (1,1) Spillover efficiency
                         .effSurfaceFinish (1,1) Surface finish efficiency
                         .effOhmic         (1,1) Ohmic losses
                         .effImpedance     (1,1) Impedancy mismatch losses
                         .eff              (1,1) Miscellaneous
   frequency    (1,:)  Frequency (GHz)
                        
   -------
   Outputs
   -------
   g            (:)    Gain (dB)
                        
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	References:	Maral, G. and M. Bousquet. (1998.) Satellite Communications
             Systems. Wiley. pp. 14-17
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