[AFMUG] antenna gain from return loss spec?

[Ken Hohhof]

If a 5 GHz band antenna has a gain spec of 23 dBi and a graph of return loss 
vs frequency, is there a way to translate this into gain vs frequency?

We put up an AF5x link today with the 23 dBi antennas and actual Rx signal 
was 6 dB less than ideal.  But that would make sense if the antenna gain was 
actually 20 dBi at the freq of operation 5230 MHz.