S parameters plot not making sense for antipodal Vivaldi antenna using antenna designer.

I am trying to simulate antipodal vivaldi antenna using antenna designer app. All results are making sense other than s parameters plots of S11. It should show a dip in the bandwidth region but the results are totally opposite: they are showing a rise indicating there in maximum return loss at operating frequency!

For all other antennas even vivaldi, the results are correct. I have operated antipodal vivaldi at 1.8 GHz using the default parameters. On all frequencies, the S11 plot of antipodal vivaldi antenna is not making sense. All other antenna elements are making sense.