I set up an S21 measurement and save the data as an S2P file. Usually I get nonzero data only for S21 real and imaginary columns, while all the other columns are zero. However I reset the analyzer once and somehow the S2P file I saved showed nonzero data for both S21 and S11 real and imaginary. What did I do to become able to get S21 and S11 at the same time?
Where if you have a full 2 port cal, all the parameters are necessary to perform error correction so all the parameters in the file are valid even if only 1 is displayed. This would be the best method as all the parameters in the file are valid.
If you only need, S11 and S21 you can get away with only performing an Enhanced Response Calibration (with port 1 as the reflection port) which will give you valid data for only S11 and S21, but with only 1 sweep. Just remember that only the S11 and S21 data are valid and error corrected with this method.