I agree, return loss should be a positive number, and I can remember endless debates in our engineering-team about it, but when you tune filters on a Network analyzer, it is nice to have the insertion loss (S12) and return loss (S11) going the other way on a single screen. My memory is not too good after having a brain tumor and a couple of strokes, but I seem to recall the HP network analyzers (HP8712???) also showing S11 as a negative number, whilst S21 as a positive number. My memory is a bit fuzzy, bit I can remember the one graph, S12 going up (less negative), and S11 going more negative on the same screen, making it easy to tune. Whether it was the default mode for the HP8712 or whether it was a custom-mode, I can¡¯t remember.
