# How do the Ripple Limit lines get determined on antenna measurements?

I'm doing an S21 ripple measurement through a VNA and I understand how the measurement is done, but I don't understand how the limit is determined. I know you can specify the limit in dB and then you specify the Hz where that limit applies.

However, how does the VNA determine where to draw the limit lines for the ripple test?

For example, when looking at this test, it puts one limit line at -1.58 dB and another at -2.58 dB. Why does it choose to put the lines here and not at the line I drew in yellow for example?

If anyone can point out what I'm missing, it would be appreciated.

It looks like the limit lines are $x$ dB away from the midpoint between the highest value in your graph and the lowest value. This makes sense for a ripple measurement, as any measurement that has a peak to peak ripple of less than (for example) 3 dB, all points on the graph will be less 1.5 db from the "middle" of the graph.