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