I use PNA N5221A. I am trying to measure S11 and S21 of of an antenna with an active matching network which needs a dc power suplly (5V, 20mA). At port 2 i use another antenna. Because of the non-linearities introduced by the transistor of the active matching network, i need low power level to verify small signal s-parameters simulation. So when i am trying to set the power level to a value smaller than-16 dbm i receive this error message:'Unleveled source 1, out 1'. I use two port standard SOLT calibration from 100 MHz to 1 GHz. What would possibly cause such an error? How accuracy is affected if i ignore the error message?