For the most accurate network measurements, vector error correction is employed, as discussed in Chapter 15. In this Appendix, we will examine the two-port model in more detail. Figure A-1 shows the forward error model for the two-port error correction. For simplicity, only half of the error terms for the two-port model are shown: the ones relevant to forward measurements. The model shown is sufficient for error-corrected S11 and S21 measurements. There is a corresponding error model for the reverse measurements, S22 and S12. S11M is the measured version of S11A, which is the actual S11 value for the device under test (DUT). The error terms included in the equation are all from the left column of Table A-1, which means the forward error model is sufficient to describe the measured result for S11.