HP 8712ET User Manual page 258

Calibrating for Increased Measurement Accuracy
Checking the Calibration
Directivity Directivity is a reflection measurement error term that
indicates the ability of the analyzer to separate the
reflected signal from the incident signal.
Corrected directivity less than −65 dB is expected after
calibrations have been performed. This is the same
value you would measure when measuring a load
standard.
Figure 6-5
6-32
Typical Directivity Error Term
ET User's Guide