HP 8712C User Manual page 273

Calibrating for Increased Measurement
Accuracy
Check the Calibration
Directivity
Directivity is a reflection measurement error term.
Corrected directivity less than -40 dB is expected after
calibrations have been performed. This is the same value
- 1 1 0
6-30
would measure when measuring a load standard.
I I I I
Figure 6-4. Typical Directivity Error Term
1 0 . 0