HP 8753E User Manual page 380

For microstrip and other fabricated standards, the velocity factor is significant. In those cases,
the phase calculation must be divided by that factor. For example, if the dielectric constant for
a substrate is 10, and the corresponding "effective" dielectric constant for microstrip is 6.5,
then the "effective" velocity factor equals 0.39 (1 + square root of 6.5).
Using the first equation with a velocity factor of 0.39, the initial length to test would be
1.95 cm. This length provides an insertion phase at 1000 MHz of 60 degrees; at 2000 MHz,
120 degrees (the insertion phase should be the same as the air line because the velocity factor
was accounted for when using the iirst equation).
Application and Operation Concepts