There are three impedances
Zo (
dielectric, with/h, thickness/h
) = impedance of the track as if it were on its own.
Zoo
(
dielectric, with/h, thickness/h
, gap/h
)
= ODD mode impedance.This is the impedance of the single track if an inverted signal is put on the second track.This second out-of-phase signal increases the capacitance seen by the first track and
reduces
its impedance
Zoe
(
dielectric, with/h, thickness/h
, gap/h
)
= EVEN mode impedance.
This is the impedance of the single track if the same signal is put on the second track.This second in-phase signal reduces the capacitance seen by the first track and
increases
its impedance
As the gap between the two tracks incraeses then the impedances of Zoo and Zoe converge to Zo