The impedance of the bottom layer in a 4-layer PCB

[buenos]

4layer PCB Z0

I want to design a 4 layer pcb.
My question is, how should I calculate the characteristic impedance of the bottom layer microstrip lines? I have equations, but I am not shure, in the hight parameter.

------------------------------------top layer
------------------------------------gnd plane
------------------------------------power plane
------------------------------------bottom layer

For the hight parameter, which should I use? The distance between the bottom-gnd, or the bottom-power?

If I split the power plane, cross a line, is it bad?

 

[rfmw]

Re: 4layer PCB Z0

power plane, since it is a ground plane for the bottom layer. Of course you shouldn't have any discontinuities in the power plane, because it would greatly affect the bottom layer microstrip impedance...

 

[buenos]

4layer PCB Z0

thanx!