microstrip characteristic impedance

I have been looking at three equations to calculate the characteristic impedance of a microstrip line. The three I am interested in are the Wheeler, Schneider, and Hammerstad and Jensen equations seen at this website:

http://qucs.sourceforge.net/tech/node75.html

I am just looking at the equations that assume an infinitely thin strip. When I plot the three equations for z0 vs. w/h, the Hammerstad and Jensen equation gives a significantly different plot for epsilon_r not equal to 1. I was wondering if someone could explain to me the difference between that equation and the other two.

Thanks

Dear mbs4,

The most accurate model is that of "Hammerstad and Jensen". However the equations written on the website has a small problem in defining Zc (final Zc should be divided by √εeff and not √εr). Also, Hammerstad model is valid for narrow and wide strips. By the way, for more accuracy, you have to take the dispersion effect in your calculations. Thanks.

Best Regards,
imodelit Moderator

Ah, thank you for pointing out that error, helped alot.

Thanks

Added after 3 hours 12 minutes:

I am trying to write a matlab program to calculate the characteristic impedance using method of moments, if anyone has any info about that, that would be great.

Thanks

Added after 3 hours 7 minutes:

Also, does Wheeler have a different formula for two parallel strips or is it that same equation as for a microstrip line?