[SI-LIST] Finding the impedance of a PCB trace

This is, of course, related to my search for a good 6-layer stackup.

What is the impedance of a PCB trace (microstrip) with the following 
parameters:
Width ......................... 6.00 mil
Height above return plane ..... 3.20 mil
Thickness ..................... 1.35 mil (1 ounce copper)
Dielectric rel. permittivity .. 4.2

These are the answers I get from various tools:

41... ohms (http://www.icd.com.au/)
41.69 ohms (http://www.emclab.umr.edu/pcbtlc/microstrip.html)
41.69 ohms (TraceSim version 1.0.0)
41.69 ohms (http://www.technick.net/public/code/cp_dpage.php?
aiocp_dp=util_pcb_imp_calculator)
41.7 .ohms (http://www.sunmantechnology.com/resources/cal_cat00.shtml)
46.14 ohms (ZTOOL Impedance Calculator V1.2)
47.31 ohms (http://www.rogers-corp.com/mwu/mwi_java/mwij_vp.html)
50.89 ohms (http://www.csgnetwork.com/boardrunimpcalc.html)
51.27 ohms (TxLine version 1.1)

Does anyone have an idea as to why these calculators give results 
that vary by 20% and which are more reliable?

Graham.




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