Hi Michael,

I used to chair that committee. I don't have the document with me, but if I remember correctly the microstrip equation
as written in the document will be larger by the square root of pi. The errata sheet lists correctly as outside the sqrt
function.

The equation was originally cited from Wadell's "Transmission Line Design Handbook" and modified by Rick Hartley for Rf
use. The Hartley equation was adopted for what ever reason, hence the removal of all the accuracy citations because we
did not use the exact equation from the source.

Hartley's equation uses the effective Er (Ereff) in place of Er. Therefore you are calculating 88 ohms instead of 50.
If you replace all Ereff values with Er, you will get 50 ohms. After many hours of calculation comparisons, I found that
the Wadell equation as written is actually quite accurate and extremely comparable to any field solver using the same
inputs.

If you have access to Wadell's book, grab it. It is very useful.

(For the record, when I plug in your values to my spreadsheet, using Wadell's equation as written, and 0.035mm for the
top and bottom of the trace (rectangle) I calculate 50.6 ohms).

I hope this helps.

Craig Sullivan
Manufacturing Engineer
MPL Incorporated
P: 607.266.0480 Ext #32
F: 607.266.0482
<mailto:[log in to unmask]>
<http://www.mplinc.com/>







-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Michael Holz
Sent: Tuesday, February 17, 2009 12:27 PM
To: [log in to unmask]
Subject: [TN] 2141A errata microstrip equation


I'm trying to calculate the characteristic impedance of a surface microstrip
using equation 4-4 found in the IPC-2141A errata.

with the inputs of t=.035mm, h=.794mm, w=1.5 and Er=4.2 I calculate
Z0=88.8 Ohms. However, when I plug these numbers into the numerous online
calculators I get varying values all close to 50 Ohms.

Has anyone else used this formula and verified it is correct?

Also, by the way the equation is written it appears that the pi^2 term at the
end of the equation outside of the square root function. Is this correct or
should it be inside the square root function?

thanks

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