# Re: [SI-LIST] : Ansoft 2D extractor question

[email protected]
Fri, 24 Apr 1998 00:49:52 -0700

Charles,

Here is some information on your quesitons about Ansoft's 2D Extractor tool:

Problem 1:

The standard equation for the impedance of a symmetrical 2 wire line in a
medium with dielectric constant of 1 is

Z=(377/pi) arc-cosh(s/d)

where s is the wire diameter and d is the center to center wire spacing.
Here it is assumed that the current flows out one wire and returns in the
other. For your values, the equation gives and impedance of 188.02 ohms.
If you set up this problem in the Ansoft 2D Extractor and identify one wire
as a signal conductor and the other as an ideal ground, you obtain an
impedance of 188.03 ohms as expected. I don't know how you set up your
boundary conditions so can't comment on how you got a value of 14 ohms.

Problem 2:

You cannot use the 2D Parameter Extractor to analyze the partial inductance
of a piece of wire by itself. When you analyze a problem with a 2D solver,
you are assuming that the geometry has the same cross section to +/-
infinity. But the partial inductance of an infinitely long wire is
infinite. It only becomes finite if there is a return current path on
another conductor or the wire is of finite length. If there is a return
current path, you can include it in the 2D model. If the wire has a finite
length, you can use the 3D Parameter Extractor to get the partial
inductance. I suspect that you were using the problem boundary to carry the
return current in your calculation and, id so, your result will depend on
the size of the boundary box.

I hope this helps.

Best regards,

Eric
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Eric Wheatley Ph.D. Electromagnetic Modeling Services
Alterra Technology Co. (760) 942-9426 (phone)
435 Dunsmore Ct. (760) 942-2366 (fax)
Encinitas, CA 92024 US [email protected]
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At 02:56 PM 4/23/98 -0600, you wrote:
>This question is for anyone who uses the Ansoft 2D extractor..
>
>I have been using this s/w for about 4 months and, in my innocence,
>have been attempting to "calibrate" the answers. For example, I
>plugged in a simple microstrip and behold - I got the right answers for
>Zo.
>
>Problem 1..
>I then tried to get the Zo of a two wire configuration. The
>wires are 20mils diameter and spaced (center to center) 50mils apart.
>
>Accepted calculations indicate that the Zo should = 188 ohms. The
>2DExtractor gives me an answer of 14 ohms!!
>
>Problem 2..
>I then tried to get the inductance of a 2 meter length of 28AWG wire.
>Measurements (@1kHz) gave an answer of 2.1uH.
>Formulae gave an answer of 3.748uH.
>2DE gave an answer of 0.5uH
>
>Can somebody explain what is going on??
>
>Thank you
>Charles Grasso
>EMC Engineer
>StorageTek
>2270 Sth 88th Street
>Louisville CO 80027 MS 4262
>[email protected]
>Tel:(303)673-2908
>Fax(303)661-7115
>Symposium Website URL: http://www.ball.com/aerospace/ieee_emc.html
>
>
>