Thanks John, 

It obvious from the ratio of s/d the magnitudes of 
the units don't matter as long as the units are the same, 
but upon what are the units based - 
inches, mils, millimeters...? 

I assume that since we are dealing with wire it's mils? 

John Nieznanski wrote:

> >From "High Speed Digital Design - A Handbook of Black Magic",
> by Howard Johnson and Martin Graham, 1993, Prentice-Hall, p. 186,
> 
>    Zo =   120    *  ln (2s/d)
>         --------
>         sqrt(er)
> 
>    where, d = diameter of conductor
>           s = separation between wire centers (s > d)
>           er = effective relative permittivity
> 
>    For widely separated wires, use er = 1.
> 
>    For wire pairs with the insulation touching,
>    use the er of the insulating material.
> 
>    Also, propagation delay (ps/in):  85 * sqrt(er)

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