Re: limit of method of moments

Larry Smith (ldsmith@faithful-188)
Wed, 29 May 1996 08:34:01 -0700

I can see where someone might say that the entire structure must be less
than 1/10 of a wavelength in order to use MoM. The MoM software that I am
familiar with will give mutual capacitances or inductances between geometries
without any regard to time. In other words, the coupling happens immediately.

Suppose a structure was 1/2 wavelength long. If time is not taken into account,
coupling might be in phase. If time is taken into account, coupling might
be 180 degrees out of phase. Big difference! My good friends at IBM, Al
Ruehli and Barry Rubin, call this the retardation effect. I'm sure they could
comment further on this.

I think that MoM can be used on structures as large as a wavelength, only if
coupling at large fractions of a wavelength are insignificantly small.

Larry Smith
Sun Microsystems

> I agree with you that your friend is wrong in stating the entire structure
> has to
> be less 1/10 wavelength to use MoM. Instead, the structure has to be
> discretized
> in such a way that each element is very small compared to wavelength.
> Typically,
> 1/10 wavelength is a good rule of thumb. One may go as high as 1/20
> wavelength
> to achieve higher accuracy. This is because many MoM formulation uses so
> called
> locally supported basis function to represent electric currents. Those
> basis
> are typically low order polynomials. Hence, it only can represent small
> change
> in currents on a local area such as a triangle or rectangle.
> -Xingchao Yuan
> Cadence Design Systems