Re: limit of method of moments
Wed, 29 May 1996 08:52:52 -0500

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
in such a way that each element is very small compared to wavelength.
1/10 wavelength is a good rule of thumb. One may go as high as 1/20
to achieve higher accuracy. This is because many MoM formulation uses so
locally supported basis function to represent electric currents. Those
are typically low order polynomials. Hence, it only can represent small
in currents on a local area such as a triangle or rectangle.

The issue of limit of MoM is a hard one in general. Since you are
referring to
HP's momentum, I can list a few limiting factors:

(1). Accuracy in computing Green's function. Momentum uses dyatic Green's
for multilayer dielectrics. To speed up the computation, HP uses
Green's function and does interpolation when the Green's function is

(2). Mesh densities. In order to achieve high accuracy, local refined mesh
necessary. I am not sure Momentum has this capability.

(3). Deembeding techniques. In order to extract circuite parameters such
scattering parameters, it is necessary to extend the ports to certain
and perform certain operations. This has a strong influence on the
of circuit parameters.

(4). Low frequency limit. Momentum uses mixed potential integral equation
technique. It generally breaks down at very low frequency unless special
care is

If you would like a practical example on the limit of accuracy for code
such as HP
Momentum, I would refer you to an artical published in European Microwave
Journal earlier this year which compared the measured results with the
soltions obtained by various simulators. For matched microstrip line, |S11|
= 0.0
analytically. Numerically, it is in the range of 0.01 to 0.001.

-Xingchao Yuan
Cadence Design Systems