# questions about moment of method

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#### pudding

##### Member level 2
Hi guys,
I need help regarding moment of method. At the last step, I already got the mattrix for a homogenous equation group. And by setting the determinant to be zero, I can get the unkowns, such as the phase constant. My questions is, for a high order equation ( The number of the basis function is large), I get a lot of zeros. I don't know how to choose. Is that normal or including error? And by the way, I plot the determinant according to the unknowns, but the value goes very high(10^300) at the poles, which make it very difficult to pick up the zeros. Does anyone know how to do that or any good books about it? Thanks
pudding

##### Full Member level 6
I understood that you are trying to solve an eigenvalue problem using MoM. But I do not understand what do you mean by zeros ? You mean zeros in the impedance matrix or zeros in the eigenvalues ?

Also, as I remember, for MoM, increasing the number of unknowns beyond a certain limit does not necessarily improve your accuracy further more it even makes your impedance matrix ill conditioned (which would justify why you are getting incorrect results). If I were you, I would start with a small number of unknowns and increase it gradually until the change in the first few eigenvalues is sufficiently small.

BR

#### pudding

##### Member level 2
Thanks for your response. By zeros I mean the zeros for determinant of the homogeneous equation. My unknowns are included in the coefficients of the homogeneous equation group. By setting the determinant to be zero, I can get the unkowns, in my case, the phase constant. By it is difficult to solve that determinant equation directly. So I plot the determinant in terms of phase constant. From the plot, I can determine the phase constants( those points which make the determinant zero). But I find there are more zeros(which make the determinant to be zero) than I expected. And from a book, "Field computation by moment methods", I find those extra zeros are the so-called extraneous roots. Those roots are not the solution for the problem, but caused by the basis functions. Do you have any idea how to determine whether it is extraneous roots or the real roots? Any characteristics for the extraneous roots? I want to get rid of those extraneous roots by computer.