Extremely low frequency antenna

You can try enabling the low-frequency stabilisation.  From chapter 3.12 in the User Manual: 

Low frequency stabilisation

 

For low frequency modelling, the low frequency stabilisation for MoM is activated in the Low frequency modelling group. Note that this option is not required at higher frequencies and does use more memory. This option should only be enabled when solving a model at low frequencies and the matrix condition number starts to deteriorate.

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Another option might be to see whether the ACA works for you.  From chapter 1.1.1 in the User Manual:

Adaptive cross-approximation (ACA)

 

The ACA is a fast method similar to the MLFMM but is also applicable to low frequency problems or when using a special Green's function. It approximates the impedance matrix by constructing a sparse H-matrix (only a few selected elements are computed).

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Without more information about what your model involves (e.g. electrical size, dielectric complexity, etc.) it might be difficult to give more information.  See if either of these work for you.  Otherwise, could you add some more detail about what it is that you are trying to model?

Hello,

As Andries indicated, FEKO has been stabalised, but only for limited solution methods. We have modeled PEC structures down to fractions of a Hz using low frequency stabilisation. At low frequencies it is also advised to rather use volume equivalence principle (VEP) dielectrics instead of the surface equivalence principle (SEP).

In general, Flux (also part of Altair EM simulation offering) is more suited to these low frequencies.