When to use Tetrahedral or Hexahedral mesh in CST eigenmode solver

Hi,
For Dispersion analysis which type of mesh is required in CST eigenmode solver?
When to use a tetrahedral mesh and when to use a hexahedral mesh?

from the help section of CST :
Which eigenmode solver method to use

The Eigenmode solver is available with hexahedral mesh and tetrahedral mesh.
With hexahedral mesh, two eigenmode solver methods are available and will be shortly described in the following: The Advanced Krylov Subspace method (AKS), and the Jacobi-Davidson method (JDM), which is capable to also solve lossy structures.
Normally, only a finite number of the lowest eigenmodes are needed. Therefore, the AKS solver uses a special filter polynomial to suppress the unwanted higher modes. The solver works in frequency domain using an iterative subspace method.
The AKS method depends on an estimation of the eigenvalue of the highest mode under consideration. This estimation is chosen automatically during an iterative estimation refinement process. If many of these passes are required, it might be advantageous to choose the JDM eigenmode solver, which is parameter free.
The solver time for the JDM eigenmode solver increases with the number of modes to calculate. Therefore, it is usually the method of choice if only a few modes are required. In many cases, the JDM solver is very robust, especially for multiple degenerated modes.
If the analyzed structure contains electrically or magnetically lossy materials which can be approximately described by a frequency independent complex permittivity or reluctivity, respectively, please choose the JDM solver, which automatically consider these materials. Consequently this method directly yields Q-factors for resonant structures, while Q-factors in loss free simulations are calculated by means of perturbation analysis as a post processing step (as done for the AKS solver or by choice for the JDM solver). In addition lumped L and C elements can be simulated with the JDM solver.
In case of tetrahedral mesh, one general purpose method is implemented and no choice of the method is to be made. A curved element order greater than One should be specified in the special tetrahedral mesh properties for a more accurate approximation of the geometry.
hope be helpful: csttutorial.blogspot.com