# Predefined Variables

The PREFEKO language includes a number of predefined variables. Generally, these variables remain constant but may be overwritten by re-assignments.

Table 1. Predefined variables list
Name Value Description
#pi  3.14159265358979... The constant $\pi$
#esp0 $8.85418781761\cdot {10}^{-7}$ Dielectric constant ${\epsilon }_{0}$ of free space.
#mu0 $4\pi \cdot {10}^{-7}$ Dielectric constant ${\mu }_{0}$ of free space.
#c0 $\frac{1}{\sqrt{{\mu }_{0}{\epsilon }_{0}}}$ The speed of light in free space.
#zf0 $\sqrt{\frac{{\mu }_{0}}{{\epsilon }_{0}}}$ The intrinsic impedance of free space.
#true  1 Used for logical true.
#false 0 Used for logical false.

PREFEKO also supports a logical function DEFINED(#variable) which returns TRUE if the variable #variable has been defined, and FALSE if not. This is useful in .pre files used for OPTFEKO or ADAPTFEKO runs. These two components insert variables at the top of the file, but it may be required to define the variable in the file for preview purposes.

For example, if a .pre file is used for optimisation with respect to the variable #a, this variable could be defined as follows:
!!if (not(defined(#a))) then
#a = 200.0e-3
!!endif