A microstrip open end can be modeled by a longer effective microstrip line length as described by M. Kirschning, R.H. Jansen and N.H.L. Koster .
The numerical error is less than % for and .
Another microstrip open end model was published by E. Hammerstad :
Here the numerical error is less than % for .
In order to simplify calculations, the equivalent additional line length can be transformed into an equivalent open end capacitance :
With being the capacitance per length and = 299 792 458 m/s being the vacuum light velocity.