The following restrictions of the aforementioned spherical diffraction model are to be considered:

• When used for outdoor-outdoor environment the model presents no variations. On the contrary for indoor-outdoor or indoor-indoor configurations, checking or un-checking the variations checkbox adds or suppresses the variable component of the corresponding corrections;
• The frequency range should be larger than 3 GHz, with caution lower frequencies may be used but not below 300 MHz due to the surface admittance and polarisation effects;
• The model was developed for open (rural) area. Therefore, additional attenuation due to obstacles like buildings found in suburban or urban environment is not included;
• The loss due to rain is not covered;
• This model is applicable only for terrestrial radio paths.