Critical distance of far field for basic radiation element

The paper provides a new method to determine the critical distance of the far-field (FF) of a basic radiation element by calculating the relative error between approximate and exact FF values in three respects: field value in maximum radiation direction; the pattern of FF and wave impedance .The result is obtained and recommended as critical distance of the basic radiation element, which is approached engineering requirement and calculation process is reduced largely. It can be widely used in antenna analysis and EMC prediction.