The EMP is one of the most important phenomena of nuclear explosions.The main features of EMP in the vicinity of airbursts have been researched. On the instant of an explosion

there exist both radial electric field and radiated electromagnetic field even in the source region. But the radial electric field is dominant and the radiated field is comparatively weak. This region can also be called saturated electromagnetic field region

because the electric field reaches a certain maximum value which is not a function of the weapon yields but merely depends on the parameters of the enviroment. Beyond the region

the radial electric field diminishes to zero and then the radiated electromagnetic field becomes dominant. The conditions for creating the saturated field and the various EMP wave forms are discussed in this paper. On the basis of Gilinsky’s assumption and considering the actual gamma time behavior instead of the "jump" wave

the expanding dipole model may be suggested and can give the satisfactory explanations of the detail wave shapes in the early parts of EMP.