Some basic principles of time domain electromagnetic field radiated by any kind of sources,especially by phased array antenna are presented. From the principles,we find the differences of radiating properties between time domain antennas and frequency domain antennas. And it is possible to describe the radiating properties of a time domain antenna using time domain signal distribution on a far field spherical face. The time domain electromagnetic fields of a ultra wide band ultra low side lobe phased array antenna have many new fashions,which are very difficult to be described in frequency domain. So it is very necessary to develop time domain methods to describe an ultra wide band ultra low side lobe phased array antenna.