Obtaining unipolar pulses at far field zone of the source

Under the pulses with non-harmonic waveform, propagate in free space, the time dependence of the field strength changes. In the far field or when the radiation is focused on the focal point, the field strength becomes proportional to the time derivative of the source field. If the field in any near zone is unipolar, then differentiation will lead to unipolarity lost. Based on these considerations, it is usually concluded that the practical use of radiation from unipolar sources is extremely limited. Since any propagation and attempts to focusing will lead to the loss of radiation unipolarity. However, this limitation does not effect situations when the field is depended on time in a near zone having a special form. In this note, conditions are given when it is possible to obtain unipolar pulses in the far field zone.