On the base of accurate physical model it is investigated a mechanism of EMP excitation due by a crossing by pulsed electron bunch of a bounded space with constant homogeneous magnetic field. It is shown that a radiation is described by three components namely by two opposite polarized pulses which have similar shape with the current pulse form and also integral component due by beam moving under Lorentz force. Pattern formation of radiated pulsed electromagnetic energy is investigated analytically and numerically.