We consider the effect of initial phase bunching of energetic electrons on the generation of triggered ELF/VLF emissions in the magnetosphere. The electrons interacted with the primary whistler wave packet form a phase-bunched beam in the velocity space, which serves as a traveling-wave antenna emitting secondary waves. We calculate this antenna field and study its spectral and amplitude characteristics. It is shown that under the typical magnetospheric conditions, the secondary wave field is by about an order of magnitude smaller than the primary wave field. Therefore this antenna field is significant as the seed wave which is further amplified by the same beam via the cyclotron resonant mechanism. The frequency band of the antenna effect is of the order of 100 Hz near the primary-wave frequency. This is consistent with the observed frequency shift of triggered emissions.