Abstract: 1. In combination, means normally unblocked for receiving incident wave energy pulses, a multi-resonant cavity supplied with the received pulses for storage of the radio frequency energy in said pulses, means normally blocked to be supplied with the received pulse energy for transmitting said received pulse energy after storage for a finite time in said multi-resonant cavity, a control circuit for disabling the receiving means and enabling the transmitting means at intervals subsequent to the reception of incident pulses, and a timing source for continuously varying the duration of said intervals.