Complex sentence: The klystron was the first significantly powerful source of radio waves in the microwave range; before its invention the only sources were the Barkhausen-Kurz tube and split anode magnetron, which were limited to very low power.
Split into simpler sentences: The klystron was the first significantly powerful source of radio waves in the microwave range. Before its invention the only sources were the Barkhausen-Kurz tube and split anode magnetron, which were limited to very low power.