An O-type traveling wave tube of the invention includes a slow wave structure having a broad band characteristic over a predetermined range of frequencies and in some cases is capable of broad band operation over a double octave of frequencies. In this the slow wave structure comprises a combination of a substantially nondispersive delay line, such as a helix, with a substantially dispersive delay line, such as a ring loop section. The ring loop sections complement in gain the gain drop off characteristic of the helix at the edges of the frequency band at which the gain from the helix circuit tapers off. In this the overall gain of the traveling wave tube is enhanced and extended over a frequency spectrum greater than that which is available solely with the helix construction.
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