  The traveling-wave tube is a critical subsystem for satellite data
transmission. Its role in the history of wireless communications and in the
space conquest is significant, but largely ignored, even though the device
remains widely used nowadays. This paper present, albeit non-exhaustively,
circumstances and contexts that led to its invention, and its part in the
worldwide (in particular in Europe) expansion of TV broadcasting via microwave
radio-relays and satellites. We also discuss its actual contribution to space
applications and its conception. The originality of this paper comes from the
wide period covered (from first slow-wave structures in 1889 to present space
projects) and from connection points made between this device and commercial
exploitations. The appendix deals with an intuitive pedagogical description of
the wave-particle interaction.
