Microwave Electron Tubes – Technologies and gridded tubes

While our theoretical knowledge of radio tubes owes a great deal to the research carried out between 1940 and 1970, our ability to design ever more efficient components has increased tenfold thanks to the mastery of powerful and sophisticated simulation tools. Until the 1970s, we had to rely on analytical expressions limited to linear regimes. It was difficult to optimize a tube near the saturation point. Things changed in the 1980s, and fantastic progress was made: a 40-fold increase in the power delivered by tubes operating above 30 GHz; a three-fold increase in the bandwidth of counter-measure tubes (now exceeding 3 octaves); a two-fold increase in electrical efficiency (with mass production of tubes with electrical efficiencies of over 70% in narrow-band and 50% in very wide-band); and reliability improvements by a factor of between 10 and 100; as a result, space amplifiers have MTBFs in excess of 5 million hours and a lifetime of almost 20 years in continuous operation!

This article gives an overview of the basic technologies involved in any electron tube: cathode operation and lifetime, electron optics design, vacuum and high voltage... Secondly, grid tubes (triodes, tetrodes and diacrodes), then IOTs (Inductive Output Tubes) are introduced: their operating principle is presented, and their performance is described. These devices are characterized by the use of a gate to modulate the beam at the signal frequency, as in a field-effect transistor.

The [E1621] article is devoted to high-power microwave tubes (klystrons, cross-field tubes (including magnetrons) and gyrotrons), and the [E1622] article to traveling-wave tubes.

At the end of the article, readers will find a glossary of terms and a list of important acronyms, as well as a table of symbols used.

The author would like to express his gratitude to the technical experts at Thales, in particular Alain Durand, Christian Robert, Michel Grezaud and Philippe Thouvenin.