IEEE Spectrum That’s right—your ultrahigh-definition satellite TV and satellite radio come to you courtesy of vacuum tubes in space. By Carter M. Armstrong
'Of course, there’s a huge difference between Telstar’s 3.5-watt, 4-gigahertz amplifier and one of the dozens of highly efficient microwave amplifiers on, say, the DirecTV-15 satellite, launched earlier this year. The latest generation of traveling-wave tubes can provide up to 180 W at frequencies up to 22 GHz, with efficiencies approaching 70 percent and rated lifetimes exceeding 15 years. Though their basic function is the same—amplifying RF signals—just about everything else has changed: the design, the testing, the materials, and the fabrication.
'That’s my point. In the six decades since vacuum tubes lost out to solid-state devices in computers, receivers, and power supplies, vacuum technology has continued to evolve and branch out into new terrain, sustaining a small but skilled corps of engineers and scientists around the world, as well as a multibillion-dollar industry. That’s because the traveling-wave tube and other vacuum devices continue to serve one purpose extremely well: as powerful sources of microwave, millimeter-wave, and submillimeter-wave radiation. (Vacuum tubes are also used in amplifiers for musical instruments and high-end audio, but the tubes I’m talking about are for generating radio-frequency waves, not audio waves.) What’s more, they do it efficiently and over broad bandwidths, and compactly and reliably, too. And by virtue of their construction and the metals and ceramics from which they’re fashioned, traveling-wave tubes are inherently hardened against radiation (unlike solid-state devices) and fairly impervious to temperature and mechanical extremes. Besides satellite communication, traveling-wave tubes are widely used in radar, electronic warfare, and other military systems.
'And now, ongoing research into a new and potentially revolutionary kind of traveling-wave tube—the ultracompact and ultraefficient cold-cathode TWT—looks poised to deliver the first practical device by the end of this decade. These are exciting times for vacuum tubes.'