SYNCOM’S REMARKABLE TRAVELING WAVE TUBE—Dr. Boris T. Subbotin

There is little doubt that several key available technologies in the 1958-9 post Sputnik era made possible the design and feasibility of the innovative and revolutionary Syncom synchronous communications satellite. One was the traveling wave tube, TWT (or TWTA). It is the device that provides sufficient radio frequency power in the satellite to be radiated by an antenna and be received on the ground. It consumes the majority of the DC power provided by a satellite and its batteries. The evolution of the space TWT has been of major importance to all space programs since the pioneering Syncom and Telstar in the early 1960’s.

There were, of course, several other technologies at that time that were essential to the Syncom concept. Included are solar cells, transistor technology, nickel cadmium batteries and the creative design breakthrough of a spinning body with axial and radial pulse jet control (the Don Williams patent). However, it was fortuitous that Hughes was also doing research in microwave power devices because of its core business in airborne and surface radars and missiles. The Research Laboratories (HRL) located in Culver City at the time, under Dr Andrei V Haeff, was the primary Hughes center for TWT developments.

The evolution and design of the TWT had taken place in many steps and places over the previous quarter century. The US centers were primarily RCA, Bell Labs, Stanford University, and HRL with several other lesser players also spanning and post WWII. The major contributors were Haeff, Nils Lindenblad, Rudolph Kompfner, John Pierce, and Lester Field. John Mendel of HRL (my classmate at Stanford) was responsible for the Syncom tube development. Some consider the TWT as the purest realization of the microwave generation principle in electron tubes. Further, it still employs the most challenging technology amongst microwave tubes** necessitating both Swiss watch precision and launch vehicle ruggedness.

If a narrow focused electron beam is sent at a speed slightly faster than a signal through a long coiled wire (delay line) in an evacuated tube, electron beam energy is transferred to the helix wire and signal amplification will occur. Electron beam design, generation, focusing, containment and collection, signal coupling in and out, suppressing unwanted reflections and oscillations, power handling and cooling, efficiency, maintaining vacuum, and numerous other design, materials, and lifetime issues are deeply involved in long life TWT design. For Syncom, Mendel developed a metal tube envelope and used a series of small annular magnets for beam focusing resulting in a S-band tube of 2 watts output.

TWTs have benefits that allowed them to still be the predominant space RF power output devices of today. Their DC to RF efficiencies have climbed from a nominal 10-15% to 65-75%, the signal amplifications from 30db to 50db ranges, and RF power outputs from 2 watts to several hundred watts. Parallel operation of TWTs for doubling output power was first used by Hughes on Intelsat II and is currently offered by suppliers. Since TWTs are not inherently coupled to resonant circuits, tubes may cover octave bandwidths. Today useful space frequency bands of operation encompass L, S, C, Ku, K, and Ka bands, from 1 GHz to over 40 GHz. Another beneficial characteristic is that they can be made low noise devices and used for receiving signals. Even just now, DARPA has issued an industry invitation to develop the next generation of TWTs.

The total number of satellite TWTs in orbit today approaches 25,000. In orbit tube failures are still improving and have been minimal, estimated at 2%, but may be governed by power supply failures rather than the TWT itself.

Hughes former Electron Dynamics Division in Torrance, CA, now L3 Communications Electron Technologies, together with the French-German Thales are the dominant suppliers of today. Not much has been published by the Russian and other suppliers.

The TWT is a remarkable device, selected with great insight, and continues to be an important part of our space heritage.

**Vacuum Electronics: Components and Devices  Edited by Joseph A. Eichmeier, Manfred Thumm

 

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