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The low-noise traveling-wave amplifier is a selfcontained microwave amplifier, consisting of a traveling-wave tube, coupler assembly, focusing structure, and power supply. The principal types are permanent-magnet-focused, single-reversal magnet, and periodic permanent magnet, with respective weights of approximately 15, 10, and 5 pounds. Noise figures ranging from 3 to 10 dB are typical, with values as low as 1 dB reported. Saturation power levels vary from less than 1 mW to 50 mW or higher, and a noise figure as low as 3.5 dB in a 10-watt tube was recently reported. Gain is typically 25 dB minimum over a full octave, and can be as high as 50 dB. Gain tracking to within 1 dB is feasible, as is phase tracking to within 5 degrees. A graphical method is presented, enabling prediction of odd-order intermodulation product levels resulting from balanced 2-tone input. Curves are given to enable rapid computation of dynamic range, which is typically between 75 and 90 dB. Double-octave operation has been demonstrated, with a noise figure in the 6-to-7-dB range. Nearly all types are designed to withstand MIL-E-5400 environmental specifications. Life tests have resulted in continuous operation for more than 20 000 hours with essentially no degradation in noise figure. Future developments will result in reduced noise figure, higher output power and dynamic range, multi-octave bandwidths, and still further reductions in size and weight.