High gain traveling-wave amplifiers

Modern techniques make it possible to build traveling-wave amplifiers with stable gains as high as 60 or 70 db. Such tubes can have relatively flat frequency response, and high gain over frequency ranges well in excess of two to one. In design it is necessary to pay strict attention to adequate isolation of the input and output so that spurious feedback paths do not cause oscillation or excessive variation of gain with frequency. In many practical applications, such tubes are required to operate with source and load impedances which produce large reflections of energy. Special design techniques result in tubes which are stable under such conditions. Another feature which can be achieved in modern tubes is a high ratio of backward loss to forward gain, so that the amplifier can very effectively isolate an oscillator. The recent design trend is toward higher currents and lower voltages, which is of direct benefit to the equipment designer, since it results in shorter tubes as well as simpler power supply designs, With the use of highly convergent electron beams, cathode current density can be kept quite low, and ion bombardment effects minimized, so that excellent life is achieved. Traveling-wave amplifiers may be readily phase modulated since small variations in the beam voltage yield large phase excursions. This characteristic is used for high level mixing. By proper gun design, it is possible to obtain good mixer action with modulation frequencies as high as a few hundred megacycles. The problems involved in obtaining the above characteristics and typical experimental results are illustrated by data on the STS-69, a half-watt twt for the 2 to 4 kmc frequency range.