![]() ![]() ![]() Most microwave gridded-tube development is now concentrated on improving the effectiveness of cooling and mechanical construction to increase reliability and to allow higher operating voltages and currents. Closer spacing between grid and cathode offers no significant improvement in transmit-time loading, but it does limit the tube’s power input. Minimization of size and reduction of spacing seem to be nearing their practical limits. Planar design lends itself to very close element-to-element spacings. By using small-diameter, annular cathodes, low-loss coatings and low-inductance grid structures, milliwatts of cw oscillator power can be obtained to 20 Gc. The center of the planar cathode will be less effective, however, if the grid wires have appreciable inductance or if the coating is lossy. This permits higher current densities and greater efficiencies. With planar design, more uniform RF potential across the cathode is possible. 1, has improved the performance of gridded microwave triodes. This file type includes high resolution graphics and schematics when applicapable. Minimum inductance is also important, which dictates that as much of the active RF circuit be outside the vacuum enclosure as possible. Generally these capacitances determine the maximum useful frequency, particularly when the tubes are used with quarter-wave resonant circuits. Minimum size is also important in order to obtain low absolute values of tube inter-electrode capacitances. The latest gridded tubes have fast warmup (about 5 s) and require only simple power supplies.įor maximum usefulness at microwave frequencies, a triode must have minimum element-to-element spacings, the greatest possible cathode-current density and the highest possible anode voltage. These improvements have resulted in tube-cavity combinations of performance equal to that of reflex klystrons but at less cost. In the past two years, the life expectancy at 9.6 Gc has gone from 100-200 hours to 1000 hours at the higher cathode current densities (0.8 A/cm 2). At about 5 Gc, a tenfold increase in output power (to about 2 W) has been achieved over the last decade together with a fivefold improvement in efficiency (to about 15 percent). New mechanical shapes and dimensions and different operating levels have extended its useful upper frequency limit to about 10 Gc. The gridded tube has undergone an evolution. ![]()
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