The gridded tube has undergone an evolution. New mechanical shapes and dimensions and different operating levels have extended its useful upper frequency limit to about 10 Gc. 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). 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/cm2).
These improvements have resulted in tube-cavity combinations of performance equal to that of reflex klystrons but at less cost. The latest gridded tubes have fast warmup (about 5 s) and require only simple power supplies.
For 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. Minimum size is also important in order to obtain low absolute values of tube inter-electrode capacitances. Generally these capacitances determine the maximum useful frequency, particularly when the tubes are used with quarter-wave resonant circuits. Minimum inductance is also important, which dictates that as much of the active RF circuit be outside the vacuum enclosure as possible.
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Planar design helps
Planar design, shown in Fig. 1, has improved the performance of gridded microwave triodes. With planar design, more uniform RF potential across the cathode is possible. This permits higher current densities and greater efficiencies. The center of the planar cathode will be less effective, however, if the grid wires have appreciable inductance or if the coating is lossy. 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. Planar design lends itself to very close element-to-element spacings.
Limits are approaching
Minimization of size and reduction of spacing seem to be nearing their practical limits. Closer spacing between grid and cathode offers no significant improvement in transmit-time loading, but it does limit the tube’s power input. 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. More efficient anodes are under development as well as more arc-resistant cathodes. Work is also being done on long-life cathodes with cw current densities up to 1.0 A/cm2—about a two-fold increase over present long-life designs.
From tests at lower frequencies, predictable oscillation requires an electron transit time of less than 5/6 cycle. Between 9 and 10 Gc, current densities of some 1.0 A/cm2 are necessary for about 1.0 percent plate efficiency. Recent work at 16 Gc confirms the predicted need for 2 to 3 A/cm2 for 1.0 percent efficiency at this frequency, evident from the curves in Fig. 2.
Tube-performance curves for a type 7910 planar triode are shown in Fig. 3. The data were obtained with the tube installed in a re-entrant coaxial-cavity oscillator, Fig. 4a, which permits adjustment of all important cavity variables. A simplified drawing of the tube-cavity combination is shown in Fig. 4b.
The developmental Y-1266 triode shown in Fig. 5 alongside its test cavity for 2.3 Gc, is one of the first designed for cw cathode-current densities above 0.2 A/cm2. Small-signal power gains of 20 dB or more were measured. The required heater power density decreases with increased cathode area; thus, tubes larger than the Y-1266 should yield higher over-all efficiencies. Present research goals at S-band are 15 to 20 W power at 25 percent over-all efficiency or better.
Measured performance of an experimental X-band tube-cavity combination is shown in Fig. 6. The triode used has a cathode area of about 0.025 cm2. This unit was designed for use as a local oscillator and weighs less than 2 oz including an integral X-band waveguide flange.
Tube-cavity mating—key performance
Optimum tube performance requires careful mating of the tube and its cavity. Up to about 2.5 Gc, the grid-separation grounded-grid amplifier (or oscillator) circuit is common. This is the condition where the grid physically and electrically separates the input and output circuits or cavities. A quarter-wave plate cavity is usually employed. The grid is grounded for RF, and there is a quarter-, half- or three-quarter-wave cathode circuit. The quarter-wave plate circuit is essential for broad-band amplifiers as this gives the maximum L-C ratio that yields minimum effective Q.