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For broad-band operation, the cathode circuit need not be a single quarter wavelength. This is because of the high value of input capacitances usually encountered. Thus, input Q is very low even with multiple quarter-wave cathode circuits. In narrow-band circuits, where extra quarter wavelengths are sometimes desirable for frequency stability, they are employed only where size and weight permit. If the device is to be used as an oscillator, a feedback circuit is needed. This can be a capacitive, inductive or a combination probe between the cathode and plate cavities.

Above about 2.5 Gc, amplifiers become more difficult to build. Gains are lower because of transit-time loading, and noise figures are higher. Quarter-wave circuits are hard to build, particularly where they must be tuned; and there is little room for the proper output probe. Multi-quarter-wavelength circuits can be used but are generally undesirable because of their complexity. They are difficult to support mechanically, and special care is needed to prevent moding at lower frequencies. The gain-bandwidth product also decreases. For all these reasons, the practical upper design limit for gridded-tube amplifiers appears to be around 5 Gc.

Oscillator considerations

The planar-gridded tube has found most application in oscillators. Above about 2.5 Gc, the reentrant coaxial-cavity configuration (Fig. 4) is commonest. This is essentially a grounded cathode with a half-wave grid-anode circuit coupled to, and inside, the anode-cathode circuit. The anode-cathode circuit is usually full wave. It is often difficult physically to provide proper feedback relations with a simple half-wave and full-wave combination. Without the proper phase and amplitude relationships between the two circuits, oscillation may insufficient or may not occur at all. At higher microwave frequencies, best performance usually can be obtained by special means as follows:

The cathode can be operated off ground by mounting it at the end of a short low-impedance line at the cathode end of the tube-cavity combination. The line length is adjusted initially for best oscillator performance and appears uncritical from tube to tube. Also, over a 10-15 percent tuning range, variable adjustment of the grid-anode circuit is not needed. The tube-test cavity in Fig. 4 provides the means for tuning the cathode. The cathode assembly can be moved so that there will be a variable length of cathode line between the finger-stock short circuit and the triode’s cathode connection.

Another way to improve performance is to optimize the characteristic impedance of the grid-anode cavity. This can be done as shown in Fig. 4, where an “anode extension adjustment” has been added to the tuning plunger to move a larger center conductor in and out of the grid cylinder. The resulting change in cavity impedance changes the phasal relationship of the RF signal. The anode extension adjustment can be set for best feedback and oscillator efficiency. The curves in Fig. 3 were obtained with all variables optimized.

Good results have been obtained with these tube-cavity combinations using hybrid coaxial waveguide cavity circuits. A quarter- or half-wave grid-anode coaxial circuit is attached to the tube, after which it is inserted into a section of waveguide. The frequency is determined by the attached grid-anode circuit. Feedback is adjusted by tuning the ends of the waveguide section with tuning stubs or susceptance probes inserted into the waveguide walls. Output power is taken from the waveguide. Although its tuning range is quite limited, this circuit appears to be the most efficient and was the one employed to obtain power at 20 Gc.

A family of tuning curves used to obtain part of the Fig. 3 results are shown in Fig. 7. These bell-shaped curves are plots of power output versus frequency for grid cylinders of various lengths Lg. With each cylinder in place, the frequency was changed by moving the anode tuning plunger and anode extension assembly. The length of the cathode line required no additional adjustment for frequencies within the grid cylinder range. From these curves it is seen that the typical half-power frequency range for each grid cylinder is about 5 percent of center frequency. The tuning range also depends upon the operating level and is less at higher outputs. If the cathode line, output probe, grid cylinder, and anode assembly are all adjustable, much wider tuning ranges are possible.

Size of cavity

Cavity size depends on grid-anode tube capacity and is chosen as follows: At higher frequencies, (4-5 Gc approx.) the grid-cylinder ID and the anode line ID should be chosen such that their difference will be equal to or less than the length of the grid cylinder. This is desirable for suppression of unwanted modes. At lower frequencies (under about 1 Gc) it is common to choose the smallest ID of the cavity to minimize size and weight. The minimum cavity size is usually determined by the size of the tube. Thus, it is desirable to use the smallest practicable tube envelope.