Finally, the effect on mismatches on the tube must be considered. This is of particular concern in the case of the amplitron because of the lack of attenuation of reverse power. Experience with amplitrons has generally indicated that a voltage standing wave ratio (VSWR) on the output is a practical high limit to maintain stability. This may not be true if the source VSWR is also as high as , i.e. , from the preceding amplitron. The klystron on the other hand presents excellent isolation between the output and input. As such its operation is relatively insensitive to mismatches at either the output or input. Since the input and output of the klystron are tuned circuits, the VSWR of the tube itself may be quite high. Figures 4-38 and 4-39 examine the effects of passive phase shift per tube on the cascaded and parallel feed amplifier configurations, respectively. For both configurations the requirement is that the RF phase at each radiating element be similar. Limits of 2° rms at any element are appropriate for MPTS as discussed in the transmitting antenna section. A tabulation of the behavior for both types of amplifier configurations is given by Figures 4-40 and 4-41 for amplifier failures. Figure 4-40 is a sketch of the probable configuration with cascaded operation and Figure 4-41 is a sketch of probable configuration using the parallel feed configurations. The following points are noted: a. In the cascade arrangement using klystrons, either removal of dc power or opening of a heater can stop a significant number of tubes from providing useful output. b. Because of source voltage variations, to operate either converter in a cascade arrangement requires regulation of the tube in a manner to control the phase shift for source voltage variations. c. In the parallel feed configuration, recognizing that equal changes in source voltages are seen by all converters in the subarray, correction for phase shifts can be made by the existing phase shifter. Sensitivity of amplifiers to voltage changes must be controlled in the design to within a specified tolerance so that all are similar.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==