Radar System Engineering

SEC.10.11] MISCELLANEOUS COMPONENTS 389
practice many annoying points must be considered, including temperature
shift, change in back-bombardment heating of the magnetron
cathode, change in pulser starting delay, and change in rectifier voltage.
The usual practice is to change the pulse lengths inversely with the pulse
rate, thus keeping average power constant. But, because of the complications
inherent in changing pulse length by switching pulse networks,
the pulser designer is always inclined to stick to one pulse width. If
pulse rate must be changed without a compensating change in pulse
width, it is usually necessary to control the rectifier output by switching
pgwer transformer taps or by varying a series primary impedance.
The present design of high-voltage, low-average-current rectifying
diodes is quite satisfactory. There are now available diodes of reasonable
dimensions, high inverse rating, and good life, covering most practically
useful ratings. Thoriated tungsten cathodes are universally employed;
these have satisfactory mechanical strength and are economical of
filament power. Efficiency could be further increased by the development
of a filamentless rectifier. This would permit operation without
filament transformers which, in some circuits, must be insulated for high
voltage. Perhaps a cold cathode discharge tube, or a barrier-layer
rectifier, will one day replace the filamentary rectifier.
Line-type pulsers for radar applications have been built and operated
successfully to cover a range of pulse power output of 1 kw to 20 Mw.
The following photographs show typical pulsers designed for airborne
service.
Figure 10.47 shows a hydrogen-thyratron pulser designed to supply