Other Radar Tubes
Many radar tubes are listed in the sections related to magnetrons, klystrons and other microwave tubes.
Here are some additional types, as the tubes used in modulators or in duplexers.
Modulator circuits had to generate sharp pulses, about 1 microsecond wide, to drive the RF transmitter;
peak power had to be very high, with voltages in the order of tens of kilovolts and currents of tens of
amperes. In the early days, the only electron tube that could handle such a power was the mercury
thyratron. Unfortunately its turn-off time was quite long, not compatible with the pulse repetition rates
usually required. The use of mercury thyratrons was also impractical, influenced by their mounting
position and by the external temperature.
Spark gaps were commonly used to generate high-power pulses. Unfortunately they asked for periodic
adjustments due to the corrosion of surfaces and were unreliable at all in airborne applications, at high
altitude and low pressure. Sealed spark gap tubes were introduced and were used in early airborne
radars. Soon later, some suitable hard tubes were introduced. Usually they were derived from
transmitting tubes, increasing the peak current emission of the cathode: tubes with two or four oxide-
coated cathodes were built.
The last step was the introduction of stable hydrogen thyratrons, characterized by fast turn-off time.
The hydrogen filling pressure was restored through the life by a titanium-hydride reservoir. Voltages in
the order of 30KV and peak currents in the order of 1000A were now handled.
The other section concerns the tubes used in the duplexer. The duplexer is the device that switches the
antenna from the transmitter to the receiver. Here special tubes were developed to replace early diode
switches. Usually TR switches are gas filled spark gaps, fired by the RF pulse of the transmitter.
Sometimes a keep-alive electrode is used, to have a faster ionization.
Radar Modulator Tubes
In the early '940s British and US radar sets quickly evolved due to the introduction of the magnetron: S and X band radars
were compact enough to be widely used in aircraft. The spark gap triggers, used in ground and shipboard transmitters were
unreliable at high altitudes. Electronic switches were readily available, often redesigning ordinary power tubes.
Left: 705A, high voltage rectifier, used to charge energy storage elements. Center: 715A, a power tetrode capable of
handling 10A peak at about 15KV. Right: 5C22 hydrogen thyratron, 325A peak current at 16KV. Hydrogen thyratrons were
much faster than mercury ones and could be used in any position. The thyratron in the picture was produced by ELSI,
Elettronica Sicula, to Raytheon specs. REL21, below, was a spark gap in use shortly before hard tubes.
High Power Hydrogen Thyratron
Here two of the high power hydrogen thyratron used in early warning radar modulators. Top row,
5948A. 25KV at 1000A pulsed, 1A average. In the middle the titanium hydride reservoir cylinder.
Bottom, one of the British equivalent, CX1140 or CV8563.
TR Switch Overview