Since the arc took some time to strike and operate in a stable fashion, normal on-off keying could not be used. Instead, a form of frequency shift keying was employed. In this compensation-wave method, the arc operated continuously, and the key altered the frequency of the arc by one to five percent. The signal at the unwanted frequency was called the compensation-wave. In arc transmitters up to 70 kW, the key typically shorted out a few turns in the antenna coil. For larger arcs, the arc output would be transformer coupled to the antenna inductor, and the key would short out a few bottom turns of the grounded secondary. Therefore, the "mark" (key closed) was sent at one frequency, and the "space" (key open) at another frequency. If these frequencies were far enough apart, and the receiving station's receiver had adequate selectivity, the receiving station would hear standard CW when tuned to the "mark" frequency.
The compensation wave method used a lot of spectrum bandwidth. It not only transmitted on the two intended frequencies, but also the harmonics of those frequencies. Arc converters are rich in harmonics. Sometime around 1921, the Preliminary International Communications Conference prohibited the compensation wave method because it caused too much interference.
The needed for the emission of signals at two differing frequencies was eliminated by the development of uniwave methods. In one uniwave method, called the ignition method, keying would start and stop the arc. The arc chamber would have a striker rod that shorted out the two electrodes through a resistor and extinguished the arc. The key would energize an electromagnet that would move the striker and reignite the arc. For this method to work, the arc chamber had to be hot. The method was feasible for arc converters up to about 5 kW.
The second uniwave method is the absorption method, and it involves two tuned circuits and a single-pole, double-throw, make-before-break key. When the key is down, the arc is connected to the tuned antenna coil and antenna. When the key is up, the arc is connected to a tuned dummy antenna called the back shunt. The back shunt was a second tuned circuit consisting of an inductor, a capacitor, and load resistor in series. This second circuit is tuned to roughly the same frequency as the transmitted frequency; it keeps the arc running, and it absorbs the transmitter power. The absorption method is apparently due to W. A. Eaton.
The design of switching circuit for the absorption method is significant. It is switching a high voltage arc, so the switch's contacts must have some form of arc suppression. Eaton had the telegraph key drive electromagnets that operated a relay. That relay used four sets of switch contacts in series for each of the two paths (one to the antenna and one to the back shunt). Each relay contact was bridged by a resistor. Consequently, the switch was never completely open, but there was a lot of attenuation.
Read more about this topic: Arc Converter
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