When tubes beat crystals: early radio detectors

Abstract

Although crystals were superior, tubes won out-until the solid-state revolution reversed tradition with a different kind of 'crystal' detector communication were avidly seeking new ways of detecting weak radio signals, two major contenders appeared: the crystal detec tor and the thermionic diode detector. The two devices were associated inseparably with two disparate personalities: the obscure H.H.C. Dunwoody, who espoused the crystal detector, and the famous J.A. Fleming, who promulgated the vacuumtube detector.Fleming's device eventually triumphed. But it is now known that Dunwoody's carborundum crystal detector was a superior device, on both practical and theoretical grounds. Later, of course, the solid-state revolution ensured that virtually all radio detectors would be "crystal" detectors, albeit a different kind of crystal. The Dunwoody and Fleming devices both detected by rectifying and converting a modulated carrier wave into a direct baseband signal. But the carborundum detector had a far steeper slope of output current versus input voltage, and it operated at a lower temperature. Both characteristics combined to make the carborundum detector inherently more efficient.The ascendancy of the vacuum-tube detector seems to have been a result of the introduction of the vacuum triode and regeneration, which made it possible to compensate for ineffi cient detection by amplification and positive feedback. Most of the new crystal devices that appeared in the early 1900sDesmond P.C. Thackeray University of Surrey wmrf* mort*» r\r\t r\f ^orKrtfUM^nm Κι»*· *\P r»o*i»«-oll«» •~"*~Z~.TT : .7~.~ minerals. They, as well as the vacuum-tube detectors, made use of signal rectification, itself a novel idea at the time. And by vir tue of this, the "sensibility"-or efficiency of detection of small signals-was potentially improved. Naturally, with primitive technology the potential was not always realized; and it is noteworthy that Guglielmo Marconi's early magnetic detector survived this erratic competition for more than a decade. Early comparisons of performance are not dependable because mea surement conditions were not always understood or controlled. There is no way of knowing now whether the detectors were com pared under optimum conditions, or indeed were performing as well as they were able to. This is particularly true in the case of many crystal detectors, where the variable properties of natural minerals and the delicate so-called cat-whisker contacts made reliable, repeatable measurements impossible.

Radio-Frequency Rectifier for Electromagnetic Energy Harvesting: Development Path and Future Outlook

“…The great advantage of the carborundum material was its density. It was so hard that the cat whisker could be firmly pressed against it, thus resulting in a very repeatable point contact [67]. This detector almost did not necessitate a new arrangement of the point contact, and could be packaged [Fig.…”

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