LearnTronics
History
Early Radio Detectors
The spark-gap receiver gives way to devices that can respond to much weaker wireless signals
Hertz's receiver proved that electromagnetic waves could cross space, but watching for a microscopic spark was not a practical way to receive messages over useful distances.
Wireless telegraphy needed a detector: a component whose electrical condition changed when radio-frequency energy arrived.
The coherer — 1890
In 1890, French physicist Édouard Branly observed that loose metal filings could change resistance when exposed to an electromagnetic disturbance. A small tube containing metal filings and electrodes became the basis of a sensitive detector later called the coherer.
The incoming radio wave did not have to provide all the energy needed to operate the output device. Instead, it changed the detector enough that a local battery circuit could respond.
The detector must be reset
Once the filings had become more conductive, they tended to remain that way. The coherer therefore needed to be mechanically disturbed or tapped so the filings would return to a high-resistance condition.
Oliver Lodge and others improved the coherer and its reset arrangements. In practical wireless sets, a small tapper or trembler could repeatedly restore the detector after each received signal.
- an antenna and tuned or semi-tuned input circuit;
- a coherer detector;
- a battery;
- a relay;
- a Morse sounder or recorder;
- and a mechanical decoherer to reset the filings.
Marconi turns the parts into a communication system
During the 1890s, Guglielmo Marconi developed practical wireless telegraph systems using antennas, spark transmitters, tuned circuits, coherer-type detectors, relays, and telegraph apparatus. In 1901 he demonstrated transatlantic wireless reception.
For LearnTronics, the important point is not the business competition around early radio. It is how several familiar device ideas were combined:
Detection is a form of conversion
The radio-frequency signal changes far too rapidly for a telegraph sounder or human ear to follow directly. A detector converts the presence or modulation of that high-frequency energy into a slower electrical change that another circuit can use.
Measurement trail 20 — sensitivity matters
A receiver does not only need to respond; it needs to respond to a small signal without being triggered constantly by noise and interference.
Ball-park detector idea
If an incoming radio signal changes a detector from very high resistance to much lower resistance, a local battery can provide the current needed to operate a relay.
Other detectors are coming
The coherer was important but crude. Crystal rectifiers, electrolytic detectors, magnetic detectors, and eventually vacuum-tube diodes would provide much more useful detection.
Before we reach the tube diode, one more nineteenth-century discovery must be made clear: what exactly are the cathode rays seen in Crookes tubes?