Crystal / Cat's Whisker Radio Detector

- details of the cat's whisker radio detector or crystal detector - how this detector worked, the different types and the history of its development.

The cat's whisker detector is one of the most iconic components used in early radio sets. Also called the crystal detector it was an iconic component used in many early radio equipments.

Today, many cat's whisker or crystal detectors sets can be seen on the antique and collectors market.

In their day, the radios using the cats' whisker detector or the crystal detector performed well and for many years they were the main type of detector used in sets used for the reception of broadcast transmissions.

The development of the crystal detector or cat's whisker itself resulted from the observations made by people researching into radio technology, although they had little understanding of the principles because they are a form of semiconductor diode, and the basic theory behind this technology was yet to be discovered.

Cat's whisker crystal detector

The crystal detector, or as it later became known, the cat's whisker detector provided a much superior form of detection and allowed a direct audible indication of the incoming signal rather than coherers that cohered to give an indication that could then be fed to headphones.

However one of the first reported uses of a crystal detector was by an Indian professor of Physics at Presidency College in Calcutta named Jagadis Chandra Bose. He demonstrated the use of a diode using galena (lead sulphide) crystals and a metal point contact. He filed a U.S patent for a point-contact semiconductor rectifier for detecting radio signals in 1901.

Then in 1906 a number of other patents were filed. One of the first fpr the year was that from Ferdinand Braun in Germany who patented his detector on 18 February 1906. This crystal detector was based on psilomelan - a hydrated oxide of manganese.

Typical crystal detector / cat's whisker detector
Typical crystal detector / cat's whisker detector

Then on 21 February 1906, L W Austen filed a patent for a tellurium-silicon detector. Then a month later in March 1906, General H H C Dunwoody in the USA patented a carborundum detector. This was followed by Greenleaf W Pickard filing a patent in August of that year for a silicon metal rectifier. This was the culmination of around four years work for Pickard who said he had been working on crystal detectors since 1902.

Operation of crystal or cat's whisker detector

There were various formats for crystal detectors. However they relied upon the fact that a PN junction as made, and this reacted as a diode rectifier. It had been noticed that current would only flow one way through the diode, and this gave it is detecting action.

AM detection using a crystal detector / cat's whisker detector
AM detection using a crystal detector / cat's whisker detector

There were a number of methods that were used to create crystal detectors. The most common was to use a crystal mounted in a crystal holder. A thin copper wire known as a cat's whisker (hence the name of the overall detector) was then connected to a holder that could be moved to allow the wire to be placed on a suitable position on the crystal.

At the point where the wire contacted the crystal a point contact diode was formed. Although it would not conduct high currents, it was ideal for receiving radio signals.

An alternative form of detector called a "Perikon" detector used two crystals in contact with each other.

Using a cat's whisker detector

In general the cat's whisker crystal detectors were reasonably reliable by the standards of the day. Those used in the early 1900s proved less reliable than the later ones. This was mainly as a result of the way they were used. They were used alongside powerful spark transmitters. It was found that the transmitter caused high voltages and current levels to be generated in the diode detector and it required repositioning once the spark transmitter had ceased operating and the receiver was required.

This effect was found to be less pronounced with carborundum detectors that used a steel spring with a higher tension on it that exerted a greater force on the crystal.

In normal domestic use, the cat's whisker crystal detector operated reliably and the wire only needed repositioning relatively infrequently.

Usage of the cat's whisker detector

The cat's whisker detector came into its own with the rise of broadcasting. The detector was relatively cheap and provided a reliable means of detecting signals. In addition to this it required not batteries like the thermionic valves (vacuum tubes). In addition it offered much better tonal quality than the valves of the early 1920s which were also very expensive.

By Ian Poole

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