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Crystal Detectors to Point Contact and Layered Signal Diodes in Electronics

OddMix.com - Technical Note - Crystal Radio - TN080417 - Karl Nagy
Picture 1. Ernest Roger crystal detector from 1920 [8 KB]
Picture 1. "Ernest Roger" Crystal detector from 1920

Many of the younger generation never had the pleasure to have, or to even see a real beautiful crystal detector as shown on Picture 1. A detector like that was the product of the early 1920's, and it was an expensive, quality, well made product. They were usually hands assembled with machine made components.

This particular "Ernest Roger" detector device is built on an insulated base, either wood or most likely of Bakelite, the most widely used and only available plastic material at the times.

Many of the younger generation never had the pleasure to have, or to even see a real beautiful crystal detector as shown on Picture 1. The new generations are much too involved with technology and playing on their computers or going to an online University. This excellent detector holder used copper, brass and bronze generously for the crystal holder and for all of the electrodes.

The most often used crystal in this and similar detectors - then and now - was pyrite or galena as shown in the 1925 listing in a table in the "American Mineralogist" publication, that has a listing of thirty three minerals catalogued with rectifying detector properties.
Picture 2. Crystal detector crystals [4 KB]
Picture 2. Early point contact diode with adjustable srew

Among the very first commercially produced diodes is the 1N21B shown on Picture 2. It is more than curious, that this "diode" is enclosed in a case on which there is a slotted screw-head is just visible on the wide side (lower right). That screw is connected to a fine "cats whisker" steel wire, a few turns of a spring-like device, terminating in a point that is in touch of the germanium semiconductor material connected to the top left diode terminal.

The arrangement is much smaller, and more diode like, then the detector on Picture 1., but it made in a very similar arrangement, which has became known as the point contact diode. From these and similar germanium diodes, evolutionary progress leads us to current PN junction Silicon, Gallium Arsenide or other more exotic semiconductor materials which are the achievement of the latest scientific age and many years of steady experimentation, research and development.
Picture 3. Point contact diode [2 KB]
Picture 3. Point contact diode

Picture 3 shows the cross section of a hermetically glass enclosed, point contact diode. If the left side, point contact terminal would be attached to a screw and a short spring, and the glass envelope would have a threaded metal part in it, the Picture 2 and Picture 3 devices would be nearly identical.

These point contact "cat's whisker" devices, are still made occasionally because of their very small capacitance. As it was discovered early on, they are highly useful in high frequency electronics. All the way up to microwave frequencies they useable. The simplest radar detectors usually made with a simple horn antenna, a diode similar to Picture 2 and Picture 3 and a transistor amplifier. If the designer keeps it simple, and don't use a local oscillator, such radar detectors are not even detectable by the police.


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