Crystal Tests for Radio

copyright © 2006 by Larry J Solomon

Crystal radio detectors have been in use since the early twentieth century, but not a lot has been written about their characteristics and function. I will not be concerned here with manufactured crystals, but rather with crystals that are found naturally in rocks. Not all "crystal detectors" are crystals -- some are minerals that do not technically qualify as crystals. This distinction, however, is of no consequence here. All these meterials will hereafter be called crystals.

Many naturally formed rocks can be used for radio applications. Since these include noncrystalline minerals, I will informally group them all together here as minerals, rather than as just crystals. In radio they function as rectifiers, or more specifically diode rectifiers. Most of these nowadays are semiconductors. This article will not cover the physics of semiconductors, diodes, or rectifiers. The purpose here is to offer my own experimental observations on the sensitivity of various crystals when used as radio detectors.

In case the reader is unaware, probing the surface of a crystal with a "cat whisker", has no effect on the frequency of a received signal. Such a probe only effects the intensity, or loudness, of a signal. So, probing various "hot spots" on a crystal has no effect on tuning various stations. The effect on signal intensity may be referred to as the crystal's sensitivity. Some minerals have better sensitivity than others, and that is all I am concerned with here. So the subject of this study is to compare the sensitivities of various crystals as I have observed them. The results are limited only to the crystals that I have tested.

One general observation is that semiconductors and metallic crystals (crystals that have some metallic content) are the best detectors. Non-metallic crystals make poor detectors, i.e., crystals such as quartz and calcite.

quartz (Herkimer "diamond")

Herkimer "diamond" (quartz)

Similar materials, such as most gemstones, are poor detectors. Additionally, opaque crystals seem to be make better radio detectors than do transparent or translucent crystals. The best natural detectors seem to be totally opaque, such as galena and pyrite, and have a metallic luster. Pure metallic elements, such as iron and gold, also make poor detectors, but when these are pocked with impurities, they can become good detectors -- such as in rusted razor blades.

Another important observation is that crystals of the same mineral do not necessarily mean that they will be equally good detectors. I have two different specimens of molybdenite. One makes and excellent detector, while the other seems not to function at all. I have observed that when this is the case, the specimen with greater metallic luster is generally the better detector. In fact, the quality of radio-signal detection seems to have as much or more to do with metalic luster than with the specific mineral content.

Heaviness or density also seems to be important for detectors. The denser minerals generally make better detectors. So, crystals that contain iron (such as pyrite and hematite), lead (such as galena), and molybdenum (such as molybdenite), make the best detectors. These crystals are generally compounds as well. Pure copper or silver do not make a good detectors. The semiconductor elements, such as germanium and silicon, used so much in electronics, are "doped" with impurities.

Although I have tested many types of crystals, the following table only contains the crystals that I've found best for radio reception.

Crystal	Chemical composition	Crystal form
galena	PbS	cubic, octahedrons
pyrite	FeS2	cubic, octahedrons
molybdenite	MoS2	radiated flakes
bornite	Cu5FeS4	massive, usually not crystalized
chalcopyrite	CuFeS2	spenoidal
zincite	ZnO	pyramidal
sphalerite	ZnS	cubic
hematite	Fe2O3	tabular rhombohedrons, scalenohedrons
pure silicon	Si	does not occur in nature

Pyrite sphere in Precision Detector stand.

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