Radio sets using all sorts of detectors have been around since the dawn of "wireless" just over a century ago. Very early in the 1900s, one particular detector gained popularity due to its simplicity and relatively high output. This was the "cats whisker" galena crystal detector – hence the name "crystal set".

Basically, this detector consisted of a galena (lead sulphide) crystal held in a metal cup which formed one end. At the other end was a piece of high-tensile wire wound into a short coil and attached to a positioning lever.

The positioning lever was manipulated so that the "pointy" end of the wire – known as the "cats whisker" – made contact with the galena crystal. As a result, it had one annoying deficiency when compared to other detectors – you had to probe around the galena crystal with the "cats whisker" until a sensitive point on the crystal was found. This was fine until something or someone dislodged the "cats whisker" from its sensitive spot, which meant that the procedure had to be repeated.

This was a nuisance which wasn’t overcome successfully until detectors like the OA47, OA79, OA91, GEX66 and 1N34A fixed point contact germanium diodes became available. These devices eliminated the "fun" of trying to find the sensitive spot on the galena crystal, as it had all been done by the manufacturer. If the set didn’t work, it was usual to look elsewhere for the fault, since these new detectors were very reliable.

However, I remember reading in "Radio and Hobbies" many years ago – in the "Serviceman Who Tells" – about a crystal set that was brought in because it had ceased to work. There isn’t much that can to go wrong with a crystal set and is usual to expect the detector diode to be OK. However, in this particular case, the diode had failed, having been destroyed by a strong signal from an amateur radio transmitter next door. Of course, modern devices are much more rugged than those early types.

Designing a high-performance crystal set isn’t quite as easy as it seems at first glance. A number of points need to be taken into consideration for a design to be successful.

The first two essential items are a good, high, long antenna and a good earth. I wrote about antennas and earthing in the March 2003 issue and readers should refer to this to achieve good results.

Unfortunately, the antenna/earth sys-
tem I’d used for several years was inadequate for crystal set operation. The antenna was only about 6m high at the highest point and about 20 metres long. Its replacement has a maximum height of 9m and is around 27m long. It is also generally higher for most of its length than the previous antenna.

Ideally, the antenna should be up to 15m high and around 30m long but achieving this on a suburban block isn’t always easy. However, in my case, the modest improvements in height and length noticeably improved the strength of the received signals.

As an amateur radio operator, I have always been well aware that the antenna in use needs to be tuned to the operating frequency. This is particularly important when the antenna is much shorter than a tuned length, which 99.9% of broadcast band receiving antennas are.

Fig.1: the circuit for a basic crystal set. Coil L1 can be air-cored (see text for specifcations) or can be wound on a 100 x 20 x 5mm flat ferrite rod using 70 turns of 22 B & S enamelled wire tapped at 10, 20, 30 & 40 turns.