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If You've Never Built a Radio Receiver, Then a Crystal Set Is a Great Place To

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If You've Never Built a Radio Receiver, Then a Crystal Set Is a Great Place To By RODNEY CHAMPNESS, VK3UG This photo clearly shows the layout of the super crystal set. It's built on a baseboard measuring 280 x 240mm. If you've never built a radio receiver, then a — made contact with the galena crys- tal. As a result, it had one annoying crystal set is a great place to start. Here's a deficiency when compared to other design that's very easy to build and get going. detectors — you had to probe around the galena crystal with the "cats whisker" until a sensitive point on the ADIO SETS using all sorts of de- Basically, this detector consisted of crystal was found. This was fine until Rtectors have been around since the a galena (lead sulphide) crystal held in something or someone dislodged the dawn of "wireless" just over a century a metal cup which formed one end. At "cats whisker" from its sensitive spot, ago. Very early in the 1900s, one par- the other end was a piece of high-ten- which meant that the procedure had ticular detector gained popularity due sile wire wound into a short coil and to be repeated. to its simplicity and relatively high attached to a positioning lever. This was a nuisance which wasn't output. This was the "cats whisker" The positioning lever was manipu- overcome successfully until detectors galena crystal detector — hence the lated so that the "pointy" end of the like the 0A47, 0A79, 0A91, GEX66 name "crystal set". wire — known as the "cats whisker" and 1N34A fixed point contact germa- 90 SILICON CHIP siliconchip.com.au nium 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, HIGH it was usual to look elsewhere for the ANTENNA IMPEDANCE HEADPHONES fault, since these new detectors were very reliable. EARTH However, I remember reading in "Radio and Hobbies" many years Fig.1: the circuit for a basic crystal set. Coil Ll can be air-cored (see text ago — in the "Serviceman Who Tells" for specifcations).or can be wound on a 100 x 20 x 5mm flat ferrite rod — about a crystal set that was brought using 70 turns of 22 B & S enamelled wire tapped at 10, 20, 30 & 40 turns. 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 Cl GERMANIUM OR SILICON Ll 3x450pF (MAX) (SEE TEXT) detector diode to be OK. However, in C3: 1pf this particular case, the diode had failed, having been destroyed by a strong signal from an amateur radio T1 transmitter next door. Of course, mod- (71L0:3.50) ern devices are much more rugged than LOW those early types. S IMPEDANCE HEADPHONES 0 High-performance sets ANTENNA Designing a high-performance crys- tal set isn't quite as easy as it seems at C5 first glance. A number of points need VR1 47pF *12-POSMON 5k to be taken into consideration for a BATTERY ROTARY SWITCH R3 design to be successful. USED TO SELECT S2 4700 The first two essential items are a ANY OF 11 TAPS good, high, long antenna and a good ON LI, OR NONE earth. I wrote about antennas and Fig.2: the super crystal set circuit uses L1 & Cl to tune the antenna circuit, earthing in the March 2003 issue and while L2 & C2 tune the received frequency. Transformer T1 is included for readers should refer to this to achieve driving low-impedance headphones. Also included is an optional detector good results. bias circuit consisting of VR1, switch S2, the 47µF capacitor, the 4700 Unfortunately, the antenna/earth sys- resistor & the battery — see text. tem I'd used for several years was inadequate for crystal set operation. The antenna was only about 6m high at with the coil or a ferrite rod inserted diodes is to achieve the best results. the highest point and about 20 metres into the coil. The coil may be tapped Note however, that silicon diodes usu- long. Its replacement has a maximum to suit as well (more of this later). In ally require a biasing voltage to operate height of 9m and is around 27m long. practice, the addition of an antenna efficiently as detectors. It is also generally higher for most of its tuning mechanism is extremely effec- length than the previous antenna. tive when it comes to increasing the Headphones Ideally, the antenna should be up signal level into the set. Good quality headphones with good to 15m high and around 30m long but When it comes to making coils, sensitivity are also needed to get the achieving this on a suburban block both the coils and coil formers need best performance from a crystal set. I isn't always easy. However, in my case, to be low loss. I've found that 60mm- have.a pair of 4k0Kriegsmarine head- the modest improvements in height diameter white PVC tubing (available phones which work well but are quite and length noticeably improved the from plumbing and hardware stores) uncomfortable to wear. I also have a strength of the received signals. is quite satisfactory for the job. By pair of Browns "F" type headphones As an amateur radio operator, I contrast, cardboard coil formers can but these have a relatively low im- have always been well aware that the attract moisture which increases coil pedance of about 15052. And I have a antenna in use needs to be tuned to losses. couple of other headphones of rather the operating frequency. This is par- It is important that both the detector mediocre quality and a pair of 8-ohm ticularly important when the antenna and antenna are matched to the tuned padded stereo headphones. is much shorter than a tuned length, circuit(s) for best performance. This is In the end, I wired up the phones which 99.9% of broadcast band receiv- achieved by simply connecting the de- socket on the crystal set so I could ing antennas are. tector and the antenna to the coil taps plug my stereo headphones into the An antenna can be tuned by having a which provide optimum matching. set with the ear pieces in series. I (loading) coil in series with the anten- Detector efficiencies can vary con- also used a speaker transformer (as na wire where it connects to the crystal siderably and you can experiment with used in valve radios) to transform set, with either a tuning gang in series various germanium and silicon signal the high-impedance output from the siliconchip.com.au APRIL 2007 91 ferrite-rod coil crystal set, the coil This photo shows the layout for the information is as follows: 70 turns of basic crystal set depicted in Fig.1. 22 B & S enamelled wire tapped at 10, 20, 30 and 40 turns on a 100 x 20 x 5mm flat ferrite core. The experimen- tal layout can be seen in one of the photographs. Super crystal set I've always wanted to design and build a "super crystal" radio receiver, so I did some experiments back in 2002 with methods of tuning the antenna. My early experimental antenna tuning system was described in the March 2003 issue of SILICON CHIP. Using crystal set to a low impedance output radio is the requirement for both cir- this device, I could detect four radio for the headphones. This combination cuits to tune to the same frequency no stations instead of the single station I proved to be as sensitive as using the matter where the tuning control is set could normally receive on my "stand- other phones on their own and is much on the broadcast band. Anyone who ard" crystal set. easier on the ears. has built crystal sets will be aware By this time, I had a reasonable idea that the station locations on the tun- of what might work well but without Crystal set experiments ing dial alter if either the antenna or being too complex. However, I didn't Over the years, I have built a variety detector connections on the coil(s) want to make a set which used exotic of crystal sets ranging a little matchbox are changed. parts, or parts that were hard to make, monstrosity (mine was anyway) to As a result of my early experiments, or one that was so complex that a rather complex twin-tuned coil varie- I fell back on the old faithful single- university degree was necessary to ties. And what did I learn from all of tuned circuit — see Fig.1. It isn't the "drive" it. this? I found the match box set a very most sensitive or selective crystal set In particular, a twin-tuned circuit poor performer, as was the twin-tuned in the world but it works and is easy would not be suitable, as getting the coil unit. to get going. two tuned circuits to "track" across the The latter unit used a 2-gang tuning In my case, I built one with a nor- broadcast band using a single control capacitor and was a failure because, at mal air-cored coil and another using would be impractical. By contrast, it that time, I didn't understand that the a ferrite rod as the former for the coil would work if I used two independ- two (identical) tuned circuits needed winding. They both worked but would ently tuned circuits but that would add to "track" each other.
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