MAKING A ROUGH SCIENCE SHORT WAVE CRYSTAL SET RADIO

THE ROUGH SCIENCE RADIO
The shortwave bands provide entertainment and news for millions (perhaps billions) of people around the world. One BBC estimate I heard claimed that 100 million short wave radios sets are in use around the world! Outlined below are plans to make a shortwave crystal set radio that (with a good antenna and earth connection) is capable of receiving stations from around the world.

Most Crystal Set radios are made to tune the medium wave frequencies because the signals are strong (giveing good volume in headphones etc.) but also because the parts are relatively easy to make. For details of such a crystal set see the web site address at the end of this section. The radio we are going to descibe here deals with a short wave crystal set that was built on location in Capraia for the BBC TV series Rough Science (used to pick up a time signal).

The radio is a little triky to make and the results are further complicated because shortwave signals are prone to fading and also great changes in signal strength between night and day. One therefore needs a lot of patience to get it to work. It does work however - I promise !!

The radio will tune from about 4-8 MHz over the shortwave. These frequencies really come alive at night and it is possible to hear several foreign shortwave stations from around the world on this simple radio.

How a radio works
The simplest radio is a crystal set which actually makes use of the radio waves electrical energy to power it and so does not need a battery. It only works well if we have a very large antenna and if the radio signals are strong. A crystal set radio is composed of five basic parts; an antenna (and earth), coil, tuning capacitor, detector and an earpiece or headphone:

Antenna
Any piece of wire or metal acts as an antenna. As the radio waves pass through the wire they create tiny voltages in it. In general the longer the wire the larger the voltage. This voltage is actually a mixture of voltages from all the radio waves that are passing through the wire, perhaps from a hundred different stations! The antenna wire is then connected to a coil and a device called a tuning capacitor.

Note: Ideally the radio should also be connected to the Earth (as short a wire as possible) so that a voltage difference can be generated to run the set. However if the signals are strong the set will still work without an earth connection but with reduced volume. If no Earth is possible then a second wire 'antenna' can be used to improve reception.

Coil and Capacitor
The coil and capacitor form a sort of filter that can select which of the radio wave voltages from the antenna should be allowed to pass. As the name suggests the coil is made up of many turns of wire. The capacitor is basically two metal plates separated by a small gap. The capacitor is designed so that it can be varied. This means we can vary the filtering and in doing so actually tune the radio. The small signal that makes it through the coil and capacitor then goes through to the detector.

Detector and earpiece
The detector is an electrical device that separates the music, or programme, from the radio signal (see below). This tiny voltage then goes to the earpiece. The earpiece is a device that converts electricity into sound and so finally the tiny signal, originally picked-up from the radio waves, comes out as sound!

More on the crystal detector
The detector is used to strip-off the program from the radio signal, we call this process demodulation. In the castaway radio we used a small crystal of galena as a detector. Galena is a beautiful naturally occurring mineral composed of lead and sulphur having a metallic lustre. The radio frequencies are too high for the earpiece or for the human ear. The crystal has the property of only letting the modulation (the music for example) of the radio signal pass efficiently and so the programme can be heard. One of the contacts onto the crystal needs to be made using a fine sharp wire. This has to be moved around on the surface of the galena to get the best signal and can only be done through trial and error and a lot of patience. Over the last hundred years a number of detector techniques have been discovered but the crystal remains the most famous. This is why this simple radio is called a crystal set.

The Castaway Radio
Our attempt at making a radio using bits and pieces that were given to us, or just happened to be lying around was successful but it took hours and hours to get it working. Dont give up.

The castaway radio was built on a flat base of wood about 30 x 30 cm. Please refer to the diagram shown above. Attached to this was a coil of about 80-100 turns of wire wound round, and along the full length of a 30 cm or so length of broom pole (2-3cm diameter). The two wires from the coil wire were then taken to two connections A and E on the diagram. The capacitor was constructed from a flat piece of metal and half a saucepan (!). The flat piece formed the bottom of the capacitor and was a half circle, just a little bigger than the base of the saucepan. About 2-3cm was cut-out from the centre. This was fixed to the wooden base using screws at the corners of the plate. A piece of plastic bag was then fixed over the plate and nailed down to stop it slipping around.
The top of the capacitor was made out of half a saucepan, the handle of which formed a convenient tuning knob. A hole was drilled in the bottom centre of the pan and a bolt used to fix it above the insulated (plastic covered) metal plate. This was tightened up so that the gap between the pan and base was as small as possible but not too tight that it could not move (a washer was used to gave the correct spacing and a compression spring really helps to keep a good tension). The bolt fitted within the central hole on the bottom plate and did not touch the bottom plate. The pan could then be moved in an arc using the handle, and the overlap between the bottom metal plate and the top of the pan could be adjusted - this constituted our tuning mechanism! A flexible wire was then connected from point E to the pan and also a wire from point A to any of the screws on the bottom plate.

Note: it is important that the movable part of the pan is electrically connected to the E (Earth) connection. Otherwise when you bring your hand close to the pan it will tend to add capacitance to the device and de-tune the radio. This makes fine tuning really very difficult. Earthing the pan means that the set will behave better.

A device to hold and make electrical connection to the Galena crystal was made next. This consisted of; a small home made spring, a bolt and a set of washers and nuts. The diagram shows the basic set-up. The Galena was griped by the washers and positioned so that a face of the crystal was exposed. A fixed coiled piece of springy wire was then used to make a connection with the face of the crystal. The earpiece was then wired-in to the crystal holder and to point E on the circuit.

We used about 50-100m of wire for the antenna, raised as high as we could and insulated from the supports using plastic tubing. A good earth connection to point E greatly improves the crystals sets response. This can consist of a short wire attached to the set and connected to a metal pipe driven into the ground. There was virtually no soil (not wet anyway) on the Island and so we found our earth didn't make any difference. We discovered however that a second antenna wire connected to where the 'earth' should have gone improved things a lot. This was very similar in length to the main antenna but spread out well away from it in a different direction.

Note: adding a second antenna, or earth to the radio will de-tune the circuit which means that you will need to re-tune the radio to pick up the same stations.

How to set-up and use the radio
After all the components had been wired-up we re-checked that all the connections were as clean as possible. Next we made a connection onto the crystal and listened in the earpiece. A click should be heard every time the antenna wire is connected or disconnected. When or if nothing was heard we tried changing the contact position on the crystal. We had to try many times to get a reliable signal. Next, while slowly adjust the tuning, we listened to see if we could pick-out any weak stations on the dial. If nothing was heard we tried using half the number of turns on the coil. We didn't need to remove the turns; we just removed one of the end connections and connect up again to the centre of the coil instead. We then tried a quarter of the coil turns if that didn't work. It was a bit hit and miss because of the great variations in our home made components and the de-tuning effects of the antennas.
The capacitor was a tricky thing to try and make. The gap between the plates needs to be very small but they should not touch. Also the plate and pan need to be large to get a descent capacitance but if they are too large there will be little variability and so it won't tune well! Short-wave signals tend to fade in and out, sometimes so quickly that it sounds as if something had disconnected from the radio set! We found that if left for a while the signal often returned. Any sounds (even a birdcall) nearby will make listening to the tiny signal in the earpiece very difficult. You need to listen to the radio in complete silence. Also the reception varies throughout the day.

Recommended reading
Simple Short Wave Receiver Construction, R. A. Penfold, B. Babani Press, 1994, ISBN 0 85934 220 4 a guide to making simple radios that work. Includes nice explanations of how the radios work and also an introduction to the radio waves and their propagation.

A Guide to Broadcasting Stations, 20Ed., Philip Darrington, Newnes 1989, ISBN 0 434 90309 4 Details of probably all the radio stations transmitting worldwide.

Web-sites
For a detailed description of making a crystal set including calculations on why the various bits are as they are:
http://www.integritynet.com.au/~purdic/crystal_set.htm target=new>http://149.96.1.33/crystal.html

Believe it or not there is a crystal set society you can join!:
http://www.midnightscience.com/

For information about the short waves and amateur radio:
http://www.rsgb.org/

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