by Cor van Rij
Welcome to a new type of Crystal set that needs no antenna or earth! The most simple to build type of all radios isprobably the 'crystal set' a combination of a tuned LC combination, a diode and a pear opf headphones. You cannot expect CD-quality audio or room filling music bit it is fun to build and a good start in teh wondefull world of the radio. Mpst crystal sets requires a long antenna and good decent ground connection. For people living in an appartment with limieted space like me this is not a feasabile option. Therefor this crystal set was developped. As you can see in the schematic there is no antenna or erath connection. Actually the coil is the antenna. In order to pick up broadcasts loud enough this needs to be a large coil.
Take a box or crate of at least 40cm x 40cm (remember larger is better) and wind some 10+ turns on it and connect it to the capacitor. Take it into a quit room or ask your girl-friend to be quit (politely if you want to enjoy this hobby for a long time with support from her) and put the headphones on. Use some high impedance head-phones or a crystal phone. If you don't have a pair of high impedance headphones use a transformer. Alternatively you can hook up an audio ampliefier to the output.
In an urban environment you should be able to hear at least a couple of stations now!
- Use any you want to wind the coil. It is not critical as long as it is insulated. Usually your local DIY shop will sell reels of bell-wire for a reasonable price. You also can use wire from an old discarded mains transformer. Usually the wire is made from metal but will look like a bit like copper wire for it is covered with a glimming yellow/brownisch copper like coating that works as insulator. You first need to scrap of the coating with a sharp knife before connecting it to the capacitor.
- The diode for detection can be a variety of types: OA91, OA85 or OA82. Preferably a germanium type for it has low voltage drop. Usually these types are easy to recognize for they have a clear glass envelope and one or two red bands at one end.
- The value of the capacitor is not critical. The higher the value, the larger the tuning range you can cover. You can use a varaible capacitor from an old AM Broad Cast Band (BCB) radio or whatever you have at hand.
- You can shift the tuning range by adjusting the number of turns on the coil. If you increase the number of turns the inductance will increase and the frequency will become lower. Decreasing the number of turns has, of course, teh reverse effect.
- Use any type of cardboard box, plastic crate or whatsoever possible. The size of the box is not critical, remember larger is better. I started to build this project on Friday night after the shops were closed. So I laid a char with it's back on the table and used it's four legs to as a support for the coil. It looks silly but it works!
If you want to build a more permanent setup you can build a simpel wooden frame. Make a cross of two long poles. Fix the two arms together with a bolt in the center. On this cross you can then wind the insulated wire, much the same way as with the box.
I build my frame with two 60cm long poles. This give a size of efcetively 42cm square. Before mounting the poles together I drilled a 1cm hole through all the poles at 1.5cm from the end. After mounting the two poles together I used this holes to guide the wire through. For my experiments I used ordinary bell-wire.
Another nice feature of this type of antennas is that they are directional eq they have the possibility to null out unwanted stations. If your listening to a far away station and your local dj is disturbing the reception with his chit chat you can all but eliminate him (vocally that is). You can do this by pointing the front (or back-side) of the atenna to him. If you want more information on this types of antennas there is tons of information available on the web. From very easy applications to the most esoteric designs.
As inspiration to start this fun project the following sources were used
- Novel crystal set requires no antenna, earth This is were the original idea came from
- AM LOOP ANTENNAS
- A magnetic loop antenna for
- Sensitivity of Multi Turn Receiving Loops
- A $10 BROADCAST BAND LOOP ANTENNA