Understanding Crystal Radio
In the early 1900s, Jagadish Bose and other experimenters found that metallic minerals could detect radio signals. This signaled the birth of radio, at least for scientists at that time. Crystal sets were the first type of radio to be developed.
In 1920, the first commercial radio broadcast was issued by the newly licensed station KDKA in Pittsburgh, PA, which broadcasted the results of the Harding-Cox presidential election.
Interest in radio quickly exploded and newspapers and magazines began carrying simple schematics that people could use to build crystal sets from household materials.
Most of these designs involved wrapping wire around a cardboard paste tube or more commonly, around an empty Quaker oats container.
In the 1940s, crystal sets enjoyed another surge in popularity in the Allied trenches and foxholes of WWII. Commercial radio sets with powerful oscillators and superheterodyne receivers were strictly banned from the front as they are easily detected and would have given away Allied troop positions to the enemy.
As crystal sets do not have oscillators, they are undetectable, and enterprising GIs figured out how to build crystal sets from salvaged wire wrapped around toilet paper tubes, rusty razor blades and pencil lead.
Touching the pencil lead to rust spots on the razor blade creates a point contact diode, and the resulting radio signals can be detected by the human ear through high impedance earphones.
The most commonly used mineral in the early crystal sets was galena. Metallic minerals have now been replaced by diodes in crystal sets and I used a germanium diode for the crystal set that I built.
Diodes allow current to flow in one direction only and I chose germanium over the more common silicon diodes because they are able to rectify a very small AC signal voltage.
This is important as crystal sets do not use any outside power source. They rely instead on receiving power from radio waves that are detected via a long antenna.
When crystal sets were first developed, they were used to transmit CW (Morse code signals in early telegraphy) but as experimentation in radio grew, crystal sets began to include voice transmissions.
Due to the lack of an outside power source, crystal sets are incapable of powering speakers and as such are best used with high impedance or crystal earphones (which are much more sensitive than standard earphones) that rely on the ability of the human ear to detect sound at as little as 10-16 W/cm2.
Crystal sets naturally receive radio signals on the AM band but can be modified to receive FM and even shortwave signals. Sets can generally only receive AM signals from a maximum distance of about 25 miles away due to their very low power.
Crystal sets are now mostly relegated to the realm of lost projects, although hobbyists still exist and even hold yearly competitions in crystal set construction.
There has been some recent research by cell phone companies into using crystal technologies to harvest stray radio signals as a means to power cell phone batteries. Prototypes are able to harvest about 5-10 milliwatts.
If you are interested in buying a kit to build your own crystal radio, or learning how to build a set yourself, read on!
Constructing a Crystal Radio
There are basically only four parts to a crystal set. There’s an antenna, which receives radio waves and converts them to electrical current, a tuning coil with a natural resonant frequency that allows signals at this frequency to pass through while rejecting all others, a diode that modulates the audio signal from the radio frequency carrier wave, and an earphone.
The diode works by rectifying the AC radio wave to a pulsed direct current that can then be picked up by the earphone.
As a kid, I built both crystal sets and pinhole cameras with empty Quaker oats containers but as of today (2011), the thick-walled pasteboard containers of my youth have given way to the thinnest, flimsiest cardboard that would not be likely to withstand such use. For this project I instead chose a more durable base for my crystal set; a clean, empty hydrogen peroxide bottle.
For this project I was unfortunately unable to find a crystal earphone locally and so I am using the handset from an old telephone set as an ear piece. This handset is far less sensitive than a high-impedance earphone but more sensitive than a set of standard earphones.
1. Gather the following materials: a geranium diode (1N34A), magnet wire or hookup wire (RS 278-1217, 18 ga, 60 feet was used for this project), a clean, dry, sturdy plastic bottle, high impedance earphones (a telephone handset was used in this project), and about 50-100 feet of stranded wire for an antenna (22 gauge was used for this project).
2. Punch two holes on the side of the bottle near the top about ½” apart. Punch two holes toward the bottom of the same side about ½” apart.
3. Thread wire through holes at top, securing tightly. Pull out about 8″ as a lead. Wrap the wire around the bottle tightly and securely until you reach the bottom. Secure the wire to the bottom through the holes.
4. Remove insulation from the top and bottom ends of the wire. If using hook-up wire just strip the insulation off. If using enamel wire you’ll need either paint thinner or sandpaper to remove the insulation.
5. Attach the diode to the open end of the bottom wire with either solder or an alligator clip.
6. If using a telephone handset, cut the end off. If the wires are color-coded use the black and yellow wires. If the wires are all one color, strip back the insulation (carefully as these wires are very fragile) and trying two at a time to the top and bottom of a battery lead, listen in the handset until you hear a click. The two wires that cause the click are the two wires you’ll need to use.
7. Attach one of the handset wires to the free end of the diode. Due to fragility, it is best to solder this connection.
8. Attach the other handset wire to the open end of the wire at top of the bottle.
9. Attach an alligator clip to one end of the wire you’re using as an antenna. Clip the other end to the induction coil on the bottle. It’s easiest to make taps along the side of the coil for this purpose, stripping the insulation away before attaching.
10. Attach an alligator lead to the wire at the top of the bottle. This is the ground wire so attach to a cold water pipe or a grounded wire.
Already a ham? Find me at KB2VSL.
