A Brief Description of Crystal Radio Operation
A Crystal Radio is a very simple AM band Receiver popular in the early days of radio. The Receiver gets all of its power from radio waves picked up by a long wire Antenna and does not need batteries or household current. The simplicity of the Crystal Radio design made it an ideal vehicle for the explosion of worldwide wireless communication in the early 20th Century.
People in isolated locations and without access to any power source, could scrounge the parts from local sources and assemble a rudimentary Receiver capable of capturing radio signals from miles away. To rural area farmers and ranchers, the homemade Crystal Radio became a prized source of information about markets, weather and news. In urban areas, for the first time in history, large groups of people were instantly aware of events taking place in their home town and around the world. As late as World War II, soldiers in the field were using their razor blades to construct what became known as the ‘Foxhole Crystal Radio’ to listen to radio broadcasts for information and entertainment.
Amplitude Modulation (AM) is a method of impressing voice or music onto an Alternating Current (AC) carrier waveform. Radio stations operating in the AM Band transmit an AC radio signal which is picked up by the Crystal Radio’s long wire Antenna. The AC signal passes from the long wire Antenna to the Crystal Radio’s Coil. By adjusting the Coil length through the use of Taps connected to a Rotary Switch, or Slider, the Radio can be tuned (resonate) to the desired frequency of a radio station.
The Crystal Radio gets its name from the Crystal Detector. The Crystal Detector is a ‘Cat’s Whisker’ wire that is dragged and pressed against a Galena Crystal until a radio station or static is found. The Galena Crystal extracts the audio signal from the AC carrier wave by only allowing the current to pass through in one direction. This rectifies the AC radio wave into direct current (DC). The DC current is then converted into sound by the Earphone. Finding and maintaining the desired signal for any length of time was difficult and the ‘Cat’s Whisker’ Diode was eventually replaced by the more modern Germanium Diode.
The Antenna is very important to a Crystal Radio’s performance since all of the Receiver’s power comes from this element. An outdoor long wire Antenna of 50 to 100 feet should provide good reception, however, an Antenna wire of 10 to 20 feet, or a simple indoor AM Loop Antenna, will pick up strong local stations. Do not leave a long wire Antenna deployed when not listening to your Crystal Radio and do not listen to a Crystal Radio if there is even a hint of a thunderstorm.
A good Ground is also a very important element of a crystal radio’s performance. The Ground provides a return circuit from the Antenna, through the radio, and into the ground. Because Crystal Radio’s operate off of the very low power output from the Antenna, a low resistance Ground will minimize the dissipation of the radio signal as it passes through the Crystal Radio to the ground. The Ground wire is attached to a metal stake driven into the ground.
The GCR A-Runner and J-Walker radio Receiver’s employ a technique for selectivity called Loose Coupling.
The Loose Coupler Crystal Radio is an interesting radio design and an excellent educational tool. Acquiring a radio station’s signal involves the tuning of the primary circuit’s Coil Taps, tuning of the secondary circuit’s Coil Taps, and the coupling arrangement between both the Primary and Secondary circuit Coils. The quality of the Galena Crystal and Cat’s Whisker Diode combination – or choice of Modern Diode, time of day, weather conditions, Antenna length, and Grounding arrangement will also impact reception. This interaction with nature and all of the radio’s moving parts allows the operator to ‘see’ how their radio Receiver works.