The Crystal Radio FAQ

started by Steve Coles

 I) Why bother?
II) Definitions
III) Simplifications & Lies
IV) Troubleshooting
V) How does a Crystal Radio Work? A lesson for beginners...

I) Why bother?

A) Crystal sets help us to understand the technical history of the 20th century.   Three, of many, examples: In very early commercial radio, crystal sets provided inexpensive receivers that allowed a large listening market to develop.  During World War II, improved crystal diodes made British and American radar receivers more sensitive than German models.  Crystal diode research provided materials and techniques used in the invention of the transistor.

B) Crystal sets provide a step-wise introduction to electronics.  Most beginner crystal sets need only an antenna, ground, coil, capacitor, diode or crystal and headphones.  Once these few components have been mastered, the student can advance step-by-step by adding a radio-frequency transformer, a one-transistor amplifier and so on until an understanding of most common electronic components is gained.

C) Many modern electronic systems still contain crystal set-like circuits.  A few examples: The detector section of AM radios, field strength meters (for measuring antenna performance), broadcast station air diodes (allowing a broadcaster to monitor a small sample of the transmitted signal), radar receiver mixers and instrument devices transferring signals generated at one ground reference voltage to processing circuits having a different ground reference.

D) Challenge and reward: A fourth grader can find satisfaction in successfully completing a simple kit while a subtle semiconductor problem can challenge a senior engineer or physicist.

II) Definitions

Each of these definitions will bring up new questions and, as Lawrence says, more interesting questions.  For instance, understanding a capacitor requires understanding conductors, insulators, electric charge, electric fields and how energy is stored in an electric field.  You might need 5 new definitions.  You'll need an electronics dictionary and a general introduction to radio.  Make sure you can understand what you're buying.  I've seen technical dictionaries at level's from fourth grade to postdoctoral.  [Any recommendations here?]  The following provide a minimum starting point for terminology:

AM (amplitude modulation).  In AM radio, music, voice or television signals are impressed on a radio carrier wave by varying the wave's amplitude.  In contrast, FM impresses music, voice or other signals on a radio wave by varying the wave's frequency.

Antenna.  For crystal radios an antenna is an electrical conductor in which a radio wave induces small currents and voltages.  A 15 meter wire suspended 3 to 5 meters above the ground and insulated from other objects usually serves well as a crystal set antenna.  You may know that a magnet moved near a coil induces a current in the coil.  You may also know that a charged rod held near an uncharged rod causes charge to separate in the uncharged rod.  The electromagnetic wave we call a radio wave induces voltages and currents in an antenna in much the same way.

Capacitor.  Two sheets of conductive material separated by an insulator form a capacitor.  Crystal-set builders sometimes make a capacitor by sandwiching a sheet of picture glass between two sheets of craft copper foil.   Commercial capacitors are often named for their insulating material, such as ceramic of mylar.  A capacitor stores energy in its electric field.  A capacitor supplies energy to the headphones during the part of the cycle that the detector diode is switched off.  Resonant circuits also use capacitors.

Cat whisker (AKA, "cat's whisker").  A cat whisker is an (often springy) piece of pointed wire.  It presses against a piece of semiconductor such as galena, iron pyrite, germanium or silicon.  Usually, it provides the anode end of a diode detector.  A variety of conductors have been employed as cat whiskers.  I've used safety pins and partially straightened phosphor-bronze ball-point pen springs.  If you have a glass enclosed germanium diode (available from Radio Shack), you can use a hand lens to see the cat whisker inside.   You may have to scrape some paint off.  You can even smash the glass and experiment directly with the cat whisker and germanium crystal individually.  I've used the crystal from the diode with other pointed wires as cat whiskers.  In principle you could use the cat whisker from the diode with some other semiconductor such as natural galena or fool's gold from a rock-hound shop.  In junction diodes the connecting wires play no role in diode action.  Therefore, junction diodes don't have cat whiskers in the same sense point-contact diodes do.

Coil.  Simply a coil of insulated wire wound around an insulating support (often a mailing tube in crystal sets).  A coil stores energy in its magnetic field.  Combined with capacitors, coils form resonant circuits.   Taps on coils assist in impedance matching.

Condenser.  Condenser is an older word for capacitor.

Conductor.  Anything that electricity easily flows through.  Conductors include wire, capacitor plates, bolts, screws, iron pipes and damp salty ground.  When a conductor fails (perhaps by breaking mechanically or oxidizing), the barrier to electricity is called an open circuit.

Crystal.  A piece of semiconductor material.   Some crystals, such as galena and iron pyrite (fool's gold) occur geologically.   Modern crystals, such germanium, silicon, gallium arsinide and even diamond and some plastics are produced by carefully-controlled laboratory or industrial processes.   For a natural crystal to show diode action, it must be mechanically clamped or set in a small cup of solder.  A wire attaches to the clamp or solder cup.  Then a cat whisker point is moved into contact with a sensitive spot on the crystal.

Detector.  As an AM radio wave is received, the negative-going half cycles cancel the positive-going half cycles.  In AM the detector is a crystal or diode that removes the negative-going (or positive-going) half cycles.

Ground.  1) In crystal set usage an electrical connection to a conductor buried in the ground.  Often a ground is achieved by a copper clamp to an iron cold water pipe where it enters a building from underground.   2) In much of the remainder of electronics, ground means a common conductor to which most voltage measurements are referenced.

Headphones.  The same idea as portable radio headphones with one important difference.  Portable radio headphones are designed to work with the ~16 ohm output impedance of the radio's integrated circuit amplifier.   Crystal set phones must have a characteristic impedance of 600 ohms or more depending on the crystal set.

Inductor.  Inductor is another word for coil.

Insulator.  1) Any material that does not conduct electricity or that conducts electricity very poorly.  2) An object made to prevent electricity from flowing through an unwanted path.  When an insulator fails, the path followed by the electricity is called a short circuit.

Leyden jar.  Early electrophysicists performed capacitor experiments using a type of capacitor called a Leyden jar.  Some general science and physics books discuss the properties of capacitors under the heading of Leyden jar.  A Leyden jar can be assembled from a fruit canning jar and two pieces of aluminum foil.

Resonant circuit.  When a coil and capacitor are electrically connected they can trade energy back and forth between the magnetic field of the coil and the electric field of the capacitor.  The trade takes place via the current flowing between the coil and capacitor.  Physics dictates that for any particular coil and capacitor, there will be a finite time for the energy to transfer from coil to capacitor, back to the coil (with reversed voltage polarity), and finally back to the capacitor with the original polarity.  The number of such cycles in a second is the resonant frequency of the circuit [in units of cycles per second or Hertz (Hz)].   Crystal sets use resonant circuits to send a desired station to the detector while bypassing all other stations to ground.

Searching a crystal.  When using a natural crystal, only small areas of the surface have the correct properties for diode action (rectification).  The cat whisker must be moved around the crystal until a "sensitive" spot is found.

Solenoid.  1) In applied electricity a solenoid is a type of electromagnetically operated actuator and has almost nothing to do with crystal sets.  2) General science and physics books often use "solenoid" to mean a single-layer coil such as we use in the resonant circuit of a crystal set.

Tap.  A tap is an electrical connection made to some point on a coil other than an end.  Taps are used for impedance matching and for changing tuning ranges.

Tuned circuit.  Tuned circuit means the same as resonant circuit.

Variable.  One of a resonant circuit parts of a crystal set--either the coil or capacitor--is made variable to allow different stations to be tuned in.

Wave.  A radio wave consists of a magnetic vibration and an electrical vibration superimposed on each other. If we could watch an AM broadcast wave approach us, the magnetic part would look something like a rope being shaken left to right and the electric part like a rope shaking up and down.

III) Simplifications & Lies

To give a first cut at visualizing these things, I made a couple statements above that ain't quite so.

A) In reality, amplitude modulation consists not in varying the carrier amplitude, but in attaching sidebands (additional frequencies) to the carrier.  Ditto FM.

B) It's often better to think of the field around a wire, rather than the current passing through it, carrying the energy.  The energy is transferred at the rate the field travels, not the much slower electron speed.

IV) Troubleshooting

1. Make sure the enamel insulation on the coil wire is in good shape, except where it must be sand papered off for slider contact.

2. Make sure enough insulation has been removed from the wire ends where they need to make contact with wires from other components.  Sand paper works OK for removing enamel insulation.

3. Make sure the headphones are crystal or high impedance (about 2000 to 4000-ohm) types.  If you must use low-impedance phones (most sold with an 1/8 inch--about 3 mm--stereo phone plug are low impedance), then use a matching transformer.  (Most Radio Shacks carry a transistor output transformer that works).

4. The antenna wire needs to be about 45 feet (15 meters) long if you're not within a few kilometers of strong stations.  Be sure your antenna wire is well insulated from nearby metal objects.  If your antenna passes into the house through anything metallic (aluminum siding, aluminum window frames, etc.), then make sure it's well insulated from the metal objects.  You can pass the wire through small-gauge flexible plastic tubing from model airplane shops.

5. Be sure you have a good cold water pipe or 4-foot rod ground connection.   Remove oxidation from the surface of the pipe or rod with sand paper before making the connection.  Don't just tape the wire on and hope for a connection.  Use a copper ground clamp from a hardware store.  The ground is just as important as the antenna.

6. Be sure the slider wire (ball, strap) presses firmly on the part of the coil you've sanded the insulation off of.

Lots of crystal set configurations work.  Try a variety.

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