Constructing a Simple Radio
The radio was invented in 1906 by Guglielmo Marconi a 21 year old Italian inventor. In this laboratory exercise we will build a very simple radio. The objective is to become familiar with the basic science and technology involved in radio broadcasting. Our radio is very similar to the first radios developed by inventors like Marconi. The radio only picks up one station, and the sound coming out is very faint, but the radio has the interesting feature that it does not require any batteries.
 
 

Step 1: Winding the Coil
For this step you will need a cardboard tube, a tack, tape, a ruler, and a spool of the red-colored wire.

  1. First we prepare the tube. Draw a line along the length of the tube. Using the tack punch a hole in the tube about 1.2 cm (1/2 inch) from the end. Punch another hole along the line at a distance of 6.7 cm (2 + 5/8 inches) from the first hole. The 6.7 cm distance is important.
  2. Obtain a spool of the red-colored wire. Find the end of the wire. Put the wire through one of the holes on the outside of the tube and pull it out from the inside. Leave 4 inches. of wire sticking out past the end of the tube.
Read steps 4 - 7 before proceeding.
  1. Begin wrapping the wire around the tube. Wrap the coils tightly and be sure that the wires are side-by-side and not overlapping. After the first 4 or 5 turns put a piece of tape over the wires. This will help to hold the wires in place as you wrap the rest of the coil. The tape will not interfere with the function of the coil.
  2. Now the rest of the coil is wound. Keep the coil wire from twisting or kinking. Winding the coil is the step that takes the most time in making the radio. It must be done carefully or your radio will not work.
  3. The wire must be wound so that it is only one snug layer. The wires should touch, but they should never cross or overlap on another. As you wind after each 0.5 cm (1/4 inch) or approximately 15 turns, tape the wire with a piece of tape. This will keep the coil from unraveling if you accidentally let go.
  4. Continue winding until you reach the other hole in the tube and the coil is 6.7 cm (2 + 5/8 inches) long. Tape the wire in place. Clip off the wire from the spool leaving 10 cm (about 4 inches). Push the leftover wire through the hole in the tube.
  5. There is one more step we need to do before the coil is done; that is prepare the ends of the wire. The wire is red because it is coated with a varnish-like material that ensures that the electric current in the wire flows along the wire (the way we want it to go) and not between the wires. This coating is called the insulation. Later we will want to make electric current flow between the coil and other parts of the radio. Electrical current only flows when contact is made between conductors. We must therefore remove some of the red varnish insulation from the ends of the wire. Take a piece of sandpaper and remove the red varnish from the last 2.5 cm (1 inch) on each end of the wire. Do this very gently so the wire does not break. Make sure that the insulation is removed from all around the wire. When the insulation is removed you can see the copper color of the wire itself.
  6. When the insulation has been removed, you can put the coil aside.

 

Step 2: The Earphone
Locate the earphone. Stretch out the wire. Notice the earphone wire is actually two wires that are twisted together. At the end, unwind the two wires so that each wire end is approximately 5 cm (2 inches) long.
 
 

Step 3: The Diode
Locate the diode. The diode is either a small blue cylinder with a black band or a small clear glass cylinder with a black or green band. Make a small hook at the end of each diode wire by bending it around the pencil. Wrap about 1 cm (about ½ inch) from the point of the pencil. If you are having difficulty you can also try making the hook using the pliers. See the drawing below:





Step 4: Preparing the Antenna and Ground Wires
We will need wires to be used as an antenna and to connect the radio to the ground.

  1. The antenna wire. Obtain 2 m (6 feet) of wire with the blue insulation. Use the wire strippers to remove the insulation from each end of the wire (2.5 cm, or about 1 inch).
  2. The ground wire. Obtain about 1 m (3 feet) of wire with the green insulation. Use the wire strippers to remove the insulation from each end of the wire (2.5 cm, or about 1 inch).
Note that the color of the wire does not matter to the functioning of the radio. We choose blue to remind us that the antenna wire points to the sky, and green for the ground wire to symbolize the ground. The wires could be any color as long as they are connected properly.
 
 

Step 5: Preparing the Wooden Base
In this step we will drill holes in the base and install the screws that will hold the wires.

  1. Locate a wooden base. Sand lightly to remove any rough edges. Use the template to mark the location of the three holes that will be drilled into the base. Mark the holes: #1, #2, and #3 as indicated on the template. Drill the holes. You must wear safety glasses when using the drill. The holes can be drilled all of the way through the wood..
  2. Next the screws which will hold the wires in place are installed. Locate the three screws. Put two washers on each screw. Install the screws. Leave the screws about 0.5 cm (about ¼ inch) from the wood. In other words, do not put the screws all of the way in yet.
  3. Put the wood on the floor. Obtain a nail. The nail is used to secure the fragile earphone wire. Pound a nail between the two screws that are furthest apart. Be sure that the wood is on the floor when pounding in the nail. Please do not pound on the table, as this makes a lot of noise. When pounding in the nail try not to go all the way through the wood. If the nail goes all the way through pound it back to the point at which it is flush with the wood surface.
Now the radio can be assembled!
 
 
 

Step 6: Securing the Coil to the Base
Use two thumbtacks to attach the coil to the wooden base in approximately the same position as the one in the model. Put the tacks in near the end of the tube. Be careful not to stick the tacks through the coils of wire! The exact position of the coil on the base is not critical but make sure you can still get at the screws with the screwdriver. If you are having trouble getting the tacks all of the way in, try using the pin punch tool.
 
 
 

Step 7: Connecting One End of the Coil with the Antenna and the Diode
We will connect three wires together at the screw marked #1 (left most screw facing the radio). The wires we will connect are: one end of the coil, the antenna, and one side of the diode. Bend a small loop in the very end of the coil wire and the antenna wire. Then locate the side of the diode opposite the stripe. The side of the diode without the stripe must face screw #1 or the radio will not work. Loosen the screw, loop the wires around the screw and tighten it down. It is a little tricky to get all of the wires to stay in place while you are tightening down. It may help to have another person hold the wires steady. It may also help to loop one of the wires between the washers. Once you tighten down, make sure the wires are securely in place. Tug on each wire gently to see that it does not fall off.
 
 

Step 8: Connect the Diode with the Earphone

  1. Tie the earphone wire around the nail. Leave enough wire to connect to the screws. This secures the earphone wire in place and keeps it from being pulled off accidentally later on when you are using your radio.
  2. At the middle screw: Screw #2 connect the wire from the side of the diode with the stripe and one of the wires from the earphone.

 

Step 9: Connect the Earphone with the Other End of the Coil and the Ground Wire
At screw #3 (the right-most screw) we will connect three wires: the remaining earphone wire, the end of the coil and the green-colored wire. As is the case for the three wires at screw #1, getting all of the wires around the screw at the same time can be a little tricky. Make sure the wires are secure so they do not fall off later when you are listening to your radio.

The radio is assembled. Now it is ready to be tested.
 
 
 

Step 10: Listening to Your Radio
A crystal radio is not very powerful. To make matters worse, concrete buildings tend to block out radio signals so you will have to go outside to test your radio. The radio may work inside near a window. For now take your radio outside to test. Attach the green ground wire to some unpainted metal like a fence, a railing, or a bike rack. Stretch out the antenna. Listen with the earphone.
 

Step 11: Fixing Your Radio
Your radio may break after you take it home. In this section of the laboratory you will demonstrate you ability to find a problem with the radio and repair it.

  1. Bring your working radio to the instructor. A "problem" will be introduced into your radio which will render it inoperative.
  2. Fix the problem with your radio and bring it to the instructor for verification of proper operation. You may need to refer to the drawing of the radio at the beginning of this laboratory or a sample radio.

 


Guidelines for Testing Your Radio





Never connect any part of your radio to a powerline or any sort of electrical outlet, or lamp socket.

Operating the radio in different places

  • Take your radio home and continue testing.
  • Take the radio to a cold water faucet. Hold the bare end of the antenna wire in your hand and touch the ground to the faucet. (To make it easier, hold the wire in place with a rubber band or tape.) Hear anything?
  • Take the radio to a telephone. Wrap a antenna wire several times around the telephone cord that comes from the wall to the telephone. Hold the bare end of the ground wire in your hand. Put the earphone in your ear and listen for a station


The Antenna

  • A crystal radio works well only if you have a good antenna (aerial) to receive the radio wave. The farther you are from the radio station, the better the antenna must be. We have tried a cold water faucet and have used the telephone line as part of the antenna. Now try a door, a wall, a waterpipe, a metal window frame, a tree, or a car antenna. Some of these will work. Others will not.
  • Do not give up! You may have to try different things to get your radio to work well. Sometimes changing the position of the antenna helps to improve reception.


The Ground Wire

  • The ground wire is important to radio operation. Cold water pipes and radiator pipes tend to be good places to attach the ground wire. Your body can also sometimes serve a ground.
  • If you are far from a radio station, it may be necessary to make the antenna longer. If your radio does not work from your home, see the instructor.


Fixing your radio

As you use your radio it may break. This is not the end of the world. The most likely thing that will happen is a wire will break off or become disconnected. To fix these types of problems look at the diagram of your radio (see lab report section) and determine where the wire should go, then reattach the wire. You will need a screwdriver to do this. Sometimes the earphone wire will break in such a way that the inner copper wire is no longer exposed. If this happens you will have to carefully cut and peal back some of the plastic covering of the wire to expose the inner copper conductor, then reattach the wire.

Useful link for other designs of crystal radios: Bizzare Stuff You Can Make in Your Kitchen: Crystal Radio

© 2001 John J. Krupczak, Jr.
All rights reserved. Reproduction in whole or in part in any form or medium without express written permission of the author is prohibited.