How to Build a Foxhole Radio

Jack Gould

How to Build a Foxhole Radio

One of the things that makes television and radio interesting is that you can use right away the information you have learned.

We are going to build a radio that is different from any you have seen. In the radio will be these things:

• An old safety razor blade

• A safety pin

• A broken point from a lead pencil

Radios of this kind were actually used during World War II. American G.I.’s in Italy put several of them together. At night when they sat near the front lines, they listened to phonograph records played on a radio station in Rome. That is why the set you are going to make is called “The Foxhole Radio.” If you live within twenty-five or thirty miles of a station, you can hear a program on your foxhole radio, too.

These are the only tools you need to build the foxhole radio:

• A hammer

• A pair of pliers

• A pocket knife

These are the parts that you will need:

• A board. Any piece of wood will do. Just make sure it is at least eight inches long and six inches wide. It can be bigger if you like.

• A piece of cardboard tubing. This should be two inches in diameter, like the tubes on which ribbon is wrapped in five-and-ten-cent stores. The tube should be six inches long.

• A spool of insulated copper wire. You may have to buy this at a radio store. Ask for No. 28 gauge; this is just the right size. The wire will cost about 75 cents.

• A pair of earphones which you can put over your head like the pilot of a jet airplane. The phones can be bought at a radio store for two or three dollars.

• Three new nails. The nails can be either an inch or two inches long. Anything you have will do.

• Four metal thumbtacks, not plastic tacks A used blade that fits a safety razor. A plain white-looking blade often works better than “blue” blades.

• A big safety pin

• A pencil with a fat lead

Let’s put all the tools and parts on a table like this and see if we have everything before we begin:

You will need to know how the parts are used and what you must do to make the foxhole radio. This is the way the set looks when it is all finished.

First, we start with the coil.

Unwind a long piece of wire from the spool and stretch it out straight across the floor.

Pick up the piece of cardboard tubing, and with one of the nails make two little holes at one end of the tube and two little holes at the other end. We’ll number the holes i, 2, 3 and 4. Now the tubing looks like this:

Push about six inches of the wire through Hole No. 2. Now pull the wire up through Hole No. 1. You can see that the two holes give us a way to hold the wire in place at one end of the tubing.

Hold the tubing in your left hand. With your right pick up the wire and carefully wind it around and around the tube. Do this until you have a total of 120 turns of wire around the tubing. Make sure the turns lie side by side on the tubing, not one on top of the other.

After you have put 120 turns of wire around the tubing, measure off six inches of wire. Then cut the wire with your pliers. Push the end of the wire down through Hole No. 3 and then pull it up through Hole No. 4. Now you have made a nice sturdy coil that will stay in place.

Put the board in front of you on a table, and lay the coil on its side on the back of the board. Take two of the thumbtacks, and fasten the coil to the board. Make sure the thumbtacks do not touch any part of the wire.

Next put the razor blade in place in front of the coil. Be very careful when you handle the blade; it is sharp. Lay the blade flat on the board, and gently fix it in place with two metal thumbtacks. Do not push the tacks all the way into the board; just start them like this:

Next is the safety pin. First, sharpen the pencil so that you have a long piece of lead showing. Break off the lead, and put it next to the point of the safety pin. Cut a piece of wire off the spool and pull off the cotton covering so that the wire is bare. Now wire the piece of lead to the point of the pin.

With your pliers bend the head of the pin back so that it will lie flat on the board.

Now the safety pin looks like this:

Place the pin to the right of the razor blade. Put one of the nails through the head of the pin, and with the hammer drive the nail into the board until it almost touches the pin.

All the parts are in place. Now we hook them together with wires. We do this by using the pocket knife to remove the cotton covering from the ends of the wire so that they are clean and the copper of the wire shines brightly. This is most important because the set will not work without clean, tight connections.

Let’s take the left half of the set first:

Take the wire from the coil and wrap the bare end around Nail A in the corner. This is Connection No. i.

Take another wire and put a bare end underneath the thumbtack that holds the razor blade. Push the tack down as hard as you can so that the bare wire is against the blade. Then take the other end of this wire and wrap it around Nail A. This is Connection No. 2.

Now the right half of the set. It looks like this:

Take the wire from the right end of the coil and wrap it around Nail В in the corner. That is Connection No. 3.

Wrap another wire around the Nail B. Take the other end of this wire and wrap it around one of the metal tips on the cord of the earphones. This is Connection No. 4.

Take another wire and wrap it around the other metal tip of the earphone cord. Now take the other end of this wire and put it under the head of the nail that holds the safety pin. With your hammer then pound this nail in so that the pin will stand up. Don’t nail it so tightly that the pin cannot move a little. This is Connection No. 5.

Our set is finished! But to make it work there are still two more things we must do.

First, we must have an aerial wire which the waves can cross and make electricity go back and forth into our coil. Whatever aerial you use, wrap one end of it around Nail A that connects with the coil and razor blade.

The longer the aerial the better. That will make more electricity move into the coil. You can use the wire that still is left on the spool and let it dangle out of a window. Or, better yet, perhaps you can get fifty or a hundred feet of any kind of wire and string it from a window to a tree out in the yard.

We also must have a connection to something that goes into the ground. The best ground connection is a cold water pipe. Take another piece of wire, scrape off the cotton covering and wind it tightly around a pipe. Take the other end of this wire and wrap it around Nail B.

With the aerial and the ground connected, we are all ready to get a program! Remember, a foxhole radio is different from the average set. So do not expect to hear music and voices booming in right away.

Put on the earphones, and do not make any noise in the room where you have your set. With your finger move the pin very gently so that the little piece of lead goes across the razor blade. Try putting the tip of the lead where the manufacturer of the blade has put his name.

Suddenly you will hear very faint little crackly noises in your earphones. Be careful. A station may come in at any moment. Move the pin just a shade more. Listen! There is somebody talking. Don’t move the pin. If you do, you may lose the station. Listen again. Do you hear two voices? You should not be surprised. Remember, we learned that a coil all by itself sometimes will let two programs come in.

A foxhole radio is a lot of fun. But why does it work?

Well, we know the incoming waves send electricity back and forth in the coil. But we know we must fix this electricity so that its changes will go in only one direction into our earphones.

The safety razor blade and the piece of lead act like an electrical gate. The blade is made of steel and the pencil point is made of carbon. When the steel and carbon touch each other, they will let electricity go through in only one direction. Once the electricity gets through the razor blade and the safety pin, it goes into our earphones.

But one of the most interesting things about radio is that if we can do something one way, we often can do it another way.