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A very simple radio with two parts

First let me warn you that this first little radio may not work in your location. It relies on having a very strong local radio station to overcome the limitations of such a simple radio. If it does not work where you are, you can either build its cousins that we will discuss later, or you can drive out closer to a local radio station, and try it there. But because it is so simple, you might try building it just to see what you might be able to pick up.

If your earphone has a jack on the end, cut it off, so you have two long wires coming from the earphone. If the wires are twisted around each other, that is OK, since we only need them to be separate at the very ends.

Remove the covering (called insulation) from the ends of the wires to expose an inch of bare wire. Often you can do this with your fingernail, but a tool called a wire stripper is made for this purpose, and can usually be purchased at the same place you got the earphone or the diode.

Wrap one bare wire around one of the diode's wires. Use some tape to keep it in place. If you know how to solder, you can solder the wires together, but it really isn't necessary for now

Tape the other diode wire to a cold water faucet. This makes a good connection to the ground, and is thus called a 'ground' connection.

Hold the remaining free bare wire of the earphone in your hand. This makes your body into the antenna for the radio. Put the earphone in your ear. If you are close to a strong AM radio station, you will be able to hear that station faintly in the earphone. You may hear more than one station at once.

If you can't hear anything, you might try a better antenna. You can tape the wire you were holding to a metal window screen, or a long wire. If one end of the long wire is thrown up on a roof or in a tree, you might get better results. Another good antenna is an outdoor TV antenna. Just touch the free earphone wire to one of the antenna terminals where it comes into the TV. If you have a good antenna, you may be able to eliminate the ground connection, using your body as a ground instead, by holding the free diode wire in your hand.

Another simple radio with two parts

Our simple radio has two main drawbacks. One is that the signals are very faint, and can only be heard if you are close to a radio station's transmitting antenna. The other is that you hear all of the strong stations at once, and it is hard to pick out just one song or voice from the mixed up jumble. The first problem is called the 'sensitivity' of the radio. Our radio is not very sensitive. The second problem is called the 'selectivity' of the radio. Our radio is not very selective.
We can solve both problems by using a trick called resonance.

Resonance is a way of taking a little bit of energy, and using it over and over again, at just the right time, to accomplish a big task. We use resonance when we push someone on a swing. It would take a lot of work to lift someone several feet in the air, but we can do this easily on a swing by giving a little push over and over again at just the right time. Timing is important: if we push at the wrong time, the swing can actually lose energy instead of getting higher.

When an opera singer uses her voice to shatter a wine glass, she is using resonance. Her voice gives the glass a little push at just the right time, over and over again, until the glass is moving so far that it shatters. In a similar way, we can slosh all the water out of a bathtub by moving a hand in the water at just the right back and forth speed. Each time the hand moves, the water climbs a little higher, until it is over the top of the tub.

Radio waves can act like the sound waves of the singer's voice, or like the waves in the bathtub. Radio waves can cause electrons to move back and forth in a wire, just like the water in the tub. If the radio waves are moving back and forth at the right frequency, then the electrons in the wire will just be crowding towards one end of the wire when the radio waves start moving them back to the other side. Just like the water in the tub, the electrons will crowd higher and higher at the ends of the wire. These electrons can do work, like moving the brass disk in the earphone to create sound.
We can use resonance to build a radio that can pick up only one station at a time, and make a louder sound in the earphone. This radio will also have some drawbacks (for one thing it will be over 1,000 feet long!) but we will solve these problems in the next radio we build.

Suppose we pick a local radio station we want to hear. For this example we will choose 740 kilohertz on the AM dial. We now need to figure out how long the wire must be to resonate at this frequency. Radio waves travel at the speed of light. This radio wave is going back and forth 740,000 times per second. This means the wave needs to go about a quarter of a mile in one direction, then turn around and go back again, over and over. The actual formula for figuring out how long the wire should be is 936 feet Frequency in Megahertz or, for our example: 936 feet .740 or about 1264 feet.

To make our radio, we take half of the wire (632 feet) and attach it to one end of the diode. We attach the other half of the wire to the other end of the diode. We attach one earphone wire to one side of the diode also, and the other earphone wire to the other end. We put the long wire up in the air by attaching each end to a tree (the trees must be about 1264 feet apart). Then we put the earphone into our ear, and listen to the radio.

Now I can think of a couple problems with this radio. It is not the most portable radio. Also, in order to change the station, we need to make the wire longer or shorter.

One solution to the portability problem is to coil the wire up by winding it on a box or a cylinder. Then we can solve the tuning problem by attaching the diode and earphone to the coil at different places (easy to do now that the whole wire is in one small place).