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Building a radio in 10 minutes.

For our 10 minute radio, we will need these parts:

A ferrite loop antenna coil

In our other crystal radios we wound the coil by hand. In this

project we use a much smaller coil with a ferrite rod inside, from

ourcatalog. The ferrite rod allows the coil to be smaller, and it

can be moved in and out ofthe coil for coarse tuning.

A variable capacitor (30 to 160 picofarads)

We carry this in ourcatalog. You can also find them in old

broken or discarded radios.

A Germanium diode (1N34A)

We carry this in ourcatalog.

A piezoelectric earphone

Also in ourcatalog.

A short wire or alligator jumper

We use an alligator jumper here for convenience. Radio Shack

part number 278-1156, or you can find them anywhere

electronics parts are sold. You can use any piece of insulated

wire instead.

About 50 to 100 feet of stranded insulated wire for an antenna.

This is actually optional, since you can use a TV antenna or FM

radio antenna by connecting our radio to one of the lead-in

wires. But it's fun to throw your own wire up over a tree or on

top of a house, and it makes the radio a little more portable.

A block of wood or something similar for a base

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Click on photo for a larger picture

You can see from the photo how simple this radio is, and why it can be

put together in a very short time.

The wires from the ferite loop are soldered to the two rightmost leads

ofthe variable capacitor. It doesn't matter which wire goes to which

lead.

The germanium diode is soldered to one ofthe leads ofthe variable

capacitor. Again, it doesn't matter which lead.

One ofthe piezoelectric earphone wires is soldered to the free end of

the germanium diode. The other is soldered to the lead ofthe variable

capacitor that does not have the diode attached to it.

Lastly, the alligator jumper is simply looped around the coil once (ifyou

have a nice long antenna and a good ground) or a few times (for

shorter antennas). We'll glue the coil and the wire down later, after we

have tested the radio. In the meantime, some sticky tape can hold it

onto the base.

That's it -- you're done!

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How does it work?

To use the radio, connect one end ofthe alligator jumper to your

antenna. Connect the other end to a good ground, such as a cold water

pipe.

We will start the tuning with the variable capacitor set in the middle of

its range, neither all the way clockwise, nor all the way counter

clockwise.

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With the earphone in your ear, slowly move the ferrite rod into the coil,

listening for radio stations.

With a long antenna, and a single loop ofthe alligator jumper, you can

tune several radio stations. In some areas, one or two stations will be

so close or so powerful that they overwhelm all the others, and you will

only hear those one or two stations.

Ifyou have a shorter antenna, the stations will sound very faint ifyou

only use one loop ofthe alligator jumper. Looping the jumper around

the coil two or three times will increase the volume. But the volume

increase comes at a price -- you will hear fewer stations.

The number ofloops in the alligator jumper trade offvolume for

selectivity. The best way to increase the volume is to use as long an

antenna as you can, and a good ground connection.

How does the ferrite change the frequency?

The ferrite rod increases the inductance ofthe coil. In our other (hand-

wound) coils, we increased the inductance by winding some more

loops, or by using a "tapped" coil, and selecting a tap that was farther

down the coil.

As the ferrite rod is inserted into the coil, more ofthe coil is affected by

the ferrite, and so the inductance increases. Increasing the inductance

moves the frequency lower. This allows us to hear stations "lower on

the radio dial".

Ferrite is used because it is magnetic, like iron or steel, but it is not a

conductor ofelectricity. Ifit were conductive, the coil would induce

"eddy currents" in it, and some ofthe energy would be lost heating up

the core. Because ferrite is not a conductor, we can use its magnetic

properties to change the inductance ofthe coil, without losing volume.

Ifyou have a long antenna, a good ground, and you are not too close

to a strong station, the variable capacitor will help in fine tuning the

stations.