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,
The ferrite rod allows the coil to be smaller, and it can be
moved in and out of the coil for coarse tuning.
A variable capacitor (30 to 160 picofarads)
We carry this in our
You can also find them in old broken or discarded radios.
We use alligator jumper wires here for convenience.
They are used to connect the ground and antenna wires to a good
ground and a long wire antenna.
We carry these in our
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
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 black painted wire from the ferrite loop is soldered to the center
lead of the variable capacitor. The unpainted wire is soldered to the
rightmost lead of the variable capacitor.
The germanium diode is soldered to the rightmost lead of the
One of the piezoelectric earphone wires is soldered to the free end of the
germanium diode. The other is soldered to the center lead of the
The red painted wire of the coil is attached to the long wire antenna
with an alligator clip lead.
The green painted wire of the coil is attached to a good ground (such as
a cold water pipe) using another alligator clip lead.
That's it -- you're done!
Click on photo for a larger picture
How does it work?
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
With the earphone in your ear, slowly move the ferrite rod into
the coil, listening for radio stations.
With a long antenna,
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.
How does the ferrite change the frequency?
The ferrite rod increases the inductance of the 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 of the 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 of electricity. If it were conductive, the coil
would induce "eddy currents" in it, and some of the energy would
be lost heating up the core. Because ferrite is not a conductor,
we can use its magnetic properties to change the inductance of the coil,
without losing volume.
If you 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
There are actually two coils of wire wound around the ferrite rod.
The large coil is connected to the variable capacitor.
The small coil is connected to the antenna and ground.
This arrangement allows the radio to be more selective, so that
strong stations don't drown out the weak ones. Really strong
local stations will still overwhelm the more distant stations,
If you have no strong local stations, you can make the stations
you receive sound louder by connecting the antenna and ground
directly to the large coil. Connect the antenna to the center
lead of the variable capacitor, and the ground to the rightmost lead
of the variable capacitor. The stations will be louder, but they
will most likely all be heard at once, since you radio will be less
selective in tuning out adjacent stations.