- Building a simple crystal radio.
- Building a radio in ten minutes.
- Building a radio out of household implements.
- Building a three-penny radio.
-
Building a very simple AM voice transmitter.
- Going further:
- License-free radio frequencies.
- Getting an Amateur Radio license.
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 our
catalog.
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
catalog.
You can also find them in old broken or discarded radios.
- A Germanium diode (1N34A)
We carry this in our
catalog.
- A piezoelectric earphone
Also in our
catalog.
- 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
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 of the variable capacitor. It doesn't matter which wire
goes to which lead.
The germanium diode is soldered to one of the leads of the
variable capacitor. Again, it doesn't matter which lead.
One of the piezoelectric earphone wires is soldered to the free end of the
germanium diode. The other is soldered to the lead of the variable
capacitor that does not have the diode attached to it.
Lastly, the alligator jumper is simply looped around the coil once
(if you 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!
How does it work?
To use the radio, connect one end of the 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.
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 of the 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.
If you have a shorter antenna, the stations will sound very faint
if you only use one loop of the 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 of loops in the alligator jumper trade off volume 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 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
stations.
For more information on radio, see the
Recommended Reading
section.
Order radio parts and kits
here.
Send mail to
Simon Quellen Field
via
sfield@scitoys.com
