shown in Figure 3. Both work well, but the ferrite core coil
has a higher Q which provides somewhat better
performance. Q is the quality factor of the coil which is a
measure of how efficient the coil is. The higher the Q, the
better the results. You can also make your own coil. I
made one using plain old #22 copper wire wound on a
four inch diameter form; in this case, a box of grits. An
oatmeal box is a good alternative since both are four
inches in diameter. Use 40 turns of wire closely spaced.
Punch holes in the box to secure the wire at the ends. You
may need to glue them down to hold the coil in place and
keep it from unwinding. Be sure to scrape the enamel
insulation from the wire ends before you solder it.
You are probably going to have to buy the variable
capacitor. These were once widely available in every
model AM radio but no longer. Again, I bought mine from
Crystal Radio Supply, but I also found one at
www.stormwise.com. The capacitance is typically 365 pF
when the plates are fully meshed, and down about 30 pF
when the plates are fully unmeshed. The shaft is easy to
turn for tuning. The one I bought is shown in Figure 2. It is
actually two capacitors that are ganged together. You will
only need one section. You can also buy a one-section
capacitor, as well.
The crystal is a germanium diode. These are no longer
widely used but you can still buy them. Part numbers such
as 1N34, 1N34a, 1N60, or 1N270 are common. Any of
these should work. I used a 1N60. Do NOT substitute a
silicon rectifier diode like a 1N4001, 1N914, or 1N4148.
It simply won’t work. The germanium diodes have a much
lower threshold of conduction of about 0.2 volts,
compared to about 0.7 volts or so for the silicon diode.
Radio signals are very small and if you want this to work,
stick with the germanium diode. One possible substitution
I did not try was a Schottky or hot carrier diode. These
small signal diodes are made for very high frequency and
microwave signals, and should work.
Headphones are also important. The widely available
earbuds or headphones used with iPods, MP3 players, and
other modern audio products have a very low impedance
( 32 ohms or so). These will load down the tuned circuit,
reducing the Q and rendering it inoperable. You must use
high impedance (many megohms) crystal or ceramic ear
BUILDING THE RADIO
I did not intend for this to be a final finished project. I
just wanted to see if I could do it for nostalgic reasons, I
guess. So, I just soldered everything together in a
haphazard way as Figure 2 shows. The only pain in the
neck connection was to the capacitor. One side of the
capacitor has a nice solder terminal for connecting the
coil and diode, but the other connection to the capacitor
is the actual metal housing. I found a solder lug in my junk
box and screwed it into the capacitor housing with a short
4-40 screw. It is mechanical connection details that
62 December 2011
sometimes discourage us from building electronic circuits.
ANTENNA AND GROUND
One thing for sure is that this radio absolutely needs a
big antenna to work. The bigger — or longer — the antenna,
the more signal it picks up and the stronger the voltage
applied to the circuit. This, of course, translates into a louder
signal in the earphone. The antenna should be the very
longest piece of wire you can afford or place. I used 100
feet of #14 copper wire which I strung on my wooden back
yard fence. You may be able to get by with 50 feet but that
depends on how strong or how close your local stations are.
I tried 20 feet of wire and it did not work; 200 feet would
be great if you can manage it. Keep the wire off the ground
and as high as possible. This radio will also not work
without a good ground. And by ground, I mean the Earth. I
used a four foot copper rod from RadioShack. Drive it as far
into the ground as you can. I connected the antenna wire to
an alligator clip that attaches to the solder connection on
the capacitor (not the solder lug). Connect the solder lug on
the capacitor to a wire that will attach to the ground rod. I
used about 15 feet of #16 speaker wire with an alligator clip
to make this connection.
USING THE RADIO
Once you solder everything together, make the
connections to the antenna and ground, and you are ready
to go. Tune the capacitor slowly while listening on the
earphone. You should be able to hear at least one local AM
station. The strongest station will stand out. If you have
several stations locally, you could hear more. I heard just
two: a local station on 590 kHz and one on 1,200 kHz
about 70 miles away. This later station is one of those clear
channel AM stations transmitting 50K watts.
Just so you won’t be disappointed, you should know in
advance that the stations you will hear will be very weak.
Good hearing is an asset. Also, most AM stations transmit
more power during the day than at night, so daylight testing
is a must. For greater volume, a longer antenna is the
answer. You can also experiment with different components.
Try a different coil and see what works best. Try another
diode. That may help. If you can find a Schottky hot carrier
signal diode (NOT a power supply rectifier diode), give that
a try. A 0.001 µF capacitor in parallel with the earphone
may help. I did not notice much difference.
The other possible addition is an amplifier. Adding a
one transistor amplifier in place of the headphone will boost
signal strength considerably. I did not do that as I wanted to
see the pure crystal set performance. It is totally self-powered by the radio signal it receives.
I was pleased with the result of this project. It seemed
so silly given the state of technology today, that I
questioned my sanity. However, for many of you it could
be the beginning of a long hobby or career. NV