Make Electronics

What Next? 265

Experiment 31: One Radio, No Solder, No Power

Figures 5-66 and 5-67 show the completed radio.

If you’ve managed to follow these instructions (one way or another), it’s time
to tune your radio to the nearest station. Move the alligator clip at the end of
your patch cord from one tap to another on your coil. Depending on where
you live, you may pick up just one station, or several, some of them playing
simultaneously.

It may seem that you’re getting something for nothing here, as the earphone
is making noise without any source of power. Really, though, there is a source
of power: the transmitter located at the radio station. A large amplifier pumps
power into the broadcasting tower, modulating a fixed frequency. When the
combination of your coil and antenna resonates with that frequency, you’re
sucking in just enough voltage and current to energize a high-impedance
headphone.

The reason you had to make a good ground connection is that the radio sta-
tion broadcasts its signal at a voltage relative to ground. The earth completes
the circuit between you and the transmitter. For more information on this and
other concepts relating to radio, see the upcoming section “Theory: How radio
works.”

Enhancements

The higher your antenna is, the better it should work. In my location, this is a
major problem, as I live in a desert area without any trees. Still, just stringing
the wire out of my window and tethering it (with rope) to the front bumper of
my car enabled me to pick up a faint radio signal.

To improve the selectivity of your radio, you can add a variable capacitor, as
shown in the following section. This allows you to “tune” the resonance of your
circuit more precisely. Variable capacitors are uncommon today, but you can
find one at the same specialty source that I recommended for the earphone and
the germanium diode: the Scitoys Catalog (http://www.scitoyscatalog.com).

This source is affiliated with a smart man named Simon Quellan Field, whose
site suggests many fun projects that you can pursue at home. One of his clever
ideas is to remove the germanium diode from your radio circuit and substi-
tute a low-power LED in series with a 1.5-volt battery. This didn’t work for me,
because I live 40 miles from the nearest AM broadcaster. If you’re closer to a
transmitter, you may be able to see the LED varying in intensity as the broad-
cast power runs through it.

7R*URuQG

7R$QWHQQD

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Figure 5-66. A signal from the antenna

can pass through the coil to ground. If the

jumper wire is attached to an appropriate

tap on the coil, it resonates with the radio

signal, just powerfully enough to energize

the earphone which is wired in series with

a diode.

Figure 5-67. The real-life version of Figure

5-66.