ionosphere throughout the day and night. After reaching theFlayer, the signals the reflect and bounce back
to the ground, where they reflect again back toward the ionosphere, and so on. With multiple “hops,” like
this, the radio waves can travel around the globe.
The interaction of AM radio waves is a bit different. During the night, AM radio waves can reach theF
layer and travel like shortwave radio, making multiple hops across the globe. But during the day, AM radio
waves are absorbed by the ionizedDlayer and cannot reach theFlayer. In effect, theDlayer acts as kind
of a “curtain” that is pulled in front of theFlayer during the day. The net effect is that AM radio signals can
travel great distances at night, but much shorter distances during the day.
43.2 The Crystal Radio
We’ll examine the operation of a simple radio receiver by looking in detail at the design of a simplecrystal
radio receiver. This is one of the first types of radio receiver, and has been in use since the 1920s. A crystal
radio can be built from just a few spare parts—in fact, soldiers during World War II would often build a
variety of crystal radio called a “foxhole radio” from wire, scrap wood, a razor blade, toilet paper tube, a
safety pin or pencil lead, and headphones.
One remarkable feature of a crystal radio is that it requiresno batteries: it runs entirely on the power
provided by the transmitter. Once you build a crystal radio, you can run it forever for free.
Tuning Circuit
The crystal radio circuit begins with a tuned LC circuit (Fig. 43.1(a)). The LC combination is designed to
oscillate at the same frequency as the AM radio signal to be received.
Recall from Chapter 39 that an LC circuit with inductanceLand capacitanceCoscillates with angular
frequency
!D
1
p
LC: (43.1)
SincefD!=2, the frequency (in hertz) is
fD1
2
p
LC: (43.2)
Antenna and Ground
Now let’s add anantenna(oraerial) and ground to the tuned LC circuit (Fig. 43.1(b)). The antenna is
typically just a long wire (50–100 ft.) strung outdoors, up into a tree or other tall structure if possible.
The ground connection is a connection to a long conductor, typically the Earth itself. A traditional ground
connection is a connection to a copper pipe driven into the ground, or a connection to a copper cold-water
pipe (which also goes to the ground).
The antenna is a large conductor that picks up radio signals of all frequencies and feeds them to the LC
circuit. But the LC circuit only resonates with those input signals that are at the frequency given by Eq.
(43.2). The ground connection essentially gives the current someplace to go; without a good ground, the
current would get “backed up” in the circuit, and the radio would not operate.
The Crystal
Now that we have a circuit resonating at the frequency of the carrier wave (the frequency at which the radio
station is transmitting), we need to extract the audio signal. In a crystal radio, this is done with adiode:a