Chapter 43
Radio
Electromagnetic waves can bemodulatedto carry information; this means that the pure sinusoidal electro-
magnetic wave is modified in some fashion to include information such as voice or music. There are two
common methods for modulating a radio wave:
- Amplitude modulation.Here the frequency of the wave is constant, but theamplitudeof the wave is
modified to carry information. In a sense, the basic sinusoidal radio frequency (thecarrier wave)is
multiplied by the sound wave, and the superposition of the two is transmitted by a device called the
transmitter. Another person has a device called areceiverthat extracts the audio (sound) information
and sends it to a speaker or headphones. - Frequency modulation.Here the amplitude of the wave is kept constant, but thefrequencyis varied
slightly about the carrier wave frequency in order to carry the audio information. Frequency modulation
has the advantage of being less susceptible to noise from phenomena such as lightning discharges, but
requires a more complex transmitter and receiver.
Radio is used in a number of ways:- AM Radio.So named because it uses amplitude modulation, AM radio is a commercial service that is
used to broadcast music, talk, news, sports, etc. It first appeared around 1920. Stations broadcast on
frequencies between 520 kHz and 1700 kHz, separated by 10 kHz. During the day, AM stations may
travel a few hundred miles, while at night they may travel across the continent by reflecting from the
Earth’s ionosphere.
Some AM stations broadcast at low power, or may broadcast only during the day. A few stations (Table
43-2) broadcast at the maximum allowed power (50 kW) day and night, and may be heard around the
country at night. - FM Radio.So named because it uses frequency modulation, FM radio is a commercial service whose
content is similar to that of AM radio. Stations broadcast on frequencies between 87.9 MHz and 107.9
MHz, separated by 0.2 MHz. FM radio frequencies are in a gap between television channels 6 and 7.
FM radio is less susceptible to “static” noise than AM, but the signals don’t travel far—–typically just
a few dozen miles at most. - Shortwave.Many countries broadcast an international service around the world in various languages.
These stations broadcast on so-called “shortwave” frequencies between 1800 kHz and 30 MHz, and
use amplitude modulation. At these frequencies, radio signals can bounce off of the Earth’s ionosphere
and travel around the world.