Chapter 4 Wireless Technology
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Electromagnetic Waves
The atmosphere is full of electromagnetic waves. Electromagnetic waves are
categorized according to frequency ranges. For example, common radio waves
used for communications start at 0.5 MHz for AM radio and span to 22 GHz for
satellite communications. The frequency of an electromagnetic wave is based
on the repeating pattern of its waveform just as it is with electrical energy. One
complete waveform is called a cycle, Figure 4-1, and frequency is the number of
times a cycle occurs in one second.
Electromagnetic waves are produced both intentionally and unintentionally
as a by-product of electrical energy. Radio and television stations produce
electromagnetic waves intentionally while many household appliances produce
electromagnetic waves unintentionally. You should recall from Chapter 2,
Network Media—Copper Core Cable that unintentional electromagnetic waves
are referred to as interference, or more specifi cally, noise.
Radio and Microwave Transmission
Radio and microwave transmission works on the principle of producing a
carrier wave as the means of communication between two wireless devices. A
carrier wave is an electromagnetic wave of a set frequency that is used to carry
data. It is identifi ed by a frequency number. For example, radio and television
stations are assigned specifi c frequencies on which they must transmit. A radio
identifi cation such as 104.5 FM represents a carrier wave of 104.5 MHz. The carrier
wave is how individual stations are identifi ed on a radio or television, Figure 4-2.
The carrier wave is mixed with the data signal. The mixing of the carrier
wave and data signal is known as modulation. The technique of modulation
is how AM radio, FM radio, and television operate. A simple radio broadcast
Figure 4-1.
Electromagnetic
frequency is based on
the repeating pattern
of a waveform. One
complete waveform
is called a cycle. The
frequency is how
many cycles occur in
one second.
Electromagnetic
wave
1 cycle
1 second
carrier wave
an electromag-
netic wave of a set
frequency that is
used to carry data.
modulation
the process of mixing
a data signal with a
carrier wave.