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1.1 Radio Waves

Radio waves are created when electrically
charged particles accelerate with a frequency that lies in the
radio frequency (RF) portion of the electromagnetic
spectrum. Other emissions that fall outside of the RF spectrum
include X-rays, gamma rays, and infrared and ultraviolet light. When
a radio wave passes a copper wire or another electrically sensitive
device, it produces a moving electric charge, or voltage, which can
be transformed into an audio or data signal.

Radio waves can be depicted mathematically as a sinusoidal curve, as
shown in Figure 1-1.

Figure 1-1. A sine wave representing a radio wave

The distance covered by a complete sine wave (a cycle) is known as
the wavelength. The height of the wave is called
the
amplitude. The
number of cycles made in a second is known as the
frequency.
Frequency is measured in Hertz (Hz), also known as cycles per second.
So, a 1 Hz signal makes a full cycle once per second. You should be
familiar with this unit of measurement: if your new
computer's CPU operates at 2 GHz, the internal clock
of your CPU generates signals roughly at two billion cycles per
second.

Note that frequency is inversely proportional to the wavelength: the longer the wavelength, the lower the frequency; the shorter the wavelength, the higher the frequency. The wavelength of a 1 Hz signal is about 30 billion centimeters, which is the distance that light travels in one second. A 1 MHz signal has a wavelength of 300 meters.

1.1.1 Radio Frequency Spectrum

To regulate the use of the various radio frequencies, the
Federal Communications Commission
(FCC) in the United States determines the allocation of frequencies
for various uses. Table 1-1 shows some of the
bands defined by the
FCC (see http://www.fcc.gov/oet/spectrum/table/fcctable.pdf).

You can get a more detailed
frequency allocation chart from
http://www.ntia.doc.gov/osmhome/allochrt.pdf.
The following conversion list should help you understand this chart:

1 kilohertz (kHz) = 1,000 Hz

1 megahertz (MHz) = 1,000 kHz

1 gigahertz (GHz) = 1,000 MHz

Wireless networks use a variety of radio frequencies. Table 1-2 shows some common
wireless network protocols and the
corresponding radio frequencies.

1.1.2 Radio Wave Behavior

Radio
waves, similar to light waves, exhibit certain characteristics when
coming into contact with objects.

Reflection
occurs when a radio wave hits an object that is larger than the
wavelength of the radio wave (see Figure 1-2). The
radio wave is then reflected off the surface.

Figure 1-2. Reflection of a radio wave

Refraction
occurs when a radio wave hits an object of a higher density than its
current medium (see Figure 1-3). The radio wave now
travels at a different anglefor example, waves propagating
through clouds.

Figure 1-3. Refraction of a radio wave

Scattering
occurs when a radio wave hits an object of irregular shape, usually
an object with a rough surface area (see Figure 1-4), and the radio wave bounces off in multiple
directions.

Figure 1-4. Scattering of a radio wave

Absorption
occurs when a radio wave hits an object but is not reflected,
refracted, or scattered. Rather, the radio wave is absorbed by the
object and is then lost (see Figure 1-5).

Figure 1-5. Absorption of a radio wave

Diffraction
occurs when objects block a radio wave's path. In
this case, the radio wave breaks up and bends around the corners of
the object (see Figure 1-6). This property allows
radio waves to operate without a visual line of sight.

Figure 1-6. Diffraction of radio waves