ELECTROMAGNETIC SPECTRUM
1.
a. The electromagnetic spectrum is an array of all types of
electromagnetic radiation arranged on the basis of their frequencies,
spectral densities, and spatial volumes.
Many types of radiation,
such as sunlight, x-rays, broadcasting waves, the microwaves, are
encountered in everyday life.
Although the many types of radiation
are seemingly different in form and do differ in terms of frequency
or wavelength, they all obey similar, though not identical, physical
laws, and their velocities of propagation in free space are
identical. This velocity is that of light.
b. Electromagnetic radiation may be visualized as a form of wave
motion with wavelength or frequency as the important parameter.
Wavelength is the distance between two consecutive peaks in the
unmodulated wave.
Frequency is the number of waves passing a fixed
point per unit time interval; the units for frequency are hertz; a
hertz equals one cycle per second.
The following prefixes are used
to describe the higher frequencies:
kilohertz
=
Kilo or thousands of hertz; such as 5 kilohertz.
megahertz
=
Mega or millions of hertz; such as 10 megahertz.
gigahertz
=
Giga or billions of hertz; such as 15 gigahertz.
1 gigahertz = 1,000 megahertz
The order of electromagnetic radiation within the spectrum, arranged
by increasing frequency, is from low (long wavelength) audio
frequencies through radio frequencies, infrared, visible light,
ultraviolet, and x-rays to high (short wavelength) gamma radiation.
c. In a discussion of radio communications, frequencies are more
commonly used than wavelengths. The radio frequency spectrum is that
portion of the electromagnetic spectrum with radiations whose
frequencies range from 3 kilohertz to 3000 gigahertz.
However, the
frequency band from 300 gigahertz to 3000 gigahertz is beyond the
limits
of
the
current
usable
spectrum
for
the
purpose
of
communications-electronics frequency management.
Figure 1 shows the
division of the managed radio frequency spectrum.
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