Unguided media transfer electromagnetic waves without using
a physical conductor. When electrons move, they
produce electromagnetic waves that can transport through the free space. In vacuum,
all electromagnetic waves move at the same speed. Unguided signals can travel
from source to target in different ways: grounded propagation, sky propagation
and line-of-sight propagation.
In ground wave propagation, radio waves travel along the surface of the earth with
a frequency less than 2 MHz.
In sky propagation,
signals propagated by refraction in the ionosphere. Support frequency in
between 2 MHz and 30 MHz.
In line-of-sight propagation signals are transmitted in straight lines from antenna to antenna
by using frequency greater than 30 MHz.
Wireless transmission can divide into three broad
classes: radio waves, microwaves, and infrared waves
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| Uu-guided Media |
Radio Waves
Radio waves can travel long
distances and penetrates walls easily. Radio waves also are omni-directional, meaning that they
travel in all directions from the source. At low frequency, radio waves can
pass through obstacles. At high frequency radio waves tend to travel in
straight lines and bounce off obstacles.
Microwaves
Microwaves are
unidirectional, means that when a transmitter transmits microwave signals, they
can be narrowly focused. Microwaves are commonly used for line-of-sight propagation.
Infrared and Millimeter Waves
Infrared and millimeter waves are commonly used for short-range
communications. They are not able to pass through solid objects. Usually
provide line-of-sight transmission. There are no frequency allocation issues
with infrared, since no license is required.
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