Radio wave
A radio wave is an electro magnetic wave. We modulate them using
three different modulations, frequency modulation, amplitude modulation,
and pulse modulation, to carry information.
http://www.howstuffworks.com/radio5.htm
For frequency modulation slight variances are made in the frequency
of the wave to represent different bits of information. This is widely
used because it is less likely to have static.
For amplitude modulation the height or amplitude of the wave is changed
to contain information. Amplitude modulation is not only used by radio
stations but it is also used to send the picture part of television.
Pulse modulation is where there are breaks in the wave to indicate
the desired information. This is usually used for morse code but can
be used for a few other things as well.
propagation
According to maxwell's equations radio waves travel at the speed
of light. The magnitude of a wave will decrease at a rate of r^2. Where
r is the distance from the origin. This is because the wave is propagates
in all directions so the same amount of energy spreads out over a greater
area.
But how can we get signals from beyond the horizon? Today we could
use satellites, but you can also bounce a radio wave off of parts of
the upper atmosphere. This can be done because the sun ionizes levels
of upper atmosphere. an electromagnetic wave resists crossing these
layers.
http://www.arrl.org/tis/info/images/MagneticField.jpg
This is an illustration of how charged particles can affect a magnetic
field. the distortion of the magnetic field around the earth is caused
by solar winds, which are comprised of charged particles that are emitted
from the sun.
For the same reason solar activity can interfere with radio wave
propagation.
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