MadSci Network: Engineering

Re: Regarding the nature of radio frequency; how far do radio waves travel?

Date: Wed Oct 24 02:09:09 2001
Posted By: Erika Gibb, Grad student, Physics & Astronomy/Origins of Life, RPI
Area of science: Engineering
ID: 1001435032.Eg

Hello Teresa, Elizabeth, Catherine, and Patrick,

A radio transmitter sends radio signals out in all directions.  Radiation
will travel until it is either scattered or absorbed.  Radio waves are on
the order of a few meters long (1 meter is just over 3 feet).  For
example, if you listen to station 101.3 that means the frequency you are
listening to is 101.3 Megahertz or 101.3 million wavelengths per second. 
The wavelength of radiation is equal to the speed of light divided by the
frequency which gives 2.96 meters or just over 10 feet.  Radiation is
efficiently scattered by anything that is close in size to the wavelength. 
The atmosphere is full of molecules and dust, all much much smaller than
the size of a radio wave.  The atmosphere is also fairly transparent
(meaning it does not absorb much radiation) at many radio frequencies. 
They essentially travel unimpeded.  However, the Earth is curved and
radiation travels in a straight path.  This means that after about 30 or 40
miles, your antenna is no longer able to pick up the radio waves.  The
transparency of the atmosphere also means that FM radio signals pass
through the atmosphere and are released into space.

AM radio uses shorter frequencies and the signal can travel much farther
(hundreds of miles at night).  This is because of the Earth's ionosphere, a
layer in the atmosphere where the Sun's light ionizes the atoms (causes
them to lose an electron).  The ionosphere reflects certain frequencies of
radio waves (less than about 30 Megahertz, though the exact number depends
on the density of electrons and varies with time) so they can travel much
farther.  Some radio stations can even be picked up better at night because
during night hours, without sunlight, the ionosphere changes and becomes
more reflective to certain frequencies.


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