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Radio Waves
Mon Aug 01, 2003
Listen in RealAudio 
Hi, I'm Bryan Yeaton from The Mount Washington Observatory, and you're listening to
The
Weather Notebook. Last month, our Brainstormer posed a question about... well, I'll let
Brian
Alex, a Tennessee listener tell you:
The question had to do with why is it that AM radio can be heard hundreds or even
thousands of
miles away.
As usual, we found a lot of smart people out there. We'll start with 2nd Lieutenant
Timothy
Dahl, of Fort Eustis, VA:
AM waves have the ability to be reflected off the atmosphere; therefore, they can reach
radios, which are farther away.
Steve Kurak of Tecumseh, Oklahoma, adds that it's all about frequency:
AM stations are on a different part of the frequency spectrum, which has more of a sky
wave
and because it's a longer frequency the wavelength is longer and so the AM signal is
subject
to refraction in the atmosphere.
We heard from lots of audio engineers and amateur radio operators. One of those was
Bill Hall
of Nashville:
The ionosphere above the earth is composed of many layers produced by the
ionization of the
sun, and the layers act as reflectors that send the AM signals, or any radio signal, back
to
the earth. It's like when you skip a rock across a pond.
Radio Engineer Larry Keyes, formerly of affiliate KANW in Albuquerque, wrote in an
e-mail that
AM waves bounce off a specific area of the ionosphere known as the F2 layer. So, what
happens
to the FM? That either goes straight off into space, or sometimes can be heard at
greater
distance than line-of-sight through a process known as "Tropospheric Ducting."
Our show is produced with funding from The National Science Foundation, and
Subaru, Driven by
What's Inside.
Today's Links
The Ionosphere and what it does
http://www.qsl.net/ki0eg/propagation/propprimer.html
Ionospheric Propagation Explained
http://homepages.picknowl.com.au/wavetel/propagation.htm
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