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If you listen to AM radio stations by night that are just impossible to
pick up by day, chances are you're benefiting from sky-wave propagation.
Propagation is just the technical word for how radio signals travel
through the air. Sky-wave propagation is the specific name given to
radio waves that travel through - you guessed it - the sky. Sky-wave
propagation takes place between sunset and sunrise - in other words,
at night, and it's the flip side to the groundwave propagation used
to transit during the day.
During the day, groundwave propagation is preferable because the radiation
from the sun causes so much ionization that radio signals sent into
the air are just absorbed into the atmosphere. When atoms in the D
region of the ionosphere are ionized, you end up with free electrons
and ions floating around in the air. It's kind of like trying to walk
through a room filled with dancing couples. When they're in pairs,
there's more room to walk through, but when they're not paired, it's
easier to get stuck in a conversation with someone. That's kind of
how the ions and electrons in the D region work - when they're not
combined, they look for something else to combine with - and that's
how they absorb radio waves. At night, however, once the sun begins
to set, the electrons and ions in the D region recombine rapidly -
leaving more room for the radio waves to travel a little farther up.
Above the D region, the F1 and F2 regions are also recombining, but
much more slowly than the D region, and so when the radio waves hit
those regions of the ionosphere, they have a chance to be reflected
or bent (some prefer refracted) back toward the earth.
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What that means for AM radio listeners is that they've suddenly become AM
DXers (DX means "distant" as in distant radio station). They've experienced
remarkable ability of AM radio signals to travel hundreds of miles
farther than during the day. Listening to AM, and scanning the AM dial
at sunrise and sunset is a fun way to discover how far a sky-wave signal
can travel to reach you. During the baseball season, I use the sky-wave
to tune in some night games played in different parts of the country.
In Northern California (where C. Crane is based), as in other places
around the country (even up into Alaska) people use the sky-wave to
listen to stations like KGO a 50,000 Watt clear station located in
San Francisco. Also, if a signal reflects off the ionosphere and then
bounces off the earth and then reflects off the ionosphere again it
can travel even farther than with just a single reflection. So you
see, sky-wave propagation makes it possible to tune in stations that
you might not even know about when tuning your radio during the day.
If you ever come across a DX web site, or even a shortwave radio Web site,
where someone is really happy about intense solar activity (or lots
of sun spots), it's because more ionization during the day makes for
better sky-wave propagation during the night.
If you want to make the most of sky-wave propagation, I'd suggest the
combination I have going of a CCRadio
plus and a Twin
Coil Ferrite AM Antenna. Since both the radio and the antenna have fine tuning capabilities,
I've been able to find a new station almost every night I tune in. It's a lot more fun than
people realize you can have with something as simple as a radio.
Happy DXing.
To view our past articles, please visit our What's
in the News Archives.
As always, please contact us with any comments or
article suggestions you might have.
If you are interested in using C. Crane's articles on your own Web site,
please let me know. I'd be happy to take a look at your Web site and
see what we can do. Good-bye for now, Carlos. About
the author
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