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 the sky. Sky-wave propagation takes place between sunset and sunrise. It's the flip side to the groundwave propagation used to transmit 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 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. 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.
What that means for AM radio listeners is that they've experienced the remarkable ability of AM radio signals to travel hundreds of miles farther than during the day. Listening to AM, and scanning the AM dial between sunrise and sunset is a fun way to discover how far a sky-wave signal can travel to reach you. During the baseball season, you can 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, which is a 50,000 watt 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, we'd suggest the CCRadio 2E. Since the radio has fine tuning capabilities, thanks to the built-in Twin Coil Ferrite® AM Antenna, you may be able to find a new station almost every night you tune in. It's a lot more fun than you might realize, with something as simple as a radio.