Radio stations use transmitters of various strengths (watts). Local rules as to how strong a station's transmitter can be,can govern how far they go. Atmospheric conditions can also affect this. Line of sight is also a factor. Stations can get around this by using repeaters (transmission towers that recieve a signal and rebroadcast) or by actually using the ionosphere (charged particles in the upper atmosphere) to act as a "mirror" if you will, to bounce signals off. For example, here in Canada, a Citizens band radio is limited in power to about 5 watts. Much less than a typical commercial radio station, yet under the right atmospheric conditions, a signal that normally only travels 16 miles or so, can actually go thousands of miles.Here in Toronto, I've heard conversations taking place in New Orleans during last years floods.

Radio waves travel at the speed of light, in straight lines, and by more than one path. Local signals that you hear from nearby CBers or FM radio stations are usually travelling by space wave or "line of sight". These travel, as the name suggest, from antenna to antenna direct. VHF and UHF (and higher) signals travel by this method. The second form of radio wave propagation is ground wave. Frequencies between about 3MHz and 30MHz travel by ground wave. Ground waves can travel a little further than space wave as the curved surface of the earth will pull the signal down and keep it along the ground. An antenna that is below the horizon can be reached by ground wave. The last form of propagation is sky wave, effecting frequencies between about 300kHz and 30MHz. These travel up into the atmosphere and are then reflected back to earth by a certain layer of the Ionosphere, depending on frequency, season, time of day, solar activity, etc. Signals can travel many thousands of kilometres by this method, reflecting back to earth, then being refracted back towards the sky, and so on. Signals can also travel to your antenna by more than one path. If you've ever listened to a Shortwave station and heard the signal fading in and out, this can be caused by the signal travelling to the antenna by two (or more) different paths. For example, if the signal reaches the antenna by ground wave and also by sky wave, the distance it has travelled by sky wave will obviously be much longer and so sometimes the signal will arrive out of phase, and so it will either add to, or subtract from, the ground wave signal. The zone located in the area where the ground wave has fully dissapated, and before the sky wave first returns to earth, is called the skip zone and receives no signal. This is why with CB you can sometimes hear stations hundreds of miles away, and yet you can't hear a station only 100 miles away - 100 miles may be too far for the ground wave, but it is too close for the sky wave. Low frequencies are best for travel by sky wave, and the maximum frequency that will efficiently reflect back to earth will vary with time of day, season, solar activity and even location. Frequencies in the VHF and UHF region do not reflect back by sky wave and generally only travel by line-of-sight, however "ducting" can sometimes cause these frequencies to travel hundreds of miles. Ducting occurs when the VHF/UHF signal is trapped in a tropospheric inversion layer. This is when the warmer air, which is usually closer to the earth, gets trapped above layers of cooler air. VHF/UHF signals can also travel long distances through scatter propagation. This is where the signal is reflected back from some object, such as rain, snow, hail, aircraft, meteors, etc.