The term Radio frequency refers to electromagnetic radiation in the range of frequencies lower than microwaves. Radio waves at these frequencies have been found useful for long-range communications. Among these frequencies, shortwave radio communications operate at frequencies that can be reflected by the ionosphere and therefore can bounce around the earth. Long-wave frequencies are used for relatively short range communications such as the most ordinary of the commercial amplitude modulation (AM) radio stations. Their transmissions ordinarily do not get long bounces.
Different short-wave frequencies (with wavelengths shorter than 120 meters) can be reflected well and transmitted by long bounces around the world during times when different conditions prevail in the ionosphere. So transmitters for such organizations as VOA (Voice of America) and BBC (British Broadcasting Corporation) will change operating frequencies from month to month and also from time to time during a 24 hour period.