DX communication is communication over great distances using the ionosphere to refract the transmitted radio beam. The beam returns to the Earth's surface, and may then be reflected back into the ionosphere for a second bounce. Ionospheric refraction is generally only feasible for frequencies below about 50 MHz, and is highly dependent upon atmospheric conditions, the time of day, and the eleven-year sunspot cycle. It is also affected by solar storms and some other solar events, which can alter the Earth's ionosphere by ejecting a shower of charged particles.
The angle of refraction places a minimum on the distance at which the refracted beam will first return to Earth. This distance increases with frequency. As a result, any station employing DX will be surrounded by an annular dead zone where they can't hear other stations or be heard by them.
This is the phenonenon that allows 'short wave radio' reception to occur beyond the limits of line of sight. It is utilized by amateur radio enthusiasts (hams), shortwave broadcast stations (such as BBC and Voice of America) and others. This is what allows you to hear AM (MW) staions from locations far from your location. It is the only backup to failure of long distance communication by satellites, when their operation is affected by electromagnetic storms from the sun. (DAN)