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.