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The ionosphere is a region of the upper atmosphere of Earth with substantial partial ionization. Its mobile Electrons and ions constitute a plasma mixed with neutral gas. For radio communication, the crucial question is whether a chosen radio wave returns toward the ground, is absorbed, or passes through. A skywave can connect stations beyond their direct view across the curved surface of Earth, while wave interference between a ground wave and a skywave can make reception fade even when the radio transmitter remains steady.

The ionosphere is useful both as a route for radio communication and as a remote laboratory: varying the probing frequency changes how deeply the radio wave penetrates the plasma. The Appleton-Barnett radio interference experiment turned an unwanted fluctuation in reception into a measurement of the overhead route. Cold-plasma radio dispersion explains why this is a tunable medium, while ionospheric virtual height explains why a measured delay is not automatically an altitude.