The Sky as a Mirror

Radio is light

Radio waves and visible light are the same phenomenon — an electromagnetic wave, an electric field and a magnetic field taking turns to create each other, travelling at 300,000 km per second. The only difference between a radio wave and a gamma ray is how fast the field oscillates. Shortwave sits between roughly 3 and 30 megahertz, which is to say wavelengths of about 100 metres down to 10.

The ionosphere

Above about 60 km, sunlight is energetic enough to knock electrons off air molecules, leaving a region of free charges: the ionosphere. Free electrons interact strongly with radio waves. Below a certain frequency the ionosphere refracts the wave so sharply that it bends back toward the ground — in effect, a mirror in the sky at 100 to 400 km altitude.

A signal can bounce off the ionosphere, off the ground, off the ionosphere again. Each bounce is called a hop, and a few hops will take a modest transmitter right around the planet. This is why shortwave was the world's long-distance medium for most of the twentieth century, and why it still is where no infrastructure exists.

Why it changes with the sun

The mirror is made of sunlight, so it moves. At night the lowest layer (the D layer, which absorbs rather than reflects) largely disappears, and low frequencies that were useless by day suddenly travel enormous distances — which is why distant AM stations appear after dark. Solar flares can black the whole band out for hours. The 11-year sunspot cycle raises and lowers the highest frequency that will bounce at all.

Which bands are open is therefore a genuine, changing, physical fact about the day and the hour — which is why a real listener checks a propagation forecast before tuning, and why the band in this game behaves differently at the top than at the bottom.

Who is on it

Everything. International broadcasters, aircraft crossing oceans beyond radar, ships, amateur operators, military traffic, and — famously — numbers stations: unattended transmitters reading strings of digits in a synthesised voice, almost certainly one-time-pad traffic to intelligence officers abroad. Nobody has ever needed to explain them, because nobody has ever admitted owning one. Shortwave is the last broadcast medium where you can hear something and have no way at all to find out who sent it.