What Is Shortwave Radio? Bouncing Signals Off the Edge of Space

Ultraviolet and X-radiation from the sun strips electrons from atoms in the upper atmosphere, producing layers of charged particles between roughly sixty and a thousand kilometres up. A radio wave entering such a layer is progressively refracted, meaning bent, and if the frequency is low enough relative to the electron density the bending is sufficient to return it to the ground, which is usually described as reflection though refraction is what actually happens. The layers are named in sequence, with the lower D layer absorbing rather than reflecting at these frequencies during daylight, and the higher F layers doing the useful work. Since the ionisation is driven by sunlight, everything changes between day and night, and the D layer largely disappears after dark, which is why distant stations that are inaudible in the afternoon appear in the evening. The usable frequency range shifts with the same cycle, so operators work higher frequencies by day and lower ones at night, and a signal can make several hops between ionosphere and ground to reach a quarter of the way around the planet.

Propagation is variable in ways that are partly predictable and partly not:

• •Time of day, with the whole usable range shifting downward after sunset and paths opening and closing around the terminator
• •Season, since the angle of sunlight changes ionisation and long winter nights favour lower frequencies
• •The eleven-year solar cycle, with high sunspot activity raising the maximum usable frequency and making long-distance paths on the higher bands routine, and solar minimum closing them
• •Solar flares and geomagnetic storms, which can cause sudden blackouts absorbing signals entirely, particularly on polar paths
• •The specific path geometry, since a signal must strike the layer at a shallow enough angle, which creates a skip zone near the transmitter where nothing is heard while distant listeners receive well
• •Antenna and power, which matter far less than the state of the ionosphere, so a low-power station can be heard worldwide on a good day and a powerful one inaudible on a bad one

International broadcasting was the largest use, and the great state-funded services transmitted news and propaganda across borders on a scale that nothing else made possible, with the British, American, Soviet, Chinese and many other services running enormous transmitter sites and directing programming at specific regions. Jamming was the countermeasure, with noise transmitted on the same frequency, and it consumed substantial resources on both sides. Beyond broadcasting, the bands carried long-distance aviation and maritime communication, diplomatic traffic, military networks and the entire international telephone service before satellites and cables took over. Amateur operators were allocated segments and have used them since the 1920s, when the authorities gave away the short wavelengths in the belief that they were useless, and amateurs promptly demonstrated transatlantic contact, which is one of the more consequential misjudgements in spectrum history. Numbers stations, transmitting sequences of spoken digits and generally understood to be one-way intelligence communications, have operated for decades and several still do.

Many broadcasters have closed their transmissions and moved to the internet, which reaches most audiences more cheaply and with better quality, so the bands are quieter than they were. The reasons for what remains are specific. The method requires no infrastructure between the two ends, so it works when networks are down, in disasters, at sea, in polar regions and in conflict zones, which keeps it in emergency and military use and is why amateur radio operators are integrated into disaster response in many countries. It reaches into places where the internet is filtered, and broadcasters maintain transmissions to such regions for that reason. It cannot be switched off by a service provider. The listening hobby continues, with interest in the unpredictability itself, since which distant station appears depends on conditions that change hourly and cannot be arranged. And digital modes have transformed amateur use, with schemes designed to decode signals far below the noise floor allowing intercontinental contacts with a few watts and a wire antenna.

Solar radiation strips electrons from the upper atmosphere, and a signal at the right frequency is bent back to the ground by that charged layer, landing thousands of kilometres away. Daylight and the eleven-year solar cycle change which frequencies work, so operators go higher by day and lower at night. Amateurs were given these bands because officials thought them worthless. The method persists because it needs no infrastructure between the two ends.