Propagation · Entry 01.2
Skywave
Refraction off the ionosphere, and why it turns a local transmitter into a long-distance one after dark.
Refraction off the ionosphere turns a local transmitter into a long-distance one after dark — and the effect is entirely predictable once you understand the layer structure.
What the ionosphere actually does
Above roughly 60 km, solar ultraviolet and X-ray radiation ionises the atmosphere into a layered plasma. The D layer sits lowest, between about 60 and 90 km, and absorbs medium-wave energy during the day — effectively blocking any skywave from reaching the higher layers and returning to earth. Above it, the E layer (roughly 90–140 km) and the F layer (140 km and above) are better reflectors. At night, without solar input, the D layer collapses. Medium-wave signals that were being quietly swallowed in daylight can now reach the F layer and refract back to the ground hundreds, sometimes thousands, of kilometres away.
| What | The consequence |
|---|---|
| D layer | 60–90 km; exists only in daylight; absorbs medium-wave skywave |
| E layer | 90–140 km; partially reflective, especially at night |
| F layer | 140 km and above; principal medium-wave reflector at night; merges into F1/F2 sub-layers by day |
The word "refract" is more precise than "reflect." The signal bends progressively as it enters a region of increasing electron density, and if the density is high enough relative to the signal frequency, the wave curves back before ever reaching a hard boundary. The critical frequency above which a signal punches straight through and escapes into space rises and falls with the sun; for medium-wave AM, the geometry is almost always favourable at night, which is why the effect is so reliable and so commercially consequential.
Edward Appleton demonstrated the existence of the reflecting layer experimentally in 1924–25, working with the BBC's Bournemouth transmitter, and took the 1947 Nobel Prize in Physics for the work — one of radio engineering's cleanest links between ionospheric physics and practical consequence. Oliver Heaviside and Arthur Kennelly had independently predicted the layer theoretically more than two decades earlier.
Chronology of ionospheric theory
- 1902Heaviside and Kennelly independently predict the conducting layer theoretically
- 1924–25Appleton and Barnett confirm the layer by measuring skywave arrival times using BBC transmissions
- 1947Appleton awarded the Nobel Prize in Physics for this work
The operating consequences
A medium-wave transmitter running daytime service on, say, 1 MHz is a local instrument: its ground wave reaches perhaps 100–200 km over average soil. At sunset, the D layer thins and dies. Within an hour the same transmitter is launching skywaves that skip a thousand kilometres and land with a field strength powerful enough to be received clearly — which is exactly what WLW Cincinnati found with its 500 kW clear-channel operation: nights became continental in reach.
The geometry of the skip zone matters. There is a region close to the transmitter, beyond the reliable ground-wave coverage but inside the point where skywave first returns to earth, where reception is poor from both mechanisms. The skip distance depends on the transmitter frequency, the radiation angle above horizontal, and the height of the reflecting layer — all of which vary, which is why the skip zone moves through the evening and across the seasons. A lower radiation angle produces a longer skip; antenna height and the electrical characteristics of the ground beneath the tower both influence this.
The same physics that creates long-range reception creates interference. A station that has been inaudible 800 km away all day becomes its own strongest competing signal at night — arriving via skywave at exactly the frequency a local station uses. This is the reason the Federal Communications Commission and its counterpart regulators under the International Telecommunication Union's frequency-coordination agreements impose separate night-time power limits and directional antenna requirements on medium-wave stations: without them, a continent's AM band would dissolve into chaos after dark. The licensed-two-ways principle — day power and night power as separate authorised quantities — is a direct administrative response to skywave physics.
Fading is the other operating reality. The ionosphere is not a smooth mirror; it is a turbulent, variable plasma. When a skywave arrives via two slightly different paths — different reflection heights, or a ground-wave and skywave arriving simultaneously — selective fading results: the two paths interfere constructively at some frequencies within the channel and destructively at others, which is why a distant AM station heard at night can sound clean one moment and distorted the next without any change in signal level. The envelope of amplitude modulation is defenceless against this.
Night skywave is, in the end, a consequence of ionisation physics: the sun stops supplying the ionisation that makes the D layer absorb, the D layer ceases to exist, and the frequencies it was blocking are free to reach higher, more stable reflecting layers and travel continent-scale distances. The transmitter has not changed; the medium through which it is radiating has changed, every single day, at sunset.