Anyone who has listened to AM radio after dark has noticed something strange: stations from far away come in clearly, while local stations they rely on during the day may fade or become crowded with interference. This is not a fault in your radio. It is a predictable consequence of how radio waves interact with the atmosphere.
Ground Wave vs Sky Wave
During the day, most AM signals travel primarily as ground waves — they follow the curvature of the earth and weaken relatively quickly with distance. A 50-kilowatt AM station might reliably serve a radius of 100 to 200 kilometres using ground wave propagation. Beyond that, signal strength drops below usable levels for most listeners.
At night, a separate mode of propagation becomes dominant: the sky wave. Radio waves bounce off a layer of the upper atmosphere called the ionosphere and return to earth at great distances from the transmitter — sometimes hundreds or thousands of kilometres away.
The Ionosphere and Its Day-Night Cycle
The ionosphere is not a fixed mirror. It is a region of ionized gas created largely by solar radiation, which means it changes dramatically between day and night. During daylight hours, a lower layer called the D layer absorbs medium-wave signals, preventing sky wave propagation. After sunset, the D layer weakens and effectively disappears, allowing signals to reflect off the higher F layer and travel enormous distances.
What This Means for Listeners
The practical result is that AM radio is a very different medium after dark. Clear-channel stations in the US — those with exclusive or dominant use of their frequency — are sometimes heard across an entire continent at night. European listeners on medium wave can pick up stations from North Africa, the Middle East, and eastern Europe after sunset.
It also creates problems. Stations on the same frequency in different countries or distant regions can interfere with each other at night, turning what was a clean signal into a garbled mess of overlapping broadcasts.
FM and DAB Are Different
FM signals operate at higher frequencies and do not benefit from sky wave propagation in the same way. They are largely line-of-sight, which is why FM coverage maps are dominated by terrain rather than time of day. DAB operates in a similar way. This predictability is part of why FM and DAB replaced AM for domestic broadcasting in most countries — the coverage area stays consistent around the clock.
Why Engineers Still Study Propagation
Propagation science is not just history. It matters for amateur radio operators who use ionospheric conditions to communicate internationally. It matters for broadcasters using shortwave to serve international audiences. And it matters for anyone planning a new AM station who needs to model their night-time interference footprint before a licence is granted.