Also known as: sporadic E, Es, E-skip
Sporadic E (abbreviated Es or “E-skip”) is an intermittent propagation mode in which small, intensely ionised patches in the ionosphere’s E layer reflect VHF radio waves back to earth, opening long-distance paths on bands that are normally line-of-sight only.1 Unlike ordinary ionospheric propagation, which fades gradually with the sun, Es appears suddenly, drifts, and vanishes — often within minutes.
How it works
Around 90–120 km up, thin clouds of unusually dense ionisation form and drift through the E region. When the electron density in a patch is high enough, it can reflect frequencies far above the normal E-layer limit — routinely into the low VHF range and, in strong events, past 100 MHz. Because the reflecting patch is small and localised, the path it supports is narrow and shifts as the cloud moves, so a station that is booming in one minute can drop out the next.
The physics of the patches is still not fully settled, but the leading explanation is wind shear: layers of the neutral atmosphere moving at different speeds concentrate long-lived metallic ions (from meteor ablation) into thin, dense sheets. Key features:
- Seasonality. Es peaks sharply in late spring and summer, with a smaller winter peak, and is far less tied to the solar cycle than F-layer skip.
- Frequency ceiling. The higher the patch’s density, the higher the frequency it reflects; openings climb from low VHF upward as an event intensifies.
- Short single hops. A typical Es hop covers roughly 800–2,200 km; occasional multi-hop paths reach much farther.
In practice
Sporadic E is famous among VHF operators as the source of summer “band openings” on the 6-metre amateur band, and it is why FM broadcast listeners and analog-TV DXers once logged stations a thousand kilometres away for a few minutes at a time. Its close cousin meteor scatter shares the same ion supply but works on individual trails rather than settled layers.
Relevance to SDR
Because Es lands in the low VHF spectrum, it is directly relevant to wideband SDR monitoring: during an event an RTL-SDR or Airspy can capture distant FM, aircraft-band, or low-VHF land-mobile signals from far outside the normal footprint. For a trunking scanner such as GopherTrunk, Es is an occasional interference source rather than a target — a distant co-channel transmitter briefly ducted in by an E-cloud can disrupt decoding of a local system. GopherTrunk itself models none of this; it decodes whatever reaches the front end, and Es simply explains the rare appearance of faraway VHF signals on an otherwise line-of-sight band.
Sources
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Sporadic E propagation — Wikipedia, on transient dense E-layer ionisation, wind-shear formation, seasonality, and VHF reflection. ↩