How signals travel

Why a signal reaches across town, the world, or nowhere at all.

Ground wave and surface waveLow-frequency signals that follow the curve of the Earth.Line of sight and the radio horizonWhy VHF and UHF reach about as far as you can see, and a little further.The ionosphereThe D, E and F layers: what they are and what they do to radio waves.Skywave, hops and the skip zoneHow HF signals return to Earth hundreds or thousands of miles away.MUF, LUF and the best frequencyThe highest and lowest usable frequencies for a path.The solar cycle and propagation indicesSunspots, solar flux, and the A and K indices.Grey line and long pathDawn-and-dusk enhancement, and signals that go the long way around.Sporadic EShort-lived patches of ionization that open VHF over long distances.Tropospheric ducting and scatterWeather that bends VHF and UHF well past the horizon.Meteor scatterBouncing signals off ionized meteor trails.Aurora propagationReflecting VHF signals from the northern lights.Near-vertical incidence skywaveSteep-angle HF for reliable regional contacts.Space weatherFlares, CMEs and storms, and how they help or wreck HF.