Propagation: ground wave, the ionosphere and VHF

Propagation is how signals travel from your antenna to someone else's. In this lesson: ground, space and sky waves, the layers of the ionosphere and what changes them, and the special cases, sporadic E and multipath.

The ground wave follows the Earth's surface. Its range is limited, because the ground absorbs energy, and the loss increases as the frequency rises. The tropospheric, or space, wave travels roughly in a line of sight, and carries most V H F and U H F contacts. The sky wave travels up to the ionosphere and is refracted back to Earth, far away. The distance to where it first returns is the skip distance, and the gap between the end of the ground wave and the returning sky wave, where nothing is heard, is the skip zone.

A radio wave has an electric field and a magnetic field. The two fields are always at right angles to each other, and the direction of travel is at right angles to both. The polarisation of the wave is defined by the plane of its electric field. So a vertical antenna produces a vertically polarised wave.

The ionosphere is made of layers of gas ionised by the Sun's ultraviolet radiation and charged particles. The D layer, at about seventy kilometres, absorbs the lower frequencies during daylight and largely disappears at night. The E layer is at about one hundred and ten kilometres. Above it are the F one and F two layers, at about two hundred kilometres and three hundred kilometres and above. At night they combine into a single F layer. The F two layer gives the longest hops, about four thousand kilometres, and multiple hops can carry signals around the world.

Ionisation changes with the time of day, the season, and the roughly eleven year sunspot cycle. The sunspot number indicates solar activity, and more sunspots mean more ionisation and better H F conditions. The highest frequency that will be refracted back over a given path is the maximum usable frequency. The lowest frequency that gets through the D layer without too much absorption is the lowest usable frequency. If the lowest usable frequency rises above the maximum, no ionospheric propagation is possible on that path.

Fading happens when a signal arrives by more than one path and the paths change, so the parts add and cancel. The short path is the most direct route around the Earth, and the long path goes the opposite way round. Sporadic E is refraction from patches of intense ionisation in the E layer. It can suddenly open V H F and the upper H F bands, at twenty four and twenty eight megahertz, with single hops of up to about two thousand kilometres, and sometimes more than one hop. At V H F and above, multipath occurs when signals reflected off buildings or aircraft arrive with the direct signal.