HF Propagation Basics: Ionosphere and MUF

On the HF bands you are not communicating along the ground. You are bouncing a signal off a layer of ionised gas a few hundred kilometres up, and whether that works depends on the sun, the time of day and the frequency you chose.

Understanding the mechanism turns HF operating from guesswork into something closer to a schedule. The bands do not open and close at random. They follow the geometry of the sunlit ionosphere.

The layers

Solar ultraviolet radiation ionises the upper atmosphere into layers at different altitudes. The D layer forms lowest, around 60 to 90 kilometres, and exists only in daylight. The E layer sits around 100 to 120 kilometres. The F layer, which splits into F1 and F2 in daylight, occupies roughly 200 to 400 kilometres and is the layer that supports long-distance contacts.

Why the low bands are better at night

The D layer is the reason. In daylight it absorbs signals at the lower frequencies, which is why 80 and 160 metres are noisy and unproductive during the day. When the sun sets, the D layer recombines and disappears within an hour or so, and the low bands open to distance.

At the same time, the F layer thins out. The maximum usable frequency — the highest frequency that will still be refracted back to earth — falls through the evening. The high bands close as the low bands open, which is why the best operating time for any band depends on the hour.

MUF and LUF

The maximum usable frequency is the highest frequency that will refract from the ionosphere to a given destination. Below it, signals get through; above it, they pass straight out into space. The lowest usable frequency is set by absorption in the D layer and by atmospheric noise.

Between those two limits is the window you can work in. The window moves constantly: it widens and rises through the morning, peaks in the early afternoon, and falls away through the evening. Skilled operators plan around it rather than discovering it.

Solar activity sets the ceiling

The sunspot cycle raises and lowers the whole system. At solar maximum the F2 layer is more heavily ionised, the MUF is higher, and the high bands support worldwide contacts with modest antennas. At solar minimum the same bands may be usable only during daylight and only to a few destinations.

The solar flux index gives a daily indication of how ionised the F2 layer is, and the K index describes how disturbed the geomagnetic field is. A high K index means a disturbed ionosphere, which usually means degraded HF conditions — particularly at high latitudes.