HF propagation for pictures

The one mechanism that matters

Above the atmosphere sits the ionosphere: layers of gas ionised by the sun. Radio waves entering it are bent, and if they are bent far enough they come back down — hundreds or thousands of kilometres away.

How far a wave bends depends on its frequency and how strongly ionised the layers are. Low frequencies bend easily. High frequencies bend less, and above a threshold they carry straight on into space and are gone.

That threshold is the maximum usable frequency, and everything else follows from it. The sun charges the ionosphere; the ionosphere sets the MUF; the MUF decides which bands work.

Why bands have a time of day

Sunlight ionises, so the MUF rises through the morning, peaks in the afternoon and falls after dark.

• Daytime: high MUF, so the higher bands — 20, 15, 10 metres — are open and carry long distances.
• Night: MUF falls below them and they close. The lower bands — 40, 80 — come into their own, because a layer that absorbed them during the day has faded.

This is why 20 metres became the SSTV gathering point. It sits in the sweet spot: open for most of the daylight hours, reaching worldwide, and not so dependent on the solar cycle that it disappears for years at a time.

A working table

The skip zone

This explains more confusing reception reports than anything else, so it is worth understanding properly.

Your signal leaves in two forms. The ground wave hugs the earth and dies out within tens of kilometres. The sky wave goes up, refracts, and returns hundreds or thousands of kilometres away.

Between the two lies a ring reached by neither — the skip zone. A station inside it hears nothing from you while one much further away copies you perfectly. Nothing is wrong with your station; the geometry simply excluded them.

The skip distance shortens as frequency falls, which is the practical reason to drop to 40 metres for a contact a couple of hundred kilometres away that 20 metres cannot reach.

The solar cycle

Solar activity rises and falls over roughly eleven years, and the higher bands live and die with it.

• Near maximum: 10 and 15 metres open regularly, and worldwide contacts on modest power become routine.
• Near minimum: those bands are quiet for long stretches; 20 metres and below carry the traffic.
• Solar flares can also cause sudden ionospheric disturbances that shut the lower HF bands down for minutes to hours. If everything vanishes at once, it is not your radio.

Greyline

At dawn and dusk there is a band of twilight sweeping around the earth. Along it, absorption in the lower ionosphere has decayed while the reflecting layers above remain charged — a combination that produces exceptional long-distance paths on the lower bands for perhaps half an hour.

It is a small window and worth planning for. A picture that will not go anywhere at midnight may cross a continent at sunrise.

What this means for pictures specifically

SSTV cares about propagation more than a voice contact does, because it needs the path to hold up for the whole transmission. A fade that costs you a word in conversation costs you a band across the middle of a picture.

• Shorter modes on marginal paths. Robot 36 needs the path to survive 36 seconds; Scottie DX needs 269. On an unstable band the shorter mode is not a compromise, it is the only one that will arrive intact.
• Analogue degrades, digital fails. When conditions are unreliable, analogue SSTV gets something through where an error-corrected mode gets nothing — the trade set out in analogue versus digital.
• Judge by reports, not by silence. A quiet frequency usually means nobody is transmitting, not that nobody can hear you.