HF propagation

Shortwave travels far because the ionosphere throws it back. That sounds simple and has consequences you only believe once you have heard them: the next town is sometimes unreachable while Japan comes in cleanly.

This page covers the four effects you actually get to hear in our simulations - and the reason Morse has the advantage.

The skip zone: near is harder than far

A signal leaves the antenna two ways. The ground wave creeps along the surface and is used up after a few dozen kilometres. The sky wave goes up, is refracted in the ionosphere and comes back down hundreds or thousands of kilometres away.

Between them lies a ring where neither arrives: the skip zone. Anyone sitting there hears nothing - although they are closer than the station copying you perfectly.

The skip zone grows with frequency. On 10 metres it can be a thousand kilometres; on 80 metres at night it nearly vanishes. So "I cannot hear you" on HF says nothing about distance.

Sporadic E: bands open without warning

At around 100 kilometres, clouds of strongly ionised air form irregularly - the E_s layer. It reflects frequencies that otherwise pass straight into space, opening 10 and 6 metres for minutes to hours over roughly 500 to 2000 kilometres.

The striking part: sporadic E does not follow the solar cycle. It occurs even at sunspot minimum, concentrated in early summer with a second, weaker season around the turn of the year. If you suddenly hear Italy on 10 metres in May, that is it.

Signals via sporadic E are typically loud and stable - unlike the fluttering aurora path below.

Aurora: the signal turns rough

During magnetic storms - a high K index in the space weather - the ionosphere over the polar regions is stirred up. Signals that must pass through arrive altered: the tone acquires a rough, hissing flutter, as if someone were tapping it rapidly with a finger.

For voice that is fatal - syllables smear. For Morse it is survivable: dot and dash differ in length, and length stands up to flutter better than timbre. That is why CW is often the only mode still carrying on auroral paths.

You can play exactly this flutter in the band conditions tool - it sits inside the fading and noise of the harder levels.

Why Morse gets through: bandwidth

This is the actual argument, and it can be checked with arithmetic. Noise spreads evenly across bandwidth: receive half the bandwidth and you receive half the noise - but all of the wanted signal, provided it fits.

A voice signal needs about 2500 Hz. A Morse signal at ordinary speed makes do with 100 Hz. The ratio in decibels:

14 dB is a factor of 25 in power. Put differently: a CW signal of 4 watts still arrives where voice would need 100 - purely from the narrower bandwidth, with no better antenna and no better receiver.

Then there is the part that cannot be calculated: the decoder is a human being. Ear and brain still pull a steady tone out of noise where an instrument finds nothing. That is why CW has no fixed sensitivity threshold in decibels - it depends on the operator, the speed and the day. Anyone quoting such a figure is quoting an average over people.

Machine modes go further by integrating over time: WSPR is decoded down to −31 dB in 2500 Hz according to the WSJT-X documentation. In exchange it transmits a beacon in two minutes, not a conversation. Morse is the compromise that does both: nearly as sensitive, and still a conversation.

And the grey line

The fifth effect has its own page because it needs a map: the grey line - the travelling twilight zone in which 80 and 40 metres carry over distances otherwise out of reach.