HF propagation basics: why a band opens and closes

A radio signal on 20 metres leaves your antenna, travels upward, and comes back down eight hundred miles away. Nothing about that is obvious, and understanding why is the difference between calling into a dead band and knowing which band is alive.

You do not need the physics in detail. What you need is a mental model with four moving parts: the sun ionises layers of the upper atmosphere, ionisation bends higher frequencies less than lower ones, a lower layer absorbs energy during daylight, and all of it changes through the day and through an eleven year cycle.

This page builds that model and then converts it into which band to try at what hour.

The layers

The D layer, 30 to 60 miles up

Forms in daylight and disappears within an hour or two after sunset. It does not refract signals, it absorbs them, and it absorbs low frequencies far more strongly than high ones. This single fact explains why 80 metres is useless at noon and excellent at midnight.

The E layer, about 60 miles up

Refracts signals over shorter distances. Its interesting behaviour is sporadic E, patches of intense ionisation that appear mostly in the warmer months and produce spectacular short-lived openings on 10 and 6 metres over several hundred to a thousand miles.

The F layer, 150 to 300 miles up

The workhorse. It splits into F1 and F2 during the day and merges at night. F2 is what carries long distance HF, and its height means a single hop can cover 2,000 miles or more.

Multiple hops

A signal refracted down can reflect off the ground or sea and go up again. Worldwide contacts are usually several hops, and each ground reflection costs energy, which is why sea paths outperform land paths noticeably.

Maximum usable frequency

Ionisation bends a signal by an amount that depends on frequency. Below a certain frequency the layer bends the signal enough to return it to earth. Above that frequency it passes straight through into space. That threshold, for a given path and moment, is the maximum usable frequency.

• MUF rises through the morning as the sun ionises the F layer, peaks in the early afternoon, and falls after sunset.
• MUF is higher toward the equator and higher during solar maximum.
• MUF depends on the path, not just on your location, because the signal refracts at the midpoint. A path to the east may be open while a path to the west is not.
• Operating just below the MUF gives the best results, because absorption is lower at higher frequencies. This is why the highest open band is usually the best band.
• The lowest usable frequency is set by D layer absorption, which is why daytime operation on 80 metres is limited to a few hundred miles.

Which band, when

The solar cycle

Solar activity rises and falls over roughly eleven years, and it changes what the higher bands can do more than anything else. At solar maximum, 10 and 15 metres are open worldwide most days and a 100 W station with a wire works everything. At solar minimum those bands are largely closed and the action moves to 40, 30 and 20.

• Solar flux index is the practical daily number. Above about 150 the high bands are lively, below about 80 they mostly are not.
• Sunspot number tracks the same thing over longer periods and is what the eleven year cycle is drawn in.
• The A and K indices measure geomagnetic disturbance, and high values are bad news. A K index of 5 or more means an active geomagnetic storm, degraded paths at high latitudes and often an aurora.
• Solar flares cause sudden ionospheric disturbances that black out HF on the daylight side of the earth for minutes to hours. A band that dies abruptly and completely, everywhere, is usually this.
• Nothing about the cycle changes what you should build. Wire antennas and 100 W work in every year. What changes is which band you find the contacts on.

Grey line and other useful effects

Grey line propagation

Along the terminator between day and night, the D layer has decayed while the F layer is still ionised. For a window of twenty to forty minutes at your dawn and dusk, low band signals travel remarkably well along that line. It is the classic way to work distant stations on 80 and 40.

Long path

A signal can arrive by the long way round the earth. If a station is peaking from an unexpected direction, that is often what is happening, and it is common on 20 metres at dawn and dusk.

Sporadic E

Intense patches of E layer ionisation, mostly in the warmer months, producing sudden strong openings on 10 and 6 metres over 500 to 1,500 miles. They last minutes to hours and are the reason 6 metres is called the magic band.

Backscatter

Signals scattered off the ground or off aurora, arriving weak, distorted and from the wrong direction. It fills in the skip zone that would otherwise be silent.

The skip zone

The ring between where ground wave stops and where the first hop lands, in which you hear nothing. It is why you can work someone a thousand miles away and not someone eighty miles away on the same band.

Using this in practice

1. Look at the daily solar numbers before you sit down. It takes ten seconds and tells you whether to expect anything on the high bands.
2. Start at the highest band with activity and work down until you find contacts.
3. Listen before concluding a band is dead. An empty band and a closed band sound the same and are not.
4. Use the digital modes as a propagation indicator. Automatic reporting networks show you which paths are open right now, and they are a far better guide than any prediction.
5. Operate at dawn and dusk for the low bands. The grey line window is short and productive.
6. Keep notes. After a year, your own log tells you more about your station and your location than any general table can.

Gear that helps you see propagation

Seeing the band

A scope and a wideband receiver turn tuning around into looking.

Icom

Icom IC-7300

A real-time spectrum scope shows activity across the whole band at a glance, which is the fastest way to tell an empty band from a closed one.

Check price$1,255.74

Nooelec

Nooelec NESDR Smart RTL-SDR bundle

A cheap wideband receiver that shows several megahertz at once, useful for watching a band come alive.

Check price$54.95

Yaesu

Yaesu FT-891

A capable 100 W radio with an excellent receiver, for stations that want the performance without the scope.

Check price$769.95

ARRL

ARRL Antenna Book, four-volume set

The reference on how height and antenna type set your takeoff angle, which decides which hop you land in.

Check price$69.95

ARRL

ARRL Handbook for Radio Communications

The general reference, with the propagation chapters that go a level deeper than this page.

Check price$30.99