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Autumn Propagation


Steve White G3ZVW looks at what the HF bands can be expected to be like this autumn.



Steve White G3ZVW looks at what the HF bands can be expected to be like this autumn.


Around the period of the equinoxes – both spring and autumn – the High Frequency (HF) bands are usually in their best shape of the year. That will be as true this year as it is any year but we are now approaching a Solar Minimum so how are they actually likely to be performing this autumn?


Solar Cycle

Solar activity and sunspot numbers are the drivers of the Solar Flux. The greater the activity, the higher the number and the better the propagation is likely to be on the upper HF bands.

On average a solar cycle lasts 11 years, during which time the highest frequency on which long-distance HF propagation can be expected to take place with any degree of reliability will go up and then down. The ‘up’ part of the cycle is usually relatively fast (a few years), while the ‘down’ part is relatively slow (several years).

We are now approaching the end of Solar Cycle 24. Scientists tell us it was the weakest Cycle in 100 years so it was certainly an inauspicious one. At the time of writing the Sun had been ‘spotless’ for about 50% of the days of 2018, Fig. 1. By the end of the year I expect the percentage to be even higher. The graph, Fig. 2, shows the Sunspot Numbers over the past 18 years. On the left of the illustration you can see the peak of Solar Cycle 23. Cycle 24 started in 2008 and HF conditions were most recently at their peak around 2012-2014 but the peak was nowhere near as big as it had been in previous cycles so the amount to DX worked was less. While Solar Cycle 24 certainly had its moments, those of us who have experienced previous cycles were not impressed with it. Now we are in the doldrums and, unfortunately, I think we are going to be there for at least another two years, so let nobody hold their breath in anticipation of a sudden and sustained improvement.

The general pattern of sunspots is that, as a Cycle progresses, they occur closer and closer to the Sun’s equator, although very rarely on it. Recent sunspots have certainly been near the Sun’s equator. When Cycle 25 begins it is likely to be with a small sunspot group at a latitude of about 30°.


Seasonal Cycle

The Earth is tilted on its axis. When it’s mid-winter in the Northern hemisphere the North Pole is tilted away from the Sun by 23.5°. This means less energy from the Sun is received by the Northern hemisphere, resulting in shorter hours of daylight and lower temperatures. The opposite conditions exist in mid-summer. The North Pole is tilted towards the Sun by 23.5° so daylight hours are longer and temperatures are higher.

Unlike the magnetic field of a bar magnet, Earth’s magnetic field is a complex shape but with the poles close to the geographic North and South Poles it is tilted by a similar amount. It is the coupling between Earth’s magnetic field and the Sun that leads to more favourable HF propagation in the periods when the axis is neither tilted towards or away from the Sun – spring and autumn.


Daily Cycle

The basic daily pattern is that after it gets light in the morning, signals on the upper HF bands will start to propagate. Distant stations to the East will be heard first because it will have been daylight to the East for longer so ionisation (and from it, radio propagation) will have had time to build up. Later in the day ionisation to the East will start to drop because the Sun will be at an increasingly lower angle above the horizon. While this is taking place, ionisation to the West will improve because the Sun will be at an increasingly higher angle above the horizon. After it gets dark, long-distance propagation on the upper HF bands can definitely be expected to take a dive. At this point in the Solar Cycle the dive can even be before it gets dark locally.


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Day-to-Day Changes

There are always short-term changes to the Sun’s output so one day’s propagation is never going to be the same as the previous or the next. Coronal Holes are commonplace and the passage of one across the surface of the Sun is always going to lead to changes in the Solar Wind, which in turn couples to Earth’s magnetic field and leads to changes in HF propagation.


This Autumn

So, what can we expect from propagation on the HF bands this autumn? The first thing I need to say is that we should not expect great things on the upper bands. The Solar Flux just won’t be high enough for the F-layer of the ionosphere – the layer that results in the longest distance propagation – to be sufficiently ionised to support refraction.

Starting at the upper end of the HF bands, I expect 28MHz will be very quiet indeed. If there is any activity it will be during daylight hours – and it will need to be daylight along the entire length of the path that the signals take. At times there could be many days in a row when there is nothing at all to be heard on the band.

Moving down to 21MHz, there should be openings but they may be short. As regards long distance (beyond Europe) working, paths to South America and Africa are likely to be the favoured ones. Signals will often be weak so if you hear faint stations chatting in Spanish during the evening, they are likely to be in South America. You shouldn’t expect any activity overnight.

14MHz will be the carrier of most DX. Some paths might remain open after it gets dark but not every day. Once it closes, even this band – widely regarded as the most reliable for long distance communication – will remain closed until daylight the next day.

As regards long distance working, 10MHz could be quite an interesting band to operate on, especially at night. Remember, though, that because the band is only 50kHz wide, we are restricted to the use of narrowband modes such as Morse and RTTY (no telephony or similar modes).

7MHz should be a good band for DX but only at night. Paths to the East should be open around sunset, making DX contacts to the Far East possible. Paths to the West –North America, for example – should be open in the early hours of the morning. Although paths to the West are likely to peak shortly before sunrise, they might remain open for a while after it gets light. During daylight hours the Critical Frequency is likely to be below 7MHz so expect short-skip paths to be difficult. During the time that it isn’t possible to work stations around the UK, longer distance single-hop refractions to continental Europe might well remain good.

During daylight hours the Critical Frequency is likely to be above 3.5MHz so this band should be good for contacts up to a few hundred miles. At night the band will open for longer distance contacts. Multiple-hop refractions will become possible so the opportunities for DX working should be good at times.

Irrespective of the time of year, 1.8MHz will be closed during daylight hours because of absorption by the D Region of the ionosphere. This being so, you shouldn’t expect to hear anything beyond the range of Ground Wave propagation (tens of miles). As darkness falls and the D Region dissipates, the band will undoubtedly open up for skywave contacts. Single-hop refractions via the E Layer are likely to yield stations up to about 1300 miles away (so all of Europe). At times, longer distance contacts should be possible overnight.



To summarise, while the HF bands are likely to be at their best during autumn, this year the best is unlikely to be good.

If you want to make the most of the meagre conditions we are likely to experience on many of the HF bands, narrow-band modes are likely to yield better results. This is because they offer a better signal-to-noise ratio than modes that require greater bandwidth. Traditionally this would mean using Morse rather than telephony but modern data modes are easy to get going on and many can be detected by a computer when a signal is too weak for a human operator to hear. PSK is a prime example of such a mode but Joe Taylor K1JT has over the years developed a suite of software offering a number of data modes optimised for different propagation types and paths. His recent development, FT8, is currently all the rage.


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