Last Updated on November 23, 2024 by John Berry
Making ionospheric predictions about whether HF ionospheric paths (or circuits) will work is straightforward. It involves running a program encompassing the method outlined in my page Background to ionospheric predictions.
Any engineer building a program for HF prediction will embed existing code for either Recommendation ITU-R P.533 or VOACAP. Possibly the best public program is VOACAP Online at https://www.voacap.com/hf/. This program is mature but perhaps includes many functions that are perhaps good for HF aficionados, but for most amateurs, are unnecessary. I’ll describe what I consider the two core uses of this program. These are two core tools from the old days of Rec. ITU-R P.533/VOACAP.
Circuit reliability
The essence of the question that this tool answers is, with what reliability can I work into <country> today. For reliability read chance of success.
You need the stations to be put on the map. VOACAP Online allows this by dragging the red and blue pins representing the TX and RX stations. These stations must then be configured using the top four boxes on the top right hand of the page. In this example, I’ve put my stations in the UK and Brazil.
At the top (right) is the transmission mode. In my case I’ve selected SSB to display SSB v. I’ve highlighted the significance of this on other pages. This effectively sets the system value for the equipment. The receiver threshold is particularly relevant.
Then there’s the power at the antenna – 100W in my example.
The Antennas button allows the station antennas to be stated. There are a host of basic antennas available from a dropdown menu. Just choose those that best defines the antenna(s) in use at each end (or likely to be in use).
Finally, there’s Settings. This is where three key other characteristics are defined: environmental noise at the receiver, Smooth Sunspot Number (though you can ask the program to use SSN from space weather predictions), and minimum take off angle.
Once defined, the tool is ready to make a prediction. Simply click the green REL | SDBW | SNR button at the bottom left.
Graphical output
After a few seconds, three graphs are displayed. The one you’re interested in is Circuit Reliability (%) on the left. This is simple to understand as the chance of success.
This graphical report allows you to read off the best frequency for this path at various times of day. So, in the UK-Brazil example, you will enjoy a 60% chance between 10:00 and 12:00 UTC on frequencies between 24MHz and 28MHz. If of course you only have 18MHz, your circuit reliability will drop to 40% between 06:00 and 08:00 UTC, though you will have another chance later in the day.
To probe another path, simply drag the pins and repeat, or vary parameters, such as a different antenna, or higher power.
And just to see the dramatic effect of dropping the receiver threshold by using FT8 as the transmission mode, go to the top right and select FT8. You’ll see then that there is always a frequency/time-of-day pair that will give you a very high chance of success!
The upper (black) line graph in the above figure is the MUF. The lower blue line is the Lowest Usable Frequency, the LUF.
MUF and FOT
The report on the Maximum Usable Frequency (MUF) and Fréquence Optimale de Travail (FOT) is the second of the useful outputs.
On the online tool, click the green box Best FREQ at the bottom left. This produces a table.
The ITU-R defines the MUF (or Basic MUF) as the highest frequency by which a radio wave can propagate between given stations, on a specified occasion, by ionospheric refraction alone. The use of the word ‘usable’ here implies longer term, rather than momentary.
And in HF communications, generally the higher the frequency, the better.
The MUF varies according to sunspot activity and sun radiation and can rise into the lower VHF frequencies on some days for some paths at some times in the sunspot cycle.
The MUF tells you the highest frequency on which you might manage to work that path. Typically, you would want to operate at a frequency not higher than say 85% of the MUF to be more sure of success – or choose the FOT.
The FOT is defined by the ITU-R as the lower decile of the daily values of the MUF at a given time over a given period. It’s the frequency that is exceeded by the MUF during 90% of the period (usually a month).
The significance of the FOT is suggested in the name – the optimum working frequency for that path at that time of day over a good number of days.
Finally
The MUF and FOT peak in the day. It’s worth looking at the variability of both and selecting the amateur band which is just below the FOT – so 21MHz at around mid-day to 13:00 UTC on the example above.
The predictions given by VOACAP Online are just that. They are based on the normal Sun. But the Sun is sometimes quirky, so sometimes even the best predictions become inaccurate.
All radio hams should at least be aware of VOACAP Online. It’s quite a brilliant execution of the old science. It’s a great program for probing and learning by varying the parameters and station locations.