HF radio propagation is the science and art of understanding how and when shortwave radio signals travel from a transmitter to a receiver. Beyond a short distance, shortwave radio signals travel by bounding off layers in the ionosphere. Often, there are several bounces. Here’s how to figure out how and when you can expect a signal to arrive at your location.
The ability of the ionosphere layers to bounce signals varies according to three cycles: daily, yearly and solar. The solar cycle is approximately eleven years. At its peak, solar activity cooks up the layers nicely and raises the useful frequencies for long distance communication. At its low, solar activity weakens, the layers do no reflect signals as well, and the useful frequencies are lowered. Over the course of a year, the planet’s solar orientation on its axis changes the amount of solar energy reaching each hemisphere. Shortwave radio listening conditions vary by season. Lastly, during our 24 hour day, the overhead position of the sun changes. In general, higher frequency paths work better during daylight, lower frequency paths work better at night.
There are many web sites that provide HF radio propagation information. Nearly all provide the basic status of sunspots, solar flux, A index and K index. The main source of information useful for understanding HF radio propagation is the Space Weather Prediction Center at NOAA. Many web sites also relay this information. Some also add an estimate of HF radio propagation conditions based on an interpretation. These are general estimates of overall HF radio propagation conditions across the planet.
Solar flux or sunspot number is a measure of the sun’s ability to enhance the various ionosphere layers. In general, the higher the solar flux, the better the HF radio propagation with higher useable frequencies. On the other hand, the A and K indices report the amount of geomagnetic activity caused by the sun. In general, the higher the A and K indices, the more HF radio propagation will degrate. This short article from the ARRL provides a good explanation. An online compendium of information useful for HF radio propagation is the NW7US web site.
HF Radio Propagation – Advanced Tools
But, perhaps the gold standard for propagation evaluation tools is the collection of programs from the Institute for Telecommunication Sciences at Boulder, Colorado. There are two main program: VOACAP and ICEPAC. VOACAP is a propagation analysis and prediction program. It was originally developed as the Voice of America Circuit Analysis Program in 1993 and released for Windows a few years later. You can read the history of VOACAP here.
In its latest incarnation, VOACAP is readily available as an online tool, shown in the picture above. It is easy to use. Just enter your location and the location of the transmitter you want to hear. In the above picture, I analyzed the HF radio propagation path between my home in Calgary, Alberta and Israel. VOACAP Online then drew a circular graph showing which frequencies and times of day provide the best probabilities of a connection. In my case, it is best for me to try to contact Israel during my mid-morning hours on the 20 meter ham band. The program says that I will have an 80-90% chance of a workable path.
The weakness of VOACAP is that it does not take geomagnetic disturbances into account. It assumes geomagnetic quiet. Geomagnetic disturbances have a particularly bad influence on signals traveling through the auroral zone and over the polar cap. In some parts of the world, many signal paths to areas of interest pass through these zones. But ITS has another program for that, called ICEPAC. It has the same roots at VOACAP, but has been modified to take better account of the effects of the auroral and polar zones on the ionosphere.
If you were to analyze the Calgary-Israel path (shown above) with a K=4 disturbed index, you would get less promising results with ICEPAC.
Anyway, these tools help you with the science of HF radio propagation. As for the art, you need to develop your own skills and judgments to get the most our of these tools. HF radio propagation is more complex than simple tools can handle, but they get you off to a good start. I would encourage you to play with VOACAP Online. Then, if you feel adventurous, you can install both VOACAP and ICEPAC on your own computer. They are free for you to download and enjoy.