- HF Frequency Planning

Communications planning is very important today if you want to get the best out of your HF link. Today there are many aids to put you on the right track. However, these aids need to be viewed in the light of what they can and cannot do. If you expect to achieve what was forecast in real time, you may be sadly disappointed, but understanding where they are coming from can make them a very useful tool indeed.

The history of HF prediction computer programs dates back to the 1960s and the development of the Ionospheric Communications Analysis and Prediction Program by a number of US government agencies. This program, written initially for main frame computers, is excellent for long range planning but not suitable for short term use, and is not very user friendly. Modelling can be very detailed taking into account antenna models for multiple frequency ranges, local site noise and ground, elevation angles, multipath distortion and solar activity levels.

IONCAP has since been improved on by the Voice of America in their VOACAP version but is still complicated to use.

Over the years a number of personal computer programs have been written to provide quick and easy predictions of Maximum Usable Frequencies (MUF) and band openings. These are much more friendly but do not provide as much detailed information. The amount of details varies from program to program. Some include antenna selection, real ground characteristics, multipath effects, LUF, etc., others do not. As with the more complex programs, their main use is in planning links and antenna types.

While the science of prediction has evolved considerably and has become a very useful tool, the science on which it is based, that of the Sun, is highly complex and still not fully understood today. Actual figures can vary quite considerably from advance predictions, as in Cycle 22 which, based on data from 1840 to 1983, was only expected to reach a maximum of just over 100 SSN but in fact peaked in November 1989 at 158.

To gain an appreciation of what is being attempted, it is necessary to have an understanding of the ionosphere and how it is affected by the sun.

When a radio wave enters the rarefied low pressure environment of the ionosphere, its direction of travel is altered, just as light is bent when it passes through water. How much it is bent depends on its frequency or wavelength. As you go up in frequency the less a wave is bent until it is no longer reflected back to earth.

To chart these changes would be straight forward were it not for the fact that the density of the ionosphere is constantly changing. At times of greater density, the amount of bending is increased, altering propagation conditions, so that higher frequencies can be reflected to earth. Such activity can be directly related to the degree of ionisation which is mainly caused by ultraviolet radiation from the sun, and visible in sunspot activity observed on the surface of the sun.

Short, sudden events seen on the surface of the sun may create disturbed propagation conditions lasting less than an hour. Greater disturbances may leave their effects for over a year. The number, size and geographical position of sunspots are also factors.

From our observation of the sun over the years, we have been able to discover a cyclical pattern of sunspot activity. The first complete solar cycle to be observed was in 1755 (Cycle 1). Cycle length is not easy to predict, as it can vary from 9 to 12.7 years. While cycle average is approximately 11.1 years, the most common cycle length is around 10.5 years with quite a number in the 11.5-12.5 year length.

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The arrival of solar maximum coincides with the reversal of the sun’s magnetic poles and is a good indication that solar maximum has arrived. Our current cycle, Cycle 23, commenced in October 1996 and is now believed to be at solar maximum as this event has already been observed. Not all cycles are equally active. Historically, there have been long periods of solar stability (the Maunder Minimum commencing in 1645 lasted 70 years), just as there have been highly active periods.

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Propagation predictions are usually based on estimates of sunspot numbers. Observations in the field from various sources are averaged out to give monthly figures. These monthly figures are then smoothed to give Smoothed Sunspot Numbers (SSN) for the month. SSNs represent the average of monthly values for 13 months, taken 6 months before and 6 months after the month in question. This is to remove the effects of short term changes. For the first 3 years of the cycle, predictions are less reliable but improve as the cycle becomes established.

Predictions based on averages, however, do not take into account the often violent nature of the sun. As a result , what is forecast may not eventuate due to solar disturbances. Generally speaking, the higher the sunspot number, the better the conditions. In practice, it has been found that the upper HF band is not usable when the SSN is below 100. Sudden increases in solar activity can make skywave communications possible at well above the predicted MUF or can disrupt communications entirely. ....to be continued

Solar Flares emitting high energy bursts of radiation from VHF to X ray frequencies and vast amounts of solar material occur mostly around solar maximum. If the earth and sun are in a favourable position, intense X ray radiation immediately increases RF absorption in the ionosphere which can cause a Sudden Ionospheric Disturbance (SID).

These holes are the birthplace of the solar wind. Another phenomenon, the Sudden Disappearing Filament (SDF), which also ejects plasma affecting propagation, is experienced mainly in the first half of a solar cycle.

When conditions are ripe, SIDs, Coronal Holes or SDFs can create ionopsheric or geomagnetic storms on earth. These storms can disrupt communications for several days with radio noise and interference, mostly at HF but VHF can also be affected. However, VHF conditions are often exceptionally good when HF is suffering from solar disturbances.

Computer prediction programs base their calculations on the Smoothed Sunspot Number (or Smoothed Solar Flux) together with the date (month/day) to determine the position of the sun and the condition of the ionosphere at critical points on the communications path. Geographic co-ordinates (lat/long) of transmitting and receiving stations determine the great circle radio path.

Usually, they perform well forecasting band openings and closings but do not take into account disturbed solar conditions and solar activity can be much higher or lower than actual peaks and troughs. Also it needs to be observed that such programs rely on updating information to give as close to real time as possible.

However, computer predictions can be improved to give a more accurate idea of current conditions when used in conjunction with the sunspot number or solar flux level for the period in question. Because sunspot and solar flux numbers can vary dramatically on a daily basis and the ionosphere is relatively slow to respond to changed conditions, it is best to take figures averaged over the previous 3 or 4 days. (to be continued)

Storm Warnings (larger image)

Solar Activity (larger image)

For those who do not have the advantage of computerised prediction for communications link planning or years of accumulated experience, the following observations based on practice may assist.

The most practical available frequencies for HF commercial operations, irrespective of the solar cycle, are between 4-16 MHz, usually 4, 6, 8, 10, 12 and 16 MHz. Other frequencies in this range are suitable but are often allocated for other purposes.

With these frequencies, choosing the appropriate bands, you can maximise your chances of reliable HF communications. If it is not possible to cover all these frequencies, the recommended minimum would be 6, 12 and 16 MHz.

Rule of thumb is: lower by night, higher by day. Day to day timing of maximum signal propagation on a given link can also vary by a few hours.