HFPred

by Steve Hunt


HFPred is an invaluable utility that allows you to predict the frequency and times from which you can receive short wave broadcasts from different parts of the globe.

It is possible to listen to radio stations from all around the world if you tune your radio to the Short Wave Bands. This is possible because signals in these bands don't travel directly from the transmitter to your radio; instead they travel upwards into the ionosphere where they are refracted (or bent), and then return to earth well beyond the horizon. In this way, the signals travel much greater distances than those in other radio bands.

You may have noticed that it is possible to hear different radio stations at different times of day. This is because the effectiveness with which any particular part of the ionosphere refracts signals is dependent on the angle it subtends to the sun, and the level of solar activity. In practice, this means that ionospheric conditions vary from hour to hour, from month to month (in a one-year cycle), and with the number of sunspots (an 11-year cycle).

At any point in time, the ionosphere is incapable of refracting signals above some maximum useable frequency (known as the MUF) at a particular angle of incidence. This means that the MUF between two points on the earth's surface is dependant on their position, the time of day, the sunspot activity, and the month of the year. For best reception, you need to listen to stations at as high a frequency as possible (but not above the MUF). It is therefore useful to be able to predict the MUF.

Frequency prediction programs have been around for many years, but to date have needed large mainframe computers or minicomputers on which to run. However the power of modern personal computers, such as RISC OS machines, now makes it feasible to run these prediction algorithms at home.

HFPred will calculate Maximum and Lowest Usable Frequencies (MUF & LUF) in two-hourly increments throughout the day, once it has been told the location of the communication sites, the sunspot number and the month of the year. In order to do this it will compute where in the ionosphere signals between the two stations will need to be refracted, and what the sun angle is at this point. It will then calculate what frequencies are capable of refraction given the sunspot number.

Using HFPred
Double-click on the !HFPred icon in the filer window; the program will be loaded, and an HF icon will be installed on the icon bar. Click Menu over this icon and the usual Info and Quit options are displayed. Click Select over the icon and the HF Frequency Prediction window will be opened.

Let's suppose that we live in New York (Lat 41N Long 74W) and are interested in listening to a radio station which is located in Hong Kong (Lat22N Long 114E). It is August, and we know that the sunspot number is currently 150 (the current sunspot number is published in a number of radio magazines). We want to know on what frequencies we are likely to hear the station at 1400Hrs GMT.

Firstly, we need to set the month to August. Click Select over the Up arrow to the right of the Month field (currently showing Jan) until the Month field shows Aug. Notice whilst you are doing this that HFPred recalculates for each new month.

Now set the Sunspots field to 150. To do this, click over the Sunspots field (currently showing 120) and edit the field in the normal manner. Note that Ctrl+U deletes the current entry.

We now need to enter the latitude and longitude of our home station (New York), and the distant station (Melbourne). Site locations can be entered in any one of three ways: we will try all three!

Click over the Home Lat field and edit the field to show 41. Press Return and the caret will move to the Home Lng field where you should enter -74 (note that Westerly longitudes and Southerly latitudes are shown as negative). Press Return again and the caret will move to the Distant Lat field. Enter 22 here and press Return, then enter 114 in the Distant Lng field. Finally, press Return again. Note that the red cross and the orange cross have moved on the map to the Home station and Distant station co-ordinates respectively.

HFPred has now calculated the MUF and LUF for the New York <> Hong Kong radio path for a day in the month of August, given a sunspot number of 150. These results are shown in the Predictions box in two-hour increments from 0000 GMT to 2200 GMT. The upper figure is the MUF in MHz and the lower figure is the LUF in MHz. HFPred has also calculated the Great Circle bearing of Hong Kong from New York (352 degrees) so that we know which way to point our antenna! We can see that a maximum useable frequency of 29MHz has been predicted for 1400 GMT, so the 11 metre short wave band (25.6MHz to 26.1MHz) would probably give good results.

Now let's try the second method of inputting Latitude and Longitude information; this time for a home station in Ireland and a distant station in Cyprus. Click Menu anywhere in the window, move the pointer right over the Home option, and click on Map. As you move the mouse pointer over the world map now, you will see that the Home Lat and Lng fields are being updated to the pointer position. Point at Ireland (Lat 54, Lng -7) and click Select. Now click Menu again, move over the Distant option and click on Map. Point at Cyprus (the eastmost island in the Mediterranean  Lat 34, Lng 32) and click Select. HFPred will now recalculate for the new path; as a check, the MUF at 0400 should be 26 and the bearing should be 110.

The third method is even easier. Click Menu, move right of the Home option, move right of the City option, and click on New York. Click Menu again, move right over the Distant option, move right of the City option and click on Hong Kong. We have now achieved the same result as we had for method 1.

To save the current predictions to disc or to output them to the printer, click Menu and move the pointer right over the Save option. The normal RISC OS Save as window will appear, allowing you to drag to a filer window or to a printer driver icon.

Of course, for pairs of stations which are almost diametrically opposed to one another, there is the option of communicating by the reverse (slightly longer) path around the world. HFPred will perform the appropriate calculations for this path if you click on the Long path button. Note, though, that for stations where the long path is not a sensible option, this option will be disabled (greyed out).

Further details
The map does not cover the whole world; its boundaries are 144W, 81N, 171E and 58S. If you want to specify sites outside this area, type coordinates directly into the writable Lat and Lng fields. The map markers will disappear, but calculations should still be valid.

HFPred will restrict Lat values to integers in the range -89 to +89 degrees, and Lng values to integers in the range -180 to +180 degrees. The Sunspots value must be an integer between 0 and 999.

Some positions of Home and Distant stations may cause a "divide by zero" error during the calculations. HFPred will warn of this. You should then move one or both stations slightly to avoid the error; this will not change the predicted frequency values substantially.

In accordance with normal Acorn style guidelines, clicking Adjust over the increment or decrement icons will have the inverse effect to Select, and clicking Adjust over a menu option will select the option but keep the menu open.

When you are in the Map mode, you can place a station by clicking Adjust rather than Select, and you will then remain in the Map mode; however, you will not be able to open a menu until after you have clicked Select to exit from the Map mode.

Copyright  RISC User magazine 1997
