VOAProp was written to satisfy my needs for a program that would show me what propagation should be like on a particular day on a particular band. It was developed from HFProp, my first attempt at such a program, which used very old and simplified propagation models designed to run on the slow personal computers of the 1980s. VOAProp uses VOACAP, a propagation model developed over 50 years by the US Navy Research Laboratory and the Institute of Telecommunications Sciences, with sponsorship from Voice of America. Calibrated over many years, using reception reports from thousands of Voice of America short wave listeners, it is probably the most accurate HF propagation model available. However, it suffers from an awkward and cumbersome user interface. Now that computers are much faster, it has become possible to run the VOACAP model interactively to produce propagation maps and charts at the click of a mouse, as HFProp did.
The VOACAP model is calibrated using the smoothed sunspot number, which is a 13-month rolling average centered on the month in question. The SSN for the current month and the months ahead can't possibly be known, so predicted values are used. It should be evident from this that VOACAP - and consequently VOAProp - cannot forecast the exact propagation on a given date. Instead, it shows the typical expected propagation under the conditions given. The real propagation may be better, or worse, than shown. As a rule of thumb, so experts say, the maximum operating frequency on any given day may vary by +/- 15% of the average predicted for that month. This can have a significant effect on the DX workable at that time. When conditions are above average, signals may be propagated further by one or two extra ionospheric hops. When they are below average, signals may not reach as far as shown, and a band that is shown as open to a few areas may be closed altogether.
It is important to understand, therefore, that VOAProp can only give a general indication of the conditions that may be expected. The VOACAP model doesn't take any account of how conditions change day-to-day in reaction to dynamic changes in solar weather. These changes are reflected (though only partially) in the values for 10.7cm solar flux, A index and K index. These indices, broadcast regularly by WWV, are used by VOAProp only to give a general assessment of what actual conditions may be like, relative to the average predicted propagation. VOACAP does not use these solar indices in any way, and therefore they have no effect on the propagation plotted on the map.
The last version of VOAProp is 1.1. If you have an earlier copy then please download the latest version.
Please note that VOAProp is made available without any warranty or support. I now use Linux in my radio shack and no longer have the tools used to create VOAProp so there will be no further development of this program.