Ten Common Mistakes in Using VOACAP


This story is supposed to be taken in a light note although these are the potential risk areas that can turn your prediction upside down :-) Some of the issues covered here seem to be rather commonplace according to my searches using Google... however, it's another story how serious these all really are: some are very serious and some are not. You may have compiled altogether different selections, and therefore, it would be nice to hear about them :-) Enjoy!

1. The use of the URSI coefficients. This seems to be a more common mistake than you might expect, and it may even happen without your knowledge! Use the CCIR Coefficients. See also Mistake 5.

2. The choice of a wrong Smoothed Sunspot Number for the month. The resource that maintains the (predicted) SSN figures based on the Lincoln-McNish smoothing function to be used with VOACAP is located presently at the address below.

These are the sunspot numbers used in the database reduction for the maps used in IONCAP and now VOACAP.


I don't know where the hfradio.org site, for example, takes its numbers from but they are not from this resource.

WARNING: Absolutely do not use a daily Sunspot Number here!

3. The use of an unsuitable Required SNR (REQ.SNR) value for the transmission mode. This, too, is difficult for a number of people - and of course it varies case by case, depending on the grade of service you wish to achieve. However, you can immediately see if there is something wrong with these values, say 73 for CW ;-) Perhaps a common start value for CW is 19/24, 40 for SSB and 67 for BC. And, yes, the values are in dB/Hz.

4. The use of a wrong transmitting power. The transmitting power to use is the power at the antenna feedpoint, not at the back of your transmitter :-) Therefore, a rule of thumb could be that you use 80% of the transmitter output power as the power at the feedpoint. I gladly admit I break this rule on a regular basis ;-) And you really have to pay attention and enter all values in kW, so 1 watt is 0.001 kW.

5. Entering a Day in the Month/SSN Groups window. This is where you can make an unconscious mistake (see Mistake 1). In Groups, there should only be the number of the month (e.g. "7.00" or "8.00" and so on) and its corresponding SSN. If you enter a day within the month (say "7.29" where 7 is July and 29 is the day), you will ruin your prediction :-) Using the day automatically forces VOACAP to use the URSI Coefficients for calculations. And it does so without any warnings!

6. Setting a wrong Required Reliability (REQ.REL.) value. This can be somewhat a matter of taste but I would suggest always to use the value of 90%. Of course, if you know what you are doing, then it's OK :) I can figure out at least one advantage with 90%: you can determine the best operating frequency just by looking for the highest SNRxx value!

7. Setting too much/little man-made noise. This is again difficult. The default value of 145 is a good starting point. Here in Finland, if you go to the countryside, you can regularly enter 164 (remote) since the only noise comes from within your receiver - or from your head ;-) In the cities, it is a different story. In the suburbs, I guess 155 is, at least in this country, suitable.

8. Setting far too optimistic a Minimum Angle. As discussed in another article, the default value of 0.1 degrees, especially for isotropic antennas, is absolutely out of question. You must use a higher value, e.g. 3 degrees. For regular antennas, 0.1 degrees is fine.

9. Selecting a wrong antenna. You have to investigate your antenna and its characteristics with the HFANT software. You cannot be always certain that the azimuth and elevation patterns are as you expect. Otherwise, you may choose an antenna which has nothing to do with the case in hand - so the prediction will be in danger.

10. The use of a wrong Method. Most of us needs only a few Methods. I use regularly Method 30, sometimes Method 25 and Method 21. If I calculate FOT-MUF-HPF curves, then I use Method 9. A good starting point is Method 30, and you may not need another Method. For the other Methods, you will need to know what you are doing since only part of the available Methods were tested and verified during the development of VOACAP. For instance, Method 9 was not among them but Methods 30, 25 and 21 were.