FAQ - Part 3


Why signal strength and not SNR?

More often than not, propagation predictions are shown with plots of Signal to Noise ratio (SNR) rather then Signal Strength. This is true of many prediction programs like HamCap, W6ELProp etc.

K6TU.NET produces propagation predictions displaying Signal Strength. Signal strength is determined from the VOACAP SDBW values which predicts the power delivered at the receiver. The power level is converted into a signal strength show in S Units. Unlike SNR, signal strength is not dependent on receive bandwidth and levels are immediately useful. For example, if a prediction shows a signal level of S5 and your current background noise is S4, you can see that the received signal is barely above the noise and CW is a good choice as the mode to pull off a contact.

Similarly, if you are entering a SSB contest and the prediction shows you can deliver an S8 or better signal into your target area, you have a good chance of being able to maintain a run frequency as you will be strong in an area you want to work.

Signal strength offers a more intuitive display of results and is instantly familiar without having to do mental math to map SNR to a useable value.

Why are there little white triangles visible on the PDF maps?

When looking at the PDF files produced by the service, you may see small triangles with faint white lines in areas of color. This is due to an anti-aliasing problem with the viewer and does not affect printed copies of the PDF. If you wish, you can alter your reader settings to make the white lines go away.

Here are instructions on how to remove the white lines with Adobe Reader X for both Windows and Mac OS X.


  • With Adobe Reader running, got to the Edit Menu and select Preferences.
  • When the Preferences window opens, select Page Display from the list of Categories on the left side of the window.
  • In the Rendering controls, make sure that Use 2D graphics acceleration is not selected, now also de-select Smooth Line Art.
  • Click on OK.

Mac OS/X

  • With Adobe Reader running, select Preferences from the Adobe Reader menu.
  • Select Page Display from the Categories list on the left on the window.
  • De-select Smooth line art under the Rendering controls.
  • Click on OK

How accurate are the predictions?

The VOACAP engine uses a statistical model of the ionosphere based on years of observation across complete solar cycles. As a result, you should expect to see some variability between the predictions and the conditions you experience at any given time. You may find that predicted signal strengths vary both plus and minus on the predicted values. Solar weather can also adversely affect the predictions if a strong flare or coronal mass ejection causes the Earth's geomagnetic field to become disturbed or enter storm conditions.

How can I make predictions on my own computer?

If you want to generate predictions using your own computer, I suggest you visit VOACAP.COM written and maintained by Jari Perkiömäki, OH6BG/OG6G. This is an excellent site and has pointers to tutorials on VOACAP, links to download the software and the ability to generate on-demand predictions.