Sunday, July 30, 2017

VOACAP Online Point-to-Point User Interface revamped

It happened last night: the familiar 24-hour circular prediction chart was replaced by versatile and detailed propagation prediction charts, 15 charts to be exact, on the front page of the VOACAP Online P2P predictions. URL:

The reason is simple: the new charts contribute to better assessment of HF propagation. You can paint the big picture with a single VOACAP output parameter but, for a more accurate picture, you will need (at least) three: REL (used by the earlier 24-hour circular chart which is still available via a separate link: but also SDBW (signal power) and MUFday. These are now offered to the ham community, in addition to band-by-band predictions, which even visualize the signal power distribution (upper decile, median and lower decile).

Each chart offers multi-colored lines for the various parameters. And thanks to the JavaScript framework (plotly.js) used for plotting these graphs, all visible legend parameters/frequencies can all be toggled on and off by clicking on the legend values on the bottom of the graphs, helping the user focus on relevant parameters/frequencies only. Also there is a versatile toolbox on the top-right corner of each graph that allows the user to save the graph as PNG, zoom in/out, compare results data on all frequencies on mouse hover, pan the chart, and more.


But let's now make a recap of what the three parameters -- REL, SDBW and MUFday -- mean to you.

The REL or Circuit Reliability. The REL is related to VOACAP's output parameters of SNR (Signal-to-Noise Ratio) and REQ.SNR (Required Signal-to-Noise Ratio), and is defined as a circuit reliability factor. It tells us the percentage of days in the month when the SNR value (which is not shown in the charts as a separate parameter) will equal to or exceed the REQ.SNR. The REQ.SNR is an internal value set by me, related to the transmitting mode selected. For CW, the REQ.SNR is set to 24 (dB-Hz), and for SSB, it's 38 dB-Hz.

SDBW or Signal Power. The SDBW indicates the dBW (the strength of a signal expressed in decibels relative to one watt) value (the green line in the chart) that can be maintained on 50% of the days (ie. on 15 days) in the month. In a similar fashion, the SDBW90 indicates the dBW or signal strength value that can be maintained on 90% of the days (ie. on 27 days) in the month. And finally, the SDBW10 is the dbW value that can be maintained on 10% of the days (ie. on 3 days) in the month. However, it does not tell us which days are good or which days are bad. The SDBW10 and SDBW90 values are the top and bottom boundaries (respectively) of the light-gray area that is now always visible in all band-by-band prediction charts. The signal power distribution is calculated for Short-Path circuits only. The SDBW values are all translated to corresponding S-Meter readings in the charts.

The MUFday will tell us what percentage of the days in a month at that hour will be below the predicted MUF (Median Maximum Usable Frequency) for the most reliable mode (MRM). The MRM is the mode with the highest reliability of meeting the Required Signal-to-Noise Ratio, or REQ.SNR (see above).

These three output parameters are being calculated via Short-Path and Long-Path.

At the same time, I changed the default setting for the Transmit & Receive Antenna and Transmit Power. Now the TX & RX antenna is a quarter-wave vertical antenna over a good ground, and the TX power is set to 1.5 kW.

So, how to use the new charts? I regularly follow this sequence:
  1. Check the bands of the best REL values for the path in question.
  2. Then check the SDBW values for the best bands.
  3. When you have your candidate bands selected, then go to the charts for those bands.
  4. In band-by-band charts, pay attention to MUFday values, and the signal power (SDBW) distribution (the light gray area). If the distribution is extremely wide, there is a chance that VOACAP unfortunately does not have a good idea of what's going on.
  5. Be sure to check the Long-Path predictions, too! On long-haul paths, Long-Path may bring nice surprises.


  1. Fantastic work, many thanks! I notice that the VOACAP HFBC page ( is currently offline. This is one I also use a lot for SWBC DX observations and monitoring. Are you planning to update that as well? Best 73, Rob vK3BVW

    1. Hi Rob, the VOACAP HFBC Online has been updated, too, but the URL has changed a bit; it's now ... actually, it's the best than ever!! :) 73 Jari OH6BG