Sunday, April 29, 2018

Minor changes in the REQ.SNR values and new TX modes at VOACAP Online

Quite recently, I slightly fine-tuned some of the REQ.SNR values and was inspired to add FT8 and WSPR as additional TX modes in the VOACAP Online propagation prediction service after watching a YouTube video where Joe Taylor K1JT talks about WSJT-X.

The video is really worth watching. Here are the details:
Work the World with WSJT-X
Joe Taylor, K1JT

On one of his slides, he shows an intriguing table, and, taking his word for it, I made slight modifications to the REQ.SNR values I had initially adopted (except for SSB). His table is this:

Weak-Signal S/N Limits

Mode (B=2500 Hz)

SSB: ~+10 dB
MSK144: -8
CW: -15
FT8: -21
JT4: -23
JT65: -25
JT9: -27
QRA64: -27
WSPR: -31

As a general rule, to calculate the threshold (or minimum) SNR values for VOACAP (which are termed as "REQ.SNR"), you would use this formula:

REQ.SNR [dB-Hz] = SNR [dB] + 10 * log(BW) [Hz]

According to sources, the minimum SNR for AM broadcasts is around 13 dB at the RX bandwidth (B). So, applying the formula where B=4200 Hz (quite typical a bandwidth for an AM radio), we will get:

REQ.SNR = 13 dB + 10 * log(4200 Hz) = 49 dB-Hz

For SSB, people say that the minimum SNR required is around 5 dB. So, using a 2100 Hz bandwidth for SSB, we'll get:

REQ.SNR = 5 dB + 10 * log(2100 Hz) = 38 dB-Hz

and, for CW, people say that a good operator can copy CW at 0 dB. So, if we use a 100 Hz bandwidth, we'll get:

REQ.SNR = 0 dB + 10 * log(100 Hz) = 20 dB-Hz

Similarly, converting Joe's SNR Table values to VOACAP REQ.SNR values at dB-Hz, we'll get:

SSB REQ.SNR = 10 dB + 10 * log(2500 Hz) = 44 dB-Hz
CW REQ.SNR = -15 dB + 10 * log(2500 Hz) = 19 dB-Hz
FT8 REQ.SNR = -21 dB + 10 * log(2500 Hz) = 13 dB-Hz
WSPR REQ.SNR = -31 dB + 10 * log(2500 Hz) = 3 dB-Hz

For SSB, I think +10 dB on Joe's table may be too pessimistic as the minimum limit. If he had used 5 dB, we would have been totally aligned.

So, this is how the REQ.SNR values have been calculated for VOACAP. I hope this clarifies the matter.

Sunday, February 4, 2018

VOACAP Point-to-Point QSO Window: know your window of opportunity to work DX!

I am pleased to announce that I have released a new functionality for exploring the windows of QSO-making opportunities at the VOACAP Point-to-Point (P2P) service at This service has been inspired by Risto OH3UU and Cesar PY2YP.

The front page of the VOACAP Point-to-Point prediction service. The new "QSO Window" button is on the right of the page. Click the image to enlarge.

Now, on the Google Map on the VOACAP P2P page, you will see not only two markers (for TX and RX) but also five new red (TX) markers, labelled from A to E. These are the sites of the "competing" locations that are trying to make a QSO with the DX station, too. Now, for you to able to see your chances in relation to the other five, the QSO Window function runs five extra propagation predictions from the five sites to the DX site (RX), and displays the results as interactive charts where you can easily compare the output values to those from your location.

To fully leverage the new functionality, you should set the location of the Transmitter Site (TX) to your QTH, and the location of the Receiver Site (RX) to that of the DX. The five red markers are additional Transmitter sites, too, and their default locations have been chosen to reflect perhaps a typical set-up:

  • A = West Coast USA
  • B = East Coast USA
  • C = South Europe (Italy)
  • D = East Europe/Russia West
  • E = Japan

The user can freely set these markers to his/her liking on the map.

When you have set up the TX and RX locations, the five extra markers, and other input values, then press the "QSO Window" button. A new window will appear with results. If no results will appear, just refresh the result page. It can be that there are too many users at that particular moment.

Please note that the five "competing" stations will use the same input values you have set to the Transmitter (TX): the same antennas per band, the same power, the same mode, path, etc.

Finding the best QSO Windows by band

The results will be displayed band-by-band as interactive graphs for the following VOACAP output parameters:

  • Median Power Power (SDBW, displayed on a S-meter scale)
  • Reliability (REL)
  • Median Signal-to-Noise Radio (SNR)

The header of the result page: Short/Long Path, and distance and bearing. Click to enlarge.
The header of the result page: The coordinates of the five TX stations (markers A to E). Click to enlarge.
On each graph, each station (TX and stations A to E) has a color of its own, and the legend below the graph shows which color belongs to which stations. As you have set your own QTH to the Transmitter (TX), your color is that of the TX station (typically blue).

The result graphs for comparing the results -- and finding the best QSO windows! Click to enlarge.

All the graphs are interactive, which means that you can hover the mouse over the graph and see the results instantly by station. At the bottom of the graph, the UTC hour will be highlighted as you compare the result values with each other. If you feel the graph is too messy and the number of the lines should be reduced, just click on the color of the line in the legend. Adn if you want to see the line again, just click the legend once more.

This is an example circuit from Finland to Rotuma in February 2018. The SNR of the TX site (Finland) peaks at 9 UTC, with the predicted SNR value of 28 (dB/Hz). So, just about readable! However, as we can see, there can be tremendous QSO walls from sites D (East Europe/Russia West), C (Southern Europe/Italy) and E (Japan).

In the Signal-to-Noise (SNR) section, all the graphs have a red dotted horizontal running through the graph. The line has been drawn at 24 dB/Hz, which is to indicate where the threshold for CW copy in terms of SNR has been set.

Friday, January 5, 2018

Getting the best operating frequency from VOACAP P2P predictions

Updated Jan 6, 2018: Primary, Secondary and Tertiary Frequencies reported.

I am happy to introduce a new functionality at the VOACAP Online P2P (point-to-point) HF propagation prediction service (, namely assessment of the Best Operating Frequencies for every hour of the day for the circuit chosen. This means that all ham radio bands are being considered and, after a careful evaluation, the three best bands will be displayed, together with some VOACAP prediction data.

To use the new service, just click on the new button labeled "Best FREQ", located just below the graphs on the front page.

On the output page, the best operating frequencies can be found in the rightenmost columns: FREQ, FREQ2, and FREQ3. On the left, you will have the UTC hours from 1 to 24. And the other columns from left to right are as follows:

  • SDBW (or Signal Power at the receiver): the dBW (the strength of a signal expressed in decibels relative to one watt) value that can be maintained on 50% of the days (ie. on 15 days) in the month. The S-Meter Signal Strength is displayed in parentheses after the dBW value.
  • REL (or Circuit Reliability): percentage of days in the month when the SNR value will equal to or exceed the REQ.SNR (the threshold value for the given transmit mode); for example, the threshold value (= REQ.SNR) for CW is 24 (dB-Hz)
  • SNR (or Signal-to-Noise Ratio): the median dB-Hz value that can be maintained on 50% of the days (ie. on 15 days) in the month. So, if the SNR value is lower than the threshold or REQ.SNR value for a mode (CW = 24, SSB = 38, AM = 49, etc.), then not so great propagation may be expected at all times.
  • MUFday: 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).
  • FOT (or Frequency of Optimum Traffic): also known as the Optimum Working Frequency for the hourly MOF (median maximum observed frequency) distribution. At the FOT, communication can be supported on 90% of the days (27 days) in the month.
  • MUF (or Median Maximum Usable Frequency): the median maximum usable frequency for a given ionospheric path, month, SSN and hour. On each day of the month at this hour, there is a maximum observed frequency (MOF) for a mode. The median of this distribution is called the MUF. Therefore, here MUF is not the maximum usable frequency in terms of communications. In other words, the MUF is the frequency for which ionospheric support is predicted on 50% of the days of the month, ie. 15 days out of 30 days.
  • HPF (or Highest Possible Frequency): at the HPF, communication can be supported on 10% of the days (3 days) in the month.

The best operating frequencies from Bouvet to my location on February 2018.
Primary = Best FREQ, Secondary = Second Best FREQ, Tertiary = Third Best FREQ

Now, three Best Frequencies are being reported in the output. The SDBW, REL, SNR and MUFday values are reported for the Best Frequency (FREQ) only. In many cases, the Best and Second Best Frequency can be quite equal in performance so it can be hard to tell which one is actually better. However, on the other hand, the Third Best can sometimes be just a theoretical (or even an impossible) choice in practice.

It should be noted, though, that FOT, MUF and HPF cannot be used alone for reliable propagation predictions. The best predictions consider Signal Power, Circuit Reliability, and Signal-to-Noise Ratio values. And this is also what the assessment of the best operating frequency for each hour is based on.

Please be warned, though, that, as a consequence, there can be hours with the "best" operating frequency which does not, in reality, support any communication! The reason can be that the Signal Power, or Circuit Reliability, or Signal-to-Noise Ratio are too low at that hour.

In order to indicate that there can be problems with the best operating frequency given, there can be three signs after the Best Frequency (FREQ): - (minus), + (plus), or * (star). The minus sign means that the Signal-to-Noise Ratio predicted is below zero (i.e., a negative value); the plus sign means the REL value is below 10% but the median Signal Power is still barely above the noise; the star sign means that the REL value is below 10% and the median Signal Power is also more or less under the noise.

The minus, plus and star signs will be applied to the Best Frequency only. So, if the Best Frequency will be flagged with any of these signs, please note that the second best or the third best frequency will not be any better in performance but likely much worse.