Thursday, July 27, 2017

SDBW, SDBW10 and SDBW90: The Predicted Signal Power Distribution

During the past few days, two more Signal Power parameters - namely, SDBW10 and SDBW90 - were added to the VOACAP D-I-Y Charts and the similar charts at VOACAP Online Point-to-Point service. As a result, these values are being used to create an area filled with the light-gray color, showing the Signal Power distribution. It's calculated for Short-Path circuits only.

The Signal Power distribution tells us what levels of signal power at the receiver are to be expected over the days in the month on the given frequency at the given hour.

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.

In our charts, we also display the REL and MUFday values in order to get as complete a picture as possible for the final propagation assessment. 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 related to the TX mode selected. For CW, the REQ.SNR is set to 24 (dB-Hz), and for SSB, it's 38 dB-Hz. And, last but not the least, the MUFday values will tell us what percentage of the days in a month will be below the predicted MUF (Maximum Usable Frequency). These values will be calculated via Short-Path and Long-Path.

Beware of extremely wide Signal Power distributions

Whenever you see that a Signal Power distribution is extremely wide, e.g. ranging from the S-Meter reading of S0 to S8 at any given hour, you are getting into the noise of the program, meaning that VOACAP will give predictions even when it has no idea what is going to happen. If we believe the prediction, then VOACAP is saying that 80% of the days of the month (bounded by SDBW90 and SDBW10) will have a Signal Power somewhere between S0 and S8. That is a spread of 48 dB! If the program could talk, it would tell you that it doesn't have a really good idea what is going to happen on that frequency at that hour.

The area filled with light-gray (= Short-Path Signal Power distribution) is bounded by the SDBW10 (top) and SDBW90 (bottom) values. Times and frequencies with extremely wide Signal Power distributions are not reliable enough. Typically, in such cases, the MUFday values are also low.