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.