I think there is some misunderstanding of bandwidth using FT-8.
A single FT8 "signal" occupies somewhere just a hair more than 50 Hz.
However, that is not the number to use for calculating the equivalent noise
bandwidth. FT8 is made up of 8 tones, only one of which is on at any time.
Each tone is separated from the next by 6.25 Hz. So, together they take up
50 Hz. There is always some keying sidebands, so that makes the occupied
bandwidth just a smidgen higher. A clean FT8 signal (no IMD, no
overmodulator, etc) can easily sit inside a 60 Hz filter.
The equivalent noise bandwidth of an FT8 detected tone is only 6.25 Hz. So
that is the noise bandwidth to use for any calculations, not 50 Hz. I.e.,
FT8 is not some miracle. It gets its sensitivity from using a very narrow
bandwidth (and slow baud rate). In addition, there is FEC.
The SNR is computed based on a 2.5 kHz passband. That is why you see
numbers like -20 dB SNR. Factually, the SNR of the 6.25 Hz wide detector is
a factor of 2500/6.25 higher, i.e., 26 dB higher.
Jim Wolf, KR9U
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