In an email to me, Russ WA3FRP brought up something interesting -- of
switching to wider shifts on the lower bands.
When shifts are too narrow, the mark and space can be so close to one
another that they no longer fade independently. When that happens,
any ATC circuit that tries to work around selective fading no longer
functions, since there is _no_ selective fading. When both tones
vanish below the noise at the same time and the decoder can no longer
decide if mark or space was transmitted.
The question is therefore (1) is selective fading on the low band
replaced mostly by flat fading, and (2) whether the use of greater
frequency diversity (larger shift) will bring back selective fading so
that at least one of the tones survives through a fade.
If all else is equal, selective fading is preferable to flat fading
for RTTY since that is what ATC circuits are designed to overcome.
Flat fading is better solved with the use of temporal interleavers and
error correction codes, such as used in MFSK16, Olivia and DominoEX --
the modern digital modes tend to spread errors in the time dimension
to overcome fading.
It is easy enough to generate a signal that has 4 tones that are
separated by 200 Hz each, for example, to try to objectively measure
selective fading at different frequency diversity. The problem is
that such a signal might not pass FCC 97.309. Short of that, it might
be sufficient to just watch a 500 Hz wide MFSK signal on a spectrum
analyzer. For starters, I think I will try to look for fast Olivia or
DominoEX on 80m to watch carefully. The slower baud rates don't work
as well since selective fades could have come and gone by the time the
tone you want to watch is transmitted.
73
Chen, W7AY
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