> Would you think it is more efficient to try shorter
> symbol lengths in the morning time when QRN is low
> or would it be better to use 100s symbols and run
> the transmission partly during higher QRN periods?

I guess, the most important factor is to avoid the
terminators, so set the symbol period so that the
message takes all day or all night.   The interleaving
helps the mode to cope with spells of QRN but if the
terminator changes the phase part way through by
more than about 90 degrees the decode will be lost.

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Paul Nicholson
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